Observations and predictions of the behaviour of large, slow-moving landslides in schist, Clyde Dam reservoir, New Zealand D.F. Macfarlane, Observations and predictions of the behaviour of large, slow-moving landslides in schist, Clyde Dam reservoir, New Zealand. Engineering Geology, 109, 報 告 者：林雅詩 指導老師：蔡龍珆 日 期： 100/05/26

2 Outline Introduction Objectives Study area Monitor basis Effect of factor Monitor result Predicting landslide responses Conclusions

3 Introduction

4 There are sixteen creeping or dormant landslides, mostly located along the Cromwell Gorge. Stabilisation work involving toe buttressing, pumped drainage, gravity drainage and/or infiltration protection was implemented on the landslides prior to lake filling.

5 Objectives

6 This paper focuses on the response to rainfall events of two of the landslides: - Ripponvale Slide - Brewery Creek Slide

7 Study area

8 Fig. 1. Clyde dam landslides location map.

9 Fig. 2. Layout and instrumentation of Brewery Creek Slide ‘Active Portion’. Brewery Creek Slide

10 Fig. 3. Active Portion cross section AA (Fig. 7), looking down valley.

11 Fig. 4. Geology and geomorphology of Ripponvale Slide, downstream segment. RV locations are survey monitoring points. A B C It has common schist outcrops in the lower slopes, a distinctive area of tension cracks and graben structures on the upper slopes, and a relatively deep seated movement zone. It lacks continuous schist outcrop along the lake edge and includes a zone of multiple shallower movement zones. It is the downstream part with geomorphology suggestive of recent surficial instability, and with a toe breakout zone well above the lake. Ripponvale Slide

12 Fig. 5. Section through Zone A, Ripponvale Slide.

13 Monitor basis

14 Installed monitoring instrumentation The aim of the monitoring programme was to: establish precedent rates of movement before the filling of the reservoir (Lake Dunstan). monitor the slide responses during and immediately after the filling period. allow on-going monitoring of slide activity. Geodetic surveys Key ray Inclinometers Piezometer Extensometer Flow measuring

15 Performance criteria

16 Clyde Slide: 4mm/year Fig. 6. Long term creep trend exhibited by large, slow moving Clyde landslide varies between 2 mm/year and 4 mm/year. 2~4mm/year Long term creep behavior

17 Byford Slide: filling Fig. 7. Long term creep movement recorded at Byford Creek slide. This shows a clear reaction to lake filling with a long term slowing of movement.

Effect of factor 18

19 Seismic stability MM VII to VIII intensity shaking may be required to initiate significant slope movements The size of earthquake that would induce a significant movement response in any of the landslides is affected by a number of factors: - include the MM intensity at the site, topographic and directivity effects, the types of landslide, geology and groundwater conditions.

20 Fig. 8. Abrupt but short term changes in piezometric levels in piezometer DL545A (curve B) due to earthquakes do not correlate with drainage flows (curve A) affected by rainfall events. Note that very small drainage flow increases can be seen matching the piezometer rises and are attributable to earthquake effects.

21 Fig. 9. Residual mass curve of Cromwell monthly rainfall 1950–2001. Rainfall patterns

22 Fig. 10. Monthly rainfall at Cromwell 1991–2001. This shows the three significant rainfall events that have occurred since lake filling was completed.

23 Monitor result

24 Fig. 11. Brewery Creek Slide Active Portion — Movement history since 1990 v Piezometer DL 50C. Discrete movement events are triggered when the piezometer level rises above EL280.9 to EL281.0m. Brewery Creek Slide, Active Portion

25 Monitored slide responses Slide monitoring data shows that the slide reacts to rainfall events by increasing its rate of movement. Zone A is significantly more sensitive to rainfall effects than either Zone B or Zone C. --because the Zone A slopes are much steeper and have shallower failure surfaces. Ripponvale Slide

Movement rates Movement rate trends 26 Fig. 12. Ripponvale Slide, Zone A. Key ray movement vs lake level and rainfall.

Responses to lake filling Lake Dunstan was filled in 3 stages between April 1992 and September No instruments in Zone A detected changed movement rates that could be attributed to lake filling effects. 27

28 Responses to rainfall The most dramatic measured increases in surface displacement rates at Ripponvale Slide have occurred in Zone A. The main cause of the increased rate of movement in Zone A is inferred to have been elevated groundwater levels within the slope consequent upon these prolonged or significant rainfall events.

29 Predicting landslide responses

30 The detailed monitoring records from the Ripponvale and Brewery Creek landslides show that the general pattern of their response to rainfall events is predictable with a high level of confidence. The onset of renewed movement and the reduction in movement rate both show a good correlation with the piezometric level in DL50C. - EL281.0m

31 It is not so easy to identify the trigger point for renewed movement at Ripponvale Slide because there are no sensitive instruments measuring movements or piezometric levels. - exceeds 100mm/month - over a period of 3 to 4 months exceeds about 300mm

32 Conclusions

33 The data show that the cumulative rainfall effect is to elevate piezometric pressures to a trigger point at which movement is reactivated. Movement rates then decline as the piezometer levels fall below the trigger point and movement ceases or returns to the long term trend over a period of months.

34 Thanks for your attention.