NORWEST HOLST Soil Engineering has just completed a major ground treatment contract for an industrial and commercial development near Glasgow.
Some 5,300 boreholes, equivalent to 100km of drilling, were sunk and grout injected for the £1.5M contract to consolidate shallow mine workings underlying part of the site of the Glasgow Business Park, next to the M8 motorway at Baillieston.
Two disused shallow coal seams under the southern section of the 20ha site being developed by Arlington Securities were found to have cavities up to 4m. This meant that six of the plots (around 6ha) were at risk from settlement, so main contractor Sir Robert McAlpine and consulting engineer Thorburn Colquhoun brought in Norwest Holst to treat the workings by injecting grout.
Eleven rotary percussive rigs were used to drill the 75mm diameter plastic cased boreholes on a 3.5m grid through the 12m to 15m thick overburden. Holes were then reduced to 64mm diameter and extended through the underlying mudstone and sandstone to the base of the coal seams between 16m and 30m depth. Boreholes were also drilled at 1.75m centres along the site's southern edge to form a grout barrier.
Grout was prepared on site by Norwest Holst. For the perimeter boreholes a stiff 6:3:1 PFA: sand: cement mix was used to create the grout barrier. The other boreholes were injected with a 12:1 PFA: cement mix. Some 30,000t of material was injected at rates of up to 400t per day.
An important aspect of the works was ensuring that a stream running along the southern boundary was not polluted by runoff from the site works. This was prevented by a system of sumps, cut-off trenches and pumps.