Stress Measurements - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 522

To determine the three-dimensional stress state with the conventional doorstopper
technique the combination of the results of tests carried out in boreholes with three
different orientations is required. The modified cell with continuous data logging allows
four in-situ stress components to be determined in one measurement with the aid of
the so-called “strain recovery curve” obtained from strain gauge readings. Then the
three-dimensional stress state can be determined from the combination of the results of
measurements conducted in just two boreholes with different orientations (Corthesy et
al. 1994). Owing to the scatter of results, however, a larger number of tests are required
in order to obtain meaningful results.
The doorstopper method has been widely replaced by triaxial cell measurements, and
therefore nowadays is only rarely used. However, in weak, highly stressed or fractured
rock this method - because of the short required length of overcoring - may be advan-
tageous in cases where the successful usage of triaxial cells fails (Corthesy et al. 1993,
Corthesy et al. 1994, Ljunggren et al. 2003).
At greater depths, the vertical stress can more or less reliably be assumed as a principal
normal stress being equal to the overburden pressure. The two other principal normal
stresses must then be horizontal and can be determined from doorstopper measure-
ments conducted in vertical boreholes. Thus, doorstopper measurements carried out
in greater depths allow the three-dimensional in-situ stress state to be determined with
certain reliability.
The deep doorstopper gauge system (DDGS), including a wireline adapter, an orienta-
tion device and a battery-operated data logger that remains attached to the doorstopper
during overcoring, enables stress measurements in dry and water-filled boreholes to a
depth of 1000 m (Thompson et al. 1997).
16.2.4 Conical Strain Cell
In the so-called “compact conical-ended borehole” (CCBO) technique the flat bottom
of a 76 mm (NX) exploration borehole is reshaped into a conical shape using a special
drilling bit. Then a cell equipped with eight strain gauge rosettes, each composed of
two or three strain gauges, is glued onto the conical rock surface at the bottom of the
borehole and subsequently overcored with the same diameter as the exploration bore-
hole (Sugawara et al. 1985, Sugawara & Obara 1986, Obara & Sugawara 1997, ISRM
1999c). Only a small length of about 30 cm is required for overcoring.
The elastic constants required for stress determination from the measured strains are
determined by uniaxial compression tests on cylindrical specimens prepared from the
overcore (ISRM 1999c).
The three-dimensional stress state can be determined from measured strains, assuming
elastic isotropic (Sakaguchi et al. 1992) and transversely isotropic stress-strain behavior
(Obara et al. 1995) of the rock mass. The most probable values of the in-situ stress
components are determined by means of the least square method.
The CCBO technique has so far only been used in dry boreholes to a depth of 40 m
(ISRM 1999c).