Creating a Borehole for the PMT

The prebored PMT must be performed in a hole or cavity created by some form of test drilling. The test drilling for a prebored PMT is an important step in the process. The borehole needs to be sized correctly for the probe size in order to obtain a complete pressure- expansion curve from the test. Table 7.2 gives some typical PMT probe sizes and nominal tolerances for the test cavity.

Table 7.2 Typical PMT probe diameters and borehole sizes (after ASTM D47I9)

According to Briaud (2013), making a quality borehole is the most important step in obtaining a high-quality test. ASTM D4719 provides some guidelines for different methods of creating a borehole in different soils, which are summarized in Table 7.3.

Briaud (2013) summarized the basic differences in drilling a borehole for a PMT and Drilling a borehole for sampling. These are summarized in Table 7.4. Recommended drilling practices for creating a quality PMT borehole by the rotary drilling are presented in Table 7.5.

Table 7.3 Recommended Method of Creating Borehole for Prebored PMT (after ASTM D47I9)

Note: I is the first choice; 2 is the second choice; NR = not recommended; NA = not applicable

Table 7.5 Recommended practices for creating a quality PMT borehole using rotary drilling (after Briaud 2013)

Test Procedures

A detailed test method is described in ASTM D4719 Standard Test Method for Pressuremeter Testing in Soil. ASTM D4719 allows for two types of test procedures. Whether a tri-cell or mono-cell probe is used, the test procedure is essentially the same. There are two common methods for performing a prebored PMT: (1) incremental stress-controlled test and (2) incremental volume-controlled test. Other procedures, such as a constant rate of stress increase or a constant rate of strain increase, are also possible but more complex and may require the use of automatic data acquisition systems.

Test Procedure A – Equal-Pressure Increment Method

In a stress-controlled test, the pressure in the probe is increased incrementally after the probe is set into the borehole to the desired test depth. After an initial volume or strain-arm displacement reading is obtained, the pressure is set with the regulator on the console and a stop watch is started. Volume or strain-arm readings are then obtained after an elapsed time of 15 s, 30 s, and lmin. After the 1-min. reading, the pressure is increased to the next level and another set of volume or displacement readings are obtained. The stepwise loading sequence is continued until the end of the test.

This procedure is the most common and the easiest test method to perform since the pressure is increased in a stepwise manner using a pressure regulator on the control console and the response of the soil to that pressure in terms of volume change or diameter/radius change is obtained. It is very common that one or more unload-reload cycles are performed in the pseudo-elastic portion of the curve to obtain the unload-reload modulus, E_UR. There is no set rule on where this should be done.

Test Procedure B – Equal-Volume Increment Method

In the equal-volume method, the pressure at the control; console is adjusted so that the injected fluid volume is the same for each increment. The pressure required to maintain this volume is then recorded along with the total volume to that increment. This test procedure is much more difficult to perform if a gas regulator is used to control the test since the pressure must be continuously adjusted to keep the volume increment constant. An alternative used in the TEXAM pressuremeter is to use a hand crank fluid piston system that injects a fixed volume of fluid with each rotation of the crank.

Continuous Loading Tests

It is also possible to perform the test under continuous loading. In continuous loading tests, the test pressure is applied to the probe in a continuously increasing manner. In this way, more data points are obtained, the test usually takes less time to complete, and the test equipment may be automated.

If compressed gas is being used to expand the membrane (either directly or using a gas/liquid interface), the gas regulator is controlled by a small motor that increases the gas pressure at a preset rate. As the gas pressure increases, measurements are taken of the probe volume or radial arm displacement. In continuous loading tests, there is no measurement of the creep as in an incremental loading test since there is no waiting period for successive load increments. The results of continuous loading tests provide a much smoother and continuous test curve in comparison with that of an incremental test. Continuous loading tests are most often used with self-boring and full-displacement tests, while incremental loading is more common for prebored tests.

An alternative method for performing continuous loading tests may be used if a liquid-filled probe is used. Instead of using a gas/liquid interface and compressed gas to perform the test, a small-liquid screw pump mechanism may be attached to the probe. A small electric motor is then attached to a screw drive to advance the pump and force liquid into the probe at a preset rate. A pressure gage or transducer placed in line provides a measure of the probe pressure, while a simple counter or linear distance transformer (LVDT) or digital dial gage provides a measure of the pump travel which can be converted to volume through a simple calibration.

Holding Tests

A special type of PMT test procedure may be used to measure creep effects by maintaining a constant pressure over a long period of time and monitoring the change in volume/diameter. This is typically referred to as “a holding test” since the pressure is held constant. The time for a holding test can be as short as 10 min or over an hour depending on the creep characteristics of the soil. Holding tests may be performed at different pressure levels to determine differences in creep behavior relative to the limit pressure.