Secant Piles Support Access Shafts for Tunnel Crossing in Difficult Geologic Conditions

A new 84 inch water pipeline project for Tampa Bay Water included a tunnel section under the Alafia River in Riverview, FL. Access shafts were required on each side of the river for the pressure-balanced tunnel boring machine. Since the 70 ft. deep access shafts extended roughly 25 ft. into the confined Floridan Aquifer (Tampa Limestone) under an artesian head, control of groundwater flows was a serious concern. Two circular cofferdams (one 40 ft. and one 24 ft. ID) were designed and built for these access shafts. Alternating, overlapping hard/soft secant piles (36 inch diameter) were utilized to act as a lateral cutoff wall. Circular concrete compression ring waters were constructed to support the secant piles at five (5) levels. In order to maintain bottom stability and to control bottom water inflows, a pressure relief well system was installed at each cofferdam. The smaller cofferdam was instrumented with an inclinometer in one of the secant piles to monitor lateral deflections. In addition, vibrating wire strain gages were installed in the concrete ring waters to measure compressive strains and the resulting stresses during the excavation process.