Tunnels & Shafts
Design of building foundations which overly tunnels requires detailed analysis of initial ground stresses acting on the tunnel and the added stress from building construction. Numerical modeling is often performed to compute the additional stress imposed on the tunnel liner. MRCE has designed foundations supporting many structures within the influence of transit tunnels in New York City and Washington, DC, and manages construction around utility pipes and small diameter tunnels.
Tunnels structures inserted into the ground by jacking, horizontal drilling, or mining come into intimate contact with the existing ground on construction, and rely on documented geotechnical conditions for method and equipment selection. Ground stresses change with time as the soil relaxes to contact the tunnel wall. Cut and cover tunnels, where precast pipe or cast-in-place box structures are constructed within an earth cut, are separated from the existing ground by drainage stone and soil backfill. Cut and cover structures require efficient excavation bracing design for rapid construction advancement.
MRCE tunnel design experience includes mined tunnels, horizontal drilling, small diameter boring machines, pipe jacking, and expansion of existing tunnel clearance and new liner structures. MRCE drilling inspection, sampling, and soils and rock laboratory testing contribute to tunnel alignment selection, liner designs, and construction contract documents.
MRCE tunnel project experience covers a wide range of tunnel contract needs, including:
- Feasibility studies and alternative alignment studies
- Tunneling method evaluation (NATM, TBM, Shield Driven, Drill and Blast and Raised Bore)
- Mined and Bored Tunnels. Tunnels with rock blasting.
- Soft ground tunnels and Mixed Face Tunnels
- Jacking, and jacking pits
- Cut and Cover structures
- Horizontal Directional Drilling (HDD)
Shafts provide access to tunnels and deep pipe networks, but also enable construction of underground structures such as escalators to subway stations, for ventilation, to launch and retrieve tunnel boring machines, or facilitate jacked tunnel construction. MRCE is familiar with the many ways shaft structures are constructed and tailors design and construction documents for each: frozen ground and deep soil mixing which provide a temporary support to enable cast-in-place permanent shaft construction, and slurry wall diaphragm walls and secant pile walls which become the permanent shaft structure.
Con Edison Harlem River Utilidor Tunnel
MRCE designed a power transmission “utilidor” tunnel under the Harlem River, between the Bronx and northern Manhattan, a heavily urbanized environment. An extensive monitoring program was put in place during construction to detect impact on the nearby Broadway Bridge, occupied office buildings, and a commuter railroad 20 feet from the northern shaft. Due to the limited turning radius of the transmission feeders, the transition zone between the shafts and tunnel required complex geometry with a large sloped, overhanging section. MRCE developed an innovative ground support design of the complex portal geometry, using numerical modeling and a detailed understanding of the rock mass. Anticipating difficult ground conditions in the marble bedrock, MRCE developed a probe drilling and pre-grouting program to ensure safe and timely tunnel construction.
Southern Reinforcement Project: Underground Transmission Line
An unusually complex electric line reinforcement project in southern New Jersey required installation of a 500-foot long trenchless crossing 30 feet below a lake and highway ramp. The challenging soil conditions consisted of loose fill and soft clay deposits. The optimal 48-inch diameter micro-tunnel profile was in the soft clay. MRCE designed an interlocking steel sheet pile cofferdam with tremie concrete plug for groundwater cut-off. The tremie plug gave the contractor significant cost savings. MRCE also designed and installed an instrumentation and monitoring program to verify that adjacent utilities, roadways and existing structures were not damaged by the trenchless micro-tunneling operation.