Model testing of passive site stabilization

Alyssa Jean Koch

doi:10.17918/etd-59

Passive site stabilization is a new technology proposed for use in mitigation of liquefaction risk at developed sites. Liquefaction is the rapid loss of strength caused by earthquakes, typically found in saturated sands or loose fills, which causes the formation to flow like later. Liquefaction has been known to be a very costly occurrence claiming many lives, damaging structures and businesses, and costing billions of dollars in repairs; however, sites can be remediated to prevent liquefaction. Traditional methods of ground remediation, such as deep dynamic compaction or vibrocompaction, may be costly, intrusive, and disruptive to the daily operations of a site. A proposed alternative for this type of ground improvement is passive site stabilization, which is non-intrusive and non-disruptive. The idea behind passive site stabilization is to slowly inject a stabilizer at the upgradient edge of a site using a trench or injection wells. Once injected, the stabilizer will flow to the target area using the natural groundwater flow or, in some cases an augmented groundwater flow. When the stabilizer reaches the target area, it will gel or set to stabilize the soil. A suitable stabilizer should have a low initial viscosity, followed by rapid gelation, long and producible gel times, be nontoxic, and be economical. Stabilizer candidates include chemical grouts, microfine cements, and colloidal silica. A feasibility study on potential stabilizers has been completed and colloidal silica has been identified as a suitable stabilizer. Colloidal silica as long, controllable, and reproducible gel times, is non-toxic, has low initial viscosity when diluted, and can be used in low concentrations. It is also cost effective, costing $60 to $180 per cubic meter of treated soil, depending on concentrations, which makes it competitive with other types of grouting. A suitable stabilizer having been identified, research focused on model testing of the method utilizing two separate and distinct methods; the geotechnical centrifuge facilities at Rensselaer Polytechnic Institute and a 1g-box model at Drexel University. The centrifuge was used to investigate the behavior of a loose sand formation treated with colloidal silica when it was subjected to earthquake motions and subsequently, the formation's resistance to liquefaction through cyclic triaxial testing. Three centrifuge models were treated with colloidal silica to investigate saturation techniques to achieve a thoroughly saturated formation. The treated models were subjected to multiple earthquake events to determine the behavior of the treated formation. Samples retrieved from the models were tested in cyclic triaxial shear to evaluate the liquefaction resistance of the formation. Results show that a 6-weight percent colloidal silica solution will withstand significant earthquake motions and that liquefaction resistance has increased in all of the models suggesting that passive site stabilization will be successful in field placement. The box model was used to investigate delivery techniques for passive site stabilization using injection wells and injection wells coupled with extraction wells. For passive site stabilization to be feasible, adequate coverage must be obtained with a minimal concentration and quantity of colloidal silica. The liquefaction resistance of the treated formation was also evaluated using cyclic triaxial testing. It was found that one and a half pore volumes introduced into the formation using a low injection rate with wells placed near the middle of the formation provided adequate stabilizer coverage. The use of extraction wells proved to be inconclusive on this small scale. Strength testing of all models tested, even those considered unsuccessful, indicated and increased resistance to liquefaction. This suggests that passive site stabilization placement will be successful in the field. The model testing is promising in developing passive site stabilization as an innovative new technology for ground improvement and for sites susceptible to liquefaction. This testing program has indicated that passive site stabilization will be successful in improving site integrity in earthquake events and will do so without disrupting the site to put stabilization measures in place.

Model testing of passive site stabilization

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Model testing of passive site stabilization

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