Break-Throughs in Pile Driving, Testing and Construction Supervision at Kinmel Bay WwTW

Upgrade works to Dŵr Cymru Welsh Water’s Kinmel Bay WwTW included a new Inlet Works, a large Final Settlement Tank, and six smaller sludge tanks. These structures were piled due to poor underlying ground conditions. In total, 250 No. 250 mm square driven precast concrete piles were constructed. A complex geological stratigraphy of Tidal Flat Deposits overlying Glacial Deposits presented significant risks to the pile design and construction; including highly variable pile base capacity, and ‘punch-through’ of piles toed into competent granular soils overlying less competent cohesive soils. Over-driving resulting in pile capacity ‘set-down’ was also observed.

To mitigate these risks without compromising the tight construction programme, Mott MacDonald Bentley (who were appointed to design and construct the works), implemented a rigorous procedure of ground investigation, trial drives, dynamic and static load testing, and intensive construction supervision. Key criteria were imposed on the pile length (governed by the stratigraphy) and driving stiffness (to differentiate between pile base conditions). This enabled verification that both individual piles, and the pile-groups met the design requirements.