Use of Micropiles for Foundations of Transportation Structures Final Report

In pile design, piles must be able to sustain axial loads from the superstructure without bearing capacity failure or structural damage. In addition, piles must not settle or deflect excessively in order for the serviceability of the superstructures to be maintained. In general, settlement controls the design of piles in most cases because, by the time a pile has failed in terms of bearing capacity, it is very likely that serviceability will have already been compromised. Therefore, realistic estimation of settlement for a given load is very important in design of axially loaded piles. This notwithstanding, pile design has relied on calculations of ultimate resistances reduced by factors of safety that would indirectly prevent settlement-based limit states. This is in part due to the lack of accessible realistic analysis tools for estimation of settlement, especially for piles installed in layered soil. Micropiles have been increasingly used, not only as underpinning foundation elements but also as foundations of new structures. Prevalent design methods for micropiles are adaptations of methods originally developed for drilled shafts. However, the installation of micropiles differs considerably from that of drilled shafts, and micropiles have higher pile length to diameter ratios than those of drilled shafts. Improved understanding of the load-transfer characteristics of micropiles and the development of pile settlement estimation tools consistent with the load-transfer response of these foundation elements are the main goals of the proposed research. A rigorous analysis tool for assessment of the load-settlement response of an axially loaded pile was developed in this study. The authors obtained explicit analytical solutions for an axially loaded pile in a multilayered soil or rock. The soil was assumed to behave as a linear elastic material. The governing differential equations were derived based on energy principles and calculus of variations. In addition, solutions for a pile embedded in a multilayered soil with the base resting on a rigid material were obtained by changing the boundary conditions of the problem. The authors also obtained solutions for a pile embedded in a multilayered soil subjected to tensile loading. They then compared the solutions with the results from FEA and also with other solutions available in the literature. Finally, they compared the results of a pile load test from the literature with the results obtained using the solutions proposed in this study. Using the obtained elastic solutions, extensive parametric studies on the load-transfer and load-settlement response of rock-socketed piles were also performed. The effects of geometry of rock socket, rock mass deformation modulus, and in situ rock mass quality were investigated. To facilitate the use of the analysis, a user-friendly spreadsheet program ALPAXL was developed. This program is based on the elastic solution obtained in this study and uses built-in functions of Microsoft Excel. ALPAXL provides the results of the analysis, the deformed configuration of the pile-soil system and the load-settlement curve in seconds. It can be downloaded at http://cobweb.ecn.purdue.edu/~mprezzi. In the context of an INDOT project, a fully instrumented load test was performed on a rock-socketed micropile. The results of this micropile load test, on a pile with high slenderness ratio and high stiffness of surrounding rock, confirmed that most of the applied load was carried by the pile shaft. The shaft capacity of hard limestone obtained from the load test at the final loading step was 1.4 times larger than the shaft capacity that is obtained using the highest value of limit unit shaft resistance suggested by FHWA. Using pile and soil properties, predictions were also made using ALPAXL. The results from ALPAXL were in good agreement with the measured data at the design load level.