The influence of close proximity blasting on the performance of resin bonded bolts.
Full scale field trials, carried out during the construction of the
Penmaenbach Tunnel in North Wales, have shown that two-speed resin
bonded rock bolts are resilient to close proximity blasting. Fully
grouted 6m long rock bolts, installed within 0.7m of the tunnel face,
have shown no significant signs of distress or failure. Instantaneous
loads of up to 40% of the characteristic strength of the bolt were
observed together with average residual load losses of 5% of the
working load, which compares favourably with the acceptable tolerance
of 10% working load stipulated by current practice.
Analysis has also shown that rock bolts with low prestress sustain
greater vibrations and proportionately higher dynamic load changes
during blasting. However, bolts with relatively high prestress loads
sustain greater induced loads.
Empirical relations have been established to describe rock bolt
behaviour in terms of induced vibration and scaled distance. In
particular, a predictive equation relating dynamic load changes in the
experimental rock bolts to scaled distance, is presented. Calculations
based on approximate bolting costs have indicated that cost savings of
up to 38% of the total bolting cost could have been effected if the
results of this work had been implemented at the design stage.
Physical modelling work has confirmed that the distribution of loads
in the fixed anchor of a resin bonded bolt are non-linear when both
static and impulsive loads are applied to the bolt head. Corroboration
of the field results has also been established with respect to the
significant influence of initial prestress load on dynamic load change.
Complementary finite element modelling work has successfully predicted
fixed anchor load distributions under static and dynamic loading
Attempts to establish a detailed relation between distance from
blast, magnitude of charge and change in residual load, for low to
medium capacity rock anchorages on the West Portal of the tunnel, were
thwarted by the poor performance of instrumentation with respect to
temperature sensitivity. However, a simplistic approach to analysis
has enabled the establishment of a tentative predictive relationship.