Field Tests - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 454

diametrically opposed sectors of the borehole wall by hydraulic jacks. The apex angle
of each of the two sectors amounts to 2 = 90°. The diameter change in the direction

of loading is measured by two displacement transducers arranged at either end of the
steel plates.
In the 1960s and 1970s a large number of borehole expansion probes for rock mass
investigation were developed for borehole diameters ranging from 34 mm to 300 mm
(Kujundzic & Stojakovic 1964, Comes 1965, Menard 1966, Rocha et al. 1966a, Takano
& Shidomoto 1966, Absi & Seguin 1967, Alas 1968, Goodman et al. 1968, Prigozhin
1968, Hustrulid & Hustrulid 1975, Worotnicki et al. 1976). Nowadays updated versions
of some of them are still available. In Table 15.1 the specifi cations of dilatometers and
pressiometers frequently used in rock are compiled.
The tests generally should not be carried out in disturbed rock zones such as those
caused by blasting of underground openings. Since the probes can operate at consider-
able depths (Table 15.1) the resulting wide choice of testing locations should be used.
The procedure of a borehole expansion test will be demonstrated by means of the ex-
ample of a dilatometer test carried out in limestone in which the diameter change

β

l/d of
the borehole due to a uniform radial pressure is measured in three directions (Fig. 15.3).

Δ

After positioning the dilatometer, an initial low pressure p 0 is applied in order to obtain
a force transmitting connection of the loading device with the rock mass. The zero
reading of the displacement or volume measurement is then taken (Fig. 15.3, upper). A
loading program including several loading and unloading cycles with increasing peak
values is recommended (DGEG 1984b, ISRM 1987b, ISRM 1996).
During each loading and unloading cycle the pressure is increased in stages up to the
peak value and subsequently reduced in steps down to the initial value p 0 (Fig. 15.3,
upper). At each stage the pressure is held constant until convergence of measured dis-
placements is achieved. At maximum pressure the applied load should be maintained
constant for several minutes to give an indication of whether the rock behavior is
time-dependent (ISRM 1987b). The maximum pressure should correspond to the ex-
pected maximum rock mass loading of the planned structure (DGEG 1984b). However,
care must be taken to avoid large irreversible deformations due to rock fracturing which
would lead to an underestimation of the determined rock mass modulus (Wittke 1990).
The test results are plotted in the form of stress-displacement curves (Fig. 15.3, lower).
These are evaluated under the assumption that the rock mass is homogeneous and its
stress-strain behavior is elastic and isotropic. Young's modulus can then be calculated
for each measurement direction as follows (Rocha 1966a, DGEG 1984b, ISRM 1987b):

(15.1)

where d is the corresponding diameter change in the

direction of measurement (Fig. 15.1). In (15.1) plain strain is assumed. Since the loaded
section of the borehole has a fi nite length, the modulus may be somewhat overestimat-
ed when using (15.1). However, this effect can be neglected if l/d ≥ 4 (Davydova 1968)
which applies to all dilatometers (Table 15.1).

Δ p is a pressure increment and Δ