Sink holes typically require two things.
1. Water
2. The water being able to carry away the soil, or the water drains or evapourates.
Broken sewers typically create sink holes in this part of the world.
Say you have a storm sewer made of concrete pipe. The concrete pipe, for whatever reason breaks. But no-one knows. In a rain even, water flows in the sewer and when it hits the break it ebbs out into the surrounding soil. (Soil being the generic name, no necessarily garden soil or top soil. )
As the flowing water takes with it the soil (or backfill around the pipe) and carries it downstream in the pipe you have a sink hole situation. (I've actually seen mini sink holes in houses. One house I was working on had a very large void under the basement floor. What had happened was that the clay sanitary drain broke and allowed the sewage to leak out. Every time someone flushed the john or used water, the sewage would seep into the soil and then suck it away as the water ebbed away into the sanitary line. (The homeowner had discovered the void under the floor when a small section of basement floor gave way. It was revealed to be pretty large actually. Where the main cast iron stack elbowed just under the basement floor and transitioned to vitreous clay pipe, the elbow had broken. Probably took years, but if unchecked, it could have undermined the foundations. But I digress.)
Over time, a void opens up and with time, it grows until finally, whatever is above is undermined.
You can have saturated soil, it's not a problem. There is nothing unusual about that. It only becomes a problem when there is a means by which the water is removed and soil can be carried away.
