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THE RIGHT CHEMISTRY

The Swedish technique of deep dry soil mixing is being pioneered in the UK on a project at Tilbury Docks.

Dry soil mixing is in essence a ground improvement technique for very soft organic silts, clays and peats previously considered untreatable, at least in the UK.

Developed in Sweden, the method was recently used for new berth construction at Tilbury Docks in Essex, where Keller Ground Engineering treated very soft, highly organic alluvial clays on a subcontract to John Martin Construction.

Consultant Mott MacDonald designed the column layout and Keller, working to a performance specification, was responsible for making sure the columns achieved the required strength.

Keller brought in technical support with staff and equipment from its sister company LCM in Sweden.

Keller senior geotechnical engineer John Judge explains: 'The design philosophy is that you are not creating weak piles, but relying on the interaction between columns and the surrounding untreated soil.'

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 can be achieved.

Judge says soil mixing is a sustainable development technique because very weak soils are modified insitu rather than being removed and replaced.

'There is a strong reliance on empirical experience in the design and getting the binder right is key to any project, ' he says.'The binder we use is typically one or more of a mix of cement, lime, gypsum or blast furnace slag.'

Projects invariably make use of laboratory mixing trials to determine the binder mix and quantities needed to achieve the required strength. Site trials follow and this, says Judge, is where the science comes into the design. Binder mix and quantities can be optimised and the results validated by testing.

Keller is keen for UK practice to adopt the reliable and well-established testing methods developed by the Swedish Geotechnical Institute. The most common of these is a pullout resistance test, which relies on embedding a vane at the base of the column. This is pulled out once the column has cured, giving a shear strength with depth profile through the column.

Different auger mixing tools are available to suit varying soil conditions. Column installation involves drilling down to full depth, which churns up the soil.

The rig driver then counter-rotates the auger tool - increasing the breakdown of the soil fabric - as the string is withdrawn and the binder injected.

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 about five minutes to complete.

At Tilbury, Keller injected a relatively high Ordinary Portland Cement-based binder concentration, because the soil profile contained intermittent lenses of peat - and peat does not react unless high quantities of binder are added.

Deep soil mixing

Gothenburg-based LCM pioneered deep soil mixing in Scandinavia and has 30 years experience with the technique.

It has installed columns up to 25m deep for a wide range of applications, from supporting embankments to reducing active and increasing passive pressures for retaining structures in weak soils.

Columns can be interlocked to provide cellular blocks and the company recently developed a tool for mass mixing large volumes of soil in blocks.

LCM says up to twice the amount of binder used to treat soft clay is needed to treat 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 should be expected in three to four weeks.

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.

Keller bought into LCM in 2000 and became its full owner at the beginning of this year. It is now keen to spread deep soil mixing technology and experience through its global operations.

Lime or cement?

The Swedish-developed technique of deep dry soil mixing is often referred to as the lime-cement column method.

Lime, however, is not an essential ingredient and in the UK Keller has that a cement-only binder works well in the half dozen applications it has done so far.

When either lime or cement (both are a source of calcium) is added, the chemistry of deep soil mixing is essentially the same as for shallow lime stabilisation (see features on pages18 and 30), but there are no known cases of heave associated with the method.

The risk of heave is significantly reduced because deep soil mixing is normally used to treat soft deeper soils in columns, in which there is in effect a soft matrix surrounding the column - if ettringite forms it will tend to be accommodated laterally.

When shallow stabilisation is used, the material is compacted to produce a stiff material and, unlike deep soil mixing, there is little free water present. With surface stabilisation, problems can arise later when surface water comes into contact with unreacted sulphates, initiating heave in the already compacted material.

Deep soil mixing usually takes place in wet conditions below the water table, so if ettringite is going to form, it will do so at an early, noncritical stage as the columns are curing.

Interestingly, the addition of lime is considered essential for shallow soil stabilisation of clays, but not for deep mixing. Shallow lime stabilisation practitioners believe lime not only dries the soft clay but also modifies its structure so that cement and other strength-giving components of the binder work more effectively.

However, deep soil mixing columns gain strength without the addition of lime, probably due to the more effective mechanical mixing of the soil and binder achieved using the deep mixing equipment.

According to Keller's John Judge, deep soil mixing comes down to getting the chemistry of the soil and groundwater right at the start and finding, through simple trials and testing, the most effective binder for the soils on a site-specific basis.

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