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HS2 Ground Stabilisation Project by Deep Soil Mixing Ltd

Shortlisted for Brownfield Awards Category 8 - Best Re-Use of Materials

Project information:

 

2020 was a year of project firsts for Deep Soil Mixing Ltd (DSM). We started 2020 starting work on HS2 where soil mixing was being used for the first time on the largest infrastructure project in the UK. The project was extremely challenging both from a health and safety perspective and on a technical level but was completed on time and within budget and soil mixing saved the client considerable cost as opposed to removing the contaminated ground to landfill and saved many months on the project programme. All of this took place whilst the UK went into the global COVID-19 pandemic.

 

Ground remediation contractor, Deep Soil Mixing Ltd believes that their soil mixing solution on HS2 is a worthy winner of category 8 best re use of materials as their soil mixing recycling methods cut costs and reduced HS2’s carbon footprint during a ground improvement project near Coleshill in Birmingham. 

 

Deep Soil Mixing’s sustainable soil mixing solution was used to geotechnically improve existing sludge beds (comprising contaminated Alluvial soft clays) at a working sewage works to allow access of construction equipment and to stabilise the ground for the installation of a new gas main pipe which needed to be relocated as part of the HS2 works.

 

The required ground stabilisation comprised of two distinct working areas, the Running Track, and the Trenching Area. For the Running Track, the improved ground had to enable vehicular access to facilitate construction of a gas main, requiring safe bearing to support a 45t tracked excavator / HGV and a uniformly distributed load of 20kPa. A minimum target performance criteria for California Bearing Ratio of 10% was set for the formation of the Running Track (prior to placement of the haul road subbase). For the Trench area, the improved ground was required to enable open cut trenching with minimal groundwater ingress to facilitate the installation of the gas main, making due allowance for vehicular and plant loading adjacent to the cut crest. 

 

The ground to be mixed was existing sludge beds (comprising contaminated Alluvial soft clays) in part of a sewage treatment works so the ground was contaminated it was also critical that there was no transfer of leachates into groundwater. By using soil mixing we were able to lock in the contaminates and there was no transfer of leachates into groundwater or the adjacent river. 

 

By utilising our soil mixing technique it prevented both the removal of contaminated material off site and the need for importing fill, which creates huge environmental benefits including substantially and reducing carbon emissions from lorry movements.  It was estimated over 1000 lorry movements were saved. Minimum spoil removal from site also means improved cost efficiencies and a shortened construction programme.

 

The project was completed in six months and included preliminary testing and site trials before work began on site. The testing was undertaken at the company’s in-house laboratory and mixing tests carried out on various soil/binder mixes. Based on the results of preliminary laboratory trials, Deep Soil Mixing Ltd’s designers, ByrneLooby (BL), defined the extent and specification of deep mixing, to ensure that the design requirements would be meet. The chosen stabilisation design mix was verified on site via field trials, and throughout construction via a detailed regime of Construction Quality Assurance recording and testing (completed on wet grab samples extracted from the mix) and verification testing (comprising CPT testing completed through the stabilised material post curing). A confirmatory sign-off report was provided on completion of the works by ByrneLooby proving that ground improvement works had meet the requirements of the design specification.

 

As a result of the soil mixing, in-service long-term settlements were reduced to a maximum of 25mm to provide increased bearing and CBR for the HS2 site. 

 

Deep Soil Mixing were working for Land and Marine Engineering Ltd (LME) who were contracted to HS2’s enabling works contractor LM JV (Laing O'Rourke and J. Murphy & Sons). Works in this area are preparing for the new HS2 railway to be built on a viaduct before it swings west though a tunnel and on towards the new station at Curzon Street in Birmingham.

 

This project was a mass mixed project to a depth of approx. 5 metres deep. Deep Soil Mixing Ltd used the ALLU system, which provides a fast, cost effective and environmentally friendly work method for the hardening and dynamic strengthening of soft soils, as well as improving the geotechnical qualities of the mixed soil. The system is designed to improve soft soils by mixing a binder into various soil classifications. The method of mass mixing used was wet soil mixing.

 

It is fantastic to be part of the largest infrastructure project in the UK and as a company we continued to work on this project throughout the first Covid-19 lockdown which was a huge benefit to the company as many other projects were put on hold in the first lockdown. This meant jobs were saved and as a company we were able to continue to grow in a period of time that was a struggle to many companies in the construction industry.

 

It was also exciting to get involved in HS2, to see the environmental ambitions and standards on site and play our part in contributing to these. Our soil mixing technique is recycling on a massive scale - great for sustainability and great for the environment.

 

Part of our R&D facility and plant depot is local to the Phase 2a route in Stafford, so it is also great that HS2 is looking to local businesses and the local people they employ and sub-contract to get involved and benefit from the project.

Simon Russell, LM Project Director said:

 

“Over the last year our team has been preparing this site for the start of construction. One of our key objectives as an HS2 contractor is to reduce the project’s carbon footprint by introducing innovative sustainable techniques wherever possible. We were extremely pleased that the ground remediation solution used by Deep Soil Mixing resulted in cost, time and environmental benefits.”

 

About Deep Soil Mixing

 

Soil Mixing can transform severely contaminated or poor-quality land into commercially viable land that can be developed. 

We do not remove materials from site, this has significant advantages of reducing CO2 emissions, improves sustainability and is recycling on a grand scale. 

 

Why we believe our work on this project merits winning this award. 

This project was innovative in its treatment of contaminated sludge beds within a working sewage works and has met safety, sustainability, and environmentally friendly standards as all the contaminated soil was treated in-situ.  It proved to be an ideal solution for this project, but we hope the success of the project can demonstrate how an engineered solution using soil mixing to treat sewage contaminated ground can also be transferred to a wide variety of other applications. In addition to the environmental advantage, stabilisation of soft soils by adding binders to reduce settlements and/or improve the stability of the land can be both a quick and cost-effective solution compared to some traditional methods including piling and dig and cart away and in 2021 and with the Government’s Climate Change strategy firmly in our minds we should not be digging out contaminated soil and removing it to landfill we should be looking for innovative solutions that treat in situ.

 

In a period when the construction industry is looking for ways to save on costs, reduce their carbon footprint and cut project time companies like Deep Soil Mixing Ltd should be recognized for the pioneering work that they do and the continual research and development the company undertakes.

 

We believe we should win this award due to the fact although we are a relatively small company - 10 employees and £1.8m t/o we have achieved success in terms of growth in turnover and growth in the success of the projects we have undertaken.  The quality of clients and the size of the projects we undertake is testament to our growing success and reputation.

 

Soil mixing is still a relatively new ground remediation solution in the UK but we are being specified for some landmark projects like HS2 and the Mersey Gateway crossing to using soil mixing on project firsts including stabilising an asbestos contaminated landslip and the use of soil mixing to stabilise ground under housing foundations. 

 

We are becoming renowned for successfully undertaking challenging & difficult projects that others may not wish to do & can demonstrate the impact this is making in the world of civil engineering & illustrate how soil mixing techniques can transform severely contaminated or poor-quality land into commercially viable land that can be developed.

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The site before soil mixing was implemented 

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The site after soil mixing 

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Soil Mixed Soil excavated for pipeline

Finite Element and Bearing Capacity Design Assessments

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