API REPORT 87-29: Offshore Lateral Pile Design Reliability
DOD Adopted ANSI Approved Approved
The uncertainties affecting the responses of single laterally loaded offshore pile are identified, analyzed and subsequently incorporated in the reliability evaluation of pile performance under lateral loading. Three modes of performance are studied, including: (i) excessive displacement at the pile head; (ii) initial yielding of steel pile (damage); and (iii) ultimate yielding of steel pile (collapse). Probabilistic models are developed to predict the respective performances. The statistics of the input parameters to the models are assessed with respect to available data. Reasonable estimates or range of statistics are also assumed wherever data is lacking. The following are major results and conclusions.
(1) Statistics (bias and error) of the model correction factor associated with current design procedures are assessed based on available field test data for single lateral pile in soft clay, stiff clay and sand. Substantial discrepancy exists on the statistics between types of response considered, soil stratum involved and particularly between piles with fixed and free head condition.
(2) Current design procedures underestimate lateral responses in clay, but overestimate those in sand.
(3) Uncertainties in the lateral load is the dominant source of overall response uncertainty, followed by those in the model. Soil parameters generally have minimal contribution except modulus k for the case of sand.
(4) For piles in soft clay designed with a safety factor of 2.0, the probability of initial yielding over a period of 20 years is about 10−2 to 10−3. That for the stiff clay case is comparable, but the probability for the sand case is down to about 10−5.
(5) The probability of ultimate yielding is generally one to two orders of magnitude less than that of initial yielding. Hence, substantial reserve capacity is apparently available after first yielding.
(6) Probability of pile head displacement exceeding 10% pile diameter is about 10−2 for the soft clay case; it becomes almost negligible for the case of stiff clay or sand.
(7) Statistics of model correction factors for lateral pile groups are assessed based on available field test data. Reliability procedures developed for the single laterally loaded pile could be extended to pile group.
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