Complex sentence: Another major feature of the underground works is a 110m-deep ventilation shaft, which after local ground conditions were found to be worse than expected, was excavated with a raise-bore system excavating the 4.25m ventilation shaft from a 0.35m pilot drill shaft.
Split into simpler sentences: Another major feature of the underground works is a 110m-deep ventilation shaft. After local ground conditions were found to be worse than expected, it was excavated with a raise-bore system excavating the 4.25m ventilation shaft from a 0.35m pilot drill shaft.