SOIL MECHANICS PRACTICAL REPORT

MODULE 11

Direct Shear Test

GROUP PI 1

Asti Diar Syafitri 1206292414

Christopher Kevinly 1206223846

Wednesson Lawijaya 1206230593

Date of Practicum : 4th of May 2014

Laboratory Assistant : Sandhamurti P.

Date Approved :

Grade :

Assistant’s Signature :

Laboratory of Soil Mechanics

Department of Civil Engineering

Faculty of Engineering

University of Indonesia

Depok

2014

11.1. Introduction:

11.1.1. Objective of the Field Test:

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. Brief 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 mathematically:

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)

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 48

o.

Table 11.1. The value of shear angle for some types of sands

(Source: Das M. Braja, “Advanced Soil Mechanics”)

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.

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. Experimental Data

Diameter of the shear box = 6.315 cm (averaged from 3 measurements)

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 11.2. Experimental Result

Time (s) Vertical Load (Kg)

5 10 15 20 25 30

15 26 45 48 68 107 96

30 29 63 62 82 121 121

45 31 70 72 94 129 133

60 33 75 80 101 138 144

75 34 77 87 110 143 150

90 34 77 91 112 148 157

105 34 78 93 114 152 159

120 78 99.5 118 154 162

135 78 110 120 154 170

150 110 124 156 172

165 110 126 156 176

180 126 158 176

195 126 158 179

210 158 180

225 180

240 180

11.4. Data Processing

The calculation itself involves the calculation of both normal force

and shear stress. All the calculation is done through computation by Ms.

Excel, but an example is provided explaining how to do the calculation.

To determine Normal Stress:

At vertical load = 5kg

Total load = 5.84215 kg

Sample Area = 31.321 cm2

To determine Shear Stress:

Max Horizontal Dial Reading = 34

Table 11.3. Calculation Result

Vertical Load 5 10 15 20 25 30

Cap & Ball Load (Kg) 0.84215 0.84215 0.84215 0.84215 0.84215 0.84215

Total Load (Kg) 5.84215 10.84215 15.84215 20.84215 25.84215 30.84215

Sample Area (cm2) 31.321 31.321 31.321 31.321 31.321 31.321

Normal Stress (Kg/cm2) 0.186525 0.346162 0.5058 0.665437 0.825074 0.984712

Max Horizontal Dial Reading

34 78 110 126 158 180

Horizontal Shear Force (Kg)

5.1 11.7 16.5 18.9 23.7 27

Horizontal Shear Stress (Kg)

0.16283 0.373551 0.526803 0.603429 0.756681 0.862041

Graph 11.1. Stress graph

In order to find the cohesion (c) and shear angle (φ), linear

regression needed to be done:

Table 11.4. Table of Linear Regression

X Y X2 Y2 XY

0.186525 0.16283 0.034792 0.026514 0.030372

0.346162 0.373551 0.119828 0.139541 0.129309

0.5058 0.526803 0.255833 0.277522 0.266457

0.665437 0.603429 0.442806 0.364127 0.401544

0.825074 0.756681 0.680747 0.572566 0.624318

0.984712 0.862041 0.969657 0.743115 0.711248

2.528998 3.285336 2.503664 2.123384 2.163248

∑ ∑ ∑

∑ ∑

∑ ∑ ∑ ∑

∑ ∑

Kg/cm2

y = 0.8451x + 0.0526

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.2 0.4 0.6 0.8 1 1.2

She

ar S

tre

ss (K

g/cm

2 ) (τ)

Normal Stress (Kg/cm2) (σ)

σ vs τ Graph

To find φ:

Φ = tan-1

(m)

Φ = 40.201o

11.5. Analysis

11.5.1. Experimental Analysis

This experiment is aimed to determine the value of cohesion (c)

and shear angle (φ) of the soil sample. Cohesion and shear angle are two

important factors which influences the shear strength of the soil (τ). This

correlation is expressed as below:

This equation shows that the larger the value of c and φ, the more τ

occur on the soil.

This experiment is prepared by measuring the internal diameter of

the shear box; this is done so the surface area of the sand can be

determined. After that, the cap and the ball should also be weighted since

both of them also contribute to the total load which acts on the soil sample.

The weight of can should be measured in order to accurately measure the

water content of the sand sample.

The sand also should be prepared. The sand should pass sieve no.

18 ASTM in order to be tested, since larger grains of sand will not provide

accurate result because of its less homogenous property, which will leads

to an inaccuracy in its stress distribution. The sand is hydrated so it can be

moulded easier.

In order to keep the soil sample stable during the filling phase, key

should be fastened well on the shear box. This is done in order to prevent

translation which may alter the structure of the soil. During the filling

process, the sample should be compacted often (once every ¼ part of the

shear box). This is done to make sure that the soil sample is packed and

the structure is good. It is not full so the cap can be installed. Since the cap

should be installed, the surface should be flat. It explains why the soil

surface should be flattened before putting the cap and ball on it.

The shear box is then moved into the apparatus for testing. No

translation should occur during this moving process in order to keep the

structure of the compacted soil stabile. After installing the shear box into

the testing apparatus, the lock can be cleared and the horizontal dial can be

set so that the dial can accurately represent the translation of the sample.

Before starting the testing, note that the force dial should shows 0 on its

value.

The loading process itself also needed to be done carefully. To

ensure uniform stress distribution, the ball of the cap should perfectly hit

the curvature at the loading beam of the apparatus. The translation rate of

1mm/min should be properly maintained, because any change of speed of

translation will potentially disturb the soil sample, initiating an early

failure.

11.5.2. Result Analysis

After testing the water content of the sand, the water content was

16.492%. Unfortunately, this value may not represent the real water

content, since the water content of the sand was measured after the sand

was exposed to open air for about 90 minutes, which makes a decrease of

water content possible. The complete detail of the error will be assessed in

the error analysis.

During the experiment, it can be observed that the shear strength

gauge that in the early loading, the increase of the shear strength increases

highly. As the translation continues the increase of the shear strength

decreases until the shear strength is in a terminal value. The terminal value

itself is the maximum shear strength; if the soil sample was translated

more, the soil cannot provide more support to hold the shear stress, so it is

said to be its maximum soil strength.

After doing the practical activity, data is gained from the six

attempts. Note that in the least loading, the dial reading shows smaller

value compared to more loaded cases. This shows that there is a positive

correlation between normal force and shear force. This fulfils the equation

given in the early part of the analysis.

From the calculation, it is can be determined that the cohesion of

the soil is c = 0.0526 Kg/cm2. This value is very small, which is provided

by the water content. The sand itself should not possess any cohesion, so

this value is dependable. The value of φ = 40.201o is also very reasonable,

since the value of φ for sand varies from 28o to 48

o. This value of shear

angle indicates that the sample is categorized as dense angular-grained

sand. In general, this field test is a success despites of the errors occurred.

11.5.3. Error Analysis

When doing this experiment, there are some minor problems faced.

One of them is when separating the sands from impurities by using a no.18

sieve. The sand was very wet at that time, so extra effort should be done in

order to separate the sand. Because the sieve was shaken too much, a little

amount of sand jumps into the container. This may influence the result,

even the influence may be very small.

The next mistake is when pouring the sand into the shear box.

During the compaction of the soil, the compaction may not be well-

distributed, which may result in stress distribution issues. One of quite

major problem in this experiment was the gap between the third and the

fourth test was too large; nearly 1 hour. This may cause a considerable

decrease of water content of the sand. This error is expressed in the graph,

since the line formed by the first three samples seems to be a parallel line

to the line formed by the last three samples. This shows a decrease of

cohesion. The decrease itself may be caused by a decrease of water

content.

The shear effect is created manually, which enables more errors to

happen. In this experiment, it is nearly impossible to create a perfectly

constant rate of translation (1mm/minute). This alteration may cause some

uncertainty on the result. Note that the gauge reading may not be accurate

too, since the limitation of the one who read the dials, or that the scale of

the dial is too large and the gauge is still moving during the reading.

11.6. Implementation

As stated before, direct shear test is done to find the value of

cohesion and the shear angle, which are very important in nearly every

geotechnical design. The advantage of using this method is that this test is

easy to be carried on, fast, and widely available. In the field, this type of

test is suitable to investigate fast and the structure which is going to be

constructed is not very heavy. This test is also done when more advanced

test is not available (in a small town, for example, where triaxial apparatus

is not available).

11.7. Conclusion

This soil investigation is carried out in order to find the value of cohesion

(c) and the shear angle (φ) of the soil.

The soil being tested possesses a value of c of 0.0526kg/m2 and φ of

40.201o

The advantage of this testing is its simplicity, speed and availability.

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

Lambe T.W. “Soil Testing for Engineers”. John Willey and Sons. New

York. 1951.

11.9. Attachment

Shear box, Cap and Ball

Direct Shear Apparatus