Case Studies: Landslides in New Zealand

The economic cost of landslides in New Zealand is difficult to analyse as in major storms landslides generally occur in association with flooding. It is the flooding which has the immediate economic impact with sedimentation on the river flats. Whether the sedimentation results from landslide erosion or fluvial erosion cannot be distinguished accurately enough to allow analysis to be carried out. In 1984 John Hawley, then Scientist in Charge of the Soil Conservation Centre, estimated the average cost of landslides in the pastoral hill country to be in excess of $20 million per year.

The immediate costs can be large, i.e. following Cyclone Bola, a three-day cyclone in March 1988 during which up to 900 mm of rain fell devastating large tracts of the Gisborne-East Coast and North Hawkes Bay regions (Table 2), the government allocated $50 million to compensate farmers for losses in production. The damage to land, housing, communications and services is estimated to have cost more than $100 million.

Landslides in urban environments are common in cut and fill situations with houses and roads being undercut or inundated with debris. The most significant urban landslide in New Zealand's history occurred in 1979 when the Abbotsford block slide, covering 18 ha, caused the destruction or removal of 14 houses in the Green Island Borough of Dunedin (Table 2). The main movement comprised a 50-m slide at a rate of about one metre per minute and occurred within an hour, sliding along a bedding plane failure surface dipping at 7 degrees. While claims under the Earthquake and War damage Act amounted to nearly $3 million, it is likely that the total cost of this one landslide to New Zealand was $10 - 15 million.

Table 2. Significant landslide events in New Zealand

Banks Peninsula and Port Hills (Bell and Trangmar 1987)

In this case study we examine the association between soil type and land instability in the fine-grained loessial and volcanic soils of the Port Hills and Banks Peninsula, Christchurch. Eroded volcanic craters form Banks Peninsula and these are mantled by loess and colluvium. Erosion of these material has been along term problem for land users in the region, especially in the Port hills, where urban development is encroaching on lower slopes. Erosion processes are related to the type of subsurface material or regolith.

Soil/regolith types and land instability

Five principal soil/regolith types are recognised:

1. In situ loess - during past glaciations of the Southern Alps fine silt-sized material was produced by the grinding action of glacial ice. This was transported by the rivers draining the glaciers out onto the wide river flood plains where it was deposited. When it dried it was blown from the floodplain onto the surrounding hillslopes. Loess occurs mainly in the summits and shoulders of the interfluves and rims of the old volcano. It also occurs in footslope positions where it is buried by colluvium.

Two types of loess occur:

1. Birdlings Flat loess occurs on the lower interfluves. It is generally highly dispersive and slakes readily when wetted. It has low cohesion between particles and a low capacity for moisture retention.

2. Barrys Bay loess occurs at the valley heads and summits of the interfluves. Barrys Bay loess is finer grained than Birdlings Flat loess, has greater cohesion and is not as dispersive.

2. Loess-colluvium - this includes all loessial materials that have been transported downslope since the material was first deposited. It is formed of fine grained sand and silt. It also includes small amounts of volcanic rock material derived from upslope outcrops. Loess-colluvium occurs on shoulders, backslopes, footslopes and toeslopes and will have characteristics corresponding to the loess from which it is derived. Loess-colluvium may overlie basement volcanic rock, loess or older colluvium. The age, distribution and composition of the loess-colluvium varies, creating layering within the loess-colluvium. Contacts between these layers often provide failure surfaces for landslides, or discontinuities above which tunnel-erosion can occur.

3. Volcanic colluvium - this material comprises weakly to moderately weathered volcanic rocks fragments which are supported and surrounded by highly weathered silty clay or clay loam. It occurs on moderately steep mid-backslopes immediately below outcrops of volcanic rocks. The surface of the volcanic bedrock underlying the volcanic colluvium is usually slightly weathered, but is highly weathered in a few localities. With distance from the outcrops volcanic colluvium grades to mixed colluvium.

4. Mixed loess- and volcanic-colluvium - mixed colluvium consists of loess-colluvium mixed with weathering products derived from the volcanic rocks that outcrop upslope. Mixed colluvium occurs on backslopes and upper footslopes below outcrops of volcanic rocks. These outcrops are often associated with seepage zones. Where mixed colluvium overlies in situ loess or colluvium the upper layer of the underlying material is firm and in winter a perched water develops above this contact. The surface of zones of mixed colluvium are often hummocky showing evidence of past slope instability. Mixed colluvium is the regolith type most prone to landslides.

5. Residual regoliths - residual regolith is formed from the weathering of volcanic bedrock at the summits and shoulders of ridges where covering material has been removed. The regolith is thin, and volcanic outcrops commonly protrude through the material. Runoff from the erosion surface is usually high during rainfall causing landslides and erosion downslope.

Six forms of erosion are associated with these regolith types:

Problems have occurred in the Port Hills area when:

1. Fill is placed on a loessial slope without prior benching, causing slide movements above the fill-buried soil contact.

2. Vertical barriers greater than 1 m high are used to prevent shallow gully erosion and surface erosion but then trigger deeper tunnel-gully erosion below the fragipan.

3. Underground service trenches are installed on sloping ground and initiate subsurface erosion as a result of seepage.

Prevention and remedial measures

Detailed engineering soil maps are available at 1:10,000 scale. However, detailed site investigations at smaller scale are required to identify and characterise the regolith materials. Once this has been done different approaches to minimising the impact of development can be used. In the case of the sensitive materials on the Port Hills these may include:

1. Minimising excavation and ground disturbance, e.g. pole frame house design.

2. Stabilising dispersive loess soils through chemical treatment, particularly using hydrated lime.

3. Backfilling erosion cavities and service trenches.

Clyde Dam (Electricorp Production 1989)

Figure 10.

In this case study we examine the problems associated with development of the Clyde Dam hydro-scheme on the Clutha River and landslides within the Cromwell Gorge. Over a period of hundreds of thousands of years the Clutha River has carved out its channel through the Dunstan and Cairnmuir Ranges. During this time the rocks of the ranges have been folded and foliated, creating seams of clay and joints within the rock. These are potential failure planes and landslides are associated with the processes that have formed the Cromwell Gorge. The thin clay seams form parallel to the layering of the rock where sliding has occurred. Where the layering dips subparallel to the hillslope (dip slope) and towards the river large rock masses can slip downslope (Figure 10). This condition occurs at the Clyde Dam site and upstream in the Cromwell gorge. Conditions are generally stable when the rock layering dips sub-perpendicular to the hillslope (scarp slope).

Cromwell Gorge landslides and the Clyde Dam

The landslides are at Clyde Dam and in Cromwell Gorge are slow creeping features, moving at an estimated < 300 mm per year and terrace surface s 15-18,00 years old at the foot of the landslides are not disturbed which is further evidence for their slow movement. However, a major concern is how the filing of the Clyde Dam hydro lake will affect the rate of movement of these landslides. It is imperative to avoid a similar situation to that which arose at Vaiont, Italy (see Table 1).

The effects that lake filling may have on the landslide are:

1. Wetting of clay seams along which sliding has occurred - this will lead to a decrease in strength and hence stability

2. Wetting of landslide toe materials will decrease the effective weight of the rock and reduce frictional resistance in this part of the landslide and decreasing stability. The affect is greater where the landslide toe is currently dry and least where it is already wet.

3. The rising water level will affect groundwater systems within the landslide and may lead to rises in water pressure thus reducing cohesion and leading to landslide instability.

Prevention and remedial measures

Geological maps are available at regional scale (1:50,000). Detailed mapping and ground studies of the distribution and nature of landslides and landslide scars identified the processes that lead to slope instability. Other approaches adopted include:

1. Monitoring - all the major landslides have surface survey points and inclinometer holes for measuring surface and subsurface ground and regolith movements, respectively. Observation wells measure variations in ground water levels. Monitoring of the dam and reservoir following filling will continue following filling

2. Toe buttressing and gravity drainage stabilisation (Fig. 10).