01 direct shear

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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



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.



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.



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



• 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



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)



7. 2.5 cm Ball Bearing

8. Ruler



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.



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).



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



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



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


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



F 3 kg

V 24 kg

V 52.91 lbs



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


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



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



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) 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

http://web.eng.fiu.edu/~prieto/geo1/Laboratories/08-Direct-Shear-Test/Index.htm

http://esig4.uwyo.edu/classes/fa2007/ce3600/8_shear/shear.htm

http://www.geoengineer.org/files/Strength-Sivakugan.ppt



http://web.eng.fiu.edu/~prieto/geo1/Laboratories/08-Direct-Shear-Test/Index.htm

http://esig4.uwyo.edu/classes/fa2007/ce3600/8_shear/shear.htm


01 direct shear

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