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Several years ago the engineers at Versa-Lok flip-flopped on this issue - a derivative of what you're talking about here, they encouraged everyone to wrap the drain tile and stone in an envelope of fabric, the idea being to keep the clay out of the drain. One year they started telling folks to not wrap the stone in fabric as the fabric would clog and create a hydrostatic pressure issue at the fabric. Then a year or two later, they were back to recommending the fabric.
My take on it is that there must be something to the possible issue of clay clogging up fabric, whether it be around the clear stone or around the drain tile itself (the sock), but that there just isn't a perfect solution.
As far as what to take back to your client - tell him it's unlikely that any significant portions of the sock would clog, enough to prevent any drainage. However, without a sock, those same particles will get into the pipe and clog that instead. Let him pick his poison.
That clay silt won't disappear if the sock isn't there.
For your purposes, you might even consider doing the envelope of fabric around the drainage stone, to address this potential issue and your client's (and his advisor's) concerns.
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