Field Tests - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 452

dilatometer measures the diameter change Δ d of the borehole due to a load increment

Δ p in different directions, usually three or four (Fig. 15.1). A pressiometer measures the

volume change

Δ V of the hydraulic cell due to Δ p.

Figure 15.1 Dilatometer test in isotropic rock mass, evaluation of Young's modulus (Wittke 1990)

A borehole jack also referred to as “stiff dilatometer” (ISRM 1996) applies a unidirec-
tional pressure to the borehole wall over two diametrically opposed sectors of the wall
via a pair of loading plates. The diameter change Δ d of the borehole in the direction of

loading is measured.
As an example of a dilatometer, a diagram of the LNEC dilatometer developed by Ro-
cha et al. (1966a) is shown in Fig. 15.2 (left). LNEC stands for L aboratorio N acional de
E ngenharia C ivil in Lisbon. Four displacement transducers are arranged in cross-sec-
tions spaced at intervals of 32 mm and directions of 45° to measure the radial displace-
ments of the wall of an NX borehole (d = 76.2 mm). The length of the tested section of
borehole amounts to l = 540 mm.

Figure 15.2 (center) illustrates the Menard pressiometer that was originally developed
for the investigation of soil and weak rock but can also be applied in hard rock (Menard
1966). It consists of the probe and a pressure applying and volume measuring unit (volu-
meter) that are connected by coaxially arranged flexible plastic tubes. The probe consists
of two guard cells and the measuring cell in between. The measuring cell contains water
that is pressurized via the inner tube. The upper and lower guard cells are pressurized
with gas via the outer tube to the same pressure as the measuring cell. The volumeter,
consisting of a water reservoir and a gas pressure bottle, is equipped with pressure gauges