In this article we will discuss about:- 1. Apparatus for Conducting Direct Sheer Test 2. Preparation of the Soil Specimen 3. Assembly of the Apparatus 4. Test Procedure 5. Determination of Shear Parameters 6. Determination of Principal Stresses 7. Merits 8. Demerits.
Apparatus for Conducting Direct Sheer Test:
The equipment for the direct shear test consists of a square or a circular shear box made of brass or gun metal. A square box of internal dimensions 6 x 6 cm is more commonly used. The shear box is split into two halves, along the horizontal plane, at its mid-height. The two halves of the shear box are held together by locking pins to facilitate assembly of the apparatus and placing the soil specimen before conducting the test. Suitable spacing screws are used to separate the two halves of the shear box, just before conducting the test.
Preparation of the Soil Specimen:
The soil specimen can be transferred into the shear box from the undisturbed soil sampler with the help of a specimen ring, which has inner dimensions same as those of the shear box. The soil specimen may also be transferred from the compacted soil sample of a compaction mold in the same manner, if the test is to be performed on a remolded sample.
Assembly of the Apparatus:
The two halves of the shear box are held together by the locking pins. The base plate of the shear box is placed at the bottom and a porous stone and a lower grid plate are placed over the base plate, as shown in Fig. 13.8. The soil specimen from the specimen ring is carefully transferred into the shear box onto the lower grid plate. The upper grid plate is placed over the soil specimen. The grid plates should be placed in such a manner that the serrations of the grid plates are normal to the direction of the shear load.
The top porous stone is placed over the upper grid plate and a pressure pad is placed on the porous stone. A pressure ball is placed in the central recess of the pressure pad. The pressure pad ensures uniform distribution of the normal stress over the entire area of the specimen and the pressure ball ensures axial application of the normal load.
Solid grid plates are used for conducting undrained test and no porous discs are used. For drained tests, perforated grid plates and porous discs are used.
Test Procedure for Conducting Direct Shear Test:
Direct shear test is conducted in the following steps:
1. The shear box is placed in a large container and is tightly held in position at the bottom in the container. The container is supported over rollers to facilitate lateral movement of lower-half of the shear box when shear force is applied to the lower shear box through a geared jack. The complete equipment of the direct shear test is shown in Fig. 13.9.
2. A normal load is applied through a loading yoke, placed over the pressure ball on the pressure pad. The required normal stress of 0.5 kgf/cm2 (or 50 kN/m2) is applied. The shear deformation dial gauge is placed in position. For drained tests, the soil specimen is allowed to consolidate under normal load.
3. The locking pins are removed and the upper box is slightly raised using spacing screws and then the shear load is applied to the lower-half of the box through a geared jack such that the lower-half moves at a constant rate of strain. The proving ring dial gauge readings are taken at regular intervals of deformation dial gauge readings.
4. The test is continued till the shear load reaches a maximum value and then decreases.
5. The shear load is then released, and the proving ring and deformation dial gauges as well as the shear box are dismantled.
6. The test is repeated with three normal stresses of 100,200, and 400 kN/m2.
Determination of Shear Parameters:
Shear stress at failure, corresponding to each normal stress, is computed from –
The area of the specimen resisting the shear force gradually decreases due to shear deformation. To calculate the shear stress at any deformation, the corrected area (Ac), computed from Eq.(13.8),is to be used.
Ac = l × lc = l × (l – δl) …(13.8)
where Ac is the corrected cross-sectional area in cm2 and δI the shear deformation in cm.
A graph is plotted between the normal stress on the x-axis and the shear stress on the y-axis, as shown in Fig. 13.10 that gives the Mohr-Coulomb failure envelope. The slope of the Mohr-Coulomb failure envelope is taken as the angle of shearing resistance (ɸ) and the y-intercept of the envelope is the cohesion (c). The shear stress-shear strain relation for dense sand and loose sand is shown in Fig. 13.11. The volume change during shear is shown in Fig. 13.12 as a function of shear strain.
Determination of Principal Stresses:
The magnitude of principal stresses and the direction of principal planes with respect to the failure plane can be determined as described below:
i. Magnitude of Principal Stresses:
Figure 13.13 shows Mohr’s circle of stresses at failure in direct shear test. ED is the failure envelope tangential to Mohr’s circle at D with an angle ɸ with the x-axis. Extension of failure envelope ED backward intersects the x-axis at F on the negative side of origin O.
Here O is the origin and C is the center of Mohr’s circle, OE = c = cohesion, DG is the shear stress at failure = τ, and OG is the normal stress at failure = σn. In ΔFDG, ∠DFG = ɸ; therefore, ∠FDG = 90 – ɸ.
Therefore, we have –
CD = DG/cosɸ = τ/ cosɸ …(13.9)
Where CD = radius of Mohr’s circle = (σ1 – σ3)/2
Substituting the value of CD in Eq.(13.9),we have –
Substituting the value of CD in Eq.(13.9) in Eq.,(13.12) –
ii. Inclination of Principal Planes:
Consider ΔPAH, <PAH = θ2 = Angle of minor principal plane with failure plane, therefore –
Consider ΔPAH, <PAH = θ2 = Angle of minor principal plane with failure plane. Substituting the values of PH and AH in Eq. (13.15), we get –
Merits of Direct Shear Test:
i. The test is simple and convenient. The preparation of soil specimen for the test is easy.
ii. Undrained tests can be conducted with shear box very rapidly. Drained tests on cohesionless soils can also be completed in a very short time compared to that in other type of shear tests. Thus, shear box test is very advantageous where time is at premium.
iii. The test does not need skilled person and even a novice can quickly learn to conduct the test with little training.
iv. The equipment used for the test is simple and less costly.
Demerits of Direct Shear Test:
i. The failure plane in the test is predetermined (horizontal), which may not be the weakest one. This results in over-estimation of shear strength of the soil, which is unsafe.
ii. The stress distribution is not uniform over the failure plane. The stresses are more at the edges and less at the center. This leads to errors in the estimation of shear parameters.
iii. Failure is progressive and shear strength of the soil is mobilized gradually.
iv. The metallic side walls of the shear box provide a lateral restraint that causes an apparent increase in the shear strength of the soil. As the side walls do not exist in the field, the test does not replicate the in situ conditions properly and it leads to inaccurate and over-estimation of the shear strength of the soil.
v. There is no provision for monitoring or control of drainage in the soil sample at any stage of the test. Drained tests can be, therefore, conducted only on highly permeable soils.
vi. Measurement of pore water pressure cannot be done in the test and hence only total stress shear parameters can be obtained in undrained tests in the shear box test.
vii. The stress conditions are known only at the time of failure. Prior to failure, the stresses are indeterminate and Mohr’s circles cannot be drawn.
In spite of the above serious limitations, the direct shear test is more popular than other types of shear tests and is more frequently used than the others on all types of soils. This is only because of the very simple test procedure and very short time needed for the test.
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