

Site Investigations

Introduction:

To design a foundation that will adequately support a structure, an engineer must understand the

type of soil deposits that will support the foundation. Moreover, foundation engineers must

remember that soil at any site frequently is non-homogeneous-that is, the soil profile may vary.

Soil mechanics theories involve idealized conditions, so the application of these theories to

foundation engineering problems involves judicious evaluation of site conditions and soil

parameters. To do so require some nowledge of the geological process by which the soil deposit

at the site was formed, supplemented by subsurface e!ploration. "ood professional judgment

constitutes an essential part of geotechnical engineering-and it comes only with practice.

Definition:

# geotechnical site investigation is the process of collecting information and evaluating the

conditions of the site for the purpose of designing and constructing the foundation for a structure,

such as a building, plant or bridge.

Purpose of soil investigation$-

• Selection of foundation type.

• %esign of foundations.

• &ontractors to quote realistic and competitive tenders.

• 'lanning construction techniques.

• Selection of appropriate construction equipment (especially for e!cavation and

foundations).

• *easibility studies of the site.

• +stimating development cost for the site.

• Study of environmental impacts of the proposed construction

Phases:

1. Collection of Preliminary Information:



This step includes obtaining information regarding the type of structures and its general use. *or

the construction of buildings, the appro!imate and their spacing and the local building code and basement require be nown. The construction of bridges requires determining span length,

loading on piers and abutments.

# general idea of the topography and the type of soil to be considered and around the proposedsite can be obtained from the following

. "eological Survey maps.

. #gronomy maps published by the agriculture departments of various states.

. The hydrological information. These include the records of stream flow, high flood levels,tidal records, and so on.

The information collected from these sources can be e!tremely helpful in planning a site

investigation. /n some cases, substantial savings may be realized by anticipating problems thatmay be encountered later in the e!ploration program.

2. Reconnaissance

The engineer should always mae a visual inspection of the site to obtain information about

. The general topography of the site, possible e!istence of drainage ditches, abandoned dumpsof debris, or other materials. #lso, evidence of creep of slopes and deep, wide shrinage cracs at

regularly spaced intervals may be indicative of e!pansive soils.

. Soil stratification from deep cuts, such as those made for construction of nearby highways andrailroads.

. Type of vegetation at the site, which may indicate the nature of the soil. *or e!ample, amesquite cover in central Te!as may indicate the e!istence of e!pansive clays that can cause

possible foundation problems.

0. 1igh-water mars no nearby buildings and bridge abutments.

2. "round water levels, which can be determined by checing nearby wells.

3. Types of construction nearby and e!istence of any cracs in walls or other problems.

The nature of stratification and physical properties of the soil nearby can also be obtained fromany available soil-e!ploration reports for e!isting structures.

3. Site Investigation



The site investigation phase of the e!ploration program consists of planning maing test

boreholes, and collecting soil samples at desired intervals for subsequent observation and

laboratory tests. The appro!imate required minimum depth of the borings should be

predetermined. The depth can be changed during the drilling operation, depending on the subsoil

encountered.

Site eploration methods:

a) Test pits

b) 4oring

c) 'robes and geophysical methods$

!" #est pits:

'its and trenches are e!cavated at site to test the soil strata. /S-002-536 recommends a clear

woring space of .m 7 .m at the bottom of the pit. Shallow pits up to a depth of m can be

made without providing any lateral support.

$" $oring:

8hen the depth of e!ploration is large, borings are used.

!uger %oring:



8ash boring is another method of advancing boreholes. /n this method, a casing about - m (3-

9 ft) long is driven into the ground. The soil inside the casing is then removed by means of a

chopping bit attached to a drilling rod. 8ater is forced through the drilling rod e!ists at a very

high velocity through the holes at the bottom of the chopping bit (figure 2.11). The water and the

chopped soil particles rise in the drill hole and overflow at the top of the casing through a Tconnection. The wash water is collected in a container. The casing can be e!tended with

additional pieces as the borehole progresses; however, that is not required if the borehole will

stay open and not cave in.

Rotatory drilling:

>otary drilling is a procedure by which rapidly rotating drilling bits attached to the bottom of drilling rods cut and grind and soil and advance the boreholes. There are several types of drilling

bit. >otary drilling can be used in sand, clay, and rocs (unless badly fissured). 8ater, or drilling

mud, is forced down the drilling rods to the bits, and the return flow forces the cuttings to the

surface. 4oreholes with diameters of 29.:-9. mm (-: in.) can be easily made by this

technique. The drilling mud is slurry of water and bentonite. "enerally it is used when the soil

encountered is liely to cave in. 8hen soil samples are needed, the drilling rod is raised and the

drilling bit is replaced by a sampler.

Percussion drilling:

This method is used for maing holes in rocs, boulders and other hard strata. /n this method a

very heavy chisel is alternately lifted and dropped in vertical hole. The material gets pulverized.

Standard penetration test:

The Standard 'enetration test (S'T) is a common in situ testing method used to determine the

geotechnical engineering properties of subsurface soils. /t is a simple and ine!pensive test to

estimate the relative density of soils and appro!imate shear strength parameters

.



Procedure:

#est )ole

%rill the hole to the desired sampling depth and clean out all disturbed material.

/f a wet drill is used, flush out all cuttings.

!ssem%ling *+uipment

#ttach the split-barrel sampler to the #-rod and lower into the hole until it is sitting on the

undisturbed material. #ttach the drive weight assembly. ?ift the 3.2 g hammer appro!imately

9.63 m and allow it to fall on the anvil delivering one seating blow. Mar the drill rod in successive .2 m increments to observe penetration.

Mar the drive weight assembly to indicate a 9.63 m hammer lift.

Penetration #esting

>aise and drop the hammer 9.63 m successively by means of the rope and cathead, using no

more than @0 wraps around the cathead. The hammer should be operated between 09 and 39

blows per minute and should drop freely. &ontinue the driving until either 9.02 m has been

penetrated or 99 blows has been applied. >ecord the number of blows for each .2 m of the



penetration. The first 9.2 m increment is the AseatingA drive. The sum of the blows for second

and third increment of 9.2 m penetration is termed Apenetration resistance or AB-valueA. /f the

blow count e!ceeds 99 in total, terminate the test and record the number of blows for the last9.9 m of penetration as the B-value.

)andling Sample

4ring the sampler to the surface and open it. >emove any obvious contamination from the ends

or sides and drain e!cess water. &arefully scrape or slice along one side to e!pose fresh material

and any stratification. >ecord the length, composition, color, stratification and condition ofsample. >emove sample and wrap it or seal in a plastic bag to retain moisture. /f the sample can

be removed relatively intact, wrap it in several layers of plastic to strengthen it and seal ends

with tape. Mar the sample AtopA and AbottomA if applicable and label it with an identificationnumber.

Correlation %et,een SP#- value/ friction angle/ and relative

density

&orrelation between S'T-B value and friction angle and >elative density (Meyerhoff

523)

S'T B

C4lows@9. m - ftDSoil pacing

>elative %ensity

CED

*riction angle

CFD

G 0 Hery loose G 9 G 9

0 -9 ?oose 9 - 09 9 - 2

9 I 9 &ompact 09 - 39 2 - 09

9 - 29 %ense 39 - :9 09 - 02

J 29 Hery %ense J :9 J 02



S#!#IC C0* P**#R!#I0 #*S#

+quipment $

Steel &one $ The cone shall be of suitable steel with its tip hardened. /t shall have an ape! angle

of 60o

K2 minutes and overall base diameter of 2.6 K9.9 mm giving a cross-sectional area

of 9 cm ( see *ig. ). The cone shall be so designed as to prevent the intrusion of soil particles

into the moving parts of the cone assembly.

*or obtaining the cone resistance, the cone is pushed downward at a steady rate of 9mm@sec

through a depth of 2 mm each time. The cone is pushed by applying thrust and not by driving.

#fter the cone resistance has been determined, cone is withdrawn. The sleeve is pushed on to the

cone and both are driven together into soil and combined resistance is also determined. The

resistance of sleeve alone is obtained by subtracting the cone resistance from combined

resistance.

There is good correlation studies available between S'T number (B) and the engineering

properties of soil. /f cone penetration results are related to the S'T number B, indirect

correlations are obtained between the cone test results and the engineering properties of soil.

*ollowing relations hold appro!imately good between the point resistance of cone (qc) and B.

) "ravels qcL :99 B to 999B



) Sands qcL 299 B to 399 B

) Silty sands qcL 99 B to 099 B

0) Silts and clayey silts qcL 99 B

8here qc is sin B@m.

H#B+ S1+#> T+ST

+

quipment$- . Hane shear apparatus.

. Specimen.

. Specimen container.

0. &allipers.

+7'+>/M+BT#? '><&+%>+

• 'repare two or three specimens of the soil sample of dimensions of at least 6.2 mm

diameter and 62 mm length in specimen.(?@% ratio or ).

• Mount the specimen container with the specimen on the base of the vane shear apparatus.

/f the specimen container is closed at one end, it should be provided with a hole of about

mm diameter at the bottom.

• "ently lower the shear vanes into the specimen to their full length without disturbing the



soil specimen. The top of the vanes should be atleast 9 mm below the top of the

specimen. Bote the readings of the angle of twist.

• >otate the vanes at an uniform rate say 9.o@s by suitable operating the torque application

handle until the specimen fails.

• Bote the final reading of the angle of twist.

• *ind the value of blade height in cm.

• *ind the value of blade width in cm.

Calculations:

eophysical methods:-

"eophysical methods indicate general boundaries of drastically dissimilar layers.

Most commonly used geophysical methods are$-

. Seismic or >efraction method

. >esistivity method

Seismic refraction method$-

# shoc wave propagates in an elastic medium with velocity,

v L (M@N)@

MI modulus and N - mass density

8ave velocity

/n soils 29 to 999 m@s

/n rocs 299 to 3999 m@s.

>esponses of a shoc wave eminating from a source are piced up by geophone at nown

distance.



The depth of roc underlying the soil or depth of water table can be obtained. Shoc

waves emanating from a dynamite charge forming a source, S travel through soil O are

piced up at geophones ,,,P..etc. at distances d,d,dP..etc.

<ne shoc wave travels directly through the top soil layer with a velocity v and another

gets refracted with a greater velocity v.

• The brea in curve represents point of simultaneous arrival of primary O refracted waves,

and its distance is nown as critical distance. The wave velocity through the roc layer is

many times greater than through soil layer.

So, time of arrival by a longer route is shorter than that by the shorter route through the

top soil.

Helocity, v L

Helocity, v L

The depth of roc 1 is obtained as,

1L



Seismic refraction method is fast.

• This method is reliable in establishing profile of different strata.

>esistivity method$-

+lectrical resistivity method is based on the measurement O recording changes in mean

resistivity of various soils. +lectrical conductivity of a soil layer depends upon the

concentration of ionized salts in the soil pores. +ach soil has its own resistivity depending

upon water content, compaction O composition.

?ow for saturated silt.

1igh for loose dry gravel or solid roc.

%ense rocs with few voids and little water content have high resistivity i.e. 99 to

9,999 ohm-m.

Soft saturated clays and organic deposits have low resistivity i.e. 2 to 29 ohm-m.

*or a homogeneous isotropic material electrical resistivity, N is given as,

*ig. electrical resistivity

• N L

• 8here,

N L mean resistivity (ohm-m)

%L distance between electrodes (cm)

+L potential drop between outer electrodes (volts)



/ L current flowing between outer electrodes (amperes)

>L resistance (ohm).

• 'rocedure$-

/t consists of using four equally spaced electrodes along a straight line. #n electric

current / is passed into the ground through end electrodes and induced potential ismeasured between the interior electrodes. The spacing is changed and the procedure is

repeated.

Mared change in potential indicates the presence of a stratum of different resistivity.

4<>/B" ?<"$-

The information on subsurface condition obtained from the boring operation is usually

presented in the form of a boring record, commonly nown as 4<>/B" ?<".

# continuous record of the various strata identified at various depths of the boring is

presented. %escription or classification of the various types of soils and rocs

encountered. #nd data regarding ground water level are presented in a pictorial manner on the log. Sample record sheet is below.



References.

) QSoil mechanics and *oundation engineeringR by .>.#rora.

) Q*oundation engineering byR 4.&. 'unmia

) /S $ 053: ( 'art /// ) I 563.

Site Exploration

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