The tunnel and shafts that make up this project shall house a force main running parallel to an existing MSD force main constructed in 1968. The alignment of the tunnel generally runs beneath River Des Peres in south St. Louis. The new force main will connect MSD’s Pump Station No. 1 to the Lemay WWTP.
The South Shaft was excavated within the Lemay WWTP to a depth of 176’ below ground surface and a diameter of 25’. The upper 50’ of the shaft consisted of soil with karst features approximately 20’ above competent bed rock. Once the South Shaft was excavated, a 150’ starter tunnel was excavated utilizing drill and blast methods to a horseshoe shape diameter of approximately 15’. The TBM was assembled within this chamber prior to beginning tunnel excavation. While tunnel excavation commenced, the North Shaft was excavated at the pump station to a depth of 128’ below ground surface and a diameter of 18’. This shaft served as the retrieval shaft for the TBM. Ground support within the soft ground portions of the shafts consisted of ring beams, lagging and liner plates. Rock bolts, wire mesh and shotcrete provided support through shaft bedrock.
The tunnel was excavated to a diameter of 11.25’ and a length of 3,100’ through a combination of dolomite and limestone bedrock. Approximately 86% of the tunnel alignment consisted of competent rock with the remaining 14% consisting of blocky rock conditions within the tunnel crown that required extensive rock bolt, wire mesh and steel strap support.
96” diameter reinforced concrete pipe shall be positioned inside the tunnel and shafts and the annular space between the pipe and excavated surfaces shall be backfilled with low density cellular grout.
The tunnel and shaft excavations were very dry, producing less than 10 gpm.