Hearting: the part nobody sees and the reason walls fall
When I took down the collapsed stretch last spring I found out why it had fallen, and it was not the faces. The face stones were decent and some of them were beautifully fitted. The middle of the wall, the hearting, was soil, small gravel and roots, and in two places I could push a steel bar a full 300 millimetres into the core without meeting stone. Rain had washed the fines out year after year, frost had lifted what was left, and eventually the two faces had nothing between them to lean on and simply folded inwards.
Hearting is the part nobody sees, so it gets rushed, and I understand the temptation: after an hour of fitting face stones the hearting feels like filling a hole. The old wall had almost certainly been built properly and then repaired by someone who tipped a bucket of whatever was handy into the gap. It held for a while. It always does, for a while.
The way I do it now, taught to me on a course and confirmed by that bar in the soil:
- Hearting is stone, nothing but stone, fist-sized down to the size of an egg. No soil, no sand, no gravel fine enough to wash.
- Every piece is placed by hand, not shovelled or tipped, and wedged so that it cannot move. I push each one in until it stops and then tap it with the hammer.
- The hearting comes up level with each course of face stones before the next course goes on, so the face stones always sit on solid stone behind as well as in front.
- Small wedges, pinnings, go into any gap behind a face stone that rocks, from the inside of the wall, never from the face.
It is slow. On the three-metre stretch I estimate a third of the building time went into hearting that no one will ever see. But a well-hearted wall is a single solid thing, and when I lean on the finished stretch nothing shifts anywhere.
The other two faults I found in the fallen section were on the faces: a running joint, where a vertical joint continued through three courses and made a natural crack, and several traces, which are stones laid lengthways along the face so they look impressive and tie nothing together. A trace has its long axis in the wrong direction; the strength of a wall comes from stones that reach inwards. Both faults are things I was doing myself in my first year, and both are tempting precisely because they make the face look good quickly.