Located next to an ancient city, Bagram Airfield-BAF is the largest U.S. military base in Afghanistan. The third phase of its drainage system project included the installation of a shallow 1,700-ft-long, 6-ft-inside-diameter drainage tunnel beneath the base’s active taxiways and the main runway. The stormdrain had to be constructed through stiff to hard clayey and silty soils without impacting airfield operations.
Schnabel evaluated and supported the contractor’s value-engineering proposal to excavate the tunnel using pipe-jacking with a microtunnel boring machine instead of the owner’s specified two-pass earth pressure balance method. Our geotechnical services included scope development so a local geotechnical engineer could perform the required site investigation and laboratory testing program, as well as reviewing the resulting geotechnical report. We developed subsurface engineering properties for design and the geotechnical interpretive report/design summary. We also reviewed the design layout of the geotechnical instrumentation and monitoring scheme to be used during construction to monitor ground response and tunnel stability for acceptability with the contractor’s proposed tunneling method and developed a settlement monitoring plan for the contractor’s use.
We provided support to the contractor throughout the project by reviewing submittals from the contractor’s microtunneling subcontractor and instrumentation monitoring results during construction. We performed structural engineering design reviews of the contractor’s reinforced pipe and shaft design. During construction, we also reviewed and responded to areas of concern, including drainage pipes passing above the tunnel, the necessity of filling pipe joints after completion of pipe jacking, and potential ground settlement in an area of relatively shallow cover.