45.1.3 Biaxial stress measurements in rock

When measuring biaxial stresses in rock, the USBM (United States Bureau of Mines) borehole deformation gauge is often used; when using this technique, the details of installation and the test procedure found in the ISRM document Suggested methods for rock stress determination [N2] should be followed.

The equipment comprises three strain gauges bonded to cantilevers that measure the changes in pilot hole diameter during overcoring. Stress components in the plane perpendicular to the borehole can be evaluated from the results. To enable the resolution of the total state of stress, measurements should be made in three mutually perpendicular holes drilled into one area. The gauge may be used in water-filled boreholes, provided the water pressure is less than 60 kPa.

45.1.4 Uniaxial stress measurements near a surface using a flat jack

When undertaking uniaxial stress measurements, the equipment and procedures described in the ISRM document Suggested methods for rock stress determination [N2] should be followed.

By means of a saw or overlapping holes, a slot should be cut into a rock surface provided by an adit or prototype underground excavation. The stresses previously acting across the slot are relieved as the rock moves into the slotted void. This movement should be measured by the convergence of marked points that are fixed on either side of the slot before it is formed. A suitable hydraulic flat jack should be embedded in the slot and the pressure in the jack increased until the convergence of the datum points is cancelled. If creep can be ignored, the cancellation pressure should be related to the stresses that were acting normally on the plane of the slot before it was formed.

The stresses measured by this means are those parallel and near to the rock surface in which the slot is cut. The location of the adit should be such that its axis is driven in a direction nearly parallel to that of the proposed prototype excavation and as close as possible into the zone of interest. Excavation of the test zone in the adit should be carefully carried out, preferably by hand excavation or using smooth blasting techniques, and the period between excavation and measurement should be as short as possible. A typical section of a flat jack test is shown in Figure 13.

The technique measures only tangential stress near the surface of an excavation. As the flat jack stresses a greater mass of rock, the results tend to give a better average measurement than can be obtained using a smaller gauge length. This method can be used in ground where other methods are not suitable. To estimate the triaxial state of in-situ stress, at least six flat jack tests should be carried out in independent directions.

NOTE All three methods require knowledge of the elastic properties. These can be obtained by biaxial testing of the over-core sample in a Hoek cell.

45.1.5 Static equilibrium method

NOTE The static equilibrium method of in-situ stress determination is based on the static equilibrium requirement that the total load on a sufficiently large area remains constant, even after an opening, such as mine drift, is made in the area. It does not require that the rock be elastic, homogeneous or isotropic.

45.1.6 Hydraulic fracturing technique

COMMENTARY ON 45.1.6

Hydraulic fracturing (see Haimson, 1978 [97]) provides a determination of the maximum and minimum stresses in a plane perpendicular to the drillhole. The particular application of the technique has been in deep drillholes and requires no knowledge of the elastic properties of the rock.

When undertaking this test, the technique described in the ISRM document Suggested methods for rock stress determination [N2] should be followed.

Key

1 Strain measurement	3 Slot cut in rock
2 Oil-filled flatjack	4 Strain measurement

45.2 Stress measurements in soils

45.2.1 General

The response of soil masses to applied loads should be made by obtaining reliable data on their strength and deformation characteristics; as these are stress-dependent, a knowledge of the in-situ state of stress assists in their evaluation by laboratory testing.

NOTE Direct in-situ measurements of the initial state of stress in soils is difficult because the disturbance created by gaining access to the ground mass is usually non-reversible, as well as being several times that produced by a stress-relieving technique. Most techniques that have been developed suffer from the disturbance that their instruments create in the ground on insertion.

It is usual to measure horizontal stress only and to make assumptions concerning the level of vertical stress from overburden depth. Total stress only may be measured, so to determine the effective stress conditions the pore water pressure at the test level has to be measured or assumed. Methods of determining pore water pressure in the field are discussed in Clause 52.

45.2.2 Hydraulic push-in pressure cells

Measurement of total stress in soft to stiff clays should be carried out using thin rectangular hydraulic cells, "push-in spade shaped cells", carefully jacked into the ground (see Tedd et al., 1989 [98]). The insertion of the cell into the ground generates a pressure in the cell which decays with time to a constant value equal to the total stress in the ground plus an over-read caused by the disturbance resulting from the insertion of the cell into the ground. The cell reading is adjusted for the over-read by a correction factor related to the undrained shear strength of the soil.

NOTE Measurement of vertical and horizontal total stresses can also be achieved from vertical boreholes in soft clays using the BRE miniature push-in earth pressure cells (see Watts, 1991 [99]).

45.2.3 Contact stress measurement

The self-bored pressuremeter (see 42.1.2) and Camkometer may be used to reduce disturbance on insertion to a minimum by fully supporting the ground they penetrate. As pressure is applied to jack the cell into the ground, a cutting tool slowly rotates and gentle water flush removes surplus materials. Once installation has been completed the Camkometer electrical load cells measure the contact pressure, from which an estimate of total horizontal in-situ stress is obtained. The pressuremeter gives an estimate of the horizontal stress from the lift-off pressures of the membrane (see Clarke, 1994 [86]). Facilities are available to measure pore water pressure with the same instruments. Ground conditions might limit the use of this technique.

45.2.4 Hydraulic fracturing

Hydraulic fracturing (see Bjerrum and Anderson, 1972 [100]) may be used to estimate minimum total horizontal stresses in a deposit of soft to firm clay (one where the expected horizontal stress is lower than the vertical). A length of borehole is sealed and a pumping-in test carried out. Pressure in the test zone is increased in increments until a sudden increase in water flow occurs, at which time it is assumed that tensile failure has occurred in the ground. The pressure at which failure takes place is related to the minimum in-situ stress by soil properties.