Ground Control 0 Comment

There are various methods and techniques available for in-situ testing of the underground rock mass. Some of the strata mechanics instrumentation that are used can be categorised as follows :

Stress Measuring Instruments,
It is not possible to directly determine the stress characteristics of rock but it can be evaluated to sufficient accuracy by calculating the stress from other rock properties such as strain and displacement. The type of instruments that are used to determine the stress properties of rock are borehole deformation strain cells. This instrument measures the diametral deformation of the borehole, perpendicular to the axis that the borehole was inserted. The stress in the rock mass is then calculated from the diametral changes that were recorded. (* Insert a simple pic of borehole deformation cell*)

Modulus of Deformation Measuring Instruments,
The measurements of modulus of elasticity values that are obtained from laboratory testing of rock samples are often higher than the actual in-situ rock mass values. For this reason, various borehole techniques have been developed and classified as follows :

  • Borehole Dilatometers : These operate by applying a uniform pressure along a section of borehole by use of hydraulic fluid contained in an expandable jack. The deformation is then recorded and used for determining the modulus of deformability of the rock material.
  • Borehole Jacks : These instruments allow the modulus of deformability to be calculated from a much higher pressure than that achieved by the dilatometers. This allows the determination of the deformability modulus of harder rocks. This technique applies a unidirectional pressure over two diametrically opposite sections of the borehole. The modulus of deformability is calculated using the resultant deformation.
  • Borehole Penetrometer : This device analyses the stress/strain characteristic of rock by forcing an indention pin into the rock mass. The modulus of deformability of the rockmass can be determined.

Displacement Measuring Instruments
Displacement measuring instruments measure the amount of displacement along the borehole axis. These techniques can give an insight into how the strata is behaving with reference to bed separation and crack formations. The instruments used to measure the displacement in the borehole is the extensometer. There are two types of extensometer used :

  • Single Position Borehole Extensometer : This instrument basically measures the relative axis displacement between two points along the length of a borehole. One point is generally located deep into the rock mass and one near the mouth of the borehole to act as a reference point.
  • Multi-Point Borehole Extensometer : These extensometer can measure the strata deformation at various points along a borehole. The results from these instruments can give an insight into have the strata is behaving in terms of bed separation and crack formation.