Project Description

The Big Walnut Augmentation/Rickenbacker Sanitary Interceptor Project involves the geotechnical investigation and design of about 38,000 feet of 168 inch and 144 inch finished diameter sewer tunnel as well as some smaller diameter connecting spurs, and preliminary studies regarding an additional connecting sewer. LFA's involvement on this $220 million project began in 1997, with completion scheduled for 2008. The tunnel will be built through unconsolidated deposits consisting of water-bearing glacial outwash (sand and gravel) containing significant numbers of cobbles and boulders, and glacial till (silty clay with sand and gravel).

Scope of Services

Working as a member of the design team headed by URS Corp., LFA provided complete geotechnical services for the design and optimization of the field boring and laboratory testing program; interpretation of the data; development of subsurface engineering properties and design criteria based on subsurface data; specialized investigation to characterize the cobbles and boulder fraction of the project area; and development of the Geotechnical Baseline Report (GBR). LFA performed an evaluation of the groundwater and hydrogeologic condition resulting in a determination that the main tunnels will be conducted with a pressure balance type tunnel boring machine in order to minimize impact to the groundwater resource and lower the risk associated with geologic uncertainty.

LFA also performed design of all underground structures, including tunnels and shafts, preparing 16 technical specifications and about 40 drawing sheets for each of two tunnel construction contracts. Tunnel design services included the structural design of the one pass precast concrete segmental lining based on the specialized construction method and equipment required, and the owners requirement for a "corrosion-proof" sewer; the design of the final support systems for the 13 shafts which will be required for the project; the evaluation of project dewatering requirements; the design of the geotechnical instrumentation to monitor response of the groundwater and the soil to the excavation methods; drawing input; provision of cost estimates, writing of specifications, and evaluations of constructibility for the tunnel. The range of expected geological conditions includes stiff to hard silty clay glacial tills, water-bearing sands and gravels, and large boulders.

During the construction phases, LFA will provide monitoring of the geotechnical instruments; comparison of the ground response to predicted conditions; resident geotechnical representative services; and integrating and analyzing the data from inspection records, soil testing, subsurface instrumentation, surface monitoring, and TBM data recording.