The site for the new hotel is located on Howard Street, immediately to the west of underlying SPT Subway tunnels. These had to remain fully operational for the duration of the works. The zone of influence was deemed to be a 45° line projected from the tunnel invert level to ground level. The distance from the tunnel to the closest pile position was approximately 3.5m, and the invert level of the tunnel was approximately 13.3m below ground level.
Stringent limits for vibration and movement were imposed on the tunnel and the surrounding ground. Vibration monitoring, tiltmeters and laser distance meters had been installed prior to commencement of any piling works.Ground Conditions
The cable percussive boreholes showed the ground conditions to comprise made ground up to 4.50m thick, overlying alluvial deposits described as dense silty sand with occasional clay bands, or medium dense and dense silty sand and gravel, encountered to a maximum depth of 34.00mbgl.
Suspected rockhead was encountered within 1 No. borehole at a depth of 34.00mbgl, described as bedrock or boulder. In-situ Standard Penetration Tests, undertaken between 3.0m and 17.0m recorded uncorrected ‘N’ values ranging between 10 and 18 indicatives of loose to medium dense strata. Between 17.0m and 33.0m uncorrected ‘N’ values ranged between 24 and 48, generally indicative of medium dense soils.