Micropile Foundations in Karst: Static and Dynamic Testing Variability

Development of a major industrial facility required support of large loads from machine foundations. The site was underlain by highly variable karstic limestone conditions, which resulted in irregular depths to rock, very soft residual soil layers, and potential for voids in the rock and soil matrix. Foundation mats on micropiles were selected for support of the machines. The benefits associated with the micropiles were the speed of installation, and relative cost and schedule savings. Two load tests were performed before the start of micropile installation. One of these tests failed prematurely. A third test, performed during the initial stages of construction, also failed prematurely. Pile Driving Analyzer (PDA) testing of micropiles was used to investigate the capacity and variability of production piles that were already installed. The authors believe this may be the first application of the PDA technology to estimate the carrying capacity of micropiles bonded into rock. Because of the lack of previous experience in this application of PDA testing, suitable testing procedures needed to be developed in the field to reduce the potential for damage of the production micropiles, and to assess the accuracy of the tests. The results of the testing program showed that PDA testing may provide very accurate estimates of the capacity of micropiles bonded into rock. This paper discusses the techniques used for PDA testing of the micropiles, and compares the results of the PDA tests to the data from static load tests. The paper also contains a brief discussion on the site conditions, and the effect of the construction methods on the measured capacity of the micropiles and their variability