Ground, levels and drainage

The trial hole comes first

Before a price means anything we want a hole in the ground. A trial hole is dug where the foundations will run, taken down past the level the drawing assumes, and looked at. It shows the sequence of strata, where firm undisturbed ground begins, whether water stands in the hole, and whether anything has been dug and refilled there before.

Edinburgh plots are not uniform and the surprises are not exotic. Old field drains cross gardens that have been gardens for a century. Made ground sits where a wall or an outbuilding came down. Tree roots reach further than the canopy suggests. Any one of those changes the depth of the dig, and the depth of the dig changes the concrete, the muck away and the time on the ground.

The datum, and the bench mark it sits on

Every level on the job is measured from one point, and that point has to be something that will not move. A temporary bench mark is fixed at the start, usually on a manhole cover, a kerb or a wall that is staying, and it becomes the site datum. Everything afterwards is a figure above or below it.

Working from one datum is what keeps a house consistent. Finished floor level, external ground level, the invert at each chamber and the formation at the bottom of the dig are all read against the same mark, so a mistake shows up as a figure that does not add rather than as a step in a floor.

The arithmetic itself is a falling subtraction, and the calculator on the front page does it line by line: finished floor level, less each layer of the oversite build-up in turn, down to formation. The difference between the top figure and the bottom figure is the depth of the dig.

What the ground gives

Foundations are poured against firm, undisturbed formation and nothing else. Where the trench reaches that at the depth drawn, the job proceeds as designed. Where it does not, the trench goes deeper until it does, and the extra concrete is real work that was not in anyone's first figure.

Soft spots are dug out rather than built over. Made ground is taken down to what lies beneath it. Where roots run through the line of a foundation, the engineer is asked what to do about them before the trench is filled, because the answer is sometimes a different foundation rather than a deeper one.

Building standards see the trench open, at the depth the ground actually gave, before any concrete is placed. That visit is the record of what was found down there once it has been covered.

Formation, and the layers that go back on top of it

Formation is the surface the building stands on, and it is the one surface nobody ever sees again. Once it is accepted, the oversite is rebuilt upwards in layers: sub-base compacted in runs, sand blinding over it, insulation laid tight and unbroken, the slab, the screed, and the floor finish last. Each layer is levelled from the datum and each one is checked before the next one covers it.

The order matters more than any single thickness. Insulation laid on an unblinded sub-base is punctured by the stone under it. A slab poured on insulation that has been walked out of line carries that line for the life of the house. There is no way back to any of it once the screed is down, which is why the checks happen as the layers go in rather than at the end of the day.

Anything that has to pass under the slab goes in first, is tested first, and is recorded first. That means drainage, any service duct and any radon or ground-gas measure the warrant calls for. The slab closes over all of it at once.

The foul run

Foul drainage is set out backwards from the connection. The level we have to meet at the boundary is fixed, so the invert at the house follows from the fall available between the two, not from a figure anybody chose.

Pipes are bedded and surrounded in granular material so the run is supported along its whole length. Inspection chambers go in at every junction and every change of direction, so the system can be rodded from the surface later.

Surface water

Surface water stays separate from foul from the first length of pipe to the last, and the two systems are never brought together anywhere on the plot. Gutters, downpipes and gullies feed the surface system; the house feeds the foul one.

Where surface water cannot go to a sewer, it goes to ground on the plot, and where it goes is settled by what the trial hole showed rather than by what would be convenient.

Soakaways

A soakaway is sized and sited to the ground found in the trial hole. Clay that holds water will not take a roof's worth of rain, and putting a soakaway into it makes a pond rather than a drain.

Its position is measured from the house and recorded for the completion file, because the next person who digs in that garden has no other way of knowing it is there. A copy of what the trial hole showed goes out from works@livingstonpark.live with the completion paperwork.