Benefits of the Pressuremeter Test:
- Can be conveniently used with drilling equipment or pushed in with direct push equipment.
- While tests can be done in soft clay or loose sands, the test is best used in dense sands, hard clays and weathered rock which cannot be tested with push equipment.
- An extensive database of load test results allows the geotechnical engineer to accurately design for shallow foundations and for lateral and vertical capacity of deep foundations.
Pressuremeter Test (PMT), ASTM D 4719: Louis Menard began his work with the pressuremeter test in 1954 while still a college student, studying first under Professor Kerisel in France, and later under Professor Ralph Peck at the University of Illinois. Menard improved and advanced a foundation test concept begun by Kogler in 1933, and then returned to France in 1957 where he started a company to build and use the PMT. He compiled a large data base of load tests and companion pressuremeter tests to refine his empirical design formulas and persuade other engineers to use the PMT. To show his confidence and encourage acceptance of the test, Menard guaranteed foundation designs based on the PMT with $10,000,000 of professional liability insurance from Lloyds of London (Hartmann, 2008).
Figure 1: Texam Control Unit and Probe
Figure 2: Typical Texam Strain Controlled Pressuremeter Test
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 (strain controlled test) (Figure 2), 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.
The PMT results include the at-rest horizontal earth pressure, the pressuremeter elastic modulus, the reload modulus, and the pressuremeter limit pressure (plastic failure), but generally require an empirical approach for foundation design or for correlation with classic geotechnical parameters such as the shear strength or Young’s modulus. While the PMT stress path can be modeled theoretically, the effects of stress history and anisotropy, testing in the direction of the minor principal stress (usually) in a material with behavior controlled by confining stress, and the disturbance of stress release and softening at the borehole wall (or stress increase for push-in probes), usually lead to an empirical approach.
Figure 3: Typical Creep Test Results
Figure 4: Ronald Stidham (GeoServices) Drilling Pressuremeter Test Hole
Good test results begin with a high quality borehole having minimal disturbance to its side walls, typically requiring mud wash rotary techniques (Figures 4 and 5). Maintaining the drilling mud level at or near the top of the borehole minimizes the horizontal stress release from drilling. During drilling, the operator should carefully monitor the rotation rate, advance rate, and mud flow rate to obtain a high quality borehole.
Modern data acquisition systems speed field testing and computer programs relieve the drudgery of data analysis, but the PMT remains one of the most labor‑intensive and time‑consuming in-situ tests. Pressuremeter tests are particularly valuable in dense sands, hard clays and weathered rock, if the DMT and CPT cannot penetrate those formations. Pressuremeter tests can also be used in remote sites that only skid rigs can access.
Figure 5: 3-inch Tri-Cone Bit next to PMT Probe
Design of Shallow Foundations
With pressuremeter data, the engineer can design shallow foundations using bearing capacity and settlement criteria. For bearing capacity criteria the design is based on an equivalent net limit pressure within 1.5 times the footing width above and below the footing depth. Settlement computations consider the immediate and consolidation components and are based on the pressuremeter modulus. A more refined settlement analysis can be performed using Briaud (1994) method that considers the entire pressuremeter curve. Moduli values are selected based on the applied stress at the layer’s depth. Both bearing capacity and settlement analyses use empirical equations based on a large data base of load test data. Adjustment factors assume that minimal disturbance occurs along the borehole’s sidewall.
Design of Deep Foundations
The design for vertical capacity of a deep foundation is based on the pressuremeter limit pressure. Tip resistance and frictional resistance are computed separately using correlation charts. The lateral load capacity of a deep foundation can be accurately designed by determining the P-y curve from pressuremeter data. Further, cyclic loading due to wind or other loads can be evaluated through cyclic loops with the pressuremeter test. Briaud (1997) developed a simple approach to lateral load analysis which can serve as a useful check for the more rigorous P-y analysis. Like shallow foundation design with PMT, deep foundation design uses empirical equations and requires minimal borehole wall disturbance.