Direct Shear Asti

SOIL MECHANICS 2 PRACTICAL REPORT

MODULE 11

DIRECT SHEAR TEST

INTERNATIONAL PROGRAM GROUP

Adam Yuta Prayoga 1206292370

Asti Diar Safitri 1206292414

Nathan 1206292420

Date of Practicum : 4/5/2014

Practicum Assistant :

Date of Approval :

Score :

Assistant’s Signature :

SOIL TESTING LABORATORY

DEPARTMENT OF CIVIL ENGINEERING

FACULTY OF ENGINEERING

UNIVERSITY OF INDONESIA

DEPOK

2014

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11.1. Introduction:

11.1.1. Objective:

To determine the value of the cohesion (c) and the angle of shear

(φ), and the water content of a soil sample.

11.1.2. Apparatus:

Direct shear test apparatus and shear box

Weight (5kg, 10kg, 15kg, 20kg, 25kg)

2 dial gauges for vertical and horizontal displacement

Specimen cutter for cutting cohesive soil

Tamper to compact cohesionless soil

Scale with an accuracy of 0.01 gr

Calipers with an accuracy of 0.01mm

Stopwatch

Can

Oven

11.1.3. Basic Theorem:

The shear strength of the soil sample can be measured directly by a

constant vertical (normal) loading on the sample and an addition of a

certain amount of shear force with a constant, slow rate to keep the pore

water pressure at zero, so the ultimate shear strength can be determined.

The normal stress can be determined by a division of the amount of

normal force and the shear-plane area or S= P/A

The shear stress can be determined by calculating the shear force

(G), which is gained from the maximum reading of the load ring dial after

being multiplied by the value of prooving ring calibration (LRC)

T=G / A

G=M ×LRC

LRC=0.15kg /¿

Universitas Indonesia

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In some reference books, it is stated that the cohesion of sand is (c)

= 0, and the shear angle (φ) of sands ranging from 28o to 48o. The table

below is used to classify the type of soil using the value of shear angle:

Table 11.1.3.1 The value of shear angle for some types of soil

11.2. Procedure:

11.2.1. Preparations

a. Measure the inner diameter of the cylinder in the shear box.

b. Balance the counterweight system so it is able to provide normal force

on the shear box.

c. Weight of the shear box’s cap, the shear box ball and the can.

d. Prepare the sands. Clean the sand from any kinds of impurities or

gravels by using sieve no. 18 ASTM.

e. Take a small portion of the sand, weight it and oven-dry it in order to

seek for the water content.

11.2.2. Practical Activity:

a. Insert the sand into the shear box. Fill ¾ part of the shear box with

sands, compact it with 15 tamper blows for each ¼ part is fulfilled. Do

not forget to put a lock on the shear box to prevent any translation.


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b. The surface of the sand should be flattened by using spatula or tamper.

The surface is then covered by the shear box’s cover and its ball.

c. Put the shear box in the apparatus, apply 5kg of loading, then release

the shear box’s key.

d. Set both of the horizontal and loading ring dials into 0

e. The shear box is given a shear force by applying the force in 1

mm/minute.

f. The horizontal dial readings should be read every 15 seconds until the

loading dial stop turning.

g. Repeat step a-f for the loading of 10, 15, 20, and 25 kg.

11.3. Data Tabulation

Properties:

Diameter of the shear box = 6.315 cm

Sample Area = 31.321 cm2

Loading rate = 1mm/min

LRC = 0.15 kg/div

Weight of cap and ball = 0.84215 kg

Water Content = 16.492%

Table 11.3.1 Data Tabulation

Time (s)Vertical Load (kg)

5 10 15 20 25 3015 26 45 48 68 107 9630 29 63 62 82 121 12145 31 70 72 94 129 13360 33 75 80 101 138 14475 34 77 87 110 143 15090 34 77 91 112 148 157

105 34 78 93 114 152 159120 78 99.5 118 154 162135 78 110 120 154 170150 110 124 156 172165 110 126 156 176180 126 158 176195 126 158 179


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210 158 180225 180240 180

11.4 Data Processing

These are how to calculate the normal stress and shear stress from the data

gained from the direct shear test. The results are input in a table to ease the result

reading.

Normal Stress (σ )

Vertical load = 10 kg

Total load = 10.84215 kg

Area of sample = 31.321 cm2

σ=10 .8421531.321

σ=0.346kg

cm2

Shear Stress (τ )

Max Horizontal Dial Reading= 78

LRC=0.15kg¿

G=78×0.15

τ=78×0.1531.321

τ=0.373kg /cm2


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Table 11.4.1 Calcuation Results

Table 11.4.2 Shear Stress and Normal Stress

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.10

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

f(x) = 0.845142857142858 x + 0.052624850511067

σ vs τ Graph

Normal Stress (Kg/cm2) (σ)

Shea

r Str

ess (

Kg/c

m2)

(τ)


Vertical Load 5 10 15 20 25 30Cap & Ball Load (Kg) 0.84215 0.84215 0.84215 0.84215 0.84215 0.84215Total Load (Kg) 5.84215 10.84215 15.84215 20.84215 25.84215 30.84215Sample Area (cm2) 31.321 31.321 31.321 31.321 31.321 31.321Normal Stress (Kg/cm2) 0.186525 0.346162 0.5058 0.665437 0.825074 0.984712Max Horizontal Dial Reading 34 78 110 126 158 180Horizontal Shear Force (Kg) 5.1 11.7 16.5 18.9 23.7 27Horizontal Shear Stress (Kg) 0.16283 0.373551 0.526803 0.603429 0.756681 0.862041

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Table 11.4.3 Linear Regression

X Y X2 Y2 XY0.186525 0.16283 0.034792 0.026514 0.0303720.346162 0.373551 0.119828 0.139541 0.129309

0.5058 0.526803 0.255833 0.277522 0.2664570.665437 0.603429 0.442806 0.364127 0.4015440.825074 0.756681 0.680747 0.572566 0.6243180.984712 0.862041 0.969657 0.743115 0.711248

3.51371 3.285335 2.503663 2.123385 2.163248

m=¿¿

m=0.8451

c=¿¿

c=0.0526 Kg/cm2

Φ = tan-1(m)

Φ = 40.201o

11.5 Analysis

11.5.1 Procedure Analysis

The objective of this experiment is to determine the value of the

cohesion (c) and the angle of shear (φ), and the water content of a soil

sample. To do this experiment several apparatus and steps are required.

The main apparatus used in this experiment is direct shear test

apparatus and shear box. Another apparatus needed are weight (5kg, 10kg,

15kg, 20kg, 25kg), 2 dial gauges for vertical and horizontal displacement,

specimen cutter for cutting cohesive soil, tamper to compact cohesionless

soil, scale with an accuracy of 0.01 gr, calipers with an accuracy of

0.01mm, stopwatch, can, and oven. All the apparatus are provided in soil

testing laboratory, Department of Civil Engineering UI.

First thing to do is sieving the sample of soil, which is going to be

tested. To sieve the sample, students use sieve no. 18 ASTM. Fine grains

of soil are preferable due to their homogenous properties that lead to more


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accurate results. Fine grains will give more accurate results because the

stress will be disturbed properly. The sample must be added by several

amount of water in order to ease the compaction process. The can used as

the container of the sample of soil also must be weighted to determine the

water content.

The next step is measuring the diameter of the shear box. Students

only measure the internal diameter because the internal diameter is used in

determining the surface of the sample of soil. After that, students weight

the cap and the ball that also act as the load acting on the sample of soil in

the direct shear test.

The last step of preparation is soil compaction. To prevent the

sample of soil from moving two keys must fastened. The filling process

must be conducted with several compactions. Every soil addition of 1/3

part of the shear box must be compacted by 15 times with constant

strength. This step is repeated until the volume has reached ¾ of the shear

box, which is signed by a dot sign inside the shear box. After the sample is

compacted, place the cap and the ball on it. In placing the cap, the pattern

of the cap must be the same with the pattern that is located at the bottom of

the shear box. This is done in order to make sure the shear stress is well

distributed.

After all the preparation steps are finished, the experiment is

started. The first thing to do is placing the shear box that contains of the

sample to the direct shear apparatus. This step must be done carefully, to

prevent the soil translation that may damage the soil structure. After being

placed, remove the key and set the dial to zero as the starting point. In the

loading process, students should make sure the ball, which is used to

distribute the load, touches the curvature. The direct shear test apparatus is

used manually; therefore the students have to maintain the translation rate

of 1mm/min properly in order to prevent error data. Students use

stopwatch and the dial to keep the constant translation rate. This is done to

prevent early failure. The horizontal dial reading is done every 15 seconds


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until the loading dial stop turning. These steps are repeated for weight of

10, 15, 20, 25 , and 30 kg.

11.5.2 Result Analysis

As we know the cohesion and shear angle are two factors that

influences the value of shear strength. The relation between the two factors

and the shear strength is shown in equation below:

τ=c+σ tanφ

where τ is the shear strength, c is the cohesion, σ shear stress, and

φ is the shear angle. From this equation, we know that the shear strength

increases while the shear angle and the cohesion also increase.

In the reading process, the shear strength gauge shows the high

increasing value of strength. However, after the direct shear test apparatus

being turned constantly, the gauge shows the small increasing value of

shear strength. The test is still continued until the shear strength gauge

shows the 3 (three) the same value. These values represent that the soil has

reached its maximum shear strength.

In the experiment, the direct shear test is conducted 6 (six) times

with different weight (5,10, 15, 20, 25, 30 kg). From the table 11.3.1, we

can see that the gauge shows bigger value in soil in more loaded condition.

For example, in the weight of 5 kg, the maximum shear strength is reached

when the direct shear test apparatus is turned for 105 seconds while in the

weight of 30 kg, the maximum shear strength is reached at 240 seconds.

Through some calculations, normal stress and shear stress can be

obtained. The combination of normal stress and shear stress is plotted to a

graph. From the graph we can see the graph the normal stress increases as

the shear stress increases. By using linear regression method, the shear

angle is obtained, which has value of 40.201o. According to table 11.1.3.1

the type of the soil tested is dense angular grains sand because the value of

the shear angle is in the range 40-45. The cohesion also can be determined

by using linear regression method, which has value of 0.0526 Kg/cm2.


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11.5.3 Error Analysis

The water content may loss during the experiment makes the shear

strength read inaccurate.

The compaction is conducted inconstantly so that the shear stress is

not well distributed.

The direct shear apparatus is not turned in constant translation rate

makes the soil fail early.

Error dial reading may occur due to human error.

11.5.4 Application

Direct shear test is used to determine the shear angle and the

cohesion. This test is very important in geotechnical design. In

geotechnical design, direct shear test is conducted in building a structure

that is not to heavy so that the construction does not require precise

calculation of shear stress. To obtain more accurate calculation of shear

stress, triaxial test is more preferable. The advantage of the direct shear

test itself is it is simpler to be done compared to triaxial test that requires

more complicated procedures and analysis.

11.5.5 Conclusion

This experiment has an objective to determine the value of the

cohesion (c) and the angle of shear (φ), and the water content of a

soil sample.

The value of cohesion obtained is 0.0526kg/m2 and while the value

of shear angle is 40.201o

Direct shear test is conducted in simple structure while the triaxial

test is conducted in building more complicated structure.

11.8. References

Budhu, Muni. “Soil Mechanics”. John Willey and Sons. New York. 2011

Craig, R.F. “Craig’s Soil Mechanics”. Spon Press. New York. 2004

Das, Braja. “Principles of Foundation Engineering”. Thomson. Toronto.

2007


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11.9. Attachments

Figure 11.9.1 Direct Shear Test Apparatus

Figure 11.9.2 Shear Box


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Direct Shear Asti