Experience in the design of slopes in large open pit mines has shown that the Hoek-Brown criterion for rock masses in situ undisturbed (D = 0) determines properties of the rock mass that are too optimistic. The effects of the damage of loud explosion as well as stress relief due to the elimination of the overburden are disruptive to the rock mass. It is believed that the properties of the rock 'disturbed', D = 1 in equations [3] and [4], are more appropriate for these rock masses.
Lorig and Varona (2000) have shown that factors such as the lateral confinement produced by different radii of curvature of the slopes (in plan) with respect to their height also influence the degree of disturbance.
Sonmez and Ulusay (1999) have analyzed five slope failures in opened coal mines in Turkey and have attempted to assign factors to each rock mass based on their assessment of the rock mass properties predicted by the Hoek-Brown criterion. Unfortunately, one of the slope failures slope appears to be structurally controlled while another is constituted by a stack of waste transported. The authors believe that the Hoek-Brown criterion is not applicable for these two cases.
Cheng and Liu (1990) report the results of back-analysis of very careful deformation measurements, from strain gauges placed before the start of the excavation in the cave of Mingtan in Taiwan. It was found that an area damaged by the explosion extended for a distance of about 2 m around all the big excavations. The calculated strength and the deformation properties of the damaged rock mass give a equivalent disturbance factor D = 0.7.
From these references it is clear that a large number of factors can influence the disturbance degree in the rock mass surrounding an excavation and that it could never be possible to precisely quantify these factors. However, based on their experience and on an analysis of all the details contained in these documents, the authors have tried to develop a set of guidelines for the estimation of factor D, and these are shown in Table 1.
The influence of this disturbance factor can be significant. This is shown by a typical example in which 
, 
and 
. For an in situ undisturbed rock mass surrounding an excavation to a depth of 100 m, with a disturbance factor D = 0, the equivalent friction angle is 
while the cohesive force is 
. A rock mass with the same basic parameters, but in a slope more than 100 m in height, with a disturbance factor D = 1, it has an equivalent angle of friction of 
and a cohesive force of 
.
Table 1: Guidelines for the estimation of the disturbance D
Description of the rock mass |
Suggested value of D |
|---|---|
The excellent quality of the controlled explosion or excavation through the Tunnel Boring Machine (TBM) results into a minimal disturbance to the confined rock mass surrounding an excavation. |
D=0 |
The manual or mechanical excavation in rock masses of low quality (without the use of explosives) translates into a minimum disturbance to the surrounding rock mass. Where the compression problems are raised in the significant plan, the disturbance can be severe unless is placed a temporary basis. |
D=0
D=0.5 |
A non controlled explosion in an excavation of hard rock causes a severe local damage, which extends for 2 to 3 m in the surrounding rock mass. |
D=0.8 |
An explosion of small-scale cuts in embankments for civil engineering works causes modest damage to the rock mass, particularly if it is used the controlled burst. However, the release of stress causes some disturbance. |
D=0.7 Explosive with controlled charges
D=1.0 Explosive with not controlled charges |
The slopes of the very large open pit mines suffer from a significant disturbance due to the heavy explosion and also due to release the stress generated by removing the overburden. In some soft rock excavation can be performed through ripping and dozing, and the degree of damage to the slope is minor. |
D=1.0 Use of explosives
D=0.7 Mechanized excavation |
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