Borewell pebbles keeps going down


We have done home borewells up to 550 feet almost 6 months back. After that in every rainy season, the pebbles around the borewell pipe went 3 to 4 feet downwards. Water also get muddy, however, after a few days, water will become normal. Again next rainy season, the same will happen. This issue continues with every monsoon. No one has any idea. Please reply, if anyone has ever faced this issue and solution to overcome it.