Geotechnical Engineering 367 Direct Shear Test for Dry Sand
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1.0
Introduction
1.1
Background Direct shear test, is one of the oldest and simplest soil shear strength test. The purpose
of direct shear test is to determine the shear strength of soil. Normally, direct shear test is used for non-cohesive soil such as sands, but it also can be used for cohesive soils such as clay. In direct shear test, apparatus needed are basically consisted of a shear box which is separated horizontally in two halves. One half is fixed, and the other is either pulled or pushed. The soil specimen is placed into the shear box.
During the test, a constant normal stress ( is applied to the soil specimen, and a horizontal shear force is applied to one half of the shear box, therefore the soil will fail along a pre-determined horizontal plane between the two halves of the shear box. Lateral deformation ( Δ L) and vertical deformation ( Δh) of the soil specimen will be recorded. All of these readings taken will be used for calculation and graph plotting to determine shear strength parameter.
1.2
Aim and Objective The objective of direct shear test is to determine the shear strength of the soil. The
shear strength of soil can be expressed by empirical Mohr-Coulomb relationship: s = c + tan ϕ while is the normal stress on failure plane of soil. With carrying out direct shear stress, two parameters can be obtained, which are the soil cohesion,
c
and the soil friction
angle, ϕ.
In this laboratory, clean dry sand will be used as soil sample. As dry sand will not having pore pressure development, the effective stress parameters c’ and ϕ’ will be equal to c and ϕ. In addition, clean sand tend to be cohensionless, therefore cohesion, c’ of clean dry sand assumed to be zero. Thus, the shear strength of clean dry sand can be determined as s =
tan ϕ.
2.0
Procedure
1. 60 mm square dimension of shear box was used in this direct shear test. Two halves of the shear box was ensured to be correctly assembled with spacer screws withdrawn and clamping screws in position.
2. The rigid plate was inserted above the lower plate, with the ridges aligned normal to the direction of travel of the assembled box.
3. The dry sand was poured and the upper surface was levelled at about 5mm below the upper edge of the shear box. The sand was placed carefully using a funnel and tube with tube end not more than 10mm above the sand surface. The sand was compacted to the required density (unit weight), while another correctly oriented and ridged plate was placed gently on the sand to fit the upper pressure plate. The sand density was calculated after filling out the information needed in the calculation sheet. No porous stones were needed on the top and bottom of soil specimen as the test is for dry sand and no drainage is required.
the displacement of the upper half of the shear box relative to the lower half of the box. Both dial gauges was set to zero. The dial gauge on the proving ring gives, after applying a calibration factor marked on the proving ring, the shear force mobilised across the shear plane.
8. The gear box drive was checked to ensure it had set to the correct displacement rate of at least 1.25 mm/min. Manual sheet was prepared and tasks of reading various dial gauges had been assigned to different group members.
9. The shear box drive motor was started. The dial gauge readings were recorded at 10 second intervals for the first minute and then at half minute intervals until the shear force is seen to either remain constant, to decrease for three consecutive readings or until the shear box is about to reach the limit of its travel. The motor was switched off and drive piston was retracted.
10. Shear box was removed, dismantled and the soil was discarded.
3.0
Results and Calculations
In this laboratory, the shear box dimension is 60 mm square. Therefore, lateral dimension, L which taken for shear strain calculation is 60 mm.
Table 1
Experimental Result and Calculated Shear Strain
Horizontal Displacement, ΔL (mm) 0.00
Vertical Displacement, ΔH (mm) 0.000
Shear Stress, τ
(kPa) 0
Shear Strain ԑ = ΔL/L 0.0000
0.02
0.002
19
0.0003
0.04 0.06 0.08 0.20 0.32
0.008 0.016 0.026 0.064 0.128
34 43 47 56 51
0.0007 0.0010 0.0013 0.0033 0.0053
0.48 0.64 0.80
0.192 0.256 0.288
46 41 37
0.0080 0.0107 0.0133
Shear Stress-Strain Curve 60
50
) a P40 k ( τ , s s 30 e r t S r a20 e h S 10
0 0.0000
0.0020
0.0040
0.0060
0.0080
0.0100
0.0120
0.0140
Shear Strain, ԑ Figure 2
Shear Stress Strain Curve for Sand Sample
0.0160
0.0180
0.0200
Graph of Normal Displacement against Shear Displacement 0.350
) 0.250 m m ( h0.150 Δ , t n0.050 e m e c 0.00 a-0.050 l p s i D l -0.150 a m r o N-0.250
n o i t a l i D
0.20
0.40
0.60
0.80
n o i s s e r p m o C
-0.350
Shear Displacement, ΔL (mm) Figure 3
Graph of Normal Displacement versus Shear Displacement
1.00
1.20
Graph of Shear Stress against Normal Stress 60
50
) a40 P k ( τ , s s 30 e r t S r a e h20 S 10
0 0
10
20
30
40
Normal Stress, σn (kPa) Figure 4
Graph of Shear Stress versus Normal Stress
50
60
