hi to everyone, i'm facing some probelms during process of concreating a cast-insitu bored pile. my method for this bored pile is reverse circulation method.the bored hole (60 metres in deep) keep collapse (because there are some soft layer which contain silty and clayey sand) despite i'm using bentonite and soil(red colour) to protect that particular boring wall. so, my questions are:- a)what is the best way to prevent bored hole from erosion/collapse if there were sandy condition? b)what is the solution if tremmie pipe cannot be installed exceed the water level of ready mixed during concreating process? c)is there any calculation of 'how many kilogrames of bentonite have to use for 1 meter of boring hole'? d)where can i buy a cheaper and better quality of bentonite? thanks and i would highly appreciate to anyone who willing to answer my questions.my e-mail add is email@example.com.