Drivability and Performance of Steel H-Piles in Schist Saprolite

Although driven steel H-piles are often an economical alternate for bridge foundations, prediction of pile drivability and length in very dense saprolite remains a great challenge pertaining to design and construction practices. A study to evaluate the performance of steel Hpiles driven into thick saprolite layers was conducted for several bridge projects in Southeastern Pennsylvania. These saprolite layers of the Wissahickon Schist Formation are completely weathered but retain the fabric and structure of the parent bedrock. A database of numerous test piles from selected projects was established to provide a quantitative understanding of pile geotechnical resistances, pile lengths and a qualitative prediction of the driving conditions. All piles were driven to absolute refusal which is a function of the hammer type, efficiency, and driving procedure. High strain dynamic tests were performed on all piles at the end of initial driving. The results of restrike testing on selected piles were used to evaluate set-up effects on piles in these intermediate geomaterials. Geotechnical resistances from case pile wave analysis program (CAPWAP) were compared against static analysis values to quantify the pile performance. In addition, statistical attributes of the pile database were summarized for pile embedment into saprolite, hammer efficiency, and maximum driving stresses, etc. The paper presents the results of the pile drivability study, identifies the influence of saprolite thickness on pile geotechnical resistances and lengths, and provides recommendations for future projects with similar design challenges.