Thevictoriancobbleco - Backfill

[TheVictorianCobbleCo]

I've been asked to sort out a problem as follows. A 6x3m pool was installed aboveground with a retaining wall 1m high all round, on average around 1.5m from each side of the pool. The walls strong, no cracks or subsidence. The paving around the pool has sunk,on investigation the builder backfilled the cavity with sharp/building sand, no compaction. (about 30 cubic metres.) Short of taking it out and replacing it with MOT 1 equivalent, can I mix the sand with Cement, dampened and compacted every 150mm to top. I've only ever used a 10/1 sand/cement ratio as a weakest mix, can I go weaker - at 10/1 it will require about 100 5okg bags cement.

[GB_Groundworks]

if it is drained properly as in weep holes with geotextile or on a drainage bed then the best way to comapct sand is with water even at that depth, if its no drained properly you just liquify it, but if you can ger water running through it it will set like concrete when dry.

otherwise its geting in there and dig it out and make a cbm and lay it back in layers like you say. where abouts in RSA are you, i spent 3 weeks there for the 97 rugby world cup. my uncle used to live rd the in Houghton in joburg

[TheVictorianCobbleCo]

The weep holes are about 150mm above the bottom, only 2 50mm dia - too high and not enough. No geotextile, no drainage base. If I put water on I think the whole caboodle will float. The Cement based mix, whats the weakest reasonable? 10:1? I'm in Somerset West which is about 50km from Cape Town on the coast. Very nice but my heart belongs in Matfield, Kent. We play Scotland next, but the guys to beat are the New Zealanders!

[lutonlagerlout]

i would say get it designed by an engineer
maybe a raft type of construction would work will but its hard to say
cheers LLL

[seanandruby]

sounds like a case of hydraulic fill. He has relied on the water to compact it . Maybe you could put a filter drain low down to a sump. i would dig out top 200 and placce a concrete raft, with expansion joints rebated top 20ml for a sealant.

[TheVictorianCobbleCo]

The swimming pool is fibreglass, so there is no inner support structure for the raft. So if I can't make a backfill mix of say 20:1 then I may as well remove the sand and then backfill properly with coarse material at the bottom and work up to fines at the top, all properly compacted, then do a 6:1 ration raft/slab 75 - 100mm thick then pave on top of that. Will try to upload a sketch.