In order to form a well 1 of large diameter without the presence of a subjacent gallery, there are presently available mining techniques which often require the ground to be frozen, or oil-drilling techniques using a supporting mud 16. In the latter case, the drilling is conventionally carried out from the top downwards. Within the scope of the present invention, a method for drilling from the bottom upwards as well as a tool 2 adapted for the implementation thereof are described. A main operation of this method consists in fact in widening a pre-existing well. The corresponding tool 2 is provided with arms 22 designed to be, firstly, folded up so that the tool 2 may be passed inside the pre-existing well in order to be lowered down to the bottom 15 of the latter and then, secondly, opened out while its teeth 23 cut into the ground 12, the tool 2 being rotated and pulled by a motor 31 supported by a mast 3 at the surface 11. It is then often advantageous thus to widen a guide well, and then to repeat the main operation on the resulting well and so on. Despite the number of passes required from then on in order to obtain the desired large-diameter well 1, the speed of progress of the tool 2 during each successive widening operation may in fact be so great that overall the well 1 is dug in a much shorter time and its cost is reduced compared to the usual methods. IMAGE
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