Scrapbook Saturday: "A Hole in the Ground"

One of the challenges in my scrapbook (see here for details) is "A Hole in the Ground".

Well, the gas company has been busy recently replacing the old metal gas pipes under our local streets with bright yellow plastic pipes, so this presented me with the ideal opportunity to complete this particular challenge. It's quite fascinating seeing how they do the replacement: mostly they seem to be feeding the plastic pipes though the old ones so as to minimise the digging required. Nevertheless, the job is still causing quite a lot of disturbance - and traffic jams as they need to use single line working with temporary traffic lights on the road.

One thing which amazes me is how they keep the gas supply going to local houses while doing all this! I wonder if the supply comes in at each end of a section of pipe so that they can replace part of it without turning it all off? - a bit like an electrical ring main. And I wonder what the taps are for (as seen close-up in the thumbnail) - ?for bleeding air out of the system? If anyone knows please tell me!