wide track type the width l of said preliminary excavation is two to four times the diameter of the pipe t typically approximately three times the diameter of the pipe the second excavation step is performed starting from the bottom 12 of the preliminary excavation said step is shown in fig1 b and consists in excavating two trenches 13 that are arranged laterally to the pipe t are inclined and converge downwards the inner sides 14 of said trenches are adjacent to the pipe t see also fig4 and go deeper than the level set for the final excavation bed 15 below the pipe t so as to form two lateral channels 16 of adequate capacity a wall 18 that separates the trenches 13 is formed in the region below the pipe t between the bed 15 and the pipe t in the subsequent and final step of the excavation shown in fig1 c the wall 18 is removed by cutting as will become more apparent hereinafter and the residual material is dumped into the channels 16 which are filled up to the level of the final excavation bed 15 the depth and width of the channels 16 and therefore their capacity are.