The cause of the landslide that damaged the water pipeline
This is technical information on the nature and extent of the landslide and pipe damage provided by LDE Consultants, Tuesday 7 January 2014.
The landslide that has occurred is some 70m in width and extends down steeply (about 35 to 40˚ but locally up to 70˚) to the base of the gully some 180m below the road. Large tension cracks up to 200mm to 400mm in width form the headscarp of the landslide and extend into the centre of the road formation to the west of the pipeline. The eastern margin of the landslide appears to pass through the location of the severed pipeline. The exposed pipe is an L-shaped section that hangs out unsupported from the slope by some 2m to 3m over a narrow and incised gut formed by significant scouring erosion from the water from the leaking pipe. Debris from the erosion is strewn over a large area at the base of the slope. The landslide appears to have been significantly more active in the lower reaches of the slope across the entire width of the landslide. The estimated depth of the landslide is some 5m in the upper and mid areas reducing to some 3m in the lower reaches. The materials forming the landslide appear to be largely silt and mudstone gravel derived from the weathering of the underlying mudstone bedrock on which the pipe rests. The estimated volume of material affected by landslippage is 50,000m3. It is understood that the landslide developed over yesterday (Monday 6th) with small cracks evident in the morning increasing to their current size over the course of the day.
The landslide is still active, with movement noted by Fulton Hogan over period over their time on site. No movement was visibly observed over the time of my assessment. However, it is likely that rate of movement is slowing with the valve now closed. Some hairline cracking was identified behind the main headscarp indicating the potential for regression of the headscarp with additional movement should this occur.
Given the very steep nature of the slope and the thickness of materials above the bedrock it is possible that the hillside was in a marginal state of stability, particularly given some of the rainfall events over recent years. The blue waterpipe housing a telecommunications line (possibly associated with the water main) appears to have been previously repaired with black insulation tape. This tends to indicate that the hillside has been subject to movement in the past, albeit slight.
Cause of landslide movement
In our opinion it is quite possible that the triggering of land movement was from the extreme wind on Sunday and the potentially significant loads that this may have placed on the slope. This may have been sufficient to result in the deformation of the ground over the pipe resulting in damage to it. The scouring erosion from the water from the pipe appears to have been significant, and possibly undermined the toe of the affected section of the slope resulting in the large scale land movement at the toe base; this in turn resulting in the loss of support for the entire slope.
Stabilisation and repair options
The only means of practical stabilisation of the landslide appears to be the removal of material at the crest of the slope. Accordingly, removal of some 3m to 4m from the crest of the affected area has been recommended. Realignment and lowering of the road was also concluded as being the best approach. Removal of a large clump of stumps, soil and debris immediately above the overhanging pipe was also concluded. Most of the earthworks to complete this is expected to be able to be completed over the next day.
With these works completed we expect that work on repairing the pipe can be carried out. In the short term, the existing pipe may be able to be repaired to reinstate flow in the pipe. We recommend that the pipe be supported by a timber or steel props pinned into the underlying bedrock.
However, the landslide is expected to still be in a marginal state of stability and is likely to be subject to movement from future rainfall, earthquake and wind events. Given that the eastern margin of the landslide appears to pass through the pipe location, relocation of the pipe to the east of this area is ultimately recommended.