Uncertainties in Pile Capacities due to the Presence of Decayed Wood Layers

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The study examines uncertainty in deep pile capacity when piles encounter highly compressible, weak decayed-wood layers, a common condition in coastal Kochi, India. Samples collected at depths of 15–18 m had 93% natural moisture content, 9.02% organic content, and very high plasticity. Laboratory tests showed low shear strength and substantial compressibility.<br /><br />Two case studies from Ernakulam illustrate the potential effects. For a 1,200 mm diameter pile, estimated capacity fell from 830 metric tonnes under the actual soil profile to 181 tonnes if terminated in decayed wood. In another case, extending a pile by only 0.75 m into the layer reduced capacity from 412 to 214 tonnes. These reductions were attributed mainly to the decayed wood’s low end-bearing resistance.<br /><br />Settlement calculations for a five-pile group also exceeded the 25 mm permissible value under all three design assumptions. Estimated total settlement ranged from 146 mm for friction piles to 294 mm for end-bearing-cum-friction piles.<br /><br />The authors conclude that decayed wood should be treated as problematic ground because of its low shear strength and high compressibility. Its depth and thickness can vary significantly even between nearby boreholes, so detailed site investigations at close intervals are essential. They recommend designing piles primarily as friction piles where such layers are present and taking particular care not to terminate piles in or just above decayed wood.

Keywords

deep pile capacity

decayed wood layer

highly compressible soil

coastal Kochi

Ernakulam case studies

end-bearing resistance

pile settlement

friction piles

site investigation

shear strength