| Abstract: | A large-scale model study was conducted for the purpose of evaluating the influence of rockbolt length upon tunnel stability. Model tunnels one foot in diameter by three feet long were loaded triaxially, following a uniaxial strain path, until five percent closure was obtained. Each tunnel was supported by 132 model rockbolts, and each test was conducted with different length rockbolts. The post-test condition of the rockbolts was documented by systematically excavating the springlines of the tunnels after each test. Results of the research were analyzed for the purpose of developing a constitutive model for the failure of fully-grouted rockbolts. Step-wise debonding and tensile failure were evaluated for both a continuum and jointed material. Excessive closure of the tunnels was correlated directly with the percentage of rockbolts that failed in tension at the head. Conversely, stability could be correlated directly with the number of bolts that debonded. Hence, rockbolt length was identified as the key design parameter, and recommendations are made for design procedures based upon ultimate strength rather than yield strength. It was concluded, as a result of the research, that selection of design values for bond strength of rockbolts can not be based upon pullout tests alone. A rockbolt design must use Mohr-Coulomb failure criteria for the grout strength since there are significant variations in normal stress along a rockbolt after it is installed. Stress redistribution around a tunnel during excavation results in increasing tangential stresses, which translate into increased normal stresses on the rockbolts, which in turn translate into increased bond shear strength. As a result, in an advancing tunnel, at the same time the bond shearing stresses are increasing (from dilation or relaxation), the available bond strength is also increasing. Hence, a prediction of changes in tangential stress after rockbolt installation is required for an efficient design, and significant cost savings can be effected by selecting a rockbolt length which will debond rather than fail in tension. |
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