Specifications for pressure testing in-tunnel air locks and bulkheads

Specifications for pressure testing
in-tunnel air locks and bulkheads Feb 2009

The performance criteria, design and specification testing of in-tunnel airlocks, bulkheads and all pressure resisting components of closed face TBMs and their resulting tunnels have come firmly into the spotlight of late. Several TBM projects are pushing at the boundaries of hydrostatic pressures against which a TBM and tunnel lining system can be expected or required to withstand. The slurry TBMs on the Brightwater Central Tunnel contract in King County north of Seattle, Washington, are operating at pressures of up to 4.4 bar with pressures of 7.5 bar predicted and therefore setting the design criteria.

Before that, the TBMs that completed the East and West drives of the Arrowhead water tunnels project in Southern California were required to cope with predicted pressures of up to 30 bar, although the TBMs were not required to withstand this extraordinary pressure in a closed mode. Cycles of extensive forward grouting to control ground water inflows and stabilize ground conditions were implemented to create the environment into which the TBMs could advance successfully and limit ground water outflows from the tunnel headings to permitted limits.

But it is the new Lake Mead water intake tunnel No 3 project in Nevada that is setting the bar currently for pressurized TBM tunneling. The machine on order for the project is required to design test to a potential full hydrostatic head of water in the under water tunnel of 17 bar. No only will this place exceptional demands on the TBMs internal pressure bulkhead equipment and airlocks, it will stretch to the limit the design and operation of hydraulic thrust rams against such tremendous face pressures; the operating seals of the machine, not only its main bearing seal but also its vital tail seal and the sealing gaskets of the precast concrete lining.

TunnelTalk is working on an article to investigate the development of the TBM and the lining systems being developed for the project's design-build procurement.

In the meantime, it is reported that factory testing of the EPBM’s integrated airlocks was partly to blame for the delayed delivery of the TBM to the West Tunnel contract on the Brightwater conveyance project north of Seattle, USA. These vessels were required to meet rigorous pressure loss verses time specifications before the machine could be accepted for delivery.

Further investigation of the issue resulted in the following feedback from
Dr Donald Lamont C.Eng., FICE, Head of Tunnel and Ground Engineering, Civil Engineering Specialist Team, British HSE – Health & Safety Executive.

"British Standard BS 6164 Clause 11.1.2 refers to BS PD 5500, which includes testing requirements as follows:

BS 6164
11.1.2 Maximum working pressure
The pressure used in design should be based on the maximum head of water likely to be encountered.

An initial estimate should be made of the pressure needed at the lowest point of excavation, allowing for any surcharge above free water level due to flow from higher ground and the effects of tidal conditions.

In exceptional circumstances, sensitive clays can act as a liquid and the pressure to be resisted is a multiple of the bulk density of the clay rather than that of the water. To allow for errors and variations in the functioning of equipment and for errors in the initial estimate, the estimate should be increased by 10% to establish the “maximum working pressure” for the purposes of design. This is the “maximum working pressure” for structures such as bulkheads and pressure equipment such as airlocks. Pressure equipment manufactured in accordance with The Pressure Equipment Regulations 1999 [22] and British Standards for pressure vessels (typically PD 5500) is subject to hydraulic testing. Structures, to which a hydraulic test cannot be applied, such as bulkheads, should be designed in accordance with BS 449-2 or the appropriate part of BS 5950 (or equivalent) using an “imposed load” equal to the initial estimate increased by 50%. The load factor required by limit state design codes is additional to this increment.

European Union standard EN 12110, Clause 6.1 requires that “air locks and pressure bulkheads designed to 1.1 times maximum working pressure shall be capable of withstanding a hydraulic pressure test to 1.5 times maximum working pressure. The results of the test shall be part of the technical documentation. Otherwise for airlocks and pressure bulkheads designed for 1.5 times maximum working pressure, verification shall be by calculation." No leakage rate is given.

Standard L96 repeats the EN 12110 requirements.

I have always accepted that in-tunnel locks leak and that what is required is sufficient compressor capacity on site to maintain the required pressure by balancing air loss against air supply. Boiler locks would not pass a hydraulic test if they leaked to any significant extent.

There is suggestion that appropriate requirement be incorporated in current revision of L96 and EN 12110.”

We shall continue the discussion as comments are received.

References

Brightwater under pressure - TunnelTalk, Jan 2009
Final breakthrough for Arrowhead - TunnelTalk, Aug 2008
New Lake Mead intake for Las Vegas - TunnelTalk, Sep 2006

Join the discussion