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BAW Code of Practice

Use of Granular Filters on German Inland Waterways (MAK) Issue 2013

Karlsruhe ∙ October 2013 ∙ ISSN 2192-5380

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BAW Code of Practice: Use of Granular Filters on German Inland Waterw ays (MAK), Issue 2013

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Table of Contents Page

1 Preliminary remarks 1

2 Terms, definitions, symbols and abbreviations 1 2.1 Terms and definitions 1 2.2 Symbols 3 2.3 Abbreviations 3

3 Principles concerning the use of granular filters 4 3.1 General remarks 4 3.2 Granular filters in a revetment 4 3.3 Use of a granular filter as surcharge drainage layer on the landside toe of a canal embankment 5

4 Planning documents 7

5 Granular filter construction types 8 5.1 General remarks 8 5.2 Filter thickness 8 5.3 Standard two-stage filters for revetments at inland waterways 9 5.4 Filters in non-uniform ground conditions 10 5.5 Connecting revetments and structures 11

6 Material requirements 11 6.1 Physical material requirements 11 6.2 Environment-related material requirements 12

7 Filter-related verifications 12 7.1 General remarks 12 7.2 Mechanical filtration stability 12 7.2.1 Principles 12 7.2.2 Cohesionless soils 13 7.2.3 Cohesive soils 13 7.2.4 Suffusive soils 13 7.3 Hydraulic filtration stability 13 7.4 Multi-stage filters 14 7.5 Suffusion resistance 14 7.6 Filter-related requirements for revetments 14 7.7 Additional design studies 15 7.8 Determining the size range of a filter 15

8 Notes regarding invitations to tender and construction works 16 8.1 Contract documents 16 8.2 Construction work 17 8.2.1 Installation in dry conditions 17 8.2.2 Installation under water 17 8.3 As-built documents 17


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References 18

Referenced guidelines 18

List of Tables

Table 1: Minimum thickness of granular filters 8

Table 2: Requirements for aggregates used in granular filters according to TL Gestein (2007) 11

Table 3: Methods for estimating the hydraulic conductivity of sands 14

List of Figures

Figure 1: Granular filter in a bank protection with revetment 5

Figure 2: Seepage conditions with flow approaching a surcharge drainage layer at the embankment toe 6

Figure 3: Surcharge drainage layer impounded by water 6

Figure 4: Two-stage granular filter in a revetment 9

Figure 5: Acceptable ranges for grading curves in a standard two-stage filter 10

Figure 6: Connecting a revetment containing a granular filter to the vertical edge of a structure 11

Figure 7: Size ranges of armourstone classes CP90/250, LMB5/40 and LMB10/60 15

List of Annexes

Annex 1: Determination of the coefficient of permeability k based on grain size distribution

Annex 2: Examples for determining grain size distributions in filters


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1 Preliminary remarks

The present Code of Practice covers granular filters used in embankments, bank and bottom protection and other structures on waterways.

The former Code of Practice (MAK), which was published in 1989 for the first time, needed a comprehen-sive revision to adapt it to technical and regulatory amendments. No verification methods regarding inter-nal erosion caused by contact erosion and suffusion are contained in the revised MAK; instead they are discussed in the rewritten Code of Practice "Internal Erosion (MMB)" (MMB, 2013). The MAK in large parts refers to the MMB, hence both Codes of Practice should be seen as a unit.

While the focus of the previous MAK issued in 1989 was on the use of granular filters in revetments for bank protection, the revised version also includes the use of granular filters as drain elements on dams and embankments. Thus the present Code of Practice MAK and the Code of Practice "Stability of Em-bankments at German Inland Waterways (MSD)" (MSD, 2011) are connected.

2 Terms, definitions, symbols and abbreviations

2.1 Terms and definitions

Surcharge drainage layer

A surcharge drainage layer is an external drainage system which stabilises the embankment with the surcharge it applies and prevents erosion by acting as a filter.

Revetment

Revetment refers to the complete structure of bank and bottom protection which includes the armour lay-er and a filter or the armour and a hydraulic barrier with a separation layer.

Note: As a rule, granular filters are only used for permeable revetments (MAR 2008).

Drain

A drain serves to collect and remove groundwater and seepage water. According to DIN 4095 (1990) the term drain refers both to drain pipes and the drainage layers. For embankments on waterways, drain pipes are not necessarily required.

Single-stage filter

Single-stage filters are granular filters that have a single layer, which consists of aggregates with a coeffi-

cient of uniformity of CU ≤ 5.

Erosion

Erosion is the migration and transport of all fractions of a soil caused by the flow of water.


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Filtration stability, hydraulic

Hydraulic filtration stability is the capability of a granular filter to sufficiently drain the inflowing water.

Filtration stability, mechanical

Mechanical filtration stability is the capability of a filter to retain in a sufficient degree the soil which it is to protect (soil retention capability).

Aggregates

Aggregates are granular material used for construction. Aggregates can be either natural, recycled or industrially manufactured. As a rule, natural aggregates of mineral origin which were only mechanically treated are used for hydraulic engineering purposes (e.g. gravel, sand, crushed rock).

Cohesive / cohesionless soils

For the verification of safety against internal erosion a distinction is made between cohesive and cohe-sionless soils on the basis of the classification according to DIN 18196:2011-05. Cohesive soils as de-fined by this Code of Practice are fine- and medium-grained soils which are of at least medium plasticity and have an effective cohesion c’. Cohesionless soils according to this Code of Practice are coarse-grained and medium- and fine-grained soils of low plasticity.

Granular filters

Granular filters are natural or industrially manufactured aggregate mixes. They can have a single layer or mixed-grain design (single-stage filter) or a multiple layer design (multi-stage filter). They must ensure both mechanical and hydraulic filtration stability.

Mixed-grain filters

Mixed-grain filters have a single layer design and consist of a mixture of aggregates with a coefficient of uniformity of CU > 5.

Multi-stage filters

Multi-stage filters have a multiple layer design and consist of different aggregates. The different filter stages must ensure stability in relation to each other.

Suffusion

Suffusion is the migration and transport of the fine fractions of a cohesionless soil through the pores of the granular skeleton of the coarse fractions; it is caused by the flow of water.


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2.2 Symbols

Symbols Designation Unit

CU coefficient of uniformity - d10 grain diameter for 10% finer by weight mm d15 grain diameter for 15% finer by weight mm d50 grain diameter for 50% finer by weight mm Dmin minimum thickness of granular filter m DPr degree of compaction % k coefficient of permeability m/s kF filter coefficient of permeability m/s kB soil coefficient of permeability m/s IP plasticity index - wL liquid limit - Index F index for "filter" Index B index for "base soil" ρ rd apparent density on an oven-dry basis according to DIN

EN 1097-6 Mg/m³

2.3 Abbreviations

Acronyms Designations

CP90/250 armourstone size class according to DIN EN13383 LMB5/40 armourstone weight class according to DIN EN13383 LMB10/60 armourstone weight class according to DIN EN13383 LA Los Angeles coefficient Wcm water absorption Fi frost resistance class Ci category for proportion of broken surfaces mLPC content of organic impurities


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3 Principles concerning the use of granular filters

3.1 General remarks

The purpose of a granular filter is to protect soil from scour and particle displacements caused by groundwater and seepage water flows on the one hand, and external hydraulic loads caused by surface water (flow, waves) on the other. Where hydraulic loads occur, it is necessary to protect both loose mate-rial and soft rock, which decompose under the influence of water, air and variations in temperature and are not resistant to erosion.

Granular filters shall meet specific technical requirements regarding the mechanical and hydraulic filtra-tion stability depending on the specific application, i.e. they must be adapted to the soil they are to pro-tect.

Granular filters are subjected to hydraulic loads caused by internal and occasionally external seepage forces. Internal hydraulic loads are caused by seepage forces acting inside the filter's granular skeleton; external hydraulic loads are caused by seepage forces acting at the surface of the granular filter. With respect to hydraulic loads, a distinction has to be made between filters located in water (e.g. inside a re-vetment, see section 3.2) and filters located on the landside (e.g. a surcharge drainage layer on the land-side toe of a canal embankment, see section 3.3).

Granular filters are subjected to mechanical loads caused, for example, by the traffic of construction equipment (wheel ruts, cracks), the installation of armour layers made of riprap (dynamic perforation), superstructures and live load. A filter design providing adequate thickness is required to resist these loads, or otherwise appropriate protective measures (e.g. installation of an amour layer) have to be taken.

3.2 Granular filters in a revetment

The purpose of a granular filter in a revetment is to prevent particle transport into or through the armour layer. The filter must be filter-stable in relation to the in-situ soil and the armour layer.

The fast-changing water levels in inshore waters (caused for example by passing ships) can generate high internal hydraulic loads in the form of high hydraulic gradients, leading to hydrodynamic transport of soil. This applies to the soil underneath the filter as well as to the filter itself. An adequate surcharge on the armour layer can prevent filter damage caused by hydrodynamic soil displacement.

The armour layer also protects the filter against erosion resulting from external hydraulic loads such as the wash of waves or flood water flow.

Figure 1 shows a revetment structure containing a granular filter on a bank slope. In this example the revetment is embedded in the bottom in order to protect the slope toe. The toe ditch required for construc-tion is refilled up to the height of the bottom of the waterway.


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Figure 1: Granular filter in a bank protection with revetment

The design of the revetment (granular filter and armour layer) is based on the principles for designing a bank and bottom protection for inland waterways as laid out in the Code of Practice GBB (GBB 2010). The standard methods for constructing revetments are described in the Code of Practice MAR 2008.

3.3 Use of a granular filter as surcharge drainage layer on the landside toe of a canal embankment

Installing a surcharge drainage layer on the landside toe of an embankment can stabilise the slope through the drain's deadweight and drain the seepage flow approaching it. The surcharge drainage layer has to be filter-stable in relation to the embankment material and ground.

With respect to hydraulic loads a distinction is made as to whether the surcharge drainage layer is sub-jected to water load from either a toe ditch running alongside the embankment toe or any other water body.

In surcharge drainage layers that are not water-loaded the internal hydraulic load is exclusively caused by seepage from the embankment. Since the surcharge drainage layer has a significantly higher hydraulic permeability than the embankment, seepage is mostly restricted to a partially saturated transition area between embankment and surcharge drainage layer as well as a saturated area of small thickness at the bottom of the drainage layer (Odenwald, 2011). Figure 2 is an illustration of seepage conditions in which a flow approaches a surcharge drainage layer at the landside toe of the embankment. Since the water-saturated flow has a low hydraulic gradient, the seepage forces both at the bottom of the surcharge drainage layer and at the landside exit of the seepage water are weak and therefore generally not rele-vant for stability issues. Heavy precipitation etc. can cause external hydraulic loads. The surface of a surcharge drainage layer must therefore be protected against external erosion, for example by applying a grass cover (see Code of Practice MSD (2011)).


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Figure 2: Seepage conditions with flow approaching a surcharge drainage layer at the embankment toe

If ponding occurs on the landside of a surcharge drainage layer, additional hydraulic loads apply. The underwater particles of the granular filter are subjected to buoyancy which may decrease the stability of the drain. If, in addition, the water level in the body of water (e.g. a flood retention basin adjacent to the embankment) drops quickly, excess pore water pressures may arise depending on the permeability of soil and drainage layer, thus weakening the stability of the embankment slope. Flows in adjacent water bod-ies (e.g. toe ditches) cause an external hydraulic load on a surcharge drainage layer impounded by water. Depending on the relevant flow rate, measures may be required to protect the surcharge drainage layer against external erosion. In most cases an amour layer (consisting of e.g. armourstones) is installed, which has to be filter-stable in relation to the surcharge drainage layer.

Figure 3 shows a surcharge drainage layer on a landside embankment slope, which is ponded at its toe by a water-bearing toe ditch. The drainage layer is protected against external erosion in the toe ditch area.

Figure 3: Surcharge drainage layer impounded by water

The increase in embankment stability achieved with a surcharge drainage layer must be verified by inves-tigations according to the Code of Practice "Stability of Embankments at Inland Waterways" (MSD, 2011).


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4 Planning documents

Specifications and information on ground conditions, hydraulic or mechanical loads and the armour layer structure (if applicable) are needed for the planning process.

Ground

The description of the ground should include the following information concerning the spatial distribution of the soil layers:

a) Grain size distribution and size range

The grain size distributions in the relevant soil layers form the basis of the investigations described in the Code of Practice "Internal Erosion" (MMB, 2013). Grain size distribution values are needed for the distinc-tion between cohesive and cohesionless soils and for deciding whether a filter is needed and which filter design is required, if appropriate.

b) Hydraulic conductivity (k value)

The coefficient of hydraulic conductivity k is relevant for verifying the hydraulic filtration stability of a gran-ular filter. In cohesionless soils it can be estimated on the basis of the grain size distribution (see section 7.3).

c) Classification according to DIN 18196

According to the classification of soils in DIN 18196 (2011) soils can be classified into cohesive and cohesionless soils. The values for liquid limit, grain size distribution and index of plasticity must be known for this.

Hydraulic loads on filters during installation and operation

In order to design an armour layer (if needed) and decide on the method of installation, information must be provided regarding the relevant natural or ship-induced flow velocities, wave heights and changes of water levels.

Mechanical loads on filters during installation and operation

The mechanical load acting on the filter during installation largely depends on the installation conditions (under water, dropping of stones, traffic of construction vehicles, etc.) and has to be considered in the filter design process, if applicable.

Structure of the armour layer

The construction type (riprap, coating, etc.), the thickness of the armour layer as well as the class of armourstone (if applicable) must be known.


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5 Granular filter construction types

5.1 General remarks

A distinction is made between the following construction types used for granular filters:

• Single-stage filters,

• mixed-grain filters, and

• multi-stage filters.

5.2 Filter thickness

Table 1 provides reference values for the minimum thickness of granular filters. They include the length of filtration required for the functioning of a granular filter and a safety margin to account for normal installa-tion loads as well as dimensional tolerances. The actual values must not fall below the specified values, unless specials tests were conducted beforehand. If required by the installation situation, larger thickness values may be used.

Table 1: Minimum thickness of granular filters

Type of filter Minimum thickness dmin for installation

in dry conditions under water

Single-stage and multi-stage filters

d50,F ≤ 30 mm 15 cm per stage 20 cm per stage

d50,F > 30 mm 15 cm per stage 30 cm per stage

Mixed-grain filters 30 cm 30 cm


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5.3 Standard two-stage filters for revetments at inland waterways

In revetments for bank protections a single filter stage is often not sufficient to achieve the required filter stability in the given sequence soil - granular filter – armour layer. In this case it is good practice to install a two-stage filter between the armour layer and the soil. Figure 4 shows the basic structure of a revet-ment containing a two-stage filter.

Figure 4: Two-stage granular filter in a revetment

The standard two-stage filter shown in Figure 5 complies with the standard method for constructing granular filters in revetments on inland waterways, which are installed underneath an armour layer con-sisting of riprap (classes CP90/250, LMB5/40 or LMB10/60 as defined in DIN EN 13383 (2002)).

If the soil is homogenous with grain size fractions of d15 ≤ 0.06 mm, there is no requirement for explicit filter verifications. The total thickness of the standard two-stage filter is taken into account in the calcula-tion for the revetment design according to GBB (2010).

If the size fraction is d15 > 0.06 mm, the standard two-stage filter can only be applied if the permeability of the first filter stage is higher than the permeability of the soil. However, the weight of the first filter stage is not included in the calculation for the revetment design according to GBB, if the hydraulic conductivity of this stage is kF ≤ 25 k.

The grading curves of the filter stages must lie within the limits specified in Figure 5. For information pur-poses Figure 5 also shows the typical range for armourstones of class LMB5/40. The standard two-stage filter has proven its reliability in many years of operation in bank and bottom protection.

If the in-situ soil fails to comply with the above requirement or if stone classes other than CP90/250, LMB5/40

or LMB10/60 are to be used in the armour layer, the filter design must comply with the requirements speci-fied in Chapter 7 with reference to the Code of Practice MMB (2013).


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Figure 5: Acceptable ranges for grading curves in a standard two-stage filter

5.4 Filters in non-uniform ground conditions

Non-uniform ground conditions exist in soils with alternating layers which have substantially different grain size distributions or permeability.

Where non-uniform ground conditions exist at the interface between ground and filter, it is very often im-possible to ensure both hydraulic and mechanical filtration stability for all of the soil layers. In that case the following requirements must be considered when using granular filters:

• If a higher-permeability soil layer is covered by a lower-permeability filter layer, the water pressure in the soil underneath the filter increases and adversely affects the stability of slopes, for instance.

• If a lower-permeability soil layer is covered by a filter layer which is too coarse, the lack of filter stabil-ity may lead to an undesirable erosion of the fine-grained soil.

Whether either a rise of the phreatic line or erosion can be tolerated has to be decided on a case by case basis. If both effects are not acceptable, the filter grading has to be adapted locally to the non-uniform ground conditions.


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5.5 Connecting revetments and structures

The connection of a granular filter to a structure has to be performed in such a way that the ground to be protected is never uncovered. Potential deformations of the connection (opening of joints) must be ac-counted for in the design. The filter has to reach every recess of the structure and sheet pile wall trough. To provide for potentially higher flow gradients in the vicinity of the structures and to prevent possible installation problems, the filter design in these areas should be as illustrated in Figure 6.

Figure 6: Connecting a revetment containing a granular filter to the vertical edge of a structure

6 Material requirements

6.1 Physical material requirements

The physical requirements for aggregates used in granular filters are laid down in the Technical Terms of Delivery for Aggregates issued by the German Ministry of Transport, Building and Urban Development (TL Gestein 2007). Table 2 shows the categories recommended by TL Gestein (2007) for the use of aggregates in hydraulic engineering.

Table 2: Requirements for aggregates used in granular filters according to TL Gestein (2007)

Requirement Category

resistance to shatter LA25 or LA30 depending on aggregate used as per Annex A of TL Gestein

water absorption Wcm0.5 Note: no requirements if granular f ilter is permanently located under w ater

freeze thaw resistance, if Wcm0.5 is not fulfilled F2 Note: no requirements if granular f ilter is permanently located under w ater

proportion of crushed or broken rock surfaces C90/3

content of coarse organic impurities mLPC0.1


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The size range required is defined by the filter design and must be observed with no tolerances permitted. The material requirements according to ZTV-W LB 210 (2006), the supplementary technical contract con-ditions for hydraulic engineering, section 210, must be considered as well. According to these require-ments the material must have a density on an oven-dry basis (defined in DIN EN 1097-6, 2013) of ρ rd ≥ 2.3 t/m³, be evenly graded and meet the requirements of DIN EN 13242. On embankment slopes with gradients of 1:2.5 and steeper, broken material has to be used.

The above mentioned material characteristics and the CE marking must be listed in the manufacturer's declaration of performance (accompanying documents with CE marking, see section 8.3).

6.2 Environment-related material requirements

The materials used must be environmentally sustainable. Due to the lack of legal provisions no generally applicable rules can currently be defined as to the sustainability of mineral materials used as filtering ma-terial.

Using steelmaking and metal slags for granular filters in revetments is currently precluded for Germany's federal waterways (BMVBS, 2010).

If other industrially manufactured or recycled aggregates are used in granular filters on waterways, the environmental hazard should be assessed by the Federal Institute of Hydrology.

7 Filter-related verifications

7.1 General remarks

The following requirements must be verified for the grain size chosen:

• mechanical filtration stability,

• hydraulic filtration stability and

• suffusion resistance.

Filter-related verifications shall be performed using representative grading curves of the soil layers. For simplification purposes similar grading curves can be combined into size ranges the respective bounda-ries of which are used as a basis for filter design.

7.2 Mechanical filtration stability

7.2.1 Principles

Mechanical filtration stability is the capability of a granular filter to retain in a sufficient degree the soil which it is to protect (soil retention capability).

A conservative verification of mechanical filtration stability must be provided for the fine-grained (left) limit of the soil's size range. For detailed examinations the verifications can be based on individual grading curves. If the size range represents cohesive as well as cohesionless soils, the design should be based


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on the curve representing the finest grading of cohesionless soils. The grading curves for cohesive soils will not be considered for design purposes.

7.2.2 Cohesionless soils

The filter design for cohesionless soils can be based on the criterion of Cistin and Ziems or on Terzaghi's method, taking into account the respective application limits. An example of this is provided in Annex 2. If the application limits cannot be observed, the method according to Lafleur has to be applied.

For verifying granular filter stability against coarse grains (e.g. armourstones) the method according to Myogahara can be used.

The different verification methods are explained in Chapter 4 of the Code of Practice MMB (2013).

7.2.3 Cohesive soils

The soil particles in homogenous, cohesive soils are fixed due to cohesive forces; thus, these soils are not susceptible to contact erosion.

Information on the need to verify safety against contact erosion and on the existing verification methods is provider in Chapter 5 of MMB (2013).

7.2.4 Suffusive soils

If a suffusive soil is protected by a granular filter which is designed to retain the entire grain spectrum, fine fractions washed out during seepage may deposit at the front of or inside the filter (clogging). The pro-cess of clogging reduces the permeability of the granular filter and thus leads to an increase of water pressure at the filter front. From this follows that a granular filter should not be designed to retain all of the fine-grained particles able to move because of suffusion.

According to section 4.3.4 of MMB (2013), the design of a granular filter for suffusive soils should be based on a method following Lafleur's approach.

7.3 Hydraulic filtration stability

Hydraulic filtration stability is the capability of a granular filter to sufficiently drain the inflowing water. To this end the hydraulic conductivity of the filter should be high enough to prevent a significant decrease in the hydraulic head inside the filter.

Hydraulic filtration stability has to be verified for the coarse-grained (right) limit of the size range. For a more detailed analysis individual grading curves can be used.

The hydraulic filtration stability against the soil to be filtered is ensured if

• kF > 25 kB for cohesionless soils and

• kF ≥ 10-5 m/s for cohesive soils.

The hydraulic conductivity of cohesionless soils can be estimated using the methods described in Table 3 based on their grain size distributions. Other methods for determining hydraulic conductivity based on grain size distributions are listed in Annex 1.


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Table 3: Methods for estimating the hydraulic conductivity of sands

Author Formula Fields of application

Hazen (1893) k = 0.0116∙d10² d10 in mm, k in m/s

CU < 5 0.1 mm < d10 < 3 mm

Beyer (1964) k = c(CU)•d10² c(CU) see Annex 1 d10 in mm, k in m/s

2∙10-5 m/s < k < 4∙10-3 m/s 0.06 mm < d10 < 0.6 mm

7.4 Multi-stage filters

A multi-stage filter is required if one filter stage alone is not sufficient to guarantee the mechanical filtra-tion stability between the soil and the armour layer.

If the soil has a very broad size range and a single-layer filter construction fails to ensure the mechanical and hydraulic filtration stability needed, a filter construction with multiple layers is advisable. In this type of construction mechanical filtration stability is guaranteed by filter stage 1 and hydraulic filtration stability by filter stage 2 (see Annex 2).

Starting from the ground, the water permeability of a multi-stage filtering system has to increase with every stage.

7.5 Suffusion resistance

To ensure long-term functioning of a granular filter, its grain size distribution must not change even in conditions of seepage, i.e. a loss of fine grain size fractions must be prevented. To ensure this, the granu-lar filter must be resistant to suffusion. The suffusion resistance is to be verified according to section 4.2 of the Code of Practice MMB (2013).

7.6 Filter-related requirements for revetments

To assess the mechanical filtration stability between an armourstone layer and the granular filter layer to be installed below, Myogahara’s method (MMB 2013, section 4.3) can be applied. For this purpose the size ranges of the armourstone classes CP90/250, LMB5/40 and LMB10/60 shown in Figure 7 can be used, which have been identified on the basis of the "Technical Terms of Delivery for Armourstones" (TLW, 2003) or DIN EN 13383 (2002). For a conservative verification the right limit of the armourstone size range is relevant.


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Figure 7: Size ranges of armourstone classes CP90/250, LMB5/40 and LMB10/60

7.7 Additional design studies

It is possible in individual cases to conduct special tests, e.g. model tests, to determine filter gradings which deviate from the design criteria described in sections 7.2 and 7.3. Model tests for determining filter gradings shall be performed taking into account the maximum hydraulic loads that can occur in situ. The filter gradings thus determined are specific to applications that fulfil the experimental boundary conditions.

7.8 Determining the size range of a filter

The size range of a filter, i.e. the range of all possible grading curves, depends on the size range of the soil for which the filter is designed. Generally it is constructed based on the envelope of all representative grading curves of the soil to be filtered.

The left boundary of the size range of the soil to be protected is relevant for verifying mechanical filtration stability. The corresponding verification methods are described in section 7.2. The grading curve identi-fied at the same time represents the right boundary of the size range of all possible filter grading curves. All filter grading curves shall be finer-grained than this limit.

The verification of hydraulic filtration stability is based on the right boundary of the size range of the soil to be protected. The relevant conditions are given in section 7.3. The grading curve thus identified repre-sents the left boundary of the size range of possible grading curves. All filter grading curves shall be coarser-grained than this limit.

Grading curves that are within these limits represent a filter grading in conformity with the criteria men-tioned above.

There may be cases where one filter stage alone is not able to ensure both mechanical and hydraulic filtration stability, for example in soil layers with broad size ranges. In such cases it is possible that a filter designed for mechanical filtration stability against the left limit of the size range fails to meet the require-


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ments of hydraulic filtration stability against the right limit of the size range. To evaluate which stability (function) should be given priority, the following criteria must be considered in each individual case:

• extent and harmfulness of potential excess pore water pressures owing to the absence of hydraulic stability (soil statics),

• extent and harmfulness of potential soil erosion owing to the absence of mechanical stability, and

• proportions of coarse and fine grading curves in the size range of the soil.

If no impairment of the hydraulic or mechanical stability is allowed in the specific application, it can be advisable to define smaller sections with narrower size ranges and perform a separate filter design for each section. Alternatively, a multi-stage filter structure could be considered, which would provide hydrau-lic filtration stability (prevention of excess pore water pressures) by using a coarser filter stage.

Annex 2 includes an example for determining the size range of a granular filter.

8 Notes regarding invitations to tender and construction works

8.1 Contract documents

The supplementary technical contract conditions for hydraulic engineering (ZTV-W) for embankment and bottom revetments (section 210) (ZTV-W LB 210, 2006) and earth works (section 205) (ZTV-W LB 205, 1992) include rules for the installation of granular filters and therefore should be an integral part of the contract.

When designing the size range of a granular filter the following parameters shall be specified and ob-served with no tolerances permitted:

• left limit: grain size fractions d10 and d100, in between: linear curve in semi-logarithmic representation and d5 > 0.06 mm, and

• right limit: grain size fractions d0 and d85, in between: linear curve in semi-logarithmic representation in order to restrict the oversized grain d100 ≤ 5∙d85.

The standard two-stage filter has to comply with the limits for grain size distribution shown in Figure 5.

If there are connections to structures, the related information must be provided in the contract document, including drawings, where appropriate.

If the granular filter is to be installed under water, the contract provisions must include the requirement that the intended installation method with regard to the

• preparation of the planum

• description of the equipment to be installed and the installation method, and

• intended control mechanisms

is indicated in the tender.

The quality assurance for granular filters complies with ZTV-W LB 210 (2006). The tender must include the manufacturer's declaration of performance.


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8.2 Construction work

8.2.1 Installation in dry conditions

Separation of the mixture must not occur during the installation of mixed-grain filters.

The filters must be installed in layers with maximum thicknesses of 30 cm and compacted if necessary.

Special care has to be taken when installing the filters in the area of a connection to structures, where compaction is difficult to achieve (e.g. below protruding elements). Gaps caused by settlement can lead to concentrated seepage.

According to ZTV W LB 205 (1992) filters in internal drainage systems must be compacted with a degree of compaction of DPr = 100%, like the embankment material. The filter requirements (static efficiency, accessibility) determine whether compaction of an external drainage system is required.

8.2.2 Installation under water

Non-uniform filter material (CU > 5) tends to separate when falling through water. As a result, the filter becomes a so-called "reverse filter" which usually does not provide the required grain sizes at the transi-tion between the adjacent layers. Hence, in practice only uniform grain mixtures (CU ≤ 5) are permitted when installing granular filters under water by dropping.

Non-uniform filter material may be installed under water only if prior tests have proven that a mixture sep-aration can be prevented by using an appropriate installation method.

The installation method must guarantee a complete installation in compliance with the required minimum thicknesses.

Damage of the granular filter due to return flow and waves caused by shipping must be prevented. In the area of the construction site the authorized maximum speed of navigation should therefore be limited to vs = 6 km/h.

For the placement of a granular filter by dropping, application-related suitability tests are required (ZTV-W LB 210, 2006).

If dropping a granular filter in flowing waters, substantial horizontal drifting of the material must be ex-pected even at low flow velocities. Hence, as a rule, granular filters to be placed in free-flowing waters should be deposited with an excavator shovel. Furthermore, a sufficiently stable position of the filter mate-rial against the flow up to the armour layer cover must be ensured.

8.3 As-built documents

With regard to future maintenance and repair activities, as-built documents containing all of the most im-portant information must be prepared to document the actual construction work. They include documenta-tion of the following:

1. type of ground in accordance with DIN 4020 (2010) and DIN 18196 (2011),

2. filter and revetment or drain structure,


18

3. filter design,

4. type of filter material and grain size distribution of filter stage(s),

5. accompanying commercial documents with CE marking,

6. connections to structures,

7. the location of the works and

8. details of particular significance regarding installation (e.g. any defects detected during quality checks or reservations during acceptance, divers’ reports).

References

Beyer, W. (1964): Zur Bestimmung der Wasserdurchlässigkeit von Kiesen und Sanden aus der Korn-verteilungskurve (Determining the water permeability of gravels and sands based on the grain distribution curve) - WWT 14: pp. 165-168, Berlin

Hazen, A. (1892): Some physical properties of sands and gravels with special reference to their use in filtration. 24th Annual Report Massachusetts State Board of Health: Pub.Doc. No.34, S. 539-556

BMVBS (2010): ): Erlass "Einsatz von industriell hergestellten Wasserbausteinen an Bundeswasser-straßen" (Decree "Use of industrially manufactured armourstones on German federal water-ways"), Federal Ministry of Transport, Building and Urban Affairs, 14 September 2010, Bonn

Odenwald, B. (2011): Numerische Berechnung der Dammdurchströmung, BAWMitteilungen (Numerical computation of seepage flow through embankment dams), Newsletters of the BAW, No. 94, pp. 59-76, Federal Waterways Engineering and Research Institute, Karlsruhe

Referenced guidelines

DIN EN 1097-6 (2013): Tests for mechanical and physical properties of aggregates - Part 6: Determina-tion of particle density and water absorption, Beuth Verlag, Berlin

DIN EN 1997-2 (2010): Eurocode 7: Geotechnical design - Part 2: Ground investigation and testing; in conjunction with: DIN 4020, Geotechnical investigations for civil engineering purposes - Supplementary rules for DIN EN 1997-2 and DIN EN 1997-2/National Annex - Nationally determined parameters - Eurocode 7: Geotechnical design - Part 2: Ground investigation and testing in: Handbuch Eurocode 7, Geotechnische Bemessung, Band 2, Erkundung und Untersuchung (Manual “Eurocode 7, Geotechnical design, Volume 2, Investigation and testing”), Beuth Verlag, Berlin

DIN 4095 (1990): Planning, design and installation of drainage systems protecting structures against wa-ter in the ground, Beuth Verlag, Berlin

DIN 18196 (2011): Earthworks and foundations - Soil classification for civil engineering purposes, Beuth Verlag, Berlin


19

DIN EN 13242 (2008): Aggregates for unbound and hydraulically bound materials for use in civil engi-neering work and road construction, Beuth Verlag, Berlin

DIN EN 13383 (2002): Armourstone - Part 1: Specification, Beuth Verlag, Berlin

GBB (2010): Grundlagen der Bemessung von Böschungs- und Sohlensicherungen an Bundeswasser-straßen (GBB) (Principles for the Design of Bank and Bottom Protection for Inland Waterways (GBB)), Federal Waterways Engineering and Research Institute (BAW), Karlsruhe

MAR (2008): Merkblatt Anwendung von Regelbauweisen für Böschungs- und Sohlensicherungen an Bin-nenwasserstraßen (MAR) (Code of Practice "Use of Standard Construction Methods for Bank and Bottom Protection on Inland Waterways (MAR)"), Federal Waterways Engineering and Research Institute (BAW), Karlsruhe

MMB (2013): Merkblatt Materialtransport im Boden (MMB) (Code of Practice "Internal erosion (MMB)"), Federal Waterways Engineering and Research Institute (BAW), Karlsruhe

MSD (2011): Merkblatt Strandsicherheit von Dämmen an Bundeswasserstraßen (MSD), (Code of Prac-tice "Stability of Embankments at German Inland Waterways (MSD)"), Federal Waterways Engi-neering and Research Institute (BAW), Karlsruhe

TLW (2003): Technische Lieferbedingungen für Wasserbausteine (TLW), Drucksachenstelle bei der WSD Mitte, Am Waterlooplatz 9, 30169 Hannover (Technical Terms of Delivery for Armourstones (TLW), printing office at the WSD Central Region, Am Waterlooplatz 9, 30169 Hanover)

TL Gestein (2007): Technische Lieferbedingungen für Gesteinskörnungen im Straßenbau, TL Gestein-StB 04, Fassung 2007, Forschungsgesellschaft für Straßen- und Verkehrswesen, Köln (Technical terms of delivery for aggregates in road construction, TL Gestein-StB 04, Version 2007, Society for research on roads and traffic, Cologne)

ZTV-W LB 205 (1992): Zusätzliche Technische Vertragsbedingungen Wasserbau (ZTV-W), Erdbau, Leis-tungsbereich 205; Drucksachenstelle bei der WSD Mitte, Am Waterlooplatz 9, 30169 Hannover (Supplementary Technical Contract Conditions for Hydraulic Engineering, (ZTV-W), Earth works, Section 205; printing office at the WSD Central Region, Am Waterlooplatz 9, 30169 Hanover)

ZTV-W LB 210 (2006): Zusätzliche Technische Vertragsbedingungen Wasserbau (ZTV-W), für Bö-schungs- und Sohlensicherungen, Leistungsbereich 210; Drucksachenstelle bei der WSD Mitte, Am Waterlooplatz 9, 30169 Hannover (Supplementary Technical Contract Conditions for Hydrau-lic Engineering (ZTV-W) for Embankment and Bottom Revetments, Section 210; printing office at the WSD Central Region, Am Waterlooplatz 9, 30169 Hanover)

BAW Code of Practice: Use of Granular Filters on German Inland Waterw ays (MAK), Issue 2013 – Annex 1

A1-1

Annex 1: Estimation of the coefficient of hydraulic conductivity k based on grain size distribution

Table A1.1: Procedure for determining hydraulic conductivity k in m/s based on grain size distribution (d in mm)

Method Formula Appropriate soil type

Parameters and limits

Beyer (1964)

( ) ( )210U dCck ⋅= sands and gravels 20CU < ; mm0.6dmm0.06 10 <<

m/s104km/s102 35 −− ⋅<<⋅ Hazen (1893)

( )210d0.0116k ⋅= sands CU < 5; mm3dmm0.1 10 <<

Kaubisch (1986)

3.59P0.12P0.0005 2

10k −⋅−⋅=

P in percent by weight

clayey soils P = pelite content = Σ(U+T) (d < 0.06 mm)

400C5 U <<

m/s10k 6−< 10% < P < 60%

USBR (in Odong, 2008)

( )2.320d0.0036k ⋅= mixed-grained

soils mm0.00220d

mm0.0210d

>

<

m/s10km/s10 68 −− <<

Seiler (1973)

210

U d1000

)(Ck ⋅= 10χ 5 < CU ≤ 17

225

U25 d1000

)(Ck ⋅χ

= gravels – sands 17 ≤ CU ≤ 100

Zieschang (1964)

( )21021 dcck ⋅⋅= clayey, silty to gravelly sand mm0.6dmm0.06

25;C

10

U

<<≤

m/s105km/s101.6 35 −− ⋅<<⋅

Table A1.2: Coefficient c according to Beyer

CU 1.0 - 1.9 2.0 - 2.9 3.0 - 4.9 5.0 - 9.9 10.0 - 19.9 > 20 c(CU) 0.011 0.010 0.009 0.008 0.007 (0.006)

Table A1.3: Dependence of empirical coefficient c1 according to Zieschang on type of loose material

Type of loose material CU Scope c1 pure sand, gravelly sand 1 - 3 d10 = 0.1 - 0.6 mm 0.0139 pure sand, gravelly sand 3 - 5 d10 = 0.1 - 0.6 mm 0.0116 slightly silty sand for d0.01 < 2% ≥ 5 d10 = 0.1 - 0.6 mm 0.0093 slightly clayey silty sand with d0.01 < 3%

≥ 5 d10 = 0.08 - 0.6 mm 0.0070

clayey silty sand with d0.01 < 4%

≥ 5 d10 = 0.06 - 0.6 mm 0.0046


A1-2

Table A1.4: Dependence of empirical coefficient c2 according to Zieschang on mica content

mica content c2 no or traces of mica 1.0 slightly micaceous 0.8 strongly micaceous 0.5

Table A1.5: Correction factors χ10(cu) and χ25(cu) according to Seiler for 5 ≤ CU ≤ 100

CU [Ones] [Tens] 0 1 2 3 4 5 6 7 8 9

χ10 (CU) 0 21.5 19.0 17.0 15.0 13.5 1 12.0 10.5 9.4 8.4 7.5 6.7 6.1 5.7

χ25 (CU) 0.88 0.88 0.89 2 0.90 0.92 0.94 0.96 0.98 1.0 1.02 1.04 1.06 1.08 3 1.10 1.13 1.16 1.19 1.22 1.25 1.25 1.31 1.34 1.37 4 1.40 1.44 1.48 1.52 1.56 1.60 1.65 1.70 1.75 1.80 5 1.85 1.90 1.95 2.00 2.05 2.10 2.18 2.26 2.34 2.42 6 2.50 2.58 2.66 2.74 2.82 2.90 2.98 3.06 3.14 3.22 7 3.30 3.40 3.50 3.60 3.70 3.80 3.92 4.04 4.16 4.28 8 4.40 4.54 4.68 4.82 4.96 5.10 5.26 5.42 5.58 5.74 9 5.90 6.08 6.26 6.44 6.62 6.80 7.02 7.24 7.46 7.68 10 7.90

References for Annex 1

BEYER, W. (1964): Zur Bestimmung der Wasserdurchlässigkeit von Kiesen und Sanden aus der Korn-verteilungskurve (Determining the water permeability of gravels and sands based on the grain distribution curve) - WWT 14: pp. 165-168, Berlin

HAZEN, A. (1893): Some physical properties of sands and gravels with special reference to their use in filtration. – 24th Annual Report Massachusetts State Board of Health: Pub.Doc. No.34, S. 539-556

KAUBISCH, M. (1986): Zur indirekten Ermittlung hydrogeologischer Kennwerte von Kippenkomplexen, dargestellt am Beispiel des Braunkohlenbergbaus, Dissertation (Indirect determination of hydro-geological parameters of dumping complexes in lignite mining), Dissertation, Mining Academy, Freiberg)

ODONG, J. (2008): Evaluation of Empirical Formulae for Determination of Hydraulic Conductivity based on Grain-Size Analysis, The Journal of American Science, 4(1), 2008, ISSN 1545-1003


A1-3

SEILER, K.-P. (1973): Durchlässigkeit, Porosität und Kornverteilung quartärer Kies-Sand-Ablagerungen des bayrischen Alpenvorlandes (Permeability, porosity and grain distribution of quaternary gravel and sand deposits in the Bavarian foothills of the Alps), Gas- und Wasserfach (gwf), Was-ser/Abwasser, Volume 114, Issue no. 8, pp. 353-358

ZIESCHANG, J. (1964): Die Bestimmung der Wasserdurchlässigkeit von Lockergesteinsgrundwasser-leitern (Determination of water permeability of loose material aquifiers), Zeitschrift für angewandte Geologie, No. 10, pp. 364-370)


A2-1

Annex 2: Examples for determining grain size distributions in filters

1 Preliminary remark

The following examples show the calculation steps used to design a granular filter.

The boundaries of the size ranges depicted for soil 1 and soil 2 in Figures A2.1 and A2.3 represent the grading curves for the finest and coarsest grain size fractions in a soil layer. As such, the boundaries do not represent a graphically designed envelope of all grading curves identified within a soil layer.

In general it should suffice to complete the following design steps for the left and right boundaries of a size range. It is advisable, however, to check and verify individual grading curves within the size range, too, if they substantially differ in shape from the other grading curves of the soil layer.

2 Design example for soil 1

Figure A2.1: Size range with characteristic grading curves for soil 1

The left boundary of the size range for soil 1 represents a slightly silted fine to medium sand (soil group SU in DIN 18196).

The grading curve at the right boundary represents a narrowly graded, coarse to medium sand, which according to DIN 18196 belongs to soil group SE.


A2-2

Step 1: Examination regarding cohesiveness according to section 2.1 of MMB

If no detailed laboratory tests are available, the distinction between cohesive and cohesionless soils is made according to the definition provided in section 2.1 of MMB, based on the soil classification pursuant to DIN 18196. A soil assigned to soil group SU generally has no or little plasticity (low plasticity) and is thus categorised as cohesionless. Pure sands (soil group SE) are never cohesive.

For the example this implies:

left boundary of size range: soil group SU according to DIN 18196 → cohesionless right boundary of size range: soil group SE according to DIN 18196 → cohesionless

Step 2: Validating suffusion resistance of in-situ soil according to section 4.2.6 of MMB

Determination of coefficient of uniformity CU:

left boundary: CU = d60,B / d10,B

= 0.2 / 0.08 = 2.5 CU < 8, continuous grading curve

right boundary: CU = d60,B / d10,B = 0.4 / 0.16 = 2.5 CU < 8, continuous grading curve

Evaluation:

Both curves meet the criterion for the simplified verification method according to Ziems. Consequently both soils are considered resistant to suffusion without further verification.

Note: For soils which are not resistant to suffusion the mechanical stability of the granular filter has to be designed according to the method developed by Lafleur (MMB, section 4.3.4).

Step 3: Filter design according to the criterion for mechanical filtration stability (corresponds to the verification of safety against contact erosion pursuant to MMB, section 4.3)

Filter design for mechanical filtration stability is based on the grading curve representing the finest-grained fractions of the relevant soil layer. Typically this will be the left boundary of the size range rele-vant for the filter design.

Selection of the method:

The relevant grading curve at the left boundary exceeds the application limit defined by the Terzaghi method (max CU < 2), because CU = 2.5. For this reason the method according to Cistin/Ziems is used. Depending on the respective application limits other methods are conceivable as well (see MMB).


A2-3

Design:

Input parameters required for the method according to Cistin/Ziems:

• soil:

from Figure A2.1: d10,B = 0.08 mm d60,B = 0.2 mm CU,B = d60,B / d10,B = 0.2 mm / 0.08 mm = 2.5 d50,B = 0.18 mm

• filter to be determined:

chosen: CU,F ≈ 2.5 For the filter a coefficient of uniformity similar to that of the soil is required.

Calculation steps:

• Based on Figure 7 in MMB section 4.3.3.3, with CU,B = 2.5 and CU,F = 2.5 the value of the permissible grain size ratio between the soil and the filter is max A50 = 12 (intermediate values may be interpolat-ed).

• Subsequently d50,B can be determined with max A50 and the known value for d50,B by transforming the equation max A50 = d50,F / d50,B into d50,F =max A50 • d50,B = 12 • 0.18 = 2.2 mm.

• The appropriate d60,F must be chosen to ensure the condition CU,F ≈ 2.5 is met (see above). In this example d60,F = 2.7 mm is chosen.

• Transforming the equation for CU,F = d60,F / d10,F therefore results in d10,F = d60,F/CU,F = 2.7 / 2.5 = 1.08 mm (d10,F = 1.1 mm is chosen),

• where d10,F = d10,R = 1.1 mm, d60,F = d60,R = 2.7 mm, and d50,F = d50,R = 2.2 mm

the right boundary of the size range including all possible filter grading curves can be drawn (Figure A2.2).

Step 4: Filter design according to the criterion for hydraulic filtration stability (MAK, section 7.3)

Filter design for hydraulic filtration stability is based on the grading curve for the coarsest-grained frac-tions of the examined homogenous area. Typically this will be the right boundary of the size range rele-vant for the filter design.


A2-4

Design:

Input parameters required: hydraulic conductivity (k value) of the in-situ soil (kB)

The k value of the soil can be estimated on the basis of its grading curve either directly in laboratory tests or analytically by using empirical calculation formulas while respecting specific scopes (mostly depending on CU and the grading curve) (MAK, Annex 1). In the following example, analytical methods are used.

Determination of CU for method selection:

d10,B = 0.16 mm d60,B = 0.4 mm CU,B = d60,B / d10,B = 0.4 mm / 0.16 mm = 2.5

Both conditions for applying the method according to Hazen (CU < 5 and 0.1 < d10 < 3 mm) are met. Thus the k value can be calculated as follows:

Calculation steps:

• Based on Hazen's formula for k, where k = 0.0116 • d10

2 and d10 = d10,B = 0.16 mm the hydraulic conductivity of the in-situ soil kB is kB = 0.0116 • 0.162 [mm] = 3.0 • 10-4 m/s. (Note: formula is not true to dimension!)

• The filter permeability required is a result of the claim kF ≥ 25 • kB resulting in ≥ 25 • 3.0 • 10-4 m/s. ≥ 7.5 • 10-3 m/s.

Transforming the equation = 0.0116 • d102 results in

d10,F = (kF/ 0.0116)0.5 = (7.5 • 10-3 / 0.0116)0.5 = 0.8 mm,

• if CU,F = 2.5 is chosen, the result for d60,F is d60,F = CU,F • d10,F = 2.5 • 0.8 mm = 2.0 mm,

• where d10,F = d10,L = 0.8 mm and d60,F = d60,L = 2.0 mm,

the left boundary of the size range including all possible filter grading curves can be drawn (see Figure A2.2).


A2-5

Figure A2.2: Result of granular filter design

Step 5: Specification of parameters for invitations to tender

To describe the permissible size range for the granular filter, the invitation to tender must specify the characteristic diameters and their respective percentage of mass proportions.

Left boundary of size range: d5 =0.06 mm d10 = 0.8 mm d100 = 4.0 mm

Right boundary of size range: d0 = 0.9 mm d85 = 4.5 mm d100 =22.5 mm


A2-6

3 Design example for soil 2

Figure A2.3: Size range with characteristic grading curves for soil 2

The left boundary of the size range for soil 2 represents a sandy, coarse silt. Laboratory tests on plasticity and the geotechnical soil identification gave only low plastic properties. Pursuant to DIN 18196 the soil can thus be classified as sandy silt with low plasticity assigned to soil group UL.

The right boundary of the grading curve represents a narrowly graded fine to medium sand of the soil group SE pursuant to DIN 18196.

Step 1: Examination regarding cohesiveness according to section 2.1 of MMB

Where no detailed laboratory tests are available, the distinction is made according to the definition pro-vided in section 2.1 of MMB, based on the soil classification pursuant to DIN 18196. Accordingly, soils of low plasticity belonging to the groups UL, TL are categorised as cohesionless. Pure sands (soil group SE) are never cohesive.

For the example this implies:

left boundary of size range: soil group UL according to DIN 18196 → cohesionless right boundary of size range: soil group SE according to DIN 18196 → cohesionless


A2-7

Step 2: Validating suffusion resistance of in-situ soil according to section 4.2.6 of MMB

Determination of coefficient of uniformity CU:

left boundary: CU = d60,B / d10,B = 0.055 / 0.012 = 4.6 < 8, continuous grading curve

right boundary: CU = d60,B / d10,B = 0.18 / 0.09 = 2 < 8, continuous grading curve

Evaluation:

Both grading curves meet the criteria for simplified verification according to Ziems. Hence, both soils are resistant to suffusion.

Note: For soils which are not resistant to suffusion the mechanical stability of the granular filter has to be designed according to the method of Lafleur (MMB, section 4.3.4).

Step 3: Filter design according to the criterion of mechanical filtration stability (corresponds to the verification of safety against contact erosion pursuant to section 4.3 of MMB)

Filter design for mechanical filtration stability is based on the grading curve representing the finest-grained fractions in the homogenous area under consideration. Typically this is the left boundary of the size range relevant for the filter design.

Selection of method:

The relevant grading curve at the left boundary exceeds the application limit of the Terzaghi method (max CU < 2), because CU = 4.6. For this reason the method according to Cistin/Ziems is used. Depending on the respective application limits other methods are conceivable as well (see MMB).

Design:

Input parameters required for the Cistin/Ziems method:

• soil:

from Figure A2.3: d10,B = 0.012 mm d60,B = 0.055 mm CU,B = d60,B / d10,B = 0.055 mm / 0.012 mm = 4.6 d50,B = 0.045 mm

• filter to be determined:

chosen: CU,F ≈ 4 For the filter a coefficient of uniformity similar to that of the soil is required.


A2-8

Calculation steps:

• Based on Figure 5 of MMB, section 4.3.3.3 with CU,B = 4.6 and CU,F = 4 the permissible grain size ratio between the soil and the filter is max A50 = 18.

• Subsequently d50,F can be determined with max A50 and the known value for d50,B by transforming the equation max A50 = d50,F / d50,B into d50,F = max A50 • d50,B = 18 • 0.045 = 0.81 mm.

• The appropriate d60,F must be chosen so as to ensure that the condition CU,F ≈ 4 is met (see above). In this example d60,F = 1.1 mm is chosen.

• Transforming the equation for CU,F = d60,F / d10,F therefore results in d10,F = d60,F/CU,F = 1.1 / 4 = 0.28 mm,

• where d10,F = d10,R = 0.28 mm, d60,F = d60,R = 1.1 mm, and d50,F = d50,R = 0.81 mm

the right boundary of the size range including all possible filter grading curves can be drawn (see Figure A2.4).

Step 4: Filter design according to the criterion of hydraulic filtration stability (MAK, section 7.3)

Filter design for hydraulic filtration stability is based on the grading curve for the coarsest-grained frac-tions of the examined homogenous area. Typically this will be the right boundary of the size range rele-vant for the filter design.

Design:

Input parameters required: hydraulic conductivity (k value) of in-situ soil (kB)

The k value of the soil can be estimated on the basis of its grading curve either directly in laboratory tests or analytically by using empirical calculation formulas while respecting specific scopes (mostly depending on CU and the grading curve) (MAK, Annex 1). In this example analytical methods are used.

Determination of CU for method selection:

d10,B = 0.09 mm d60,B = 0.18 mm CU,B = d60,B / d10,B = 0.18 mm / 0.09 mm = 2

The first condition for applying the method of Hazen is met (1 < CU < 5), however, the second condition (0.1 < d10 < 0.5 mm) is not met. Therefore the method according to Beyer is applied (applicable for CU < 20 and 0.06 < d10 < 0.6 mm).


A2-9

Calculation steps:

• Based on Beyer's formula for k, where k = c(CU) • d10

2 and c(CU) = 0.010 (value obtained from Annex 1) and d10 = d10,B = 0.09 mm the permeability of the in-situ soil kB is kB = 0.010 • 0.092 [mm] = 8.1 • 10-5 m/s.

• The required permeability of the filter is a result of the claim kF ≥ 25 • kB resulting in ≥ 25 • 8.1 • 10-5 m/s ≥ 2.0 • 10-3 m/s.

• By transforming the equation k = c(CU) • d102 and provided that c(CU) = 0.009 (value obtained from

Table A1.1, Annex 1 for CU,F = 4, see step 3) d10,F results in d10,F = (kF/ 0.009)0.5 = (2.0 • 10-3 / 0.009)0.5 = 0.47 mm.

• Given the chosen CU,F = 4 this implies for d60,F

d60,F = CU,F • d10,F = 4 • 0.47 mm = 1.9 mm,

• where d10,F = d10,L = 0.47 mm and d60,F = d60,L = 1.9 mm

the left boundary of the size range including all possible grading curves can be drawn (see Figure A2.4).

Figure A2.4: Grading curves obtained in the first design attempt (result meaningless, because right boundary is finer-grained than left boundary)


A2-10

Result: There is no filter grading that meets both conditions!

The filter grading that is relevant for maintaining mechanical filtration stability (index R) is finer than the grading curve relevant for ensuring hydraulic filtration stability (index L). This means that a mechanically or geometrically stable filter fails to satisfy the hydraulic criterion. Thus, the filter permeability would not be sufficiently high to drain outflowing water without causing harmful seepage forces.

Possible solutions:

In a second calculation process a steeper grading curve (a smaller coefficient of uniformity CU,F) could be provided in an attempt to identify a filter grading that meets both conditions. If this solution fails to work, the design of a multi-layer filter is an option (multi-stage filters, step 5).

Step 5: Design of a multi-stage filter (in this case two-stage filter)

In a two-stage filter, the first filter stage is responsible for ensuring the mechanical filtration stability in relation to the in-situ soil, whereas the hydraulic filtration stability may be neglected. In verifications of soil statics harmful excess pore water pressures occurring in the first filter stage shall be considered. For in-stance the weight of the first filter layer must not be fully considered when calculating the surcharge on the in-situ soil (e.g. in revetment design).

In a second calculation process the second filter layer is designed, which is both geometrically and hy-draulically filter-stable in relation to the first layer and thus ensures the hydraulic efficiency of the multi-stage filter. The weight of this filter stage is fully applicable in surcharge calculations for verifications of soil statics. Furthermore, this filter layer ensures protection against erosion for the area of the first filter layer through which water is seeping.

Design of filter stage 1:

The design is based on the grading curve for the finest-grained fractions in the size range of the in-situ soil. The calculation steps are according to the approach described in step 3. Since the input parameters do not change, the results are identical. The right boundary of the size range of filter stage 1 can thus be described by means of the following characteristic values:

Characteristic values at the right boundary for filter stage 1:

d10,F1,R = 0.28 mm d60,F1,R = 1.1 mm and d50,F1,R = 0.81 mm CU,F1,R = 4

Theoretically the left boundary for filter stage 1 can extend up to the grading curve representing the coarsest-grained fractions in the size range of the in-situ soil. It should have approximately the same co-efficient of uniformity as the right boundary.


A2-11

Chosen characteristic values at the left boundary for filter stage 1:

d10,F1,L = 0.125 mm d60,F1,L = 0.50 mm and d50,F1,L = 0.39 mm CU,F1,L ≈ 4

Design of filter stage 2:

Filter design for mechanical filtration stability is based on the left boundary for filter stage 1.

Chosen: CU,F2,R ≈ 4

• Based on Figure 5 in MMB, section 4.3.3.3 with CU,F1,L ≈ 4 and CU,F2,R ≈ 4 the permissible grain size ratio between filter stage 1 and filter stage 2 is max A50 = 17.5.

• Given this max A50 and the known value for d50,F1,L it is possible to determine d50,F2,R by transforming the equation max A50 = d50,F2,R / d50,F1,L into d50,F2,R = max A50 • d50,F1,L = 17.5 • 0.39 = 6.8 mm.

• The appropriate d60,F2,R must be chosen so as to ensure that the condition CU,F2,R ≈ 4 is met. In this example d60,F2,R ≈ 9 mm.

• Transforming the equation for CU,F = d60,F2,R / d10,F2,R results in d10,F2,R = d60,F2,R/CU,F2,R = 9 / 4 = 2.3 mm,

• where d10,F2,R = 2.3 mm, d60,F2,R = 9 mm, and d50,F2,R = 6.8 mm,

the right boundary of the size range including all possible filter grading curves can be drawn (Figure A2.5).

The right boundary for filter stage 1 is the basis for calculating the hydraulic filtration stability.

k value for the right boundary for filter stage 1:

d10,F1,R = 0.25 mm → 0.1 < d10 < 0.5 mm, CU,F1,R = 4 → CU < 5

The application limits of the method of Hazen are respected (see Annex 1).

• Where d10 = d10,F1,R = 0.25 mm, according to Hazen the result is: kF1,R = 0.0116 • 0.252 [mm] = 7.3 • 10-4 m/s.

• The hydraulic conductivity required for filter stage 2 is calculated based on the condition kF2 ≥ 25 • kF1, which results in kF2 ≥ 25 • 7.3 • 10-4 m/s ≥ 1.8 • 10-2 m/s.


A2-12

Calculation steps:

• By transforming the equation k = 0.0116 • d102 the corresponding diameter d10,F2 is calculated as fol-

lows:

• d10,F2 = (kF2/ 0.0116)0.5 = (1.8 • 10-2 / 0.0116)0.5 = 1.24 mm, with the chosen CU of CU,F2 = 4 the result for d60,F2 is d60,F = CU,F2 • d10,F2 = 4 • 1.24 mm = 5.0 mm,

• where d10,F2,L = 1.24 mm and d60,F2,L = 5.0 mm,

the left boundary of the size range including all possible grading curves can be drawn (Figure A2.5).

Figure A2.5: Multi-stage filter


A2-13

Step 6: Parameters for invitations to tender

To describe the permissible size range, the invitation to tender must indicate the characteristic diameters and their percentage of mass proportions for each filter stage.

Filter stage 1: Left boundary of size range: d5 ≥ 0.06 mm d10 = 0.12 mm d100 = 1.5 mm

Right boundary of size range: d0 = 0.19 mm d85 = 2 mm d100 = 10 mm

Filter stage 2: Left boundary of size range: d0 = 0.6 mm (chosen) d10 = 1.25 mm d100 = 14 mm

Right boundary of size range: d0 = 1.8 mm d85 = 20 mm d100 = 100 mm

BAW Code of Practice · BAW Code of Practice: Use of Granular Filters on German Inland Waterways...

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