CASE STUDY: ROCKS AND WEATHERING Flashcards
Where is the case study?
Abbotsford Landslide, Dunedin, New Zealand.
When is the case study?
8th August 1979.
What is the background information?
A 1951 report based on a geological survey from the University of Otago had stated that the Abbotsford soil was unsuitable for building on because of its instability. However this report was either lost or ignored as during the 1960s housing and then the Southern Motorway were built in Abbotsford.
Describe the Abbotsford soil.
The land at Abbotsford is unstable due to the combination of different types of soil. The bedrock is covered by a layer of mudstone that can swell when wet and shrink when dry, and has a slippery surface. The next layer is sand, topped off by a mixture of soils. This combination of soils can become very unstable, especially when wet.
What occurred?
From 1978, several families in Abbotsford began to notice hairline cracks appearing in their homes - in the brickwork, concrete floors and driveways.
During 1979, workmen discovered that a leaking water main had been pulled apart.
Geologists discovered that water had made layers of clay on the hill soft, and that the sandstone above it was sliding on this slippery surface.
The danger became clear and an early warning system was put in place.
A state of Civil Defence Emergency was declared on the 6th of August and several homes began to be evacuated.
Finally, on the night of 8th of August, the landslide occurred.
What were the results/outcomes?
69 homes were destroyed or relocated and around 200 people were displaced.
7 hectares of the housing area slid almost 50 metres down the hillside, and around 18 hectares were affected in total.
There were no deaths.
The total cost from the destruction of the homes, infrastructure and relief organisation amounted to over £7 million.
An insurance scheme designed to cope with such disasters, as well as other government and voluntary relief measures, meant that many of the residents were compensated for their loss. However, other costs such as depressed house prices in the surrounding area, psychological trauma, and the expense of a prolonged public enquiry, were not immediately appreciated.
The landslide was essentially a block slide of sandstone resting on a bed of weaker clay. Displacement of 50 m took place in about 30 minutes, leaving a small rift 30m deep at the head of the slope. It involved around 5.4 million metres cubed of material. At first the land moved as a slow creep, followed by a rapid movement at about 1.7m/minute.
What were the causes?
Water had collected in the impermeable clay, reducing its strength and cohesion, and caused the sandstone to slip along the boundary of the two rocks.
Deforestation in the area had reduced evapotranspiration in the area and there was less binding of the soil by plant roots.
Urbanisation in the previous 40 years had modified the slopes by cutting and infilling, and had altered surface drainage (speeding up the removal of surface water).
Quarrying of material at the toe of the slope in the 1960s and 70s had also removed support from the base of the slope.
The trigger of the landslide is believed to have been a combination of leaking water pipes and heavy rainfall.
What were the lessons learnt?
Dangerous landslides can occur on relatively gentle slopes if the right conditions exist (the slope at Abbotsford was only at an angle of about 7˚).
Attention to early warning can help preparedness and reduce the loss of life.
Human activity can destabilise slopes.
Low-frequency, high-magnitude events may be difficult to predict, but mapping and dating of old hazards may indicate areas of potential risk - a regional landslide hazard assessment should be made where there is evidence of previous landslide activity.
A landslide insurance scheme eased the cost of the event - however the money was only available after the event, not beforehand, and the insurance only covered the houses, not the land damage.
What are some methods of controlling falls?
- Flattening the slope.
- Benching the slope.
- Drainage.
- Reinforcement of rock walls by grouting with cement, anchor bolts.
- Covering of wall with steel mesh.
What are some methods of controlling slides and flows?
- Grading or benching to flatten the slope.
- Drainage of surface water with ditches.
- Sealing surface cracks to prevent infiltration.
- Subsurface drainage.
- Rock or earth buttresses at foot.
- Retaining walls at foot.
- Pilings through the potential slide mass.
