UFC 3-260-02
30 June 2001
reading. (If an electrical pressure transducer is used to measure the pore pressure, valve G may be
safely left open during the entire saturation procedure).
(3) Using the technique described above, increase the chamber pressure and the back
pressure in increments, maintaining the back pressure at about 0.034 MPa (5 psi) less than the
chamber pressure. The size of each increment might be 0.034, 0.069, or even 0.138 MPa (5, 10,
or even 20 psi) depending on the compressibility of the soil specimen and the magnitude of the
desired consolidation pressure. Open valve G and measure the pore pressure at the base
immediately upon application of each increment of back pressure and observe the pore pressure
until it becomes essentially constant. The time required for stabilization of the pore pressure may
range from a few minutes to several hours depending on the permeability of the soil. Continue
adding increments of chamber pressure and back pressure until, under any increment, the pore
pressure reading equals the applied back pressure immediately upon opening valve G.
(4) Verify the completeness of saturation by closing valve F and increasing the chamber
pressure by about 0.034 MPa (5 psi). The specimen shall not be considered completely saturated
unless the increase in pore pressure immediately equals the increase in chamber pressure.
c. Chamber Pressure. After verification of saturation, and remeasurement of )H, close all
drainage lines leading to the back pressure and pore water measurement apparatus. Holding the
maximum applied back pressure constant, increase the chamber pressure until the difference
between the chamber pressure and the back pressure equals the desired effective confining
pressure as follows. With valves A and C closed, adjust the pressure regulator to preset the
desired chamber pressure. The range of chamber pressures for the three specimens will depend on
the loadings expected in the field. The maximum confining pressure should be at least equal to the
maximum normal load expected in the field so that the shear strength data need not be
extrapolated for use in design analysis. Record the chamber pressure on data sheets. Now open
valve A and apply the preset pressure to the chamber. Application of the chamber pressure will
force the piston upward into contact with the ram of the loading device. This upward force is
equal to the chamber pressure acting on the cross-sectional area of the piston minus the weight of
the piston minus piston friction.
d. Operation.
(1) Start the test with the piston approximately 2.5 millimeters (0.1 inch) above the
specimen cap. This allows compensation for the effects of piston friction, exclusive of that which
may later develop as a result of lateral forces. Set the load indicator to zero when the piston
comes into contact with the specimen cap. In this manner, the upward thrust of the chamber
pressure on the piston is also eliminated from further consideration. Contact of the piston with the
specimen cap is indicated by a slight movement of the load indicator. Set the strain indicator and
record on the data sheet the initial dial reading at contact. Axially strain the specimen at a rate of
about 1 percent per minute for plastic materials or about 0.3 percent per minute for brittle materials
that achieve maximum deviator stress at about 3 to 6 percent strain; at these rates, the elapsed
time to reach maximum deviator stress would be about 15 to 20 minutes.
(2) Observe and record the resulting load at every 0.3 percent strain for about the first
3 percent and, thereafter, at every 1 percent or, for large strains, at every 2 percent strain;
sufficient readings should be taken to completely define the shape of the stress-strain curve so
frequent readings may be necessary as failure is approached. Continue the test until an axial strain
of 15 percent has been reached; however, when the deviator stress decreases after attaining a
L-6
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