I don't pile the brash to make a dead hedge. I find it makes a stronger hedge to build one by feeding the stems in.
I first of all pile all the cut stems with tails together, in separate piles small enough to pick up by the tails and drag to the next section of hedge. I say this because some helpers just pile branches in a great heap and make them difficult to move. Each cut stem has the lower 2 or 3 foot cleaned of twigs and side branches. Start by laying stems on the ground and then feeding the next on top by pushing the clear stem through to the ground. All these are done at a low angle and it doesn't take long to make a long line. You can then feed from the other end by laying the stems in the opposite direction, again push the clear 'tail' through all the other stems to the ground.
I have built a dead hedge to any height required very quickly and because the stems aren't just piled up but are knitted together it makes for a narrow yet strong barrier. Brambles quickly grow through and make it impenetrable. I have some done 20 years ago and all I do is add to it each year to compensate for the ones that have rotted. I do this with all coppiced stems as the woodland appears 'looked after' and mini dead hedges can make compartments and micro-climates which trap fallen leaves. Birds love them and before long they have deposited seeds and you have young trees appearing.