Measurement of Shear Strength Parameter of Soil with Direct Shear Test

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18 Ιουλ 2012 (πριν από 5 χρόνια και 3 μήνες)

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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



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
):