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Anaheim Hills Landslide

Anaheim Hills Landslide. ‘Avenida de Santiago’ January 17th 1993. Introduction. Southern California, the Orange County Winter storms cause widespread slope failures Disrupt infrastructure Excess rainwater causes rising groundwater levels Re-activates ancient landslide

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Anaheim Hills Landslide

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  1. Anaheim Hills Landslide ‘Avenida de Santiago’ January 17th 1993

  2. Introduction • Southern California, the Orange County • Winter storms cause widespread slope failures • Disrupt infrastructure • Excess rainwater causes rising groundwater levels • Re-activates ancient landslide • ‘Avenida de Santiago’ most high profile

  3. Why did it occur? • North-facing hillside topography • Geology • Rising Groundwater • Human error

  4. Geology • Northward dipping section • Sandstone and Siltstone (Puente Formation) • Miocene age • Bedding dips 7° to 25° to North • Strikes range NE to NW • Failure in Puente Formation • Parallel to bedding • Sandstone • Medium to Coarse grained • Poor cemented • Weak • Inter bedded with Siltstone

  5. Geology • Compressional uplift of tertiary sedimentary section due to blind thrust faults • San Andreas fault • Strike slips and thrust faults • Multiple earthquakes • Late Quaternary time contributes to landslide

  6. Weather • Intense rainfall December 1992 and January 1993 • Equal to average annual rainfall (38cm) • Raised groundwater level in landslide mass

  7. Human Error • Early Geotechnical Investigation • No groundwater evaluation • Piezometer • Poor borehole investigations • Few widely spaced and shallow • Responsive, not proactive • Local authorities authorised Geological Investigation upon observation of cracks • Neglected evidence of previous landslides

  8. The Landslide • Soil Slumps and Soil Block slides move at 2/3cm per day • Translational • 400m wide x 600m long • After management solutions slide deemed stationary by mid-April 1993

  9. The Landslide • Cracks and Fissures developed at head • Compressional features damaged infrastructure lower down slope • Immediate Engineering implications

  10. Geotechnical Investigation

  11. Geotechnical Investigation • Ridge Top Graben at LD-3 source of landslide • Slip occurs at Sandstone bedrock as identified by borehole logs

  12. Engineering Prevention Strategies • Dewatering to control groundwater level • Removal and recompaction of potentially compressible soils • Complete or partial removal of ancient landslides • Stabilisation of potentially unstable cut slopes • Case study $2.9 million for one house! • Specific structure foundation designs

  13. Dewatering • GHAD final solution • 30 million gallons water per year • Dewatering wells and horizontal drains • Lower and control groundwater levels • Substantially improved stability of landslide mass • Constantly monitored

  14. Conclusion • Re-activation of Ridge-Top Graben by seismic shaking & rising groundwater • Dewatering is a ‘stop gap’ • $3.5 million budget • Constant road resurfacing • New fissures appeared in 1998 • Escape route plan

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