Reliability-Based Design and Quality Control of Bored Piles

Abstract

Bored piles are a type of deep foundations which have been and are being widely used in construction engineering such as high-rise buildings, bridges, jetties, and so on. Although bored piles have remarkable advantages over driven piles, the quality of bored piles is frequently affected by many causes of imperfection, which mainly come from the inadequate ground investigation and construction procedures. It can be said that design and quality control of bored piles are two closely related stages. A quality control procedure has to be clearly addressed in the design stage; and decision making in the design, for many cases, has to be based on the testing results of a quality control procedure. Therefore, two major objectives need to be solved in this thesis as: (1) Propose models to calibrate resistance factors for the Load and Resistance Factor Design (LRFD) approach and find a suitable model aiming to directly determine reliability of a bored pile considering some types of defect that may occur in the bored pile. (2) Select a quality control method and evaluate its reliability when applied to bored piles. The thesis consists of chapters, in each of which a new model is proposed, and then is applied for a specific case study. The logicality and succession of the theoretical issues between chapters are systematically presented. In Chapter 2, a history of the development of design approaches is presented, including the Allowable Stress Design (ASD), the Limit State Design (LSD), and the Reliability-Based Design (RBD). Advantages and limitations of each design approach are discussed in detail. This chapter focuses on analyzing the LSD with the use of partial safety factors following the ultimate limit state. In which, the calibration of resistance factors under the framework of the LRFD is one of the main objectives of this thesis. The level II and level III reliability methods are used to calibrate these resistance factors. In Chapter 3, the quality control approaches of bored piles are briefly introduced as an important part of the design and construction process. The post-construction tests comprise planned and unplanned tests, in which planned tests are typically non-destructive test methods. Of these methods, the Cross-hole Sonic Logging (CSL) method, the most widely used method for testing the integrity of bored pile concrete, is chosen aiming to evaluate its reliability. The inspection probability, which is used as a measurement of reliability for the CSL method, was formulated based on the encountered probability and the detection probability. For an assigned target inspection probability, the magnitude of a defect that can be detected is a function of the pile diameter and the number of access tubes arranged. A necessary number of access tubes is recommended in this study. This finding is a good reference associated with design engineers and project managers in making decisions for the design of bored piles. In Chapter 4, the calibration models of resistance factors, following the framework of the LRFD, are proposed and presented with respect to some technical aspects of bored piles. The calibrated resistance factors aim at achieving target reliability levels for a set of load factors that were already specified in the structure code. For a calibration procedure, the model uncertainty is considered and represented through the resistance bias factor. At first, calibration models of a common resistance factor using three reliability methods are presented. The reliability methods consist of the First Order Second Moment (FOSM) method, the First Order Reliability Method (FORM), and Monte Carlo Simulation (MCS). In this study, the calibration model using MCS is proposed, aiming to gain a more precise resistance factor and to reduce the calibration time. Sixteen calibration cases are considered; each calibration case is represented by a soil type, a prediction method, and a construction method. The resistance factors obtained from the proposed calibration procedure have a good correlation with those from other calibration procedures that were officially accepted in practice. This confirms that the proposed calibration model is valid and applicable. One interesting finding is that the calibrated resistance factor strongly depends on the ratio of the coefficient of variation to the mean of the resistance bias factor with a linear relation. This is an important basis for calibrating the shaft and base resistance factors separately. Next, a calibration procedure for separate shaft and base resistance factors is proposed, because the degrees of uncertainty of shaft and base resistances are different. The use of a common resistance factor as mentioned above clearly does not reflect this difference. In order to calibrate shaft and base resistance factors separately, the shaft and base resistance bias factors need to be determined. By the proposed calibration procedure, many couples of values for the shaft and base resistance factors would be derived; all of which satisfy the target reliability levels. Therefore, a ”correlation ratio” is proposed aiming to represent the correlation between uncertainty degrees of shaft and base resistance bias factors. To which, a unique couple of values for the shaft and base resistance factors is finally obtained. Through a case study at the site of the Los Angeles Memorial Coliseum (the US), using shaft and base resistance factors may lead to a more economical design than a design using a common resistance factor. In Chapter 5, the increase of pile resistance with time, compared to the initial resistance, is usually referred to as ”set-up” effect. The initial resistance is also called the reference resistance; and the portion of increasing resistance with time is called the set-up resistance. Although the bored pile set-up effect is not as dramatic as the driven pile set-up effect, incorporating the set-up effect into the LRFD for bored piles is more or less necessary. By this incorporation, an economical design can also be gained. Therefore, a calibration procedure for the reference and set-up resistance factors is presented and applied for a case study at the site of the new SR20 eastbound bridge in Florida (the US). Due to the compatibility in the calibration algorithm, the calibration procedure used for the set-up effect is completely the same as that for the shaft and base resistance factors. The calibration model of a common resistance factor as mentioned in Chapter 4 is normally based on the initial empirical distributions of the resistance bias factor. In general, these distributions have been built up by a large amount of data collected from many different sites. Therefore, applying a common resistance factor, which is calibrated from the initial empirical distribution, for a specific site may not be completely consistent. Through the experimental outcomes of pile loading tests at a designed site, the Bayesian inference enables to reduce uncertainty with respect to the initial empirical distribution in terms of load test results within a site. To which, a posterior distribution of the resistance bias factor is then derived. A re-calibration process of a common resistance factor is subsequently carried out and an updating resistance factor is obtained. As a result, a more precise design using the updating resistance factor can be reached. The Bayesian inference is applied for a case study at the site of the 330 MW Uong Bi Extension No. 2 Thermal Power Plant in Quang Ninh province (Vietnam). The derived results and some comments are presented in Chapter 6. It can be seen that, the LRFD uses the resistance factors that were obtained through the calibration process and satisfies the specified target reliability levels. This approach does not require the explicit use of the probabilistic description of random variables and therefore it has been familiar to design engineers in terms of its simplicity. In current practice, however, clients and project managers are more and more interested in the reached reliability level or the probability of failure of a pile foundation. Therefore, applying the RBD aiming to directly estimate reliability levels for a specific bored pile foundation is of interest. In Chapter 7, the reliability of a single bored pile is directly determined by the use of a coupling calculation between the finite element package (Plaxis version 9.0) and the numerical probabilistic toolbox (Prob2B). The reliability is assessed, not only for an intact bored pile but also for a defect bored pile, by assuming different types and magnitudes of defect that may occur within the pile body. Two failure modes, the Geotechnical Failure (GF) mode and the Structural Failure (SF) mode, are proposed in this study. The GF mode pertains to the geotechnical resistance of bored piles and the SF mode is related to the compressive stress in bored pile concrete. Both modes are evaluated through reached reliability levels for bored piles that are subjected to a specified combination of loads from the superstructure. Based on which, the reliability of an axially loaded pile is comprehensively assessed. Through a case study at Pier T10 of the An Dong bridge in Ninh Thuan province (Vietnam), some findings and comments are presented in this chapter. Finally, the conclusions and recommendations are stated in Chapter 8. Some models proposed in Chapters 4, 5, and 6 can be well applied for driven piles.