Anybody have any experience of laying vehicle detection loops under granite slabs?
We need to replace failed loops at a couple of rising bollard systems within a pedestrian zone (public realm, so strictly no slot cutting) and are extremely apprehensive to repeat the previous method of installation – standard loop cable zigzagged around the edges of the slabs at a depth of 10mm with some grout over the top for luck (for the record - I had nothing to do with the original installation)
We are considering the – costlier - option of lifting the slabs and laying pre-formed loops a little below (10mm) the slabs, but this does mean quite a depth (minimum of 80mm) and this is the point where one of you lot shouts NO that’s never going to work or says “yeah, should work, we tried that –not a problem, yet”
Seems our choice is to repeat the earlier flawed but easy installation every year or so when it will undoubtedly fail again (one set began failing within 2 months of use) or, if feasible, go for a more thorough approach - which will be much more expensive and disruptive, but should mean a considerable gap before they will need replacing again.
I suspect it would work as the field from a conventional loop can be quite extensive so there's every chance that the extra depth won't affect it. I do know that some granite is magnetic so whether that would have an impact or not I don't know. Is there anybody there from the detector companies?
Nice idea, but I've had a few chats with various suppliers (Clearview, Telent, AGD, Sensys) regarding video and wireless detection - all goes well until you mention the safety loop that surrounds the bollards to prevent the big chunk of metal coming up under vehicles or pedestrians.
Yeah, should work. We've tried in under granite setts, and preformed concrete blocks with no problems at all. Lay the loop within some 20mm round pvc conduit in the shape of the loop for added protection, this also stops the loop cores creeping. Most slotcutting should be at 70mm anyway so 10mm more shouldn't be a problem. Have used ths for stoplines and SCOOT as well as some access control stuff that I do as well and haven't ever had a problem.
We have installed loops for rising bollards under setts on the High Street in Edinburgh. Looks very neat and works well. I'd imagine the depth is greater than the 80mm you are proposing and we haven't had any problems.
Our rising bollard supplier advises 100mm as the limit however I know we have a site where loops were 'mistakenly' installed in conduit at a greater depth under 100mm granite slabs, its actually been OK for 18 months since installation. We have another site with loops at 80mm in conduit which has also been OK for 12 months. I believe the granite may be chinese.