Find a friendly structural engineer to assess the integrity of the bridge and the age/solidity of the sandstone itself?
Then make sure to use glue-in staples, since expansion bolts will fail quickly on sandstone. Ideally, also put in an extra bolt low down, somewhere away from the routes, and get it pull-tested?
And if this is your first time bolting anything, get someone to help who has some experience.
Used long (100mm ish) glue-in bolts. Expanding anchors are not great in sandstone. If anyone asks, it was bolted years ago by person or persons unknown.
there's a limit to what a structural engineer can tell you and even more of a limit on any assurances they can give....
Short of having cores drilled and samples tested - it would be worthwhile to drill a couple of pilot holes as deep as you can: get a feel for the size of the blocks and if the stone is solid (grout or fill with lime(y) mortar) - you can occasionally find (pretty unlikely on a bridge) pitch-faced ashlar facing pined to brick......
you might be able to get some history (pos even drawings) from the local authority; for armature local history research purposes obviously.
you might even find useful stuff in local history books that could tell you about the construction.
The main thing might be to think about the size of the stones you're bolting, and if they could be pulled out in a fall? You couldn't happily place bolts in a brick wall - I have been told.
Cheers, exactly the same time of bridge as the New Mills one so we are pretty hopeful of getting some half decent routes up. Permission, not so hopeful.
In hindsight, Willie, if he were alive today, wouldn't use expanding anchors to bolt at Blantyre Towers. I believe the vast majority of the expanding anchors he used went in fine but I think occasionally he had trouble with them not gripping properly. It remains to be seen how long the current bolts will last before they start to work loose, if they start to take lots of falls at say a crux. I imagine there will need to be an on-going checking and replacement (with glue-ins) program.
The comments in the crag notes about the quality of the bolts used were there to put people off using cheap short mild steel bolts which would only have a very limited life-span. I'll reword that paragraph.
Nice write-up JLS! Have only been to the Towers a couple of times, used the 7a Max guide book, and really enjoyed it! Near enough for after-work (an hour from W Embra depending on traffic), big long routes without giant runouts, some great moves, and the option of a top-out for a lovely view or a photo. And there's a pie shop in Blantyre! A unique venue (can be midgy in summer).
In reply to BrendanO: Cheers. Yeah, the Towers turned out to be a good sport venue. Probably havent yet really received the attention they deserve. A bit too hard for beginers, a bit too easy and esoteric for hardcore climbers. That may change. They've seen a bit of action this spring with a few teams pitting themselves against the challenges and a recent send of an old project at 7c.
In fairness depending on the hardness of the sandstone expansion anchors should work just fine, especially if 12mm and longer than 130mm with double collars. Many of the original bolts in the Angus quarries (all sandstone are expansion and have been in use for nearly 30 years.
Usually the sandstone used for civil projects like retaining walls and viaduct piers is of a high quality though may look worse on the surface through weathering.
If there's no good reason not to though just use glue ins.
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