Vip-polymers has specialised in pipe seals for potable and wastewater since its formation in 1923 and first entered the tunnelling market in the mid- 1980s.
Younger engineers, used to the precision-placement of segments capable with modern machinery and expertise, may have baulked at the realities of the pre-gasket world. The rule of the day was ‘just plug it’ with ropes, tar – by any means necessary.
As a major manufacturer of Tunnel Segment Gaskets up until the mid 2000s VIP took a strategic decision to withdraw for a period of time before re-entering the market in 2012
Back to Tunnelling
Having lost none of its operational and technical staff the company focused on its pipe industry work away from tunnelling.
“With the passage of time the reason we took the strategic decision to pull away from tunnels had changed,” says Steve Casey who has been with VIP for 31 years and sales and technical director for the past 22 years. “So we decided to take a look back at what was happening in tunnelling. And there was a problem that no one seemed to be openly talking about: segments fitted with Cast-in gaskets were cracking with regular occurrence.”
“Cracking of segments can be caused by many reasons but one was identified by VIP as a result of solid corners in the segment gaskets,” says Casey who delayed their planned re-entry to tunnelling while they assessed the issue.
“Solid corners are created during the shot joining process, which joins lengths of extruded profile to create angled corners, resulting in rubber filling the gasket voids at these corners. When you have a solid corner, there is only so far you can compress it before you have a block of rubber. Rubber is incompressible in a confined or determined space such as in the segment groove in which the gasket fits. If the volume of rubber in the gasket exceeds this space then you cannot close the segment fully as per the tunnel design. If you cannot close the segment and you are still applying closure pressure, the load will then follow the weakest path and crack through the segments.” The external segment edge is the most vulnerable, whether glue in groove or cast in gasket, as this is prone to micro cracking during concrete casting if thermal gradient and shock are not well managed during rapid curing regimes.
“The importance of thermal control of the concrete curing and early age demoulding damage has long been recognised in segment production, but micro cracking caused by thermal shock only becomes noticeable when the gaskets exert stress on the weakened section. Understanding the complete integrated process is essential to avoid defects that arise from elements other than the gasket itself,” says Casey.
The linear lengths of the gasket compress fine, due to its internal structure of voids, but the shot-joined solid corner will not. This also creates variable and hard spot point loads around the circumferential joint, defeating the design principles of distributed loading which can overstress Steel Fibre Reinforced segments. Installation of packers to open the joint is an expensive ‘band aid’ used to reduce contact pressure passing through the gasket however, this in turn reduces the amount of sealing pressure the gasket is able to withstand; so trying to solve one problem by reducing closure with packers potentially creates another.
Compressible, not soft
In recent years the potential issue with corners has started to become recognised. “I’d like to think”, says Casey, “this is in part due to us highlighting the potential problem and subsequently many specifications for a number of projects in the past few years have required ‘soft corners’ for gaskets.”
Casey goes on to say, “this in my view is a mistaken term. “Hardness is a property of rubber, you can have IRHD (a measure of hardness for rubber) 40, 45, 50, all the way up to 80. Whether you have a malleable ‘soft’ rubber or a higher-rated, intractable one, it is equally incompressible within a given space. You do not need to be an engineer to understand that a solid block of rubber will not compress into a groove if the volume of rubber exceeds the volume of the space into which it is being forced into.
“The specification/term of ‘soft corners’ is an error and is very misleading” in Casey’s view, “the corner must be capable of compressing into the groove. It must be a compressible corner.”
To be compressible the gasket must be designed in such a way to allow it to collapse whilst providing the necessary sealing function.
Readers will have seen cross-sections showing the intricate patterns of voids that are cut through gaskets to allow them to collapse in on themselves and into the segment groove. For glued gaskets this is also the case, but less of a concern. With glued gaskets you can deepen the base of the arches of the gasket corner, for example, to retain the same volume of rubber in the corner as there is in the extruded profile. Voids inside castin gaskets need to be isolated from the concrete during the manufacture of the concrete segment with a solid base to the gasket, otherwise the voids would get filled during the concrete pour.
This is easily done along the length of the gasket and every supplier of gaskets does it. However, it is in the more complex joining work for the corners that the trade secrets are held and the battle lines drawn.
Casey says that a number of companies, but not all, have also recognised the need to remove rubber from the corner joint and have other approaches, albeit, he believes, not to the same success as VIP’s patented joint. His solution was to devise a new method of joining the corners, a cross between the usual methods of shot-joining and a film system that he calls a ‘shot-film system’ which VIP has patented.
“It’s a trade secret and not something I am willing to share publicly,” says Casey. “We found that when we tried to use existing joining methods whilst eliminating solid corners, the join would split. It was the process of developing our new joining system which was a combined effort of our laboratory, quality and product development teams that delayed VIP’s launch into the supply of Cast-in gaskets by almost two years.”
“We needed a joint that would collapse into the groove at the corner, but would have the required tensile and elongation properties for service. Tunnel Segment Gaskets do not undergo excessive stretch if fitted, lubricated and installed correctly, a more critical feature is that the gasket should be able to be compressed without excessive loads while retaining pressure resistance, and this is what our new joining system allows”
Shieldhall and launch to market
During the final phases of gasket development, Shieldhall, the major wastewater project in Glasgow, Scotland, was about to get underway.
The Shieldhall specification was developed following previous project experiences and called for ‘soft corners’.
“As with all specifications a load deflection test was required with one gasket compressed against another. However, we believe that a load deflection on the corners should also be provided to get the true load deflection value which includes the corners and not just on the length of the gasket itself. Although this was not in the specification we did in fact undertake these tests which were independently witnessed”
Additionally, the tunnel lining manager, Sam Simons, engaged by Costain-Vinci JV, required gaskets with corner angles to exactly match the corner angle on the segments and an additional angle on the Z-axis (so the visible face of the gasket slopes with the segment) on the key and adjoining segments.
“This gave an additional complexity to the joining system, but the hard development work had been done so modifying the joining system to provide a Z-axis corner was solved in a very short period of time”, says Casey
“And the rest is history,” he says. “On the back of Shieldhall and witnessed tests at our facility, we won 70 per cent of the multiple Thames Tideway Tunnels and have just been awarded the contract for the Hinkley Nuclear Power station inlet and outlet tunnels.
“As a result of the excellent success witnessed during construction at Shieldhall, Simons was very impressed approached us to represent the company for Tunnel Segment Gaskets in the USA. He now represents us as our North American agent, so do feel free to ask him about the product. We came up with the term together at one of the BTS annual dinners – Compliant Compressible Corners.” “Compliant – because the whole gasket, including corners, must be compliant to the specification”.
The future
Looking ahead there are a few areas that gasket suppliers will obviously be looking at making improvements. Here are just two:
Cast-in is generally becoming a preferable system to gluing with experienced segment manufacturers because it is clean, efficient, accurate and cost effective, but it is still not perfect. Work is ongoing to find alternative ways of clipping the gasket to the segment mould to make them “fool-proof”; as it stands gaskets can inadvertently be improperly clipped to the mould, resulting in an undesirable wave-like fit to the finished segment which leads to inconsistent performance across the seal and is unacceptable. Vigilance from the team in the casting yard is still absolutely necessary.
Rubber is cost-significant to the process. It is readily apparent that a 24mm gasket will be cheaper than a 35mm equivalent. In previous decades, gaskets of up to 50mm were used to ensure pressure resistance was retained in instances where segments were miss-aligned. Tunnelling is at a higher standard than it has ever been, and segment placement is increasingly well-controlled, therefore gaskets can be reduced in size. However, designers will look at other, clever ways to take more rubber out of the product while retaining the specified seal requirements.
Finally, readers wanted to know if VIP has its sights on any other areas of tunnelling, TBM tail seals or other machinery seals. “Not at this time” says Casey. “We are focused on Tunnel Segment Gaskets and further development in that area and we have made significant investments in our manufacturing capability by introducing a new extrusion line along with cutting and joining equipment – however, with design technical input from Sam Simons we are reviewing all parts of the ring build process to develop new and cost effective solutions.
“As an example, we have recently developed a gasket for a project we are currently supplying in Mumbai. The tunnel will be going through an area of particularly difficult ground under a river and the client and contractor wanted a gasket that could be used on the same segment and in the same gasket groove whilst maintaining the same sealing performance. The risk is the possibility of a greater offset between the segments and although the probability of this occurring is very low, the client wanted extra reassurance of the gasket performance.
“We calculated the area and volume around the gasket we had available and designed a “mushroom” shaped gasket that had a larger top surface width whilst locating into the existing gasket groove. With the use of FEA and physical load and pressure testing in the laboratory culminating in a trial on the rings at the tunnel site, we finalised a design that met with the brief we were given. Our major concern was that we didn’t create a block of solid of rubber which in turn would potentially give rise to over-loading and cracked segments, as we have discussed earlier about the corner. The trials on site were very successful and we are now supplying just over 400 ring sets to this design for which we have a patent pending.”
