The design of the Nant Ffrwd mainline bridges with flexible integral frame abutments

As part of the A465 Head of the Valleys Sections 5&6 project two integral bridges with a maximum span of 70m across the Nant Ffrwd valley were designed to support the new mainline carriageway construction. The topography and founding rock levels were such that abutment walls up to 19m in height were required. To provide significant material and carbon efficiencies the abutments were designed to be slender with their flexibility accounted for in the design. In the UK for full height integral frame abutments, PD 6694–1 (BSI, 2020) presents a limit equilibrium/simplified method to determine K* the enhanced horizontal earth pressure coefficient generated from strain ratcheting, where the values of earth pressures generated are based on the full height of the abutment and do not consider the flexibility of the abutment. For slender abutments this can be conservative for the design and unrealistic to the soil structure interaction. Where the foundation is rigid and the abutment flexible there will be parts of the abutment which will not deflect or rotate from expansion of the bridge deck and will thus not generate earth pressures that approach the limiting K* values. This paper will discuss the design of the Nant Ffrwd bridges and the methods used to determine the magnitude of earth pressures and their distribution based on the stiffness of the foundation and the structural elements.