600 Ponds and Basins

AbstractThis section discusses the design and installation of concrete basins, synthetic pond and basin liners, and shotcrete or concrete pipeways. It focuses on environmental considerations, primarily on the need for long term leak-free designs that protect the soil and ground water. It also gives guidance on where to go for help in order to understand the environmental regulations.

Contents Page

610 Environmental and Safety Concerns 600-3611 Environmental

612 Safety620 Concrete Basins 600-4621 General Design Considerations

622 Design Criteria623 Loads And Analysis624 Construction

630 Pond and Basin Liners 600-12631 Liner Uses and Selection632 Flexible Membrane Liner Materials

633 Design and Construction634 Inspection635 Common Problems636 Leakage Monitoring and Detection637 List of Manufacturers and Installers638 Company Experience

640 Shotcrete and Concrete Paved, Grade-level Pipeways 600-43Chevron Corporation 600-1 June 1997

650 Description of Closure Technologies 600-44660 Model Specification, Standard Drawings, and Engineering Forms 600-45

600 Ponds and Basins Civil and Structural Manual661 Model Specification662 Standard Drawings670 References 600-45671 CRTCs Materials and Equipment Engineering Reports on Liners672 Other ReferencesJune 1997 600-2 Chevron Corporation

Civil and Structural Manual 600 Ponds and Basins610 Environmental and Safety Concerns

611 EnvironmentalMany federal, state, and local regulations govern the installation, operation, mainte-nance, and cleanup of facilities which are set on or in the ground. Most of these regulations are still in the process of being developed or interpreted. However, the following generalizations can be made:

Understand the rules and regulations which govern your area. Consult your local operating company environmental specialists or CRTCs HES Group for assistance. In some cases, it is also important to have an idea where regulations are headed in the future. The environmental specialists can also help you with any permits that regulatory agencies require.

Avoid the installation of new basins or ponds if at all possible. While ponds and basins may seem like the most economical alternatives now, they may be expensive to maintain or to close and clean up in the future as they become more and more regulated.

For any ponds or basins which must be installed, consider secondary contain-ment and leak detection. This is required by regulations in some cases, but may be prudent even if not required.

Do not underestimate the testing, management, and disposal costs of excavated material, especially in existing plants. Again, consult your environmental specialist on what can be disposed of onsite versus offsite. Our objective should be to minimize offsite disposal.

Section 650 lists techniques for clean-up and closure of ponds or other waste sites. Consult CRTCs Material and Equipment Engineering for up-to-date technology in this area and for other entities in the Corporation that have undertaken similar jobs.

612 SafetyThe following safety precautions should be taken when excavating for pond liners or for concrete basins:

Maintain the proper slope stability or shoring as discussed in the Safety in Designs Manual.

Understand the hazards of the material being handled. It should be sampled in several locations to determine:

The type of protective clothing to wear The degree of breathing protection requiredChevron Corporation 600-3 June 1997

600 Ponds and Basins Civil and Structural Manual620 Concrete BasinsThis section primarily discusses basins designed to retain water. Other fluids, in particular molten sulfur pits, need special design and material considerations. In sulfur pits, the vapors from the sulfur mix with moisture and condense on the exposed concrete surfaces. The resulting acid attack rapidly deteriorates normal concrete materials. Consult a materials engineer for selection of a proper lining for molten sulfur pits.

621 General Design ConsiderationsBelowground concrete basins are frequently used for waste water treatment systems, such as oil/water separations, biological treatment, and clarifiers. They are also used for drainage sumps, cooling tower forebays and basins.

The types of facilities mentioned above are commonly installed with part or all of the structure at or below grade level. This is done to meet hydraulic gradient requirements, provide convenient access for operators, and to take advantage of lateral soil pressure on the outside of basin walls to resist hydrostatic pressure within the basin.

Reinforced concrete is commonly used for construction of these facilities for the following reasons:

Concrete provides physical properties that assure long service life with low maintenance if properly designed and constructed.

Concrete offers great design flexibility. It can easily be formed to provide desired water flow requirements and to support mechanical equipment, piping, and appurtenances.

With proper design, concrete basins can hold large volumes of liquids and remain water tight.

Basin ProblemsThe structural problems that most commonly occur with concrete basins are:

Differential settlement of the basin. Minor to severe cracking and leakage can result if this occurs.

Inadequate concrete coverage for reinforcement. This can lead to corrosion with spalling and deterioration of concrete.

Expansion and contraction of the basin from filling or draining. Problems may occur at the connection of wall and basin slab or at wall connections.

Inadequate basin design for all possible operating conditions, particularly when the basin is empty. A high water table or flooding could result in high external soil/water pressures and possibly flotation.

Loss of water tightness where piping penetrates the basin walls.June 1997 600-4 Chevron Corporation

Civil and Structural Manual 600 Ponds and BasinsControlling CracksAt the design stage, the best techniques for controlling basin cracks and reducing leakage from the basin include:

Getting the best information possible on the soil conditions at the site. Soils that are fairly uniform over the entire basin area do not generally present a problem, because large differential settlements are not expected. Varied soil conditions across the site can cause severe problems if not recognized and prop-erly accounted for in the design.

Providing adequate reinforcement in the walls to meet anticipated shrinkage and temperature requirements.

Paying careful attention to the following structural details:

Interconnection of basin walls Wall-to-slab connection All wall penetrations Abrupt changes to basin geometry or loading where planes of weakness

can develop in the structure or differential settlement can occur.

Secondary ContainmentTo meet specific site requirements or to comply with applicable regulations, it may be necessary to provide secondary containment and leak detection for concrete basins. Refer to Standard Drawing GD-S1119, Standard Secondary Containment and Leak Detection Details for Concrete Basins and Appendix F (Secondary Containment for New Construction and Existing Facilities).

Precast Concrete BasinsAlthough this section primarily deals with relatively large cast-in-place basins, there is often a need for small basins, sumps, or boxes. The availability of precast units should be explored to determine if this is an economical way to meet the need.

622 Design CriteriaFollowing is a discussion of some of the critical elements that must be considered when designing basins.

Recommended StressesACI 350R, Concrete Sanitary Engineering Structures, discusses the importance of using conservative allowable stresses to minimize cracking in reinforced concrete basins. Tables are included which provide recommended maximum stress at service loads for various bar sizes and severity of exposure. The commentary for ACI 318R (10.6.4) states that Several bars at moderate spacing are much more effective in controlling cracking than one or two larger bars of equivalent area. For this reason, the recommended stresses for flexural crack control decrease with increases in bar sizes.Chevron Corporation 600-5 June 1997

600 Ponds and Basins Civil and Structural ManualDesign ConditionsBesides the normal operating conditions for the basin, consideration should also be given to the following cases:

Basin empty with active soil pressure and possible hydrostatic pressure acting on the basin walls. This is a condition that will probably occur after initial construction when backfill is complete, and again whenever basin maintenance requires it to be drained.

High ground water or flooding conditions with the basin empty. Basin to be checked for possible flotation.

Design RecommendationsACI 350R makes the following recommendations for design of basin walls in contact with liquids:

Walls 10 feet and higher should be at least 12 inches thick.

Maximum reinforcing bar diameter should not exceed 6% of the thickness of the structural member.

Maximum reinforcing bar spacing should not exceed 12 inches.

Shrinkage and temperature reinforcement perpendicular to the main flexural reinforcement should be a minimum of 0.3% for walls less than 12 inches thick. For walls more than 12 inches thick, the requirements of ACI 318 should be followed.

623 Loads And AnalysisThe primary loads that must be considered for design are:

Vertical loading on the bottom slab of the basin. If the basin is pile supported, the use of uplift piles may have to be considered.

External lateral loading on the basin walls due to hydrostatic pressure, buoyant loads, and lateral soil pressure.

Dead loads from equipment or structures that the basin will support.

Hydrostatic loads from fluids in the basin.

For analysis and design of belowground concrete basins, refer to References 1 and 2 in Section 672. Perhaps the most useful for typical Company installations is Reference 3, Rectangular Concrete Tanks, which may be obtained by writing or calling:

Portland Cement AssociationOrder Processing5420 Old Orchard RoadSkokie, Illinois 60077847-966-6200June 1997 600-6 Chevron Corporation

Civil and Structural Manual 600 Ponds and BasinsThis publication includes:

Moment coefficients for slabs with various edge conditions (free, fixed, hinged) Moment coefficients for rectangular concrete tanks Shear coefficients Top and base slab design Muticell tank design Details for wall-to-base slab

Using the moment coefficients from Reference 3 of Section 672, the designer is able to calculate moments at critical points in individual wall panels under hydro-static pressure increasing from zero at the top to a maximum at the bottom. The same coefficients are applicable for design of walls for square tanks. Where there will be moment distribution at the edges of walls, such as with rectangular basins, separate tables are provided for determining critical design moments.

624 Construction

Corrosion ConsiderationsUnless the fluids in the basin are expected to include concentrations of corrosive substances, concrete will be quite durable in service. Resistance to chemical attack of the concrete can be improved by providing the following:

Good quality concrete to make it dense, strong, and impermeable

Adequate cover for reinforcement

Thorough compaction of the concrete

Smooth surface finishes for contact surfaces

Epoxy-coated (with electrostatically-applied powder) reinforcing steel The use of a surface coating or sealing system. Refer to ACI 515.1R, A Guide

to the Use of Waterproofing, Dampproofing, Protective, and Decorative Barrier Systems for Concrete.

Consult a CRTC Materials and Equipment Engineering coatings specialist if an internal coating is being considered.

Mix ProportioningRefer to ACI 350R for the following requirements for concrete mixes for construc-tion of basins:

Compressive strength Cement type Maximum water-cement ratio Air entrainment SlumpChevron Corporation 600-7 June 1997

600 Ponds and Basins Civil and Structural ManualConstruction JointsWhen a pour must be stopped at intervals during construction of the basin, a construction joint is created. Construction joints should be located so as to least impair the strength of the basin structural components and provide logical separa-tions for the sequence of construction. ACI 350R (Reference 4) recommends vertical spacing of construction joints from 10 to 15 feet and horizontal spacing of 20 to 30 feet unless expansion joints are serving as construction joints. All rein-forcement should be continuous through construction joints with waterstop and keyway if required. Provision should be made for installation of sealant on the inside face of construction joints. Refer to ACI 504R for recommended details for construction joints.

Expansion JointsExpansion joints are required to allow for basin expansion, contraction, differential foundation movement, or unbalanced applied loads. As a general rule, expansion joints should be provided at a spacing of not more than 50 to 60 feet for basins exposed to the atmosphere and 80 to 100 feet for basins completely underground. The recommended expansion joint widths for different temperature ranges of the concrete, shown in Figure 600-1, are taken from ACI 350R. Expansion joint details are illustrated in Figure 600-2. Joints typically are constructed using the following components:

Preformed joint filler Joint sealant Waterstop

Preformed joint filler preserves space into which the concrete may expand. Mate-rials for the joint filler are selected that are compressible to one half their original thickness yet fully recover when compression loads are reduced. Materials such as sponge rubber and cork conforming to ASTM D1752 are commonly used.

Joint sealants are discussed in considerable detail in ACI 504R. For expansion joints, sealants must have the following properties: Good bond Low shrinkage

(1) NR = Not Recommended

Fig. 600-1 Recommended Expansion Joint Widths

Spacing Between Joints (ft)

40 60 80 100

Conditions: Recommended Widths (in.)

Underground, 40F 1/2 3/4 7/8 1

Partly protected above ground, 80F 3/4 7/8 1 NR(1)

Unprotected, exposed roof slabs, etc., 120F 7/8 1 NR(1) NR(1)June 1997 600-8 Chevron Corporation

Civil and Structural Manual 600 Ponds and Basins High strength in rubber-like materials Resistance to flow and stress relaxation Ability to change shape without changing volume Not susceptible to permanent deformation

The bond breaker noted on Figure 600-2 is a surface application used to prevent the sealant bonding to the joint filler so that there can be independent movement of both sealant and filler.

WaterstopsWaterstops come in a variety of shapes. Four types are illustrated in Figure 600-3 and discussed below.

Fig. 600-2 Expansion Joint DetailsChevron Corporation 600-9 June 1997

600 Ponds and Basins Civil and Structural ManualFig. 600-3 WaterstopsJune 1997 600-10 Chevron Corporation

Civil and Structural Manual 600 Ponds and BasinsRibbed. Ribbed waterseals are available both with and without the center bulb. The ribs on the waterstop are designed to provide maximum anchorage to prevent water seepage.

Dumbbell. Dumbbell waterseals are used for joints when little movement is antici-pated. The ball at the end of the seal acts like a stopper in a bottle. Tension of the seal pulls the ball tighter against any crack that may develop.

Centerbulb. Flat waterseal shapes are used where limited joint movement is expected. Where greater joint movement is anticipated, centerbulb seals are gener-ally specified. Flexure of the bulb accommodates much larger movements without excessively stretching the waterseal material.

Labyrinth. Labyrinth waterstops are used in places where it is desired to form a structural key between separate poured sections.

Materials and Construction. Materials used for waterstops are usually either PVC, neoprene, or styrene butadiene rubber. PVC is not recommended for joints that are exposed to low temperatures and where significant joint movement is expected.

For basin construction, it is recommended that waterstops be 3/8 to 1/2 inch in thickness with a minimum width of 9 inches. The effectiveness of joints in preventing seepage depends on proper location of the waterstop within the joint. Careful attention should be given to the method used to firmly fix flexible water-stops to the reinforcement or forms, to prevent movement during the placing of concrete.

Figure 600-4, taken from ACI 504R, illustrates common performance problems associated with waterstops and recommendations for avoiding these problems.

Fig. 600-4 Waterstop Performance Problems and Solutions (Courtesy of American Concrete Institute)Chevron Corporation 600-11 June 1997

600 Ponds and Basins Civil and Structural Manual630 Pond and Basin LinersA flexible membrane liner (geomembrane) is a sheet of synthetic material, either plastic or rubber, which is used to control the migration of fluids. In order to comply with federal, state, or local environmental regulations, ponds, lagoons, and other surface or below grade impoundments often require some type of imperme-able liner to prevent fluids from seeping out into the surrounding soil and contami-nating groundwater. Double liners with leachate collection are required in some cases. Flexible membranes provide an alternative to materials such as clay, soil-cement, asphalt, or concrete, which may also be used in some applications or form part of a multi-liner system with membranes. This section gives information on synthetic liner materials; the design and construction of membrane-lined impound-ments; inspection; common problems; leak monitoring; and a summary of Company experience with these materials.

Geomembrane liners for tanks are discussed in Section 500 of the Tank Manual.

631 Liner Uses and SelectionThis section gives examples of membrane installations and lists factors in selecting a membrane liner.

Examples of Membrane InstallationsThe flexible membrane liner industry was developed out of efforts to conserve water in reservoirs, canals, and irrigation systems by preventing it from seeping out into the surrounding soil. While this is still a major use of membranes in terms of the volume of material installed each year, the use of membranes for pollution control and environmental protection has received much more recent attention. Examples of membrane installations for environmental reasons within Chevron include pond liners, waste impoundments, and secondary containment membranes beneath and around storage tanks. Refer to Section 638 for the service history of various liners the Company has used.

Pond Liners. Waste water treating ponds, evaporation ponds, settling basins, oil field brine ponds, and so on, are frequently lined with membranes to prevent the effluent from seeping out into the soil. In this type of installation, the membrane provides the primary containment, meaning that there is always fluid in contact with the membrane when the pond is in service. The surface of the pond is usually left uncovered, although floating covers can be manufactured out of the same membrane materials. The membrane liner under the pond may be buried under a protective soil cover, or may be left exposed.

Waste Impoundments. Municipal landfills and other waste dumps usually have a membrane liner under them, particularly if the waste is considered hazardous. Membrane liners are required by the U.S. Environmental Protection Agency for compliance with the Resource Conservation and Recovery Act of 1976, known as RCRA.

These regulations prevent the migration of waste or leachate. For this type of instal-lation, the membrane again acts as the primary containment. Double liners with June 1997 600-12 Chevron Corporation

Civil and Structural Manual 600 Ponds and Basinsleachate collection are required in some cases. A final cover may also be included in the design of a permanent waste impoundment. A membrane is often used as a part of the final cover to prevent rain water from percolating down through the waste.

Secondary Containment Around Storage Tanks. Many states have passed laws requiring secondary containment around all underground storage tanks which contain petroleum products or hazardous chemicals. Basically, the requirement is that if the tank leaks, there must be a secondary system capable of preventing the fluid from escaping into the soil or groundwater. Membranes can be used to provide secondary containment by forming a big bag around the tank. Chevron Marketing has elected to use double wall tanks to provide secondary containment at service stations but has used a membrane liner around the piping from the tanks to the pumps. They currently use double wall piping.

Chevron also uses membranes to provide secondary containment under the bottoms of aboveground storage tanks. The design philosophy is that any leak which occurs from corrosion of the tank bottom should be contained and some method of leak detection should be employed which informs operations that the primary contain-ment has failed. Refer to Standard Drawing GF-S1121, Standard Secondary Containment and Leak Detection Details for Storage Tanks and Section 500 of the Tank Manual.

Factors in Selecting A Flexible Membrane LinerMany different flexible membrane liner materials are available, and no one material is perfect for every job. The process of selecting the right membrane for a specific installation involves matching the requirements to the characteristics of a particular liner. Several key important characteristics (discussed in Section 632, under Prop-erties of Liner Materials and Seaming Methods) are: Mechanical strength Weathering resistance (if necessary; not as important for buried liners) Chemical resistance to the waste Permeability Ability to be seamed Cost and availability

Selection of the appropriate membrane material must be based on the physical and chemical requirements of the intended application:

Composition of the fluid or waste

Site conditions (maximum and minimum temperatures, wind velocities, soil type, slope lengths and angles)

Four principal membrane materials, described in depth in Section 632 under Avail-able Membrane Materials, are currently available:

Polyethylene (HDPE, LLDPE, VLDPE) Chlorosulfonated Polyethylene (CSPE)Chevron Corporation 600-13 June 1997

600 Ponds and Basins Civil and Structural Manual Chlorinated Polyethylene (CPE) Polyvinyl Chloride (PVC)High density polyethylene (HDPE) is the most popular membrane material in the industry and within the Company. HDPE has excellent weathering characteristics, good physical properties at thicknesses of 60 mils and up, and very good general chemical resistance. Some of the drawbacks to HDPE are its relatively high stiff-ness and high coefficient of thermal expansion.

Other polyethylene membrane materials are available. Low density polyethylene (LDPE), linear-low density polyethylene (LLDPE), and very low density polyeth-ylene (VLDPE), are more flexible than HDPE but are generally much inferior in chemical resistance and mechanical properties.

Hypalon is the Du Pont trademark for chlorosulfonated polyethylene (CSPE). Hypalon is reinforced with fabric (scrim) to give it tear resistance. Hypalon has good weathering resistance and good chemical resistance to acids and alkalis but poor chemical resistance to oils, fuels and solvents.

Chlorinated polyethylene (CPE) is available with and without scrim reinforce-ment. CPEs physical properties are similar to Hypalon, and it has good weathering resistance. However, its chemical resistance is generally not as good as Hypalon. CPE has no memory (the ability to return to original shape after deformation) and should not be used on slopes without a scrim reinforcement.

Polyvinyl chloride (PVC) is a widely used membrane material because it is rela-tively cheap. A plasticizer is added to give PVC flexibility, but the plasticizer is gradually lost during service, making the material brittle. PVC has poor weathering resistance and durability, and its overall chemical resistance is inferior to other membrane materials.

632 Flexible Membrane Liner MaterialsThis section has four parts. The first part describes what flexible membrane liners are made of and how they are manufactured. The second part, Properties of Liner Materials, describes the test methods used to evaluate these properties. The third part describes seaming methods, and the fourth part lists available membrane mate-rials.

Components and ManufacturingLiners are made of polymers and various compounding ingredients. They are often reinforced with fabric. Liner manufacturers use one of three fabrication methods: calendering, spread coating, and extrusion.

Basic Polymers. The basic polymers used in manufacturing flexible membrane liners vary from relatively soft rubbers to relatively hard plastics. All of the charac-teristics of the membrane, including its mechanical strength, chemical resistance, ability to be seamed, and so on, depend on the polymer used and the method of manufacture. The polymers used to make membranes can be divided into three major categories: elastomers, thermoplastics, and thermoplastic elastomers.June 1997 600-14 Chevron Corporation

Civil and Structural Manual 600 Ponds and BasinsElastomers. Commonly called rubbers, elastomers are chemically cross-linked (cured) under heat and pressure. They are thermoset, which means that after curing, they will not melt when heated. Cured elastomers also have memory, or the ability to elongate under stress and return to their original shape when the stress is removed. Examples of cured elastomers which are used as membranes include butyl rubber, neoprene, and ethylene-propylene rubbers.

Thermoplastics. Commonly called plastics, these differ from elastomers because they will melt when heated. This characteristic is important because it allows membranes made from thermoplastics to be seamed by the use of heat. Thermoplas-tics generally have poor memory and will elongate permanently under stress. Examples of thermoplastics used as membranes include plasticized polyvinyl chlo-ride (PVC), high density polyethylene (HDPE), and elasticized polyolefins (XR-5).Thermoplastic elastomers. These polymers are uncured elastomers which will melt when heated, like a thermoplastic, but which also have the memory characteris-tics of an elastomer. These are excellent properties for a membrane. Examples of thermoplastic elastomers used as membranes include Hypalon (chlorosulfonated polyethylene, CSPE), chlorinated polyethylene (CPE), and polyester elastomers. In some cases, particularly with Hypalon, the elastomer would have much better chem-ical and heat resistance if it were cured. However, when used as a membrane, it is left uncured to allow for easier seaming.

Reinforcement. Membranes are often reinforced with fabric, commonly called a scrim, which is sandwiched between layers of the polymer. The purpose of the scrim is to increase the tensile strength and decrease the elongation of the membrane, improving its dimensional stability under stress and increasing the punc-ture and tear resistance of the membrane. A reinforced membrane is generally much tougher and easier to handle than an unreinforced membrane made from the same polymer.

Scrims are typically open weave nylon or polyester fabrics and are specified by count, denier, and weave pattern. The count is the number of fibers per inch in each direction. The size of the fibers is indicated by the denier, which is a measurement of the weight per unit length of the fiber. Therefore, a typical 10 x 101000d scrim would have ten 1000-denier fibers in each direction.

The most common weave patterns are plain and leno weaves. A plain weave has single fibers in the fill direction passing over and under the fibers in the warp or machine direction in an alternating manner. A leno weave is similar, except that the fibers in the warp direction are arranged in pairs and are twisted around each other in between each fiber in the fill direction as it passes over and under. This prevents slippage of the fibers and increases strength. Scrims may have different strengths in the warp and fill directions, and this will affect the properties of the membrane.

The size of the openings between fibers in the scrim is important because strike through of the polymer is necessary to obtain good ply adhesion. Polymers gener-ally adhere much better to themselves than to the scrim, so scrims with higher Chevron Corporation 600-15 June 1997

600 Ponds and Basins Civil and Structural Manualcount and denier are more susceptible to delamination. As a practical rule, scrims heavier than 10 x 101000d are rarely used.

The scrim must be completely encapsulated by the polymer to prevent it from acting like a wick and drawing liquid into the membrane, causing delamination. All factory edges of the membrane should have about 1 inch of unreinforced polymer (called selvage) to ensure that the scrim is not exposed. Edges where scrim is exposed by cutting during field installation must be sealed, usually by flood coating.

Manufacturing Methods. Membrane manufacturers start with the basic polymers which are supplied by chemical and rubber companies, such as Du Pont and Dow Chemical. Chevron Chemical Olefins Division manufactures and supplies HDPE resin pellets to several membrane manufacturers. The polymer suppliers do not usually manufacture membranes, but there are exceptions.

The first step in manufacturing is to mix the basic polymer with various compounding ingredients. These ingredients may include plasticizers to make the material more flexible, carbon black to increase its resistance to degradation from ultraviolet light, other antidegradants such as antioxidants or microbiological inhibi-tors, cross-linking agents, and inert fillers. Because compounding varies from manu-facturer to manufacturer, finished membranes of the same basic polymer can have different properties and are not necessarily equivalent. Also, manufacturers may blend two or more polymers (called an alloy) to enhance certain membrane proper-ties.

Calendering. The compounded polymer is converted into rolls of sheeting approxi-mately 4 to 8 feet wide, usually by a process called calendering. The mixed compound is passed through a series of heated rolls which compress it into a sheet. If a reinforcing scrim is needed, it will be sandwiched between sheets of the compound during calendering. Typically, unreinforced membranes are calendered in a single ply, but some manufacturers may prefer to use two plies to prevent pinholes through the sheet. Reinforced membranes are typically three plies (compound/scrim/ compound) or sometimes five plies (compound/scrim/ compound/ scrim/compound).Spread Coating. Another process used to produce membranes is spread coating, in which the compound is softened with solvent and spread over the reinforcing scrim or a sheet of release paper. However, this process is much less common than calen-dering.

Extrusion. This is used primarily for polyethylene. The compound is extruded through a die at the desired thickness. Many different die configurations exist, and sometimes the hot extrudate is overlapped to make a wide sheet. This overlapping process (SLT method) uses a relatively small die. Only unreinforced membranes can be manufactured by extrusion.

Other companies, such as Poly-America and GSE Lining Technology, Inc., use the blown film method of extrusion. Polyethylene pellets are heated and extruded into a large blower which forms a large, cylindrical bubble. This method forms a contin-uous sheet with no overlaps.June 1997 600-16 Chevron Corporation

Civil and Structural Manual 600 Ponds and BasinsProperties of Liner MaterialsThe important properties to consider in selecting a membrane are its mechanical strength, weathering resistance, chemical resistance, and permeability. Manufac-turers should provide this information in the form of a data sheet for the membrane which gives the membranes properties as determined by standard test methods. Figure 600-5 lists the standard test methods most commonly used for membranes. The engineer must compare the information on various membranes and determine which one is best for the particular job. Unfortunately, the manufacturers informa-tion is often incomplete or not specific enough, and additional testing may be required.

Refer also to Figure 600-8, presented later in the section, where properties and char-acteristics of various membrane materials are compared.

Mechanical Strength:

Tensile Strength and Elongation. High tensile strength is desirable to prevent the membrane from rupturing under stress from hydrostatic pressure, shifting soil, and other mechanical forces. High elongation is also important to allow the membrane to conform to irregularities.

Tensile strength and elongation are determined by one of several test methods, depending on the type of membrane. For unreinforced membranes, a strip or dumb-bell-shaped sample is usually tested according to ASTM D 638 or D 882 for plas-tics or ASTM D 412 for elastomers. Tensile strength may be reported in either pounds per square inch or pounds per inch width (not divided by thickness) and elongation should be reported in percent. For reinforced membranes, a standard 4-inch wide sample called a grab tensile is usually tested according to ASTM D 751 and the tensile strength is reported only in pounds (not divided by width or thickness). Elongation should be reported in percent but is frequently omitted, making it difficult to compare the strengths of reinforced and unreinforced membranes. Probably the best way to compare strengths is to multiply the values for unreinforced materials by a 4-inch width and the material thickness to get pounds, but this is only an approximation as the specimen shapes are different.

Tear Resistance. High tear resistance is also desirable to prevent cuts and gouges, or other irregularities in the membrane, from becoming large rips. Generally, rein-forced membranes have much higher tear resistance than unreinforced membranes, but again the numbers can be difficult to compare because of differences in the test methods.

Unreinforced membranes are tested according to ASTM D 1004 for plastics or D 624 for elastomers. Both of these tests measure the initial tearing force on a 90-degree angle specimen. The results may be reported in pounds or in pounds per inch of thickness. Reinforced membranes are tested according to ASTM D 751 which uses a tongue tear specimen to measure the tear propagation force, and the results are reported in pounds. These values may be compared to those for unrein-forced materials (in pounds), although this comparison probably makes unrein-forced materials look slightly better than they are because the force to initiate a tear is normally higher than the force to propagate one.Chevron Corporation 600-17 June 1997

600 Ponds and Basins Civil and Structural ManualFig. 600-5 Standard Test Methods Applied to MembranesASTM D 638 Tensile Properties of PlasticsASTM D 882 Tensile Properties of Thin Plastic SheetingASTM D 751 Coated FabricsASTM D 412 Rubber Properties in TensionASTM D 1004 Initial Tear Resistance of Plastic Film and SheetingASTM D 624 Rubber PropertyTear ResistanceFTMS 101B Puncture Resistance (Method 2031)ASTM D 1149 Rubber DeteriorationSurface Ozone CrackingASTM D 3041 Coated FabricsOzone CrackingASTM D 471 Rubber PropertyEffect of LiquidsASTM D 3083 Flexible PVC Sheeting for Pond, Canal, and Reservoir LiningASTM D 1204 Linear Dimensional Changes of Nonrigid Thermoplastic Sheeting or Film at Elevated

TemperatureASTM D 573 RubberDeterioration in an Air OvenASTM D 746 Brittleness Temperature of Plastics and Elastomers by ImpactASTM D 1790 Brittleness Temperature of Plastic Film by ImpactASTM D 2136 Coated FabricsLow Temperature Bend TestASTM D 814 Rubber PropertyVapor Transmission of Volatile LiquidsASTM E 96 Water Vapor Transmission of MaterialsASTM D297-81 Rubber ProductsChemical Analysis. Section 15-Density; Section 34-Referee Ash

MethodASTM D413-82 Rubber PropertyAdhesion to Flexible SubstrateASTM D518-61 Rubber DeteriorationSurface CrackingASTM D792-66 Specific Gravity and Density of Plastics by DisplacementASTM D1146-53 Blocking Point of Potentially Adhesive LayersASTM D1203-67 Loss of Plasticizer from Plastics (Activated Carbon Methods)ASTM D1239-86 Flow Rates of Thermoplastics by Extrusion PlastometerASTM D1239-55 Resistance of Plastic Films to Extraction by ChemicalsASTM D1248-84 Specification for Polyethylene Plastics Molding and Extrusion MaterialsASTM D1505-85 Density of Plastics by the Density-Gradient Technique, Section 08.01ASTM D1593-80 Specification for Nonrigid Vinyl Chloride Plastic SheetingASTM D1603-76 Carbon Black in Olefin Plastics, Section 08.02ASTM D1693-70 Environmental Stress Cracking of Ethylene PlasticsASTM D2240-81 Rubber PropertyDurometer HardnessASTM D3015-72 Recommended Practice for Microscopical Examination of Pigment Dispersion in Plastic

CompoundsASTM D4218-82 Carbon Black Content in Polyethylene Compounds by the Muffle Furnace

TechniqueASTM D4545-86 Practice for Determining the Integrity of Factory Seams Used in Joining Manufactured

Flexible Sheet GeomembranesEPA Test Method 9090 Compatibility Test for Waste and Membrane MaterialsJune 1997 600-18 Chevron Corporation

Civil and Structural Manual 600 Ponds and BasinsPuncture Resistance. High puncture resistance is another desirable property of a membrane and is needed to prevent rocks or other sharp objects from making holes in the membrane. Again, reinforced membranes usually have higher puncture resis-tance than unreinforced membranes.

The standard test most frequently used is Federal Test Method Standard (FTMS) 101B (Method 2031), which measures the load, in pounds, required to puncture the membrane with a standard penetrator. The values for reinforced membranes are easy to compare, but values are rarely reported for unreinforced materials.

Seam Strength. Seam strength is usually tested in a manner similar to testing tensile strength, except that the sample has a seam perpendicular to the direction of the tensile force. This is called the seam tensile strength and is covered by ASTM D4545.

The two primary destructive test methods for determining geomembrane seam weld strength are the peel test and the shear test. For both tests, 1-inch wide strips are placed in a testing machine (tensiometer) as shown in Figure 600-6.

For a weld to be considered acceptable, its strength measured in the destructive tests must exceed a specified percentage of the liner sheet strength. Typically, five tests are run for peel and for shear and four out of five test specimens must pass, in each test, for the entire sample to pass.

Figure 600-7 shows a series of schematic drawings of various types of breaks. Sepa-rate break codes exist for many of the different welding techniques, although all welds, from solvent to thermal, can be classified either film tear bond (FTB) or non-FTB.

Desirable breaks are the FTB classification. These breaks occur in the base mate-rial, which indicates that the weld is stronger than the sheet material. If the strength of these types of breaks exceeds the specified minimums, usually 70% to 80% of the specified sheet strength, then the test sample passes. Weld breaks are classified as test failures if the strength fails to meet the specified minimum for FTB breaks or if the break is a non-FTB break and the break strength is below 100% of the

Fig. 600-6 Schematic Showing Peel and Shear TestsChevron Corporation 600-19 June 1997

600 Ponds and Basins Civil and Structural ManualFig. 600-7 Locus-of-Break Codes and Descriptions of Seam Breaks for Fillet Weld SeamsJune 1997 600-20 Chevron Corporation

Civil and Structural Manual 600 Ponds and Basinsminimum specified sheet strength. Non-FTB breaks pass if the break strength exceeds 100% of the minimum specified sheet strength.

For reinforced materials, the peel strength is often limited by the ply adhesion, and values may be considerably lower than the base material tensile strength. Prefer-ably, as with the seam tensile test, the break occurs in the base material instead of along the seam.

Peel tests are often used in the field to check seam quality during installation. The seam may be judged by a pass/fail criterion based on whether the base material or the seam breaks (FTB or non-FTB).Weathering Resistance. Weathering resistance includes the ability of the membrane to resist deterioration from exposure to both low and high temperatures, sunlight (ultraviolet light, UV), ozone, water, and soil. Most membranes have good weathering resistance but some are not designed to be used exposed, meaning without a protective earth cover. PVC should not be used as an exposed liner because it becomes brittle and cracks after prolonged exposure to ultraviolet light. Most other membrane materials contain carbon black to prevent UV deterioration.

Low temperature flexibility is important in cold climates. Several standard tests determine the temperature at which the membrane becomes brittle and may crack. For unreinforced plastics and elastomers, an impact test (ASTM D 746 or D 1790) is used, and for reinforced membranes, a low temperature bend test (ASTM D 2136) is common. The brittleness temperature determined by these tests indicates the minimum service temperature to which the membrane should be exposed.

Heat resistance of a membrane is indicated to some extent by the standard tests for dimensional stability (ASTM D 1204) and heat aging resistance (ASTM D 573). However, these tests only demonstrate the materials resistance to hot ambient air. The maximum service temperature for membranes is more often determined by their chemical resistance to the liquid which they will hold. Chemical resistance data refers to immersion tests run at elevated temperatures.

For elastomers (reinforced or unreinforced), resistance to ozone cracking is an important property if the liner will be exposed. Standard tests such as ASTM D 1149 or D 3041 are used to determine resistance to ozone. These tests are not normally run on plastics, because they are not susceptible to ozone cracking.

Resistance to water adsorption is normally tested by measuring weight change after immersion according to ASTM D 471. Some materials, such as PVC, may lose weight due to the extraction of plasticizers or other components, but most materials will show a slight weight gain.

A test for resistance to soil burial is included in ASTM D 3083, which is a specifica-tion for PVC. This test is frequently applied to other materials as well to determine their resistance to degradation by microbiological attack. Most materials contain microbiological inhibitors and will show little change in properties after soil expo-sure.

Chemical Resistance. The chemical resistance of a membrane depends a great deal on the basic polymer used, since the different polymers have different degrees of Chevron Corporation 600-21 June 1997

600 Ponds and Basins Civil and Structural Manualresistance to acids, alkalis, and hydrocarbons. However, the chemical resistance of the membrane also depends on how the polymer is compounded and manufactured, so one manufacturers material may not be as good as anothers. Membrane manu-facturers usually provide chemical resistance charts which can be used as a guide-line in selecting the right membrane. However, these charts are very general in nature, and it is sometimes difficult to determine the basis for their recommenda-tions. Usually, an immersion test of candidate materials in the specific environment is necessary.

EPA Test Method 9090 determines the effects of chemicals on flexible membrane liner materials in a surface impoundment, waste pile, or landfill. The liner material is immersed in the chemical environment (a sample of the actual waste) for at least 120 days at 23C and 50C. Data from these tests are intended to assist with the decision of whether a given material is acceptable for the intended application. The EPA 9090 test measures for changes in tear resistance, puncture resistance, tensile properties, hardness, elongation at break, modulus of elasticity, volatiles and extract-ables content (mainly for potable water applications), ply adhesion (for scrim rein-forced membranes), and hydrostatic resistance.Chemical resistance may also be determined by immersion testing according to ASTM D 471. Changes in weight and volume, hardness, and tensile properties, including seam strength, are determined after immersion for several different periods of time. It is difficult to set fixed criteria, particularly for weight and volume change. Tensile properties after immersion should generally be at least 80% of initial values. One qualitative method for judging acceptability is to look for a leveling off of properties with time. Another method is to test a range of materials and pick the best ones.

CRTCs Materials and Equipment Engineering has performed many membrane-waste compatibility tests and has a good database for chemical resistance.

Stress Cracking and Environmental Stress Cracking of Polyethylene. Stress cracking (for plastics) is defined as brittle failure at stresses below the yield stress of the material. Certain grades of polyethylene are much more susceptible to this mechanism than other grades. Much research has been done in the polyethylene pipe industry on improving the stress cracking resistance of polyethylene pipe and more research is currently underway evaluating geomembrane liner grades of poly-ethylene resins.

The Company has experienced stress cracking failures at field seams in HDPE liner systems. In the cases evaluated, very high stresses, rather than inferior polyeth-ylene, were the major cause. Stresses from thermal expansion and contraction (cyclic stresses) concentrate at the welded seams and form cracks. The cracks prop-agate through thickness and the failure has the appearance of a tear with little visible plastic elongation at the failure.

Stress cracking failures in HDPE geomembranes can be avoided by leaving adequate slack in the liner system for thermal contraction at low temperatures and by conscientious design (i.e., orienting seams vertically down slopes and leaving slack around penetrations and odd geometries).June 1997 600-22 Chevron Corporation

Civil and Structural Manual 600 Ponds and BasinsEnvironmental stress cracking (ESC) is stress cracking as defined above, but also accelerated by a polar liquid, such as igepal (which is used in accelerated ESC tests). The Companys Model Specification CIV-MS-4797 requires HDPE geomem-branes to exhibit greater than 1500 hours ESC resistance in an accelerated ESC test. This requirement is especially important as liquids such as aromatic and aliphatic hydrocarbons and MTBE (methyl tertiary butyl ether) are known to promote ESC in susceptible grades of polyethylene. To date, the Company has not experienced any ESC failures of geomembrane liners.

Permeability. Membranes are not impermeable to all liquids, although their perme-ability may be very low. Since the purpose of the membrane is to prevent the migra-tion of liquids, it is important to select a membrane with a low permeability for the liquid it has to contain. If the liquid is water, the choice of membranes is generally easy to make because almost all membranes resist water permeation quite well. For other liquids, however, the selection is more difficult. Membrane manufacturers generally provide little or no information on the permeability of their materials. It is frequently assumed that if the membrane has adequate chemical resistance to the liquid being contained, then it will also have a relatively low permeability to that liquid. This may or may not be true.

The standard test methods for determining permeability (ASTM D 814 and E 96) basically consist of sealing a cup or jar with the membrane material and measuring the weight loss of the apparatus with time. These tests are sometimes not very accu-rate, primarily due to the difficulty of obtaining a good seal. CRTCs Materials and Equipment Engineering has developed a permeability test cup with an improved sealing surface. Other types of tests using sealed pouches of membrane have also been developed.

Unfortunately, permeability standards for synthetic membranes are not well defined. Federal regulations for hazardous waste surface impoundments and land-fills do not give specific standards for synthetic membranes; they must prevent the migration of hazardous constituents into such liner. They do specify that compacted soil liners must be at least 3 feet thick with a hydraulic conductivity of less than 1 10-7 cm/sec. Federal NON-hazardous landfill regulations actually give a membrane thickness standard: minimum of 30 mils thick, or 60 mils if the membrane is HPDE. The specified minimum thickness for a compacted soil liner in this case is 2 feet. California regulations for secondary containment membranes for underground storage tanks (Title 23, Div. 3, Chapter 16) specify that the membrane must have a maximum permeability of 0.65 gram/meter2/hr by method ASTM E96. However, this was meant for containment around storage tanks and may not neces-sarily be applicable to pond liners. In addition, we currently have little data on which we can base selection of membranes to meet this requirement.

Seaming MethodsThere are several methods for making seams in membranes, and the best method depends on the type of membrane, primarily the basic polymer from which it is manufactured. Also, seaming methods for a given type of membrane may be different for factory and field seams. Basically, seaming methods can be classified into three categories: thermal, solvent, and adhesive.Chevron Corporation 600-23 June 1997

600 Ponds and Basins Civil and Structural ManualThermal Seams. These seams are made by heating the polymer until it melts and then fusing the two pieces together. Thermal seams can be made only in thermo-plastic materials (including thermoplastic elastomers). They have the advantage of being relatively fast and easy to make and are fully cured as soon as the material cools.

Several thermal welding methods are available and are summarized below:

Extrusion Welding. A bead of hot polymer is extruded between or on top of the two sheets of membrane to fuse them together. This method is used for field seams of HDPE membranes.

Hot Wedge or Hot Air Welding. The material is melted and then the sheets are pressed together. Typically, this process is carried out by one machine which contains a hot air gun or a hot wedge followed by rollers which press the sheets together. This method can be used for factory and field seams and is popular for some thermoplastics and thermoplastic elastomers including HDPE, polyesters, and elasticized polyolefins.

Dielectric Welding. A high voltage electric current fuses the sheets of membrane. This method is used only for factory seams because of the large, heavy equipment needed and is popular for reinforced thermoplastic elastomers such as Hypalon and CPE.

Ultrasonic Vibration Welding. This is a relatively new method for seaming HDPE, XR-5, PVC, Hypalon, and other thermoplastic materials. We believe that the process may have some potential, but we recommend that this method not be used until further study and development are completed.

The Welding Institute recently completed a preliminary study of ultrasonic welding of several thermoplastics and got generally poor results. Several other installers and manufacturers agree that the results of ultrasonic weld machines have been mixed, and they still prefer extrusion and hot wedge welding techniques.

Weather Constraints. Thermoplastics, such as HDPE, should not be welded when the temperature drops to 45F or lower. When it is this cold, the molten extrudate does not fuse to the membrane, and the welds will peel off the membrane when peel tested. If the weld technician adjusts his welding equipment for hotter molten extrudate, fusion will improve, but the weld will overheat. If it is overheated enough, the weld will become brittle and will fail at low strengths when tested.

Solvent Seams. These seams are made using a solvent such as trichloromethane to partly dissolve the polymer and then using rollers to press the two sheets of membrane together. Solvent seams take longer to cure than thermal seams because the seam does not reach its full strength until all of the solvent has evaporated. A bodied solvent is one which contains some of the polymer already dissolved into it to fill in the gaps between the two sheets. Bodied solvents are frequently used for reinforced materials, since the surface of the membrane may have a texture corre-sponding to the scrim pattern. Solvent seams are popular for field seaming of ther-moplastics and thermoplastic elastomers such as PVC, Hypalon, and CPE.June 1997 600-24 Chevron Corporation

Civil and Structural Manual 600 Ponds and BasinsAdhesive Seams. Adhesives may include contact adhesives and gum tapes. Adhe-sives are used mostly for materials which are difficult to seam, such as elastomers like neoprene, but may be used for almost any membrane. Generally, if other seaming methods are available, they will be preferred over adhesives. Elastomers are sometimes seamed with a vulcanizing (cross-linking) adhesive or tape which requires heat and pressure to cure.

Available Membrane MaterialsA wide variety of flexible membrane liner materials is available for different appli-cations. Figure 600-8 compares the characteristics of most of the common membrane materials, including their mechanical properties, general chemical resis-tance, most frequently used seaming methods, and so on. This information is taken from various publications and manufacturers literature and should be helpful as a general guideline for comparing different materials or for writing specifications. More information about each material is presented below, including typical applica-tions.

HDPE. HDPE is a thermoplastic and is never reinforced with a fabric scrim. Chevron Chemical produces a resin, HiD 9642, which is used for liner manufac-ture. Polyethylene is the most widely used liner material because of its combination of good mechanical properties, especially high tensile strength and elongation, and good broad-range chemical resistance, including resistance to petroleum products. HDPE also has excellent weathering resistance if carbon black is added for ultravi-olet light stabilization, and is suitable for use as an exposed liner.

The primary disadvantage of HDPE is its relatively poor tear and puncture resis-tance, especially in thin sheets. This problem can be minimized by using relatively thick sheets, and 80 mils is a common thickness for HDPE. Unfortunately, the thick sheets are not very flexible (especially in cold weather), and are difficult to handle.Another drawback to HDPE is its relatively high coefficient of thermal expansion (1.2 x 10-4 ft/ft/C). Extra material, or compensation, must be left in the liner while it is being installed to permit contraction when temperatures decrease. This is espe-cially true for exposed liners in locations like Wyoming and Montana. Seam fail-ures have occurred in membranes in the winter months in these states because of stresses from liner shrinkage.

HDPE is generally manufactured by extrusion and is supplied in large rolls up to 30 feet wide and several hundred feet long. It is not normally prefabricated by factory seaming. Field seams are made by extrusion welding, hot air or hot wedge welding.

HDPE has been used for most of the tank bottom secondary containment membranes installed by Chevron to date and also for a large number of pond liners. Low density polyethylene (LDPE, LLDPE, and VLDPE) membranes are also avail-able, but these are generally much inferior in both overall mechanical properties and chemical resistance compared to the high density polyethylenes.Chevron Corporation 600-25 June 1997

600 Ponds and Basins

Civil and Structural Manual

June 1997600-26

Chevron Corporation

Fig. 600-8 Membrane Materials (1 of 2)

Chemical Resistance

Oils Fuels/Solvents

t Excellent Excellent

Excellent Good

Poor/Fair Poor

Poor Poor

Fair Poor

Excellent Excellent

Poor Poor

Good Fair

Poor Poor

Poor PoorMaterial Available Products TypeFabric Reinforced

TypicalThickness, Mil

ApproximateCost(1)

(material only), $ Acids Alkalis

HDPE GSE , Schlegel,NSC, Poly/America

Thermo-plastic

No 20-100 0.15 - 0.65 Good Excellen

Elasticizedpolyolefin

Seaman XR-5, Plasti-Steel Petrochem 10

Thermo-plastic

Yes 30 0.62 Fair Good

PVC Watersaver, Staff, Palco

Thermo-plastic No 20-30 .30 - .40 Fair Poor

Hypalon (CSPE) Watersaver, Palco, Burke Thermo-plastic elastomer

Yes 36 0.70 Good Good

CPE Staff Thermo-plastic elastomer

NoYes

20-30 36

0.35-0.40 0.60

Fair Fair

Polyester Cooley (Du Pont Hytrel)

Thermo-plastic elastomer

Yes 30 1.80 Fair Fair

EDPM Carlisle Cured elastomer

No 30-60 0.50-0.60 Good Good

Neoprene Carlisle Curedelastomer

No 30-60 0.85 - 1.10 Fair Good

Butyl Rubber Carlisle Cured elastomer

No 30-60 0.85 - 1.10 Fair Good

Asphalticurethane

Commercial IndustrialMembrane

Yes 70 1.25 Fair Fair

Civil and Structural Manual

600 Ponds and Basins

Chevron Corporation600-27

June 1997

Seams

tory Field Notes

e Thermal(2)

rmal Thermal

rmal Solvent Oil resistant gradesavailable

rmal Solvent Partiallycures withaging

rmal Solvent Poormemory

rmal Thermal

canized Adhesive Difficult to seam orrepair

canized Adhesive Difficult toseam orrepair

canized Adhesive Difficult toseam orrepair

e None Sprayapplied

F(1) In 1996(2) Extrusion weld(3) NR No data reported

Mechanical Properties TemperatureResistance, F

MaterialTensile Strength, lb/in. Elongation

TearResistance,lb

PunctureResistance,lb

WeatheringResistanceLow High Fac

HDPE 30-150 500 10-50 80-400 -40 180 Excellent Non

Elasticizedpolyolefin

300-400 NR(3) 60-125 300 -30 220 Good The

PVC 46-69 300 6-8 NR(3) -20 130 Poor The

Hypalon (CSPE) 200 NR(3) 80 170 -40 160 Good The

CPE 34-43 200

250 NR(3)

3.5-4.5 35

NR (3)

170-20 -40

130 130

Good The

Polyester 250 NR(3) 50 100 -50 250 Excellent The

EDPM 42-84 300 4-8 NR(3) -75 300 Excellent Vul

Neoprene 45-90 250 4-8 NR(3) -30 200 Good Vul

Butyl Rubber 36-72 300 4-8 NR(3) -40 200 Good Vul

Asphalticurethane

160 NR(3) 4.5 70 -40 140 Good Non

ig. 600-8 Membrane Materials (2 of 2)

600 Ponds and Basins Civil and Structural ManualElasticized Polyolefins. Elasticized polyolefins (XR-5, Petroguard 3) are thermo-plastics blended with a special resin modifier such as Du Ponts Elvaloy to make them more rubberlike. Although originally introduced as unreinforced membranes, most of these materials are now reinforced with a scrim. They have excellent tensile strength, tear resistance, and puncture resistance. Elasticized polyolefins also have good weathering resistance.

The chemical resistance of elasticized polyolefins is generally good, especially to oils such as crude oil. However, they will be attacked by aromatic solvents (benzene) or fuels with a high aromatic content (gasoline). Thermal seaming methods are most commonly used, with dielectric welding preferred for factory seams and hot air welding for field seams.

The largest use of elasticized polyolefin membranes has been for lining oil field ponds where hydrocarbons are often part of the effluent. These materials are cost-competitive with HDPE, which is the other common choice where hydrocarbons are present.

PVC. PVC is also a thermoplastic and a widely used membrane liner material because it is relatively inexpensive. It is normally used unreinforced, although rein-forced PVC membranes are available. Conventional PVC contains 25 to 35% plasti-cizer, which is usually some type of oil, to make it more flexible. The plasticizer gradually will be lost during service, and the material eventually will become more brittle. High temperatures will accelerate the loss of the plasticizer.

Initially PVC has good tensile strength and elongation, but the strength tends to increase and the elongation to decrease as the plasticizer is lost. Its tear and punc-ture resistance are relatively low compared to reinforced membranes. PVC has poor weathering resistance and should never be used as an exposed liner. It is resistant to dilute concentrations of many chemicals, but overall, its chemical resistance is infe-rior to other liner materials. Special oil-resistant grades of PVC which have fair resistance to crude oils are available, but they are not resistant to fuels or solvents.

Factory seams in PVC are usually made by thermal methods such as dielectric welding, but field seams are normally made using a solvent. PVC membranes were used mostly for waste impoundments until HDPE took over. PVC is still used for some waste containment and water containment.

Hypalon (CSPEChlorosulfonated Polyethylene). Hypalon is a Du Pont trade-mark for chlorosulfonated polyethylene (CSPE), a synthetic elastomer which has been used for many different applications. The Hypalon used for membrane liners is uncured, which gives it the properties of a thermoplastic elastomer, although it does cure slowly in service. Hypalon is almost always reinforced because the uncured polymer has relatively low strength. Reinforced Hypalon, however, has good tensile strength, tear resistance, and puncture resistance. Hypalon has very good weathering resistance and is frequently used as an exposed liner.

There are three common grades of Hypalon membranes: potable water grade, aquatic grade (for fish ponds), and industrial grade. Industrial grade has the best overall chemical resistance. It is resistant to a wide range of chemicals, including strong acids and alkalis, but has very poor resistance to hydrocarbons. Chemical June 1997 600-28 Chevron Corporation

Civil and Structural Manual 600 Ponds and Basinsresistance charts can be misleading, because cured Hypalon actually has very good resistance to oils, fuels, and some solvents, but uncured Hypalon will soften and swell in the presence of oils and can be dissolved by fuels and solvents. Unfortu-nately, cured Hypalon is very difficult to seam, which is why all Hypalon liners are made from uncured Hypalon.

Factory seams in Hypalon are usually dielectrically welded, while field seams are made with solvents. Because Hypalon cures partially in service, repairing it requires special seaming techniques.

Hypalon is a very widely used membrane liner material and its cost is moderate. It performs very well in many waste water and chemical services, but even small amounts of oil in the effluent can cause it to fail prematurely, and it is not recom-mended for any service where oil contamination may be present.

Chlorinated Polyethylene (CPE). CPE is another thermoplastic elastomer and may be used with or without fabric reinforcement. The reinforced CPE membranes have much better tensile strength, tear resistance, and puncture resistance than the unreinforced ones, but in some cases, the unreinforced membrane may be adequate. The mechanical properties of reinforced CPE are comparable to reinforced Hypalon, but CPE has relatively poor memory (elongates and does not return to its original shape). CPE has good weathering resistance and can be used as an exposed liner. The chemical resistance of CPE is generally not as good as Hypalon, but many CPE formulations do have fair resistance to oils and can be used in services where low levels (a few hundred ppm) of hydrocarbon contamination are expected or the liner will only be exposed to the hydrocarbon for a short time, such as for an emergency spill containment.

Seams in CPE membranes are usually dielectrically welded in the factory and solvent-seamed in the field.

Polyester. Currently, there is only one polyester elastomer availableDu Pont Hytrel, manufactured by Cooley, which is a thermoplastic material usually used as a reinforced membrane. It is relatively expensive, but its major selling feature is excellent resistance to all types of oils, fuels, and most solvents, which other flex-ible, fabric-reinforced membranes do not have. It is also resistant to a wide range of chemicals but can be attacked by strong acids or alkalis. It has excellent weathering resistance and good tensile strength, tear resistance, and puncture resistance.

Both factory and field seams are made by thermal methods, typically dielectric welding in the factory and hot air welding in the field. The major use of polyester elastomers to date has been for secondary containment around fuel storage tanks. Chevron Marketing has used Hytrel membranes for secondary containment around service station piping.

Neoprene. Neoprene is another elastomer which has been used for membrane liners. It is generally unreinforced, although reinforced membranes are sometimes available. Its mechanical properties are comparable to other cured elastomers, and it has good weathering resistance. Neoprene also has good resistance to oils but can be attacked by some fuels and solvents. It has been used to contain waste water which has some hydrocarbons present. Neoprene is difficult to seam. Vulcanized Chevron Corporation 600-29 June 1997

600 Ponds and Basins Civil and Structural Manualseams can be made in the factory, but field seams must be made with special adhe-sives.

Butyl Rubber. Butyl rubber membranes have been used for potable water impound-ments for many years. The membrane is an elastomer, usually without reinforce-ment. Its mechanical properties are comparable to other elastomers. Butyl rubber has good weathering resistance and is also resistant to a wide variety of chemicals, but it swells badly in the presence of hydrocarbons. Like most elastomers, butyl rubber is difficult to seam. Vulcanized seams are used in the factory, but field seams require special adhesives.

Asphaltic Urethane (Commercial Industrial Membrane) (CIM). CIM is a spray-applied asphaltic urethane, usually fabric-reinforced. The fabric is laid in place first and then the compound is sprayed over it. The main advantage to this type of membrane is that there are no seams, but other application problems such as pinholes, thin spots, or improper curing can occur. CIM is used primarily for water containment. It has fair resistance to a variety of chemicals but poor resistance to hydrocarbons. Its mechanical properties (tensile strength, tear and puncture resis-tance) are relatively low compared to other reinforced membranes.Other Materials. Most of the available membranes are made from one of the generic types of materials described above. However, new materials are being devel-oped, and special blends of different polymers are sometimes used to obtain desired properties. Manufacturers are occasionally reluctant to give out information on the composition of their products, which can make it difficult to predict performance. Laboratory testing can provide some additional guidance, but actual service experi-ence is the best indication of a good material.

633 Design and ConstructionThe design and construction of a lined pond or impoundment really begins with site selection. The site conditions, including the soil, groundwater, climate, etc., together with the composition of the effluent to be contained dictate the membrane liner requirements. Any applicable codes and standards must also be considered to assure compliance. Specific requirements for site preparation, leakage monitoring systems, and installation and inspection of the membrane must be spelled out in detailed specifications and drawings. See the Companys Specification CIV-MS-4797, Pond and Basin Geomembranes, in Section 2000 of this manual.

Most liner failures occur at field seams. During design, the engineer should generate a membrane panel layout that minimizes the number of field seams and avoids horizontal seams on slopes. Seam intersections should be staggered to avoid more than two weld intersections at one point.

Site PreparationBefore the membrane liner is actually installed, extensive design and construction work must be completed to properly prepare the site and ensure that the liner will perform well in service. Normally, the site preparation work is done by an earth-work contractor and is completed before the start of liner installation. The engineer June 1997 600-30 Chevron Corporation

Civil and Structural Manual 600 Ponds and Basinsmust work with the earthwork contractor to establish the site requirements and ensure that they are met. The final excavation should be inspected jointly by the engineer, the earthwork contractor, and the membrane installation contractor to ensure that it is satisfactory.

An analysis of the soil conditions will be required to determine the stability of the subgrade and the compaction requirements. This is essential to ensure that settling or shifting of the subgrade will not cause the liner to rupture. The angle of the side slopes must be derived both for the stability of the slope and the strength of the liner material, especially the field seams. For some materials with slow curing adhe-sive or solvent seams, shallow side slopes may be required. Generally, side slope angles steeper than 2 to 1 (horizontal to vertical) are seldom recommended, and slopes of 2-1/2 or 3 to 1 are common.

The surface of the soil must be smooth and free of rocks, roots, or other sharp objects which may puncture the liner. If the soil is coarse textured or rocky, a 6-inch layer of sand and/or a geotextile is recommended to provide a smooth bedding for the liner.

An anchor trench must be dug around the outer perimeter to secure the top edge of the liner. Anchor trench designs and dimensions vary. Typical trenches are U shaped, 1 foot wide, and 1 to 2 feet deep and within a few feet of the top edge of the side slope. However, if the slopes are long and steep, 2:1 or greater, or if a large portion of the liner will be exposed to winds, larger anchor trenches are warranted. Larger, V shaped designs are common for larger trenches.

The surrounding area should be contoured to provide drainage of surface runoff away from the pond or impoundment.

The type of vegetation in the area must be investigated to determine soil steriliza-tion requirements. Some types of vegetation can puncture membranes and grow right through them. The composition of the soil and any previous contamination must also be determined to establish whether or not gas generation under the liner is a potential problem. A venting system and vent holes at the top of the liner may be required. Figure 600-9 shows one kind of air-gas vent.

Membrane Installation

Panel Placement and Anchoring. Panels should never be unrolled and posi-tioned if they cannot be seamed and anchored the same day. The first step in installing the membrane is to place the unrolled panels in position before deploy-ment. Drawings should be provided to indicate the correct positions of prefabri-cated panels, and each panel should be clearly identified. The panels should be unrolled or unfolded as close to their final position as possible to minimize handling and avoid damage. Panel placement must allow for expansion or contrac-tion which may occur due to temperature changes. This is extremely important for exposed liners at locations that have hot summers and cold winters (Montana and Wyoming, for example).Once the panels have been correctly positioned, they must be anchored in place immediately and loose edges weighted down with sandbags. Panels should be Chevron Corporation 600-31 June 1997

600 Ponds and Basins Civil and Structural Manualseamed together as soon as possible after placement. Two major blow-outs (loss of material due to wind upheaval and carry away) have occurred during installation of panels by the Company because of the Contractors casual approach to membrane ballast (weights) during deployment.Field Seaming. Field seaming requires specialized training and experience to consistently do well. Only trained and experienced weld technicians should be employed.

Methods for making field seams vary for different membrane materials, but several general requirements apply to any seaming technique. First, edges to be seamed must be clean and dry. Some seaming methods may require the edges to be wiped with a solvent or abraded with a wire brush or sandpaper before seaming. Second, the seam must have a minimum overlap width, which is typically several inches, to obtain adequate strength. Solvent or adhesive seams typically require rolling to press the two sheets together. Finally, the minimum recommended curing time must be allowed before putting any stress on the seam, including walking on or near it or inspecting it.

The welding process should be carefully monitored and field seams destructively tested to ensure high seam integrity. All field welds, including patches, should be vacuum tested for pinhole leaks.

Fig. 600-9 Air-Gas Flap-Type Vent (Courtesy of Poly-America)June 1997 600-32 Chevron Corporation

Civil and Structural Manual 600 Ponds and BasinsPipe Penetrations:

Sealing Around Penetrations. Many membranes will have to have some penetra-tions through them for pipes, sumps, and so on. Sealing the liner around these pene-trations requires extra attention, because the liner is very prone to leakage at these sites. Drawings of typical attachment details are shown in Figures 600-10 and 600-11.

Fig. 600-10 Typical Detail Showing Sealing of Membrane Around a Pipe Penetration Using a Prefabricated Boot

Fig. 600-11 Typical Details Showing Attachment of Membrane to Concrete Using Batten StripsChevron Corporation 600-33 June 1997

600 Ponds and Basins Civil and Structural ManualFor pipe penetrations, a special boot of liner material is usually prefabricated to fit over the pipe. A clamp and a sealant are used to secure the boot to the pipe, and the edges of the boot are then seamed to the liner.

For attaching the membrane to concrete, such as sumps, a combination of stainless steel or aluminum bars called batten strips and a caulking or sealant is usually used. Bolts are embedded in the concrete, then holes are punched in the membrane for the bolts to pass through. The membrane is placed over the bolts using plenty of sealant, and the batten strip is placed on top and tightened in place.

The membrane should be anchored in all areas where turbulence is expected, such as where pipes empty into a pond or near sumps, aerators, etc., to prevent the liner from being stretched or lifted. This can be accomplished by anchoring the liner to a concrete splash pad underneath it or by placing sandbags on top of the liner.

Protecting the Liner Around Penetrations. Consider using a diffuse sack, rub-sheet, concrete shot block, or other protective device to protect the liner from impact and erosion damage from flowing effluent. A protective rub sheet or other device is advisable regardless of the effluent temperature or chemical composition.

Cleanout ProvisionsSome ponds will require periodic cleanout of accumulated sludge or other deposits. The method of cleaning must be considered in the design of the pond, because liners can easily be damaged by cleaning equipment. If mechanical equipment such as front-end loaders must be used to clean out the pond, the liner will have to be protected by an earth cover. The equipment operator will have to be very careful to avoid digging through the earth cover and tearing up the liner. Just driving the equipment around on the liner could damage it if the earth cover or the bedding under the liner is rocky. A ramp would also have to be provided to get the equip-ment in and out of the pond.

Another method of cleaning out a pond is to make a slurry by adding liquid to the sludge and then vacuuming or pumping it out. This method works fairly well for some types of deposits and is much less likely to damage the liner.

634 Inspection

Visual InspectionThe entire membrane should be visually inspected both during and after installa-tion, primarily to check for obvious defects in materials or workmanship. (Shop inspection of membrane material by Company inspectors is not needed.) Some wrinkles in the liner should be expected since the materials expand and contract with temperature changes, but for the most part, the liner should lie flat. Any cuts, deep scratches, gouges, holes or damage from handling must be patched. Seams should be smooth and have the minimum specified overlap. Verify that the specified seaming method is used, including specific equipment and products. Special atten-tion should be given to inspecting the quality of workmanship on seals around pene-trations.June 1997 600-34 Chevron Corporation

Civil and Structural Manual 600 Ponds and BasinsNondestructive Seam InspectionAll field seams should be 100% nondestructively tested after they have had suffi-cient time to cure. Several test methods, described next, are available. Each method requires considerable inspector skill to ensure that all defects are detected and repaired. The inspector should also make a thorough visual inspection as he goes along to look for any other obvious defects, such as wrinkles or dirt trapped in the seam or seams which may have a narrow bonded area.

Air Lance Testing. A small diameter jet of air at about 50 psi is directed at the edge of the seam. Unbonded areas will ripple or inflate. This method is not recom-mended for stiff, thick membranes such as HDPE.

Vacuum Chamber Testing. A gasketed chamber is placed over a section of seam which has been wetted with soapy water. When the chamber is evacuated, any leaks in the seam will produce bubbles. This method is very sensitive to small leaks but is also relatively slow. This is the most widely used method.

Ultrasonic Testing. This test method has been developed for use on some unrein-forced membranes. Pulse-echo methods are the most successful. This method does not work well on reinforced membranes.

Spark Testing. A copper wire is inserted before the seam is welded. After welding, a technician equipped with a spark tester checks for pinhole leaks. If there is a pinhole gap, the machine will make an electrical connection with the wire in the seam, and an alarm will sound. This method is also widely used.

Mechanical Pick Testing. A dull-pointed object (such as a screwdriver) is run along the edge of the seam. The pick will dig into any unbonded or weakly bonded areas. The inspector must take care to avoid scratching or gouging the liner. This is a less effective method than those above.

Destructive Seam TestingNondestructive inspection of seams indicates seam continuity but does not test seam strength. Destructive test samples should be cut out at regular intervals and the seam strength checked, usually by a peel test on site. Note that the cut-out must be patched. One test for every 300 to 400 feet of seam is typical for large installa-tions. On smaller jobs, this frequency should be increased. If defective seams are found, additional tests will be required to determine the extent of the problem.

Destructive seam testing techniques are discussed in Section 632, Flexible Membrane Liner Materials, and specified strengths are included in Specification CIV-MS-4797. Destructive seam tests should be performed during installation. These tests help save time by identifying poor welders and defective welding equip-ment.

635 Common ProblemsThere are a number of common problems which can cause membrane liners to fail. All of the potential problems should be investigated and resolved during design of Chevron Corporation 600-35 June 1997

600 Ponds and Basins Civil and Structural Manualthe liner system. Some of the more common problems and ways to avoid them are discussed below.

Subgrade ProblemsSettling or cracking of the subgrade can cause the liner to stretch and eventually rupture. Good site selection and proper compaction will minimize these problems, and the liner material selected should be able to withstand the expected stresses. If there is potential for subgrade instability, a geotextile may be recommended. Geotextiles are heavy, nondegradable fabrics that can be placed under the membrane to provide additional support and protection. See Section 637 for a list of manufacturers.

Erosion of the side slopes beneath the liner near the water line may occur due to wave action. Even minor erosion will stress the liner, and if it becomes severe enough that the liner is no longer properly supported, then the liner may tear. One way to avoid this problem is to use shallower side slopes. Geotextiles and good subgrade preparation also help prevent this from happening. If the liner is buried, erosion may remove the protective covering, which could lead to damage.

If the soil is not effectively stabilized, vegetation may puncture the liner and grow through it. Soil sterilization is usually accomplished by chemical treatment, which may be specific to the type of vegetation in the area. This has not been a major problem, however.

Gas pressure may build up under the liner from chemical reactions in the soil. Soils containing organic materials will generate methane gas as they decompose. Chem-ical reactions between the liquid contained in the pond and the soil or rock under it may also generate gas if any leakage occurs. Gas pressure will cause large bubbles to form under the liner and force it up to the surface. If the possibility for gas gener-ation under the liner exists, the bottom should be sloped and vents should be placed at the top of the liner. In some cases, vent pipes may have to be installed under the liner to allow the gas to escape.

If the bottom of the liner is too close to the groundwater table, groundwater may exert pressure against the liner or flood the leakage monitoring system. The depth of the groundwater table and its seasonal fluctuations must be determined when the liner is designed.

Mechanical DamageThe potential for liner damage is often highest during installation when it is handled the most. People will walk on it, weld on it, repair equipment on it (such as welders, vacuum testing equipment, etc.), transport solvents, tools, cutting knives, etc., over it, and smoke while they work on the liner. Most specifications prohibit these activities, but Company personnel should remain onsite during installation to enforce these basic rules.

Because liner installations often take several days to several weeks to complete, the liner, or at least a portion of it, is always left unsecured during construction. Precau-tions must be taken to ensure that the liner is adequately ballasted so that it will not blow away if it becomes windy. Liner blow-outs have occurred at Company installa-June 1997 600-36 Chevron Corporation

Civil and Structural Manual 600 Ponds and Basinstions causing the loss of several hundred thousand square feet of liner. In addition to loss of time and money, blow-outs are life threatening. It is better to sacrifice some progress and take the time to ensure that the liner is adequately ballasted during installation. Worn, heavy equipment tires, sandbags, and unwrapped rolls of liner have been used as temporary ballast.

Once installation is complete, exposed liners may still be damaged by wind, hail, foot traffic, and so on. These factors should be taken into account when estimating the expected life of the liner. Any damage which is found should be repaired promptly. The liner may also be damaged by equipment used to clean out the pond, and extreme care must be taken to avoid this. Sharp objects such as rocks will punc-ture the liner, and some rodents have been known to chew holes through it.

Chemical DeteriorationSome types of membranes will degrade slowly under normal weathering and soil exposure conditions. It is especially important to select materials with good weath-ering resistance for exposed liners. The expected composition of the fluid to be contained under normal and unusual conditions must be analyzed carefully before selecting the liner material. For example, even very low levels of hydrocarbons can cause some liners, such as Hypalon, to fail. When in doubt, it is best to test candi-date materials and select the best ones. Call Materials and Equipment Engineering at CRTC for help in this area.

636 Leakage Monitoring and Detection

MonitoringMost leakage monitoring systems consist of a layer of porous material, such as gravel or coarse sand, that allows leakage to drain into a system of collection piping or monitoring wells. The leakage monitoring system must be designed as a part of the complete lining system. For a single layer lining, the leakage monitoring system is placed in the subgrade below the liner.

Environmental regulations may require dual liner designs on all impoundments or landfills containing regulated waste. For dual lining, the leakage monitoring system is usually between the primary and secondary liners. The drainage layer is typically 1 foot thick, with a minimum slope of 2%. The collection piping is typically slotted or perforated plastic pipe, laid out in a network such that any leakage will drain into monitoring wells. The size and spacing of the collection piping must be adequate to prevent any buildup of leakage in the drainage layer. See Figure 600-12.

DetectionSouthwest Research Institute has developed a method to detect and locate leaks as small as 1/32 inch in diameter in new or in-service geomembrane-lined liquid impoundments. This leak detection method uses an electrical measurement tech-nique which detects and locates leaks to within 1/2 inch or less. This method may be used in regulated as well as unregulated impoundments.Chevron Corporation 600-37 June 1997

600 Ponds and Basins Civil and Structural ManualThe major limitation to this technique is that it is ineffective in impoundments containing solid wastes greater than 1 foot deep. See Section 637 for vendor contacts and phone numbers.

RepairExposed geomembrane systems should be visually inspected routinely for damage. The seams should be given the most attention, since that is where stresses concen-trate and most failures occur. Regions that experience wide temperature variations are more likely to experience stress crack failures along weld seams. Some loca-tions are more likely to experience damage from animals (deer hooves have punc-tured 60 mil HDPE) and even vandalism.The original manufacturer/installer should do the repairs. For emergency repairs they should have no trouble mobilizing a small crew in a day or two. Some mate-rials such as PVC and Hypalon are more difficult to repair because their properties change over time. HDPE is no more difficult to repair than it is to install.

A full impoundment is much more difficult to inspect and locate leaks in. Several leak detection options are listed in the following sections. For repair, the area around the leak must be accessible. This requires either draining the impoundment or building a dike around the leak and pumping out the liquid. This has been done at several Company locations.

637 List of Manufacturers and InstallersThis section lists vendor contacts for several manufacturers and installers of flexible membrane liners. These are the companies that CRTCs Materials and Equipment

Fig. 600-12 Double Liner, Details (Courtesy of Pol