What is subsidence?
Subsidence is vertical movement of the earth downwards in the direction of the gravity vector. This movement can have a variety geotechnical causes.
Subsidence can occur in any geo-material, including soil, rock, sand and it can also happen in human-made structures and materials.
Subsidence and the formation of sink holes or glory holes
Sinkholes are formed by subsidence of the ground that continues to the point of collapse, leaving either a form of crater or even a hole into the earth. This is most commonly occurring when there are cavities under the ground, such as caves, old or active underground mine workings, existing tunnels as well as tunnels under construction. If the span of rock and earth above the opening is subject to more stress and strain than its own strength then the material starts to move or deform, which can cause subsidence. Often referred to as a ‘crown pillar’ this earth structure can start to move downwards, effectively making the cavity smaller. At times this crown pillar can move all the way to the surface meaning the actual ground level on the surface moves.
Sinkholes can also for when material below the surface is actively removed, creating a crown pillar that cannot hold the force it is subjected to. On roads this can occur due to piping – underground erosion caused by the flow of water. Active tunnelling or underground mining is a common cause of subsidence.
Regardless of the underlying cause, sinkholes do not occur instantly. Even if the moment of collapse appears to happen instantly, there is always a deformation of the material as it yields to the stress before a collapse occurs. This deformation or background subsidence can be measured using a GeoLidar subsidence monitoring system, an inclinometer or tiltmeter in the ground where it a subsiding, or by placing prisms for a survey instrument to monitor overtime.
Subsidence in a landslide
Landslides also do not occur instantly; they also show precursor movement before the moment of collapse. Even during intense rainfall there will be an accelerating rate of movement before a collapse, landslide or mudslide.
Landslides start as stable slopes where the strength of the rock and earth on the slope is stronger than the forces moving the slope or the effect of gravity on the material that makes up the slope. Once that strength reduces – perhaps due to rainfall, seismic activity, earthworks, fill, tree clearing or loading with a mass such as equipment, vehicles or machinery, then a slope will start moving.
While landslides deform, the vertical component of this movement is called subsidence. Take the example of a road or railway that is cut into a mountain slope. If part of that slope starts to deform then the part of the road on top of this moving area will be subsiding downwards.
Tensions cracks, or large cracks in a road pavement, can be a sign of subsidence and deformation. These cracks form when the rate of movement or subsidence is greater in one patch than in another. At times the downhill edge of a mountainous road can deform and subside, especially if the road is built on fill rather than solid ground. In other cases, an entire stretch of road may be free from cracks but be moving, only to find the tension cracks may be tens or even hundreds of meters away.
Measuring this movement and watching for changes in trends is also very important, as this is the most reliable way to forecast if a road will stabilise or collapse.
How much does the ground move?
In subsiding and deforming ground, rates of movement can be as low as 0.2mm per week, up to 10mm per hour. Most instantaneous movements are so small they cannot be seen by eye, but an accumulation of movement can lead to visual changes of a slope over time.
By measuring the current rate of subsidence, and tracking that rate over time, it is possible to forecast the time of collapse and even give time to engineers to design rehabilitation to stop the collapse from taking place.
How much does the ground move?
In subsiding and deforming ground, rates of movement can be as low as 0.2mm per week, up to 10mm per hour. Most instantaneous movements are so small they cannot be seen by eye, but an accumulation of movement can lead to visual changes of a slope over time.
By measuring the current rate of subsidence, and tracking that rate over time, it is possible to forecast the time of collapse and even give time to engineers to design rehabilitation to stop the collapse from taking place.
The importance of subsidence monitoring
Subsidence monitoring is important as it is the key to determining the areas that are moving, if a region is slowing down to stability, or if it is accelerating towards collapse. It is only with this monitoring data that a time of collapse can be forecast.
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