01 direct shear
TRANSCRIPT
8/2/2019 01 Direct Shear
http://slidepdf.com/reader/full/01-direct-shear 1/18
Florida International University
Department of Civil and Environmental Engineering
CEG 4011 L Geotechnical Engineering I Laboratory
Dr. Luis A. Prieto-Portar PhD, PE, SE.
Lab Report #01
The Direct Shear Test (ASTM D-3080)
Performed on xx March 2010
Team Members:
Member Attendance Writing Assignment Completed
8/2/2019 01 Direct Shear
http://slidepdf.com/reader/full/01-direct-shear 2/18
0 1- The Direct Shear Test of a Soil
1) Introduction:
The direct shear test is one of the oldest methods for testing the strength of soils. This test
can be performed under different conditions. The soil sample is normally saturated before
the test, but the sample can also be tested at the in-situ moisture content. The rate of strain
can be varied to create to a test of undrained or drained conditions. This depends on whether
the strain is applied slowly for water in the sample to prevent pore-water pressure buildup.
Several specimens are tested, at varying confining stresses, to determine the shear strength
parameters, the soil cohesion, and the angle of internal friction.
Soil Shear strength describes the maximum strength of a soil where significant plastic
deformation occurs due to an applied shear stress. The shear strength of a soil mostly
depends on the rate at which the shearing occurs. The shear strength is one of the most
important engineering properties of a soil because it is required whenever a structure is
dependent on the soil’s shearing resistance. The shear strength is applied in engineering
situations such as the design of foundations, retaining walls, and pavements in civil
engineering applications.
In the U.S., the standard defining how the test should be performed is ASTM D 3080. The
test is performed on three or four soil specimens. A specimen is placed in a shear box having
a cross-sectional area ( A); a confining stress is applied vertically to the specimen. Testing
proceeds by displacing the lower half of the split box and measuring the horizontal shear
force (T ) transmitted through the soil to the upper portion of the box. Testing continues by
displacing the lower box horizontally until the sample fails (when the shear force increases to
a maximum value and then decreases or remains constant). The load applied and the strain
induced is recorded at frequent intervals to determine a stress-strain curve for the applied
confining stress.
8/2/2019 01 Direct Shear
http://slidepdf.com/reader/full/01-direct-shear 3/18
The Direct Shear Testing Apparatus
The specimens are tested at different confining stresses to determine the shear strength
parameters:
The shear stress (τ) on the shear plane may be calculated using:
The shear strength S of a granular soil may be expressed by the Mohr-Coulomb equation:
S = c + σ’ tan Ф
Where σ’ = effective normal stress and Ф = angle of friction of soil.
8/2/2019 01 Direct Shear
http://slidepdf.com/reader/full/01-direct-shear 4/18
Typical values of the drained angle of friction, Φ, for sands are given below:
Round-grained sand Φ(degrees) Angular-grained sand Φ(degrees)
Loose 28-32 Loose 30-36
Medium 30-35 Medium 34-40
Dense 34-38 Dense 40-45
The results of the tests on each specimen are plotted on a graph with the peak (or residual) stress
on the x-axis and the confining stress on the y-axis.
2) Equipment:
1. Direct Shear Test Machine (Soil Test Inc.)
• Soil Test Engineering Test Equipment
Model: D-124-A
Serial No.: # 700
Electronic Specifications: 115 V 60 CY
8/2/2019 01 Direct Shear
http://slidepdf.com/reader/full/01-direct-shear 5/18
• Motor specifications (Mac Motor Appliance Corp)
Frame: 42-38 20 L Specs: B5412 M3 Horsepower: 1/6 hp Volts: 15/230
Amps 2/1.6 Cycle 30/50 RPM 1725/1426 Rating 70°C
2. Force Meter
ELE 88-4000 0.0001”
3. Displacement Meter
8/2/2019 01 Direct Shear
http://slidepdf.com/reader/full/01-direct-shear 6/18
Starrett No. 25-3041 0.0001”
4. Mass Balance
• Ohaus – Model: Explorer Pro
• Maximum: 22000 g
5. Spoon
6. Assortment of weights (for applying load)
8/2/2019 01 Direct Shear
http://slidepdf.com/reader/full/01-direct-shear 7/18
7. 2.5 cm Ball Bearing
8. Ruler
8/2/2019 01 Direct Shear
http://slidepdf.com/reader/full/01-direct-shear 8/18
8/2/2019 01 Direct Shear
http://slidepdf.com/reader/full/01-direct-shear 9/18
3) Procedure.
1. Remove the shear box assembly and insert the two vertical pins tokeep the two halves of the shear box together.
2. Determine the dimensions of the shear box. Determine the
dimensions for the arm of the vertical load yoke in the direct shear
machine.
3. Weigh the dry sand bowl, W 1. Fill the shear box with sand in small
layers. Weigh the bowl with sand again to record the amount of sand put into the shear box.
4.• Try to compact the sand layers. The top of the compacted
specimen should be about ¼ inch below the top of the
shear box. It is important to level the surface of the sand
specimen so that the cap will sit level with the sandsample.
5. Slip the loading head down from the top of the shear box to rest onthe soil specimen. Place the ball bearing in the gap of the loading
head.
8/2/2019 01 Direct Shear
http://slidepdf.com/reader/full/01-direct-shear 10/18
6. Put the shear box assembly in place in the direct shear machine.
7. Apply the desired normal load, N , on the specimen by hanging1Kg. dead weights to the vertical load yoke. The top crossbars will
rest on the loading head of the specimen, which, in turn, rests on
the soil sample.
8. Attach the horizontal and vertical dial gauges (0.001 in/small div)to the shear box to measure the displacement during the test.
9. Remove the two vertical pins that were keeping the two halves of the shear box together (from Step 1).
8/2/2019 01 Direct Shear
http://slidepdf.com/reader/full/01-direct-shear 11/18
9. Apply horizontal load, S , to the top half of the shear box. The rate
of shear displacement should be between 0.1 to 0.02 in/min.
Record the readings of the vertical dial gauge and the proving ringgauge, which measures the horizontal load, S for every tenth small
division displacement in the horizontal dial gauge.
Continue until the following happens at the proving ring dialgauge:
• Reaches a maximum and then falls
• Reaches a maximum and then remains constant.
10. Repeat the test (Steps 1 to 9) two more times. For each test, the dry unit weight of compaction of the sand specimen should be the same as that of the first sample.
4) Data and Calculations.
W1 (Weight of bowl + dry soil) (before) = 5.4885 lb
W2 (Weight of bowl + dry soil) (after) = 5.2430 lb
Length (L) = 2 in Width (B) = 2 in Height (H) = 1.31 in
Specific Gravity of soil (G) = 2.66
Dry unit weight of the soil:
3
1 2
3
(5.4885 5.2430) 120.0469 81
(2)(2)(1.31) 1d
W W lb lbs in pcf
LBH in ft γ
− −= = = =
Voids Ratio of the Soil:
(2.66)(62.4 )1 1 1.048
81
s w
d
G pcf e
pcf
γ
γ = − = − =
Trial 1 Sample Calculations
8/2/2019 01 Direct Shear
http://slidepdf.com/reader/full/01-direct-shear 12/18
F = 1 kg
V (Normal Force) = 8 x 1 = 8 kg
V (Normal Force) = 8 kg x 2.20046 lbs/kg = 17.64 lbs
Normal Stress:
291.16
)2)(2(
)64.17(
))((
)('
in
lbs
B L
e Normalforcload Vertical ===σ
Shear force:
S = 35 / 5 = 7 lbs
Shear Stress:
psiin Bin L
lbS
soil theof area
S force shear inlb 75.1
)2)(2(
)7(
)()(
)(,)/( ====τ
5) Tables.
Below is a diagram explaining why the total force is eight times the load.
Arm advantage is 8 since 24in/3in
ΣM(support) = 0
Load * 24 = Acting Force * 3Acting Force = (Load * 24) / 3
Acting Force = 8 * Load
8/2/2019 01 Direct Shear
http://slidepdf.com/reader/full/01-direct-shear 13/18
F 1 kg
V 8 kg
V 17.64 lbs
F 2 kg
V 16 kg
V 35.27 lbs
Trail 1 Value Units
W1 5.4885 lb
W2 5.2430 lb
Length 2 in
Width 2 inHeight 1.31 in
Gs 2.66
Units obtained in Lab
Horizontal
Displacement
Shear
Force10 35
15 40
20 45
25 55
30 60
35 65
40 68
45 70
Normal
Stress,
σ’(Lb/in²)
Horizontal
Displacement
(mm)
Horizontal
Displacement
(in)
Shear
force
S (Lb)
Shear
stress
τ (psi)
16.91 0.254 0.010 7.0 1.75
16.91 0.381 0.015 8.0 2.00
16.91 0.508 0.020 9.0 2.25
16.91 0.635 0.025 11.0 2.75
16.91 0.762 0.030 12.0 3.00
16.91 0.889 0.035 13.0 3.25
16.91 1.016 0.040 13.6 3.40
16.91 1.143 0.045 14.0 3.50
Trail 2 Value Un
W1 2.6419 l
W2 2.5178 l
Length 2 i
Width 2 i
Height 1.31 i
Gs 2.66
Units obtained in Lab
Horizontal
Displacement
Shear
force
1 101 15
2 20
3 25
3 30
4 35
5 40
6 45
10 50
10 55
13 60
16 65
18 70
20 75
22 80
25 85
27 90
31 95
35 100
39 105
48 110
59 112
Normal
Stress,
σ’(Lb/in²)
Horizontal
Displacement
(mm)
Horizontal
Displacement
(in)
Shear
force
S (Lb)
Shear
stress τ
(psi)
21.32 0.0254 0.001 2.0 0.5021.32 0.0254 0.001 3.0 0.75
21.32 0.0508 0.002 4.0 1.00
21.32 0.0762 0.003 5.0 1.25
21.32 0.0762 0.003 6.0 1.50
21.32 0.1016 0.004 7.0 1.75
21.32 0.1270 0.005 8.0 2.00
21.32 0.1524 0.006 9.0 2.25
21.32 0.2540 0.010 10.0 2.50
21.32 0.2540 0.010 11.0 2.75
21.32 0.3302 0.013 12.0 3.0021.32 0.4064 0.016 13.0 3.25
21.32 0.4572 0.018 14.0 3.50
21.32 0.5080 0.020 15.0 3.75
21.32 0.5588 0.022 16.0 4.00
21.32 0.6350 0.025 17.0 4.25
21.32 0.6858 0.027 18.0 4.50
21.32 0.7874 0.031 19.0 4.75
21.32 0.8890 0.035 20.0 5.00
21.32 0.9906 0.039 21.0 5.25
21.32 1.2192 0.048 22.0 5.50
21.32 1.4986 0.059 22.4 5.60
Trail 3 Value Units
W1 2.7485 lb
W2 2.6295 lb
Length 2 in
Width 2 in
Height 1.31 inGs 2.66
8/2/2019 01 Direct Shear
http://slidepdf.com/reader/full/01-direct-shear 14/18
F 3 kg
V 24 kg
V 52.91 lbs
8/2/2019 01 Direct Shear
http://slidepdf.com/reader/full/01-direct-shear 15/18
Normal
Stress,
σ’(Lb/in²)
Horizontal
Displacement
(mm)
Horizontal
Displacement
(in)
Shear
force
S (Lb)
Shear
stress τ
(psi)
25.73 0.0127 0.0005 2.0 0.50
25.73 0.0152 0.0006 3.0 0.75
25.73 0.0152 0.0006 4.0 1.00
25.73 0.0152 0.0006 5.0 1.25
25.73 0.0254 0.0010 6.0 1.50
25.73 0.0381 0.0015 7.0 1.75
25.73 0.0508 0.0020 8.0 2.00
25.73 0.0762 0.0030 9.0 2.25
25.73 0.1016 0.0040 10.0 2.50
25.73 0.1270 0.0050 11.0 2.75
25.73 0.1524 0.0060 12.0 3.00
25.73 0.1905 0.0075 13.0 3.25
25.73 0.2286 0.0090 14.0 3.50
25.73 0.2540 0.0100 15.0 3.75
25.73 0.2921 0.0115 16.0 4.00
25.73 0.3302 0.0130 17.0 4.25
25.73 0.3556 0.0140 18.0 4.50
25.73 0.3810 0.0150 19.0 4.75
25.73 0.4064 0.0160 20.0 5.00
25.73 0.4318 0.0170 21.0 5.25
25.73 0.4572 0.0180 22.0 5.50
25.73 0.4572 0.0180 23.0 5.75
25.73 0.5334 0.0210 24.0 6.00
25.73 0.5842 0.0230 25.0 6.25
25.73 0.6096 0.0240 26.0 6.50
25.73 0.6604 0.0260 27.0 6.75
25.73 0.7112 0.0280 28.0 7.00
25.73 0.7620 0.0300 29.0 7.25
25.73 0.8128 0.0320 30.0 7.50
25.73 0.9144 0.0360 31.0 7.75
25.73 0.9652 0.0380 32.0 8.00
25.73 1.1176 0.0440 33.0 8.25
25.73 1.2954 0.0510 34.0 8.50
25.73 1.4224 0.0560 34.0 8.50
25.73 1.4732 0.0580 34.2 8.5525.73 1.5240 0.0600 34.4 8.60
Units obtained in Lab
Horizontal
Displacement
Shear
Force
0.5 10
0.6 15
0.6 20
0.6 25
1 30
1.5 35
2 40
3 45
4 505 55
6 60
7.5 65
9 70
10 75
11.5 80
13 85
14 90
15 95
16 100
17 105
18 110
18 11521 120
23 125
24 130
26 135
28 140
30 145
32 150
36 155
38 160
44 165
51 170
56 170
58 17160 172
15
8/2/2019 01 Direct Shear
http://slidepdf.com/reader/full/01-direct-shear 16/18
0
1
2
3
4
5
6
7
8
9
10
0 2 4 6 8 10 12 14
S h e a r S t r e s s ( l b / i n
)
Normal Stress (lb/in2)
Shear Force vs. Shear Stess
Φ=57.58°
16
8/2/2019 01 Direct Shear
http://slidepdf.com/reader/full/01-direct-shear 17/18
7) Conclusions.
A direct shear test is used to find the shear strength parameters of a soil. Stress failure is caused by
slippage of soil particles, which may lead to sliding of one body of soil relative to the surrounding
mass. The shear stress and displacement is not uniformly distributed within the soil, therefore, as
the soil is initially displaced the shear stress increases at a fast rate and then as more displacement
occurs, the rate decreases. This can be seen in the plot of shear stress versus horizontal
displacement where the slope of the graph is steep initially and then decreases as displacement
increases.
There are advantages and disadvantages to using a direct shear test. Some of the advantages are
that it is cheap, fast and simple, especially for the testing of sand and failure occurs along a singlesurface, which approximates observed slips or shear type failures in natural soils. However, the
main disadvantage is that the failure plane is forced and may not be the most critical plane which
failure can occur. Other disadvantages are that non-uniform stress conditions exist in the specimen,
and the principal stresses rotate during shear, and the rotation cannot be controlled. While
conducting this experiment, it was determined that some factors might have induced errors in the
data that was being recorded. Before weighing the sand sample in the porcelain dish, a part of it
spilled. This might have led to some overestimation in the values for the weight of the sand placed
in the shear box. Another factor that might have induced error was the way in which the gauges
were being read. They were read simultaneously and at a very fast pace. This might have led to
some inaccuracies while recording the readings.
Also, the tools used for the experiment were not perfect. The horizontal reading gauge could not be
placed in a perfectly horizontal position, which definitely caused some underestimation of the
horizontal shear displacement recorded. Finally, the sand placed in the shear box didn’t have a
perfectly flat surface. This probably led to some inconsistencies while recording the values for the
vertical shear displacement. From the experiment the maximum shear stress was found to be 3.5
psi, 5.6 psi and 8.6 psi for the first, second and third trials respectively.
17
8/2/2019 01 Direct Shear
http://slidepdf.com/reader/full/01-direct-shear 18/18
8) References .
Prieto-Portar, Luis. “08. The Direct Shear Test” Florida International University. 1 Apr. 2008<http://web.eng.fiu.edu/~prieto/geo1/Laboratories/08-Direct-Shear-Test/Index.htm>.
"Shear Strength in Soils". <http://esig4.uwyo.edu/classes/fa2007/ce3600/8_shear/shear.htm>.
Sivakugan, N. "Shear Strength of Soils." <www.geoengineer.org/files/Strength-Sivakugan.ppt>.
18