The case of piles wholly embedded in a settling soil mass – What’s with all the negativity?

Negative skin friction forces (NSF), or drag load, would need to be considered in the geotechnical design of piles where the soil surrounding a pile undergoes settlement or consolidation after the pile has been installed. Typically, in the Melbourne region, piles that are prone to NSF forces generally terminate in the ‘stable zone’ below the consolidating layer (e.g. bedrock) such that there’s sufficient geotechnical resistance to sustain the combined serviceability load and NSF forces, and to satisfy the serviceability requirement. For the new road bridge over the Murray River at Echuca-Moama (the ‘Dhungala Bridge’), the geotechnical design considered the development of NSF forces in the abutment piles resulting from potential long-term consolidation of the ground due to the construction of fill embankments at the abutments. With the piles wholly embedded in the settling ground, the traditional method of determining a sufficient ‘stable zone’ beneath the consolidating layer to resist the drag load could not be applied and an alternative approach, the ‘Unified Design Method’, was adopted to allow for the drag load in the design.