Mix it up

Colin Critchlow explains how soil mixing techniques are gaining in popularity to meet government targets

Developers, public bodies, building contractors and investorsare recognisingthat environmentally-friendly construction provides not only long-term positive environmental benefits but also immediate financial payoffs. With the UK government calling for a 50 per cent reduction in construction excavated material being sent to landfill, together with the increase in landfill charges, improvement of existing ground is now high on thesustainable construction agenda, helping to reduce the industry’s carbon footprint when it comes to developing sites.

Soil mixing is a technology that is widely used throughout the USA,the Far East and Europe includingGermany and Scandinavia who have been using it for over50 years. It is now gaining inpopularity in the UK, helping tomeet environmental targets. As a ground remediation solution, soil mixing reduces project times, costs and the carbon footprint on construction projects compared to other more traditional ground engineering methods such as various types of piling, excavate and cart away, dynamic compaction, surcharge etc.

By making use of the existing ground and landfill and improvingit to achieve a specified bearingcapacity and shear strength, there is no need to excavate, cart awayor dispose of the existing material off site and then bring in costly and bulky fill materials. It is not just about improving unsuitable groundbut aims to turn poor quality soils and fill into acceptable bearing strata for numerous applications. This process is typically referred to as in-situ soil stabilisation and there are two common types of soil mixing – wet soil mixing and dry soil mixing. Dry mixing is used predominantly in very wet ground whereas wet mixing can be used in both wet and dry ground successfully as it produces a more homogenous mass and binder distribution.

Cementitious-based mix designs are most commonly used for soilstabilisation treatment, however, avariety of other additives including Bentonite, lime or blast furnace slag can also be used to meet specific project requirements.

Soil Stabilisation has uses in both the environmental and the geotechnical industries. It can be used to increase soil strength; decrease leachate of contaminants; decrease soil permeability; alter the soil’s pH; and reduce the contaminant ratio.

Applications are numerous for both remediation works and alsobeing utilised by developers andmain contractors on new build projects. Improving the strengthand durability of soil by mixing an appropriate amount of binder agent with the soil, allows these solutions to be used for bearing panels, columns or mass mixingfor retaining walls, flood defence walls, cut off walls, retaining walls,king post retaining walls andground and mass soil stabilisationand embankment stabilisationand reinstatement. Additionally,they can work in the marineenvironment and frequency transfer reduction from railways and roads, bridge abutments, ground bearing slabs, industrial and commercial building sub foundations etc.

In addition to the environmental advantage, mass stabilisation of soft soils by adding binders to reduce settlements and/or improve the stability of the land can be both a quick and costeffective solution compared to the traditional method of piling, as well as being suitable for some contaminated ground. If developers have a site with soft ground being present including peat and alluvial deposits, glacial till, bogs, etc soil mixing will improve the soil using the mass stabilisation soil mixing system.

Soil mixing gives the ground new properties, enabling it to takemuch heavier loading. Whether it’s best to use wet or dry mix toget proper stabilisation, to get the proper effects of the binder depends on the ground itself and will be determined by specialist ground investigation and precontract sampling and testing.

The success of the system on any site depends on carrying out a detailed geotechnical survey,taking samples and having thesethoroughly laboratory analysed andevaluated. In some cases, it might be necessary to carry out field trials before a decision is made as to how the system is best used.

The possibilities for this system in the UK and Ireland are now verywide indeed and the UK is waking up to its benefits in a positive way.

Case study
Soil mixing in action
Ground Engineering Contractor, Deep Soil Mixing Ltd was part of the team working on the prestigious new Mersey Gateway project between Widnes and Runcorn in Cheshire for Merseylink CCJV using soil mixing technology not before used in the UK.

The work undertaken by Deep Soil Mixing Ltd was the innovative ground engineering and soil stabilisation method of soil mixing using a ten tonne Cutter Soil Mixer attached to a 65 tonne RTG rig. This was the first time this bi-rotational mixing head was used for soil mixing in the UK. The use of this technology follows trials carried out by the Deep Soil Mixing team on behalf of Merseylink CCJV.

Following the trials, Deep Soil Mixing was awarded the multimillion pound project to install over 1000 soil mixed panels at a number of locations across the north landside area of the Mersey Gateway project and these panels are designed to stabilise the ground in areas around the various bridge structures so that further construction work could be undertaken.

Colin Critchlow is director of Deep Soil Mixing Ltd. With three offices in Bedfordshire, Staffordshire and Belfast, Deep Soil Mixing is a leader in the field of developing soil mixing, and its soil mixing technology has helped on several projects over the last few years from road construction through to dealing with contaminated and poor quality ground, flood alleviation and slope stabilisation.

For more information, please see www.deepsoilmixing.co.uk