Anaheim Hills LandslideAnaheim Hills Landslide

‘‘Avenida de Santiago’Avenida de Santiago’

January 17th 1993January 17th 1993

IntroductionIntroduction Southern California, Southern California,

the Orange Countythe Orange County Winter storms cause Winter storms cause

widespread slope widespread slope failuresfailures

Disrupt infrastructureDisrupt infrastructure Excess rainwater Excess rainwater

causes rising causes rising groundwater levelsgroundwater levels

Re-activates ancient Re-activates ancient landslidelandslide

‘‘Avenida de Santiago’ Avenida de Santiago’ most high profilemost high profile

Why did it occur?Why did it occur?

1.1. North-facing North-facing hillside hillside topographytopography

2.2. GeologyGeology

3.3. Rising Rising GroundwaterGroundwater

4.4. Human errorHuman error

GeologyGeology Northward dipping sectionNorthward dipping section

• Sandstone and Siltstone (Puente Formation)Sandstone and Siltstone (Puente Formation)• Miocene ageMiocene age

Bedding dips 7Bedding dips 7°° to 25 to 25° to North° to North Strikes range NE to NWStrikes range NE to NW Failure in Puente FormationFailure in Puente Formation

• Parallel to beddingParallel to bedding SandstoneSandstone

• Medium to Coarse grainedMedium to Coarse grained• Poor cementedPoor cemented• WeakWeak• Inter bedded with SiltstoneInter bedded with Siltstone

GeologyGeology

Compressional uplift of tertiary Compressional uplift of tertiary sedimentary section due to blind sedimentary section due to blind thrust faultsthrust faults

San Andreas faultSan Andreas fault• Strike slips and thrust faultsStrike slips and thrust faults• Multiple earthquakesMultiple earthquakes• Late Quaternary time contributes to Late Quaternary time contributes to

landslidelandslide

WeatherWeather Intense rainfall December 1992 and Intense rainfall December 1992 and

January 1993January 1993• Equal to average annual rainfall (38cm)Equal to average annual rainfall (38cm)

Raised groundwater level in landslide Raised groundwater level in landslide massmass

Human ErrorHuman Error

Early Geotechnical InvestigationEarly Geotechnical Investigation• No groundwater evaluationNo groundwater evaluation

PiezometerPiezometer

• Poor borehole investigationsPoor borehole investigations Few widely spaced and shallowFew widely spaced and shallow

Responsive, not proactiveResponsive, not proactive• Local authorities authorised Geological Local authorities authorised Geological

Investigation upon observation of cracksInvestigation upon observation of cracks Neglected evidence of previous landslidesNeglected evidence of previous landslides

The LandslideThe Landslide

Soil Slumps and Soil Slumps and Soil Block slides Soil Block slides move at 2/3cm per move at 2/3cm per dayday

TranslationalTranslational• 400m wide x 600m 400m wide x 600m

longlong After management After management

solutions slide solutions slide deemed stationary deemed stationary by mid-April 1993by mid-April 1993

The LandslideThe Landslide

Cracks and Fissures developed at headCracks and Fissures developed at head Compressional features damaged infrastructure Compressional features damaged infrastructure

lower down slopelower down slope Immediate Engineering implicationsImmediate Engineering implications

Geotechnical InvestigationGeotechnical Investigation

Geotechnical InvestigationGeotechnical Investigation

Ridge Top Graben at LD-3 source of landslideRidge Top Graben at LD-3 source of landslide Slip occurs at Sandstone bedrock as identified by Slip occurs at Sandstone bedrock as identified by

borehole logsborehole logs

Engineering Prevention StrategiesEngineering Prevention Strategies

Dewatering to control groundwater levelDewatering to control groundwater level Removal and recompaction of potentially Removal and recompaction of potentially

compressible soilscompressible soils Complete or partial removal of ancient Complete or partial removal of ancient

landslideslandslides Stabilisation of potentially unstable cut Stabilisation of potentially unstable cut

slopesslopes• Case study $2.9 million for one house!Case study $2.9 million for one house!

Specific structure foundation designsSpecific structure foundation designs

DewateringDewatering

GHAD final solutionGHAD final solution 30 million gallons 30 million gallons

water per yearwater per year Dewatering wells and Dewatering wells and

horizontal drainshorizontal drains Lower and control Lower and control

groundwater levelsgroundwater levels Substantially Substantially

improved stability of improved stability of landslide masslandslide mass

Constantly monitoredConstantly monitored

ConclusionConclusion Re-activation of Ridge-Re-activation of Ridge-

Top Graben by seismic Top Graben by seismic shaking & rising shaking & rising groundwatergroundwater

Dewatering is a ‘stop Dewatering is a ‘stop gap’gap’

$3.5 million budget$3.5 million budget Constant road Constant road

resurfacingresurfacing New fissures appeared New fissures appeared

in 1998in 1998 Escape route planEscape route plan