RAPID: Blast Testing to Investigate Resin-Injection Treatment for Liquefaction Mitigation

Brigham Young University, Provo UT

Investigators

Abstract

Earthquake shaking can cause loose sands below the water table to liquefy and behave like heavy liquid, causing buildings, bridges and ports supported on the sand to settle, slide or tip over. Direct and indirect economic losses resulting from liquefaction are substantial costs to society. To deal with this hazard, engineers can compact all the liquefiable sand in place or use deep foundations that penetrate through the liquefiable sand, but these methods are very expensive, especially for residential areas. A more economical strategy, being intensively studied around the world, is to compact a surface layer 12- to 20-ft thick that can safely support a building even if liquefaction still occurs in the deeper layers below it. To investigate this approach, without waiting for an earthquake, we will compact a surface layer 20-ft thick and use an array of small explosive charges to liquefy the sand below. The field test site will be in Christchurch, New Zealand, where liquefaction damage caused over 10,000 homes to be condemned. Settlement from liquefaction will be compared with an identical test at an adjacent untreated area. The results from these tests will allow engineers to determine if this economical solution is actually safe to employ in practice. The soil compaction will be peformed using an innovative approach in which expanding polyurethane is injected into the ground. Unlike most soil improvement techniques, this method can be safely used under existing structures. The New Zealand government is paying the cost of the soil improvement while the US National Science Foundation will pay the costs of the liquefaction test. Leveraging funding from both countries expands the scope of work beyond what either country would accomplish independently and increases the impact of the research internationally. These benchmark case histories will help engineers to design safe, yet cost effective foundation solutions to minimize earthquake damage. In addition, U.S. graduate student will gain significant international experience by participating in this study. Liquefaction of loose saturated sands results in significant damage to civil infrastructure in nearly every earthquake event. Direct and indirect economic losses resulting from liquefaction are substantial costs to society as was recently demonstrated in New Zealand. Over 10,000 homes were damaged as a result of liquefaction and the economy is struggling to recover. Engineers in New Zealand are experimenting with ground improvement methods that thicken and stiffen a non-liquefiable surface crust to minimize differential settlement from liquefaction below the crust. An innovative, new liquefaction mitigation method known as resin-injection was recently developed and tested in New Zealand at three sites where 18 other ground improvement methods were previously tested using blast induced liquefaction in 2013 as part of a $5 million study. Polyurethane resin is injected from the bottom-up through vertical pipes installed in a triangular grid pattern. The low-viscosity resin penetrates the surrounding soil through planes of weakness or by hydrofracturing. Expansion of the resin densifies the surrounding soil and increases the lateral earth pressure. This method can be used to treat soils below existing structures and is effective in silty sands that are difficult to densify with vibratory methods. An extensive set of post-treatment in-situ tests demonstrate that this new technique is very effective relative to previous methods; however, none of the resin injection test sites has been subjected to blast testing to evaluate the performance of the improved ground overlying the liquefiable sand at depth. This makes it impossible to compare the overall system performance of this treatment method with the other systems previously evaluated in the 2013 trials. To remedy this deficiency, we will conduct blast liquefaction tests at one of the resin injection sites and an adjacent untreated area. Prior to blasting, instrumentation will be installed to measure pore pressure generation and dissipation, soil settlement vs. time and vs. depth, shear wave velocity vs. pore pressure ratio, lateral earth pressure vs. pore pressure ratio, and blast-induced shear strain. This project will: (1) provide a direct comparison of excess pore pressure and soil settlement in the resin-treated layers with an untreated site, (2) add to the database defining the effect of surface crust thickness and stiffness on the settlement and distortion of buildings on the crust, and (3) provide fundamental in-situ measurements of lateral earth pressure and shear modulus variation with the generation and dissipation of pore pressure. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.