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 Bedding dips 7° to 25° to North
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 Responsive, not proactive
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