Field Study of Tunnel Prereinforcement

A program of field instrumentation was implemented at two tunnel sites to examine the mechanisms by which prereinforcement and, in particular, spiling reinforcement work. The investigation was designed to address questions primariliy related to the magnitude, distribution, and time history of the deformation-induced tension and bending of spiles as a result of excavation. Deformation induced tension was shown to be the major mechanism by which spiling reinforcement displayed its effectiveness. Results also indicated that the reinforced arch thickness was strongly dependent on ground type, while arch capacity was largely a function of opening size, shape, and depth. A study of reinforced ground-support system interaction revealed two methods by which the loads on internal supports can be predicted. One method considered stress relaxation within the reinforcement, while the other examined the characteristic shape of spile stress distributions.