Last reviewed 1 Oct 2026 · 6 min read
Pipe materials
A sub-surface drain pipe must admit water, carry it to the outlet, resist crushing and chemical attack and last for decades.
| Material | Features | Typical use |
|---|---|---|
| Clay tile (burnt clay pipe) | Short lengths (30 cm) laid with open butt joints through which water enters; durable and inert; heavy and brittle; slow to lay | The traditional material; used for small diameters in stable soil |
| Concrete pipe | Plain concrete pipes with open joints (or perforated); heavy and labour-intensive; vulnerable to sulphate-rich water and acid soils unless sulphate-resistant cement is used | Large collectors and mains |
| Corrugated plastic tubing (PE / PVC) | Light, flexible, supplied in coils of 100–200 m; perforated (slots or holes) all round; laid quickly by trenching machines; flexible joints; now the commonest material | Laterals and collectors (50–200 mm) |
| Smooth-wall PVC pipe | Rigid, with slotted perforations | Collectors and mains, heavy-duty |
| HDPE / PE smooth pipe | Flexible, durable | Large mains, outlets |
| Asbestos-cement, steel, cast iron | Obsolete or special (outlets, crossings, pump discharge) | Special locations |
| Bamboo, brushwood, stone | Traditional, cheap, short-lived | Temporary or local use |
Properties required of plastic drains (summary)
- Stiffness (resistance to deflection under soil and traffic loads) — checked by a pipe stiffness test; ability to withstand installation stresses (low temperature, stretch).
- Perforation area (inlet area) — about 15–60 cm² per metre length (typical 20–50 cm²/m) depending on the pipe, with slots narrower than the filter material.
- Durability — resistance to chemicals in drainage water and soil, UV (before burial) and rodents at the outlet.
- Dimensions and tolerances — nominal diameters 50, 65, 80, 100, 125, 160, 200, 250, 315 mm (corrugated), with specified wall thickness and corrugation depth.
- Hydraulic roughness — Manning's – for corrugated, for smooth PVC.
The function of an envelope
A drain envelope is a layer of permeable material placed around the pipe. It serves to:
- Filter — prevent soil particles from entering the pipe and clogging it (especially fine sand, silt and low-cohesion soils).
- Hydraulic function — provide a permeable zone around the pipe, increasing the effective drain radius and reducing the entrance resistance (head loss at the drain).
- Bedding — give the pipe uniform support and correct alignment.
- Mechanical — improve the stability of the trench, especially where pipes are laid in unstable soils.
An envelope is not needed in stable, cohesive soils (clays with a high clay content) and in soils that carry little fine sand; it is essential in fine sands and silts, which can enter the pipe easily, and in unstable, structureless soils.
Types of envelope materials
| Type | Description | Comment |
|---|---|---|
| Gravel (graded sand-gravel) | A layer, typically 7–10 cm thick, of clean graded gravel (about 2–20 mm) placed round the pipe | The traditional envelope; effective but costly, needs a gravel supply and a laying machine |
| Organic materials | Straw, peat, coir, flax, bark, wood chips, rice husk | Cheap; decompose with time (a short-term filter effect, 5–10 years), so they are used in temporary conditions or in soils that stabilise by themselves |
| Synthetic fibre envelopes (pre-wrapped) | Polypropylene or polyester loose-fibre "sock" (thin or thick) wrapped around the corrugated pipe | The commonest envelope today; thin (about 1–3 mm) or thick (6–10 mm) |
| Geotextile (non-woven) filter fabric | A fabric wrapped around the pipe; can clog if the soil is fine and iron ochre is present | Used in specific soils |
| Natural soil | The soil itself; in cohesive soils no envelope is needed |
Ochre (iron hydroxide) clogging of envelopes and perforations in some iron-rich, anaerobic soils is a known problem; coarse gravel is more tolerant than fine synthetic material.
Design of a gravel envelope (filter criteria)
A gravel envelope must be coarser than the soil so that it does not clog, and finer than the pipe openings so that it does not enter the pipe. The classical filter criteria use the particle-size distribution (the , , etc.) of the soil (base) and the envelope (filter):
- Piping (retention) criterion: — the filter's pores must be small enough to hold the soil particles back.
- Permeability criterion: — the filter must be more permeable than the soil.
- Opening criterion: (about 1.2–2) — the filter particles must be too large to enter the perforations.
- Uniformity: the envelope should be well graded, with few fines ( about 5 % passing a 0.075 mm sieve) and no clay lumps.
(Values are indicative of the common USBR/US Soil Conservation Service practice; local specifications may modify them.)