The standard consolidated undrained test is compression test, in which the soil specimen
is first consolidated under all round pressure in the triaxial cell before failure is
brought about by increasing the major principal stress.
It may be performed with or without measurement of pore pressure although for most
applications the measurement of pore pressure is desirable. Triaxial Shear Test is the
fastest Triaxial Shear Test to obtain shear strength parameters (c,Φ) of soil.
IS 2720(Part 11):1993 Determination of the shear strength parameters of a specimen tested in unconsolidated undrained triaxial compression without the measurement of pore water pressure (first revision). Reaffirmed- Dec 2016.
The machine is set in motion (or if hand operated the hand wheel is turned at a constant rate) to give a rate of strain 2% per minute. The strain dial gauge reading is then taken and the corresponding proving ring reading is taken the corresponding proving ring chart. The load applied is known. The experiment is stopped at the strain dial gauge reading for 15% length of the sample or 15% strain.
Compression Gauge Reading | Load Gauge Reading | Compression of Sample | Strain | Corrected Area | Load | Deviator Stress (σ _{1} - σ _{3} ) |
Vertical Stress (σ_{1}) |
- (σ _{1} / σ _{3} ) |
---|---|---|---|---|---|---|---|---|
0 | ||||||||
50 | ||||||||
100 | ||||||||
150 | ||||||||
200 | ||||||||
250 | ||||||||
300 | ||||||||
350 | ||||||||
400 | ||||||||
450 |
Table 1 : Recordings during Triaxial Shear Test
Sample No. | Wet bulk density gm/cc | Cell pressure kg/cm^{2} | Compressive stress at failure | Strain at failure | Moisture content | Shear strength (kg/cm2) | Angle of shearing resistance |
---|---|---|---|---|---|---|---|
1 | |||||||
2 | |||||||
3 |
Table 2 : Triaxial Shear Test Result
It is assumed that the volume of the sample remains constant and that the area of the sample increases uniformly as the length decreases. The calculation of the stress is based on this new area at failure, by direct calculation, using the proving ring constant and the new area of the sample. By constructing a chart relating strain readings, from the proving ring, directly to the corresponding stress.
The strain and corresponding stress is plotted with stress abscissa and curve is drawn. The maximum compressive stress at failure and the corresponding strain and cell pressure are found out.
The stress results of the series of Triaxial Shear Tests at increasing cell pressure are plotted on a Mohr stress diagram. In this diagram a semicircle is plotted with normal stress as abscissa shear stress as ordinate.
The condition of the failure of the sample is generally approximated to by a straight line drawn as a tangent to the circles, the equation of which is ԏ = c + σ tan(Φ). The value of cohesion, c is read of the shear stress axis, where it is cut by the tangent to the mohr circles, and the angle of shearing resistance (Φ) is angle between the tangent and a line parallel to the shear stress.
For normally consolidated soils, c= 0; however,
for over-consolidated soils, c> O.
A typical range of values of A at failure for clayey soils is given below:
Type of Soil | A at Failure |
---|---|
Clays with High sensitivity | 0.75 to 1.5 |
Normally Consolidated Clays | 0.5 to 1.0 |
Over Consolidated Clays | -0.5 to 0.0 |
Compacted Sandy Clay | 0.5 to 0.75 |
Table 3 : Triaxial Shear Test Result