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Ground Stabilization in Shallow Depth Soft Ground Tunneling

by Khosrow Bakhtar, Bakhtar Associates, Newport Beach, United States,
William Martin, Bakhtar Associates, Newport Beach, United States,
Bruno Dietl, Bakhtar Associates, Newport Beach, United States,
Paul Byrnes, Bakhtar Associates, Newport Beach, United States,



Document Type: Proceeding Paper

Part of: Detection of and Construction at the Soil/Rock Interface

Abstract:

A new approach was employed for ground stabilization to drive a semi-circular cross-section tunnel with overall dimensions of 24 ft (7.3m) wide, 12 ft (3.7m) high, and 164 ft (50.0m) long. The tunnel was mined through sandy-silty-clayey soil, SM and SC based on the Unified Soil Classification System, with the invert located in thin-bedded claystone and soft to medium hard sandstone. The beds were typically highly folded and fractured, with occasional small faults. The tunnel under-crosses the Pacific Coast Highway, a major state freeway, with a maximum crown depth of 8 ft (2.4m) below the surface. Location of the tunnel is within an active seismic zone, 1.2 miles (1.9km) from the Newport Inglewood Fault, which required seismic loads to be considered in design. The shallow depth of cover combined with the presence of a soil-rock interface along the tunnel invert presented a major challenge for the engineers involved to develop a design approach which would be feasible and cost-effective, while minimizing the risk of excessive settlement of the road. This paper describes the technique employed to stabilize the surrounding soil mass and the method used to design and construct the foundation footings at the soil-rock interface. Furthermore, the paper provides a practical example on how to differentiate between soil and rock for structural design purposes.

Subject Headings: Soil stabilization | Soft soils | Soil classification | Soil properties | Tunnels | Seismic design | Structural design

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