Projects

The PMT is typically performed by inserting a cylindrical probe into an open borehole, supporting it at the test depth, and then inflating a flexible membrane in the lateral direction to a radial strain of as much as 40% depending on the probe design.  The PMT operator may expand the pressuremeter probe in equal pressure increments (stress controlled test) or in equal volume increments , typically stopping the test when initial volume of the probe has doubled or when reaching the maximum allowable pressure.  About 40 data points are obtained from a strain controlled test versus and about 10 data points from a stress controlled test, thus a better defined curve can be obtained from strain controlled tests.  Creep tests can be performed near the yield point of the test to evaluate time effects of the modulus (Figure 3).  Ideally the PMT provides an axisymmetric, plane strain test (the horizontal plane), typically drained in sands and silts, and undrained in cohesive soils.  Early PMT probes employed guard cells at their top and bottom to force the measurement cell located between them to expand only in the lateral direction.  Briaud (1992) showed that the error in test results did not exceed 5% for single-cell probes (Texam in Figure 1) with a length at least six times its diameter.  Researchers have also used self‑boring and push-in probes with some success in specific types of soils.  Probes may also be designed with very stiff membranes for testing at high pressures and lower strain in soft rock.