BAUER FOUNDATIONS has introduced a new ground improvement technique to the UK.
Combined soil stabilisation with vertical columns (CSV) is way of improving the loadbearing and settlement characteristics of soils that are too soft for conventional vibro stone column techniques, or where there is inadequate end bearing for vibro concrete columns.
Historically, the solution would have been to use a more traditional approach such as piling.
Small diameter columns (typically about 150mm) of a dry cement-sand mixture are installed using a displacement auger. The auger runs through a material container, turning against the drilling rotation to transport the dry mix into the ground. It does not generate any spoil.
Groundwater or pore water is used for the hardening/curing process. Bauer's Colin Serridge explains: 'The degree of improvement results from a combination of densification and pressurisation, load transfer and dewatering effects, depending on specific soil conditions (ie very soft to soft cohesive soil, loose granular soil or organic soil). '
Common applications are in soft rock and weak soils for railway and highway embankments, as well as structural raft slabs, groundbearing floor slabs and main foundations for industrial and residential buildings.
CSV columns are unreinforced and are principally designed for axial loading only, although there is scope for installing raking columns, for example at embankment edges.
Introduction of the method to the UK comes after its successful use on a section of the new high-speed railway between Furth and Wurzburg in Germany.
A third line alongside the existing tracks was needed for the high-speed trains, which meant the embankment the railway runs on had to be widened.
But on a 500m length of proposed route, the site of the new 4. 5m high embankment was found to be underlain by very soft to soft clays and silts with inadequate bearing capacity.
The original plan was to use a sheet pile wall to secure the existing embankment and a combination of soil replacement and stone columns. Instead, Bauer proposed using the CSV system to reduce the predicted settlement of between 100mm and 300mm at the embankment crest due to the extension.
Serridge says that maximum settlements using the CSV system with column spacing of one or two columns per square metre were anticipated to be about 30mm.
Over the two month contract, working from a 300mm granular blanket underlain with a geotextile layer, Bauer installed 6000 columns to depths between 5m and 5. 3m, penetrating 0. 5m into the competent claystone beneath.
Columns were installed without any restrictions to rail traffic and caused no significant movement beneath the existing rail tracks because installation caused no vibrations, Serridge says. The load from the new embankment and part of the existing embankment was transferred into competent soil strata, significantly reducing settlements.
The method also avoided the need for extensive dewatering and temporary works associated with the original excavation and replacement scheme.