Abstract

During the slope instability process the level of deformations within the rock mass increases over time as the competency of the rock degrades and relevant parameters such as the Shear Modulus G* diminish. Similarly, to the curve obtained in a laboratory testing for the intact rock but at much greater scale for the rock mass there will be three different stages; linear, non-linear with small deformations and non-linear with large deformations being the start of the last stage the onset of the progressive stage and the imminence of slope failure. At the open pit scale, it means that in the 1st stage cracks may be non-noticeable yet in the field while during the 2nd stage cracks will be visible during field inspections due to a progressive lack of confinement and during the 3rd stage large deformations will develop progressively until the onset of failure has been reached with the total collapse of the slope.

It has been investigated that during the slope instability process deformations will change its behavior over time from linear to non-linear ranging varying from 0.01% to 0.1% of deformation for deformable to very deformable rock mass types respectively. For purposes of simplicity in this paper it has been considered that weak/deformable rock masses will have a transition limit from linear to non-linear behavior at 0.1% of deformation. These changes affect important parameters of the rock mass such as the shear modulus that must be quantified for small to large deformations because they can be relevant to predicting the time of collapse for different rock mass qualities. For simplification some assumptions were made for the analysis method and are based on field observations of several slope instabilities, they mainly considered that; approximately at 0.1% of deformation the rock mass start to become plastic at a low rate, and the loose of confinement in the rock mass gradually increase between 0.25% - 0.7% of deformation where the failure process initiate. With these considerations, the analysis method has been used to match existing real historical data of deformations taken within a few areas of instability to understand at which average percentage of deformation it was triggered the failure mechanism as to calibrate the design/assumed elastic parameters such as Em. The correlation obtained from the second objective will help out to verifying the above-mentioned indicators. This information will be used to set up empirically the basic indicators of collapse and be used to infer the preliminary threshold alerts for similar rock mass types used in previous research.

Finally, conclusions on the results obtained from the empirical model probed with two historical instabilities occurred in a mine located in the north of Peru and recommendations for performance monitoring will be provided. At the same time, it will be established the basis for future research on the influence of the shear modulus in the velocity and deformation of the failure mass for prediction of the time of collapse.

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