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

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The paper analyzes more than 25 instrumented bored-pile load tests from the HS2 project to improve estimates of pile base capacity in extremely weak to very weak Mercia Mudstone Group (MMG). The tests covered different pile sizes, construction methods, support fluids, base-opening times, and rock conditions. Pile base resistance was interpreted from load–displacement data, generally using capacity at a displacement of 10% of pile diameter.<br /><br />The results support two complementary design approaches. For MMG with unconfined compressive strength (UCS) below about 2 MPa, an effective-stress method using a bearing capacity factor of 22 gave reasonable estimates and can reduce reliance on rock samples for UCS testing. For stronger, very weak MMG, a UCS-based method derived from Zhang and Einstein provides a reasonable, generally cautious estimate. A modification factor may be appropriate where rock quality and base condition are favourable; a factor of 1.5 was considered justifiable for GSI of at least 50 and good base-hardness results.<br /><br />Construction and geology affect performance. Base-opening times exceeding 48 hours appeared to reduce capacity, while bentonite showed no clear adverse effect when considered separately from opening time. Better-quality rock and thorough base cleaning were associated with a stiffer response. Soft clay layers, water ingress, and other local conditions may explain lower-than-predicted capacities.<br /><br />Overall, the study broadens conventional pile-design guidance by incorporating construction practice, base condition, and rock quality within the zone below the pile toe. The proposed methods are based on a project-specific test dataset and should be applied with attention to local ground conditions.

Keywords

HS2 project

bored-pile load tests

pile base capacity

Mercia Mudstone Group

unconfined compressive strength

bearing capacity factor

Zhang and Einstein method

base-opening time

bentonite

rock quality