DFI JOURNAL | VOL. 16 | ISSUE 1 | 1 Research Paper Side Resistance of Drilled Shafts in Weak Fine Grained Sedimentary Rock Timothy D. Stark 1* , Ahmed K. Baghdady 2 , Abdolreza Osouli 3 , Heather Shoup 4 and Michael A. Short 5 Abstract: Load transfer mechanism in side resistance of rock socketed drilled shafts has been studied for the past four decades using results of axial load tests and theoretical methods. Various models for predic- tion of side resistance have been proposed. Only few studies (e.g., Horvath et al. 1983; Rowe and Armit- age 1987; Hassan et al. 1997, Miller 2003, Abu-Hejleh et al. 2003) have been completed on socket side resistance of drilled shafts in weathered and fractured fne-grained rock. These studies, however, were based on only a limited number of load test data. A survey of current predictive models has been con- ducted. This survey shows most of the current models include strong and intact rocks in their databases. Almost all of the current models use a power function to correlate side resistance of rock socket to rock unconfned compressive strength. A database of side resistance of large diameter drilled shafts in only weak fne-grained rocks, such as, weak shales, mudstones, and siltstones (i.e., Intermediate Geomaterial frst introduced by O’Neil et al. 1996, Hassan et al. 1997, and O’Neill and Reese 1999) has been com- plied in this study. The range of weak rocks considered herein corresponds to an unconfned compressive strength of 0.48 to 4.8 MPa. Analysis of this database shows that a linear model best predicts the side resistance of drilled shafts in weak fne-grained sedimentary rocks. Keywords: rock socketed drilled shafts, side resistance, weak fne-grained sedimentary rocks, unconfned compressive strength Introduction Drilled shafts are commonly used to support large structural loads where surfcial soils do not provide suffcient bearing capacity for shallow foundations. To support these loads, drilled shafts are often socketed into weak fne-grained rocks, which are at the boundary between clays and rock that have unconfned compressive strengths of 0.48 to 4.8 MPa (10 ksf to 100 ksf) (Kulhawy and Phoon, 1993; Hassan et al. 1997). A rock socketed drilled shaft distributes applied ax- ial loads to side and tip resistance. Allocation of axial load between these two components of resistance depends on 1 Professor of Civil and Environmental Eng., Univ. of Illinois at Urbana-Champaign 2 Senior Project Manager, SESI Consulting Engineers, 12A Maple Avenue, Pine Brook, NJ, 07058, USA 3 Associate Professor of Civil and Environmental Eng., Southern Illinois Univ. at Edwardsville 4 Central Offce Geotechnical Engineer,Central Bureau of Materi- als Illinois Dept. of Transportation 5 Materials Engineer, District 3, Illinois Dept. of Transportation * Corresponding author, email: tstark@illinois.edu © 2022 Deep Foundations Institute, Print ISSN: 1937-5247 Online ISSN: 1937-5255 Published by Deep Foundations Institute Received 08 April 2021; received in revised form 03 July 2021; accepted 26 August 2021 https://doi.org/10.37308/DFIJnl.20210408.235 relative stiffness of the shaft concrete and the surrounding rock, length of the rock socket, and allowable axial displace- ments. Drilled shafts in weak sedimentary rocks obtain most of their axial capacity by mobilizing side resistance along the drilled shaft/socket interface (Horvath, 1978; Horvath and Kenney, 1979; Horvath, 1982). Side resistance is usu- ally mobilized at small displacements along the shaft/socket interface, and it remains constant after failure (Rosenberg and Journeaux 1976). Since the 1960s, many full–scale load tests have been conducted on drilled shafts socketed in rock. However, only Williams (1980a) compiled a database that focuses on drilled shafts in weak fne-grained rocks. Therefore, most available design methods were developed using databases that include load tests in both weak and strong rocks. Only a few researchers (e.g., Miller 2003; Abu-Hejleh et al. 2003; Abu-Hejleh and Attwooll 2005) have studied the applicability of available predictive models to drilled shafts in weak fne-grained rocks. Although their work provides valuable information on this matter, their databases include a limited number of load tests against which predictive meth- ods for estimating side resistance can be evaluated. In this paper, some of the available predictive models for side resistance are reviewed and compared to the avail- able load test data developed herein, and then the axial load transfer via side resistance is discussed for weak fne-grained