Reliability-based Durability Design Method of Pile Foundations
Published:2015-01-02 Hits:749
      The durability problem of pile foundations is one of the most popular and fundamental problems in underground engineering. Reinforced concrete (RC) pile foundations are often applied in chemical erosion environments, which are not only subjected to various mechanical stresses, but also subjected to serious chemical corrosion. The deterioration of RC piles is in the form of carbonization, neutralization, cracking, spalling and corrosion during their lifetimes. Moreover, pile foundations have bad maintainability and always require high maintenance costs. Therefore, pile foundations require high design standards to satisfy security requirement of superstructures. However, current concrete durability research mainly focuses on aboveground structures, and traditional global factor of safety (FOS) design methods can’t meet these requirements anymore. In such a circumstance, a series of researches on the reliability-based durability design method of pile foundations, including pile corrosion test, investigations of failure modes, statistics of variation coefficient and calibration of resistance factors have been carried out by our research group, and a systematic reliability-based durability design method was developed for pile foundations. Through the use of the reliability-based durability design method, the uncertainty of pile foundations can be modeled explicitly by the probabilistic methods, and the safety in pile foundation design can be controlled by limiting the chance of unsatisfactory performance during their lifetimes to a sufficiently low level. The detailed research, in this aspect, that performed by our research group can be summarized as follows:
      1) According to the actual service conditions of pile foundations, the chloride ion immersion test and salt spray test were carried out under the flexural-tensile stress state, the effect of erosion time, environmental temperature, stress level, the chloride ion concentration and the erosion pattern on the diffusion rule of chloride ion and flexural strength of piles were studied according to test results.
      2) The investigations of RC pile foundations were carried out in Zhoushan, Wuhu, Anqing, Ningxia and Xining. The regional and local characteristics of durability feature, such as concrete rebound strength, carbonation depth, and chloride ion concentration distribution were studied according to the test results. The durability feature and the failure modes of pile foundation were collected and studied on the basis of service time.
      3) A time-dependent reliability model was established basing on the statistical data and the failure models of pile foundations. Considering different target life cycle and resistance variation, the target reliability index for different failure models were derived respectively by using the time-dependent reliability method. The first order second moment (FOSM) method was adopted to determine the partial factors for both ultimate limit state and servicing limit state design, and then the reliability-based durability design expressions were also presented. A systematic reliability-based durability design and evaluation methods were proposed for pile foundations.
      The reliability-based durability design overcame the disadvantage of the global factor of safety (FOS) method, which can’t explicitly consider the level of uncertainty involved in a design, and the time-dependent deterioration of pile foundations was sufficiently considered in the design process. The research results have been successfully applied to power transmission and transformation concrete foundations deign, and the significant economic and social benefits were achieved. 
1) To consider the pile durability requirements during the design process; 2) To enhance the probability of meeting its service life; 3) To solve the key technical problems of durability design of pile foundations; 4) To establish a theoretical system and application program of reliability-based durability design method. 
Durability experiments of RC piles in corrosion environment, investigation of durability status of pile foundations, durability strength test, failure model analysis, time-dependent reliability theory and first order second moment (FOSM) method were employed herein to study the reliability evolution of pile foundations during their life-cycles and to calculate the partial factors for both ultimate limit state and servicing limit state design.
Significant Results and Potential Impact
1) A series of time-dependent reliability equations for different failure models of pile foundations were established; 2) Service life predication methods based on capacity limit criterion and reliability theory for pile foundations were developed; 3) A systematic reliability-based durability design method was proposed for pile foundations; 4) The methods for improving the durability for pile foundations according to actual projects were proposed. 
Principal Investigator:
Jingpei Li, Chunfeng Zhao Fayun Liang, and Cheng Zhao.
National Natural Science Foundation of China (51178341); Science and Technology Projects of State Grid Corporation of China (GC71-13-001, GC71-14-004).
Key Publication
(1) Jingpei Li, Zhuwen Yue, Wei Shao, et al. Calculations of designed lifetime of PHC Pipe piles in marine enviornment. Journal of the Chinese Ceramic Society, 2014, 42(4): 476-485. (in Chinese)
(2) Jingpei Li, Yi Liu, Yunhong Zhou. Service life prediction of horizontal bearing capacity of PHC pipe pile in marine environment. China Civil Engineering Journal, 2013, 46(12): 109-117. (in Chinese)

Fig. 1. Damage mechanism of pile foundations durability.

Fig. 2. Erosion degradation of pile foundations.

Fig. 3. Corrosion experiments of RC piles.