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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

Anaheim Hills Landslide

‘Avenida de Santiago’

January 17th 1993


Introduction
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


Why did it occur
Why did it occur?

  • North-facing hillside topography

  • Geology

  • Rising Groundwater

  • Human error


Geology
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


Geology1
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


Weather
Weather

  • Intense rainfall December 1992 and January 1993

    • Equal to average annual rainfall (38cm)

  • Raised groundwater level in landslide mass


Human error
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


The landslide
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


The landslide1
The Landslide

  • Cracks and Fissures developed at head

  • Compressional features damaged infrastructure lower down slope

  • Immediate Engineering implications



Geotechnical investigation1
Geotechnical Investigation

  • Ridge Top Graben at LD-3 source of landslide

  • Slip occurs at Sandstone bedrock as identified by borehole logs


Engineering prevention strategies
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


Dewatering
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


Conclusion
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