Waller Creek, which winds through The University of Texas at Austin campus, was prone to flash flooding, particularly along the lower portion where it flows through the eastern edge of downtown to Lady Bird Lake. The Waller Creek Tunnel project was developed to provide flood relief for much of the downtown area.

Designed to siphon water from the lake to the creek during dry times, the tunnel would reverse flow during heavy rain to reduce downtown flooding and allow development in what was previously flood plain. Developed primarily in Austin Chalk, with Eagle Ford Shale present at both the upstream and downstream ends, the main tunnel was constructed using sequential excavation methods (SEM) with multiple roadheaders, and used cast-in-place concrete as the final lining.

The 5,200-ft long conveyance/storage main tunnel is comprised of three different cross-sectional diameters of 26.5 ft, 22.5 ft, and 20.5 ft respectively. In addition, two large-span connection chambers are located at the 4th Street and 8th Street Inlet locations. The work/mucking shaft at 4th Street is 30-ft in diameter and 70-ft deep and was finished as the 4th Street Inlet Structure.

Schnabel’s initial work on the project included a constructability review focusing on many of the challenges faced on a heavy civil construction project located in a highly urban setting. At the contractor’s request, we redesigned initial support for the work shaft, and for SEM excavation of the tail tunnel, conveyance/storage tunnel, and two large-span connection chambers. We paid particular attention to the arch capacity of the chalk in the connection chamber spans, which were over 60 feet in length, and to the swell potential of the Eagle Ford Shale.

The designs used both traditional empirical calculations and numerical analysis using PLAXIS, and resulted in the development of four ground support categories for each diameter of tunnel. We were also tasked to evaluate the merits of substituting polypropylene fibers for steel fibers for use in the planned shotcrete mix, and to evaluate substituting friction-anchored Omega bolts for traditionally grouted DYWIDAG rock dowels.