There was no problem at all with heavy muddy boots tramping over the steel fabric and insulation whilst laying the pipes.  The fabric was stood on 50mm chairs, the pipes were clipped to it with cable ties, and the insulation underneath is 300mm deep EPS.  The slab is 100mm deep, so the pipes are just above the centre of the slab, which in practice works very well indeed.  We still get around 8% heat loss from the UFH to the ground through the 300mm insulation, but that's pretty typical for UFH;