Borehole pressuremeter testing in Sydney is an in-situ test that was developed to measure the deformation and strength characteristics of soil. The pressuremeter is typically used in investigation work to assess the in-situ stress deformation features of gravel, rock, sand, clay and silt deposits below the ground level or above the groundwater level. With the help of the pressuremeter, the stress deformation features are obtained from the soil’s natural state under radial stress that increases gradually.
Before starting the test, proper planning is required to decide the test’s location and the borehole diameter. The test is normally done at varied depths in fresh boreholes, or in a self bored hole. The pressuremeter consists of an expandable probe with two guard cells at the bottom and top and a measuring cell at the centre (tri-cell) or a single cell (mono-cell).
Pressuremeter test process
During the test, the probe is typically inserted in a pre-bored hole. The probe is then expanded in volume either by air pressure or by liquid until the soil fails or the expanded volume of the measuring cell reaches its maximum volume. The guard cells are used to reduce the end effect on the measuring cell, monocells are longer and do not require this adjustment. Drill casing can be provided to prevent caving in the borehole. The bottom of the casing is kept at least one metre above the desired test depth. Depending on the condition of the soil, it is possible to drill a hole below the casing and do a successive pressuremeter test.
Result analysis
Analysing the results of the test uses the pressure and volume results obtained by doing pressuremeter testing the curve for stress vs. deformation are then plotted for each test completed. There are three phases to the deformation curves. The first one is the re-establishing phase. The second one is the pseudo-elastic phase while the third one is the plastic phase. After drilling the borehole and the augers/ rods and bit are withdrawn, the walls of the borehole relax and this reduces the cavity volume and effects the stiffness of the borehole wall. As the pressuremeter probe is inflated, the walls of the borehole are pushed back to their original state, this is the re-establishing phase of the curve.
Why is a pressuremeter test necessary?
The pressuremeter test can be used to help in the design of foundations for different types of soils including residual soils and rock. The foundations’ settlements can be estimated using a deformation modulus which can be derived from the pseudoplastic phase of the load-deformation diagram. The pressuremeter can give in-situ lateral stresses in the ground, the stress-strain behaviour and the strength of the soil at different depths. The test can take up to 60 minutes after drilling operations. The results are available within the shortest time possible to arrive at quick conclusions regarding the suitability of the site to be adopted.
Advantages
Pressuremeter testing in Sydney is performed on virtually undisturbed soil and rock. You can obtain a lot of fundamental soil and rock properties from a single test and derive further properties; no empirical correcting factors are needed. The test is controlled by semi-automatic systems and is independent of the operator influence. It is fast and results are obtained quickly.
