ASCE Geo-Institute/Seattle Section Geotechnical Group - October Meeting

Presentation: An End Bearing Method for Evaluating Instrumented
Becker Penetration Test Data

Khaled Chowdhury, PhD, PE, GE

ABSTRACT

Becker Penetration Tests (BPT) with instrumentation are regularly performed to characterize the density of embankment dams and foundations that consist of sandy gravel, gravelly soils or rockfill, where other methods such as the Standard Penetration Test (SPT) and the Cone Penetration Test (CPT) are difficult to perform. An embankment dam constructed in the 1950s was recently evaluated using instrumented BPT and sonic borings, as well as earlier large diameter in-situ ring density tests and BPTs, original construction records, and observations and in-situ testing as part of a dam modification project. Three commonly used BPT methods for conversion of BPT blow counts to equivalent SPT N₆₀ values were evaluated. Wide differences in the resulting equivalent SPT N₆₀ values between (1) the Harder and Seed (1986) BPT method and the instrumented BPT-based methods of (2) Sy and Campanella (1994) and (3) DeJong et al. (2017) and Ghafghazi et al. (2017) were observed in this project. This finding is consistent with several other projects that were reviewed. These different equivalent SPT N₆₀ blow counts from different methods would result in significantly different estimates of expected seismic performance of an embankment dam or other structures.

Based on an evaluation of the origins and development of the three BPT and instrumented BPT interpretation methods listed above, and further analyses of available data, an alternative “end bearing” method was developed to determine site-specific equivalent SPT N₆₀ values from BPT. This proposed method systematically and transparently analyzes the collected BPT instrumentation data and provides equivalent SPT N₆₀ values considering in-situ soil characteristics; a significant improvement to commonly used procedures. The end bearing method is based on transparently separating the shaft friction and residual force of the BPT piles by using CAPWAP-RSA and CASE methods to accurately determine the end bearing capacity of soil. In addition to site characterization for seismic analyses, this method can be used for deep foundation design and capacity verification.

 BIOGRAPHY

Khaled Chowdhury is a Senior Geotechnical Engineer at the USACE South Pacific Division Dam Safety Production Center in Sacramento, California and the USACE HQ National Earthquake Program Policy Advisor. Khaled has over 22 years of experience in evaluation, design, and construction of infrastructure projects. He currently provides technical leadership on several major dams and levees evaluation and design projects nationwide, addressing static and seismic potential failure modes. Khaled contributed or is currently contributing to development of several USACE and California DWR guidance and regulatory documents on dams and levees. He received his BS in Civil Engineering from Bangladesh University of Engineering and Technology and ME in Civil Engineering (Geotechnical) from Texas A&M University. Khaled earned his PhD from the University of California, Berkeley under supervision of Professor Ray Seed. Khaled’s research and practice areas include site characterization, soil liquefaction, residual strength, seismic deformation analyses, seepage cutoff walls, and levee and dam design and construction. Khaled co-teaches numerical methods in geotechnical engineering course at UCLA with Dr. Ethan Dawson. Khaled is the lead instructor for USACE Prospect Course on seismic stability of embankment dams. Khaled can be reached at Khaled.Chowdhury [at] usace.army.mil.