Evaluation of Construction Methods for a 135-Inch Diameter Tunnel Crossing of a Major Highway Bridge

The crossing of Highway IH-69 with the City of Houston’s 108-in. water line presented unique design and construction issues. The tunnel design, which spans a major overpass, has a length of 1,150 LF with a primary liner diameter of 135-in. Along with the length, poor soil conditions posed major challenges to completing a successful installation. The soil conditions vary from full face of clay on the west, a full face of sand along the bore, to a mixed face consisting of clay and sand on the east. A 400-foot section of the drive showed coarse non-cohesive soils under the water table. This unstable material creates significant risk to the overpass structure and safe tunnel construction. The tunnel methods considered to successfully complete the crossing included soil stabilization with chemical grout coupled with a tunnel boring machine (TBM) with face closure abilities, or utilization of a pressurized face TBM. This tunnel diameter presents difficulties in selecting the means and methods of construction as it falls in the grey area defining the advantages between pipe jacking or building the tunnel primary liner in ring sets. During construction, the contractor proposed installing the portion under the bridge by open cut instead of tunneling. This method was evaluated but sidelined, as it was not perceived to reduce risk. Ultimately, the selected construction method consisted of using an earth pressure balance machine with jacked casing for the entire tunnel limits. Tunnel construction began in January 2020 and is past the IH-69 ROW as of early March.