Shoring the Insertion and Pulling Shafts
Proper shoring of the insertion and pulling shafts is essential for the safety of the workers and the surrounding environment. The trench shoring or bracing should be constructed to comply with OSHA standards. Some of the available means of shoring these shafts are: trench box, solder pile and lagging, steel sheet piles, corrugated pipes, etc. Also, if space is available, sloping the sides of the shaft to provide stability is an option. The judgment and the supervision of a competent person (as defined by OSHA) or a qualified geotechnical engineer is needed to ensure safe shoring.
In the pulling shafts, the winch will thrust against the manhole wall or one side of the pulling shaft. This side must be able to withstand the pressure from the pulling winch. Therefore, a thrust block or structure is needed to distribute the force over a larger area. This is especially important for the static pull applications, for which the contractor should construct a thrust block against which the pulling system thrusts during pulling and bursting. The thrust block, shoring, and soil behind the shoring must be able to withstand the stresses from the pulling system. The passive earth pressure of the soil has to exceed the stresses generated by the pulling system with an acceptable factor of safety.