The process of ejecting out the soil slurry through joints and cracks under the action of wheel loads is known as Mud Pumping.
Water may get percolated into the soil sub grade through defective joints and cracks and may soften it. Due to effect of repeated loads due to traffic, soft patches get somewhat settled and small initial spaces are developed under the slab. With further percolation of water, the softened sub grade forms soil slurry which starts ejecting out through joints and cracks in the vicinity, when heavy wheel loads cause pavement slab to deflect or depress a little. The process of ejecting out or forcing out the soil slurry results in hollowing of underneath spaces and consequently loss in sub grade support.
It is more pronounced when sub grade is made from fine grained soils.