Post Driving Prediction of Pile Capacity

Abstract

Because soil is a heterogeneous media, pile installation response does not always proceed according to plan. When driving performance variability occur, the capacity of the piling must be determined and the cause of these discrepancies analyzed. Procedures for evaluating available soil borings, geophysical information and pile driving records to determine pile capacity are detailed. BACKGROUND During installation of a four pile jacket in the East Cameron area of the Gulf of Mexico, difficulties were encountered in reaching the design penetration of 323 feet. Piles 1 and 4 reached refusal after the final section was added when driving with a Menck 3000 hammer. A Menck 7000 hammer was tried on pile 4 and also met refusal. The typical definition for refusal used by installation contractors for a Menck hammer is 150 blows per foot for five feet or 250 blows per foot for restarting a pile. This definition differs form the API RP 2A(Ref. 1) recommendation of 300 blows per foot for five consecutive feet or 800 blows for one foot of penetration. This lower blow count for refusal is favored to prevent damage to the hammer which can be caused by excessive pounding with the hammer. In this paper the former definition for refusal was used. Adding the final section and driving to refusal gained less than 10 feet of additional penetration and left a weld joint at a poor location for proper connection to the jacket. It was decided to drive piles 2 and 3 and to terminate pile driving after installation of the fourth section at a penetration of about 258 feet. This final penetration would leave the heavy wall pile section high in the profile, but in an acceptable location to provide the design lateral support because of proper underdrive design. The predicted pile capacity based on the nearby boring provided a capacity of about 5400 kips. This capacity of 5400 kips was sufficient for minimum design requirements but a determination of the as installed capacity consistent with normal design procedures was required to determine maximum allowable loading conditions. Final pile penetrations (measured from the seafloor) for piles 1,2,3 and 4 were 268.5, 259, 258.5 and 263.5 feet respectively. QUESTIONS TO BE ANSWERED The difficulties during installation on this project raised several questions on the design and installation of a jacket. The questions that were asked of Seafloor Engineers as geotechnical consultants and are addressed in this paper are as follows:What is the pile capacity of the piles in their as built condition?Based on a review of the boring log near the location, no driving problems were anticipated. Why did driving problems occur?