45 In-situ stress measurements
COMMENTARY ON CLAUSE 45
Measurement of in-situ stress in soils and rocks may be made, although the equipment used means that the results only normally provides an estimate of stress and not an exact measurement. To enable both total and effective stresses to be estimated, it is usual to measure the pore water pressure in addition to the total stress.
The stresses existing in a ground mass before changes caused by the application of loads or the formation of a cavity within the mass are referred to as the initial in-situ state of stress. These stresses are the result of gravitational stress and residual stresses related to the geological history of the mass.
Data on the initial in-situ state of stress in rock and soil masses before the execution of works are important in design. The most favourable orientation, shape, execution sequence and support of large and complex underground cavities, and the prediction of the final state of stress existing around the completed works, are all dependent on knowing the initial in-situ state of stress. Measurements of in-situ stress have shown that in many areas the horizontal stresses exceed the vertical stress, which in turn often exceeds that calculated, assuming that only gravity is acting on the ground mass.
45.1 Stress measurements in rock
Over-coring should be used for measurement within the rock mass, whereas slotting should be used for surface stress measurements. With the exception of the static equilibrium method (see 45.1.5), the methods described are based on stress changes, achieved by over-coring or slotting a previously instrumented test area. Measurements taken should be adjusted to take account of the redistribution of stresses as a result of formation of the borehole or slot and when the measurement is made in the zone of influence of the main access, such as an adit. Stress measurements may also be determined from the measurement of displacements of the walls of a tunnel, or of an exploratory adit, close to the working face.
NOTE The techniques often require that the material in which the measurements are made behaves in a near elastic, homogeneous and isotropic manner and that it is not prone to swelling as a result of drilling water, or excessively fractured. Analyses are available that evaluate measurements made in anisotropic material but these are not widely used. For the over-coring methods, the elastic behaviour is assumed to be reversible, the elastic constants being obtained from field or laboratory tests.
Stress measurements may be made using electrical strain gauges, photoelastic discs, solid inclusions and systems for measuring the diametrical change of a borehole. Some equipment is designed to measure stress change with time, or stress change due to an advancing excavation, whereas other equipment is designed to obtain an instantaneous measurement of stress. The technique selected should be chosen in relation to the rock material, mass quality and water conditions.
To determine the triaxial state of stress at a given point, measurement should be made in at least six independent directions. It is, however, desirable to have the extra data for better evaluation by statistical methods of error distribution.
The report on the results of in-situ stress measurement should include the following:
- a) location of test and direction and depth of the drill holes, method of drilling and diameters of cores;
- b) depth below ground level of the point of measurement;
- c) geological description of the rock mass;
- d) strain readings to the nearest 10 micro strain;
- e) the modulus of elasticity, E, and Poisson's ratio, v, of the rock determined from static laboratory testing of core preserved at in-situ water content, over the appropriate stress path, from each stress measurement area;
- f) the six components of stress (σx, σy, σz, τxy, τyx, τzx) at each point to the nearest 100 kPa;
- g) the three principal stresses and the directions (to the nearest degree), related to both a borehole or adit axis system and a global axis system;
- h) colour photographs of the cores or test location (see Annex H); and
- i) date of measurement and data at which excavation passes the point of measurement.
45.1.2 Determination of the in-situ triaxial state of stress in rock
The most widely adopted method for the determination of the in-situ triaxial state of stress in one set of measurements uses the CSIRO hollow inclusion stress cell. The strains are measured over relatively small gauge lengths, approximately 10 mm, on a small test area, and should, wherever possible, be correlated and cross-checked with data obtained from other tests involving a larger test area, such as a flat jack test in the side walls of a suitably shaped and oriented adit.
NOTE 1 The method is one of over-coring a cell containing nine or twelve electrical strain gauges installed on the walls of a pilot drillhole. The test is relatively cheap and quick to perform. A full description of the equipment and the test procedure is given in the standard instruction manual from Mindata  and the ISRM document Suggested methods for rock stress determination [N2].
NOTE 2 The stress ceil, shown in Figure 11, contains three or four oriented rosettes each of which has three gauges and a temperature compensating gauge or thermistor. Nine or twelve independent strains are recorded, of which six are used to determine the total state of stress, and the other measurements are used as a check and for estimating errors.
A second instrument that can be used in deep water-filled boreholes drilled from the surface is the Borre Probe. This has been used in the UK to depths of up to 250 m (see Whittlestone and Ljunggren, 1995 ). The probe shown in Figure 12 contains three oriented rosettes, a temperature gauge and a dummy gauge. Strain changes during overcoring are recorded by a downhole data logger without connection to the surface.
It is usual when carrying out measurements from underground openings to carry out several tests in each borehole at increasing depths in order to investigate the change in stress distribution as a result of excavation. If possible, two holes should be drilled at orthogonal directions to take account of any anisotropic characteristics of the rock.
NOTE 3 All methods require a knowledge of the modulus of elasticity and Poisson's ratio of the rock. These can be obtained by biaxial testing of the overcore sample in a Hoek cell in the field, or laboratory testing of the core preserved at in-situ water content, under the appropriate stress path.
|1 Centering tip||8 Trip wire across body of gauge|
|2 Piston rod||9 Centering lugs|
|3 Piston||10 Grout exit holes|
|4 Shear pins||11 Rubber seals|
|5 Centering lugs||12 Orientating pins|
|6 Main body||13 12-core cable|
|7 Strain gauge rosettes|
|1 Cover to protect strain gauges during running of tool||4 Datalogger|
|2 Plastic cantilever arms and strain gauge rosettes||6 Connection for weight and wireline|
|3 Triggering mechanism|