A method of sinking a shaft, particularly in unstable ground, comprises driving an annular shield into the ground and controlling the sinking speed of the driving shield by varying the cross-sectional area of the shield, and lining the excavated shaft behind the shield as it is advanced. An excavating device for forming the vertical shaft comprises an annular driving shield which is displaceable relative to an annular platform by fluid pressure operated piston and cylinder jacks connected therebetween. The structure includes an annular shield tail which is connected to the shield and extends rearwardly of the shield and the platform and the shield itself is provided with an inwardly directed taper and has prop means associated therewith for varying the affective taper and the affective cross-sectional area of the shield. The excavated shaft is lined behind the shield and the tail portion thereof as the shield is advanced. An annular machine platform is braced against the lining by telescopically movable braces which are fluid pressure driven to engage against the lining.