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Hi Hilliard,
I am involved in a very similar project (T=2.5)in a sensitive area. I made enquiries as I was going to tackle the solution the exact same way you were i.e. Secondary wwtu followed by a sand polishing filter.
I engaged the services of Dr Robert Meehan in relation to a geological matter on the site and this was the recommendation in relation to the low t-value issue for the site in question. i.e. no water table and no bedrock @2.3m b.g.l
Detailed hydro geological investigations into the subsoil on the site show that the material is
dominated by GRAVEL, which is either sandy and silty, or sandy, silty and clayey. The material is
unmottled illustrating adequate permeability and no seasonal saturation. This is corroborated by
the absence of water ingress into the pits on-site, and the abesence of subsurface drains and
wetland indicators in the general area. The material was found to have very low ‘T’ value
percolation rates, as seen in the tests of Mr. Tim Clifford.
• This material found under the site is therefore suitable for discharge to ground. As the material is GRAVEL dominated, site improvement works by machine tracking and compaction of the subsoil
before emplacement of the polishing filter should rise the percolation values above 3. This is
contingent on re-testing of this subsoil at the time, however.
• If the re-tested, compacted subsoil is not found to have ‘higher’ values, between 3 and 75, such
material should be imported into the site and also tested.
• This should mean that treated wastewater is adequately treated before it meets the bedrock
interface and issues out at the springs and seeps at the base of the subsoil cliffs below the site
In summary (my opinion): No imported sand was required as there was sufficient subsoil that needs to be compacted in layers and retested to ensure the 900mm unsaturated subsoil had the <3 T-value <75 . (CoP Annex F Page 96). Once this has been achieved install the pressurised system for even distribution and that should do the trick.
Extra Ramblings :)
I don't believe you need to do the P-test unless you can't achieve the 900mm of subsoil due to bedrock or water table. If you have high rock/ WT and conduct a P test and achieve a P>3 & < 75 then you need to base the size of the SPF on this value. If however you still don't have the required amount of subsoil that includes the P layer and you import subsoil with a T-value 3-30 then I would design the SPF on the higher of the P value or imported subsoil.
I hope it helps.
tim |
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