Civil & Environmental Engineering Department

1

Measurement of Shear Strength Parameter of Soil with

Direct Shear Test

Figure 1 Direct Shear Device

Advantages of Direct Shear Test

Simple, fast

Disadvantages of Direct Shear Test

Cannot control pore pressures. Therefore tests are assumed to be drained.

Failure on horizontal plane only, which may not be the weakest plane.

Non-uniform stress conditions inside shear box.

Principal stress rotations occur

Vertical and horizontal stresses are principal stresses before shear.

Vertical and horizontal stresses are not principal stresses at failure.

Figure 2 Shear Strength Envelop

EGCE 324L (Soil Mechanics Laboratory) Fall 2008

Instructor: Binod Tiwari, PhD Date: 10/30/2008

Civil & Environmental Engineering Department

2

Application

Direct shear test gives shear strength parameters (cohesion and friction angle) of

soil. Shear strength parameters are important in all types of geotechnical designs

and analyses.

Equipment

Strain controlled direct shear device with two displacement LVDT and a load cell

Balance sensitive to 0.1 g

Moisture cans

Oven

Procedure

1. Take the shear box, and set two vertical pins to keep the two halves of the

shear box together.

2. Set a porous stone at the base and fill the box with the dry sand (make 1 inch

thickness). Compact the sand gently.

3. Set another porous stone on the top.

4. Set top platen on top of the porous stone.

5. Put the shear box assembly into the direct shear device.

6. Fill up the outer jacket with water.

7. Turn on the software and follow the instruction.

8. Apply dead load to the load hanger to make normal stress of approximately 50

kPa. You need to hold the cross bar to make it rest right on top of the top platen.

9. Remove both vertical pins.

10. Set up the dial gauges for vertical displacement and horizontal displacement.

Make sure that the shear box is connected to the electricity line and is on.

11. Consolidate the specimen for 100% consolidation and apply horizontal load to the

box at the strain rate that is calculated based on the consolidation data. Set

that speed both in the computer and the shear box.

12. Record horizontal displacement, vertical displacement, and shear force at 15

seconds interval.

13. Shear stress increases, peaks and then drops or may remain flat. Once peak/or

maximum shear stress is attained, continue for a while and stop the test. Be

cautious not to let the shear box touch the wall of water jacket.

14. Take the sample out, take weight and put it into the oven to measure the water

content.

15. Take another specimen and repeat the procedure for the normal stress of 100

kPa.

16. Take the third and fourth specimens and repeat the procedure for the normal

stresses of 150 kPa and 200 kPa respectively.

Calculations

1. Calculate area and volume of the specimen.

2. Calculate bulk unit weight of the specimen.

EGCE 324L (Soil Mechanics Laboratory) Fall 2008

Instructor: Binod Tiwari, PhD Date: 10/30/2008

Civil & Environmental Engineering Department

3

Bulk unit weight (γ)=

V

W

3. Calculate dry unit weight of the specimen.

Dry unit weight(γ

d

) =

w+1

γ

4. Calculate initial and final void ratio.

e =

1−

d

ws

G

γ

γ

take G

s

= 2.65

5. Calculate normal stress (σ’)

Area

LoadNormal

=

'

σ

6. Calculate shear stress.

Area

ForceShear

=τ

7. Plot τ versus shear strain (shear displacement/original height of specimen).

8. Plot vertical strain (displacement/initial height) vs shear strain.

9. Plot normal stress (in x-axis) vs shear stress for all tests.

10. The equation of the best fit line will give you c’ and φ’.

Report

1. Submit all pertinent calculations and graphs.

2. Report the values of c’, and φ’ based on four shear tests.

3. Present final void ratio vs shear stress ratio (τ/σ’).

EGCE 324L (Soil Mechanics Laboratory) Fall 2008

Instructor: Binod Tiwari, PhD Date: 10/30/2008

Civil & Environmental Engineering Department

EGCE 324L (Soil Mechanics Laboratory) Fall 2008

Instructor: Binod Tiwari, PhD Date: 10/30/2008

4

Soil Mechanics Laboratory

Direct Shear Test Laboratory Data Sheet

I. GENERAL INFORMATION

Tested by: Date tested:

Lab partners/organization:

Client: CSUF Project: 324 Lab Direct Shear

Boring no.: NA Recovery depth: NA

Recovery date: NA Recovery method: NA

Soil description: Clean sand

II. TEST DETAILS

Sample length/width: 4 in. Sample Height:

Initial Sample Mass: Final Sample Mass:

Wet Mass of the Specimen: Dry Mass of Specimen:

Normal force, N:

Normal stress,

σ

:

Deformation rate: Deformation indicator type: LVDT

Shear force measurement instrument type: Load cell

Horizontal dial gauge conversion factor, K

H

: 1

Vertical dial gauge conversion factor, K

V

: 1

Proving ring dial gauge conversion factor, K

F

: 1

III. MEASUREMENTS AND CALCULATIONS

Horizontal

Deformation

Reading

(G

V

)

Vertical

Deformation

Reading

(G

H

)

Force

Reading

(G

F

)

Horizontal

Displacement

(ΔH)

Vertical

Displacement

(ΔV)

Shear

Force

(F)

Shear

Stress

(τ)

Shear strength (

τ

f

):

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