Introduction Experimental Setup Data Analysis Results Theory

Direct Shear Test

Need and Scope:

The value internal friction angle and cohesion of the soil are required for design of many engineering problems such as foundations, retaining walls, bridges, sheet piling. Direct shear test can predict these parameters quickly.

Concept:

The concept of direct shear is simple and mostly recommended for granular soils, sometimes on soils containing some cohesive soil content. The cohesive soils have issues regarding controlling the strain rates to drained or undrained loading. In granular soils, loading can always assumed to be drained. A schematic diagram of shear box shows that soil sample is placed in a square box which is split into upper and lower halves. Lower section is fixed and upper section is pushed or pulled horizontally relative to other section; thus forcing the soil sample to shear/fail along the horizontal plane separating two halves. Under a specific Normal force, the Shear force is increased from zero until the sample is fully sheared. The relationship of Normal stress and Shear stress at failure gives the failure envelope of the soil and provide the shear strength parameters (cohesion and internal friction angle).

Direct Shear Test

Experimental Setup:

Strain controlled direct shear machine consists of shear box, soil container, loading unit, proving ring, dial gauge to measure shear deformation and volume changes. A two piece square shear box is one type of soil container used. A proving ring is used to indicate the shear load taken by the soil initiated in the shearing plane.

1. Direct shear box apparatus, and Loading frame (motor attached)

2. Dial gauge for vertical deformation measurement

3. Dial gauge for horizontal deformation measurement

4. Proving ring for Shear force measurement. Loads are kept in loading frame for application of Normal stress

5. Components of shear box with porous stone, filter paper etc.

Testing Procedure (IS 2720 Part 13):

1. Check the inner dimension of the soil container, and put the parts of the soil container together.

2. Calculate the volume of the container. Weigh the container.

3. Place the soil in smooth layers (approximately 10 mm thick). If a dense sample is desired tamp the soil.

4. Weigh the soil container, and find the weight of soil. Calculate the density of soil.

5. Plane the top surface of soil, and put the upper grating stone and loading block on top of soil.

6. Measure the thickness of soil specimen.

7. Apply the desired normal load and Remove the shear pin.

8. Attach the dial gauge which measures the change of volume. LC (least count) of dial gauge needs to be noted down.

9. Record the initial reading of the dial gauge and calibration values.

10. Check all adjustments to see that there is no connection between two parts except sand/soil.

11. Start the motor. Take the reading of the shear force and volume change till failure. Proving ring is used to measure the shear force. PRC (proving ring constant) needs to be noted down.

12. Add 5 kg normal stress 0.5 kg/cm^{2} and continue the experiment till failure.

13. Record carefully all the readings. Set the dial gauges zero, before starting the experiment.

Direct Shear Test

Observation Sheet:

Size of the sample = 60 mm x 60 mm x 25 mm

Area of the sample (Cross Sectional ) = 36 sq.cm

Volume of the sample = 90 cm^{3}

Weight of the sample (gm) =

In-situ Density of the sample (gm/cc) =

In-situ water content (%) =

Least count of dial gauge (Horizontal) =

Least count of dial gauge (Vertical) =

Proving Ring No. =

Proving ring constant =

Normal stress (Kg/ sq.cm) = * *

Normal stress = (Applied Normal Load x Lever Ratio) / Cross sectional Area of sample

Test 1: Applied Normal Stress = **0.5 kg/cm**^{2}

Test 2: Applied Normal Stress = **1.0 kg/cm**^{2}

Test 3: Applied Normal Stress = **1.5 kg/cm**^{2}

Calculations:

1. Shear stress (τ) on the horizontal failure plane are calculated as τ = S/A; Where S is shear force. A is the cross sectional area of the sample, which decreases slightly with the horizontal deformations.

2. Corrected area (A_{corr}) needs to be calculated for calculating the shear stress at failure.
A_{corr} = A_{0} (1-δ), where δ is horizontal displacement due to shear force applied on specimen. A_{0} is the initial area of the soil specimen.

3. i. Shear Stress = (Proving ring reading x Proving ring constant)/A_{corr}

ii. Horizontal displacement = Horizontal dial gauge reading x Least count of horizontal dial gauge

iii. Vertical displacement = Vertical dial gauge reading x Least count of vertical dial gauge

4. Shear stress at failure needs to be calculated for all three tests performed at three different normal stresses to plot the failure envelope.

Direct Shear Test

Graphs:

1. Shear stress Vs Horizontal displacement relationship for tests performed at Normal Stress of **0.5, 1.0 & 1.5 kg/cm**^{2}

2. Shear stress Vs Normal stress (Failure envelope).

Shear stress Vs Horizontal displacement curve

Shear stress data at failure for tests at Normal Stress of 0.5, 1.0 & 1.5 kg/cm^{2}

***All three tests were performed at deformation rate =0.25mm/min**

Shear stress Vs Normal stress (Failure envelope)

Shear strength parameters of soil

Cohesion (c) = 0 kPa

Internal friction angle (Φ) = 34 deg

General Remarks

1. In the shear box test, the specimen is not failing along its weakest plane but along a predetermined or induced failure plane, i.e. horizontal plane separating the two halves of the shear box. This is the main draw back of this test. Moreover, during loading, the state of stress cannot be evaluated. It can be evaluated only at failure condition, i.e Mohr's circle can be drawn at the failure condition only. Also failure is progressive.

2. Direct shear test is simple and faster to operate. As thinner specimens are used in shear box, they facilitate drainage of pore water from a saturated sample in less time. This test is also useful to study friction between two materials - one material in lower half of box and another material in the upper half of box.

3. The angle of shearing resistance of sands depends on state of compaction, coarseness of grains, particle shape and roughness of grain surface and grading. It varies between 28 deg (uniformly graded sands with round grains in very loose state) to 46 deg (well graded sand with angular grains in dense state).

4. The volume change in sandy soil is a complex phenomenon depending on gradation, particle shape, state and type of packing, orientation of principal planes, principal stress ratio, stress history, magnitude of minor principal stress, type of apparatus, test procedure, method of preparing specimen etc. In general loose sands contract and dense sands expand in volume on shearing. There is a void ratio at which either expansion contraction in volume takes place. This void ratio is called critical void ratio. Expansion or contraction can be inferred from the movement of vertical dial gauge during shearing.

5. The friction between sand particle is due to sliding and rolling friction and interlocking action.

The ultimate values of shear parameter for both loose sand and dense sand approximately attain the same value so, if angle of friction value is calculated at ultimate stage, slight disturbance in density during sampling and preparation of test specimens will not have much effect.

Direct Shear Test

Theory:

In direct shear test, square/rectangular specimens are used unlike most of the other shear strength tests. Cylindrical (Circular) specimens are used in most of the shear strength tests such as Triaxial, UC test etc. Both shapes of specimen have their own pros and cons. In square/rectangular specimen, it is easy to calculate the magnitude of qualitative shear stress along the surface, whereas in circular section, we get uniformity of shear stress in the specimen.

When soil sample is subjected to horizontal load, stresses are also produced at the vertical boundaries. However, vertical sides are normally greased in direct shear setup to prevent any shear stress at these boundaries during normal stress application stage and this lubrication presumably carries over to the shearing stage at given normal stress. Hence, no shear stress can exist at both vertical and horizontal surface, and consequently the shearing stresses must also be non-uniform on the horizontal faces.

In direct shear test, strain responsible for shear resistance is different from displacement between the two halves divide by specimen thickness because most of the distortion occurs in a thin zone of unknown thickness. Therefore, it is not easy to calculate other than qualitative stress-strain data from the test.

Since only normal and stress stresses can be calculated through this test, it is difficult to draw Mohr circle giving state of stresses. However, by drawing a perpendicular to failure envelope at a particular normal stress, a semi-circle can be plotted. Its center will be point on normal stress axis(x-axis) cut by perpendicular line which represents the radius of semi-circle.

The porous stone is not needed for tests on dry soils, but it is very essential for tests on moist or saturated soils. Mostly, shear strength of soil is measured under Wet condition (moist sample) in Direct shear test.

Change in volume is directly proportional to change in thickness of soil sample since cross-sectional area remains constant. Dense soils usually show contraction initially, and then dilate (expand in volume); however, loose soils show only contractive (decrease in volume) response; as shown below.

*Limitations*

• It is difficult to control the drainage conditions.

• Lateral pressures and stresses on planes other than the plane of shear is not known.

• Pore water pressure cannot be measured.

• The shear failure plane is predefined which might not be weakest one.

• Shear stress distribution on the failure plane is non-uniform.

• The contact area between the soil and in the two halves of the box decreases as the test proceeds.

• Field conditions cannot be simulated.

*The direct shear can be used to measure the shear strength parameters at the interface of soil and foundation material by keeping the foundation material in the bottom half and the soil can be placed above it.*