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Temporary works: Props forward

With construction activity, especially in London, reaching pre-recession levels, Mabey Hire has launched a new major projects division to deal with demand for temporary propping projects. Report prepared in association with Mabey Hire.

The number of cranes on the London skyline has often been seen as a barometer for the capital’s economic weather. If that is the case, the twice-yearly London Crane Survey compiled by Deloitte Real Estate recently suggests that confidence among developers is finally reaching levels last seen before the credit crunch.

But while Deloitte looks upwards and across the skyline when it attempts to gauge the health of the economy, propping and temporary works company Mabey Hire tends to look downwards. And the company’s major projects manager Matthew Green thinks the abundance of large scale propping contracts in the capital is equally encouraging.

Dollar Wharf

Top dollar: 1067mm diameter props were used on the Dollar Bay excavation on the Isle of Dogs

“I think people are seeing the fact that there is a healthy market for basement/temporary propping projects and although it’s quite competitive - especially in London - the volume of work is immense,” he says. “No single company can undertake all these potential contracts, whether you’re a proprietary or structural steel supplier.”

More traditionally associated with propping, formwork and more general groundwork schemes, the company has been so impressed with the scale of work available that it has set up a new division to
cater to this market.

Mabey Hire Major Projects, as Green explains, will develop new products and offer new capabilities for the preponderance of basement propping
jobs available.

The company will pit its proprietary offering against the structural steel suppliers who more typically support
this market.

“We want to ask companies why they are using structural steel when we can supply an off-the-shelf proprietary system that can compete on cost and capability and make obvious savings on time, labour and wastage,” says Green.

 

Mabey White Collar building

BIM: A major project during CAD development

“The ethos from day one was to move forward slowly, undertake a few projects and deliver them to an excellent standard thus building up experience and confidence with our clients,” says Patrick Flannery the company’s major projects sales director.

Under such an approach, it was important for Mabey Hire to deliver a handful of exemplar projects to convince prospective clients that it has the expertise to compete in this market. Such a project materialised when it was asked to provide a propping solution for an excavation for the Battersea Power Station redevelopment in London (see box).

“I would say in the last five years, the type of excavations that proprietary equipment can have increased in size considerably, so the products have had to increase in capacity to deal with those,” explains Green.

“Off the back of Battersea we developed a large capacity propping system which consists of a 1067mm diameter tube.

“I would say in the last five years, the type of excavations that proprietary equipment can accommodate have increased in size considerably”

Matthew Green, Mabey Hire

“This now allows us to do unsupported spans up to 40m within an excavation where previously it was limited to around the 20m to 29m mark.”

Proof that experience is everything in temporary works was found when Mabey Hire applied the lessons from the Battersea project to a basement excavation for O’Halloran & O’Brien at Dollar Bay on the Isle of Dogs. Green describes this as a more stereotypical excavation.

“The Dollar Bay excavation measured 60m by 34m,” he says. “Because we had undertaken the Battersea contract we were able to offer our 1067mm diameter props, which could accommodate the unsupported span of circa 34m.”

Green says that one of the advantages of using an unsupported tube across larger spans is that it obviates the need for mid-span plunge columns underneath the prop. This allows for everything to be propped at a higher level and allows the construction team to excavate freely underneath and even begin construction below without the temporary works being in the way.

“As soon as you have built a portfolio of products, it opens up doors for other projects”

Matthew Green, Mabey Hire

“As soon as you have built a portfolio of products, it opens up doors for other projects that you wouldn’t have been able to do before,” he says.

“A bespoke product becomes an off-the-shelf product, so you can design and specify these jobs a lot easier than you can if you have to do each of these projects as a one off.”

Having a body of work behind you is of course one way that you can persuade clients to work with you, but Green and Flannery also want the new division to provide a problem solving service for customers.

“I think what we’re offering is an excellent design service combined with dedicated project management support,” says Green. “We want to build relationships, and by doing that you’re working closely with the customer from a design, logistics and delivery point of view. You’re really trying to remove their problem and make sure they get what they need on site when they need it.”

Site Installation

As Mabey Hire moves forward, it continues to grow and develop its offering in this specialist market. Over the last few months, the major projects team has worked closely with other Mabey Hire divisions so that knowledge and experience can be shared throughout the company. One key area that the major projects team has focused on is the contracting element of the Mabey Hire business.

“It became very apparent that we could look to offer our customer a service that no other proprietary supplier could provide - site installation. By working closely with our contracting team, we can look at undertaking full sub-contract orders which involve the complete installation of equipment,” says Flannery.

“It became very apparent that we could look to offer our clients a service that no other proprietary supplier could provide - site installation”

Patrick Flannery, Mabey Hire

“This further strengthens our market position and provides the major projects team with a competitive edge,” he says. The ability to work with sub-contract orders is something that Mabey Hire is very familiar with, and on high value contracts Flannery thinks it makes sound commercial sense to embrace such a method of working.

Although each of the projects that Green describes has employed a different propping solution, the one thing they all have in common is the fact that they are in London.

“Ninety five per cent of what we tender is located in the London area,” he says. “We have looked at projects in Leeds, Glasgow and Birmingham, but the majority are up and around central London.”

The other thing that strikes him about major projects work is the level of design involvement and the time a project can take to deliver.

“There’s a great deal more thought and detail needed from an engineering and design aspect,” he says.

“We could spend several months on a single project from initial tender to potentially securing the work to then completing the design package”

Matthew Green, Mabey Hire

“We could spend months on a single project from initial tender to potentially securing the work to then completing the design package, which often needs to be checked and approved by the client or a third party interest.”

Green thinks this differs quite markedly from a more traditional Mabey Hire project.

“A typical shoring excavation for a petrol tank, or something of that nature, is fairly quick and easy and quite run of the mill.

“You do a design, the contractor agrees with it, the equipment goes to site and you’re in and out of the ground in, say, a couple of weeks.

“Major project contracts could be in the ground for three to five months at a time so it is really just a question of understanding what the customer requires, not just from a design but also from a construction, commercial and logistical point of view.”

In association with

Mabey Hire logo

Load monitoring

Mabey instrumentation

Monitoring: Mabey Hire Live Instrumentation will work with the Mabey Hire Major Projects division

In conjunction with the launch of its Major Projects division, Mabey recently launched an instrumentation division.

Mabey Hire Live Instrumentation will provide a monitoring service to reassure large civils customers and clients that structures will retain their integrity during engineering work (NCE 2 April).

Typically, this service will monitor flat jacks, as was the case when the company was commissioned to analyse and mitigate the effects of Crossrail tunnel boring machines as they passed under a viaduct in Canning Town, east London. The division will also carry out noise and vibration monitoring for compliance with environmental regulations.

However the company also has plans to apply load monitoring technology in the Major Projects division as well.

“Monitoring prop loads has not been something we’ve previously done but, in conjunction with our development team, we are now creating solutions to meet those requirements,” says Green.

“They have undertaken the product development in-house and developed and designed a solution with which we can measure the prop load in these systems. Prop load monitoring will not always be a requirement but we can now offer this additional service to our clients,” says Green.

Battersea Power Station redevelopment

Mabey Battersea

Blue is the colour: The Battersea Power Station site presented numerous constraints

The former Battersea Power Station has stood empty since 1983 but has recently been bought by a Malaysian consortium of investors which is turning the premises into a luxury accommodation and leisure development.

Phase 1 required the construction of a huge basement sub-structure to allow for the building of the first new complex consisting of residential and commercial spaces.

Although large in area, the site footprint presented numerous constraints, with the existing power station to the west, the river Thames to the north, an overground railway to the west and a busy road network to the south. 

A particular challenge was a double-storey basement for an energy centre building which is being constructed on the site.

The excavation measured 90m by 32m wide and 9m deep. During the project, O’Keefe Construction required support to three sides of the excavation - the east, south and west elevations - to prevent ground movement and therefore allow for the safe construction of the new structure.

The retaining wall comprised a permanent sheet piled solution, which on the east and south elevations required two levels of support, primarily due to the surcharge loadings generated by the power station foundations adjacent to these elevations.

The west elevation, which sits next to the Network Rail overground railway only required one level of support, as the existing ground level on this side was lower than the east and south.

The design required two levels of support, with both levels requiring a perimeter steel waling beam. As the existing ground levels varied across the site, the upper level propping had to provide support at two different levels; the east, and south east elevations at 3.5m and the south west and west elevations at 1.8m.

This presented a number of design challenges, particularly on the southern wall where there were diagonal square cross section brace props

As the waling beams were at different levels, a bespoke structural steel beam section connecting the two proprietary waling beams was required to transfer the large shear forces which were generated by the diagonal props. This detail was designed and fabricated in-house by the Mabey Hire development team and was done in such a way that it could be installed on-site with no welding required. 

On the east and west elevations, the shear forces generated from diagonal square cross section braces also had to be dealt with, and to do this, special shear keys were designed.

The shear keys connected the waling beam to the steel sheet piles and transfer the shear force into the retaining wall which provided the resistance required.

The upper level support consisted of Mabey Hire Super Shaft Brace and Super Shaft Brace Plus waling beams and the Super Bracing Strut Plus which allowed for prop spans of circa 31m at centres of approximately 7m. 

The low level waling was only required on the east and south east elevations, but due to the presence of deep pile caps and varying excavation levels, raking props could not be adopted and therefore horizontal props also had to be used. 

As the low level propping sat adjacent to the power station, the waling loads were high 380kN/m and this meant that the prop loads would be in the region of 270t with spans at circa 31m. 

To achieve this requirement, Mabey Hire had to develop, design and fabricate a new propping system within a five week time frame to ensure that the site was not held up as the excavation progressed.

The Mabey Hire development team came up with a solution that combined the existing hydraulic unit from the Super Bracing Strut with a new 1067mm diameter tubular section. This new prop type ensured that the loads and spans could be achieved while maintaining a modular proprietary system.   

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