The Swedish method of dry soil mixing looks set to extend the application of ground improvement.
The Swedish method of deep dry soil mixing (DSM) to form limecement columns is a geotechnical purist's solution to very soft soils, in that it modifies their behaviour rather than bypassing them by using structural elements as in piling.
DSM is in essence a ground improvement technique, but one that is applicable in soils such as very soft organic silts, clays and peat, that previously were considered largely untreatable - at least in the UK.
And that is good news, because ground improvement methods are generally highly cost effective compared to harderengineered solutions.
UK specialist contractor Keller Ground Engineering, with technical support from its Swedish sister company LCM, is using the method on a subcontract at Tilbury Docks in Essex, UK.
Here DSM is being used to treat very soft, highly organic alluvial clays to reduce active earth pressures on new quay walls.
Consultant Mott MacDonald designed the DSM column layout and Keller, working to a performance specification, is responsible for making sure the columns achieve the required strength.
'The design philosophy is that you are not creating weak piles, ' says Keller senior geotechnical engineer John Judge. 'Instead you are relying on the interaction between columns and the surrounding untreated soil.'
Column installation involves drilling down to full depth, which churns up the soil. The rig operator then counter-rotates the auger tool - increasing the breakdown of the soil fabric - as the string is withdrawn and the binder injected.
Rotation of the auger tool mixes the soil with the binder and an immediate reaction starts.
Binder volumes, rate of rotation and speed of withdrawal are critical to creating a good quality column and these parameters are all digitally recorded during the process.
A typical column takes around five minutes to complete.
Each DSM rig is connected to a satellite silo unit housing the grout powder. This is delivered into the column along a delivery hose under compressed air to a nozzle above the soil mix auger.
At Tilbury, Keller is injecting an OP cement-based binder at a density of 150kg/m 3. This relatively high concentration is needed because the soil profile contains intermittent lenses of peat - which will not react unless high quantities of binder are added.
Judge believes the applications to be almost limitless and is confident that in the next few years DSM will be more widely used 'in ways that haven't yet been thought of. It's simply down to the imagination of the engineers.'
Typically, columns are designed to have undrained shear strengths of no more than 150KPa, which means loads of up to 70kN on 600mm diameter columns.
Judge says there is a strong reliance on empirical experience in the design and that getting the binder right is key to any project.
The binder is typically one or more of a mix of cement, lime, gypsum or blast furnace slag.
Projects invariably make use of mixing trials to determine the binder mix and quantities needed to achieve the required strength. These are quite basic tests involving mixing buckets of soil with various binder combinations and seeing which react best. Site trials follow.
It is the site trials, he says, where the science comes into the design. Binder mix and quantities can be optimised nd the results validated by testing.
There are reliable and wellestablished testing methods developed by the Swedish Geotechnical Institute (SGI).
The most common is a pull-out resistance test that relies on embedding a vane at the base of the column. This is then pulled out once the column has gone off, giving a shear strength with depth profile through the column.
Alternatively shear strengths can be determined by pushing a vane through the top of a completed column. A drawback to this approach, says Judge, is that the testing vane has been known to deviate through the column side, 'and you don't know whether a low reading is indicative of a weak spot within a column or simply measuring the strength of the untreated soil'.
Breaking new ground
Keller bought into Swedish contractor LCM in 2000 and has been quietly developing UK experience ever since.
The most demanding contract so far has been at the Queen Elizabeth dock reclamation in Hull.
Here Keller treated 6m-deep tidal mudflats with lime cement columns to allow construction of a 6m high, 450m-long bund.
Keller constructed up to 60 columns in each four-hour window between tides, rolling out a geogrid-reinforced load transfer platform over the fresh columns to provide the working platform for the next tidal shift.
This alternative design to a conventional embankment built with staged surcharging reduced the programme by a phenomenal two years.
Keller became full owner of LCM at the beginning of this year and is spreading DSM technology and experience throughout its global operations.
Gothenburg-based LCM pioneered DSM in Scandinavia and has 30 years' experience with the technique. It has installed columns from 500mm to 1m in diameter, and up to 25m in depth. Columns can be interlocked to provide cellular blocks and the company recently developed a tool for mass mixing large volumes of soil in blocks.
CM says binder quantities commonly used range from 80-100kg/m3 in soft clay and 150-200kg/m3 in peat.
Pure lime is suitable for inorganic soil, but lime slows down the rate of strength gain and in a lime column no more than 50% of final strength will be reached in three to four weeks.
However, in cement-based columns, strength starts to increase after a few hours and rises rapidly in the first week, reaching about 90% of final strength in three weeks.
LCM's Helena Eriksson says: 'In Sweden we like to say that we can almost make stiff columns out of water.'
The success of the method in Sweden's liquefiable 'quick' clays shows that the exaggeration is almost justified.