Liquefaction

Introduction• Primary cause of many landslides and other ground failures associated with earthquakes• Definition—“The transformation of granular material from a solid state into a liquefied state as a consequence of increased pore water pressure.”• Solid surface material suddenly becomes liquid• Personal experience—bouncing on a beach

Explanation• Initially, grains settle somewhat randomly with lots of pore space between them• Support is by grain to grain contact

Effects• Loss of support– Buildings sink or tilt– Settling is usually uneven• Water expelled from ground• Buried objects rise to the surface– Fuel tanks– Septic tanks– Caskets

Types of Liquefaction• Flow landslides• Lateral-spreading landslides• Quick condition failure

Flow Landslides• Moderately to steeply sloping terrain• Loose, saturated granular deposits• Movement– Large blocks– Long distances

– Liquefied flows– Relatively rapid• Greatest threat to human life of all liquefaction

Lateral Spreading• Gently or very gently sloping terrain• Liquefiable material at the surface or at shallow depths• Lateral displacements of several feet• Effects– Ground cracks– Differential settlement• Causes considerable property damage because structures are usually built on flat areas

Quick Condition Failure• Flat-lying areas• Loose granular material extends from the surface to considerable depth below the water table• Structures & other loads sink• Buried tanks and other vessels rise buoyantly

LiquefactionPort Royal, Jamaica Earthquake

• June 7, 1692, just before noon• Magnitude estimated to be about 8• Aftershocks persisted for months– 3 months after the quake there will still 3 aftershocks per day• Widespread liquefaction—lateral spreading– Port Royal is built on a giant sand bar– Sand was loose and water-saturated• 1/3 of Port Royal vanished• 2500 people died

Port Royal, Jamaica, 1692

LiquefactionExample—Lateral SpreadingExample—Lateral SpreadingExample—Lateral SpreadingExample—Lateral Spreading

California—1906Nigata, Japan—1964Nigata, Japan—1964Nigata, Japan—1964

Turnagain Heights—SouthwestTurnagain Heights—Center

Turnagain Heights—NortheastTurnagain Heights—Close-Up

Turnagain HeightsTurnagain HeightsTurnagain HeightsTurnagain HeightsTurnagain Heights

San Fernando—1971San Fernando—1971Loma Prieta—1989Loma Prieta—1989Loma Prieta—1989

Identifying Liquefaction-Prone Areas• Silts, sands, and gravels– Clay free– Water saturated

• Potential increases with amount of water in fine silt layers• In developed areas they may be hard to identify because development has changed the external appearance• Common around historic margins of bays and harbors, and artificial fill

Reducing the Risk• The primary answer is zoning– Identify areas with a high potential for liquefaction– Prevent building in those areas– Difficult because these are often areas extremely desirable for development• In some cases, it may be possible to remove the hazardous material– Only if there is a thin layer of poor material– Only if there is a very large budget