30 June 2001
desired. No membrane is used inside the mold, as the membrane can be readily placed over the
compacted specimen after it is removed from the split mold. The specimen should be compacted
to the desired density in accordance with paragraph L-4.
L-5. Q TEST WITH BACK-PRESSURE SATURATION.
a. Equipment Setup. For the Q test with back-pressure saturation, the apparatus should be
set up similar to that shown in Figure L-1. Filter strips should not be used and as little volume
changes as possible should be permitted during the test. Complete the steps outlined in
paragraph L-4 and the following steps:
(1) Record all identifying information for the sample project number or name, boring
number, and other pertinent data on a sheet.
(2) Place one of the prepared specimens on the base.
(3) Place a rubber membrane in the membrane stretcher, turn both ends of the membrane
over the ends of the stretcher, and apply a vacuum to the stretcher. Carefully lower the stretcher
and membrane over the specimen. Place the specimen cap on the top of the specimen and release
the vacuum on the membrane stretcher. Turn the ends of the membrane down around the base
and up around the specimen cap and fasten the ends with o-rings or rubber bands. With a
35.5-millimeter- (1.4-inch-) diameter specimen of relatively insensitive soils, it is easier to roll the
membrane over the specimen.
(4) Assemble the triaxial chamber and place it in position in the loading device. Connect
the tube from the pressure reservoir to the base of the triaxial chamber. With valve C on the
pressure reservoir closed and valves A and B open, increase the pressure inside the reservoir and
allow the pressure fluid to fill the triaxial chamber. Allow a few drops of the pressure fluid to
escape through the vent valve (valve B) to ensure complete filling of the chamber with fluid. Close
valve A and the vent valve.
b. Back-Pressure Procedure. Then apply a 0.02-MPa (3-psi) chamber pressure to the
specimen with all drainage valves closed. Allow a minimum of 30 minutes for stabilization of the
specimen pore water pressure, measure the change of deformation )H, and begin back-pressure
procedures as follows:
(1) Estimate the magnitude of the required back pressure by theoretical relations.
Specimens should be completely saturated before any appreciable consolidation is permitted for
ease and uniformity of saturation as well as to allow volume changes during consolidation to be
measured with the burette; therefore, the difference between the chamber pressure and the back
pressure should not exceed 0.034 MPa (5 psi) during the saturation phase. To ensure that a
specimen is not prestressed during the saturation phase, the back pressure must be applied in small
increments, with adequate time between increments to permit equalization of pore water pressure
throughout the specimen.
(2) With all valves closed, adjust the pressure regulators to a chamber pressure of about
0.048 MPa (7 psi) and a back pressure of about 0.013 MPa (2 psi). Record these pressures on a
data sheet. Next, open valve A to apply the back pressure through the specimen cap.
Immediately, open valve G and read and record the pore pressure at the specimen base. When the
measured pore pressure becomes essentially constant, close valves F and G and record the burette