A geotechnical engineer’s perspective on recent events

By Doug Harbutt

What is a geotechnical engineer’s perspective on recent events?
For me, the 14th February deluge from Cyclone Gabrielle brings to the fore the vulnerability of many house sites situated at the base of steep slopes across NZ and the devastating effects of relatively shallow landslips.

Typically, these slips occur in coastal communities below steep escarpments comprising weak soils. Based on media footage, the Muriwai slips appear to be reasonably shallow, say less than 1 -1.5 m thick but the impact of these fluidised debris flows is detrimental.

Why do slips occur?
Landslips are commonly triggered by high rainfall episodes. There are numerous landslip types but they typically fall into either shallow landslips or deep-seated failures.

Shallow slips are generally between 0.5 – 1.5 m deep and are more prevalent than deep-seated landslips. The lower strength near surface soils, which are relatively weaker than deeper soils simply become so saturated they no longer hold structure. Whereas, deep-seated failures are visually more spectacular and are quite often initiated by spiked water pressures within underground water seepage zones, which under normal rainfall conditions do not affect the slope.

How can they be prevented?
There are numerous methods to prevent landslips such as terraced retaining walls, gravity walls and soil nailing the slope face. But in some situations, if a slope is too steep, high, or construction access is restricted, these systems become cost prohibitive and the only pragmatic solution is to accept the risk and safeguard buildings by shifting structures to a safe distance away from the crest of the slope. For buildings situated at the base of steep slopes, it may be possible to construct a debris barrier, wall, or earth bund system.

What is the remedial process?
The reactive response involves an initial walkover/triage assessment followed by a suite of ground investigations to confirm the ground conditions. The extent of these investigations depends on the complexity and scale of the landslip setting. Once investigations are complete a ground model is developed, from which engineering slope stability analyses are completed and a selection of suitable repair solutions are found. The design solutions are bundled into a comprehensive design report and submitted to council for building consent. Once approved, construction can begin, and the engineer will be involved with inspecting each construction element in order to certify the remedial works.