Stabilising a site in Northern Ireland set Keller a number of challenges
Arguably most famous as the resting place of Ireland’s patron saint, the small town of Downpatrick in Northern Ireland has also been home to an unusual ground improvement project.
Ground engineering specialist Keller has completed a six-week temporary works project in the town worth £400,000, designed to strengthen the ground for safe excavation.
The work was part of a £14.5M project to replace the current Down Leisure Centre building with new facilities. Some of those, including a new swimming pool and pump house, will be operational by summer 2017.
Main contractor Felix O’Hare will excavate the site and install the pool and pump house for the local authority, County Down.
Before that could happen though, the poor ground conditions needed to be improved. Given that it consisted of a few layers of made ground and peat before giving way to soft marine silts, the site would have been too weak to support the plant without intervention.
GE visited the site near the end of the project in September to see the improvement work first hand.
Starting in August, Keller worked to strengthen the ground so it could support a platform for the rig. It also needed to provide enough lateral strength to allow excavation to the required depth, which was 2m for the pool site but 7m for the pump house as the permanent plant will sit below ground level.
Even for experienced specialists at Keller, the poor ground conditions proved to be the biggest challenge on the project.
“The ground itself there, the best way to describe it is ‘heavy water,’” explains project manager Martin McAlinden.
“Any consecutive rain for maybe two hours and the whole area just turns to a bog. It’s just horrible.”
The area consisted of 1m of made ground, which was used as landfill in the Victorian and Edwardian eras, followed by 1m of peat. These first couple of layers then led to 18m of soft marine clay and silt.
The poor conditions made it near impossible to excavate the site without intervention. The ground would have been too soft to support itself and the contractor would not have had the lateral resistance needed to dig down safely – any excavated area could have easily collapsed. The site obviously needed supporting, but less obvious was the method which should be used to support it.
The team rejected the idea of using temporary sheet piles. Usually a good option, they do require sufficient ground strength and resistance at depth to be effective. In this case the soft marine clay and silt did not give way to harder ground until a depth of 18m. This meant the sheet piles would not have met the required resistance.
Given the impracticality of creating 18m long sheet piles, it could not be considered as a method for this project. Instead, the team rose to the challenge and chose to strengthen the ground using wet soil mixing, which is still relatively unusual in the UK.
According to McAlinden, soil mixing is ideal for strengthening looser sand and coarser silt material. It is particularly popular in areas with a seismic risk – it was trialled in Japan around 20 years ago and is commonly used in New Zealand, where McAlinden spent three years of his career.
Soil mixing is also popular in Scandinavia and Poland as it works well with peat, which is commonly found in European soils. However, it has yet to really take off as a soil strengthening method in this country.
“It’s a technology which isn’t used as much as it should be in the UK,” says McAlinden, whose last soil mixing project in the UK was almost two years ago.
“In the UK, and possibly in Ireland also, they don’t like to do things differently.”
Designed to increase soil strength, wet soil mixing involves adding cementious grout to existing ground, usually in columns of 1.5m diameter.
Similarly, dry soil mixing strengthens ground by adding dry binder. This method is most suited to soft soil with a high water content.
Which begs the question – as the water content of the ground was a serious problem, why use wet soil mixing instead of dry?
As McAlinden explains, a perched water table on the upper made ground meant that the water was unable to drain downwards and created boggy conditions.
However, when he collected samples of the marine silt and clay, he discovered that the ground underneath was quite dry due to the perched water table above. This meant that the conditions were perfect for wet grout.
Health and safety was also a factor in the decision to use wet soil mixing over dry, as dry soil mixing involves using cement powder which could have spread across the site.
In total the team created more than 900 interlocking columns at 1.2m centres in the pump house area and 1.3m centres on the pool site.
The strength provided by the columns meant that no ground was removed during the process, even the made ground on the surface of the site.
Each column took 10 to 15 minutes to complete and around a day to cure.
Interestingly, no bearing capacity or factor of safety was required as the columns were completely temporary and were not designed to support the final structures.
The columns provided an undrained sheer strength of 100kN/m2 across the site, which was enough resistance for the plant.
To create the grout cement and water were pumped into a mixer at a 1:1 ratio, 1t of cement to 1,000l of water. The grout could only be held in the mixer – or batcher – for three hours before it cured, so the team had to be quick.
Site workers channelled the grout at 20bar from the mixer into a pump and out to a rig.
It was then pumped down the 10m drill rods on the high torque, £750,000 SR80 Soilmec rig to meet the 2m long and 1.5m diameter mixing head, made from steel with hard facing.
The mixing head, imported from Poland, featured counter rotating blades – which moved at 36rpm – to ensure homogenous distribution of the binder as it burrowed into the ground.
Drilling down to the desired depth – 2m for the pool site and 7m for the pump house – was made easier by the electronic panel on the rig informing the operator of the mixing head depth.
As the mixing head was drilled downwards, the grout was released through eight nozzles at a rate of 180l per minute while on the way up it was released at a lower rate of 120l per minute.
According to McAlinden, it was far easier to pump more grout on the way down than when drawing the mixing head back towards the surface.
The amount of grout released added up to 300l/m3, which was a set requirement for the job.
Despite the challenging ground conditions, McAlinden stresses that he has enjoyed the unusual project in Downpatrick.
“It’s a community job,” he says. “It’s always nice to give back.”