Evaluation of the Effectiveness of Prefabricated Vertical Drains Using Full-Scale In Situ Staged Dynamic Testing

Prefabricated vertical drains (PVDs) represent one soil improvement alternative for liquefiable sites. The primary function of PVDs is to expedite the dissipation of excess pore water pressures, ensuring the excess pore pressure ratio (r_u = Δu/σ'_vo) remains below a prescribed maximum value. Full scale dynamic testing using a vibratory hammer excitation source was performed at an active liquefaction remediation site in South Carolina to assess the effectiveness of the PVDs in dissipating excess pore water pressures. Tests were conducted in both the untreated and treated site conditions. For the treated site condition, PVDs were installed in a triangular grid pattern with a center-to-center spacing of approximately 1.5 meters. The field testing program for evaluating the effectiveness of the PVDs consisted of: (1) shear wave velocity measurements via crosshole seismic testing, (2) dynamic loading in the untreated condition, (3) monitoring and recording pore water pressures and accelerations during the installation of the PVDs, and (4) dynamic loading in the drain-treated condition. Instrumentation was installed to capture the response of the test area during staged dynamic excitation in the untreated and treated conditions, and during the installation of the PVDs. This paper presents preliminary results from the in situ staged dynamic testing. The experimental set up is described along with the in situ site characterization. Pore water pressure ratio, acceleration, and shear strain time histories from the untreated and treated conditions are presented.