South Sister St. Marys, Tasmania

Landslide potential

expert opinion

DAVID STAPLEDON
GEOTECHNICAL CONSULTANT
10 Deakin Grove
Glenside SA 5065
AUSTRALIA
30 January 2005

 

Thank you for the material sent on 18 Jan and received a few days ago. I have looked at all of it but can comment in detail only on the potential for landsliding. This follows, after which I comment briefly on the letter of Dr Ingles.

ASSESSING RISK OF LANDSLIDING

Throughout the world, before dam and reservoir projects are built, their sites are studied and explored carefully for months and years by various means, usually including geological mapping, and digging or drilling of test holes. One of the most important objectives of these studies is to ensure that the project works are not located in areas likely to be affected by landsliding. Despite these studies, many unexpected landslides have occurred at such projects. Some have started or occurred during early roadworks or other excavations, and others after completion and storing of water. Some of the slides have been 'first time' events, in area free from past landsliding, but may have been 'reactivated sites' (See Attachment A).

In 2002, the International Commission on Large Dams published the results of case history studies of 145 landslides which occurred at reservoir projects. These showed that only about 36% of the slides had been predicted from the pre-construction studies and At least 75% of the slides were found to have been of the 'reactivated' type.

From these results, and my own 53 years of experience with engineering projects on slopes, I believe that the following risk assumptions should be made for sites to be disturbed by clearing and excavation.

In the light of these beliefs, I was surprised by the very relaxed or 'off-hand' approach to the landslide risk assessment in the E-mail exchanges between Peter McIntosh (Forest Practices Board) and Andrew Crowden (Forestry Tasmania) in July and October 2003.

ATTACHMENT B*

[*Email from Dr. McIntosh (FPB) to Mr. A. Crowden (FT), dated 19 August 2003, obtained under Freedom of Information Act.]

In his message, Mr Crowden describes some conditions at the coupe and then states '... I think the excluded area will exclude the majority of the potential landslip problems.' Dr. McIntosh replies 'This coupe has all the potential for landslides - thin dolerite talus over sst and coal measures, but if you haven't seen any signs of instability, including bare soil, tension cracks, curved trees, slumps, backwalls, seepages harvest according to code provisions should be OK. If you need an FPB check I will look at it in late August or early September. Your draft prescriptions are supported'

MY COMMENT.

The emphasis is mine. To me, the emphasised words are indicative of either current or very recent active landsliding. It seems to me that despite his note that the coupe has 'all the potential for landslides' Dr McIntosh believes 'harvest according to the Code provisions is OK' as long as we don't see evidence of 'current or very recent active landsliding'.

ATTACHMENT C*

[*Email from Dr. McIntosh (FPB) to Mr. A. Crowden (FT), dated 20 October 2003, obtained under Freedom of Information Act.]

In this message, Dr McIntosh's 4th paragraph states 'In the north-east corner of the coupe there are ancient landslide features at two levels. Neither is active – all trees were upright. At the higher level (west of the Telstra track) there is a concentric scar in the dolerite, which appears to have slumped away. Immediately east of this, below the Telstra track, is another scar developed in sedimentary rocks. The basin caused by this second landslide is the source of the domestic water intake. This area should be protected from harvest with a minimum of 50m between the harvest area and landslide scars. This measure will protect the water quality of the seepages feeding the intake'.

MY COMMENT/QUESTIONS.

Is his statement 'Neither is active' based solely on upright trees on the features, these trees being the same sizes (ages) as those on the surrounding slope? Is his apparent confidence that these 'ancient features' do not indicate a significant landslide risk during or after logging, based on long experience with successful logging (without causing landsliding) in similar geological, topographic, drainage and climatic situations? If so, I would respect his judgment. If not, I consider his recommendation to ignore these landslide features is irresponsible.

Further, I assume that his recommendation to leave 'a minimum 50m between the harvest and the landslide scars' refers only to the second landslide scar, and that this 'not to be harvested zone' is aimed simply at preventing excessive siltation of the 'basin'?

Dr McIntosh's 5th paragraph states 'No other landslides were noted. The E. brookeriana reserve along the eastern boundary of the coupe will protect the steeper walls from landslide risk.'

I assume that the 'steeper walls' referred to here are on slopes below the lower boundary of the proposed coupe?

LETTER FROM Dr. INGLES

I understand that Dr Ingles is a renowned soil specialist, and consider that his opinions on soil degradation should be taken seriously.

David Stapledon

REFERENCE
International Commission on Large Dams (2002). Reservoir Landslides – Guidelines for Investigation and Management.

appendix

GEOTECHNICAL ENGINEERING OF EMBANKMENT DAMS

by

ROBIN FELL
Professor of Civil Engineering
School of Civil Engineering, University of New South Wales, Sydney

PATRICK MacGREGOR
Chief Engineering Geologist
Snowy Mountains Engineering Corporation, Sydney

DAVID STAPLETON
Geotechnical Consultant & Professor of Applied Geology
University of South Australia, Adelaide

A.A. BALKEMA / ROTTERDAM / BROOKFIELD / 1992

pp 56, 2.6.1 'First time' and 'reactivated' slides

A distinction can be made between ‘first-time’ landslide activity which develops at ‘intact’ sites like that shown on Figure 2.29 and that which is simply reactivation of an old landslipped mass which has become stabilised or is subject to only minor creep type movements.

This distinction is important because there is a very large difference between the predictability of the two kinds of sliding.

2.6.1.1 Reactivated slides
Experience has shown that in more than 50** percent of cases, disturbance of an old landslide eg. By cutting into or inundating its toe, causes reactivation. Hence if there is clear evidence that all or part of a slope is an old slide mass, a high risk of reactivation by construction activities or operation must be assumed, at least until geotechnical conditions in the slope are sufficiently well known to prove otherwise.

** In our latest book (in press) this figure is 75 percent, based in 1 COLD (2002)

First-time slides
First time slides are much more difficult to predict. Reliable prediction is not possible without considerable knowledge of the geotechnical conditions in the slope, and of the nature of the proposed disturbing activities. For example, there is not enough subsurface data on Figure 2.29 for one to say definitely that Wedge A B D will slide, with or without some excavation above Point A.

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