Backfilling a french drain behind a retaining wall

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Hello - I've just built a retaining wall in my sloping back garden which will be backfilled to create a level platform for a summerhouse, and am now installing a french drain along the back of the wall. The wall is about 1m high and the subsoil grades back from the footing of the inside of the wall at about 45 degrees, and I'm struggling to backfill the french drain. It's being done with a 20mm pea gravel, burrito wrapped in non-woven fabric membrane, ideally to a width of about 12 inches, and virtually to the full height of the wall. Does anyone have any tips on how to fill the inside of the membrane with pea gravel and the outside of the membrane with subsoil/crusher run such that the pea gravel maintains about 12" width to the top of the wall?. As I pour in the pea gravel, it rolls back against the 45 degree slope and the drain gets wider and wider. If I pour in subsoil first, it pushes the membrane back towards the back of the wall. and narrows the width of the drain. I did wonder about some shuttering, but then I'm not sure if I could get it out again afterwards. I don't really want to fill the entire space behind the wall with pea gravel as it would be over a metre wide by the time I reach the top of the wall. Hope this explanation makes sense and that someone has a nifty trick to get round this problem..... At the moment I'm adding a bit of gravel to one side of the membrane, then a bit of subsoil to the other side, then pulling and pushing the membrane, gravel and subsoil by hand until they end up in about the right place and then moving along and up the wall, and it's taking forever.
 
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You could shutter in upward stages. If you used something like a scaffold plank, fill, tamp the subsoil just enough to hold form to just below the top of board,you may have to damp it , then remove board carefully and fill gravel the opposite side...repeat as necessary.
 
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Thanks lostinthelight - tried it today with a couple of 4 x 2 stacked on top of one another (didn't have a scaffold board handy) and it worked far better than my previous attempts. Still a bit of a scramble, but obviously it should get easier as the filling continues upwards and the angle decreases.
 

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