The inlet wall is the permeable back wall of a spring box, built of dry-stacked stone or gravel and never cemented, backed by at least 30 cm of clean 20-40 mm gravel, and it works only while it stays more permeable than the ground it drains. A clean gravel wall passes a 10 L/min spring with about 6 mm of head loss, but if fines clog it to a hundredth of that permeability the loss rises to about 0.63 m and the spring backs up, so grade the layers from fine at the soil to coarse at the wall.
Part of Spring Capture
The inlet wall is the face of a spring box through which spring water enters. It must pass water freely and hold back soil, and it is the part of a box most likely to fail slowly by clogging.
The Wall Must Stay More Open Than The Ground Behind It
Water crosses a porous layer at a speed set by Darcy’s law: the flux v (flow per unit area) equals the permeability K times the hydraulic gradient i, the head lost per metre of travel. Rearranged, the head lost crossing a layer of thickness L is Δh = v × L ÷ K. A clean gravel layer has a large K, so the loss is tiny; if soil particles migrate into the gravel and clog it, K collapses and the loss rises in proportion.
Worked example. A spring flows at 10 L/min = 1.67×10⁻⁴ m³/s through a wall 1.0 m wide and 0.8 m high, an area of 0.8 m². The flux is v = 1.67×10⁻⁴ ÷ 0.8 = 2.1×10⁻⁴ m/s (0.21 mm/s). Take a gravel layer L = 0.3 m thick, the minimum in the Spring Capture guide. The permeabilities are assumed for illustration; real values for a given gravel vary widely, so measure the loss in the field rather than trust a table.
| Gravel condition | Assumed K (m/s) | Gradient i = v ÷ K | Head lost over 0.3 m |
|---|---|---|---|
| Clean coarse gravel | 1×10⁻² | 0.021 | 6.3 mm |
| Somewhat clogged | 1×10⁻³ | 0.21 | 63 mm |
| Badly clogged | 1×10⁻⁴ | 2.1 | 0.63 m |
A loss of 6 mm costs nothing. A loss of 0.63 m means the water inside the wall must stand 0.63 m higher than the water in the box to push the same flow through, which is as bad as raising the overflow 0.63 m over the eye: the spring is choked or breaks out beside the box. The lesson is that the wall’s permeability must be protected, and the way to protect it is to stop fines getting in.
Grade The Layers So The Soil Cannot Move Into The Gravel
Where the spring ground is fine sand or silt, the 20-40 mm gravel of the guide, laid directly against it, lets the fine soil wash into its large voids and clog them. The cure is a graded filter, the same one used in the collection chambers (geotextile or progressively finer gravel). The classic rule, from Terzaghi’s work on filters (recalled, not from the Spring Capture guide), is that each layer’s 15%-passing grain size should be no more than five times the 85%-passing grain size of the layer it protects:
D15 (filter) ≤ 5 × D85 (protected layer)
and, to keep the filter much more permeable than the layer it protects, D15 of the filter should be at least about four times D15 of the protected layer.
Worked example. Suppose the soil at the eye has D85 = 0.5 mm.
- Layer A against the soil must have D15 ≤ 5 × 0.5 = 2.5 mm. Coarse sand to fine gravel of 1-4 mm (assume D15 about 1.5 mm and D85 about 3.5 mm) qualifies.
- Layer B against layer A must have D15 ≤ 5 × 3.5 = 17.5 mm. A 10-20 mm gravel (assume D15 about 11 mm and D85 about 18 mm) qualifies.
- The guide’s 20-40 mm gravel against layer B needs D15 ≤ 5 × 18 = 90 mm; a D15 of about 22 mm qualifies easily.
So two thin layers (a few cm each) between the soil and the 20-40 mm gravel solve the problem for this soil. The grain sizes above are nominal; sieve or hand-sort your real gravel, or use a geotextile where one can be salvaged, and check by watching the water: it should run clear, and the box should not silt up.
What You Need
- Dry-stacking stone — flat-sided pieces with gaps between them
- Gravel in three sizes — fine, medium and the 20-40 mm backing, washed clean
- A sieve or hardware cloth — to sort the sizes
- A tamping rod and a plumb line
- Clay or compacted earth — for the cap above the gravel (Backfill & Protection)
Method: Stack, Pack, Cap
Step 1: Stack the wall dry, with joints narrower than the smallest gravel (20 mm) so gravel cannot fall into the box. It never gets mortar.
Step 2: Place the filter. From the soil outwards, lay the finest layer against the ground, then the medium, then the 20-40 mm gravel against the wall, each at least a few centimetres thick, and the whole gravel zone at least 30 cm.
Step 3: Build up together. Raise the wall and the gravel in step, so the cut face behind is never left open for long.
Step 4: Keep the permeable zone over the whole emergence area, from the floor up to a little above the highest point at which water comes out. Above the overflow level the wall is solid.
Step 5: Cap the gravel with clay so surface water cannot run into it.
Step 6: Check the clarity of the water after the first rains; if it silts the box, the filter has failed.
Common Mistakes
- Cementing the wall. It blocks the spring.
- Unwashed gravel. Dust clogs the wall at once.
- Coarse gravel straight against fine soil. The soil moves into the gravel and clogs it.
- A wall narrower than the emergence zone. Water that cannot enter finds another way out.
Safety
The cut face behind the wall
The ground behind the wall is an open cut while you build. Do not leave it unsupported deeper than 1.2 m: slope or shore it, keep spoil at least 0.6 m back, and build the wall and the gravel up together so you are not working below an exposed face (Excavation).
What’s Next
- Learn next: Backfill & Protection — capping and protecting the gravel zone
- Combine with: French Drain Collection — the same graded-filter logic in a trench