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Demand driven innovation

metropolitanrail586

Call for deeper basements and tunnelled infrastructure has driven hand in hand development of both new ground engineering and propping solutions. 

Use of proprietary propping or structural steel for deep basements and large infrastructure excavations has now become standard practice but that has not always been the case. Client demand for ever more complex and deeper excavations had driven over 150 years of innovation in the ground support industry to create the solutions we use today.

“The first use of ground shoring can probably be traced back to the 1860s in London with the construction of the sewer networks,” says RMD commercial director John Breen. “Use of timber for excavation support was common place but the depth and size of excavations was limited by the size of timber available.

“Timber generally limited the excavations to a span of around 10m.”

retaining walls camden town030

retaining walls camden town030

Early excavations for the London Underground used timber supports for cut and cover or wide, battered excavations where space allowed

The excavations were usually undertaken by miners or navvies who came from working on coal mines and canals and they brought the ground support techniques from those sectors to the building industry.

The start of construction of the London Underground resulted in larger excavations with the early routes built using cut and cover along street alignment using the same timber support techniques. However, later lines were built at greater depth using tunnelling shields but the traditional excavation support was still used to take the work from ground level down to the tunnelling depth.

Most of these projects used timber for temporary support and cast linings for the permanent support.

“What engineers achieved with what we now see as primitive techniques is amazing,” says Breen. “Many of the sewers and train tunnels excavated using these methods are still in use today.”

Arup director Dinesh Patel adds: “In the 1920s most basements were for banks with vaults and were a maximum of two levels deep – up to 8m. Controlling ground movement for this type of structure was not an issue.

“These basements were usually constructed using the trench and dumpling method, which is similar to modern underpinning techniques, to form a wall that then allowed the ‘dumpling’ in the middle to be dug out.”

According to Patel, the work was dangerous but undertaken by skilled miners and favoured sites with good ground.

“There was no piling at this time as equipment did not exist and labour was cheap,” says Patel.

According to Breen, the industry stayed much the same under the end of World War II when the use of steel sheet piles was brought over from the US.

“The need for better support was driven by the need to undertake repairs to war-damaged structures,” he says.

“Basements excavated with timber and steel sheet pile support went to considerable depth.”

Patel adds: “The economic recovery of the UK construction market in the mid-1950s meant that major developments were undertaken on bomb sites. These were the first structures to have deep foundations and, as previous buildings had shallow foundations, the grabs could deliver deep walls without concern over obstructions.”

Structural steel started to be used as excavations became deeper and wider support spans were needed but timber frames and walers were also common.

Many excavations were still undertaken by hand but larger digs were undertaken with caissons using clam shells or top down methods.

“Walls for basements between 1955 and 1970 were commonly built using diaphragm wall techniques using grabs,” says Patel.

groundfroce 2

groundfroce 2

Demand for deeper excavations and lower risk of ground movement drove innovation in the propping market

“In the 1950s tripod rig piling was introduced to install piles up to 600mm in diameter using a steel casing shell to drive piles. It was very manual work and dangerous but was widely used up until the 1970s.

Change was also happening in ground support. “In the 1960s trench boxes made by Krings in Germany started to be brought into the UK market,” says Breen.

SGB, which was later bought by Vibroplant, brought in the first models and these evolved in the 1970s and 1980s. Mechplant and Shoreco also offered such systems and MGF started with trench boxes. Mabey’s business was founded by reusing Bailey Bridges for civilian use after the war.

“Trench boxes limited the dig depth to 5.5m to 6m with some of the largest ones offered by Shoreco, which became a leader in high capacity propping,” says Breen.

Mechplant was the first to offer proprietary propping solutions and it is believed that the business started in the 1980s with the use of adapted military rams.

Further developments in piling brought the need for further ground support innovation.

“Introduction of the Piglet rig in the 1970s gave rise to construction of single piles and the rise in use of contiguous piled walls, then the arrival of continuous flight auger (CFA) piles in the 1980s enabled secant pile walls to be built,” says Patel.

“Development of more powerful rigs opened up the opportunity to build hard/firm secant pile walls and core through older piles.

“This meant that deeper basements were possible from the 1980s. Before that the maximum depth was 12m to 15m.

“The deeper basements called for new propping solutions and created the opportunity to build from the top down or bottom up. Top down called for smaller diameter piles and less temporary works while bottom up needed larger diameter piles and created more space for fast excavation.”

Speaking about the innovation of proprietary propping solutions, Breen says that there was a need to find a new solution as buildings were getting bigger, basements deep and excavations for new facilities, like water treatment plants, were getting larger.

“The old systems were clumsy and involved intense engineering – there was a need for a more streamlined solution,” he says.

“Early proprietary props had a capacity of 50 to 100t but we are now heading towards 500t capacities and beyond.

“The challenge of adopting proprietary in the early days was the analysis as it all had to be done by hand,” adds Breen. “The quality of steel was also an issue.”

However, the advent of computer analysis, greater understanding of ground movement and quality improvements, along with demand for greater efficiency and cost effective solutions have driven proprietary propping to become almost ubiquitous in the UK excavation market.

“The change to proprietary propping was welcomed by the industry but, as with anything new, it took a while to take off but it is now the norm,” says Breen.

Patel adds: “The technology we have today is very flexible and can meet difficult site requirements.

Basements of 20m or more are now common.”

 

This article was produced in association with:

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