On June 16, 1964, a magnitude 7.5 earthquake rocked Niigata, Japan. Though Japan had seen its share of earthquakes, this one was different. Buildings and structures were toppled intact and large sand boils appeared throughout the affected region. An investigation found most of the structures failed because of bearing capacity failure. The sand boils were the result of groundwater being pushed up from below due to increased pore pressure caused by the settling sands.
Never before had an earthquake of such magnitude hit an area so susceptible to vibration. Being a coastal city, most of the structures were built on saturated loose sands. The combination of vibration and saturated loose sands was a recipe for widespread destruction because of foundation failures. This event sent a wake-up call to structural engineers and geologists around the world about the devastating effects of liquefiable soils.
Technically, soil liquefaction is the phenomenon in which loose soils are subjected to sudden lateral loads, such as during an earthquake. The soils collapse to a denser state and may cause a loss of bearing capacity, extensive permanent lateral displacement and surface settlement.
The highest risk for soil liquefaction is when there is fine or loose sand, high groundwater, seismic activity, or any combination of these. During an earthquake, vibrations cause soil grains to reorient, consolidating some soils and increasing pressure on the water trapped in the soil pores. The increased pressure causes the water to flow through the sand, creating a momentary quicksand condition. Even the briefest change in soil condition can spell disaster for structure foundations.
The Utility Impact
While states on the West Coast, such as Alaska and California, are well-known for earthquakes, the U.S. Geological Survey indicates most of the United States is subject to seismic activity. Not all seismic events create liquefiable conditions; however, electric utilities, including Public Service of New Hampshire (PSNH), have determined the risk is significant enough to warrant safeguards against these dangers, particularly in plants and substations.
In the wake of the Fukushima, Japan, earthquake in 2012 and the resultant nuclear plant damage, utilities in New England are requiring facilities to be evaluated using a design earthquake with a return period of up to 2,500 years. In New England, this equates to a magnitude 7.0 seismic event.
PSNH has taken steps to address conditions under which soil liquefaction in such an earthquake could occur and how it could potentially undermine its substation equipment foundations. A PSNH design review team and its consultant TRC Companies Inc. reviewed the geotechnical conditions at a PSNH site and developed a design approach to address this geotechnical challenge.