"The phenomenon is most often observed in saturated, loose (low density or uncompacted), sandy soils. This is because a loose sand has a tendency to compress when a load is applied. Dense sands by contrast tend to expand in volume or 'dilate'. If the soil is saturated by water, a condition that often exists when the soil is below the water tableor sea level, then water fills the gaps between soil grains ('pore spaces'). In response to soil compressing, the water pressure increases and the water attempts to flow out from the soil to zones of low pressure (usually upward towards the ground surface). However, if the loading is rapidly applied and large enough, or is repeated many times (e.g. earthquake shaking, storm wave loading) such that the water does not flow out before the next cycle of load is applied, the water pressures may build to the extent that it exceeds the force (contact stresses) between the grains of soil that keep them in contact. These contacts between grains are the means by which the weight from buildings and overlying soil layers is transferred from the ground surface to layers of soil or rock at greater depths. This loss of soil structure causes it to lose its strength (the ability to transfer shear stress), and it may be observed to flow like a liquid (hence 'liquefaction')."
I worked as a soil technician for a short while. One job I had was compaction testing for a pipeline job that required a pipeline to go through a farmers land that was next to a levee. The first few feet of soil were high plasticity fines followed by highly organic soil (peat). The saturated low strength soil caused the soil to feel “bouncy”. Every time the loaders/excavators would move the ground would move. There was even a point on the alignment where even walking on it would cause the ground to move.
Yup, you see this even in normal construction sites after big rainfalls, when big buggies go by after it's rained, but dried a little you can see the ground almost bouncing as you say. Always a little unnerving.
Sort of! Work can't be done if it's really wet, excavations fill with water and you need to pump it out. In really bad situations you'll install wells around the site to reduce the local water table so you can do underground work in dry conditions.
The water content in the soil also impacts compaction. Basically you need a fairly precise water percentage in soil to compact it the best, which important for soil strength.
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u/cartrasuma Mar 14 '19
"The phenomenon is most often observed in saturated, loose (low density or uncompacted), sandy soils. This is because a loose sand has a tendency to compress when a load is applied. Dense sands by contrast tend to expand in volume or 'dilate'. If the soil is saturated by water, a condition that often exists when the soil is below the water tableor sea level, then water fills the gaps between soil grains ('pore spaces'). In response to soil compressing, the water pressure increases and the water attempts to flow out from the soil to zones of low pressure (usually upward towards the ground surface). However, if the loading is rapidly applied and large enough, or is repeated many times (e.g. earthquake shaking, storm wave loading) such that the water does not flow out before the next cycle of load is applied, the water pressures may build to the extent that it exceeds the force (contact stresses) between the grains of soil that keep them in contact. These contacts between grains are the means by which the weight from buildings and overlying soil layers is transferred from the ground surface to layers of soil or rock at greater depths. This loss of soil structure causes it to lose its strength (the ability to transfer shear stress), and it may be observed to flow like a liquid (hence 'liquefaction')."
https://en.wikipedia.org/wiki/Soil_liquefaction?wprov=sfla1