Base Capacity Design for Piles in Extremely Weak to Very Weak Mercia Mudstone

The Mercia Mudstone Group (MMG) is the bedrock for a large portion of central and southern England, and often the material on which the foundations for infrastructure in the area are built upon. The High Speed 2 project, a major intercity high speed railway line connecting London to Birmingham, offered the opportunity to analyse the behaviour of bored piles founded in the MMG under axial loading. The joint venture between Balfour Beatty and VINCI (BBV), with support from design partners Mott MacDonald and SYSTRA, has been awarded the delivery of Lots N1 and N2 covering a 90km stretch, south of Birmingham. This paper covers the results from over 25 pile load tests located between Birmingham and Coventry, focusing on the bearing capacity response of the pile toes within the MMG strata. The pile testing schedule encompassed piles of varying diameter/length and different piling techniques including variation in temporary support measures and pile opening time.  The analyses undertaken considered the impact of construction methodology, geology, and rock quality on load- displacement behaviour. Based on the test results, updated methods for calculating pile base capacity are proposed for extremely weak to very weak MMG. These methods serve to broaden the conventional design methodology and provide potential to optimise design to include effects of construction methodology, opening time, base hardness, and Geological Strength Index (GSI) of strata within 5 diameters below toe. Two methodologies are proposed, dependent on the strength of the MMG; the first is the use of an effective stress approach (for materials closer to extremely weak), and the second an update on the relationship with Unconfined Compressive Strength (UCS) approach, building on the work of Zhang L. & Einstein H.H (1998) (for materials closer to very weak).