lab manual soil

8/12/2019 Lab Manual Soil

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Soil mechanics laboratory manual

5.3 Direct

shear test

BS 1377: part 7 1990

Scope of the test

The direct shear test is

used to measure shear

strength, friction angle

and cohesion of soils for

stability analysis of

foundation, slopes, and

retaining walls

The test may ta!e place underdrained, undrained or consolidated"undrained conditions

#ig $3"1

1"#rame %& '("007

'"Thyristor controlled dri)e unit

3"*earbo+

"&oad ring

$"-eight hanger

."&e)er arm /beam with counter balance

7"isplacement transducer

("&oading yo!e

During the practical we will execute the unconsolidated undrained test!


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Description of test

The direct shear test is used to determine the shear strength of soils on predetermined failure surfaces

The principle of the direct shear test is illustrated in #ig, $3' The soil sample confined inside the

upper and lower rigid bo+es is sub2ected to the normal load This load is applied by the yo!e which is

placed on the loading cap and by putting weight on the hanger the specimen is loaded a+ially Because

ofthe length of the beam the applied weight has to multiply with a factor 11


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#ig $3'

The shear force T shears the sample4 this force is applied by the motorised dri)e unit and measured with

help of the load ring

5f 6 is the area of surface , the shear stress-

acting on surface is e8ual to T6, and the normalstress is e8ual to 6 The soil shear strength is the shear stress - that causes the soil to slip on

surface 5t can be defined by ;ohr"oulomb theory:

- = C + tan 5

-here c is the cohesion and 5 is the friction angle

uring the test, the stress state is not completely defined: - and are only measured on the hori


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to a)oid segregation of fine particles, and is therefore referred to as sand


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#ig $33

" 6pply the normal force by placing the load hanger on the ball bearing The force is appliedby

placing the slotted weights on the bottom of the hanger #or greater normal forces the slotted

weights can put on the hanger from the le)er arm /see fig $3"1

" Select the shearing speed, for sand a rate of 1 mmmin, for sand the effect of the displacement rate

on the friction angle is generally negligible within the range 3 to 01 mmmin" #or cohesi)e material the shearing speed depends of the type of test, for an undrained test a rate to

appro+imately 1mmmin should be fast enough to approach the undrained condition

" 6d2ust the position of the bo+ such that it is in contact with the screw applying the shearing force

and the arm of the top half of the shear bo+ is in contact with the load"measuring de)ice

" 5nstall the measuring de)ices to obtain the )ertical and hori


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etermine the and 5 )alues by plotting the ma+ Shear stress against the normal stress


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Calibration chart for load measuring ring !.5 "# compression.

Temperature at calibration '0 8

Cing serial number 11$$"7"130(0

*auge reading 0001mm &oad !

3(7' 0

$1'' 0

770' 1

10'9. 1

1'(3( '

1$.0 '

1(0 '

'0.(' 3

'33'' 3

'$9.

'9'9.

Typical )alues of effecti)e cohesion intercept cand effecti)e friction angle 5c for )arious fine"grained

soils /drained test

ase record water

ontent

>lasticity

inde+ >5 /

c

/!>a

5c

/deg

Dimola anal

Trondheim emban!ment

Slope failure in )ariegated clay shale

$3

"

'0

'7

'"1

'

9

("'0

7

'(

31"3$

'&ondon clay failures 31 $' 1' '0

#ield test in Eslo clay 30"3( '3 (( '

Daolin " 3' '$(

Se)en Sisters i!es ( .7 13( 1$

Table 1

Andrained shear strength of clays

onsistency description Andrained shear strength

/!mFery soft

Soft

Soft to firm

#irm

#irm to stiff

Stiff

ery stiff or hard

G '0

'0 H 0

0 H $0

$0 H 7$

7$ H 100

100 H 1$0

I 1$0


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$.% Consolidation test BS 1377: >art $: 1990

Scope of the test&

onsolidation can be defined as the plastic deformation or )oid"ratio reduction of a soil mass, which are

functions of time and e+cess pore pressure

-hen fine grained soils are sub2ected to changes in load due to construction, their deformation ta!es

place not only at the time of the load application, but also continues for )ery long time periods which

may last se)eral years The long"term settlement of fine grained soil layers is primarily controlled by

consolidation, a physical process in which the interstitial water that is under e+cess pressure slowly

diffuses through the compressible matri+ of soil particles 6fter the e+cess pore pressure has completely

dissipated, fine"grained soils can also deform due to their )iscous nature

The properties that characterise the amplitude and rate of deformation are determined in the

consolidation test

#ig .1"1

1" onsolidation frame'" onsolidation cell

3" isplacement transducer

" &oading yo!e

$" ounter balance weight

." Beam

7" Beam support 2ac!(" -eight hanger


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Description of test

6 prepared soil specimen is put in a consolidation cell /fig .1"'4 which is mounted on the cell

platform from the consolidation frame

The loading yo!e is placed on the loading cap and by putting weight on the hanger the specimen is

loaded a+ially Because of the length of the beam the applied weight has to multiply with a factor

/epending to which hole of the beam the hanger is connected The stress is held constant until the

primary consolidation has ceased This can ta!e a few hours to a few wee!s, depending of the load and

sample material

uring this process water drains out of the specimen, resulting in a decrease in height which canbe

measured with the displacement transducer at suitable inter)als

Sample preparation

The inside diameter of the cutting ring shall be not less than $0 mm and not greater than 10$ mm The

height of the ring shall be not less than 1( mm and not greater than 0 times the internal diameter

Andisturbed specimens shall be prepared with the minimum change of the soil structure and moisture

content The method of preparation shall depend on whether the sample recei)ed in the laboratory is

contained in a tube of the same internal diameter as the specimen to be tested, or in a tube of larger

diameter, or as a bloc! sample

#or the practical you will

get a clay bloc! sample,

from which you will

prepare a specimen with

help from the cutting ring

#ig .1'

" ;easure the diameter and height of the cutting ring, with an accuracy of 01 mm

" -eigh the ring to an accuracy of 01 gram

" &ubricate the inner face of the ring lightly with silicon grease, to minimise side friction

" >lace the sample on a glassplate

" >ush the cutting ring into the sample cutting away surplus soil from the outside of the ring as the

sample enters it, until the top surface pro2ects a few millimetres abo)e the top of the ring


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" ut of the soil pro2ecting abo)e and below the ring with the wire saw /see fig .1' and flat ten

both sides carefully with the spatula

" Cemo)e soil particles stic!ing to the outer side of the ring

" -eigh the specimen with ring


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#ig.1"

6fter ' hours, the decision must be ta!en whether or not to apply the ne+t load increment

5f the dial reading )ersus log"time shows a flattening out from the steep part of the cur)e to a straight line

which is less steeply inclined, as in figure .1", it indicates that the primary consolidation phase is

complete and that the ne+t load increment may be applied 5f the straight line representing secondary

compression has not yet been established, the load should be left unchanged for another ' hours-hen it has been established the loading stage may be terminated:

" 6pplied the second load on hanger, to gi)e the re8uired new stress /37 !>a

The procedure has to be carried out at the same way as done for the first load increment

Cepeat this procedure for a third load increment

" 6fter completion the last load increment ta!es out the consolidation ring


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" Cemo)e the porous discs carefully, any soil adhering to them should be scraped off and returned to the

sample

" -ipe the outside of the ring dry and weight the sample with the ring

" >lace sample with ring in the o)en for ' h

" Ta!e the dry weight, to calculate the moisture content and dry"weight

(eporting

alculate the bul! mass density and moisture content before and after the test

alculate the dry density /if no material has been lost during the test

>lot the settlement )ersus log"time cur)e, and analysis has to be made following asagrandes method, to

determine the coefficient of consolidation C for each increment of loading

The principle of the method is illustrated in fig .1"

&ocate the corrected


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=1 =eight of specimen at start of a loading increment ='

=eight of the specimen at the end of that increment t$0

time for $0 @ consolidation, e+pressed in minutes

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