ICL Landslide Teaching Tools PPTtool 4 064 1

ICL Landslide Teaching Tools PPT-tool 4. 064 -1. 1 (1) Case History: The 1979 Abbotsford Landslide, Dunedin, New Zealand Graham Hancox GNS Science (Lower Hutt, New Zealand) e-mail: g. hancox@gns. cri. nz

ICL Landslide Teaching Tools PPT-tool 4. 064 -1. 1 (2) OUTLINE OF PRESENTATION: = Hancox, G. T. 2008. The 1979 Abbotsford Landslide, Dunedin, New Zealand: a retrospective look at its nature and causes. Landslides, 5(2): 177 -188 = Introduction – when, where, and what happened. = Description of Abbotsford Landslide: - Its geomorphic/geological setting. - Movement history and final failure. - Landslide features and effects. = Causes of the landslide and Stability Analysis. = Lessons from the landslide, and implications for urban developments. = 22 GNS Science

ICL Landslide Teaching Tools PPT-tool 4. 064 -1. 1 (3) Abbotsford Landslide - Introduction: = The Abbotsford Landslide occurred on 8 August 1979, after creeping movements since October 1978. = Located in the Abbotsford suburb of southwest Dunedin. = The failure resulted in destruction of 69 houses, and overall cost of $10 -13 million – but no injuries or loss of life. ► Location of the Abbotsford Landslide in the Abbotsford suburb of southwest Dunedin. = The disaster was investigated by a Government Commission of Inquiry 3 to determine its cause. GNS Science

ICL Landslide Teaching Tools PPT-tool 4. 064 -1. 1 (4) Abbotsford Landslide – Geological Setting: = Located in area of soft Tertiary rocks (sands, mudstone) overlain by bouldery colluvium. These rocks dip 6 -9º to the southeast. = Landslide formed on dipslope on western side of Miller Ck. = Also other active landslides in area, and prehistoric Sun Club Slide (within the sliding block). ► Geological map showing the location of the Abbotsford Landslide and other features. = Key features in the area are an old sand quarry 4 GNS Science (Harrison's Pit) at the

ICL Landslide Teaching Tools PPT-tool 4. 064 -1. 1 (5) Abbotsford Landslide – Early movements: ► Aerial photos taken 3 days before the final movement on 8 August. This shows locations of the initial movements at the toe in the old sand quarry (Harrisons Pit), breaks in water mains (c), and the developing graben cracks. 5 GNS Science

ICL Landslide Teaching Tools PPT-tool 4. 064 -1. 1 (6) Abbotsford Landslide – Early movements: ► Aerial photo showing the graben cracks at the head of the landslide, extending north through farmland. Drill hole (D) in progress and backhoe digging test pit (TP, visible bottom right) during the pre-slide investigations. 6 GNS Science

ICL Landslide Teaching Tools PPT-tool 4. 064 -1. 1 (7) Pre-Slide Investigations: ► Establishment and monitoring of survey marks and two survey lines. ► Recording of damage to houses, services (water pipes, overhead wires). ► Mapping of ground deformation features (tension cracks, shear cracks, compression ridges, settlements, subsidence). ► Drilling of 6 small-diameter drill holes (N-size, triconed and cored), with tube inclinometers installed in 4 drill holes. ► Several shallow (4 -6 m deep) test pits and push-tube samples. 7 GNS Science

ICL Landslide Teaching Tools PPT-tool 4. 064 -1. 1 (8) Deformation and Investigations before Landslide: Deformation and Damage: ► Area affected by pre-slide ground movements. ► Tension and shear cracks, with compression roll in the toe area ► Damaged houses; breaks in water mains and stormwater pipes. Investigations: ► Monitoring of survey marks and two survey lines. ► Six drill holes – 4 with inclinometers. ► Piezometers in four holes blocked or sheared off by slide movement. ► Several shallow test pits. 8 GNS Science

ICL Landslide Teaching Tools PPT-tool 4. 064 -1. 1 (9) Abbotsford Landslide – Movement History: ◄ ◄ 150 -200 mm/day 670 mm/day ► Graph of early movements of the landslide (from 2 survey lines) showing total crack widening and rates, effects on services, and rainfall data from 18 June to 8 August. ► Movement rate from June to 25 July was ~5 -20 mm/day. It accelerated rapidly on 28 July to ~100/day during moderate rainfall. . . and then continued to slowly speed up. 9 GNS Science

ICL Landslide Teaching Tools PPT-tool 4. 064 -1. 1 (10) Abbotsford Landslide – Movement History: ◄ ◄ 150 -200 mm/day 670 mm/day ► Hazard and response planned on 5 August when movement accelerated from 150 -200 mm/day. Plans made to evacuate landslide hazard zone by 12 August – 450 mm/day predicted by then. ► Movement accelerated to 670 mm/day by 7 August… change surveyed, but data not plotted and used to revise risk assessment and evacuation before the final movement on 8 August. 10 GNS Science

ICL Landslide Teaching Tools PPT-tool 4. 064 -1. 1 (11) Abbotsford Landslide: The Main Failure ► The main failure occurred on 8 August 1979 – 30 years ago. ► In the final movement a large block of land with 28 houses and 17 people on it slid 50 m SE into Miller Creek over 30 minutes. ► The movement formed a graben at the slide head 15 -30 m deep and 70 -150 m wide, in which 15 houses were destroyed. ► The failure occurred before all people had been evacuated from the landslide hazard zone. ► Slide movement rate much faster than predicted on 5 August and evacuation plans made. Survey data (plotted later) showed movement accelerated to 430 -670 mm/day on 6 -7 August, much faster than the rate predicted for 12 August (450 mm/day). ► People on the landslide were evacuated safely – but they were probably lucky to survive without injury or loss of life. 11 GNS Science

ICL Landslide Teaching Tools PPT-tool 4. 064 -1. 1 (12) Abbotsford Landslide – Main Features: = Landslide was a large block slide - 800 x 400 m - up to 40 m thick - volume c. 5 million m 3. = Slide mass moved 50 m down 7 degree bedding plane clay layer in 30 minutes = Formed graben 70 -150 m wide at slide head - in which 20 houses were destroyed. ► Air photo of Abbotsford Landslide on 9 August showing the main features. The old sand quarry = Compression at slide toe, and a leaking DCC water main above it were suspected of causing the failure. 12 zone GNS Science

ICL Landslide Teaching Tools PPT-tool 4. 064 -1. 1 (13) p SC G H SP ► Abbotsford Landslide from the air on 12 August - showing the graben (G) with earthworks, sand pit (SP), water ponded in Miller Creek (p), old Sun Club Slide (SC), and 20 undamaged houses (H) on the slide block (these were later moved to new sites). 13 GNS Science

ICL Landslide Teaching Tools PPT-tool 4. 064 -1. 1 (14) Abbotsford Landslide – Main Features: ► View looking south showing the deep graben at the slide head, here partly regraded (1), houses stranded on slide block, and the Sun Club Slide. Stream diverted from slide (2) 14 GNS Science

ICL Landslide Teaching Tools PPT-tool 4. 064 -1. 1 (15) Abbotsford Landslide – Main Features: ► View over landslide looking northwest – toe area foreground shows the former sand pit and old Sun Club Slide. 15 GNS Science

ICL Landslide Teaching Tools PPT-tool 4. 064 -1. 1 (16) Abbotsford Landslide – Main Features: ► Closer aerial view of the Sun Club Slide area showing deformation features, water ponded (p) behind slide debris in Miller Creek, old sand pit (sp), graben (g), and the head scarp (hs). 16 GNS Science

ICL Landslide Teaching Tools PPT-tool 4. 064 -1. 1 (17) Abbotsford Landslide – Main Features: Photo: Don Bird, EQC ► Twenty houses were destroyed when the graben opened at the slide head. One house (h) was moved 50 m southeast and down 12 m. In all 69 houses were affected, some on the moving slide block were undamaged and later moved to new sites. 17 GNS Science

ICL Landslide Teaching Tools PPT-tool 4. 064 -1. 1 (18) Abbotsford Landslide – Main Features: ► The graben area with earthworks in progress, and stranded houses. 18 GNS Science

ICL Landslide Teaching Tools PPT-tool 4. 064 -1. 1 (19) Abbotsford Landslide – Main Features: ► House destroyed by the collapse on the southern edge of slide block. 19 GNS Science

ICL Landslide Teaching Tools PPT-tool 4. 064 -1. 1 (20) Abbotsford Landslide – Main Features: ► Features at slide toe in Sun Club grounds (part of old slide area) – shows compression ridges (cr), one of which destroyed the volley ball court (TC). 20 GNS Science

ICL Landslide Teaching Tools PPT-tool 4. 064 -1. 1 (21) Abbotsford Landslide – Main Features: ► Features at slide toe showing more damage caused by compression ridges (cr) – here they destroyed the tennis court (TC), swimming pool (SP), and Sun Club Rooms (SC). 21 GNS Science

ICL Landslide Teaching Tools PPT-tool 4. 064 -1. 1 (22) Abbotsford Landslide – Main Features: ► More damage at the slide toe – another court destroyed and pine trees tilted by a large compression ridge. 22 GNS Science

ICL Landslide Teaching Tools PPT-tool 4. 064 -1. 1 (23) Post-Slide Investigations: to determine cause of landslide (Aug 1979 -Feb 1980) ► 18 small-diameter (N-size) cored drill holes (11 – 59 m deep). ► Inclinometers in 8 holes, with standpipe groundwater monitoring. ► Piezometers in 6 drill holes. ► Six back-hoe test pits (3– 6 m deep). ► Seventeen Calweld shafts (900 mm, 10– 30 m deep). ► Soil testing: Atterberg limits, consolidation, densities, grading, mineralogy, permeability, and triaxial (sand) and shear box (clay layers) strength). ► Geomorphic and geological mapping. ► Geological history and historical records of landslides and ground movements in area, rainfall and earthquake data, pre and post-slide groundwater levels, natural erosion, activities of man (removal of vegetation, toe excavation, leakage of water pipes, urbanization of area). ► Limit equilibrium slope stability analyses. ► Evaluation of conditions that contributed to the cause of the landslide, and the main factors that triggered movements leading to the final collapse. ► Results submitted to a Government Commission of Inquiry –this began in March 1980 and reported in November 1980. 23 GNS Science

ICL Landslide Teaching Tools PPT-tool 4. 064 -1. 1 (24) Main Features: ► Map and Cross Sections of the of Abbotsford Landslide showing geomorphic features, slide blocks, investigation drill holes and test pits, and the failure surface (slide plane). 24 GNS Science

ICL Landslide Teaching Tools PPT-tool 4. 064 -1. 1 (25) Abbotsford Landslide – The Causes: 1. Unfavourable Geology and Topography: - significant (a) Landslide formed on a dip slope -Tertiary sediments dip c. 7– 10º towards Miller Creek. (b) Low (near residual) strength (Ø' = 5– 10º) along bedding at top of Abbotsford Formation. (c) Progressive failure over >20, 000 years reduced strength of clay layers to near residual – began after downcutting by Miller Creek. (d) Prehistoric Sun Club Slide (~10, 000 years BP) illustrates inherent instability of slope on west side of Miller Creek. Stability further reduced by stream erosion at toe. ► Factors that may have contributed to the Abbotsford Landslide. 25 GNS Science

ICL Landslide Teaching Tools PPT-tool 4. 064 -1. 1 (26) Abbotsford Landslide – The Causes: 2. Increased pore water pressure: - significant Geotechnical evaluations suggested that the landslide slope was sensitive to changes in groundwater level. A rise in the water table in the slide area was likely because of: (a) Wet weather at the end of July and early August 1979 and, more importantly, increased rainfall over the last 10 years prior to the failure (following a 20 -year drier period). (b) Leakage from broken water and stormwater pipes in the affected area, and known long-term leakage from a Dunedin City Council 750 mm water main at the top of the slope. ► Factors that may have contributed to the Abbotsford Landslide. 26 GNS Science

ICL Landslide Teaching Tools PPT-tool 4. 064 -1. 1 (27) Abbotsford Landslide – The Causes: 3. Quarrying of sand at the toe of the slope: - significant Excavation of ~300, 000 m 3 (6% of the slide mass) from Harrison’s Pit between 1964– 1969 was considered to have reduced stability of the slope – especially directly above and adjacent to the pit. This was not sufficient to cause immediate failure of the slope. Damage to one house (in gully to SW) possibly started in 1968… seen by EQC to beof natural vegetation: - insignificant 4. Removal due to local factors. First unequivocal signs of damage more likely to have Removal of native bush and scrub cover in the area more than 100 been 1972. years ago (before 1894), possibly had a minor effect on stability due to slightly increased surface runoff, and hence a rise in the water table. The lack of data about previous vegetation and runoff characteristics made it impossible to quantify these factors. ► Factors that may have contributed to the Abbotsford Landslide. 27 GNS Science

ICL Landslide Teaching Tools PPT-tool 4. 064 -1. 1 (28) Abbotsford Landslide – The Causes: 5. Urban development (excluding DCC water main): - insignificant Building of houses, roads etc. resulted in very small increased weight on the slope over 15% of the slope area. Increased interception of rain (by houses, roads) possibly offset deficiencies in storm water reticulation. Breaks in water mains, and pipe for East Christie St stream (1978 and 1979) resulted from early slide movement –most detected and repaired. Vibrations from motorway too small to have affected slide area. 6. Seismic Activity: - insignificant Not thought to be relevant. There were no significant earthquakes (MM 5 or greater) in the area in the 3 years prior to the slide. The strongest historical earthquake that has affected the area (M 5, MM 7) occurred in April 1974 is not known to have affected the slope. The effects of prehistoric earthquakes are unknown. ► Factors that may have contributed to the Abbotsford Landslide. 28 GNS Science

ICL Landslide Teaching Tools PPT-tool 4. 064 -1. 1 (29) Stability Analysis (DSIR)*: 1. Geometry – pre and post failure contours. 2. Material strength parameters – laboratory test data. - Shear plane clay layer: c´ = 0; Ø´ = ~8 - Green Island Sand: c´ = 0; Ø´ = ~28 3. Piezometric levels – pre-slide piezometer readings and drillers logs; and post slide observations of seepage and piezometer levels 4. Stability analyses using limit equilibrium method – Janbu’s generalised procedure of slices, which allows for controllable interslice forces (Janbu 1973). * Reference: Salt, Hancox, and Northey (1980). 29 GNS Science

ICL Landslide Teaching Tools PPT-tool 4. 064 -1. 1 (30) Abbotsford Landslide – The Slide Plane Slide plane: located within a thin clay layer forming the interface between Abbotsford Formation Mudstone (grey) and overlying disturbed Green Island Sand (yellow brown) in Test Pit 19 a. Slide plane – along clay layer 1. 74 m below interface between Abbotsford Mudstone (grey) the Green Island Sand in TP 34. Showed well developed slickensides. Shear strength determined by residual shear tests: c´ = 0; Ø´ = ~8. 30 GNS Science

ICL Landslide Teaching Tools PPT-tool 4. 064 -1. 1 (31) Stability Analysis: Sections ► Stability analysis carried out using 2 D limit equilibrium method (Janbu 1973, which produces controllable interslice forces). Sections lines and pre-slide contours and inferred piezometric conditions as shown. 31 GNS Science

ICL Landslide Teaching Tools PPT-tool 4. 064 -1. 1 (32) Stability Analysis: Key Results 1. Removal of sand from Harrisons Pit (1964 -69): - Reduced toe restraint in southern part of landslide - Reduced stability of Section Line 4 (through pit) by ~1. 5% - Reduced stability of the overall slope by ~1% 2. Rise in groundwater level due to: (a) more rainfall over last decade; (b) leakage from water main/pipes/stormwater. - 0. 3 m rise in groundwater levels reduced stability of the overall slope by 1%. . - 2 -3% decrease in stability for 1 m rise in groundwater levels in area. . - Final failure occurred after 11 year period of higher than average rainfall. . . - Leakage from DCC water main over last 3 years was 6 -9 litres/min ~equal to 40 -50% increase in rainfall over that period. 32 GNS Science

ICL Landslide Teaching Tools PPT-tool 4. 064 -1. 1 (33) Stability Analysis: Increased Long-term Rainfall ► Plots of long-term annual rainfall in the landslide area. The failure occurred after 11 years of higher than average rainfall. Low permeability materials in the area more likely to have been affected by long-term changes in rainfall, and reduce stability of the slope, than short term changes. 33 GNS Science

ICL Landslide Teaching Tools PPT-tool 4. 064 -1. 1 (34) Abbotsford Landslide - Conclusions: 1. The Commission of Inquiry (Co. I) found that unfavourable geology (weak clay layers in a 7º dip-slope) was the main and underlying cause of the landslide. 2. The old sand quarry at the toe of the slope and a leaking water main above the slide area were found to be manmade factors that contributed to the failure. The Co. I report did not distinguish between these factors. 3. Slope stability analysis by DSIR showed that after sand excavation (c. 300, 000 m 3) reduced stability by c. 1%, so the water table had to rise 0. 3 m less for failure to occur. The quarry preconditioned the slope, making it more susceptible to failure, but did not control the timing of the failure. 34 GNS Science

ICL Landslide Teaching Tools Conclusions - contd: PPT-tool 4. 064 -1. 1 (35) 4. Leakage from the DCC water main over the 3 years prior to the landslide was 6 -9 litres/min – this was equal to a 40 -50% increase in rainfall over that period. 5. The DSIR concluded that a long-term rise in groundwater levels (due to increased rainfall over the previous decade and leakage from the water main) controlled the timing of the failure – and in this sense triggered the final movement of the landslide in August 1979. 6. Investigations before final movement accurately defined depth and rate of movements. Critical survey data not used to revise the 5 August risk assessment. 7. The predicted rate of 450 mm/day by 12 August exceeded on 10 August - 670 mm/day surveyed but not plotted up. 35 GNS Science

ICL Landslide Teaching Tools PPT-tool 4. 064 -1. 1 (36) Conclusions - contd: 8. Evidence of increased ground movement 2 days before the final rapid movement was overlooked. Monitoring data should always be quickly plotted up, and used to update a risk assessment and the planned response. 9. In retrospect, the 17 people who were on the landslide when the final movement occurred should have been evacuated on 7 August, when the landslide accelerated rapidly. Their survival without injuries or deaths was more a matter of chance than good management. 10. After 30 years the Abbotsford Landslide still provides a valuable reminder of the dangers of excavations at the toes of slopes, leakage from water mains in urban areas, and the need for geotechnical investigations to precede urban developments (now covered by the RMA). 36 GNS Science

ICL Landslide Teaching Tools PPT-tool 4. 064 -1. 1 (37) THANK YOU FOR YOUR ATTENTION Abbotsford Landslide after 30 years - the Graben area as it is today Acknowledgements: I wish to acknowledge the contributions made by my former DSIR colleagues to reports on which this presentation was based, especially Graham Bishop, Ian Mc. Kellar, Pat Suggate, Graham Salt, Roy Northey, Peter Barker, and Nick Perrin. GNS Science P. O. Box 30368, Lower Hutt, New Zealand Ph: +64 -4 -5701444 www. gns. cri. nz 37 GNS Science

ICL Landslide Teaching Tools PPT-tool 4. 064 -1. 1 (38) Abbreviations • Co. I – Committee of inquiry • DCC - Dunedin City Council • DSIR – Department of scientific and industrial research • EQC – Earthquake Commission • M – local magnitude (of earthquake) • MM – Modified Mercalli scale of felt ground shaking intensity • RMA – Resource management act 38 GNS Science