Tough Specifications
The tunnel alignment places the new line directly beneath busy Houston streets, very near the congested downtown district. The grade and elevation result in cover depths up to 55 ft. through dense silty sands and very stiff clays. To assure the longevity of the finished line, project designers, Lockwood, Andrews & Newnam of Houston designed the sewage carrier pipes to withstand all loads assuming no long-term strength from the primary tunnel liner. Other important considerations included achieving corrosion resistance and a leak-free waterway. Due to the depths and surface congestion/development, only tunnel construction methods were permitted. Specifications allowed direct jacking with HOBAS or RCP pipes and conventional tunneling with a primary liner and a waterway (sewage) carrier. Sewage pipes within the primary liner could be cast-in-place concrete, RCP or HOBAS. All concrete required protections with PVC or polyethylene liners.

Substantial Savings
Successful bidder, Affholder, Inc. of the St. Louis area, based his proposal on a conventional tunnel approach with HOBAS pipes as the waterway carrier due to the significantly lower installation costs, which included:

• Approximately 12% smaller primary tunnel.
• Over 20% less tunnel excavation and haul away volume.
• Lighter, less expensive equipment needed to handle and install the 4 ton, 20 ft. long HOBAS pipes.
• Easy rigging by lifting HOBAS pipes through a single grout bushing hole.
• Fast insertion of HOBAS pipes; up to 800 ft. in a shift.
• Fewer access shafts required due to long push distances possible with HOBAS pipes.
• On demand delivery to avoid jobsite storage due to very limited space available in downtown Houston.

Successful Installation
Affholder placed the HOBAS pipes into the tunnel by pushing them consecutively in a "train" from the access shafts. Friction between the smooth HOBAS exterior and the muck car rails was so low, that long pushes up to 2,700 ft. were completed easily within the allowable 400 ton capacity of the pipe. Affholder's crew preplaced wooden blocks in the tunnel ceiling ahead of the advancing pipe to fix line and grade during the annular space grouting. The grouting was accomplished in 3 lifts, gently floating the pipes upward a fraction of an inch to the restraining blocks. The completed installation is nearly round with maximum deflections of about 1%. Affholder's project manager, Jerry Shaw, concluded that using HOBAS pipes is the best way to build a tunnel project.