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Cracking The Pumphouse Peanut

A series of peanut-shaped diaphragm walls has helped create a basement pump house for a new power plant in Kent. GE reports

Geotechnical specialist Bachy Soletanche has used its expertise to aid civils contractor Laing O’Rourke in constructing the underground pump house for E.On’s new combined heat and power (CHP) facility on the Isle of Grain in Kent.

In a £5M groundwork contract, Bachy’s in-house design team generated a new concept, ideal for the conditions on site.

The project requires the pump house to be located in very soft silt, up to 18m thick, and close to the existing pumping station.

In these conditions it soon became apparent that many traditional forms of construction, such as a braced sheet pile cofferdams, were not going to be suitable.

Using a traditional circular shape for the pumphouse would require lots of propping and would be too expensive.

A cost-saving solution and a fundamental re-think of the structural concept was vital.

At that point Bachy proposed a cellular diaphragm wall solution, nicknamed the “peanut design”, derived from the company’s experience in the Far East.

The three-celled peanut design is divided by cross walls, which take away the need for propping.

The key to the innovative three celled solution was the constructability of the four large Y-shaped panels where the adjacent cell walls and the cross walls meet.

The concentrated loads coming from three directions meant that the joint had to be monolithic.

To construct a Y-shape, the panels were excavated with three bites of the grab, one for each leg.

“Very tight vertical tolerances needed to be achieved”

These excavations were considerably larger than the other panels on the project.

The effect of the large size was two-fold. Large excavations are generally less stable and each stage of the construction process takes longer to complete, only adding to the risk of instability.

Verticality of each bite of these panels is also vital to ensure the same shape of panel is excavated at the bottom of the hole as it is at the top.

Poor verticality causes many problems including making it impossible to install the prefabricated reinforcement cage into a hole that is the wrong shape.

“The need to transfer large horizontal hoop stresses from panel to panel throughout the depth of the structure meant that very tight vertical tolerances needed to be achieved,” says Bachy Soletanche engineer David Puller, who designed the cellular diaphragm wall.

“Without them the bearing surface between adjacent panels would be insufficient. Using hydraulic grabs fitted with real-time electronic monitoring equipment, the required verticality of one in 120 was successfully achieved throughout.”

The revised design had the significant advantage of allowing most of the diaphragm wall to be the permanent walls of the pump house.

The cross walls of the cellular scheme also served as two of the necessary divisions between the compartments of the pump house structure.

The design itself comprised a 1m thick wall to a maximum depth of 45m to permit an internal excavation depth of 18m.

piling_peanut_2

The internal cell has a radius of 18.2m, which incorporated the three cells. Although the “peanut” principle was accepted, many challenges still remained.

The actual design had to cope with 5m high openings in the cell cross walls at base slab level and it also had to be checked for asymmetrical loading conditions which would result from differential excavation levels in adjacent cells.

At Grain, specialist hydraulic grabs excavated the panels.

Throughout the construction process the panel excavation was kept topped up with bentonite, to prevent the sides of the hole collapsing.

In the particularly weak silty soils at Grain this is a very important technical aspect of the process.

Being so close to the coast, the groundwater level is particularly shallow, further reducing the stability of the open panel excavations.

“A number of mitigating steps were taken to reduce the risk of instability,” says Puller. “Where possible, the panel width was limited to 5m, a very close control of bentonite quality was maintained throughout, and raised guidewalls were employed.

“These guidewalls surround the top of each panel and by raising them above the ground level an additional head of bentonite was able to be maintained, which in turn exerts a greater stabilising force against the sidewalls of the excavation.”

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