The Liquefied Foundation
Niigata and How Solid Sand Turns into Quicksand in Seconds
Four-story reinforced-concrete apartment blocks at the Kawagishi-cho housing complex in Niigata, Japan, lay tilted eighty degrees onto their sides after the Magnitude 7.5 earthquake of June 16, 1964. Their window glass remained unbroken and their structural walls stood free of cracks. Residents trapped inside the toppled buildings opened their apartment doors and climbed out onto the upward-facing exterior wall. They walked safely down the painted concrete facade to the street as if descending a gentle ramp. Nearby, the control tower of Niigata Airport had sunk three meters straight into the ground. Empty steel sewage tanks buried underground had popped out of the pavement into the open air.
Geotechnical engineers H. Bolton Seed from the University of California, Berkeley, and Kenji Ishihara from the University of Tokyo arrived days later to inspect the Kawagishi-cho complex. They found that the buildings had not suffered structural failure from violent shaking. Instead, the reclaimed river sand beneath the shallow concrete mat foundations had lost 100 percent of its bearing strength in less than twenty seconds. Describing the surreal geometry of the intact blocks resting on their backs, Seed wrote in his field assessment that the structures never broke apart, but simply lost their footing and settled onto their backs as if sinking into a tub of mud.
Niigata sits at the mouth of the Shinano River, where centuries of river sediment and twentieth-century harbor reclamation had deposited loose, uniform layers of fine quartz sand. A shallow water table saturated those sand beds just one meter below the surface. Under static conditions, a bed of wet sand supports heavy buildings through the principle of effective stress formulated by geotechnical pioneer Karl Terzaghi. Total downward weight is carried by two components: grain-to-grain contact stress between solid quartz particles, and hydrostatic pore-water pressure filling the voids between them. Only the solid grain contacts provide frictional shear resistance. Water itself has zero shear strength.
During the 1964 earthquake, four rapid mechanical steps transformed Niigata's solid ground into a heavy liquid:
- * Loose Packing Contraction: Before the earthquake, the rounded river sand grains rested in an open, unstable arrangement with roughly 45 percent pore space filled with groundwater.
- * Undrained Cyclic Shearing: When fifteen seconds of horizontal seismic shear waves shook the ground at 0.16 $g$, the sand grains jostled out of their loose arches and tried to settle into a denser packing configuration, squeezing the water trapped in the microscopic pores.
- * Pore-Pressure Spike ($u \to \sigma$): Because seismic shaking occurred far faster than water could percolate upward through the fine sand pores (undrained loading), the trapped water absorbed the entire weight of the overlying soil and buildings, driving pore pressure $u$ up until it equaled total overburden stress $\sigma$.
- * Zero Effective Stress Flotation ($\sigma' = \sigma - u = 0$): With pore water pushing the sand grains apart with a force equal to the weight pressing them together, contact friction vanished completely; the sand-water mixture became a dense suspension weighing 1.9 grams per cubic centimeter in which top-heavy concrete buildings sank and rotated like unbalanced barges.
Wherever high-pressure water found a crack in the surface crust, geysers of sand and groundwater spouted two meters into the air for twenty minutes after the earthquake stopped. Finally the sand grains settled onto a compacted floor. After pumping the sand out from beneath one tilted Kawagishi-cho block and righting it with hydraulic jacks, engineers found the concrete frame so undamaged that the building was put back into service.