sir, this is the problem that we experienced. thats why i leave the decision to my operator. encountered sand strata, the soil gives in and we hardly penetrated because the sands keep on falling. as per recommendations from them, we look for sticky mud and bring bags of it in the site. when they use it, it helps but some other project we got, we resorted to Drill Drive because we cant stable the hole even we used bags of muds. i heard about bentonite, is it good? because as far as i know, bentonite only used in Rotary Drilling method. hoping for your replies.
The bentonite only works if you keep your static level of the mud about 25 feet above the cave in area.
If the cave in are in near the surface you should install a conductor pipe. We use 10" PVC but most drillers will use steel. When the conductor pipe is set into clay it can be very difficult to pull so we sometimes coat the outside with a polymer so it slides out without the clay sticking to the surface.