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How do you estimate shaft friction for bored piles in London Clay when SPT data is sparse?

Working on a 15-storey RC frame in Lambeth — the SI only has 4 boreholes across a 0.3ha footprint. Trying to decide between the empirical α-method or going full effective-stress. The clay is heavily overconsolidated at depth but I'm not confident in the s_u profile. How would you approach the undersampling?

#geotechnical#piles#london-clay·Graduate MEngNardia Osei-Bonsu · 3 days ago

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Shaft friction carries most of the load in service; base resistance only mobilises at large settlement. Undrained shear strength s_u rises ~7–10 kPa/m with depth — so design the profile, not a single average line.

AI-generated reviewed by Dr. Sarah Mitchell, CEng MICE

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22
CEng MICEDr. Sarah MitchellAccepted answer AI-reviewedKI 91
For overconsolidated London Clay the α-method is conservative at depth — once you're below ~6m the effective-stress (β) approach gives a better fit with the real shear-strength profile. With only 4 boreholes I'd anchor s_u to the Skempton plasticity correlation, then sanity-check against any SPT N-values you do have using s_u ≈ 4.5N. Crucially: don't average across the footprint — London Clay's s_u increases ~7-10 kPa/m, so a single design line will over-predict the shallow piles and under-predict the deep ones. Split the profile.
14
FICEProf. Idris Adeyemi AI-reviewedKI 88
Agree with Sarah. One addition: with sparse SI the governing risk isn't your method, it's the variance. I'd specify a preliminary pile test (or at least a maintained-load test on an early working pile) and write the redesign trigger into the spec. First-principles judgement here is recognising that the SI is the weak link, not the calc.

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