Site survey for efficient power generation
ESB International, an Irish state-owned electricity company, developed an 860-MW natural gas combined-cycle gas turbine (CCGT) power plant at Carrington, Manchester, UK.
The plant will operate at about 57% fuel efficiency, compared with the 30–35% efficiency of the former power station on the same site. As part of the development, ESB also planned to construct a 3-km gas pipeline from the National Transmission System to the CCGT site. The pipeline route passes through land with several different owners, including Peel Holdings, Maro Developments, Shell and National Grid Gas.
We were tasked with carrying out a geophysical and intrusive investigation of the proposed gas pipeline route to determine the location of all the below-ground services, foundations, geological constraints such as faults and other obstructions that would render it infeasible to lay a 600-mm diameter steel gas pipe along the proposed corridor. We also acted as principal contractor throughout the site work and managed all the contractors and the health and safety aspects.
Phase 1: Geophysical survey
Our surface geophysics survey covered a 50-m-wide corridor along the pipeline route. For the buried services, we used state-of-the-art radio-detection instruments jointly with a fully documented inspection cover lifting exercise and a ground penetrating radar (GPR) survey using a 400-MHz radar system. Our specialists combined all the utility tracing, visual inspection and GPR information with a topographic base plan of the site and existing utility record drawings to produce a comprehensive plan of the services crossing the pipeline route. Electromagnetic techniques using EM31 and EM61 instruments complemented the radio-detection and GPR techniques.
Phase 2: Detailed investigation
After completing the geophysical survey, we opted for a targeted intrusive investigation to establish the nature and depth of the features identified in the geophysical survey. The intrusive techniques included hand-dug, machine-excavated and vacuum-excavated trial pits; we targeted more than 100 locations along the proposed pipeline to prove the anomalies. We conducted a concurrent borehole investigation of the sandstone bedrock along the route that revealed a discontinuity. We used a host of geophysical techniques (resistivity imaging, seismic refraction and surface wave ground stiffness) to determine the nature and structure of this possible fault. The seismic survey showed a sudden drop in the seismic velocity corresponding to a vertical displacement of about 10 m.
Using specialist downhole seismic, GPR and magnetometer instruments, our specialists completed further geophysical testing on a series of boreholes drilled near the former pump house and along the water coolant culvert. The results revealed that the piles extended about 11 m below ground level and were not reinforced. This information aided the design of the redevelopment plans. We finished our programme of work on schedule and within budget.