The Oakland County Water Resources Commissioner is undertaking the Middlebelt Transport and Storage Tunnel Project in the Evergreen Farmington Sewage Disposal System to address system bottlenecks and to prevent potential sanitary sewer overflows.
The project plan includes a tunnel along Middlebelt Road in Farmington Hills, Michigan to transport and store wet weather flows. Wade Trim Associates is the prime engineer for the project, and NTH Consultants, Ltd. (NTH) was retained as a subconsultant to provide geotechnical expertise and underground design services.
The proposed tunnel is 9-foot inside diameter and is approximately 7,500 feet long. Several shafts are proposed including a tunnel launch shaft, recovery shaft, and intermediate shafts for manholes or chambers. An understanding of geotechnical conditions is critical for any underground project, and this is especially true for tunneling work. NTH reviewed historical geotechnical data in the project area to establish the expected subsurface conditions. Using this information, NTH then developed a detailed scope of work for the geotechnical investigation and utilized a subconsultant to perform the field work and lab testing, and to prepare the Geotechnical Data Report. The field investigation included over 1,500 feet of subsurface drilling and sampling. NTH worked closely with the subconsultant to address difficult subsurface conditions that made drilling challenging in some locations.
NTH carefully reviewed the Geotechnical Data Report prepared by the subconsultant and made revisions as needed. NTH used this information to prepare several tunnel alignment options, and worked carefully with the project team to ensure hydraulic issues were accounted for. As the design progressed, it became clear that some of the proposed structures, including the tunnel recovery shaft, would be located in areas of potentially high groundwater inflows. NTH therefore recommended that a pump test be performed at the site to help determine the best ways to control the groundwater flow during construction at these difficult locations. This information was specifically critical in establishing how the recovery shaft could be installed and excavated most effectively.
NTH prepared the Geotechnical Baseline Report (GBR) for the project, which is an especially important document for tunneling projects. The GBR is used to properly manage project risks and associated costs by defining the geotechnical baseline conditions for bidders. The GBR provides contractual statements to describe the geotechnical conditions that are anticipated during underground construction. The GBR also discusses how the ground conditions will impact the anticipated means and methods of construction.
Using the geotechnical information obtained, NTH worked with the project team in selecting the most appropriate tunnel alignment and shaft locations. NTH also used the geotechnical information to analyze the tunnel liner requirements, the type of tunnel boring machine needed, ground stabilization measures and shaft constructability.