All rock excavations by means of natural gravity caving, mechanical excavation or blasting cause redistribution of the static stress and strain in the remaining rock mass. It is critical to measure the change of the strain state. Such a measurement can serve as quantification of the damage and of the effectiveness of a blast design or an excavation method. Secondly, the measurement can be used to monitor slope or mine structure stability to improve the mine safety.
At present, there is no method available to calculate strain changes over a large area of a rock slope or an underground structure. New technologies developed over the last few decades; such as GPS, radar systems, three dimensional photogrammetry and multi-sensor displacement monitoring, are also used nowadays to monitor displacement. If a strain calculation method is developed, the data may be used to calculate distributions of three dimensional strain tensors in the slope or highwall. Without a strain calculation method, the data can only give displacement change at discrete monitoring spots.
This paper presents a method for measuring the change of the static strain state at a remaining rock mass or ground associated with operations of excavations, such as blasts, mechanical means or gravitational caving of a portion of the ground. The method is based on measuring coordinates of selected points with recent accurate survey techniques before and after an excavation. The paper also shows that for small deformation the different reference points for survey before and after an excavation can be used, which provides great convenience in practical measurement.
The calculation of the strain change is done by solving a set of simultaneous equations for the displacement gradients. With the number of survey points larger than the minimum number required, the number of the independent equations is greater than the number of unknowns and minimizes measurement errors.