Cracks above a cut slope after heavy rain
A house sits on a 25° slope underlain by loess, a wind-blown silt with high porosity and low cohesion. Above the house, a 4 m cut slope was made for the driveway, removing the toe of the natural slope. In 48 hours, 250 mm of rain falls, about half the annual total for the area. The homeowner reports new cracks parallel to the slope, a tilting retaining wall, and muddy seepage at the base of the cut. A neighbour's paddock has hummocky ground and a previous landslide scar. The council's hazard map shows the area as 'moderate landslide susceptibility' but the house has building consent. The owner asks whether the cracks are just settlement from the rain and whether they can stay the night. The rain is forecast to ease, but another 40 mm is expected in six hours. The house is 15 m from the cut slope.
1. Are the cracks just settlement? What should the owner do tonight?
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They are more likely landslide precursors than ordinary settlement. Cracks parallel to the slope, a tilting retaining wall and muddy seepage are classic signs of a moving mass; the cut slope removed support at the toe, and 250 mm of rain raises pore pressure in loess. Settlement cracks are usually local, vertical or diagonal around foundations, not continuous downslope. The previous landslide scar and hummocky ground show the area has moved before. With another 40 mm forecast, the safe action is to evacuate for the night and get a geotechnical inspection before returning. Do not try to fill cracks or anchor the wall without advice. Report to council and keep people and vehicles off the slope.
2. Explain how the cut slope and rainfall combined to increase landslide risk. Distinguish the trigger from the preconditions.
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Preconditions: loess is porous, weakly cemented silt with low cohesion; the 25° slope is already steep; the 4 m cut removed the toe that buttressed the slope, increasing the driving stress; the old landslide scar shows a history of instability. Trigger: 250 mm of rain in 48 hours infiltrates, raises the water table and pore pressure, reduces effective stress and soil strength, and adds weight. The cut made the slope closer to failure, so a smaller rainfall event could trigger movement than on an intact slope. The forecast 40 mm keeps pore pressure high, so risk remains after the rain eases. This is why the cracks should not be dismissed as just settlement.