PDB Debrief: Under Slab Location
Last month we looked at placing underslab conduit and piping correctly to ensure new installations are properly located. This month, we will look at locating existing conduit and piping in slabs to avoid conflicts and damage when renovating an existing building.
Issue: Locating existing underslab conduit.
Start with a meticulous visual examination of the space to determine the likelihood and possible routing of underslab utilities. Are there conduit stub ups, floor boxes, or other indicators of in slab electrical? How about other utilites like an
underground water line stub? Research the probable routing to the stub up. Oftentimes, embedded items will create a weakness that allows the slab to crack along the route of the line.
Review record drawings (and we know these are not always reflective of actual conditions as most designers will show these diagrammatically only. It will be a special set of plans that actually documents these close to their installed locations). Ground Penetrating Radar or Electromagnetic imaging to “X-Ray” the slab.
Core or cut the slab just slightly less than the actual thickness. This can be checked with a small pilot hole or after the first cut. An experienced craftsman will know when the slab has been breached as the lubrication water will empty from the hole immediately. Many conduits are originally placed at the bottom of the slab in a hurried rough-in, while cutting short might avoid damaging them.
Of course, on elevated slabs always verify the presence of pipes and conduits exposed on the bottom of the slab and transfer these to the top of slab.
Deenergize circuits and utilities in an area to create a safeworking environment. You'll know when you remove the concrete if something has been damaged.
Finally - NEVER cut into a pre-stressed or post-tensioned slab without a full investigation of the tendon locations and layout. Know the building structure before proceeding.