Fig. 1 Photograph showing formation of “sand volcano” as a result of blast induced liquefaction and upward flow of water to the ground surface in unimproved ground. The upward flow of water brings sand grains to the surface.
Liquefaction research by Civil & Environmental Engineering Professor Kyle Rollins is being used to help build safer structures in Christchurch, New Zealand. During large earthquakes, loose sand below the water table can “liquefy” and be transformed from a stable solid to an unstable fluid similar to quick sand. As a result, buildings, bridges and houses which were on stable foundations before an earthquake can tilt, settle and slide following liquefaction. This problem was particularly severe during a series of earthquakes which devastated Christchurch, New Zealand in 2010 and 2011. Over 7500 homes were condemned and many structures in the central business district had to be demolished. Government leaders in New Zealand have funded a multi-million dollar research program to determine the most cost-effective methods to deal with the liquefaction problem. Various techniques for improving the ground have been proposed including dropping weights on the ground, mixing the soil with cement, ramming gravel columns into the ground and installing deep pile foundations through the liquefiable layers. The cheapest techniques involve improving a10 ft-thick surface layer so that liquefaction in a 20 ft-thick layer below it won’t cause major problems.
But how does one determine if these solutions will perform as intended in a real earthquake? New Zealand investigators employed technology developed at BYU in 1998 to produce liquefaction in the underlying layer using small explosive charges. (see video of test blast below) The explosive charges were successful in liquefying the loose sand layers and caused “sand volcanos” to form at the surface in untreated areas as shown in Fig. 1. A total of 14 different test areas were evaluated using blast induced liquefaction from October 14 through Oct 24, 2013. The weight of a structure was simulated by using steel plates and concrete weights as shown in Fig. 2. Rollins assisted in designing the charge weights and charge layout along with the monitoring equipment to determine the thickness of the liquefied layer and the settlement of the ground. “It’s particularly gratifying to see our research being used to help the city of Christchurch get back on its feet” said Rollins.
Because this research also has important implications for seismic design in the US, the National Science Foundation awarded Rollins with a $200,000 grant to conduct tests on additional remediation methods alongside his Kiwi colleagues. Collaboration allows both sides to leverage their funds and maximize the benefits. Rollins’ tests aim to determine how settlement following liquefaction can create downward friction on pile foundations which extend through a liquefied zone and into a denser soil stratum at depth. Downward friction can exert an extra force on the pile that can lead to unanticipated settlement of the foundation. Rollins and graduate Research Assistant Erick Hollenbaugh flew to New Zealand in late October to oversee the construction of the test piles (see Fig. 3) and to install instrumentation for the tests. Results from these tests provide the first physical measurements of this phenomenon on concrete piles and should help resolve contradictory design recommendations.
Fig. 2. Photograph showing loaded areas over unimproved ground which settled nearly 8 inches as a result of the blast induced liquefaction. In contrast, settlement was substantially reduced in improved ground test panels and settlement was more uniform.
Fig. 3 Photo showing construction of 50-ft long “augercast” concrete pile foundation which was instrumented to measure downward or negative skin friction on the pile resulting from liquefaction induced settlement.