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A report on site exploration, geotechnical engineering
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A REPORT ON
Site Investigation
Department Of Civil Engineering
INSTITUTE OF TECHNOLOGY
NIRMA UNIVERSITY
Ame!a"a! #$% &$'
Septem"er %('&
Prepared By
Neera) *atri '& MCLC '%
+ragne, +atel '& MCLC '-
Sa.in +atel '& MCLC '/
Te)a, +atil '& MCLC %%
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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:
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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
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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:
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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
.
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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
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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
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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
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) 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$- . Hane shear apparatus.
. Specimen.
. Specimen container.
0. &allipers.
+7'+>/M+BT#? '><&+%>+
• 'repare two or three specimens of the soil sample of dimensions of at least 6.2 mm
diameter and 62 mm length in specimen.(?@% ratio or ).
• Mount the specimen container with the specimen on the base of the vane shear apparatus.
/f the specimen container is closed at one end, it should be provided with a hole of about
mm diameter at the bottom.
• "ently lower the shear vanes into the specimen to their full length without disturbing the
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soil specimen. The top of the vanes should be atleast 9 mm below the top of the
specimen. Bote the readings of the angle of twist.
• >otate the vanes at an uniform rate say 9.o@s by suitable operating the torque application
handle until the specimen fails.
• Bote the final reading of the angle of twist.
• *ind the value of blade height in cm.
• *ind the value of blade width in cm.
Calculations:
eophysical methods:-
"eophysical methods indicate general boundaries of drastically dissimilar layers.
Most commonly used geophysical methods are$-
. Seismic or >efraction method
. >esistivity method
Seismic refraction method$-
# shoc wave propagates in an elastic medium with velocity,
v L (M@N)@
MI modulus and N - mass density
8ave velocity
/n soils 29 to 999 m@s
/n rocs 299 to 3999 m@s.
>esponses of a shoc wave eminating from a source are piced up by geophone at nown
distance.
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The depth of roc underlying the soil or depth of water table can be obtained. Shoc
waves emanating from a dynamite charge forming a source, S travel through soil O are
piced up at geophones ,,,P..etc. at distances d,d,dP..etc.
<ne shoc wave travels directly through the top soil layer with a velocity v and another
gets refracted with a greater velocity v.
• The brea in curve represents point of simultaneous arrival of primary O refracted waves,
and its distance is nown as critical distance. The wave velocity through the roc layer is
many times greater than through soil layer.
So, time of arrival by a longer route is shorter than that by the shorter route through the
top soil.
Helocity, v L
Helocity, v L
The depth of roc 1 is obtained as,
1L
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Seismic refraction method is fast.
• This method is reliable in establishing profile of different strata.
>esistivity method$-
+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)
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/ 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.
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References.
) QSoil mechanics and *oundation engineeringR by .>.#rora.
) Q*oundation engineering byR 4.&. 'unmia
) /S $ 053: ( 'art /// ) I 563.