Encasing a soil pipe

[Rogeranerley]

We've just finished a garage extension and I've excavated our 100 year old 6 inch clay soil pipe and now need to infill with 150 mm of concrete as the pipe runs under the garage approach.

We're virtually on sandstone bed rock with only several inches of clearance below the pipe. To ensure it's securely encased, it's been suggested that I first lay a dry mix up to half the pipe's diameter and leave to cure before pouring wet concrete on top.

Is this sound advice, and what should be the mix of cement, sharp sand and balast?

Thanks

 

[theprinceofdarkness]

If you mean a mix without any water in it, then it will be weaker then a wet mix as the sand will have air voids in it. I would just use a weak mix (1:6) of wet concrete. Its only got to be stronger then the soil. One possible problem could be ground water building up behind this "dam" across the sandstone, Perhaps the dry mix is to allow the ground water to percolate through it. What ever method you use I would lay some transverse pipes(could be cardboard tubes) across the sandstone so water could percolate from side to side of this concrete block.
Frank

 

[Rogeranerley]

Thanks for your advice.

I've some small diameter clay pipes of similar age of apparent good strength which we took out of a sock away some years back which look to be the answer. Roughly how far apart should these transverse pipes be set?

As to the mix, what ratio of ballast should I add to the sharp sand, or is the sand sufficient? And will this sand-only mix suffice for the whole trench infill?

 

[theprinceofdarkness]

1:1 1/2 :6, cement, ballast, sand. A sand only mix I expect would do , just depends on how heavy the traffic will be, hard to say for me, I always over engineer stuff. Its easier then trying to re-do a job later on. Pipes 3'?, 4'?, 5'?, apart? Sort of enough to stop a solid wall of water building up the depth of your concrete (12"+?).
Frank

 

[Rogeranerley]

Just been speaking to my BC officer about transverse piping. It's not something he's any experience of and suggested checking its use elsewhere.
Grateful, therefore, if you could advise of your or others experience of this