Stress Measurements - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 527

zones, or by stress changes, leading to plastic zones. As a result, inelastic deformations
may occur, and the pressure required to return the pins to their original position will
differ from the assumed conditions (Wittke 1990, Fairhurst 2003).

Figure 16.14 Pin separation versus slot excavation time t and versus flat jack pressure p,

identification of cancellation pressure p c (ISRM 1987a)

Δδ

It goes without saying that compensation tests cannot be carried out in zones of ten-
sion, where the slot opens during cutting.

16.5 Hydraulic Methods

16.5.1 Hydraulic fracturing (HF)
The hydraulic fracturing (HF) technique, also referred to as “hydrofracturing”, was
originally developed by the oil industry to stimulate oil production by increasing po-
rosity and permeability of the ground by means of fractures induced by water inject-
ed into packer-isolated borehole sections. Using the theory of elasticity Hubbert &
Willis (1957) found out that the pressure recorded during borehole pressurization is
directly related to the stress existing in the rock mass around the borehole. Scheid-
egger (1962) and Fairhurst (1965) were among the first who adapted the HF method
for the purpose of stress measurements. A detailed description of equipment, setup,
data interpretation and stress determination can be found in ISRM (1987a), ASTM
(1989), Haimson (1993) and ISRM (2003).
The HF technique requires the same equipment as permeability tests using packers
(Section 15.8). The tests are normally carried out in boreholes with diameters of 76
- 96 mm. Images of the borehole wall are essential for selecting a suitable test section
with a typical length of 0.5 - 2.0 m. In the test section no discontinuities or fractures
should be visible at the borehole wall. The test section is sealed off using two inflata-
ble rubber packers called “straddle packers” (Fig. 16.15, left).