Engineering Rock Mechanics

Tests on rock masses 189

5 or more per hole)

MPBX sensor head

Rubber sleeve over

Transducer lead wire

Data acquisition sys

Note timber platform

for support during

erection not shown

4 screws for set

up and removal

1

8.3

chors

Dcformation, mm

Rock surface deformation as a function of bearing pressure

Figure 11.13 The plate loading test for estimating rock mass deformability (after

Brown, ISRM Suggested Methods, 1981).

and displacement transducers located beneath the plate. It can be seen in
Fig. 11.13 that there is hysteresis present in the loading-unloading cycles,
with the attendant problem of establishing the actual modulus value.
Usually, the hysteresis is directly associated with the presence of discon-
tinuities. Large tests have been attempted, such as the surface chalk moduli
testing programme conducted using a water tank loading the rock surface
at Munford, UK (Burland and Lord, 1969) or the underground chalk moduli
testing using the reaction ring of a tunnel boring machine at Chinnor, UK
(Hudson et al., 1977, illustrated in Fig. 11.3.
Testing the strength of a rock mass is also difficult because of the high
loads involved. One seminal paper, illustrating the existence of the size
effect in coal pillars up to 6 feet wide by testing them to destruction, was
published by Bieniawski (1968). In this type of test, rock pillars in an under-