As part of a joint venture, TBI was responsible for construction of 27,900 feet of 13-foot-diameter sewer tunnel utilizing three Lovat TBMs at a tunnel depth of approximately 160 feet. Two of the drives were driven in predominantly alluvial soils, below the water table, using EPB TBMs in the closed mode, with extensive soil conditioning required to maintain face pressure and control loss of ground and settlement. The third drive was driven in siltstone/claystone with a conventional TBM. The tunnel support was precast segmental bolted and gasketed lining. The inflow through the lining was less than 30 gpm over the length of the tunnel. Three work shafts/diversion structures were constructed utilizing slurry wall initial support; seven 14-foot-diameter maintenance shafts were drilled to tunnel depth in advance of the tunnel drives, with tunnel to shaft connections made after the tunnel was driven past the lean grout plug installed at the base of the drilled manhole. The tunnel alignment ran through a densely populated area of downtown Los Angeles and passed under three major freeways, requiring an extensive ground stabilization grouting program to be carried out in advance of tunneling.The owner had serious concerns with regard to ground settlement and the effect it could have on utilities located above the tunnel.
The nature of the soils was such that there was a high likelihood settlement would occur. Nearly as great a concern was the adverse public relations that would result from any publicity about property damage or traffic disruption. TBI was able to eliminate the problem by utilizing face pressures and grouting techniques that resulted in zero measurable ground surface settlement over the entire length of the 5.6-kilometer tunnel.
The relatively small diameter of the tunnel and need to have the capability of performing TBM tool changes under 2.5 bar of pressure, coupled with the tight project schedule, made the compressed air setup a critical part of the operation. The TBM was too small to incorporate an integral lock system, and compressing the entire tunnel based on the porosity of the sands would have required an air compressor volume that would have been difficult to deliver and maintain. Therefore, TBI developed a “portable” airlock system which was towed by the TBM. Whenever compressed air was required to perform tool changes or cutterhead maintenance, the lock structure was affixed to the TBM plenum bulkhead through a series of tension members and a rubber diaphragm installed between the tunnel liner and the lock structure to form an airtight seal. This allowed the team to compress only that portion of the tunnel which was forward of the trailing gear and keep the amount of air required to maintain face stability and water control to a minimum.
Due to the high water table and variable ground conditions along the length of the tunnel, conditioning the soil to maintain a sand plug and to control the consistency of the muck was a major challenge. A major research and development effort was undertaken upon award of the contract, wherein TBI took samples of the various soil types along the alignment and had each tested to determine a “formula” of ground conditioning additives that would best treat each of the different types of soil. From this research a table was developed that classified each soil type by gradation and moisture content and prescribed the formula best suited to attain the optimum conditioning.