According to industry standards the lifespan of major engineering projects tunnels and bridges and tunnels is 120 years. Please see reference from the below report; http://www.aspirebridge.com/magazine/2009Fall/perspective_freyermuth_fall09.pdf
'On an international basis, British Standards have required a minimum service life design of 120 years since 1988. This requirement has also been implemented in other parts of the world where British Standards are used. For purposes of this discussion, the service life period will be considered to be the time period to be achieved “with proper maintenance but without any major repair work.” This would preclude, for example, complete removal and replacement of the bridge deck (or tunnel lining).'
The tunnel at Stonehenge would be built to these standards i.e. the main bores would only be 'guaranteed' until approximately the year 2040 when major reconstruction work would be required. 120 years is only a blip in the Stonehenge story, but one which could leave an indelible mark across the landscape. Surely we can think of something better?
i.e a P&R at Solstice Park and a parallel bridleway/service road to the south and introduction of average speed cameras on the existing A303!