Pressure one-side

ORNL also subjected samples treated by CVI of carbon to 100 MPa stresses in bidirectional bending of plates." These samples were then tested for hermeticity by pressurizing one side wifli 206 kPa of hydrogen and measuring the through-thickness gas leakage rate, and it was found that excellent permeation resistance could be achieved. 

In removing the shields if they have not been squeezed by earth pressure, each section can be lifted out intact by crane and, if needed, placed in a new position. Where they have been locked in by earth pressure, one side plate only is lifted, thus collapsing the shield which then comes clear and is ready for setting up in a new section of trench. Usually the bottom of the trench is back filled up to the lower steel struts before the shield is removed. This type of shield has been used in poor ground in trenches up to 9 ft deep, but is suitable for deeper trenches if the top few feet are battered back. 

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