Support of underground excavations by resin grouted rock bolts is common in mining and civil engineering. However, the interactions between bolt and rock are not fully understood. Laboratory tests have been conducted for evaluating the shear strength of rock bolts under confining stresses, as it is the case in-situ. Particularly for shale very high values for cohesion and friction were found. Reliable input parameters for numerical modeling rock bolt behavior by FLAC have been determined and were successfully applied.
Support of underground excavations in rock is often provided by rock bolts. Particularly in mining the primary support system is fully grouted rock bolts with 100 Mio fully grouted bolts per year used annually alone in the US (Campoli and Shapkoff, 2004). Provided the installation procedures are followed, fully grouted rock bolts are a very effective measure for controlling the deformations of an underground opening. The development of fast setting resin gave civil or mining engineers a tool to design high support pressures by bolts that transfer load almost immediately upon installation. For head and tailgate development in coal mining often rectangular cross-sections are employed and the design philosophy is to control the roof displacement with bolts based on the beam theory (STMRAC, 1998). Also for civil engineering tunnels the use of rock bolts may provide the required primary support until the final lining is installed. Furthermore, the use of rock bolts often is the least time consuming procedure for support installations, which is particularly of interest for tunnel excavations with open hard rock TBMs (Alber and Ruehl, 2005).
However, there are still some shortcomings with respect to the design approaches to underground excavation support by fully resin-grouted rock bolts. Often, the support pressure (MPa) is estimated by dividing the maximum bolt capacity (kN), which is the lesser value of either the yield strength of the steel rebar or pullout load, divided by the number of bolts per square meter tunnel surface (Hoek and Brown, 1980). The pullout load is estimated by consulting design values from publications (Stillborg, 1994; Bieniawski, 1987) or from rock bolt pullout tests (ASTM 04435–84, 1998; DIN 215121, 1993). Those values may be used for estimating the final deformations by the "rock support interaction" or "convergence-confinement" analysis with easy to use software such as Rock Support (Rocscience, 2004) This method, however, neglects the effect of normal stress on the bond behavior between the interfaces steel rebar/resin/rock mass, which may, as shown below, be significant for the load behavior of fully grouted rock bolts. With the use of advanced numerical modeling, e.g. FLAC (Itasca, 2004), those effects may be incorporated into a logic for designing support systems for underground excavations.
The conceptual mechanical approach for the interactions between the reinforcement and the rock mass within FLAC accounts for the shear behavior of the grout annulus (Figure I).