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8/9/2019 Class6 Shearstrength Directsheartest 150416085805 Conversion Gate02

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Civil Engineering - Texas Tech University

CE 3121: Geotechnical Engineering Laboratory

Class 6

Shear Strength

(Direct Shear Test)

Sources:

Soil Mechanics – Laboratory Manual, B.M. DAS (Chapter 15)

Soil Properties, Testing, Measurement, and Evaluation, C. Liu, J. Evett


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Shear strength in soils

Introduction

Definitions Direct shear test

Introduction

Procedure

Calculation

Results and Figures

Class Outlines


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Shear Strength

The strength of a material is the greatest

stress it can sustain

The safety of any geotechnical structure is

dependent on the strength of the soil

If the soil fails, the structure founded on it can

collapse


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Slope Failure in Soils

Failure due to inadequate

strength at shear interface


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Shear Failure in Soils


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Transcosna Grain Elevator Canada(Oct. 18, 1913)

West side of foundation sank 24-ft


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Significance of Shear Strength

Engineers must understand the nature ofshearing resistance in order to analyze soilstability problems such as;

Bearing capacity Slope stability

Lateral earth pressure on earth-retainingstructures

Pavement


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Shear Strength in Soils

The shear strength of a soil is its resistance toshearing stresses.

It is a measure of the soil resistance to deformationby continuous displacement of its individual soilparticles

Shear strength in soils depends primarily oninteractions between particles

Shear failure occurs when the stresses between the

particles are such that they slide or roll past eachother


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Shear Strength in Soils (cont.)

Soil derives its shear strength from two

sources:

Cohesion between particles (stress

independent component)

Cementation between sand grains

Electrostatic attraction between clay particles

Frictional resistance between particles (stressdependent component)


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Shear Strength of Soils; Cohesion

Cohesion (C), is a measure of the forces thatcement particles of soils

Dry sand with no cementation Dry sand with some cementation

Soft clay

Stiff clay


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Shear Strength of Soils; Internal Friction

Internal Friction angle (f ), is the measure of theshear strength of soils due to friction


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Mohr-Coulomb Failure Criteria

This theory states that a material fails

because of a critical combination of normal

stress and shear stress, and not from their

either maximum normal or shear stressalone.

The relationship between normal stress and

shear is given as

f tanc sfrictioninternalof angle

cohesionc

strengthshears

f


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Shear

Strength,S

Normal Stress, n = = h

C

f = f

Mohr-Coulomb Failure Criterion


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General State of Stress

σ1

σ1 major principle stress

σ3 σ3Minor principle stress

Confining stress


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State of Stresses in Soils

σ1

Shear

stress σ3σ3

Normal stress σnConsider the following situation:

- A normal stress is applied vertically

and held constant

- A shear stress is then applied until

failure


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Determination of Shear Strength

Parameters

The shear strength parameters of a soil are

determined in the lab primarily with two types of tests;

Direct Shear Test

Triaxial Shear Test

Soil

Normal stress σn

Shear stress σ3

3

1


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Direct Shear Test

Direct shear test is Quick and Inexpensive

Shortcoming is that it fails the soil on a

designated plane which may not be the

weakest one

Used to determine the shear strength of both

cohesive as well as non-cohesive soils

ASTM D 3080


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Direct Shear Test (cont.)

The test equipment consists ofa metal box in which the soilspecimen is placed

The box is split horizontallyinto two halves

Vertical force (normal stress)is applied through a metalplaten

Shear force is applied bymoving one half of the boxrelative to the other to causefailure in the soil specimen

Soil

Normal stress σn

Shear stress σ3


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Direct Shear Test


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Direct Shear Test


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Direct Shear Test


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Direct Shear Test Data

S h e a r s t r e s s

Residual Strength

Peak Strength


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Direct Shear Test Data

Volume change

H


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Direct Shear Test (Procedure)

1.Measure inner side or diameter of shear box and find the area

2.Make sure top and bottom halves of shear box are in contact andfixed together.

3.Weigh out 150 g of sand.

4.Place the soil in three layers in the mold using the funnel.Compact the soil with 20 blows per layer.

5.Place cover on top of sand

6.Place shear box in machine.

7. Apply normal force. The weights to use for the three runs are

2 kg, 4 kg, and 6 kg if the load is applied through a lever arm, or 10kg, 20 kg, and 30 kg, if the load is applied directly.

Note: Lever arm loading ratio 1:10 (2kg weight = 20 kg)


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Direct Shear Test (Procedure)

8. Start the motor with selected speed (0.1 in/min) so that the

rate of shearing is at a selected constant rate

9. Take the horizontal displacement gauge, vertical displacement

gage and shear load gage readings. Record the readings on

the data sheet.10. Continue taking readings until the horizontal shear load peaks

and then falls, or the horizontal displacement reaches 15% of

the diameter.


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Calculations

1. Determine the dry unit

weight, gd

2. Calculate the void

ratio, e

3. Calculate the normal

stress & shear stress

1d

wGs

e

g

g

A

V

A

N ;


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Figures

S h e a r

s t r e

s s , s

Peak Stress

N1 = 10 kg

N2 = 20 kg

N3 = 30 kg

Horizontal displacement, H

s3

s2

s1


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Figures (cont)

S

h e a r

S t r e s s , s

( p s f )

C

f

(1,s1)

(3,s3)(2,s2)

Normal Stress , psf


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Figures (cont)

V e r t i c a l d i s p l a c e m e n t

Horizontal displacement

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