Serge Borel, Michel Bustamnant, Luigi Gianeselli, Laboratoire Central des Ponts et Chaussees. This paper was first published in GE’s January 2004 edition.
Tests on piles are generally performed to check that their structural integrity is satisfactory and that their real capacity is in reasonable accordance with the forecast load-settlement curve of the anticipated working load. For the engineer the load test is considered satisfactory when it has been possible to determine:
- the ultimate resistance Qu
- the pile settlement under working load Qw
- the shaft Qs and toe Qp resistances under Qu
In current practice unfortunately, it is rare that all of these loads, Qw, Qs, Qp are available because the load test has not been taken to failure and liecause of lack of instrumentation. To define the ultimate resistance Qu and possibly the shaft resistance Qs authors such as Van der Veen (1953), Brinch Hansen (1963), Chin (1970), Davisson (1972),Mazurkiewicz (1972), Fuller & Hoy (1977) and others, have proposed methods based on mathematical or graphical approaches. A comprehensive comparative study of the nine methods most commonly used in practice has been carried out by Fellenius (1980).
Because ultimate resistance Qu remains the key value in most piling problems, it is worthwhile to recall that various definitions of this concept are given by different authors and codes. For example:
Qu,F - plunging failure, observed sometimes insitu and corresponding to a rapid increase of settlement without an increase in load (Figure 1)
Qu,asym - asymptotic ultimate load derived mathematically or graphically (Figure 2) from the measured load settlement curve
Qu,conv - conventional ultimate load defined by many codes as the load causing a gross settlement equal to 10% of the equivalent pile diameter D (Figure 2), or the load at which the penetration creep rate reaches a given value.
The failure derived by the Chin method falls into the mathematicalasymptotic category and has been defined further as Qu,CHIN.
This paper examines the performance of Chin’s recommendations on load tests with limited settlement and the ability of his method in predicting the proportion of shaft load.