This weekend on a whim I lifted an inspection chamber in our garden and got a nasty surprise - backed up sewage for what looks like a while. I now own 17 metres of drain rod and have established there are now no blockages in our run - or at least I hit something very solid when rodding towards the mains which I think must be the opposite sewer wall.

However, on the surface of the water are some polystyrene beads that clearly got in when we were building. Since we've been in now for almost 2 years this seemed odd. What I've noticed is that when rodding they follow the rod but get sucked back when the rod is removed and a small amount of water (with beads floating on) never leaves the bottom of the inspection chamber - implying the base of the chamber isn't quite high enough to encourage positive flow and drain fully. I'm worried this means we might get more 'backlog' in the future.

What I want to know is what a decent survey company could actually establish in the circumstances?

We know our invert level must be higher than the mains junction as there isn't 2 years worth of waste in there and water levels do drop overnight (plus the mains connection was so deep they had to shore up the hole). However we are in heavy clay and the final stretch was moled due to (protected) tree roots. My fear is that a camber or dip may have developed in the moled section that allows liquids to pass but nothing that floats.

If I'm right, a conventional laser survey will just tell us the invert levels at each end and a CCTV survey would just establish that the pipe itself is empty.

Does anyone offer mapping using rodded gyroscopes or similar to plot the 'flatness' of pipe along it's length?

I doubt geophys is accurate enough to establish relative depths to the required precision either.

Aside from hand digging the 12-ish metres of expensively moled pipe up again, what are my options?

djh, thanks for the tip about insurance - it hadn't occurred to me that it might be covered.

I know roughly where the drain is, but there's also a water supply pipe in the same area but at shallower depth so not sure I'd be able to determine the difference.

What I was told about moling is that certain obstacles can cause the mole to glance off so you don't get always get a completely straight hole/pipe. We know the two ends are in the right place but there's no guarantee the route isn't banana shaped.

The problem I'm seeing is that the main inspection chamber doesn't clear completely - water leaves and then some of the same water seems to flow back along the pipe (or at least the stuff floating on top comes back). This implies the pipe isn't pointing purely downhill, which seems a bad idea for a waste pipe.

My dad has just reminded me that when the builders were pouring the ground slab they stopped up all pipe connections with spare offcuts of polystyrene from the insulated formwork. New theory is that a lump of polystyrene may have been lodged somewhere in the run and might have been breaking up gradually - hence the beads.

I'm now going to wait it out for a week and see what's changed by next weekend....