PURPOSE:To raise the operability of a measure for bottom-expanded pit by a method in which, a sensor for the opening amount of a cutter is attached to a bucket, a sensor for lowering amount of the bucket, signals from the sensors are transmitted by a transmitting and receiving device, and the detected values are image-displayed and printed out. CONSTITUTION:A sensor 1 for opening amounts of cutter is attached to a bucket 101, and the detected signals S1 of the opening amount of cutter unit are transmitted to a receiver 20. A sensor 10 for lowering amounts of bucket is attached to the rotary base 23, and the detected signals S2 of the lowering amount of the bucket 101 are sent out. The signals S1 and S2 are transmitted by the transmitter-receiver 20-30, and the detected values are put in a computer unit 40 for image-displaying and printing-out. The bucket 101 is lowered into a well 200, and the cutter unit is slowly opened to excavate a conical bottom in order to form a bottom-expanded pit 201. A highly accurate bottom-expanded pit can thus be easily formed.
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