Mott MacDonald

The construction of the reinforced concrete raised intake structure was required to extend the sustainability of the Tarbela 3 (1728MW) and Tarbela 4 (1410MW) hydroelectric power plants. There were several key constraints: construction of the shafts was in a large cofferdam; the monsoon season determined the construction schedule; monsoon inundation of the cofferdam; and reservoir water levels controlled by external agencies. Throughout our five-year construction phase, there were constant changes made to the schedule, increasing risk, while activities were affected by monsoons. It was an opportunity to innovate, however. The new adaptive strategy was to complete the 18m diameter shaft superstructures while they were under pressure. To achieve this, our solution was to install a temporary steel plug at the top collar of each 18m diameter shaft to isolate the construction from the pressurised shaft and lower intake tunnel. Usually, bulkheads are installed vertically or tilted but this temporary steel bulkhead was installed horizontally across the shaft collar to plug the shaft. This innovative solution isolated the construction of the superstructure from the pressurised shaft minimising effects of the monsoons, and enabling construction to stay on track. While the innovation of the horizontal steel bulkhead was significant, further innovation was required to facilitate its offsite manufacture, transportation, installation and removal. To achieve this, the bulkhead was delivered in modular form, with holding down bolts and joint seals designed to allow for the static and potential dynamic back pressures of operation. Originally, the work plan called for the shutdown of Tarbela 3 for 15 months to allow construction of its new raised intake structure. This would have prevented farmers accessing vital water for irrigation and reduced the amount of power generated by the plant. Isolating construction enabled us to reduce down time and the hydroelectric power plant to remain operating. Additional benefit derived from a robust and stable bulkhead, which provided a useful working platform, making it easier and safer for the construction of the whole superstructure. The shutdown of Tarbela 3 was drastically reduced, from 15 to just six months, enabling construction to take place during the dry season when the dam reservoir was at its lowest level. The reduction in shutdown time reduced the client’s overall project costs. For a relatively small investment, about US$3M, the solution led to a windfall of a not insubstantial US$200M+ of power generation revenue over the nine months the plant did not have to shutdown. The original estimate of the project was about US$710M, but our solution reduced the net cost in real terms to about US$500M.

This category rewards innovation that is boosting productivity, safety or quality in the design and construction of tunnel shafts, the construction process of tunnel linings, and the excavation process of a tunnelling scheme through new products, equipment or methods. The innovative product, equipment or methodology must have been used on a specific project and be able to demonstrate benefits to the overall project outcome and have delivered greater client satisfaction.

VINCI Construction Grands Projets is a subsidiary of VINCI, a global player in concessions and construction, employing more than 211,000 people in some 100 countries. The mission of VINCI is to design, finance, build and operate infrastructure and facilities that help improve daily life and mobility for all. Following on from successful projects, VINCI Construction Grands Projets London office was established in 1992 to ensure the development of local long-term UK partnerships. Established for more than 25 years, VINCI Construction Grands Projets is involved in the construction of major civil engineering structures with key customers.