Drilling methods for soil sampling in clays
Drilling methods for soil sampling in clays
(OP)
Hello all
I've just been assigned to a project in Vietnam, where a site investigation in unconsolidated deposits (predominantly clay) is currently in progress. Drilling is with small coring rigs, advancing the hole with single barrel core barrels and bentonite as drilling fluid. This is my first time to see such a method for geotechnical exploration, although I have quite a bit of expereince with this method for drilling water wells. I have also worked with diamond-bit rigs in granite using only water (bentonite was forbidden because we had to hydraulically test the fractures). For water wells, we always utilize downhole logging methods to give us data concerning groundwater. There is no such provision here. How can we get reliable data about water strikes and artesian conditions without geophysical logging? I've given instructions to check fluid levels every morning, evening, and whenever it seems they are losing fluid or the returning fluid flow is increasing, but these drillers are not experienced enough to tell. And how much contamination is taking place with Shelby tube and split spoon samples? Again, the deposits are mostly clay, and the lab tests are the basic indices, direct shear box, and oedometer consolidation.
Nick
I've just been assigned to a project in Vietnam, where a site investigation in unconsolidated deposits (predominantly clay) is currently in progress. Drilling is with small coring rigs, advancing the hole with single barrel core barrels and bentonite as drilling fluid. This is my first time to see such a method for geotechnical exploration, although I have quite a bit of expereince with this method for drilling water wells. I have also worked with diamond-bit rigs in granite using only water (bentonite was forbidden because we had to hydraulically test the fractures). For water wells, we always utilize downhole logging methods to give us data concerning groundwater. There is no such provision here. How can we get reliable data about water strikes and artesian conditions without geophysical logging? I've given instructions to check fluid levels every morning, evening, and whenever it seems they are losing fluid or the returning fluid flow is increasing, but these drillers are not experienced enough to tell. And how much contamination is taking place with Shelby tube and split spoon samples? Again, the deposits are mostly clay, and the lab tests are the basic indices, direct shear box, and oedometer consolidation.
Nick





RE: Drilling methods for soil sampling in clays
As for water pressures - you would install standpipes - pvc tubing with slotted ends - suitably wrapped with geotextile cloth. Properly seal the hole with bentonite plugs (balls of bentonite you drop in). Though the hole is full of bentonite, if the standpipe is wrapped, I don't see this being a problem - you will measure the 'pressure' inside the standpipe. See Hvorslev book on Soil Exploration.
Enjoy Vietnam - I haven't worked there but have worked in neighbouring locales and with Vietnamese contractors.
RE: Drilling methods for soil sampling in clays
I didn't think it was too much of a problem for the samples. Except perhaps for very permeable sands, there might be some contamination.
We are taking SPT/split spoon, and driving in Shelby tubes for 'undisturbed' samples.
Standpipes are not in our contract (I guess the person creating the scope of works just assumed they were going to drill with casing), and the foundation engineer hasn't shown much concern about water levels and seepage, although the water table in the area is known to be quite high.
Nick
RE: Drilling methods for soil sampling in clays