BASE CAPACITY DESIGN FOR PILES IN EXTREMELY WEAK TO VERY WEAK MERCIA MUDSTONE-PPT Presentation

PPT Presentation

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The presentation examines the base capacity of piles founded in extremely weak to very weak Mercia Mudstone, drawing on axial load tests for High Speed 2 viaducts. The test programme included 25 instrumented sacrificial bored and CFA piles, 900–2,100 mm in diameter and 20–40 m long, tested using O-cell or top-loading methods.<br /><br />The study considers how geological conditions and construction quality affect pile performance. Relevant factors included support-fluid type and temporary support methods, the time the pile base remained open, water ingress, borehole collapse, concrete overbreak, and base hardness. Crosshole sonic logging, Thermal Integrity Profiling, construction records, and recorded anomalies were used to assess pile integrity and workmanship.<br /><br />Test results were interpreted using established methods, including approaches by England, Fellenius, and Fleming. Instrumentation data were processed to assess load–displacement behaviour, normalise responses over time, and distinguish shaft resistance from base resistance. Where necessary, results were extrapolated.<br /><br />The researchers then back-analysed measured base capacities against design predictions, considering methods for estimating end-bearing capacity in rock. The presentation highlights the importance of accounting for workmanship and rock quality through a correction factor, and of selecting an appropriate bearing-capacity factor for the mudstone’s strength and effective stress conditions. Overall, it emphasises that reliable design of piles in weak Mercia Mudstone depends on combining suitable test interpretation with careful consideration of geology and construction practice.

Keywords

Mercia Mudstone

pile base capacity

axial load tests

High Speed 2

bored piles

CFA piles

end-bearing capacity

load-displacement behavior

construction quality

geological conditions