Untensioned, fully grouted, steel bars (dowels) up to 30 m long and 45 mm in diameter were used to reinforce rock slopes several hundred metres high, excavated for a hydroelectric project near Revelstoke, British Columbia. These dowels were successfully used to prevent surface rock from loosening, to prevent sliding on interconnected discontinuities and to prevent sliding of relatively intact rock on a thin shear zone. Because the interaction between rock slopes and dowels is not well understood, critical slopes were extensively monitored. Based on the experience from Revelstoke and elsewhere, it appears to be worthwhile, in most circumstances, to reconsider the need for tensioning rock anchors.
Untensioned, grouted, steel bars (dowels) up to 30 m long were used to reinforce rock slopes several hundred metres high, excavated for a hydroelectric project near Revelstoke, British Columbia. Dowels were used to prevent surface rock from loosening, to prevent sliding along interconnected discontinuities and to prevent sliding of relatively intact rock along a thin shear zone. During the early years of construction, tensioned rock bolts and high capacity tensioned anchors were the primary means of rock reinforcement (Moore and Imrie, 1982). Dowels were first used to prevent further loosening of fractured and weathered surface rock in the excavations for a highway to bypass the dam on the east bank. The ease of installation and obvious effectiveness of the dowels in the bypass road excavation encouraged the use of high capacity dowels in the east abutment and powerhouse excavations and later in the west wall of the powerhouse. Because the interaction between rock slopes and passive anchors is not well understood (Azuar et al, 1979), these critical slopes were extensively monitored. The results of this monitoring, brief descriptions of the rock reinforcement system and some general considerations about the use of passive anchors are contained in this paper.
The east abutment and powerhouse excavation is an example of where dowels were used to prevent deep seated sliding along interconnected discontinuities in a highly fractured rock mass. Although no outward dipping shears existed in the east bank rock, foliation shears dipping gently into the slope were common, as were steep fractures parallel to the slope. A major gouge-filled shear zone, dipping into the slope as well as many other smaller, cross-cutting shears, existed in the slope (Fig. 1). This major shear zone is the most obvious expression of a regional fault which occurs at the damsite. Early movements on this fault resulted in a zone of mylonitized rock hundreds of metres thick, and the most recent movement about 40 million years ago resulted in the fractures and shears, as well as, pervasive chloritization of the gneissic rocks at the site. By the time the east abutment excavation was started it was clear from these geological conditions that the 100 m high, 0.5H:iV slopes would require substantial rock support. The slopes could not be flattened without undercutting the newly completed highway.
