New Premier Inn Hotel, Glasgow case study

Project information

Location: Howard Street, Glasgow

Customer: Roger Bullivant Ltd

Product used: Micropiles

Project Background & Challenges

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.

The methodology

Roger Bullivant Limited had carried out a trial pile in the summer of 2018, exactly replicating the eventual piles adjacent to the Subway infrastructure. The trial pile was installed over a weekend during night shifts when the Subway wasn’t operational. No alarms were triggered during this installation; henceforth the construction methodology was approved.

A second test pile was then installed, again replicating the proposed main works piles. This pile was tested to 2.5 x SWL (MTL = 2,125kN) with a maximum settlement recorded at around 20mm.

The solution

Ischebeck Titan micropiles comprise a continuously threaded, hollow-stem steel reinforcement tendon, sacrificial drill bit and a pressure grouted OPC grout body combined, the profiled surface of which transfers the compression and/or tension forces into the ground.

Following the installation of sacrificial pile casings a number of 103/78 Ischebeck Titan micropiles were then drilled to a depth of around 35m. This ensured a nominal one metre socket into the underlying sandstone bedrock, which provided a maximum working load of 850kN (compression), 200kN (tension) and 50kN (lateral). The micropiles were installed using a Sonic SD3 drill rig in a one-visit drilling operation. The installation procedure used a weak flushing grout to stabilise the bore hole, consequently eliminating the use of full-depth casings.

A further static load test demonstrated excellent results, well within tolerances set by the Project Engineer.

The result

All pile positions which fell within the zone of influence of the underlying SPT tunnel were permanently cased to a depth equivalent to the invert level of the tunnel prior to the installation of the micropiles. All positions were permanently cased using a rotary percussive air flushed drilling system. All piles which encroached this line of influence were formed by installing a 324mm diameter casing through all overburden material, terminating beyond the underside of the tunnel invert level at a depth of 14.0m.