By F Lizzi, technical director, Fondedile
This paper was first published in the March 1980 issue of GE
The design of cast-in-place piles (with respect to load-bearing capacity and anticipated settlements) in heterogeneous soils is a very old geotechnical problem which has not yet been satisfactorily solved’. Theoretical approaches, based on laboratory or in situ tests, are still a long way from giving an acceptable picture of the very complex phenomenon of soilstructure interaction on which the loadbearing capacity of a pile depends.
More recent experimental approaches with devices such as penetrometers, pressuremeters, etc., cannot give particularly reliable results for the essential reason that these instruments, introduced into the subsoil, are structurally quite different from the piles; therefore any similitude of data derived from these instruments is not fully acceptable.
The very large safety factors usually associated with the above systems of design are a clear demonstration of their inadequacy. The direct load test, therefore, remains to date the only way for a safer approach to the problem. Load tests for large diameter bored piles (currently known as “drilled shafts”, “caissons”, or “drilled piers”), suffer several technical and operational problems, as well as very high costs.
The experimental approach suggested in this Paper is based on the use of a small diameter pile (referred to as a “pilot pile” ) as a test instrument, constructed with the same materials and same technology as the actual piles to be designed. The pilot pile, monitored with strain gauges at the toe and along its length, is subjected to a gradual load test up to final failure.
The data collected from the instrumentation is used for the design of the larger diameter piles, in accordance with a similitude relationship derived strictly from two “congruence equations”, and applied to both the pilot pile as well as to the actual piles. The cost of one (or more) pilot pile and of the associated load test need not be higher than any other in-situ experimental approach and is therefore fully acceptable, considering the overall cost of the foundations, sometimes drilled to considerable depths.