Construction methods · Site investigation and testing

Liquefaction assessment

Loose saturated sands and silts can lose strength in an earthquake. The assessment compares the cyclic stress the design earthquake imposes with the cyclic resistance inferred from SPT or CPT, and tells you whether piles must be designed for lateral spreading and downdrag.

How bridges get built · Site investigation and testing · Liquefaction assessment

Also called: cyclic resistance, seismic ground hazard

How it is done

Establish the design ground motion for the site class.
Compute cyclic stress ratios with depth; compute cyclic resistance from corrected N-values or cone resistance, adjusted for fines content.
Identify layers with a factor of safety below 1 and estimate settlement and lateral spread.
Design piles through liquefiable layers for the loss of support, downdrag and kinematic loading; consider ground improvement.

Where it fits

Moderate and high seismic zones with young alluvium, reclaimed fill, estuaries and river banks.

Where it does not

Ignoring thin liquefiable seams under footings; they matter more than deep ones.

Plant, pace and money

PlantNone on site; it is analysis on the investigation data.

ProductivityDays of engineering.

CostSmall; the consequences it prevents are not.

Risks and controls

What goes wrong

Fines corrections are the largest uncertainty; use both SPT and CPT where the layer is critical.

Quality assurance

Independent check of the design motion and of the corrections applied.

Examples

Niigata 1964 and Kobe 1995 bridge failures were liquefaction failures; the game raises seismic zones on estuary and reclaimed sites.