Special tests

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The pressuremeter is normally used to carry out a cavity expansion test in a vertical hole. There are other more specialised tests that can be made and this section gives some examples.

Horizontal tests

The picture in figure 1 shows an example of a self boring pressuremeter working horizontally. The location is more than 200 metres below ground in a test tunnel researching the properties of Boom Clay as a possible barrier medium for the long term storage of nuclear waste. It was not permitted to use water as a drilling fluid, so the SBP was adapted to drill with air. The camera flash is reflecting off some of the returning soil particles.

Horizontal testing has also been carried out with pre-bored pressuremeters and inclined holes are common-place. If one axis is arranged to be vertical when the pressuremeter is used horizontally then this can inform the analysis, because the vertical insitu stress is normally known.

Figure 1 horizontal self boring in Boom Clay
Fig.1 Horizontal self boring in Boom Clay (click for full-size)

Creep tests

The picture in figure 2 shows a test carried out with an HPD in a rock glacier. At intervals during the test the pressure was held constant for one hour duration. For each step the creep displacements, expressed as a percentage of the cavity diameter, were plotted against log elapsed time. The slope of this trend gives a stress dependent rate.

In this material the creep is substantial and made it difficult to obtain an unload/reload cycle, even after a long creep hold.

Figure 2 creep testing in switzerland
Fig.2 Creep testing in Switzerland (click for full-size)

Consolidation Tests

The SBP can carry out a holding test to obtain consolidation parameters. It is a modification of a normal undrained expansion test. Near the point where the cavity would be unloaded it is instead held at that expansion and the excess pore water pressure (pwp) that has been generated is allowed to dissipate. As it does so the effective stress at the cavity wall starts to rise and the cavity wants to expand.

This triggers an automatic control system to reduce the total pressure at the cavity wall to compensate. The net result is that the cavity remains at a constant diameter for as long as the test is  conducted. There is a closed form solution for this situation [Reference 32] that uses the parameters derived from the expansion phase of the test and the time taken for 50% of the generated excess to  dissipate. In figure 3 the dissipation data from two pwp cells, their mean and the total pressure response are plotted in a normalised form. Any of the profiles can give a value for the horizontal consolidation, but it is normal to use the mean of the two pwp sensors.

Figure 3 consolidation testing in london clay
Fig.3 Consolidation testing in London Clay

Permeability testing

The two figures below show the result of a permeability test carried out with a self boring pressuremeter. The procedure exploits the ability of the pressuremeter to bore a pocket in the ground that it exactly fits. The stress conditions are, more or less, representative of the insitu state and are acting on the body of the probe, giving an excellent seal. As a consequence the drill string now provides a pipe from the surface down to the bottom of the probe allowing access to the formation. For low permeability material the pipe work is filled with water, is sealed off and is connected to the output of a small constant flow pump. This then pressurises the water column. Figure 4 shows steps of pressure, and the flow rates required to establish each step. Figure 5 plots the flow rates against pressure, and gives a linear trend. The slope of this trend is a function of the permeability and a shape factor.

This is one result at this location, for one geometry – the tested pocket is zero length and the permeability is the mean of the horizontal and vertical characteristics. If time allows, then the probe can be pulled back to give a pocket of some length and the test repeated. This gives a second permeability value where the horizontal characteristic is having a greater influence. Further pulling back allows additional values to be obtained. By a best fit process it is possible to identify the anisotropy factor for the horizontal and vertical conditions. In practice reconciling the data is more complex than this implies because as more and more of the material is exposed to the test then a scale effect related to the variability of the fabric becomes apparent [Reference 105].

The permeability testing is an addition to the conventional expansion test, and is a way of obtaining more data from one self boring episode. If $k$ is higher than 10-7 m/sec then the same concept can be used, but constant flow is not required and a falling head test can be carried out, measuring the height of the water column in the SBP drill rods.

Figure 4 permeability testing, raw data

Fig.4 Permeability testing, raw data (click for full-size)

Figure 5 permeability testing result

Fig.5 Permeability testing, result (click for full-size)

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