CONTRACTORS building a chapel annexe at Loreto Convent in Llandudno, North Wales are having to be very careful not to shatter the peace of the elderly nuns there.
Occupied by the Institute of the Blessed Virgin Mary (the Loreto Sisters), the convent lies between the northern end of the seaside resort and the Great Orme headland.
The site of the two-storey annexe is on steeply sloping ground in the middle of the convent. There is a 4m height difference between the chapel and the annexe, which meant cutting into the slope and constructing a retaining wall to form two sides of the new building.
Excavation was based on digging through competent shallow rock, with no effect on the chapel above. But suitable rock was not found and the excavation had to be backfilled with imported stone to avoid undermining the chapel foundations. Backfill was graded to slope down from the chapel.
Consultant Shepherd Gilmour had to rethink and approached geotechnical contractor Van Elle's Gateshead office to design and install a soil nailing scheme to support the retaining wall excavation.
Van Elle, acting as principal contractor for the soil nailing and excavation work, sank a rotary cored borehole to investigate the bedrock.
It then designed the soil nailing works, which involved installation of 80, 6m long Dywidag R32N MAI hollow section soil nails using a sacrificial drill bit.
The system allows simultaneous drilling and grouting using an Ordinary Portland Cement grout, injected through a grout swivel on the rig and flushing ports in the drill bit.
A rotary percussive drilling head and mast mounted on the extendable boom of a tracked rig was used to install the nails on a grid. The nails retain a height of 5.25m, inclined at about 75infinity.
Staged excavation kept the face stable during work, with nails installed as it deepened.
Maccaferri rock fall netting was fixed to the face from the top of the slope.
Because of limited space, careful co-ordination of the earthworks and removal of spoil had to be maintained throughout.
Perhaps more importantly, workers had to bear in mind the convent residents - retired, often elderly, nuns in their 'place of quiet contemplation'