Class 6 Shear Strength - Direct Shear Test ( Geotechnical Engineering )

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


Shear strength in soils

Introduction

Definitions

Direct shear test

Introduction

Procedure

Calculation

Results and Figures

Class Outlines


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


Slope Failure in Soils

Failure due to inadequate

strength at shear interface


Shear Failure in Soils


Bearing Capacity Failure


Transcosna Grain Elevator Canada

(Oct. 18, 1913)

West side of foundation sank 24-ft


Significance of Shear Strength

Engineers must understand the nature of shearing resistance in order to analyze soil stability problems such as;

Bearing capacity

Slope stability

Lateral earth pressure on earth-retaining structures

Pavement


Shear Strength in Soils

The shear strength of a soil is its resistance to shearing stresses.

It is a measure of the soil resistance to deformation by continuous displacement of its individual soil particles

Shear strength in soils depends primarily on interactions between particles

Shear failure occurs when the stresses between the particles are such that they slide or roll past each other


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 (stress

dependent component)


Shear Strength of Soils; Cohesion

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

Dry sand with no cementation

Dry sand with some cementation

Soft clay

Stiff clay


Shear Strength of Soils; Internal Friction

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


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 stress

alone.

The relationship between normal stress and

shear is given as

f tancsfriction internal of angle

cohesionc

strengthshear s

f


Shear

Strength,S

Normal Stress, n = = g h

C

f = f

Mohr-Coulomb Failure Criterion


General State of Stress

σ1

σ1 major principle stress

σ3 σ3 Minor principle stress

Confining stress


State of Stresses in Soils

σ1

Shear

stress σ3 σ3

Normal stress σn Consider the following situation:

- A normal stress is applied vertically

and held constant

- A shear stress is then applied until

failure


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


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


Direct Shear Test (cont.)

The test equipment consists of a metal box in which the soil specimen is placed

The box is split horizontally into two halves

Vertical force (normal stress) is applied through a metal platen

Shear force is applied by moving one half of the box relative to the other to cause failure in the soil specimen

Soil

Normal stress σn

Shear stress σ3


Direct Shear Test


Direct Shear Test Data

Sh

ear

str

ess

Residual Strength

Peak Strength


Direct Shear Test Data

Volume change

DH


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 and fixed 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 10 kg, 20 kg, and 30 kg, if the load is applied directly.

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


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.


Calculations

1. Determine the dry unit

weight, gd

2. Calculate the void

ratio, e

3. Calculate the normal

stress & shear stress

1d

wGse

g

g

A

V

A

N ;


Figures

Sh

ear

stre

ss, s

Peak Stress

N1 = 10 kg

N2 = 20 kg

N3 = 30 kg

Horizontal displacement, DH

s3

s2

s1


Figures (cont)

Sh

ear

Str

es

s, s

(p

sf)

C

f

(1,s1)

(3,s3) (2,s2)

Normal Stress , psf


Figures (cont)

Ver

tica

l d

ispla

cem

ent

Horizontal displacement

Class 6 Shear Strength - Direct Shear Test ( Geotechnical Engineering )

Engineering

Transcript of Class 6 Shear Strength - Direct Shear Test ( Geotechnical Engineering )