Mercia Energy from Waste - Anchored Secant Pile Wall

Following on from a successful tender, Byland Engineering, working for specialist piling subcontractor Van Elle Limited, carried out the Eurocode compliant detailed design of a permanent anchored secant pile wall for the Mercia Energy from Waste (EfW) plant near Kidderminster, Worcestershire.

The ground conditions at the site comprised Made Ground and Superficial deposits overlying grade IV to grade I weathered Mercia Mudstone.  Permanent groundwater levels were expected to be at 1m below top of wall. 

The secant pile wall encapsulates the facility over a linear length of approximately 400m, with a temporary retained height of 8.7m reducing to 8.0m in the permanent condition.  A permanent haul road for waste delivery around the perimeter required the embedded wall to be designed for a 20kPa back of wall surcharge and accidental vehicle impact load.

The hard/firm secant pile wall comprised 750mm diameter primary female piles using C8/10 concrete and formed by cost effective continuous flight auger (CFA) techniques.  To ensure verticality over the retained height the hard secondary male piles were cut into the female piles using rotary bored piling techniques with an 880mm OD temporary cased diameter over the full retained height and 750mm diameter open bore below formation level. The secant pile wall was propped at capping beam level by 157 No. permanent double corrosion protection (DCP) ground anchors at 35° below the horizontal and 2.3m centres.  The anchors comprised a 165mm diameter bore filled with a neat water cement grout and 3 No. 15.2mm diameter drawn wire strands.  The design ground to grout bond value in Mercia Mudstone calculated by empirical methods and by reference to publised data were validated by testing of 2 No. preliminary sacrificial ground anchors in advance of construction.  In addition, each works anchor was subject to acceptance testing to 150% of the working load. 

Besides detailed design calculations, Byland Engineering detailed the temporary works guide wall and produced numbered pile and ground anchor layout drawings and schedules for construction.  Following on from the successful completion of the works Byland Engineering/Van Elle also undertook the design and construction of a small contiguous pile wall and bearing piles within the basement.