Development of Design Options for the Installation of 10 Km of Sewer in Very Soft Lacustrian Clays

摘要

· This paper describes a project that demonstrates the benefits of: · Careful evaluation and acceptance of project alternatives, · Development of a refined alternative, · Using a consultant with specialist expertise and experience · Conception and implementation of a performance specification, · Use of advanced techniques in a developing environment, · Implementation by an experienced designer/contractor. The Project, for Owner EAAB, the water and sewer authority for Bogota, proposed to install a large interceptor sewer running through some rural but largely urban areas. The soils conditions were principally soft Lacustrian deposits of clayey silt or silty clay with a high water content and low shearing resistance extending, most unusually, to 700 m depths in may areas. The conventional scheme that had been developed was based on the construction of an open cut sewer using sheet steel piling with a large pump station close to the start of the sewer section. A review determined that there was sufficient fall between the upstream connection and the outfall to allow a gravity line to be installed. The recommendation was a change of concept on the project to a deep gravity sewer installed by pressure balance tunneling (microtunneling) with depths of installation of between 13 to 24 meters. The estimates developed showed EAAB a USD 10 million saving using tunneling over the projected costs of the pumped scheme at USD45 million. The deep tunneled sewer also eliminated the operation and maintenance costs of the pump station and greatly reduced disruption, potential settlement problems, and with a shortened period of construction compared to the open cut approach. Challenges to be addressed on the design and construction of the project were ongoing global settlements in the general Bogota area, the potential for differential settlements at all interfaces including sewer/shaft interfaces, sewer and shaft settlement with 700 m of underlying soft clay to consider and a high level of seismic activity. The bid was let as a "design and construct" contract based on tight performance specification and quality control regime. The successful bidder opted for a jacked pipe solution at USD 35M, which was within the 3% of the cost estimated. The work was completed with out any real problems within the schedule and contract price.