The Menard pressuremeter is the best known type of pressuremeter for use in soils. It consists of two main components; a Probe and a Pressure-Volume meter.
The ‘Probe’ is the cell that fits in the borehole and consists of a steel cylinder with rubber membranes stretched over it so as to form three independent cells (Fig. 1).
The central measuring cell contains a liquid under gas pressure while the top and bottom ‘guard’ cells are pressurised by gas only. Volume deformation of the borehole is measured only in the central cell adjacent to which conditions of uniform stress are considered to exist. The rubber membranes are protected by thin steel strips to prevent their being punctured by sharp stones. The cells are made to fit standard borehole sizes, such as AX, BX etc.
The action of the probe in the borehole is governed by the ‘Pressure Volumeter’ at the surface. This allows water under pressure to be pumped into the central cell and gas into the guard cells. The water is pressurised by carbon dioxide gas brought on site in compressed gas bottles.
The pressure volumeter is connected to the cell by tubes through which water and gas pass. The system contains a known amount of water. As the borehole deforms the cell expands, taking in more water and the water level in a ‘sight tube’ at the surface correspondingly alters thus providing an assessment of the volumetric deformation of the borehole. Pressure is usually applied in increments and the corresponding volume changes noted. A typical pressure/volumetric deformation curve is given in (Fig. 2).
The curve generally shows three sections. The first part of the curve shows quite large volumetric deformation for limited pressure and represents the recompression of the relaxed soil around the borehole. The second straight line part represents ‘elastic’ deformation of the soil. The third section shows increasing volumetric deformation for equal increments of pressure and represents plastic deformation leading to shear failure. This may reach the ‘limit pressure’ at which the soil is deemed to have failed. The pressuremeter is capable of applying pressures of the order of 100 kPa.
Generally the pressuremeter is used in boreholes that will stand without casing. If the borehole shows a slight tendency to collapse support may be given by drilling mud.
In granular soils the pressuremeter (menard test) may be used within special slotted casing which is strong enough to support the borehole yet sufficiently flexible to deform significantly under the loads that the pressuremeter can apply. Results must be corrected to allow for the influence of the casing.
The results obtained may be used to assess bearing capacity and settlement characteristics of the soil.
The results may be influenced by the disturbance of the ground during boring. To overcome this Menard has developed a pressuremeter which may be washed bored into place. A self-boring pressuremeter, the ‘Camkometer’ has also been developed. Because the device bores itself into place the soil tested can be held not to have been stress relieved so that soil load on the cell is equal to the in situ stress.
Pressuremeter testing in rock requires the generation of much larger pressures on the sides of the borehole if significant deformation is to be measured. Pressuremeters for work in rock thus tend to be more robust than those in soil, will expand less and have much more delicate instrumentation to measure very small diametrical displacements.