Lateral ground displacement
Lateral movement occurs when earthquake shaking causes a mass of soil to lose cohesion and move relative to the surrounding soil. Lateral movement can be entirely horizontal and occur on flat ground, but it is more likely to occur on or around sloping ground, such as adjacent to hillsides and waterways.
In most cases, lateral movement involves an intact block of land sliding downhill – a phenomenon called a block slide or bulk lateral movement. However, a lateral movement can also stretch the ground as it moves – this is known as lateral spread.
Lateral spread
When lateral spreading occurs, the ground tears, opening surface cracks and fissures across the slope. This type of stretching of the ground can introduce significant lateral forces into foundation elements and built structures. If the foundation is not strong enough to resist the movement, the lateral spread causes it to extend.
Lateral spreading near a waterway can cause damage to the surrounding land and the buildings it supports. Typically, the degree of lateral movement lessens as the distance from the waterway increases.
In the case of an unreinforced concrete floor slab, it is likely to crack in several places perpendicular to the direction of spread. In its technical guidance Repairing and rebuilding houses affected by the Canterbury earthquakes, MBIE states that, if the floor plate of the dwelling is not strong enough, lateral spreading may cause an extension of the floor plate (that is, the concrete floor slab may crack or the timber floor may fracture generally at joints between framing members).
Lateral spread can cause severe damage to a foundation unless it is reinforced to withstand the movement.
Block sliding
A block slide or bulk lateral movement occurs when a section of ground moves as a complete whole without any significant stretch, strain or change in local surface shape or characteristics. This kind of movement tends to result in blocks of land moving laterally towards a free edge, such as a stream or waterway.
The effect is caused by a slope failure on a hillside that is not underlain by liquefied soils. In this situation, the weight of the soil block, often in combination with the destabilising effect of groundwater and/or seismic shaking, exceeds the shear strength of the ground. As a result, a section of ground moves relative to its original position.
This kind of movement can result in blocks of land moving laterally. Buildings in the middle of the block sustain little damage while those located at the edges of the zone of movement exhibit irreparable damage.