Rory Lennon and Phil Thurlow of Halcrow Special Structures. This paper was first published in GE’s December 2006 issue.
Before the wide application of reinforced concrete in practice in the early part of the twentieth century, large gravity retaining structures built in the UK often required load-relieving measures such as the embedment of a horizontal timber grillage supported by timber piles, into the retained material.
This would be ruled out today by the high standards of construction which are achievable from the greater availability and affordability of materials strong in flexure, such as reinforced concrete and steel, and specialised machinery such as pile drivers, required to build walls from the above materials.
This work is based on MathCAD spreadsheet overturning and sliding limit point stability analysis of an existing river facing wall which retains material for a wharf structure. The retained material comprises soft natural alluvium and over this, broken slate fill. The overburden pressure on the alluvium is partially relieved by a horizontal timber grillage platform, which in turn is supported by a grid of vertical timber piles, driven through the alluvium and founded on bedrock.
Each pile in the grid is driven in segments of two or three, and segments are connected end to end by sleeve (v notch) joints, the number of pile segments depending on the depth to bedrock at each particular pile location. This work aims to demonstrate that the timber piles in the alluvium provide benefits which are additional to their acknowledged original function, ie to support the horizontal timber grillage relieving platform.
Figure I represents the wall and soil system which is assessed in this work and is a simplified version of the actual engineering problem. The system of forces in the established Coulomb active trial wedge calculation for a cohesionless soil is developed to include the resistance from pile friction and pile dowel resistance in the failure wedge. This additional resistance gives a reduced coefficient of active pressure Ka for application in further overturning, sliding or numerical modelling calculations.
While this work is described in the context of a gravity wall retaining piled material, the methodology can in fact be applied to any failure plane system which could benefit from additional friction or bearing resistance gained through embedded components.