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Hunting down hotel quarry

Old backfilled quarries and underground tunnels meant careful piling was needed during the construction of a new hotel in the heart of Glasgow.

Construction in city centres is rarely trouble free, but there are parts of Glasgow which provide particular headaches. The area around the city's new Concert Hall is a case in point.

Sandstone beneath this part of Glasgow was quarried by earlier developers, and used for many of the city's 19th century buildings.As a result, the area is honey- combed by old excavations which have been infilled and built over, with little evidence on the surface. Fortunately, archived records of quarrying activities exist.

Developers building the six-storey with basement fourstar Lange Hotel on the corner of Port Dundas Road and Renfrew Street, soon discovered there was a possibility of buried quarries beneath the site.

'Desk studies indicated that there were two quarries below the site, separated by a narrow ridge, but there wasn't much more information, ' says consultant URS Thorburn Colquhoun's Alan Swanson.'One of the first priorities was to locate the edges of the quarries, the slopes and the condition of the rock at the base.'

Part of the site of the eastern quarry had been occupied by a 1930s building with a basement which went down to the old quarry floor.However, as the hotel developers wanted to make full use of the whole site, the new building is basically triangular, extending over the ridge and projecting over the western quarry at the south west corner.

Compounding these problems are the twin tunnels for the Glasgow Underground which run directly under the site, with the crown of the tunnels only 19m below the surface.

URS Thorburn Colquhoun undertook a detailed survey of the tunnel before work started, accompanied by representatives of the Strathclyde Passenger Transport Executive (SPTE) and the demolition contractor. No serious defects were found.

Keller Ground Engineering also carried out preliminary down-the-hole hammer rock drilling in the tunnel area to confirm the SPTE survey. But SPTE placed stringent restrictions on acceptable vibrations on the tunnels during both the site investigation and construction work.

The overall picture that emerged from the site investigation was that the two quarries were markedly different.

The eastern quarry - beneath the site of the old building - was shallower, with comparatively gentle sloping sides. Infilled with a mixture of ash, sand, clay and rubble, sandstone was encountered at depths between 3m and 7m.The upper 1m to 3m of the rock was weak and decomposed, with competent material at between 6.5m and 7m.

The western quarry was deeper, its base at around 10.5m to 11m and it had much steeper sides.The upper 3m to 4m of fill consisted of a mixture of ash, brick and rubble, with sand and clay below. Sandstone on the narrow ridge between the two quarries was only 1m below the surface, but again was weak and decomposed, with good rock about 3m down.

URS Thorburn Colquhoun designed the foundations and proposed a combination of piers over the shallower sections and bored piles in the deep quarry.Piling work was carried out by Keller Ground Engineering as subcontractor to the main contractor, Dawn Construction.

Two types of piles were used. In the western quarry the diameter was 190mm and 200mm, with working loads from 200kN to 425kN.These were installed by a Casagrande M6A drilling rig using a down-the-hole hammer, cased through the overburden and socketed into the sandstone.

Using a flexible tremie pipe, the boreholes were filled with a 1:1 sand/cement grout and reinforced for their full length. The piles are between 0.6m and 1.5m apart, with most placed in the south west corner of the site, where the quarry is deepest.Pile length was limited to 15m in this area because the underground tunnels run beneath this part of the site. Drilling effect on the tunnel linings was carefully monitored during the work.

Piles in the eastern quarry, were 450mm diameter bored cast insitu reinforced concrete piles put down using a Soilmec R412 track mounted rig.Penetrating up to 4m into the rock, they were constructed principally in pairs on either side of the original south eastern side of the old building and in two groups on the northern section of the site. Design loads vary from 1,200kN to 1,500kN.

All the surface areas around the concert hall had been covered by decorative stone flags and these had to be removed from the working area before construction could start.

The area is busy, with pedestrians heading to the nearby bus station and Queen Street railway station, so for safety the site was fenced in with high hoarding.

'There wasn't a great deal of room to manoeuvre, ' says Keller Ground Engineering's Nick Sharp.'Getting the large Soilmec rig on site was quite difficult and we had to be careful to keep the sound to acceptable levels.The tunnels created a bit of a problem but for the most part construction was straightforward.'

Despite restrictions, the piling work was completed within the three weeks specified. The tunnels were resurveyed and no defects found.

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