A WORKER WAS CRUSHED TO DEATH by falling shotcrete on 7 March 2014, 10m into a tunnel in Holborn, Central London. Rene Tkacik, 44, was carrying out SCL works during construction of a crossover cavern between the two main western running tunnels in Holborn for Crossrail, when nearly a tonne of concrete from the crown fell on him.
An inquest was heard in February last year, which revealed that Tkacik, a Slovakian national, was an experienced construction worker. However, the jury ruled his death had accidental contributory factors including him not being able to understand all briefi ngs. No one faced prosecution for the death.
In the wake of the accident, the risks of spraying concrete were brought to the public’s attention with contactors keen to try safer methods that will protect site workers from the dangers of sprayed concrete lining failures.
TRADITIONAL METHODS
Shotcrete lining forms an integral part of conventional tunnelling and is widely applied for underground excavations. Shotcrete gains strength in a similar way as ordinary concrete gains strength, with the classic S-shaped curve.
Strength monitoring of shotcrete is vital; important for two key reasons. The first is re-entry time: this is based on measuring the developing strength of shotcrete until an adequate strength value is reached. The second reason is that shotcrete linings are loaded at early age as the tunnel advances, and it is important that the shotcrete gains strength at a rate corresponding to the rate of loading, so that safety is maintained at all times and the lining is not overstressed.
Strength monitoring is traditionally performed by needle penetrator and Hilti nail guns, followed by testing of cylinders cored from the lining or from a panel sprayed at the same time as the lining.
The main drawback with these methods are that they are local, destructive, some produce waste – unused Hilti nail gun cartridges are classified as hazardous waste – and they also introduce additional risks. Furthermore, checking the early strength of the shotcrete in the crown is particularly difficult; a scaffold or a MEWP is needed.
“The issue with the needle penetrator and the Hilti nail guns is that you have to get right up to the piece of concrete you’re testing; somebody has to go there physically and push a needle into it or fire a nail into it,” says Aled Davies, Senior Tunnel Engineer, Costain. “Therefore, potentially if you’re digging a tunnel, the shotcrete that is of most concern is right up next to the tunnel face. That’s not somewhere you want to be standing, so you’ve got to rely on test panels and you’re hoping that the tiny little area you’re testing on the test panel is representative of 10m3 of shotcrete.”
The development of Strength Monitoring Using Thermal Imaging (SMUTI) was driven by dissatisfaction of current strength tests for monitoring sprayed concrete. SMUTI is a patent-protected invention created by Benoit Jones from the University of Cambridge also, also inventor of SMUTI and Director of Inbye Engineering.
The approach is based on developing temperature histories for the shotcrete lining using onsite thermal imaging. Knowing this temperature history enables the engineers to calculate the amount of hydration that has taken place in the concrete, and hence its strength.
For the last 25 years the Arrhenius function has been used. It can be said that for a given concrete mix, the rate of hydration at any time is dependent on only the temperature at that time and the degree of hydration that has already occurred.
In order to use the Arrhenius equation the normalised affinity needs to be known, as well as the activation energy and the temperature.
“First, we need to do some laboratory tests and work out a couple of thermodynamic parameters for the cement and mixtures that are being used. We then do some calibration on site with some sprayed panels to work out the relationship between the amount of chemical reactions that have taken place and the strength, and then we input all of those parameters into the software,” explains Jones. “So when we are spraying the concrete, and afterwards, we can use a thermal imaging camera to measure the temperature of the shotcrete, and put that data of times and temperatures since we’ve sprayed it into the software and it tells us what the strength is. It allows us from a remote and safe position to check the whole sprayed concrete lining.
“It’s a very straightforward procedure, it’s as easy as taking a photo.”
Since SMUTI allows workers to directly monitor the compressive strength development of sprayed concrete while remaining at a safe distance, it provides a vast improvement on traditional tests that are local and rely upon a small test panel, which may not be representative since the panel and the lining may have a very different temperature history due to the different size, time of spraying and environmental conditions.
This is a step-change in safety and quality control of shotcrete tunnelling. “When performing a local test on a panel, it’s basically assumed that the whole lining is behaving in the same way as the panel, but the SMUTI method gives you much more assurance that the whole lining is doing what it is expected to be doing. It could also flag up issues that you wouldn’t know otherwise,” says Jones.
THE TRIAL BEGINS
A Costain and Laing O’Rourke joint venture, with technical support by London Bridge Associates, has recently trialled SMUTI to test the strength of sprayed concrete at its Bond Street Station Upgrade (BSSU) project, on behalf of London Underground. The project is the first in the world to test the new technique in a production environment.
The BSSU project is a GBP 302M, seven-year project which will open in 2017. Bond Street Tube station is one of the busiest stations on the Underground network; there are over 173,000 passengers that use the station every day and numbers will increase to over 225,000 when Crossrail arrives in 2019.
Bond Street is set to benefit from a number of improvements, including: a new entrance on Marylebone Lane, on the north side of Oxford Street; new escalators serving the Jubilee line; lifts to provide step-free access from street to all platforms; 30 per cent more capacity, plus more passageways to reduce journey times; and improved interchange between the Central and Jubilee lines. Earlier this year, following extensive laboratory testing, production trials began on-site at BSSU. First, the concrete in 12 sprayed concrete panels was tested simultaneously using the established in-situ tests and SMUTI.
This provided sufficient data to carry out trials in a production environment and in March, BSSU implemented the SMUTI system alongside existing test methods for the first time.
“We were already concerned that we could do more as an industry to make the sprayed concrete process even safer and it just so happens that SMUTI came at the right time for us,” says Davies. “We were carrying out sprayed concrete work at Bond Street when Benoit Jones was available to come and demonstrate the technology.”
Jones adds: “From my point of view, I was invited to come and talk to the guys on the site here and we had several conversations early on with the whole site team – Costain Laing O'Rourke and London Underground.
“It was important to me to be able to work with people who understand how technology needs to be developed through using it.
“In a sense this is what we could call a Beta trial; before this job started we knew SMUTI could work, but we needed to test the reliability and how easy it is for the shift engineers on site to actually do it and input the data into the software. “To work with people who understand that and who work with you to help improve the system, well that was a big attraction for me.”
In order to develop and improve the method, Jones says that on site trials and research are vital. The BSSU project also highlighted areas that need further development. “There’s definitely work needed to improve the software interface. Currently reports are generated as a PDF, so it will be beneficial to also be able to download to excel or data files, that you can then manipulate the data better in excel or other spreadsheet packages.
So there are things that we can develop from now on. In terms of the reliability of the data we are still in the process of churning the numbers on that”
