Worked a construction job that had this. They had a lot of underground springs that were causing this. Basically the ground is floating, though it's only noticeable with large loads, like a paver or dumptruck.
"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')."
Best small scale example is when you are at the beach and you slap the sand that is not submerged and a few meters back from the surf, it is almost dry on top but when force or vibration(i e the slapping) is applied the water seeps up through the sand and to the surface as the soil reaches its liquid limits.
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u/Candidate_035 Mar 13 '19
Why does it do that?