protecting our ports and harborshome
Earthquakes and Tsunamis . . . building more resilient port and harbor communities
Overviewtransparent images Hazardstransparent images Vulnerabilitytransparent images Mitigation Optionstransparent images Port/Harbor Atlastransparent images Planning Tooltransparent images Resources
pixel

Liquefaction in the Pacific Northwest

Introduction

liquefaction examples

Liquefaction is a phenomenon associated with earthquakes in which sandy to silty, water-saturated soils behave like fluids. As seismic waves pass through saturated soil layers, the structure of the soil distorts and void spaces between soil particles (pores) collapse, causing deformation and ground failure. In general, young, loose sediment and areas with high water tables are the most susceptible to liquefaction.

Types of Ground Failure Caused by Liquefaction

Sandboils:

Sandboils or blowouts are a common sign of liquefaction. Sandboils form when sediment that is below the surface and has a high water content is blown to the surface due to increases in pore pressure.

Lateral spread:

Lateral spread involves the movement of large blocks of soil due to the liquefaction of underlying sediments. Horizontal movement of 10 to 15 feet is common, but movement up to 150 feet is also possible with this type of ground failure.

Flow failures:

Flow failure is the second type of ground failure and the most destructive. Flow failures occur on steeper slopes and can move considerable distances. In this type of failure, blocks of intact material ride on top of a layer of liquefied soil.

Loss of bearing strength:

This third type of ground failure occurs when liquefaction causes loss of bearing strength to soil layers. Buildings and other structures situated above a liquefied soil layer may tilt or collapse due to lack of bearing strength.

Impacts to Ports and Harbors

Liquefaction most commonly occurs in earthquake-prone, low-lying areas with saturated soils. Generally, flat areas around rivers, lakes, bays, and oceans are highly susceptible to liquefaction. Port and harbor facilities are therefore especially vulnerable to liquefaction effects. Port facilities contain large retaining walls and structures and heavy equipment to facilitate shipping activities. These structures and equipment increase the strain on the under-lying material and an earthquake can drive this strain to liquefaction and failure. Many port and harbor facilities have been damaged in the past from earthquake-induced liquefaction. The pictures below demonstrate the effects liquefaction can have on port facilities.

port damage port damage
Damage to Port Facilities Caused by the 1995 Kobe Earthquake in Japan

Source: Federal Emergency Management Agency (FEMA). 1995. Multi-hazard Identification and Risk Assessment: A Cornerstone of the National Mitigation Strategy. Washington, D.C.,

Soil Liquefaction Web Site from the University of Washington.

Top

Image Credits