New failure load criterion for large diameter bored piles in weathered geomaterials

In the literature, various "failure criteria" or methods of estimating the failure load in pile loading tests have been proposed. The criteria, based on varying assumptions, were intended for different methods of pile testing and were verified on tests of a variety of pile types and sizes. Most of the criteria were not developed for slow maintained loading tests of large-diameter (greater than 0.6 m) and long bored piles. Piles of this kind have considerable resistance, and it is often impractical to reach failure load as defined by the various criteria. In this paper, a total of 38 large-diameter bored piles (drilled shafts) that were tested, ranging from 0.6 to 1.8 m in diameter, varying from 12 to 66 m in depth, and founded in weathered geomaterials (rocks and saprolites), are critically reviewed and studied. Among them, a selection of seven pile load tests is examined in detail by using different existing failure criteria and specifications. The tests were chosen for their high degree of mobilization of pile capacity and the availability of reliable load-movement relationships. Specific aspects of pile behavior, such as the mobilization of toe resistance and shaft shortening, are also investigated using 31 loading tests to develop a new failure load criterion. The writers were heavily involved with the construction, testing, and analysis of 15 of the 38 piles. From the results of the study, a new nonsubjective, semiempirical method is proposed for estimating the approximate interpreted failure loads for piles founded in weathered geomaterials. The method is based on a moderately conservative estimation of the movement required to mobilize toe resistance and incorporates observations of shaft shortening from pile loading tests. Generally, the new method may allow more effective and consistent designs for large-diameter bored piles in weathered geomaterials.