Direct Shear Test

CEP 701 PG Lab

Mohr-Coulomb Failure Criterion(in terms of total stresses)

f is the maximum shear stress the soil can take without failure, under normal stress of .

tancf

c

failure envelope

Cohesion

Friction anglef

Mohr-Coulomb Failure Criterion(in terms of effective stresses)

f is the maximum shear stress the soil can take without failure, under normal effective stress of ’.

’

'tan'' cf

c’

’

failure envelope

Effective

cohesion

Effectivefriction angle

f

’

u '

u = pore water pressure

Mohr-Coulomb Failure Criterion

'tan'' ff c

Shear strength consists of two components: cohesive and frictional.

’f

f

’

'

c’ c’cohesive

component

’f tan ’ frictional component

Normal stresses and shear stresses on any plane can be obtained with the following equations

Principal stresses

or

Mohr Circle of stress

Soil element

’1

’1

’3’3

’

222

22

'3

'1

'3

'1'

'3

'1

Cos

Sin

Resolving forces in and directions,

2'3

'

1

2'3

'1'2

22

Mohr Circle of stress

2'3

'

1

2'3

'1'2

22

Soil element

’1

’1

’3’3

’

Soil elementSoil element

’1

’1

’3’3

’1

’1

’3’3

’

’

’

2

'3

'1

2

'3

'1

'3 '

1

Mohr Circle of stress

2'3

'

1

2'3

'1'2

22

Soil element

’1

’1

’3’3

’

Soil elementSoil element

’1

’1

’3’3

’1

’1

’3’3

’

’

’

2

'3

'1

2

'3

'1

'3 '

1

PD = Pole w.r.t. plane

’,

Soil elements at different locations

Failure surface

Mohr Circles & Failure Envelope

X X

X ~ failure

YY

Y ~ stable

’

'tan'' cf

NEED AND SCOPE

In many engineering problems such as

• design of foundation,

• retaining walls,

• slab bridges,

• pipes,

• sheet piling,

The value of the angle of internal friction and cohesion of the soil involved are required for the design.

Direct shear test is used to predict these parameters quickly.

Direct shear test

1. This test is performed to determine the consolidated - drained shear strength of a sandy to silty soil.

2. The shear strength is one of the most important engineering properties of a soil, because it is required whenever a structure is dependent on the soil’s shearing resistance.

3. The shear strength is needed for engineering situations such as determining the stability of slopes or cuts, finding the bearing capacity for foundations, and calculating the pressure exerted by a soil on a retaining wall.

Direct shear test

Apparatus

1.      Direct shear box apparatus

2.      Loading frame (motor attached).

3.      Dial gauge.

4.      Proving ring.

5.      Tamper.

6.      Straight edge.

7.      Balance to weigh upto 200 mg.

8.      Aluminum container.

9.      Spatula.

PROCEDURE

• Check the inner dimension of the soil container.

• Put the parts of the soil container together.

• Calculate the volume of the container. Weigh the container.

• Place the soil in smooth layers (approximately 10 mm thick). If a dense sample is desired tamp the soil.

• Weigh the soil container, the difference of these two is the weight of the soil. Calculate the density of the soil.

• Make the surface of the soil plane.

• Put the upper grating on stone and loading block on top of soil.

Direct shear test

Preparation of a sand specimen

Components of the shear box Preparation of a sand specimen

Porous plates

Direct shear test is most suitable for consolidated drained tests specially on granular soils (e.g.: sand) or stiff clays

Direct shear test

Leveling the top surface of specimen

Preparation of a sand specimen

Specimen preparation completed

Pressure plate

Direct shear test

Test procedure

Porous plates

Pressure plate

Steel ball

Step 1: Apply a vertical load to the specimen and wait for consolidation

P

Proving ring to measure shear force

S

Direct shear test

Step 2: Lower box is subjected to a horizontal displacement at a constant rate

Step 1: Apply a vertical load to the specimen and wait for consolidation

PTest procedure

Pressure plate

Steel ball

Proving ring to measure shear force

S

Porous plates

PROCEDURE

8. Measure the thickness of soil specimen.

9. Apply the desired normal load.

10. Remove the shear pin.

11. Attach the dial gauge which measures the change of volume.

12. Record the initial reading of the dial gauge and calibration values.

13. Before proceeding to test check all adjustments to see that there is no connection between two parts except sand/soil.

14. Start the motor. Take the reading of the shear force and record the reading.

15. Take volume change readings till failure.

16. Add 5 kg normal stress 0.5 kg/cm2 and continue the experiment till failure

17. Record carefully all the readings. Set the dial gauges zero, before starting the experiment

Direct shear test

Shear box

Loading frame to apply vertical load

Dial gauge to measure vertical displacement

Dial gauge to measure horizontal displacement

Proving ring to measure shear force

Direct shear testAnalysis of test results

sample theofsection cross of Area

(P) force Normal stress Normal

sample theofsection cross of Area

(S) surface sliding at the developed resistanceShear stressShear

Note: Cross-sectional area of the sample changes with the horizontal displacement

Direct shear tests on sands

Sh

ear

str

ess

,

Shear displacement

Dense sand/ OC clayfLoose sand/ NC clayf

Dense sand/OC Clay

Loose sand/NC Clay

Ch

ang

e in

hei

gh

t o

f th

e sa

mp

le Exp

ansi

on

Co

mp

ress

ion Shear displacement

Stress-strain relationship

f1

Normal stress = 1

Direct shear tests on sandsHow to determine strength parameters c and

Sh

ear

stre

ss,

Shear displacement

f2

Normal stress = 2

f3

Normal stress = 3

Sh

ear

stre

ss a

t fa

ilu

re,

f

Normal stress,

Mohr – Coulomb failure envelope

Direct shear tests on sandsSome important facts on strength parameters c and of sand

Sand is cohesionless hence c = 0

Direct shear tests are drained and pore water pressures are dissipated, hence u = 0

Therefore,

’ = and c’ = c = 0

Direct shear tests on clays

Failure envelopes for clay from drained direct shear tests

Sh

ear

stre

ss a

t fa

ilu

re,

f

Normal force,

’

Normally consolidated clay (c’ = 0)

In case of clay, horizontal displacement should be applied at a very slow rate to allow dissipation of pore water pressure (therefore, one test would take several days to finish)

Overconsolidated clay (c’ ≠ 0)

Interface tests on direct shear apparatusIn many foundation design problems and retaining wall problems, it is required to determine the angle of internal friction between soil and the structural material (concrete, steel or wood)

tan' af c Where,

ca = adhesion,

= angle of internal friction

Foundation material

Soil

P

S

Foundation material

Soil

P

S

Advantages of direct shear apparatus

Due to the smaller thickness of the sample, rapid drainage can be achieved

Can be used to determine interface strength parameters

Clay samples can be oriented along the plane of weakness or an identified failure plane

Disadvantages of direct shear apparatus

Failure occurs along a predetermined failure plane

Area of the sliding surface changes as the test progresses

Non-uniform distribution of shear stress along the failure surface