SHOTCRETE SUPPORT DESIGN IN BLOCKY GROUND - TOWARDS A DETERMINISTIC APPROACH

The precise mechanisms by which shotcrete acts to stabilize the ground around a tunnel are not well understood and are the focus of ongoing research and discussion. Consequently design of shotcrete support is s till more of an art than a science, with the majority of designs based on past experiences in similar settings, on empirical recommendations or on monitoring and modification, during construction. In an attempt to provide an analytical alternative, and to gain some insight concer ning the mechanisms which control shotcrete failure in blocky ground, four deterministic design models are presented which illustrate shotcr ete's effectiveness at supporting structurally defined blocks and wedg es, thereby avoiding progressive loosening and destabilization of the rock mass around an opening. These models may also be used to verify o r refine design recommendations which have been based initially on emp irical guidelines, and to give an indication of the approximate factor of safety. Examples of typical design charts are given. Results of an alyses based on these models show that failure of shotcrete is general ly a two-step process, governed initially by loss of adhesion to the r ock, followed by flexural failure. These results indicate that while a dhesion is of critical importance in maximizing the peak ground suppor t capacity of the shotcrete, flexural strength and toughness play a cr itical role in the post-peak region, once adhesion is lost.