site investigation 1

8/7/2019 Site Investigation 1

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SITE INVESTIGATION


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DefinitionThe process of determining the layers ofnatural soil deposits that will underlie a

proposed structure and their physicalproperties is generally referred to as siteinvestigation.


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The purpose of a soil

investigation program1. 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 the probable settlement of a structure.

4. Determination of potential foundation problems (forexample, expansive soil, collapsible soil, sanitarylandfill, and so on).

5. Establishment of ground water table.

6. Prediction of lateral earth pressure for structures likeretaining walls, sheet pile bulkheads, and bracedcuts.

7. Establishment of construction methods for changingsubsoil conditions.


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EXPLORATION PROGRAMThe purpose of the exploration programis to determine, within practical limits,

the stratification and engineeringproperties of the soils underlying thesite. The principal properties of interest

will be the strength, deformation, andhydraulic characteristics. The programshould be planned so that themaximum amount of information can be

obtained at minimum cost.


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Steps of subsurface exploration

program [Stage 1]1.Assembly of all available information on

dimensions, column spacing, type and use ofthe structure, basement requirements, andany special architectural considerations of theproposed building. Foundation regulations inthe local building code should be consultedfor any special requirements. For bridges the

soil engineer should have access to type andspan lengths as well as pier loadings. Thisinformation will indicate any settlementlimitations, and can be used to estimate

foundation loads.


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program [Stage 2]2.Reconnaissance of the area:

This may be in the form of a field trip to

the site which can reveal information onthe type and behavior of adjacentstructures such as cracks, noticeable sags,and possibly sticking doors and windows.

The type of local existing structure mayinfluence, to a considerable extent, theexploration program and the bestfoundation type for the proposed adjacent

structure.


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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 mannerthe stratification, types of soil to be expected,and possibly the location of the groundwatertable. One or more borings should be taken to

rock, or competent strata, if the initial boringsindicate the upper soil is loose or highlycompressible. This amount of exploration isusually the extent of the site investigation for

small structures.


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program [Stage 4]4.A detailed site investigation:

Where the preliminary site investigation

has established the feasibility of theproject, a more detailed explorationprogram is undertaken. The preliminary

borings and data are used as a basis forlocating additional borings, which shouldbe confirmatory in nature, and determiningthe additional samples required.


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Depth of BoringThe approximate required minimumdepth of the borings should bepredetermined. The estimated depthscan be changed during the drillingoperation, depending on the subsoil

encountered. To determine theapproximate minimum depth of boring,engineers may use the following rule:


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Depth of Boring1. Determine the net increase of stress,(W under a

foundation with depth as shown in the Figure.2. Estimate the variation of the vertical effective stress,W'Y, with depth.

3. Determine the depth, D = D1, at which the stressincrease (W is equal to (1/10) q (q = estimated netstress on the foundation).

4. Determine the depth, D = D2, at which (W/W'Y = 0.05.

5. Unless bedrock is encountered, the smaller of thetwo depths, D1 and D2, just determined is theapproximate minimum depth of boring required.Table shows the minimum depths of borings forbuildings based on the preceding rule.


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Depth of Boring

Determination of the minimum depth of boring


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Depth of Boring

Depth of Boring


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Depth of BoringFor hospitals and office buildings, the followingrule could be use to determine boring depth


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Depth of BoringWhen deep excavations are anticipated, thedepth of boring should be at, least 1.5 timesthe depth of excavation. Sometimes subsoilconditions are such that the foundation loadmay have to be transmitted to the bedrock.The minimum depth of core boring into thebedrock is about3m. If the bedrock is

irregular or weathered, the core borings mayhave to be extended to greater depths.


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Spacing BoringThere are no hard and fast rules for thespacing of the boreholes. The following

table gives some general guidelines forborehole spacing. These spacing can beincreased or decreased, depending on

the subsoil condition. If various soilstrata are more or less uniform andpredictable, the number of boreholescan be reduced.


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Spacing BoringApproximate Spacing of Boreholes


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SOIL BORINGThe earliest method of obtaining a test holewas to excavate a test pit using a pick and

shovel. Because of economics, the currentprocedure is to use power-excavationequipment such as a backhoe to excavate thepit and then to use hand tools to remove a

block sample or shape the site for in situtesting. This is the best method at present forobtaining quality undisturbed samples orsamples for testing at other than vertical

orientation.


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SOIL BORING


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Boring toolsAuger boring Power drills


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Boring tools


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Boring tools


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Boringtools


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Preparation of Boring Logs1. Name and address of the drilling company2. Drillers name3. Job description and number4. Number, type, and location of boring

5. 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 sampler

7. Elevation of water table and date observed, use of casing and mudlosses, and so on

8. 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 ROD


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SOIL SAMPLINGTwo types of soil samples can be obtained duringsampling disturbed and undisturbed. The mostimportant engineering properties required for

foundation design are strength, compressibility, andpermeability. Reasonably good estimates of theseproperties for cohesive soils can be made bylaboratory tests on undisturbed samples which canbe obtained with moderate difficulty. It is nearlyimpossible to obtain a truly undisturbed sample of

soil; so in general usage the term "undisturbed"means a sample where some precautions have beentaken to minimize disturbance or remolding effects.In this context, the quality of an "undisturbed"sample varies widely between soil laboratories.


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Disturbed vs UndisturbedGood quality samples necessary.

AR


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Disturbed vs Undisturbed


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Common Sampling Methods


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ROCKSAMPLING

Rock cores are necessary if thesoundness of the rock is to be

established. small cores tend to break up

inside the drill barrel.

Larger cores also have a

tendency to break up (rotateinside the barrel and degrade),especially if the rock is soft orfissured.


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Rock coring


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ROCKSAMPLING - Definition


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Rock Core Drilling

Done with eithertungsten carbide or

diamond core bits Use a double or triple

tube core barrel whensampling weathered or

fractured rock Used to determine Rock

Quality Designation

core barrel


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Rock Quality Designation RQD


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Rock Quality Designation

RQD

Rock Quality Designation (RQD) is defined as the percentage of rock

cores that have length equal or greater than 10 cm over the total drill

length.


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Example onCore Recovery & RQD

Core run of 150 cm

Total core recovery

= 125 cm Core recovery ratio =

125/150 = 83%

On modified basis,95 cm are counted

RQD = 95/150=63 %


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GROUND WATER TABLE LEVELGroundwater conditions and the potential forgroundwater seepage are fundamentalfactors in virtually all geotechnical analysesand design studies. Accordingly, theevaluation of groundwater conditions is abasic element of almost all geotechnicalinvestigation programs. Groundwaterinvestigations are of two types as follows:

Determination of groundwater levels andpressures.

Measurement of the permeability of thesubsurface materials.


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FIELD STRENGTH TESTSThe following are the major field tests fordetermining the soil strength:

1. Vane shear test (VST).2. Standard Penetration Test (SPT).

3. Cone Penetration Test (CPT).

4. The Borehole Shear Test (BST).

5. The Flat Dilatometer Test (DMT).

6. The Pressure-meter Test (PMT).

7. The Plate Load Test (PLT).


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FIELD STRENGTH TESTS


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Standard Penetration Test (SPT)


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Standard Penetration Test (SPT)Corrections are normally applied to the SPT

blow count to account for differences in:

energy imparted during the test (60%hammer efficiency)

the stress level at the test depth

The following equation is used to compensatefor the testing factors (Skempton, 1986):


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Cone Penetration Test (CPT)


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The Plate Load Test (PLT)


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The Plate Load Test (PLT)Scale Effect in Foundation Design


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Subsoil Exploration Report1. A description of the scope of the investigation2. A description of the proposed structure for which the subsoil exploration has

been conducted3. A description of the location of the site, including any structures nearby,

drainage conditions, the nature of vegetation on the site and surrounding

it, and any other features unique to the site4. A description of the geological setting of the site5. Details of the field explorationthat is, number of borings, depths of

borings, types of borings involved, and so on6. A general description of the subsoil conditions, as determined from soil

specimens and from related laboratory tests, standard penetrationresistance and cone penetration resistance, and soon

7. A description of the water-table conditions8. Re commendations regarding the foundation, including the type of

foundation recommended, the allowable hearing pressure, and any specialconstruction procedure that may he needed; alternative foundation designprocedures should also be discussed in this portion of the report

9. Conclusions and limitations of the investigations


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Subsoil Exploration ReportThe following graphical presentations should he

attached to the report:

1. A site location map2. A plan view of the location of the borings

with respect to the proposed structures andthose nearby

3. Boring logs4. Laboratory test results

5. Other special graphical presentations


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Example Tableof Contents fora GeotechnicalInvestigation

(Data) Report

site investigation 1

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