The Effect of Excavation Technique on Slope Stability
The stability of a slope depends to a great extent upon discontinuities (Fig. 1). Even in a carefully excavated slope stress relief may induce joint opening which allows ingress of water and plant roots to induce long-term decay (Fig. 1b). Rock excavation often requires blasting to break the rock before it can be excavated. If this blasting fractures the rock from which the final slope is to be made, than these fractures will reduce stability (Fig. 1c).
Most engineering geologists have seen slopes in which the rock mass structure was basically favourable for stability but, once excavated, became
unstable due to excessive and poorly executed blasting. For permanent slopes care should be taken that the blasting for excavation does not disrupt the slope. While much disruption may be avoided by employing experienced and able blasting contractors working to a clearly written and closely defined contract, various working techniques may be employed to reduce rock mass disturbance.
The fracturing that may be induced in the final slope as the result of excavation blasting is minimized by establishing a ‘buffer zone’. No blasting takes place within this zone which is partly fractured by blasting from the area above. The fractured rock in the buffer zone is pulled down using excavating machinery to form the final slope.
The buffer zone may be 2–3 m wide (Fig. 1d). Damage to the designed slope from the bulk blast may also be prevented or reduced by the technique known as ‘pre-splitting’.
In this a row of boreholes is drilled along the plane of the design slope. The holes are parallel, about 1 m or so apart. They are charged with suitable explosive which is then detonated. This causes a crack to run between the holes so that the plane of required slope is marked by a continuous crack (Fig. 1e). The rock in the excavation is then removed by normal drilling and blasting techniques; fracturing caused by the normal blasting is found not to pass through the boundary fracture plane.
The method produces very fine, clean rock faces. However, it must be noted that the inclination of the pre-split fracture must be that of the stable slope whose stability is determined by the discontinuities within it. Pre-splitting does not make a slope stable but prevents it becoming unstable as the result of fracturing following blasting. The method works best in uniform rock conditions.
It may not work at all if the rock mass is particularly anisotropic as the result of geology or weathering, for in such a case the boreholes may not be parallel and success in crack-forming would vary because of the varying response to shock of the different rock materials.
In a rock face that has been well pre-split the observer should see parallel lines of half boreholes on the face. The efficacy of the pre-split may be assessed by comparing the length of boreholes drilled to the length of borehole seen in half section. Thus a ‘Pre-splitting Index’ can be easily defined as:
The nearer the index approaches 100%, the better the pre-split. If, however, the design slope follows a discontinuity plane then a good ‘split’ may be obtained without a high index for the blast would tend to open the pre-existing discontinuity rather than create another one. On some contracts very good slopes may be formed only to be damaged by later minor works. Thus blasting out a drainage trench at the foot of a face could, if carelessly done, damage the toe of the slope above.
While techniques may be applied to reduce bulk excavation disturbance in the rock mass forming the final slope, some disturbance is inevitable. Some problems may be caused by error. Thus, for example, infrequent shear planes, near impossible to detect in investigations, may give local stability problems. Completed slopes must be examined to discover such features that may give rise to short- or long-term stability problems and remedial measures applied. It is important, particularly in road excavations, that, for reasons of safety and economy, that remedial works be implemented before
the road is opened.
However, experience suggests that the greatest source of excavated rock slope instability is poor workmanship in the use of explosives. It is noticeable that many very old slopes excavated by hand and by blasting by gunpowder in short boreholes are stable, while their modern counterparts in similar geological conditions but excavated using much more powerful explosives and deeper boreholes are unstable.
There is little point in spending time in investigation and elegant rock slope design if the excavation technique employed destroys any possibility of achieving a stable slope.