I'd say it all depends on soil type, water table elevation, method of advancing hole (cased first, hollow stem auger, solid stem auger, water added, etc., support zone for the pile (side friction, end bearing ?). Reference : my experience. Reason why might help me also.
Silty Sand, dense to medium dense, No water table present, 6 ms temporary casing during solid stem Auger boring, Bentonite added during boring, bored piles 1000 mm to 1200 mm in Dia., Length varies from 25 ms to 43 ms from Cut-off level, side friction & End Bearing.
I'd not use hollow stem augers or uncased solid stem augers. I'd use driven casing, washed out, take sample, etc. and repeat as needed. Much less chance of causing problems. At 2 meters maybe use anything you want. It likely depends on how stable improvement the initial job was on material near the pile.
If the bored pile is concreted, 1 m away is a fair distance to do a borehole. A temporary casing is a must in a silty SAND layer to avoid caving in. Casing installation should be pushed and no allowance for vibration that creating a loose sand and consume a lot of concrete.
