A Swedish ground improvement technique has cut two years from the construction programme on a land reclamation project in Hull.
Ground improvement systems available in the UK have, until now, had limited success in treating very weak soils such as peat or soft organic silts and clays.
But a Swedish system known as dry soil mixing (DSM) is gaining increasing acceptance and Keller Ground Engineering, working with its Swedish company LC Markteknik, can probably claim to have achieved the greatest success to date.
Dry soil mixing is an insitu method of modifying the strength of very weak soils through the direct introduction in columns of powdered grout, typically comprising lime or lime/cement mixes.
'DSM is basically a ground improvement system, ' says Bob Essler, Keller's engineering director. 'We provide a stabilised soil that interacts with the surrounding unstabilised soil. The applied load is carried partly by the columns and partly by the ground between them. When the ground is exceptionally weak the columns are designed to transfer loads acting, in effect, as low strength piles.'
Typically, says Essler, you can achieve loads of up to 70kN on 600mm diameter columns with progressively higher loads on larger diameters. Grout mixes are determined on a site by site basis using laboratory trials to establish the soil reactivity to various binders and to determine the most effective mix.
Treatment depths, says Keller, are limited to around 25m.
On one of their largest contracts to date, Keller is using dry soil mixing as part of a land reclamation contract on the foreshore of the River Humber.
Working for Birse Construction, Keller is treating extremely soft tidal mudflats to provide a foundation for a 6m high, 450m long bund wall to reclaim land for future redevelopment at the Queen Elizabeth dock in Hull.
Ground conditions comprise very soft clays varying in depth between 4m to 6m overlying glacial till.
Birse won the contract on a design of conventional staged placement of fill to build up the embankment bund, originally approaching Keller to develop a soil mixing solution to improve the upper 2m of the silts to form a construction platform.
Keller proposed extending the soil mix treatment to the glacial till topped with a geogrid to form a load transfer platform. This would remove the need to allow for pore water pressure dissipation as the bund height was increased, potentially reducing the construction programme by two years.
A joint presentation to the client Associated British Ports (ABP) to change the construction process was approved, and the Environment Agency consented to the new proposals, subject to a full-scale site trial to prove both short and long term performance.
Short term strength improvements were particularly critical as the 35t DSM rig would have to track over freshly constructed columns - quite a risk when working between tides. A minimum undrained shear strength of 45kPa within 12 hours was required. Sixteen columns were constructed in a single afternoon shift and tested insitu.
The results achieved the necessary short term strength gains and subsequently the long term increase in design strength to an undrained cohesion of 150kPa to 200kPa was also easily met.
'The cost of the mobilisation of the rig from Sweden for the trial was very much at our risk, ' says Essler. 'We were confident we could reach the required performance but the results even exceeded our own expectations.'
Following the trial in July 2001, Keller's design was adopted by Birse's design engineer Halcrow.
Work, based on a performance specification, started in early August and was completed at the end of September.
Keller had returned to site for a second phase, which is expected to be completed by July. It is currently achieving up to 60 columns in the three to four hour window between tides.
During production, each DSM rig is connected to a remote satellite silo unit housing the grout powder. This is delivered under compressed air along a delivery hose to a nozzle above the soil mix auger. The auger is inserted into the ground and once at the intended maximum depth of treatment, the powder is blown and mixed into the soil while rotating the auger tool at over 150rpm, forming the column as it is gradually removed from the ground. Each column takes less than five minutes to complete.