Page | 1 Compiled by: Prof.B.S.Cha whanM.Tech(Geo-Tech Engg)!""#.Profe" "orCE$Go%er nmen#.Engineerin g College&a %eri-'* (+,,+*-Till da# e) GE-TEC&/0C!1 E/G0/EE20/G-00C&!PTE2-*3(*) S4S425!C E E6P12!T0/$efini#ion: The selection of the type and depth of the foundation, the bearing capacity and sett lement analysis and the determinati ons of the earth pressures, all depend upon soil properties. The field and laboratory investigations required to obtain the necessary soil data for design are called soil exploration. Soil exploration is not only needed for the design and construction of new structures but is also needed, sometimes for remedial measures if a structure starts cracking due to settlement. E7plora#ion P rogramme: The objectives of an exploration programmed are: . !ete rmin ation of the ex tent in the hori"o ntal di rect ion and thick ness in the vert ical dir ecti on of the earth typ e of soil up to the required depth. #. !ete rmination o f the g round w ater t able b elow t he grou nd sur face . $. %btaining of samples of ground water , soil and rock of a si"e and co ndition adequat e for posit ive identif ication of the materia l. &. 'ield obs ervation and testing fo r evaluati ng the in(situ soil propert ies to substantiate the labora tory determinations. ). *n case the su rvey is me ant for ex plori ng the soun ding of ma teria l, it is ess entia l to colle ct data reg ardin g cubic cont ent of each deposit. Me#hod" of e7pl ora#ion: There are many methods of sub(surface exploration. These methods can be classified into three categories: . Se mi (direct me thods. #. !ir ect met ho ds . $. *n di re ct methods. . Semi(d ire ct me thods+ The semi(direct methods include all operations that involve drilling of bore(holes into the ground for purpose of taking out the samples. uger borings, displacement borings, wash borings, percussion drilling and continuous sampling all fall under the category of semi(direct method. *t must be understand at the stage itself that the -advan ce of bore(holesand -taking of the samplesare tw o separate, through interconnecte d processes. #. !i re ct me thod: The direct methods include the digging of trench and trial pits. ccessible borings are also included in this category. $. *ndi re ct method: /eo(physical methods and soundings or probabings fall into the indirect method. Pha"e" in S8b-" oil E7plora#io n: The subsoil exploration involves four distinct phases . 'ac t f ind ing and geo log y s urv ey. #. 0e connaissance ex pl or at ion. $. !e tail ed ex pl or ation. &. Spec ial ex pl or at io n. . 'ac t finding and ge olo gy sur vey : 1efore the actual explorations at site are started, it is always advisible and beneficial to know the geological history of the area from the geological reports and maps. Soil survey maps of the area are also sometimes available and they can help in providing information about the soils that the engineer may encounter in the field. The geological survey means subsurface exploration and sampling which will provide information regarding: . !e pt h to groun d wat er #. 2r es sure of ground water$. Seque nce thic knes s, extent an d approxi mate ide ntif icati on of encoun tered str ata. *t include i3 collection of samples of ground water, soil and rock in different quantity and conditions, for positive identification and laborato ry tests and ii3 making observ ation s and for fie ld tests to determi ne condi tion of sampl es and to esti mate physic al properties of the subsoil. 4aluable information about the general(geologic characte r of a site may be obtained from maps and publications of central and institutiona l geological surveys and from the central and state agencies connected with the construction and planning of highways, buildings, dams and other large projects. The 5entral water and 2ower commission, central 0oad research *nstitute and 5entral 1uilding research *nstitute 6500* and 510*3 are among the many sources of such information. Compiled by: Prof.B.S.Cha whanM.Tech(Geo-Tech Engg)lec#8rer in CE$ST90T2/2-(+**'-++,*+,+**)GEC2aich8r(+;,*+,+**-,*,+*) 1
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The selection of the type and depth of the foundation, the bearing capacity and settlement analysis and the determinations of the
earth pressures, all depend upon soil properties. The field and laboratory investigations required to obtain the necessary soil data for
design are called soil exploration.Soil exploration is not only needed for the design and construction of new structures but is also needed, sometimes for remedial
measures if a structure starts cracking due to settlement.
E7plora#ion Programme:
The objectives of an exploration programmed are:
. !etermination of the extent in the hori"ontal direction and thickness in the vertical direction of the earth type of soil up to the
required depth.
#. !etermination of the ground water table below the ground surface.$. %btaining of samples of ground water , soil and rock of a si"e and condition adequate for positive identification of the material.
&. 'ield observation and testing for evaluating the in(situ soil properties to substantiate the laboratory determinations.
). *n case the survey is meant for exploring the sounding of material, it is essential to collect data regarding cubic content of each
deposit.
Me#hod" of e7plora#ion:
There are many methods of sub(surface exploration. These methods can be classified into three categories:
. Semi(direct methods.
#. !irect methods.$. *ndirect methods.
. Semi(direct methods+
The semi(direct methods include all operations that involve drilling of bore(holes into the ground for purpose of taking out the
samples.
uger borings, displacement borings, wash borings, percussion drilling and continuous sampling all fall under the category of
semi(direct method.
*t must be understand at the stage itself that the -advance of bore(holes and -taking of the samples are two separate, through
interconnected processes.#. !irect method:
The direct methods include the digging of trench and trial pits. ccessible borings are also included in this category.
$. *ndirect method:
/eo(physical methods and soundings or probabings fall into the indirect method. Pha"e" in S8b-"oil E7plora#ion:
The subsoil exploration involves four distinct phases
. 'act finding and geology survey.
#. 0econnaissance exploration.
$. !etailed exploration.
&. Special exploration.. 'act finding and geology survey:
1efore the actual explorations at site are started, it is always advisible and beneficial to know the geological history of the area
from the geological reports and maps. Soil survey maps of the area are also sometimes available and they can help in providing
information about the soils that the engineer may encounter in the field.
The geological survey means subsurface exploration and sampling which will provide information regarding:
. !epth to ground water
#. 2ressure of ground water $. Sequence thickness, extent and approximate identification of encountered strata.
*t include i3 collection of samples of ground water, soil and rock in different quantity and conditions, for positive identification
and laboratory tests and ii3 making observations and for field tests to determine condition of samples and to estimate physical
properties of the subsoil.
4aluable information about the general(geologic character of a site may be obtained from maps and publications of central and
institutional geological surveys and from the central and state agencies connected with the construction and planning of highways,
buildings, dams and other large projects.
The 5entral water and 2ower commission, central 0oad research *nstitute and 5entral 1uilding research *nstitute 6500* and
510*3 are among the many sources of such information.
Compiled by: Prof.B.S.Chawhan M.Tech(Geo-Tech Engg) lec#8rer in CE$ST90T2/2-(+**'-++,*+,+**)GEC2aich8r(+;,*+,+**-,*,+*)
6e3. 8igh embankment like lavees, earth dams, etc. 6f3. 5oncrete dam
1ack slope
!C#($m6d3. 5ut
5ut level
!C1
!C To firm strata or <.A)(mC#8 6h3. Tunnel
6g3 0etaining wall.
5ig.+. $ep#h of e7plora#ory boring".
$. !etailed explorations.
The purpose of the detailed explorations is to obtain comprehensive soil profiles and representative soil samples of the important
deposits. This helps in lowering deposits of good construction materials. Sometimes undisturbed samples of the soil strata are also
taken during this phase and sent to the laboratory for testing. Test pits and trenches are usually dug, for shallow foundation. nyone
of the semi(direct methods can be used for these exploratings.&. Special explorations.
Special explorations are carried out to obtain undisturbed soil samples from the critical soil layers. The samples are usually
<cms in diameter. ccessible borings for visual inspection are drilled during this programme. 'ield tests, if any, required to beconducted are carried out during this phase of investigation.
2ower driven augers, wash borings, percussion and rotary drilling methods are employed to advance the bore holes.
2ore(water pressure is determined in the materials of low permeability and pie"ometer or hydrostatic pressure cells are
employed for this purpose.
5onsolidation characteristics of a clayey layer, unconfined or triaxial compression strength of a soil and direct or torsion testing
of a soil is done on the samples obtain during this phase of investigation.
Me#hod" of E7plora#ion:
a. Boring:
The semi(direct methods are the auger borings , displacement borings , wash borings , percussion drilling and continuous
sampling.
a. !8ger Boring":
Soil uger is device that helps in advancing a borehole into the ground.
1ore hole is advanced by hand or power operation auger with periodic removal of material. *n some cases continuous auger may
be used requiring only one removal. 5asing is generally not used with auger boring. The following figure shows both hand(operated
and power operated.
5ig. ;(a). Po"#-hole a8ger
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. 8and driven augers are used in soft to stiff cohesive soils, in sandy silty soils above water table.
#. 7ith hand auger, depth is usually limited to ?m.
$. 2ower driven augers can be used to great depths, even to $<m and used in almost all types of soils above water table.
&. This method provides almost continuous disturbed samples.). 9ndisturbed samples can be obtained at required depths by using proper samplers.
b. <a"h boring:
The following figure shows a wash boring explorations. =
A
?
@
5
) &
<
$
/ B
#
#
$
&
)
?
(2ump,#(Sump for wash water and collation of wash samples,$(T(section for return,&(!rill rod,)(2artial drill rod,?(T(section of
water swivel,A(swivel,=(Single hook for multiple blocks for pulling of casing,@(>anila hoisting rope,<(;otor,(>ipple,#(
casing,$(casing coupling,&(!rill rod coupling,)(!rive shoe,?(!rill bit,A(Three or four(legged derrick standard pipe or timber.
5ig. .<a"h boring e7plora#ion
1ore hole is advanced by chopping, twisting action of a light chopping bit and jetting action of a drilling fluid, usually water, under pressure, changes in soil strata are indicated by changes in the rate of progress of boring, examination of out coming slurry andcuttings in the slurry, casing are used whenever necessary to prevent cave(ins.
!pplica#ion":
. 7ash boring can be used in all types of soil except these containing boulders.
#. This can be used great depths, adopted easily at inaccessible locations.
$. 9ndisturbed samples whenever needed can be obtained by use of proper samplers.
&. Samples obtained are in highly disturbed and slurry form.
c. $i"placemen# Boring:
This is the simplest method of boring. 9se is made of the slit, cap or piston samplers.The operation involves+
Compiled by: Prof.B.S.Chawhan M.Tech(Geo-Tech Engg) lec#8rer in CE$ST90T2/2-(+**'-++,*+,+**)GEC2aich8r(+;,*+,+**-,*,+*)
. *t can be used in all types of soils and rocks, can be used great depths.
#. 1eing increasingly used because of fast rate of progress in all soil types, but difficult to use at inaccessible locations because of
heavy machinery.
$. 9ndisturbed samples can be obtained at desired depths by using suitable samplers.
&. The bore(holes in this drilling range from <cms diameters.
). *t is uneconomical for holes of less than <cms diameter. f. Con#in8o8" Sampling:
*n this technique, the boring is accomplished entirely by sampling. *n exploration of rocks, core drilling is done for obtaining the
rock samples continuously. *n the soil too, when core barrels and piston samplers are used continuously to obtain the soil samples,the boring proceeds side by side.
!pplica#ion":
. *t is used extensively in detailed and special foundation explorations for important structures.
#. *t provides a more reliable and detailed information on soil conditions than any other method.
S#abili=a#ion of Bore &ole":
The problem of preventing caving of the sides and bottom of a hole and to avoid disturbance of the soil to be sampled is common
to all boring methods.9ncased, dry(bore holes are generally stable when they are shallow and above the ground water table, but danger of
caving increases rapidly with depth and the presence of free ground water. *n firm cohesive soils the hole may remain open for a
limited(length of time. 1ore(hole without any provisions for stabili"ation and extending below ground water level are often piston
samplers.
They are a number of methods for stabili"ation the bore(holes.
. Stabilisation with water.
#. Stabilisation with drilling fluid.
$. Stabilisation with 5asing.&. Stabilisation by free"ing.
). Stabilisation by /routing.
. S#abili"a#ion wi#h wa#er:
Suitable in rock and often in stiff cohesive soils. 7ater alone is not suitable or preventing caving of borings in soft or
cohesionless soils.
#. Stabilsation with drilling fluid:
n uncased bore(hole can be stabili"ed by filling it with a proportioned drilling fluid or mud, which when circulated also serves
to remove the churne(up soil from the bottom of the hole. !rilling fluid: satisfactory drilling fluid can be obtained by mixing locally available clays with water but it is often
advantageous and necessary to add commercially prepared products such as vol(clay or quaver. These commercial clay products
consists of highly colloidal, gel forming, thixotropic clays primarily bentonite with certain chemicals added to control dispersion
thixotropy, gel strength and viscosity.The stabili"ation effect of drilling fluid is caused part by its higher specific gravity compared to water and partly by the formation
of relatively impervious lining or mud coke on the sides of a bore hole.
$. Stabilisation with 5asing:
5asing or the lining of the bore hole with steel pipe provide the safest through relatively expensive method of stabili"ation of the
bore(hole. !ifferent types of casing are available.
&. Stabilisation by free"ing: bore(hole may be stabili"ed by free"ing the soil around it as the boring progress. This may be accomplished by replacing water
with kerosene or brine cooled by means of Hdry iceI. This method cannot be used when the ground is dry or nearly so and when there
is a strong water flow. So for it has been used for experimental purposes.
). Stabilisation by /routing:
bore(hole passing through "ones in rock cavities, faults and broken rock etc may be stabili"ed by filling the lower part of the
hole with cement grout and then re(boring the concrete plug.
Typical Boring 1og: boring log is a diagrammatic report on a bore hole. *t should be carefully written and should provide the following
information.
. *dentification of the job.
#. >umber of boring on the plan 6hori"ontal control3.
$. /round elevation of boring 6vertical control3.
&. !etailed information regarding the type of the soil available at various depths. This is written as per visual examination.
). /round water information.
?. Strength test values, if any tests are conducted in the field. These values must be shown on the boring log at the relevant
elevations at which the tests are conducted. 6The tests usually conducted in the field are the unconfined compression test, the vane shear test and the standard penetration test3.
Compiled by: Prof.B.S.Chawhan M.Tech(Geo-Tech Engg) lec#8rer in CE$ST90T2/2-(+**'-++,*+,+**)GEC2aich8r(+;,*+,+**-,*,+*)
The following figure shows a data field in a boring log.
/B > values wn B.B 2.* qu6tEm# 3
2roperty line 5lay fill grey coloured .<m
<m #m 7.T
1.8 .)m & ##J ))J #< &.@
1.1.0oad /rey sandy silt with little
&m clay
&m 1.8# $m ? #$.)J )=J #< ).<
&<m#m #m $.)m
1.8$ &.)m #)
1.8& ;ain 0oad ;edium dense fine sand
#m >
?.<m ?m $<
$<m
'oot path
5ig. >.(a). Si#e plan of boring for a ban? b8ilding. 5ig.> (b). Boring log of a bore hole wi#h / and labora#ory re"8l#".
The type foundation to be used for the various portions of the structure, the depth of foundation to be provided, the allowable
pressure to be used in designing the member and the expected settlement should be state unambiguously.
So8nding Te"#":
*n this method some sounding device like split spoon sampler, a cone or a rod is formed, statically or dynamically, into the soil.The energy consumed in penetration into soil is indication of consistency of soil.
Standard penetration test, dynamic cone penetration test and static cone penetration test are commonly adopted standard tests. !pplica#ion":
. This method can be in any type of soil and to any depth.#. !epth by static cone penetration is limited by the capacity of equipment.
$. 2resently $tonne and <tonne machine are in use.
&. Standard penetration test is done in a bore hole.
). Sounding methods are generally faster and cheaper than boring.
!. $ynamic So8nding.
7hen the probe is advanced with the help of dynamic load like a falling weight it is called dynamic sounding. B. S#a#ic So8nding.
*f the probe is subjected to a dead weight or hydraulic pressure and its penetration is measured, it is called static.
C. S#andard Pene#ra#ion Te"#(SPT):
This is most widely used sounding method. The S2T test is used extensively in soil exploration programmes.
The test consists of driving a standard split spoon sampler and S2T test equipments.
The test consists of driving a standard split spoon, )<.=mm outside diameter and $)mm inside diameter, into soil under the blows
of a drop weight 6hammer3 of ?)kg falling freely through A)cm.The number of blows required for $<cm of penetration of sampler in the soil is designated as > values and is termed as standard
penetration blow count.
ttempt has been made to correlate >(values to density of cohesionless soils and unconfined compressive strength of cohesive
soils. ttempt has also been made to correlate compressibility of soils to >(values. Such relations are utili"ed in arriving at allowable
bearing pressure of soil. Such approaches can be and are subjects of criticism. 1ut past experiences indicate that standard
penetration test can provide reliable and safe design criteria when used with caution keeping in mind the limitations of the test and its
application.
Correla#ion" #o mea"8red %al8e" of SPT:
The >(values of S2T as measured in field may to be correlated. There are two types of corrections generally applied to measured >(4alue.
Compiled by: Prof.B.S.Chawhan M.Tech(Geo-Tech Engg) lec#8rer in CE$ST90T2/2-(+**'-++,*+,+**)GEC2aich8r(+;,*+,+**-,*,+*)
The overburden correction is gradually presented in eqn63 M6#3M6$3 for the following respective two cases,. 7ater table at great depth greater than ?.)m below ground level with assumed density of soil as .AtEm$ 6?.Ak>Em$ 3.
#. 7ater table at ground level and assumed submerged density of soil as .<tEm$ 6@.=k>Em$ 3.
/ of SPT and Con"i"#ency of "oil:
5onsistency of a soil granular soil is indicated by its relative density and that of a cohesive soil by its unconfined compression
strength.Table( represents empirical relationships currently in use in obtaining relative density of cohesionless soils from > values of
S2T. 'rom > values angle of internal friction, N of soil can be obtained from table(.
Table(. mpirical values of N, !r and unit weight of granular soils based on the S2T with correction for saturation effect and for overburden, if any
This is the fundamental equation of resistivity method and gives the resistivity in terms of measurable quantities. *f the electrodes are placed at equal distances,
i.e.,22
32
41
r r r r === , then equation 6&3 reduces to
1.2 r 0π ρ =
7here, *
4 0
cd = (((((((((((((((((((((((((((((((((((((((((((((((((6)3
7here, 0 is measured in P and r in cm+ therefore the units of resistivity Q is Pcm. The value of 0 equals the voltage measured
between the central electrodes divided by current * measured by an ammeter.
Some resistivity values of various soils are shown in below table+
Type of ;aterial ρ 6P(cm3
7et to moist clayey soils )<($<<
7et to moist silty clay and silty soils $<<(&)<
;oist to dry silty and sandy soils )<<()<<<
7ell fractured to slightly fractured bed rock with moist soil(filled cracks. )<<<($<<<<
Sand and gravel with silt $<<<<
Slightly fractured bedrock with dry soil filled cracks, sand and gravel with layer of silt. $<<<<(#&<<<<
;assive bed rock and hard rock+ 5oarse dry sand gravel deposit. K#&<<<<
!d%an#age" $i"ad%an#age"
. The method is very rapid and economical for large projects
where only average conditions are required.
. This method is that it is capable of only detecting strata
having quite different properties, i.e., rock and water table.
#. *t is good for depths upto $<m and instrumentation is
simple.
$. *t provides no simple for definite identification and therefore
should always be supplemented by borings.
&. *t needs a specialist for proper interpretation of results as
the results are known to be influenced by surface irregularities.
B. Sei"mic 2efrac#ion Me#hod:
The following figure shows the seismic refraction waves+
/eophones 5onnected to automatic time recorder with multiple channel
Shot point B B# B$ B&
/.B !
Boose, dry top soil
8 4
8 # 1 5 5lay, compacted soil weathered rock
4 #
;assive sound rock 5ig. A(a). ne "ho# poin# wi#h m8l#iple geophone me#hod.
Compiled by: Prof.B.S.Chawhan M.Tech(Geo-Tech Engg) lec#8rer in CE$ST90T2/2-(+**'-++,*+,+**)GEC2aich8r(+;,*+,+**-,*,+*)
from eqn 63 and 6#3 it is clear that 4 p is greater than 4 s. 'igure #6a3 M 6b3 shows a typical soil formation with the direction of waves. The exploration by seismic refraction method is
generally conducted in one of the two ways+
. ;ultiple geophone with multiple shot point fig.#6b3, and#. %ne geophone with multiple shot point fig.#6b3.
*n one shot point, as soon as the charge goes off the recorder starts recording the time 6tC<3 and records the time taken for the
first arrival of the wave at various geophone. time(distance record 6B in T , BLB# in T # , etc3 is made from recording of time starts
when the hammer strikes the first peg at B and the recorder reads, the time taken for the first arrival of the wave at the geophone.
>ext, time is recorded from when the hammer strikes the peg at6BL B# 3 distance from the geophone till the first arrival of the wave
at the geophone and the process is repeated for all the pegs. This also produces a time(distance record like the previous method.
'rom fig#6a3 and 6b3, it is easy to visuali"e that the waves will travel either through the top layer along ! or partly through thetop layer and partly through the second layer 615!3.
*n the latter, the path length is more than that in the former but the velocity of travel in 15 is higher. Therefore, the thickness of
the top layer and the velocities in the layers will influence the arrival time by the two routes. The time distance plot gives the velocity
of wave prorogation through the strata and also the thickness of strata.
*n both the methods, B is usually kept quite small 6about $m3, so that B and T will give the velocity in the top layer. 7ith the
velocity of top layer known, it can be seen whether the BLB# and T # data shows the first arrival is through second layer. This also
becomes evident when time(distance data is plotted fig@$3. The velocity 4 # and thickness 8 can be obtained from the time distance
data.
1
1
T
B4 = (((((((((((((((((((((((((((((((((((((((63
*f( )
1
21
24
B BT
+= then the wave travelled through top soil and reached as first arrival, otherwise it travelled partly through top
layer and partly through the second. >ext, from fig6$3, the third point 6BLB#LB$ ,T $ 3 does not lie on the same straight line indicating that the velocity of propogation is
different from 4 .
Therefore,
2
321
1
1
3
2
4
B B B
4
8
T
++
+= ((((((((((((((((6#3
ly **
2
4321
1
1
4
2
4
B B B B
4
8 T
++++= ((((((((6$3
%r2
4
344
BT T =−
%r34
4
2T T
B4
−= ((((((((((((((((((((((((((((((((((6&3
Substituting the values of 4 # in equation 6#3,
2
* 1
2
321
31
4
4
B B BT 8
++−= (((((((6)3
'rom figure6&3, the first intersection is at U and it means that a wave travelling by ! route and 15! route would be take the same
time to reach a point at a distance of U from the shot point. *n another wards,
21
1
1
1 2
4
U
4
8
4
U +=
2*
2
12
1
U
4
4 4 8
−=∴ ((((((((((((((((((((((((6?3
Compiled by: Prof.B.S.Chawhan M.Tech(Geo-Tech Engg) lec#8rer in CE$ST90T2/2-(+**'-++,*+,+**)GEC2aich8r(+;,*+,+**-,*,+*)
split spoon sampler consists of a thick walled steel tube split lengthwise. *t carries a cutting edge called a cutting shoe at the
low end and a head at the upper end. The maker of the sampler currently available in the market have an outer diameter ranging from
#inches to &.)inches and the internal diameter ranging from .)inches to &inches. The overall length in all cases is #&inches. The
*ndian Standard recommends a si"e, outer diameter )<.=mm, and inner diameter $)mm. The sampler is lowered into the bottom of the
borehole with the help of drilling rods and is driven into the soil with hammer blows. fter penetration to full depth, it is pulled out.The cutting shoe and the head are removed and the soil is examined visually. Then it can be bottled and sealed for shipment to the
laboratory for further examination.
split spoon sampler is used in the standard penetration test is conducted. 2roperties of cohesive soils are obtained from theundisturbed samples and for sampling thin(walled tubes popularly known as Shelby tubes are used.
1. Thin-<alled T8be "ampler": Bength as specified method
;ounting hole <mm min dia
#mm ;in Thickness as specified
5ig.. Thin-<alled T8be Sampler.
thin(walled tube sampler is a seamless tube with very thin walls. The *ndian Standard *nstitution recommends that the tubes
shell be &<mm to #)mm in outside diameter and should be made of steel, brass or aluminum. The si"es and wall thickness asrecommended by *S* of these tubes are given below,
Table(. Sampling tube dimensions.
%utside diameter in mm &< )< )) ?
)
A) #)
7all thickness in mm #.) #.
)
#.
)
A
A $.
)
The length of tubes shall be <times the diameter for sampling in sandy soils and it shall be <()times the diameter for sampling
in clays, subject to a maximum of ?<cms.The lower end of the tube is sharpened to form a cutting edge. %ne typical tube is shown in above figure.
The tube can be fitted to a head attached to a drilling rod. Static force is applied to push the tube in the bottom of the bore hole
dug to the required depth.
C. Pi"#on Sampler":Sometimes it happens that the soil tends to drop out from the sampler while being withdrawn. *n such cases piston samplers are
available in the market. !r.%sterbarg developed a piston sampler whose principle is explained in below figure, and this is the most
advantageous amongst all the piston samplers.
!rill rod
Sampler head
2ressure cylinder
8ollow rod
'ixed piston
5ig. +."#erberg" Pi"#on Sampler.
*f a pit has been dug to the level where a sample is needed, a cake of the soil can be cut with a spatula and sealed on all sides.This probably is the best method of sampling but expensive.
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Some sample disturbance by any one of the sampling techniques 6!isturbed sample, undisturbed sample, representative samples
or non(representative samples3 however sophisticated they may be, usually occurs. s for as the sampling by the use of the tube
samplers is connected the area ratio is an index to the sample disturbance.
rea ratio is defined as+
100*2
22
i
ie
r
!
! ! ,
−
= ((((((((((((((((((((((((((((((((63
7here, !eCexternal diameter of the sampling tube.
!iCinternal diameter of the sampling tube and it represents approximately the ratio between the volume of displaced soil and the
volume of the sample. *f this ratio is W<J, the disturbance is considered negligible. The area ratio of the split spoon sampler is
approximately $<J and hence a sample obtained is a disturbed sample. The ratios for the sampling tubes mentioned in above table(
are less than <J and hence a sample obtained with these tubes is an undisturbed sample. !rea ra#io (!r ) 2eco%ery 2a#io (2r ) Clearance 2a#io (C r ):
!rea 2a#io(!r ):
!c !i !<
5utting edge or !rive shoe
!< !c D#<< !i
7here,
!oC%utside diameter of sampling tube, !iCinside diameter of sampling tube, !cCinside diameter of cutting edge.
!cW!iW!o.
rea ratio, 100*2
22
i
ie
r
!
! ! ,
−
= ((((((((((((((((((((((((((6#3
*nside 5learance ratio, 100*c
cicr
!
! ! * −= (((((((((((((((((((((((((((((6$3
%utside clearance ratio, 100*
c
co
r
!
! !%
−= (((((((((((((((((((((((((((6&3
. rea ratio for undisturbed samples obtaining is maximum permissible area ratio is #J for )<mm diameter samplers, )J for
A)mm diameter samplers and #<J for <<mm diameter samplers.
#. *nside clearance ratio for undisturbed samples obtaining should be between <.) to $J for different types of samplers. Bow value
of clearance ratio is recommended for sampling in sands while clearance ratio upto $J may be needed in obtaining good sample
recovery in certain clays.Table(#. *nside 5learance ratio for Samples for undisturbed sampling.
Sl.>
o
*nside 5learance ratio of sampler Soil Type
bout <.)J Sands, silts and soft clays# bout .)J Stiff and hard clays below ground water level
$ bout $J Stiff expensive clays below ground water level
2eco%ery ra#io(2r ):
The practical requirement to be satisfied by a sample for accepting it is an undisturbed sample is given by recovery ratio. The
overall condition of a soil sample taken in a tube is represented by the total recovery ratio is defined as+
Total recovery ratioCBength of the sample before withdrawalE2enetration of sampler below the bottom of the bore hole.CBE8O<<
/ross recovery ratioC!istance from the top of the sample to the cutting edgeE2enetration of sampler below the bottom of the bore
holeCB s E8
The gross recovery ratio should be used in computing the recovery ratio for soil sample.
The net recover ratioC>et length of the sampleE2enetration of sampler below the bottom of the bore holeCBn E8 Compiled by: Prof.B.S.Chawhan M.Tech(Geo-Tech Engg) lec#8rer in CE$ST90T2/2-(+**'-++,*+,+**)GEC2aich8r(+;,*+,+**-,*,+*)