Theoretical basis of the borehole deepening method of absolute stress measurement

Knowledge of the initial state of stress in rocks provides a key to the solution of many problems in rock mechanics. The initial state of stress is part of the basic data required for rational design of structures in rock, since its redistribution when engineering activities are conducted is a primary load on the rock portion of the engineering system. Knowledge of initial and subsequent changes in the state of stress is also relevant in the study of active faults and crustal strain.

The purposes of stress measurements are not generally well served by methods which disclose only one of the stress components at a point. Recently, interest has centered on overcoring methods which though expensive and difficult have the important advantage of yielding data on three or more stress components.

In a previous paper, the authors introduced a new method--termed borehole deepening--based on measurement of the deformations that occur close to the bottom of a borehole when it is deepened by further drilling without overcoring. The proposed method is at a disadvantage when compared with overcoring procedures in that the quantities to be measured are somewhat smaller and the measuring environment is more hostile. However, the borehole deepening method presents three distinct advantages in comparison with overcoring procedures: (1) Using the same borehole, both the initial and the subsequent changes in the state of stress can be obtained by borehole deepening. Stress changes with time are very significant for stability monitoring as well as in predicting earthquake activity in regions close to known active faults and volcanoes. (2) The necessity of completely unloading a portion of the rock mass where deformation measurements are being carried out in the overcoring methods is ill-suited to nonlinearly elastic materials such as rocks. The borehole deepening method, on the other hand, is based on a change of stress from one level to another, rather than complete strain relief and is therefore less affected by nonlinear behavior. (3) Numerous stress measurements can be carried out economically in a single borehole. Thus, the regional stress distribution can be more readily obtained. Such information is considerably more significant than a single stress measurement at a point; considering the variability of rock masses and the magnitudes and direction of the stresses acting on them from one region to another and within the regions themselves, the reliability of a single measurement when extrapolated for the entire region is not very reasonable. This advantage of more numerous measurements makes it possible to predetermine the initial state of stress with confidence before actual access and construction. It also makes it possible to consider studying regional tectonic systems. Integrated regional measurements of in-situ state of stress would provide the necessary information for plotting the variation of crustal stresses; this would be of great interest in evaluating the several theories of orogenesis and in delineating tectonically active regions.

This chapter is concerned with the theoretical basis for the borehole deepening method--the "software" for converting deformation measurements into stress values.