Direct Shear Test

DIRECT SHEAR TEST SOIL MECHANICS (CE-403)

CREATED BY OBAID KHALID ANUM RAZA

APPENDIX A LABORATORY SESSION # 1 DIRECT SHEAR TEST Introduction This test is performed to determine the consolidated-drained shear strength of a sandy to silty soil. 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 needed for engineering situations such as determining the stability of slopes or cuts, finding the bearing capacity for foundations, and calculating the pressure exerted by a soil on a retaining wall. The test is carried out on either undisturbed samples or remoulded samples. To facilitate the remoulding purpose, a soil sample may be compacted at optimum moisture content in a compaction mould. Then specimen for the direct shear test could be obtained using the correct cutter provided. Alternatively, sand sample can be placed in a dry state at a required density, in the assembled shear box. A normal load is applied to the specimen and the specimen is sheared across the pre-determined horizontal plane between the two halves of the shear box. Measurements of shear load, shear displacement and normal displacement are recorded. The test is repeated foe two or more identical specimens under different normal loads. From the results, the shear strength parameters can be determined.

Standard Designation ASTM D 3080 - Standard Test Method for Direct Shear Test of Soils Under Consolidated Drained Conditions

Object & Scope: The object of the test is to determine the shear strength parameters for a given soil using the direct shear test.

Materials And Equipment: • • • • •

Direct shear box Metallic grids Wooden strip Locking pins Direct shear machine

• • • •

Serrated porous stone Metallic ball Metallic mould Spatula

Wooden strip

Metallic mould

Shear box

Locking pin

Metallic ball

Metallic grid

Spatula

Theoretical Background: The strength of a soil depends of its resistance to shearing stresses. It is made up of basically the components; 1. Frictional – due to friction between individual particles. 2. Cohesive - due to adhesion between the soil particles The two components are combined in Colulomb’s shear strength equation, τf = c + σf tan ø Where τf = shearing resistance of soil at failure c = apparent cohesion of soil σf = total normal stress on failure plane ø = angle of shearing resistance of soil (angle of internal friction)

The direct shear test is very common and most popular popular test for strength of soil. soil. However it has some some advantages and disadvantages as follow Advantages: •

A quick and inexpensive test to obtain the shear strength parameters of both fine and coarse grained soils either in undisturbed or remolded state.

•

Ease of sample preparation.

Disadvantages •

Drainage cannot be controlled and pore water pressure can not be measured

•

Not very much representative of failures in fine grained soils where pore pressure plays a significant role on shear strength. Much recommended for fully drained cases.

•

The failure plane is always forced horizontal which is not the weakest plane in the case of in situ

•

conditions. This issue can be resolved by using triaxial testing.

Test procedure: •

1.

Sample preparation: Sample is to be taken from undisturbed soil.

2. Take special care to avoid the loss of moisture content. •

Procedure:

1. With the help of the metallic mould prepare the square shape sample. 2. Place the square metallic mould with the sample in the shear box in which one metallic grid is already placed. 3. With the help of wooden strips shift the sample in the shear box. And compact it.

4. Place another metallic grid on it in such a way that its grid direction is perpendicular to the previous one placed below. 5.

Put the metallic ball on the metallic grid.

6. Now put the whole whole assembly of the shear box on on direct shear apparatus. apparatus. 7. Adjust the apparatus; bring the proving ring to zero. 8. Apply the desired normal load, N, on the specimen. This can be done by hanging dead weights to the vertical load yoke. The top crossbars will rest on the loading head of the specimen, which in turn, rest on the soil specimen. 9. Apply the horizontal load “S” to the top half of the shear box. The rate of the shear shear displacement should be between 0.1 to 0.2 in/min. 10. Note down the reading when the proving ring dial gauge reading reaches a maximum and then falls, or reaches maximum and then remain constant. 11. Repeat the test for taking more readings by incr easing the dead weights.

Observations: 1. Proving ring calibration factor for shear force is 0.15 Kgf/division. 2. Dimension of the sample = 3. Area of cross section of sample =

Sr.

Normal

Dial Gauge

Shear Force “F”

Normal Stress

Shear Stress

No.

Force

Reading

=Dial Gauge Reading

σ= N/A

Τ=F/A

“N”

X 0.15

(Kg/cm2)

(Kg/cm2)

( Kg)

(Kg )

1. 2. 3. 4. 5. 6.

Calculations: •

Normal and Shear forces at the state of shear failure of the specimen were measures/ observed and noted in the observation table.

•

σ= N/LB .. ……………………

Eq.1

τ= S/LB ……………………….

Eq. 2

Normal and shear stresses were calculated (dividing the measured forces by the area of specimen); noted in the observation table.

•

Plotting the calculated values of the shear and normal stresses on rectangular coordinate of σ-τ sheet attached, Mohr-Coulomb shear failure envelop is drawn. The straight line is achieved using approach of best fitted curve if required.

•

Using the geometry of curve the value of C and φ are calculated.

Graph: Plot a graph of Shear stress vs. Normal stress. This graph will be approximately a straight line. The y-intercept of this line gives the cohesion. The angle of internal friction of the soil can be determined from the slope of the straight line. For purely cohesion less soil this line passes through origin. And angle of internal friction; φ = tan

-1

(τ/σ)…………………………….. Eq. 3

Normal Stress vs. Shear Stresses Stresses

Limiting Values:

http://www.fs.fed.us/rm/pubs_other/wo_em7170_13/wo_em7170_13_vol2.pdf (Pg 434of book) (pg no. 113 of pdf file)

Result: The shear strength parameters of the soil sample tested through direct Shear Test in laboratory are found to be C= φ=

References: 1. http://www.fs.fed.us/rm/pubs_other/wo_em7170_13/wo_em7170_13_vol2.pdf (Pg 434of book) (pg no. 113 of Pdf file)

2. Above graph is taken from soil mec hanics II work book.

APPENDIX B LABORATORY SESSION # 1 DIRECT SHEAR TEST 1. Limitations / Problems: Distilled Water was not available during the preparation of Soil sample for direct shear test.

How to solve it: To solve this problem in the soil lab we have to provide an assembly for the distillation of water. The assembly contains the following equipments. •

Distillation flask

•

Thermometer

•

Condenser ( containing inlet & outlet pipe of c ool water)

•

Conical flask ( in which clean water stored)

The process: The distillation process utilizes a heat source to vaporize water. In the distillation process, water is first heated until it reaches its boiling point and begins to evaporate. The temperature is then kept at constant. The stable temperature ensures continued water vaporization. Next, the evaporated water is captured and guided through a system of tubes to another container. Finally, remove from the heat source, the steam condenses back into its original liquid form. The process removes most minerals, most bacteria and viruses, and chemicals having higher boiling point than water. For this reason, distillation is sometimes valued as a method of obtaining pure water.

Consequences: The water we use at laboratory for testing of soil is probably one of the most important factors to achieve the ideal result for the test. The polluted water can look the same as pure water. Cloudy or muddy water contain harmless sediments which may decrease the strength of soil then we cannot get the ideal results from soil testing. 2. Limitations/ problems : The practical is not performed on the undisturbed soil which not according to the code.

Description: Since bring the undisturbed soil need special care and also expensive, it is not possible to bring the soil sample from situ for lab practice work. Hence the practical is performed on the sample available in lab.

APPENDIX C LABORATORY SESSION # 1 DIRECT SHEAR TEST Test procedure: •

Sample preparation:

1. Take a sample. 2. Grind the sample to make it finer. 3. Sieve the material with 40 no. sieve. 4. Add the distilled water in a such a quantity that all the sample get wet and mix the water thoroughly.

•

STEP 1

STEP 2

STEP 4

STEP 5

STEP 3

STEP 6

Procedure:

1. With the help of the metallic mould prepare the square shape sample of the area 25cm2. 2. Place the square metallic mould with the sample in the shear box in which one metallic grid is already placed.

3. With the help of wooden strips shift the sample in the shear box. And compact it. 4. Place another metallic grid on it in such a way that its grid direction is perpendicular to the previous one placed below. 5. Put the metallic ball on the metallic grid. 6. Now put the whole whole assembly of the shear box on on direct shear apparatus. apparatus. 7. Adjust the apparatus; bring the proving ring to zero. 8. Apply the desired normal load, N, on the specimen. This can be done by hanging dead weights to the vertical load yoke. The top crossbars will rest on the loading head of the specimen, which in turn, rest on the soil specimen. 9. Apply the horizontal load “S” to the top half of the shear box. The rate of the shear shear displacement should be between 0.1 to 0.2 in/min. 10. Note down the reading when the proving ring dial gauge reading reaches a maximum and then falls, or reaches maximum and then remain constant. 11. Repeat the test for ta king more readings by increasing the dead weights.

Step 1

Step 4

Step 2

Step 5

step 3

Step 6

Step 7

Step 10

Step 8

Step 11

Calculations and Reference tables: Formulas used in calculations are mention in appendix A. Eq 1, Eq 2 and Eq 3 will be used.

SAMPLE NO 1: (Sand)

G29-S1

Step 9

Step 12

Observations and tables: 1. Proving ring calibration factor for shear force is 0.15 Kgf/division. 2. Dimension of the sample =5cm x 5cm 3. Area of cross section of sample =25 cm2

Sr.

Normal

Dial Gauge

Shear Force “F”

Normal Stress

Shear Stress

No.

Force

Reading

=Dial Gauge Reading

σ= N/A

Τ=F/A

“N”

X 0.15

(Kg/cm )

(Kg/ cm )

(Kg)

(Kg)

1. 2. 3.

2

2

4

15

2.25

0.16

0.09

8

35

5.25

0.32

0.21

12

54

8.1

0.48

0.324

Moisture content: This sample is made with adding 14 .54% of moisture content. Pan no. = 44 Empty pan wt =8.09gms Wet wt + pan wt = 23.84 gms Dry wt. + pan wt. =21.84gms =21.84gms Mw = 2 gms Md = 13.75 gms

Graph and Figure: The graph is plotted between Normal Stress (x-axis) and Shear Stress (y-axis)

0.35 y = 0.731x 0.731x - 0.026 0.026 R² = 0.999

0.3

0.48, 0.324

0.25 s s ) e 2 r t S m c r / g a k e ( h S

0.32, 0.21

0.2 0.15 0.1

0.16, 0.09

0.05 -1E-16 0

0.1

0.2

0.3

0.4

0.5

0.6

-0.05

Normal Stress (kg/cm2)

Obtained Result: The shear strength parameters of the soil sample tested through direct Shear Test in laboratory are found to be C = -0.026=0 (approx) φ = 36.17˚

Comparison with expected result: It is shown from the above value of φ = 36.17˚, that the given sand sample is dense sand of uniform grain size. (appendix A)

SAMPLE NO 2: (Sand)

G29-S2


Sr.

Normal

Dial Gauge

Shear Force “F”

Normal Stress

Shear Stress

No.

Force

Reading

=Dial Gauge Reading

σ= N/A

Τ=F/A

“N”

X 0.15

(Kg/cm2)

(Kg/ cm )

(Kg)

(Kg)

1. 2. 3.

2

4

45

6.75

0.16

0.27

8

55

8.25

0.32

0.33

12

64

9.6

0.48

0.384

This sample is made with adding 21.42% of moisture content. Pan no. = 38 Empty pan wt =8.13gms Wet wt + pan wt = 16.29 gms Dry wt. + pan wt. =14.85 gms Mw = 1.44 gms Md = 6.72 gms

Graph and Figure: The graph is plotted between Normal Stress (x-axis) and Shear Stress (y-axis) 0.45

y = 0.356x + 0.214 R² = 0.999

0.4

0.48, 0.384

0.35

0.32, 0.33

0.3

s s ) e 2 r 0.25 t S m c / r g a e k 0.2 h ( S 0.15

0.16, 0.27

0.1 0.05 0 0

0.1

0.2

0.3

0.4

0.5

0.6


Obtained Result: The shear strength parameters of the soil sample tested through direct Shear Test in laboratory are found to be C = 0.214 φ = 19.6˚

Comparison with expected result: With the increase in moisture content for the same sample of soil the internal angle of friction is decreased.

SAMPLE NO 3: (Sand)

G29-S3


Sr.

Normal

Dial Gauge

Shear Force “F”

Normal Stress

Shear Stress

No.

Force

Reading

=Dial Gauge Reading

σ= N/A

Τ=F/A

“N”

X 0.15

(Kg/cm2)

(Kg/ cm )

(Kg)

(Kg)

2

1.

4

35

5.25

0.16

0.21

2.

8

45

6.75

0.32

0.27

3.

12

87

13.05

0.48

0.522

This sample is made with adding 8.67% of moisture content. Pan no. = 10-A Empty pan wt =15.36gms Wet wt + pan wt = 25.76 gms Dry wt. + pan wt. =24.93gms =24.93gms Mw = 0.83 gms Md = 9.57 gms

Graph and Figure: The graph is plotted between Normal Stress (x-axis) and Shear Stress (y-axis) 0.6 y = 0.975x + 0.022 R² = 0.887

0.5

s s ) e 2 r t S m c r / g a k e ( h S

0.48, 0.522

0.4

0.3 0.32, 0.27 0.16, 0.21

0.2

0.1

0 0

0.1

0.2

0.3

0.4

0.5

0.6



Comparison with expected result: It is shown from the above value of φ = 44.27˚, that the given sand sample is dense sand of mixed-grain size. (appendix A)

SAMPLE NO 4: (Clay)

G29-S4


Sr.

Normal

Dial Gauge

Shear Force “F”

Normal Stress

Shear Stress

No.

Force

Reading

=Dial Gauge Reading

σ= N/A

Τ=F/A

“N”

X 0.15

(Kg/cm )

(Kg/ cm2)

(Kg)

(Kg)

2

1.

4

55

8.25

0.16

0.33

2.

8

73

10.95

0.32

0.438

3.

12

73

10.95

0.48

0.438

This sample is made with adding 19.11% of moisture content. Pan no. = 37 Empty pan wt =8.03gms Wet wt + pan wt = 22.55 gms Dry wt. + pan wt. =20.22gms =20.22gms Mw = 2.33 gms Md = 12.19 gms


0.45

0.32, 0.438

0.48, 0.438

0.4 0.35 0.16, 0.33 s 0.3 s ) e 2 r t S m c 0.25 r / g a k e ( h 0.2 S

0.15 0.1 0.05 0 0

0.1

0.2

0.3

0.4

0.5

0.6



Comparison with expected result: It is shown from the above value of φ = 18.62˚, that the given given clay sample lies between very soft organic clay and soft, slightly organic clay.

SAMPLE NO 5: (Clay)

G29-S5


Sr.

Normal

Dial Gauge

Shear Force “F”

Normal Stress

Shear Stress

No.

Force

Reading

=Dial Gauge Reading

σ= N/A

Τ=F/A

“N”

X 0.15

(Kg/cm2)

(Kg/ cm )

(Kg)

(Kg)

2

1.

4

44

6.6

0.16

0.264

2.

8

70

10.5

0.32

0.42

3.

12

73

10.95

0.48

0.438

This sample is made with adding 21.077% of moisture content. Pan no. = 14-A

Empty pan wt =8.51gms Wet wt + pan wt = 21.55 gms Dry wt. + pan wt. =19.28gms =19.28gms Mw = 2.27 gms Md = 10.77 gms


0.45

0.32, 0.42

0.48, 0.438

0.4 0.35 0.3 s s ) e 2 r t S m c r / g a k e ( h S

0.16, 0.264

0.25 0.2 0.15 0.1 0.05 0 0

0.1

0.2

0.3

0.4

0.5

0.6



Comparison with expected result:

We have noticed that by increasing the moisture content in the same clay sample the internal friction angle is increased.

Direct Shear Test

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