A stone aqueduct is a channel faced with flat stones bedded in cement mortar mixed 1 part cement to 3 parts sand, with the gaps filled and the surface smoothed. It loses only 2-5% of its water, lasts 20-50 years and suits permanent drinking-water supply, and the finish matters: at a 0.5% slope the same section runs at about 0.86 m/s with smooth mortar and about 0.58 m/s with a rough face.
Part of Aqueducts and Channels
A stone aqueduct is a durable channel built of stones set in mortar. It is slower to build than a ditch but loses little water and can last for decades with maintenance.
Mortar Joints Hold The Water In, And The Stones Hold The Mortar
The aqueducts page lists stone-and-mortar lining at 20-50 years of life, moderate cost, 2-5% seepage, and the best use as a permanent drinking-water supply. It describes the method: lay flat stones on the bed and sides, fill the gaps and cover them with cement mortar (1:3 cement:sand), and smooth the surface. A page elsewhere (Building Gravity-Fed Aqueducts) describes lime mortar giving similar seepage and a much longer life of 50-200 years or more; the lime route suits where cement is scarce.
The stones do not make the channel waterproof; the mortar does. A stone lining with open joints leaks like a dry wall.
What You Need
- Flat stones, sound and not crumbling, roughly matched in thickness
- Cement and clean sand for mortar, mixed 1:3 by volume
- A prepared trench at the right grade, with a firm compacted base
- Trowels, a straightedge and a template of the channel section
- Water for curing (Curing)
Method: Bed, Fill, Smooth, Cure
Step 1: Prepare the trench to the section and grade, compacting the base.
Step 2: Lay the stones in a bed of mortar on the floor, then up the sides, flat faces inward, and press each one in.
Step 3: Fill the gaps between stones with mortar, packing it in so there are no voids.
Step 4: Smooth the surface with a trowel along the flow, flush with the stone faces.
Step 5: Cure damp for several days and keep the work shaded.
Step 6: Test with a slow flow of water, and repoint leaks.
Worked Example
Speed depends on the roughness of the surface. For the 30 cm bottom, 1:1 sides, 15 cm depth and 0.5% slope, Manning’s equation with the roughness of cemented rubble masonry (0.017-0.030; Chow 1959) gives:
- Smooth mortar, n = 0.017: V = 0.86 m/s, Q = 0.0675 m² × 0.86 = 58 L/s
- Typical, n = 0.025: V = 0.58 m/s, Q = 39 L/s
- Rough, n = 0.030: V = 0.48 m/s, Q = 33 L/s
For a 5 L/s intake, seepage at 2-5% loses 5 × 0.02 = 0.10 to 5 × 0.05 = 0.25 L/s, delivering 4.75-4.9 L/s, against 3-4 L/s from an unlined channel.
Diagnosing Problems
| Symptom | Likely cause |
|---|---|
| Wet patch beside the channel | Open joint or cracked mortar; repoint |
| Cracks across the channel | Settlement or a missing movement joint |
| Stones loose | Mortar failed or stone undermined |
| Frost damage | Water in the joints freezing |
| Water running slowly | Rough finish, silt or weeds |
Common Mistakes
- Soft ground under the base. The channel settles and cracks.
- Voids in the joints. They leak and freeze.
- Crumbling stones. They break up under frost and flow.
- No curing. The mortar is weak and porous.
Safety
Mortar and heavy stone
Wet cement burns skin and eyes; wear gloves and eye protection. Lift stones with the legs and keep fingers out of the gaps when placing them.
What’s Next
- Learn next: Lining Materials — how stone compares with the other options
- Combine with: Covered Aqueducts — roofing a stone channel