John Gannon of Byland Engineering has won the BGA pile prediction contest.
Entrants were asked to predict pile head deflections at working load and 1.5 working load for 450mm and 600mm diameter cfa piles.
Gannon's results of 1mm and 2mm settlement for the 450mm pile and 3mm and 7mm for the 600mm pile were closest. Actual deflections were 0.99mm and 1.84mm for the 450mm pile and 3.77mm and 8.52mm for the 600mm pile.
Here Gannon describes his method:
Fleming's (1992) method of single pile settlement prediction uses six key input parameters with pile geometric properties to describe an algorithm which presents the pile load-settlement relationship.
Three superimposed effects are at work: pile shaft-ground response, pile base-ground response and pile shortening.
Of the six key inputs, three are most important: ultimate shaft capacity (Qsu), ultimate base capacity (Qub) and modulus of the ground beneath the pile base (Eb).
Three approaches were used to assess ultimate shaft capacity and one approach for ultimate base capacity (Table 1).
Shaft capacity in the made ground was neglected. In the clay, a conventional total stress 'alpha' approach A was used to predict ultimate 'adhesion'. The rock strata are weak rocks at the difficult boundary between soils and rocks proper.
Therefore approach B, based on effective stress soil mechanics 'beta' theory, and approach C, based on weak rock practice, were used and an appropriate intermediate value selected for use in the settlement prediction (see Ciria Report 181 Pile foundations in weak rock for more details).
Base capacity and base modulus of the rock strata were based on guidance in the Ciria publication. The base modulus was chosen to reflect the contractor's prior knowledge about load testing proposals. For the 450 mm diameter pile, which was not to be tested, a low modulus value representative of less care being taken in forming a clean pile-ground base contact was assumed.
For the 600mm diameter pile, which the contractor knew would be load tested, a higher modulus, representing a higher standard of construction practice, was assumed.
References Fleming WGK (1992). A new method for single pile settlement and prediction Geotechnique Volume XLII Number 3 Gannon JA, Masterton GGT, Wallace WA, Muir Wood D (1999).
Pile foundations in weak rock. CIRIA Report 181.