A gallery trench is a long, deep, covered collecting trench cut along the contour of a hillside to intercept groundwater over a length, like a French drain but dug deeper and longer to reach a thicker water-bearing layer. Its yield scales with its length and with the square of the saturated thickness it cuts, so a 20 m gallery in a sand of assumed permeability 1×10⁻⁴ m/s with 2 m of saturated thickness yields about 5 L/min, and because the cut is deeper than 1.2 m it must be shored: a 2 m deep cut is 24 m³ shored but 144 m³ if sloped.
Part of Spring Capture
A gallery is a horizontal collector, a trench or tunnel with a pipe or open channel in it, that gathers water along its length from the ground on one or both sides. Hillside galleries collect where a spring is spread over a long outcrop or comes from many fissures.
A Longer Collector Takes More Water, Down To The Impermeable Base
The flow to a long drain from the ground on one side follows a standard hydrogeology result (the Dupuit equation for a drain in an unconfined aquifer, recalled from textbooks, not derived here):
q = K × (H² − h²) ÷ (2 × L)
q is the flow per metre length of drain in m³/s per m, K the permeability in m/s, H the saturated thickness of the aquifer above the impermeable base in metres, h the depth of water left in the drain, and L the distance from the drain over which it draws water. It assumes the drain reaches the base and that L is a distance you can only estimate; treat the answer as an order of magnitude.
Worked example (all inputs assumed). K = 1×10⁻⁴ m/s (a sand), H = 2 m, h = 0.1 m, L = 50 m:
q = 1×10⁻⁴ × (2² − 0.1²) ÷ (2 × 50) = 1×10⁻⁴ × 3.99 ÷ 100 = 4.0×10⁻⁶ m³/s per metre = 0.24 L/min per metre
For a gallery 20 m long drawing from one side, 20 × 0.24 = 4.8 L/min, about 5 L/min. Two things to read from the formula: flow is directly proportional to K, which can vary by factors of 100 between soils, and it rises with the square of H. Double the saturated thickness from 2 m to 4 m and the flow goes up by (16 − 0.01) ÷ (4 − 0.01) = 4 times. A deeper gallery that reaches the base of a thicker layer pays much more than a longer shallow one, which is the argument for going beyond the 0.5-1 m of a French drain (French Drain Collection). Measure a pilot trench to replace the assumed K (Seep Springs).
A Deep Cut Is Cheaper Shored Than Sloped
Below 1.2 m a trench needs sloped or shored sides. For wet ground, the slope is 1.5 horizontal to 1 vertical on each side (Excavation gives the source). For a gallery 20 m long, 2 m deep and 0.6 m wide at the bottom:
- Shored, vertical sides: 20 × 0.6 × 2 = 24 m³
- Sloped at 1.5:1: the top width is 0.6 + 2 × (1.5 × 2) = 6.6 m; the cross-section is (0.6 + 6.6) ÷ 2 × 2 = 7.2 m²; the volume is 7.2 × 20 = 144 m³
The sloped cut moves six times as much earth. Shoring with boards and props costs timber and the risk of working in a braced trench, but is the usual choice for a narrow deep cut. Brace as you dig, setting each board before anyone works below it.
Tunnels Are Not Recommended Here
A gallery can be driven into a hillside as a tunnel instead of dug as a trench. This guide does not recommend it for a first collection system: a tunnel is a confined space with collapse and bad-air hazards combined, and it needs timbering, forced ventilation, a harness and line and an attendant for every entry (the protocol in Safety Protocol), with the same rule that a person who collapses is hauled out from outside, never rescued by a second person entering. Cut a covered trench and keep out of the ground.
What You Need
- Timber boards and props — to shore the trench as it deepens
- Collection pipe — perforated, with a graded-filter envelope (Inlet Wall)
- Gravel in graded sizes, clay for the cap
- A sealed chamber at the downhill end (Collection Chambers)
- Inspection points along the run, so the pipe can be rodded and checked
- A level to hold the fall of at least 0.5%
Method: Contour, Cut, Shore, Lay, Cap
Step 1: Survey the line along the contour, across the flow, so the gallery cuts the slope’s seepage.
Step 2: Dig test pits at intervals to find the depth of the impermeable base.
Step 3: Cut from the downhill end uphill, shoring as you go. Keep spoil 0.6 m or more back and downhill.
Step 4: Set the pipe and filter on the base, with a fall of at least 0.5% towards the chamber, and place graded gravel round it.
Step 5: Cap with clay and backfill, mounded to shed surface water.
Step 6: Fence and divert (Fencing, Uphill Diversion).
Common Mistakes
- A gallery that stops above the impermeable base. It misses the lower flow.
- Digging first and shoring later. The face goes before the boards.
- No inspection points. The pipe silts and cannot be cleaned.
- Deep trench left open overnight. Rain brings the wall down.
Safety
Deep wet trenches kill
A cut deeper than 1.2 m needs sloped or shored walls; do not enter an unsupported cut. Keep spoil at least 0.6 m back, work with a partner on the surface, set boards before working under them, and leave the trench if the face cracks or runs. Nobody enters a hillside tunnel without the full confined-space protocol.
What’s Next
- Learn next: Fencing — protecting the collection zone
- Combine with: Uphill Diversion — keeping runoff out of the gallery