OBJECTIVE
To
find the shear of the soil by Undrained Triaxial Test.
NEED
AND SCOPE OF THE TEST
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 perform with or without measurement of pore pressure although for most
applications the measurement of pore pressure is desirable.
PLANNING AND ORGANIZATION
A constant rate of strain compression
machine of which the following is a brief description of one is in common use.
a)
A loading frame in which the load is applied by a yoke acting through an
elastic dynamometer, more commonly called a proving ring which used to measure
the load. The frame is operated at a constant rate by a geared screw jack. It is
preferable for the machine to be motor driven, by a small electric motor.
b)
A hydraulic pressure apparatus including an air compressor and water
reservoir in which air under pressure acting on the water raises it to the
required pressure, together with the necessary control valves and pressure
dials.
A triaxial cell to take 3.8 cm dia and 7.6 cm long samples, in which the sample can be subjected to an all round hydrostatic pressure, together with a vertical compression load acting through a piston. The vertical load from the piston acts on a pressure cap. The cell is usually designed with a non-ferrous metal top and base connected by tension rods and with walls formed of perspex.
Apparatus for preparation of
the sample :
a)
3.8 cm (1.5 inch) internal diameter 12.5 cm (5 inches) long sample tubes.
b)
Rubber ring.
c)
An open ended cylindrical section former, 3.8 cm inside dia, fitted with
a small rubber tube in its side.
d)
Stop clock.
e)
Moisture content test apparatus.
f)
A balance of 250 gm capacity and accurate to 0.01 gm.
Experimental Procedure
1.
The sample is placed in the compression machine and a pressure plate is
placed on the top. Care must be taken to prevent any part of the machine or cell
from jogging the sample while it is being setup, for example, by knocking
against this bottom of the loading piston. The probable strength of the sample
is estimated and a suitable proving ring selected and fitted to the machine.
2.
The cell must be properly set up and uniformly clamped down to prevent
leakage of pressure during the test, making sure first that the sample is
properly sealed with its end caps and rings (rubber) in position and that the
sealing rings for the cell are also correctly placed.
3.
When the sample is setup water is admitted and the cell is fitted under
water escapes from the beed valve, at the top, which is closed. If the sample is
to be tested at zero lateral pressure water is not required.
4.
The air pressure in the reservoir is then increased to raise the
hydrostatic pressure in the required amount. The pressure gauge must be watched
during the test and any necessary adjustments must be made to keep the pressure
constant.
5.
The handle wheel of the screw jack is rotated until the under side of the
hemispherical seating of the proving ring, through which the loading is applied,
just touches the cell piston.
6.
The piston is then removed down by handle until it is just in touch with
the pressure plate on the top of the sample, and the proving ring seating is
again brought into contact for the begging of the test.
Observation
and Recording
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.
Operator
:
Sample No:
Date
:
Job :
Location : Size of specimen :
Length
:
Proving ring constant :
Diameter : 3.81 cm
Initial area L:
Initial
Volume :
Strain dial least count (const) :
| Cell
pressure kg/cm2
1 |
Strain dial 2 | Proving
ring reading
3 |
Load
on sample kg
4 |
Corrected
area cm2
5 |
Deviator
stress
6 |
| 0.5 | 0
50 100 150 200 250 300 350 400 450 |
||||
| 0.5 | 0
50 100 150 200 250 300 350 400 450 |
||||
| 0.5 | 0
50 100 150 200 250 300 350 400 450 |
| Sample No. | Wet bulk density gm/cc | Cell pressure kg/cm2 |
Compressive
stress at failure
|
Strain at failure | Moisture content | Shear strength (kg/cm2) | Angle of shearing resistance |
| 1.
2. 3. |
a)
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.
b)
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.
c)
The stress results of the series of triaxial 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.