One of the most important functions of a drilling fluid is to stabilize and maintain a borehole opening. Borehole stability in coarse soils starts with creating an impermeable barrier between the soil and the drilling fluid or slurry via the filter cake (or polymer gel membrane for non-rotary mud drilling such as foundation drilling). Then, positive hydrostatic pressure from having the hole full of fluid to a level above the water table exerts itself against the impermeable barrier to keep the formation stable.
If either of these two components (impermeable barrier or filter cake, and positive hydrostatic head pressure) is missing or not functioning correctly (as in cases of high fluid loss), borehole stability is compromised and the hole will fall in when drilling through coarse soils. Drilling fluid professionals spend a large portion of their time troubleshooting drilling projects and answering questions from drilling contractors, and here are some frequently asked questions regarding borehole stability.