Becker Penetration Testing for Liquefaction Evaluation for Existing Dams-Two Case Studies

Becker Penetration Testing was conducted at Echo Dam, Utah, and Cle Elum Dam, Washington, to provide continuous profiling of the coarse-grained foundation materials for evaluation of these materials with respect to their potential for liquefaction under earthquake loads. The potential for liquefaction of the foundations soils at Echo Dam was investigated after lower Standard Penetration Test (SPT) blow counts were encountered during previous drilling programs conducted at the site. The potential for liquefaction of the foundation soils at Cle Elum Dam was investigated after lower shear wave velocities were detected in cross-hole shear wave investigations. Becker Penetration Test holes and Becker Sample holes were drilled at both sites using an AP1000 Becker rig, monitored with a Pile Driving Analyzer (PDA). The measurements of force and velocity from the PDA were used to measure the energy transmitted from the hammer/helmet system into the casing. From this, the efficiency of the Becker hammer, ie. ratio of transmitted energy (ENTHRU) to hammer rated energy, was calculated. At Echo Dam, the PDA was used to assess the skin friction of the casing at selected depths by performing Case Pile Wave Analysis Program (CAPWAP) analyses. (CAPWAP analyses were not performed at Cle Elum Dam.) Friction pull-back tests were also performed at select depths for comparison to the CAPWAP results. Correlation of Becker blow counts to equivalent SPT N60 values was conducted using both the Sy and Harder methods. 10 pp., 8 figures, 4 references.