Seismic Hazards Evaluation and Lateral Pile Design in Diatomaceous Earth-Paper

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The Klamath Falls wastewater treatment plant expansion required seismic hazard assessment and pile design in unusual diatom-rich lake deposits beside Lake Ewauna. These soils are very low-density and water-rich, and conventional liquefaction correlations may not describe their behavior reliably.<br /><br />Three undisturbed samples were tested using cyclic direct simple shear (CDSS), with post-cyclic strength and consolidation also evaluated. Testing indicated clay-like cyclic softening rather than classical liquefaction. At the estimated loading from the 1993 Klamath Falls earthquakes, samples did not reach the defined failure strain, consistent with the absence of observed permanent ground deformation at the plant. Under the maximum considered earthquake (MCEG), a sample failed during the first cycle and developed an excess pore-pressure ratio of about 65%. The tests supported a site-specific cyclic resistance relationship and suggested lower seismic impacts than unadjusted penetration-based methods.<br /><br />Liquefaction settlement estimates from standard SPT procedures were calibrated against the 1993 earthquake observations by increasing clean-sand-equivalent blowcounts. The resulting MCEG estimate was about six inches of settlement in the diatomaceous layer. A lateral-spread hazard zone extending approximately 100 feet inland from the lake bank was identified, with estimated displacement of about six inches; critical facilities were located outside this zone.<br /><br />For 24-inch auger-cast piles, seismic lateral capacity was assessed by modifying soft-clay p-y curves using either adjusted p-multipliers or post-cyclic residual strength and strain parameters. The estimated capacity under seismic conditions was 55–63% of static capacity, with the p-multiplier method providing the lower-bound result.<br /><br />The study concludes that site-specific CDSS testing can reduce uncertainty and avoid overstating seismic hazards in diatomaceous soils, supporting more economical foundation design.

Keywords

Klamath Falls

wastewater treatment plant

seismic hazard assessment

diatomaceous soils

cyclic direct simple shear

liquefaction settlement

pore pressure ratio

lateral spread

pile design

site-specific cyclic resistance