SITE INVESTIGATION

SITE INVESTIGATION 1Definition of Site InvestigationIt is a process for determining engineering and mechanical properties of natural soil layers.

The process consisting of boring, sampling and testing. 2The purpose of a soil investigation program 1.Selection of the type and the depth of foundation suitable for a given structure.2.Evaluation of the load-bearing capacity of the foundation.3.Estimation of settlement of a structure.4.Determination of potential foundation problems (for example, expansive soil, collapsible soil, sanitary landfill, and so on).5.Establishment of ground water table.6.Prediction of lateral earth pressure for structures like retaining walls, sheet pile, and braced cuts.7.Establishment of construction methods for changing subsoil conditions.3The Importance of SI

Site suitability. Safe, practical and economic design. To determine difficulties. To study the suitability of construction material (soil or rock). 4EXPLORATION PROGRAM The purpose of the exploration program is to determine engineering properties of the soils. strength, deformation, hydraulic characteristics.

The program should be planned for maximum information, minimum cost.5Steps of Soil Exploration

Boring: Refers to drilling or advancing a hole in the ground. The test would include hand auger, motorized hand boring (wash boring), deep boring (rotary drilling), and/or trial pits.

Sampling: Refers to removing soil from the hole. The samples can be classified as disturbed or undisturbed sampling. Disturbed samples are usually used for soil grain-size analysis, determination of liquid limit, specific gravity of soil as well as compaction test and California bearing ratio (CBR). The undisturbed samples are collected at least every 1.5 m and if changes occur within 1.5 m intervals, an additional samples should be taken.

Testing: Refers to determining the properties from the soil. The test can be perform either at laboratory or at field. Laboratory testing would normally be moisture content, sieve analysis, liquid limit, compaction test, CBR and so forth. Field test would include Standard Penetration Test (SPT), Cone Penetration Test (CPT) and Vane test.

6SI consists of two partsSurface Investigations:Site inspection to assess general site condition if there is any anticipated problems that might arise during the construction later on. Usually the engineer is required to inspect the site to appreciate actual site and ground problems with particular reference to terrain, vegetation, swamps, water runoff, stratigraphical formations where it is exposed. Sub-surface Investigations:Ground or soil investigation by means of boring, sampling, testing, and etc. To determine the stratigraphy and pertinent properties of soil underlying the project site.

7Wok Procedure for SIDesk study to collect all the relevant data and information, Reconnaissance of site works, Planning program after reviewing Ground or soil exploration includes boring, sampling and testing, Laboratory testing (also field if necessary), Preparation and documentation of SI report,

8Steps of subsurface exploration program [Stage 1] Desk Study1.Assembly of all available information on dimensions, column spacing, type and use of the structure, basement requirements, and any special architectural considerations of the proposed building. Foundation regulations in the local building code should be consulted for any special requirements. For bridges the soil engineer should have access to type and span lengths as well as pier loadings. This information will indicate any settlement limitations, and can be used to estimate foundation loads.99Steps of subsurface exploration program [Stage 2] 2.Reconnaissance of the area: A reconnaissance is a preliminary examination or survey of a job site. First step is to collect and study any pertinent information already available. After collecting and studying the data available, the engineers should visit the site in person, observe thoroughly and interpret what is seen. Results of reconnaissance help to determine the scope of subsequent soil exploration. It is important to locate any underground utilities to assist in planning and carrying out subsequent subsurface exploration.

10Some typical steps in site Reconnaissance Details on the ground surface for an early observation, Topographical characteristics, e.g. flatland, hilly, swamps or pit area, The possible location of the ground water tables (GWT), Interviewing the local residents for further information, Taking a lot of photographs of the proposed site.

11Steps of subsurface exploration program [Stage 3] 3.A preliminary site investigation: In this phase a few borings are made or a test pit is opened to establish in a general manner the stratification, types of soil to be expected, and possibly the location of the groundwater table. One or more borings should be taken to rock, or competent strata, if the initial borings indicate the upper soil is loose or highly compressible. This amount of exploration is usually the extent of the site investigation for small structures.12Steps of subsurface exploration program [Stage 4] 4.A detailed site investigation: Where the preliminary site investigation has established the feasibility of the project, a more detailed exploration program is undertaken. The preliminary borings and data are used as a basis for locating additional borings, which should be confirmatory in nature, and determining the additional samples required.1314Site Exploration MethodsTest PitsBoreholesProbes (In-situ Tests)GeophysicalChoice of method depends on budget, sampling requirements, extent of investigation, and often site conditions15Methods of InvestigationTest pits: Permits visual inspection of subsurface conditions in natural state.Max. depth limited to 18 -20 feet.Especially useful for gravelly soil where boreholes may be difficult.Sampling/testing done on exposed surfaces.

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Hand operated augersPower driven augers17Stratigraphy and FindsLayerSoilSoil ColourFindsChronologyL1Sandy soil Gray 7.5YR 5/1Modern Rubbish (filled soil)1980sL2Sandy soilPinkish white 7.5YR 8/2Modern rubbish (filled soil)1980sL3Sandy soilReddish yellow 7.5YR 7/6Modern rubbish (filled soil)1980sL4Sandy soilGray 7.5YR 6/1Modern rubbish (filled soil)1980sL5Loamy soilReddish yellow 5YR 6/6Nil (original decomposed soil)L6Loamy soilReddish yellow 5YR 6/8Nil (original decomposed soil)L7Loamy soil, with some decomposed bed rock textureLight red 2.5YR 6/8Nil (original decomposed soil)Test Pit Wall Photograph Western Wall Section

Test Pit Wall DrawingWestern Wall Section Drawing

SOIL BORING18

Depth of Boring The approximate required minimum depth of the borings should be predetermined. The estimated depths can be changed during the drilling operation, depending on the subsoil encountered. To determine the approximate minimum depth of boring, engineers may use the following rule: 19Depth of Boring1.Determine the net increase of stress, under a foundation with depth as shown in the Figure. 2.Estimate the variation of the vertical effective stress, ', with depth. 3.Determine the depth, D = D1, at which the stress increase is equal to (1/10) q (q = estimated net stress on the foundation).4.Determine the depth, D = D2, at which /' = 0.05.5.Unless bedrock is encountered, the smaller of the two depths, D1 and D2, just determined is the approximate minimum depth of boring required. Table shows the minimum depths of borings for buildings based on the preceding rule.20Depth of Boring21

Determination of the minimum depth of boring Depth of Boring22

Depth of Boring Depth of Boring For hospitals and office buildings, the following rule could be used to determine boring depth 23

Depth of Boring When deep excavations are anticipated, the depth of boring should be at, least 1.5 times the depth of excavation. Sometimes subsoil conditions are such that the foundation load may have to be transmitted to the bedrock. The minimum depth of core boring into the bedrock is about 3m. If the bedrock is irregular or weathered, the core borings may have to be extended to greater depths.24Spacing Boring There are no hard and fast rules for the spacing of the boreholes. The following table gives some general guidelines for borehole spacing. These spacing can be increased or decreased, depending on the subsoil condition. If various soil strata are more or less uniform and predictable, the number of boreholes can be reduced.25Spacing Boring26

Approximate Spacing of Boreholes SOIL BORING The earliest method of obtaining a test hole was to excavate a test pit using a pick and shovel. Because of economics, the current procedure is to use power-excavation equipment such as a backhoe to excavate the pit and then to use hand tools to remove a block sample or shape the site for in situ testing. This is the best method at present for obtaining quality undisturbed samples or samples for testing at other than vertical orientation.27Boring tools 28 Auger boring Power drills

Boring tools29

Boring tools30

Boring tools31

Preparation of Boring Logs1. Name and address of the drilling company2. Drillers name3. Job description and number4. Number, type, and location of boring5. Date of boring6. Subsurface stratification, which can he obtained by visual observation of the soil brought out by auger, split-spoon sampler, and thin-walled Shelby tube sampler7. Elevation of water table and date observed, use of casing and mud losses, and so on8. Standard penetration resistance and the depth of SPT9. Number, type, and depth of soil sample collected10. In case of rock coring, type of core barrel used and, for each run, the actual length of coring, length of core recovery, and ROD3233

SOIL SAMPLING Two types of soil samples can be obtained during sampling disturbed and undisturbed. The most important engineering properties required for foundation design are strength, compressibility, and permeability. Reasonably good estimates of these properties for cohesive soils can be made by laboratory tests on undisturbed samples which can be obtained with moderate difficulty. It is nearly impossible to obtain a truly undisturbed sample of soil; so in general usage the term "undisturbed" means a sample where some precautions have been taken to minimize disturbance or remolding effects. In this context, the quality of an "undisturbed" sample varies widely between soil laboratories. 34Disturbed vs Undisturbed

35Good quality samples necessary.

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