Study On Reliability Based Design Methodology Of Multi-pile Composite Foundation

Due to the uncertainty of soil properties, reliability theory, which has been widely used in structural engineering, has been seldom applied in geotechnical engineering. In this thesis, some reliability-related problems in geotechnical engineering are studied with multi-pile composite foundation as main target.Firstly, the basic theory of reliability analysis is studied. An iteration process to determine the parameters for a beta probability density function and an empirical equation to simplify this iteration are presented. As a result, the estimation of the bounds of random variables is efficiently made. Furthermore, beta distribution function is introduced to determine the membership function of a fuzzy failure event for the first time. Therefore, the membership function is smooth within its domain, and is characterized by center symmetry. By utilizing the character of performance functions which are relatively simple in geotechnical problems, an explicit formula to estimate the fuzzy failure probability is derived. In order to overcome the disadvantage that the convergence of First Order-Second Moment (FOSM) method is not guaranteed, this thesis proposes an Equal Step Iteration Method (ESIM), of which the convergence is proved as well.Secondly, a Reliability-Based Design (RBD) methodology is presented to determine the replacement ratio (percent coverage) of piles in a multi-pile composite foundation. Based on the probabilistic and statistic theory, a random variable, denoted as the factor of deep soil mixing quality, is proposed to quantitatively assess the influences of mixing quality on the strengths of cemented soil and pile shaft. Furthermore, the applications of this random variable in conventional designs and RBD method are analysed. In this study, the reliability of bearing capacity and settlement of multi-pile composite foundation are analyzed when the characteristics of soil are adopted as Mohr-Coulomb's shear yield criterion, associated flow rule and bilinear elasto-plastic model for stress-strain relationship. According to the relationship between the reliability index and the replacement ratio of piles, the replacement ratios of multi-pile composite foundation can be evaluated in a rational manner.