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.

The pile research program was designed for fundamental understanding of pile and soil behavior as a basis for rational design, primarily for deepwater piles in soft clay. Thus, the experimental program was planned to cover a wide range of static and cyclic loadings over a long time. For extrapolation to large prototypes, fully instrumented field tests were performed on a 30-inch diameter pipe pile and on 1.72 and 3-inch diameter pile-segment models, tested in situ. For the 30-inch pile, loading was applied by four large two-way hydraulic rams activated by a 50-hp servo-eontrolled hydraulic power system. The pile was assembled in the field from three pieces: a fully instrumented 180-ft test section and two 90-foot follower sections. After driving, the tip was 234 ft below the sea floor. The soil was a very uniform deposit of soft clay. All transducers, cables, and cable connections, plus the computer-based control and data recording system, were custom-designed for maximum stability and reliability over the full 2.5-year test program. Special strain modules were welded along the inside of the pile and were backed up with 60-inch long extensometers lowered to seats after the pile was driven. Lateral pressure cells were welded flush with the pile wall to measure total pressures. Commercial pressure transducers were similarly installed to measure pore pressures for deducing effective pressures and to monitor the progress of soil consolidation in the critical zone adjacent to the pile. Axial displacements at points along the pile were determined from integration of the measured strains. Shear transfer was deduced from differences in strain along the pile length. During driving of the pile, static lateral pressures were observed at all pressure stations as a function of pile penetration. Static tension and compression load tests were performed within two hours of completion of the installation to provide the immediate strength after driving. Residual stresses were observed at various times after driving. Effects of consolidation and setup were clearly shown by four complete series of static and cyclic load tests over 2.5 years. Extensive records were made of soil shear versus pile displacement along the pile. Analytical modeling and empirical correlations were done to characterize the behavior and furnish recommendations for design. One of the most revealing elements of the program was the detailed study of the thick soil layer adhering to the pile after extraction. Introduction About 1980, Conoco Inc. was designing the Hutton tension-leg platform for the North Sea and was considering a second platform for deep water off the Mississippi delta. The foundation conditions in the second case were quite different, with the expectation of very soft clay. The behavior and reliability of tension pile anchors in such material was of prime concern, particularly considering the superposition of cyclic storm loading on the steady-state tensile loading. Intuitively, degradation of soil resistance could result in catastrophic failure. The need for a high degree of reliability in design and the lack of an experience base for the expected soil and loading conditions led Conoco engineers to seek a test of performance that would support design of full-scale prototypes. To gain the maximum benefit, a fully instrumented pile test was recommended.

The results of a geotechnical study for the proposed Rio Cariben offshore development are presented. The profile essentiallyconsists of firm to very hard clays, with a surface layer of calcareous sand. Back artalyses of spud can penetration at a site close to the site under study were used to evaluate engineering parameters for the carbonate sands. In addition, analyses of the resistance of the surface sandswas also evaluated based on the results of several in situ and laboratory tests.A discussion of the problems of evaluating liquefaction susceptibility in calcareous sands is presented, principally in terms of shortcomings of the offshore SPTprocedures employed, INTRODUCTION As part of the preliminary preparations for the engineering analysis, design and planning fm the development of the Rio Caribe offshore platform, a 500 fi (152 m) geotechnical boring was completed at the proposed location of the structure, Both in situ and laboratory test data were obtained for the purpose of character rizing the geotechnical parameters of thefoundation soils', The objective of the field and laboratory testing program was to provide information for evaluating the foundation bearing capacity for piles under axial and lateral loads for static, cyclic and dynamic loading. The offshore geotechnical boring and laboratory testing program were performed by Fugro-McClelland under contract to LAGOVEN, S.A. The field work was supervised by INTEVEP, S,A, who also peformed corroboratorylaboratory testing. The purpose of this paper is to review the data available From the field and laboratory testing program and to present the engineering parameters that were used for the static, cyclic and dynamic foundation analyses. The, data are taken from the various Fugro-McClelland field and laboratory reports produced for LAGOVEN and from the INTEVEP laboratory test results. SOIL PROFILE The simplified soil profile for the site is shown in Fig. 1. A surface calcareous sand exists to a depth of 9.1 m, after which the complete profile consists essentially of stiff to hard clays. Based on initial liquefaction analyses, the sand layeris to be consideredliquefiable to a depth of 15 ft (4.57 m) due to the expected ground accelerations at the site 2. The variation of Atterberg Limits of the recovered samples is indicated in Fig. 2. The liquid (LL) and plastic (PL) limits are reasonably constant over the depth of the boringand give an average plasticity index (LL-PL) of around 40%. Laboratory measurements of the submerged unit weight (y') are also presentedin Fig. 2, as is the smoothed trend used in the analyses performed, STRENGTH PROFILE The variation of the measured undrained shear strength in the cohesivesoils is indicated in Fig. 3. The measured shear strengthshave been obtained from several types of test and the scatter is that generally associated with offshore investigations. For the initial static analyses, the design profile indicated in Fig. 3 was used. For the subsequent dynamic and pseudo-dynamic analyses, the relationship from DSS and triaxial results (Eq. 1) has been used, Depending on the type of evaluation, this relationship ismodified to take into account the effects of rate of loading and cyclic degradation.