One of the goals of this project is to investigate two liquefaction mitigation techniques-prefabricated vertical drains (PVD) and colloidal silica grout-that are well suited for remediating hydraulic fills prone to liquefaction at port facilities. These techniques represent relatively new and innovative technologies that still require validation and verification. PVD for liquefaction remediation consist of corrugated, perforated, plastic pipe encased in a geotextile fabric and can range from 75 to 150 mm in diameter. These relatively large diameters provide the large flow capacity required to drain the soil quickly and rapidly dissipate excess pore pressures. Passive site stabilization using colloidal silica grout involves slow injection of stabilizing materials at the up-gradient edge of a site and delivery of the stabilizer to the target location using natural or augmented groundwater flow. Upon delivery to the target location, the stabilizer starts to gel or set rapidly at a predetermined time to bind the soil particles and stabilize the soil mass.

The University of Texas (Rathje) is taking the lead on investigating PVDs, while Drexel University (Gallagher) is taking the lead on investigating colloidal silica. Both UT and Drexel are working in cooperation with UC Davis (Boulanger) to perform a series of centrifuge tests at the NEESR@UCDavis geotechnical centrifuge , while UT and Drexel are working together to perform field tests using the NEES@UTexas mobile shakers . 

The objectives of the experiments are to investigate the effectiveness of both soil improvement methods in mitigating liquefaction hazards. These data will be use to develop performance-based design guidelines and constitutive and numerical models of soil behavior for improved soils.

The first centrifuge test to investigate PVDs was performed in March 2007. The centrifuge specimen represents a lateral spread site of loose sand sloping 3° towards a channel cut along the transverse axis (short direction) of the specimen box. Although this configuration does not mimic a wharf facility specifically, it does model the most significant concern related to liquefaction and port facilities: lateral spreading. The soil upslope of the channel on one side of the specimen box was improved via PVDs, while the other side of the channel was left untreated. This configuration allows for a direct comparison between treated and untreated soil under the same excitation. Additional centrifuge tests are planned throughout 2007.

Following the centrifuge test program, the performance of prefabricated vertical drains and colloidal silica grout will be investigated via a full-scale field test using a large, mobile shaker operated by the NEES Equipment Site at the University of Texas at Austin. The field tests provide an opportunity to evaluate the two techniques in natural soils using the same installation methods employed in practice. At present, four candidate field sites are being considered as test-beds: (1) Port of Seattle, (2) Port of Oakland, (3) Port of Los Angeles, and (4) Naval Air Station North Island in San Diego, CA. Available geotechnical data is being reviewed to assess the suitability of the site for full-scale field tests.

Supplementing the centrifuge and field tests are laboratory element tests at MIT and Georgia Tech to improve our understanding of the constitutive properties of the soils treated with colloidal silica grout. Resonant column and simple shear tests are being conducted on three different concentrations of colloidal silica gel and sand mixtures. The behavior of the gelled samples is being compared to pure sand samples and the effect of the concentration of colloidal silica gel by percent weight is being investigated. The influence of cyclic shear strain on the dynamic properties is also being examined.

The first centrifuge test on prefabricated vertical drains (PVD) was successful in inducing significant pore water pressure, liquefaction, and lateral spread movements. At lower intensity levels, the presence of the PVDs minimized pore pressure generation as compared with the untreated section. At larger shaking levels, water discharged from the drains, yet pore pressure ratios close to 1.0 were observed. Nonetheless, movements were limited for the section treated with drains, while significant lateral spreading occurred on the untreated section. These results are somewhat surprising in that the performance of the site was improved even though large excess pore pressures were observed.

 

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