The Determination of Factor of Safety against Liquefaction and Post-Liquefaction Settlement

P. R. Klibbe, BE, MEngSc, MIEAust, NPER

Bechtel Australia, Mining & Metals, Wickham Street, Fortitude Valley, QLD, Australia, 4006

Paper Topics

The paper describes the liquefaction triggering relationships by NCEER (1997) and Youdet al. (2001, 2003), Idriss and Boulanger (2008) and Boulanger and Idriss (2014), used to assess the susceptibility of loose sands to liquefy.

Then describes the techniques to normalize the in situ sounding results for site-specific fines content and carbonate content of the sands for use in the analysis.

The paper then demonstrates the use of CPTu and SCPTu soundings and shear wave velocity measurements to develop soil capacity for the site seismic input data.

Then shows how the factor of safety against liquefaction calculations are performed using the CPTu data.

Then the paper shows how post-liquefaction settlements are calculated based on the normalized CPTu results and the factor of safety.

Presentation Topics

This presentation demonstrates how the CPTu and SCPTu soundings and shear wave velocity measurements were undertaken and the results processed.

Then describes the laboratory test results used to normalize the in situ sounding for site-specific fines content and carbonate content.

The presentation will then show the spreadsheet calculations used to develop soil capacity for the site seismic input data and show how the factor of safety against liquefaction calculations are performed and the post-liquefaction settlements are calculated.

Finally the results are plotted and the selection of settlement criteria and foundation systems are selected.

Plan

Subsurface Conditions and Investigations

The subsurface profile comprises compacted sandy fill over approximately 8 to 10m of natural poorly-graded silty sand, underlain by bedrock composed of calcarenite and sandstone and interbedded mudstone and gypsum to more than 70m depth.

The investigations comprised the following works -

– 40 boreholes

– 100 piezocone penetration tests (CPTu)

– 20 seismic piezocone penetration tests (SCPTu)

– 17 seismic dilatometer tests (SDMT)

– as well as geophysical surveys

Typical Soil Profile

CPT Investigations

There is a potentially liquefiable zone between approximately 8 to 10 meters depth between the very dense, possibly cemented zone, and bedrock.

Additional supplemental investigations were undertaken when the CPT soundings encountered refusal at a depth of less than 10 meters. The cone was withdrawn from the hole and a drill rig was used to drill 0.5 meters below the depth of refusal.

The CPT rig then continued the push until a total depth of 10 meters was achieved.

The two pushes of the CPTs were then combined into a single CPT log.

CPTu sounding

CPTu interpreted data

Cumulative CPTu and SCPTu interpreted data

Carbonate content and Fines content of the sands

Relative density was assessed using a correlation for carbonate sands and an equivalent penetration resistance was determined for silica sand.

Perform liquefaction calculation using input of equivalent silica sand penetration resistance.

Normalized CPT cone resistance was used to determine an apparent fines content and soil behavior index.

Perform liquefaction calculation using the soil behavior index correlated with the fines content.

Analysed data

Spreadsheet calculation used for each CPT sounding containing the entire data set.

Plots of factor of safety against liquefaction, incremental settlement and cumulative settlement were obtained as each result is loaded.

Plot of results

Contours

Application

Contour plots of post-liquefaction settlement were produced using the results from the spreadsheet calculations.

The results show the post-liquefaction settlement varies from negligible in areas (<25mm) up to being significant (>120mm) in some locations.

The selection of the appropriate foundation system was then made based on the structure and the post-liquefaction settlement magnitude at each facility location.

Foundation systems included bored piles, stone columns and stiffened rafts.

Location Maximum Range

A 30 to 50mm 5 to 50mm

B 60 to 70mm 5 to 70mm

C 90 to 130mm 30 to 130mm

D 30 to 40mm 5 to 40mm

E 70 to 80mm 5 to 80mm

F 10 to 20mm 5 to 20mm

G 30 to 50mm 5 to 30mm

Thank you

Bechtel Australia, Mining & Metals, Wickham Street, Fortitude Valley, QLD, Australia, 4006