Tunnelling in contaminated ground

摘要

As part of the $6.3B East Side Access Project (ESA) in New York, four bored tunnels and two cut and cover sections will be constructed through contaminated soils and groundwater. The tunnels will be constructed under an active railroad yard (Sunnyside Yard) and the main line of the Long Island Rail Road (LIRR). The results of environmental site investigations revealed that the tunnels will pass through or nearby existing soil and ground water contamination. The project site is designated as a Class 2 inactive hazardous waste disposal site. Therefore, it is critical that any construction at the site would not result in movement of the contaminated plumes. The contaminations include lead, polychlorinated biphenyls (PCB) and carcinogenic polycyclic aromatic hydrocarbons as the principal contaminants of concern in the soil. A floating diesel fuel plume contaminated with PCBs and various dissolved phase petroleum and chlorinated volatile organic compound plumes were also identified. Semi-volatile organic compounds (SVOCs), toluene, and pesticide-impacted soils were also detected in the soils. Investigations also detected the presence of several ground water plumes of chlorinated volatile organic compounds (VOCs), semi-volatile organic compounds (SVOCs), and BTEX plumes in the yards and nearby areas. A three-dimensional steady state ground flow model was developed and supplemented by parametric sensitivity analysis to assess the construction impact on the ground water regime and the potential migration of the plumes. It was concluded that to prevent migration of the plumes, the groundwater drawdown should not exceed 610 mm after taking into considerations seasonal fluctuations. Therefore, diaphragm walls will mainly be used for the construction of the cut and cover sections. They will be used as the primary excavation support and as groundwater cut-off. They will be keyed 150 mm into rock to create a full seal. Pressurized face (EPB or Slurry) TBM will be used to construct the soft ground tunnels. The use of a pressurized face TBMs will limit the movement of ground water by controlling in situ stress imbalances. In addition, they provide their own resistance to soil and to ground water inflow by the use of sealed, pressurized chamber at the TBM's cutting head. The shield is sealed and pressurized to stabilise the face. Polymer or foam will be used to condition the soil and to help balance the ground water and soil pressures. The tunnel liner will consist of 300 mm thick one pass precast reinforced concrete fully bolted segmental liner equipped with 35 mm thick elastomeric gaskets. The lining is designed to be watertight as it emerges from the TBM by injecting grout through the TBM tail skin. Any residual leaks will be dealt with by further pressure contact grouting through the liner grout ports and if necessary by internal caulking. Groundwater from the excavation will be removed, sampled, treated, and disposed of properly. A comprehensive groundwater monitoring program was developed to effectively predict movement of any plumes and to provide ample time to implement any remedial measures. The project design will provide successful construction without spreading the existing contamination.