What is the direct shear test and why it is required?
Shearing strength of the soil is the magnitude of maximum shearing resistance offered by the soil when subjected to shear stress. The friction and interlocking of soil particles cause shearing resistance in soil.
This Shearing resistance between soil may be due to chemical bonding and the cementation of soil particles.
The shearing strength of the founding soil determines the type and design procedure of various civil structures.
The shearing strength of a soil material can be quantified using the Mohr-Coulomb Failure Criterion resulting in the following equation,
Shear strength formula
Various methods can be implemented to determine the shear strength of soil material under various conditions.
Among which one of the most common and simplest methods is the direct shear test using shear box apparatus which can be used to determine the shear strength parameters, cohesion (c), and internal angle of friction (φ) under various normal stress and shear strain rate conditions.
This lab test can be conducted for both undisturbed and remolded specimens under 3 different drainage conditions (unconsolidated undrained, consolidated undrained, and consolidated drained). This test can be conducted for both cohesive and non-cohesive soil samples.
- Sample Trimmer or Core Cutter, Rammer, Spatula, Straight Edge
- Loading Frame, Weights, Proving Ring, Micrometer Dial Gauge-2
- Shear box, grid plates (1 pair Plain, 1 pair perforated), 1 pair porous stones plates, 1 pair metal plates, 1 base plate, loading pad with steel ball, connecting pins, spacing screws
- Stop Watch, Weighing Balance and Desiccator
Procedure for Direct shear test
The test procedure consists of following steps,
Preparation of specimen
- Undisturbed samples can be obtained using sample trimmer or core cutter rammed into the field site and dugout. The sample obtained is thus leveled using a spatula.
- Remolded sample can be prepared by compacting soil inside the shear box under various in situ condition depending on soil type or either outside the box and trimmed into required size
- Size of sample vary from 60mm*60mm to 100mm*100mm depending on the size of the shear box and usually 20-25mm thick.
- The prepared sample is placed inside the shear box (2 halves secured with connecting pins and spacing screws) in the order of base plate-metal plate-plain grid plate- soil sample-plain grid plate-metal plate and loading pad with steel ball, from bottom to top.
- This order is for undrained unconsolidated test
- Serration of grid plates are perpendicular to the direction of shear
- The initial water content of the sample is needed to be measured
- The whole shear box is then secured inside the shearing apparatus (carriage box) provided with a water jacket to prevent the drying of the sample.
- Proving ring is placed forming contact with u shaped bend on the upper half of the shear box.
- Loading yolk is placed forming contact with the steel ball of the loading pad, which is used to apply normal stress.
- 2 Dial gauges are placed accordingly to measure longitudinal(shear) and vertical(normal) displacement of the sample during the test.
- Dial gauges and proving ring are set to zero.
- The upper half of the shear box is raised using spacing screws such that a gap of 1mm is left between the two halves so that a definite shear plane is defined on the sample, followed by the removal of connecting pins.
- Now the test is carried out by applying required normal stress (initial 25KN/m2) and longitudinal displacement at a rate such that no drainage can occur in the sample during the test.
- The shear load readings indicated by the proving ring assembly and the corresponding longitudinal displacements and vertical displacements should be noted at regular intervals (recommended 30sec) until the failure of the sample which is indicated by a kickback of pointer in dial gauge or until the longitudinal displacement exceeds 12mm. (20 % of the dimension of the specimen in direction of shear)
- After the end of the test. the final water content of the specimen is to be measured.
- This test shall be carried out for 3 to 4 samples of similar density and water content with varying normal stress which should correspond to the field values and design requirements.
Note: For the consolidated undrained condition and consolidated drained conditions the plain grid plate and metal plates are replaced by perforated grid plate and porous plates simultaneously.
Additionally, the specimens are initially compacted and consolidated completely by applying normal stress along with measuring vertical displacements to check the completion of the consolidation process (same normal stress which is applied during the consolidation process should be applied during the shear test as well).
Aftermath, for the consolidated undrained test, shear stress is applied to the specimen at a rate that prevents drainage of water from specimen whereas in case of consolidated drain test the shear stress is applied at such rate which results in the dissipation of at least 95% of pore pressure during the test.
Tables and Calculation for Direct shear test
- Soil Specimen Measurements
Dimensions:………………. Area of specimen: ……………..
Thickness: ………………. Volume of specimen: …………….
the initial wet weight of specimen: ………………….
Moisture content:…………….. (Average of …………tests)
The final wet weight of the specimen: …………….
The moisture content at shear zone: ………………
- Consolidation stage (For consolidated undrained and consolidated drained test)
Hanger load: …………. Applied load: ……………….
Normal stress: ………..
|Date and Time||Vertical Dial Reading||Vertical Dial Difference||Thickness of Specimen|
- Shearing Stage (constant strain shearing)
Rate of Shearing ………………………… mm/min
LC of Dial gauge ………………….mm
|Date and Time||Displacement Dial Reading||Displacement, δ||Area Correction||Corrected Area A||Proving ring Dial Reading||Shear Force||Shear Stress||Vertical Dial Reading||Vertical Dial Difference||Thickness of Specimen|
II. Shear Stress = Shear force / Corrected area
Plot Shear stress and longitudinal displacement curve and find :
(a) Maximum shear stress and
(b) Corresponding shear displacement
Summary of Results – Report for direct shear test
|Test No.||Normal Stress||Shear Stress at Failure||Shear Displacement at Failure||Initial Water Content||Final Water Content||Remarks|
Plot Shear stress at failure and normal stress curve and find :
(a) Cohesion intercept and
(b) The angle of Shearing resistance
- The density of soil sample is calculated from the mass of soil and volume of the shear box as = W/V.
- The dial gauge is converted to the appropriate length and load units.
- The vertical ( normal) stress is computed as follows;
The shearing rate must be slow enough to allow nearly complete dissipation of excess pore pressure during the drained tests.
The shearing rate must be slow enough to prevent the buildup of pore pressure during undrained tests.
Sample must be handled carefully during placement.
The grooves of the grid plate must be normal to the direction of shear strain.
The upper half of the shear box must be lifted carefully without disturbing the sample resulting in cracks.
Connecting pins must be removed before applying shear stress.
Advantage of direst shear test:-
- The direct shear test is simple.
- Since the thickness of the sample is small quick drainage and hence rapid dissipation of pore pressure is possible.
- For conducting drained tests on cohesionless soils, it is identically situated.
- The apparatus is relatively cheap.
what are the major drawbacks in the direct shear test for soils?
- 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, which is the main drawback of this test.
- State of stress can be evaluated only at failure condition i.e. Mohr’s circle can be drawn at the failure condition only, thus we cannot determine the state of stress during loading.
- The distribution of stresses along the shear plane is not uniform. The stresses are more at the edges which leads to progressive failure concluding that full strength of the soil is not mobilized throughout the entire failure plane.
- The continual decrease in the contact surface of the two halves during the test produces a small error on the shear and normal stress and affects the Mohr-Coulomb failure envelope.
- There is no control over the drainage condition hence pore pressure cannot be measured. As a result of which this test gives satisfactory results in drained conditions only.
Why is the direct shear test typically not performed on clayey soils
Direct shear test is usually conducted for granular soil especially under drained condition.
This test has no proper measure to control the drainage condition or to determine the pore pressure thus during undrained conditions there is no method to determine the effective normal stress (σn = σ – u).
As a result of which we have to use the total normal stress (σ) resulting in values of the shear parameter (c and φ) being overestimated.
On the other hand, during drained test pore water pressure is gradually dissipated (u=0) resulting in effective normal stress being equal to total normal stress (σn = σ).
Thus a more reliable value of shear parameters can be determined. This concludes that the direct shear test is usually suitable in undrained conditions.
Additionally, the longitudinal strain applied for the undrained test condition of any soil type is about 1-2mm/min. whereas for the drained conditions, it depends on the soil type. For sandy soil longitudinal strained applied for shear is about 0.2mm/min whereas for clayey soil it ranges from 0.02 to 0.005mm/min.
The slower rate of application of longitudinal strain results in longer test duration (2- 5days) for the clayey soil which seems to be uneconomical and lengthy in comparison to other suitable tests. As a result, a direct shear test is not typically carried out for clayey soils.
I hope this article remains helpful for you.
Happy Learning – Civil Concept
Civil Engineer – Jenish Shakya