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Diaphragm walls: Going deeper underground

isle of grain, peanut dwall (3)

Construction of major projects like Crossrail and Tideway is hard to imagine without diaphragm wall techniques. Nadine Buddoo looks at how the method was brought to the UK.

With the emergence of the Beatles and the advent of colour TV in UK homes, the 1960s are often characterised as a decade of revolution. Alongside significant shifts in everything from youth culture and fashion to women’s rights and technology, the swinging sixties also produced major developments in geotechnical engineering with the first diaphragm walls being constructed in the UK.

In 1961, the first was built at Hyde Park Corner, London for an underpass, which was a key feature of the London County Council’s Park Lane Improvement Scheme. It was delivered by Italy-based Icos. 

Initial testing of diaphragm wall construction dates back to 1948, but it was not until 1950 that the first of these was built by Icos, using bentonite slurry as a means of support. This contract for a cut-off wall on a dam site was followed by  construction of the Milan Metro in the late 1950s, when Icos used diaphragm walls extensively in cut-and-cover construction.

metro de saint denis

metro de saint denis

Initial development of diaphragm walling came out of techniques for either building or remediating dams and cut-off walls.

By 1961 the introduction of diaphragm walling in the UK presented a means of constructing deeper and larger walls than had previously been possible through piling.

Even a few decades later in the early 1990s when continuous flight auger (CFA) piling was commonly used across the industry, the rigs were a fraction of the size of the rigs available today.

“They simply couldn’t build big secant walls like we do today with cased CFA techniques,” says Bachy Soletanche chief engineer David Hard.

“Diaphragm walling allowed heavier reinforced walls to be built.”

Much of the initial development of diaphragm walling came out of techniques for either building or remediating dams and cut-off walls.

“That’s where a lot of the grab and bentonite slurry technology came from,” explains Hard. “The techniques were then developed into use for retaining walls for basements.”

When the technique was initially debuted in the UK, the technology available was considerably more rudimentary than today’s standards.

Back then, the grab would be on a boom crane with a Kelly mast attached to the front. Round cable grabs traditionally used for piling were converted to a rectangular grab in order to produce the wall.

“That was quite simply the only technology available,” admits Bachy Soletanche managing director Chris Merridew.

Historically, the technique has found more popularity in mainland Europe than in the UK.

“It’s never been as big here as it has been in France,” says Hard. “But with our parent company being French, they use a lot of diaphragm walling techniques that you wouldn’t traditionally see in the UK, so that’s allowed us to bring it in.”

But as with any new technique, the success of diaphragm walling in the UK has been dependent on getting buy-in from the industry.

Nonetheless, the use of diaphragm walling opened the way for solutions that had previously been limited on UK projects; for example, the construction of walls that could be left exposed.

“That’s something you wouldn’t have necessarily done with secant walls because there’s lots of piles and potentially lots of joints,” says Hard. “The technique definitely provided different options for deeper basements, thicker walls and bigger structures.”

Hard points to the Jubilee Line Extension as a good example of this. 

“At Westminster Tube station, as you travel down the escalators you’re looking right at the diaphragm walls,” he says.

lee tunnel dwalling 2011

lee tunnel dwalling 2011

Evolution of technology and equipment in recent decades has given the technique an advantage overtraditional piling methods in certain ground conditions

Despite typically representing a smaller part of the UK market, the technique has been used in major infrastructure projects for deep shafts and metro stations. However, the New Tyne Crossing in the North East is an example of the technique being used for cut and cover approaches.

“That’s a classic use for diaphragm walls. It doesn’t have to be a very deep project. If you’ve got a long, linear process, the technique is particularly advantageous,” says Hard.

Alongside the benefits for linear tunnelling projects, Merridew insists the technique offers superior watertightness in comparison to other, more traditional, techniques. He also describes it as one of the most stable methods to use when constructing deep shafts.

The evolution of technology and equipment in recent decades has also given the technique an advantage over traditional piling methods in certain ground conditions. Sites with very granular ground, for example soft rock or Thanet sand, are typically difficult to dig, but machines like Bachy’s Hydrofraise rig have been instrumental in combating that challenge.

“Some of the shafts on Thames Tideway and the Channel Tunnel, for instance, involved digging through chalk, granular material and soft rock,” explains Merridew.

“With the Hydrofraise, you can excavate that quite rapidly using the technique.”

The modern hydro-mill also trumps the traditional rope grab in terms of the verticality tolerances that can be achieved, ultimately making the wall more structurally efficient.

As machine technology has evolved, so too has the technology involved with forming diaphragm wall panels.

The development of stop ends has led to various methods to guarantee the joint between panels. Bachy’s CWS system – on which a paper was first published in GE’s September 1985 issue – allows the inclusion of a waterbar which promises maximum watertightness between adjoining panels.

Despite the benefits of diaphragm walling and the rapid improvement of many of the processes involved, use of the technique is not without its challenges. One of the key issues is the amount of plant needed coupled with the resulting space requirement.

In addition to space limitations, the fact that the technique is not as commonly used in the UK as it is in Europe means that it is often more expensive than alternative solutions.

So how is the industry working towards overcoming some of these limitations?

Hard believes significant changes in the materials used for the technique will emerge in the coming decades, with polymer already being eyed as a replacement for bentonite.

“I also think concrete will move on massively,” says Hard. “It’s a carbon intensive material so it’s all about making it a more environmentally acceptable product.”

Traditional reinforcement cages may also evolve with the possibility of using fi bre reinforced materials to make the process more efficient.

Recent work between the European Federation of Foundation Contractors and the Deep Foundations Institute also suggests that key changes in concrete and tremie concrete are imminent.

Merridew sees this collaboration as emblematic for the future of the technique and the wider industry.

“We are starting to see problems globally, and the industry is taking leadership to improve the quality of the materials being used and to standardise some of the methods,” he says.

“I think there will be more of this international collaboration around various techniques, materials and specifi cations. Over the next 50 years, the world is going to get smaller.”

With these advancements on the horizon, it’s clear to see that the recent resurgence in diaphragm walling could mark a long-term shift for the UK industry as clients seek new ways to deliver deeper and larger structures that are in line with pressing time, cost and carbon concerns.

This article was produced in association with:

bachy soletanche logo icon above colour

bachy soletanche logo icon above colour

 

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