A Simple Model to Predict Soil Resistance to Driving for Long Piles in Deepwater Normally Consolidated Clays

Abstract

ABSTRACT As the exploration and development for ot%shoreoil and gas reserves moves info the deepwater environment of the continents/ slope, a good model js necessaty to evaluate drivability of large-diameter, long pjles jn normally consolidated clays. Procedures are available to predict soil resistance to driving in stiff to hard overconsolidated clays of the North Sea and the Arabian Gulf Use of these procedures in normally consolidated clays grossly overpredicts soil resistance. Such gross overpredictions msutt in the mobilization of larger hammers or thicker pile wall than necessary for pile installation. A simp/e model k proposed to estimate soil resistance to driving. Case histories of recent deepwater pile installations are presented to illustrate the adequacy of the new procedure in deepwater normally consolidated clays. INTRODUCTION As shown on Figure 1, a pile drivability assessment requires two independent analyses. First, a series ofwave equation analyses is performed at different penetrations to estimate the driving resistance that a particular hammer-pile-soil system can overcome. The second step in the process is an estimation of soil resistance to driving that the pile is likely to encounter. These two independent analyses are then combined to predict the pile driving response for the particular hammer-pile-soil combination. The first analysis is very straightforward and is dependent on pile dimensions, hammer characteristics, and the load transfer (damping and quake) properties of the soil. Computation of the soil resistance to driving is analogous to the computation of the ultimate static capacity of piles, except that the static undrained properties of the soil are degraded to account for remolding as a result of pile driving. Available procedures to compute soil resistance to driving in clays were developed based on experience in stiff to hard overconsolidated clays of the North Sea and the Arabian Gulf (TooIan and Fox, 1977; Semple and GemeinhanX, 1981; Stevens, et al., 1982). These procedures grossly overpredict soil resistance in normally consolidated clays. Therefore, in the drivability assessment, the overprediction of soil resistance leads to selection of bigger hammers or thicker pile wall than is actually necessary. Before going into the mechanics of the soil-pileinteraction during driving, a few comments regarding pile driving experience in the normally consolidated clays of the deepwater are in otier. First, the soil resistance during continuous driving is low and increases slowly with depth. Based on hindcast analysis of the pile driving data, the dynamic resistance is typically between 20 and 40 percent of the ultimate static capacity. Another observation, which is even more important, is that the piles drive unplugged, i.e., the soil plug is generally within a few feet of the mudline. Soil resistance to driving is overpredicted when piles are considered to be plugged during driving in normally mnsolidated clay. In this paper, all discussions will be limited to the coringcase, that is, the piles will be considered to be unplugged during driving. EXISTING PROCEDURES In clay, for a coring pile with an internal shoe, Toolan and Fox (1977) computed soil resistance as the summation.