Landslide Damage: What to Do and How It Gets Fixed

If a landslide has damaged your property, or threatens to, it can be unsettling and difficult to know where to start. The important thing is to understand what caused the movement before deciding how to fix it.

Many slope failures can be stabilised, but the right solution depends on the ground, water, slope geometry and the risk to people, buildings and neighbouring properties. This guide explains what causes slope failures, the stages involved in assessing and repairing one, and the options engineers commonly use, so you know what to expect whether you are dealing with the damage directly or through your insurer.

Why slopes fail

Understanding what triggered a slip matters, because good remediation addresses the cause, not just the visible damage.

Slopes usually fail when the ground has lost strength or the forces acting on it have increased. That can happen on steep natural hillsides, cut slopes formed during earthworks, or areas where soil has become saturated after heavy or prolonged rain. Clay and silt soils can be particularly sensitive to changes in water content.

Erosion can also undercut a slope over time, especially on coastal cliffs, riverbanks and stream edges. Earthquakes can trigger movement in ground that is already marginal or unstable. Human activity can contribute too, including vegetation clearance, uncontrolled stormwater discharge, added loading near the top of a slope, or poorly engineered excavation.

Often, more than one factor is involved. Identifying which causes apply to your site is the first step, because it shapes the investigation, the design and the type of stabilisation that will actually work.

The remediation process, stage by stage

Stabilising a slope usually follows a sequence of distinct stages. Knowing those stages helps you understand where you are in the process, what each step is for, and what information is needed before decisions are made.

Stage 1. Geotechnical assessment

The process usually starts with a geotechnical assessment. An engineer visits the site to observe the slope, the damage and any obvious signs of movement, water or instability.

A ground investigation may then be needed. This can include boreholes, test pits, soil sampling and laboratory testing to understand the soil and rock below the surface and what is likely driving the instability. The outcome is usually a geotechnical report setting out the ground conditions, the current and likely future stability of the slope, and the key considerations that will guide the repair.

On larger or more complex sites, geophysical survey can help image the subsurface and identify possible slip surfaces before drilling. That helps target the investigation to the areas where it will provide the most useful information.

Stage 2. Remediation strategy

Once the cause and scale of the problem are understood, the engineer recommends a remediation approach suited to the site. This means balancing safety, effectiveness, cost, constructability and the level of risk to buildings, people and neighbouring properties.

The right option depends on the slope and the failure mechanism. Retaining walls are often used to support soil, with options ranging from timber pole walls and crib walls to gabion baskets, concrete walls and masonry block walls.

Where unstable material needs to be tied back into deeper, stronger ground, palisade walls, soil nails or rock bolts may be more appropriate. These systems help restrain the moving ground and are often combined with drainage, erosion control or revegetation.

For rockfall or shallow surface instability, shotcrete can be used to protect or armour the slope, although it is not suitable for every site, especially where the slip is large, deep or environmentally sensitive.

These are common approaches, but they are not the only ones. The right solution depends on what is causing the movement, what needs to be protected and what can realistically be built on the site.

Stage 3. Detailed design

Once the remediation approach is selected, the work moves into detailed design. This is where the strategy is turned into calculations, drawings and specifications that can be priced, consented, built and certified.

Stage 4. Consent support

The design may need building consent, resource consent, or both. Your engineer will confirm what approvals are likely to be required and prepare the supporting engineering documentation, including the geotechnical report and design drawings.

Where planning advice is needed, that sits with a planner. Our role is to provide the engineering evidence that supports the consent process.

Stage 5. Construction monitoring

Construction is carried out by contractors, with the engineer monitoring the work at the stages that matter. This helps confirm that the remediation is being built in general accordance with the design and that any ground conditions exposed during construction are dealt with properly.

Stage 6. Certification and close-out

Finally, the completed work usually needs engineering sign-off and, where required, council sign-off. This confirms the remediation has been constructed in general accordance with the design and provides the documentation needed to close out the project.

Where insurance is involved, this evidence can also support the claim by showing what caused the damage, what work was required and how the remediation was completed.

Where we come in

Cook Costello has assessed and remediated slope failures across New Zealand, including after major storm and earthquake events. Our geotechnical and geophysics teams can investigate what caused the movement, assess the ongoing risk, and recommend an appropriate stabilisation approach.

Where remediation is required, we can take the work through investigation, design, consent support, construction monitoring and engineering sign-off, working alongside your insurer where there is a claim. Because we focus on the cause of the movement, not just the visible damage, the repair is designed to address the underlying problem rather than simply tidy up the surface effects.

If your property has been affected by a landslide, or you are worried a slope may be unstable, tell us what has happened and we will talk you through your options.

Request a landslide assessment

OR

Tell us about your site