2 Groundwork, pipes and fill
Getting the ground right around and under everything else. Fill has to compact, so it needs fines. Frost protection must not hold water, so it needs few.
- 2.1Pipe and duct beddingBed and fill around drains, ducts and water pipes. 2–8 mm gravel or 0–4 mm sand, 100 mm under and 150 mm over the pipe.
- 2.2Backfill against foundations and wallsFilling the dig-out next to a wall. 0–32 mm crushed in layers, with 8–16 mm drainage stone against the wall.
- 2.3Structural fillRaising the ground under a slab or paving. 0–32 or 0–63 mm crushed, compacted in thin layers.
- 2.4Frost protection layerSand or gravel that does not lift in frost. Under 3 % finer than 0.02 mm, down to the local frost depth.
The layers in this chapter thickness after compaction
| Layer | Size | Thickness | Compacted |
|---|---|---|---|
| Pipe bed and surround (2.1) | 2–8 or 0–4 | 100 mm under, 150 mm over | by hand |
| Backfill (2.2) | 0–32 | 150–300 mm layers | plate, by hand near walls |
| Structural fill (2.3) | 0–32 or 0–63 | 100–200 mm layers | 95 % Proctor |
| Frost protection (2.4) | 0–8 to 0–32 | to frost depth | as 2.3 |
| Leveling pad (9.1) | 0–32 | 150 mm | 95 % Proctor |
Compacted crushed stone weighs about 2.1 t per m³ in place, in the middle of the 2.0–2.3 t/m³ that FHWA gives for natural aggregate. Every page cites its sources at the bottom; all are standards, public bodies or universities.
Why fines matter
Fill must compact without voids, so it needs fines to fill the gaps between the stones. Frost protection needs the opposite. Frost lifts the ground when water rises through fine soil and freezes into ice lenses, so a frost layer needs few fines: under 3 % finer than 0.02 mm (US Army TM 5-852-3). Clean stone without fines drains but does not lock together, so it belongs next to walls and pipes, not under loads.
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