GRW/Drainage/TBT-GRW-018

Land Drain and French Drain Installation

Groundworks & Earthworks › Drainage › Land Drain and French Drain Installation

Presenter		Project
Location		Date

Land drains and French drains collect and divert groundwater to prevent waterlogging and foundation damage.
Installation involves trench excavation, laying perforated pipes, surrounding with granular fill, and backfilling.
Trench depths for land drains vary from 600mm to over 1.5 metres depending on the design requirements.
Trenches deeper than 1.2 metres require support or battering to prevent collapse.
Granular fill materials such as gravel and clean stone create manual handling demands during placement.
Existing buried services must be located before excavation — drainage routes often cross service corridors.
Waterlogged ground conditions during installation make the trench sides less stable and increase collapse risk.
Geotextile wrapping around the drainage aggregate prevents soil migration and long-term clogging.
Connections to outfall points, soakaways, or existing drainage require permits where discharging to watercourses.
Incorrect falls, levels, or connections result in drainage failure that is expensive to rectify after backfilling.

Trench collapse	Even shallow drainage trenches in soft waterlogged ground can collapse without warning, burying workers.
Buried services	Drainage trenches cross paths with gas, electricity, water, and telecoms services that must be avoided.
Manual handling	Handling bags of aggregate and positioning drainage pipe in confined trenches causes back and shoulder injuries.

Don't

Locate all buried services using CAT, Genny, and service plans before excavating
Support or batter trench sides for any excavation deeper than 1.2 metres
Inspect the trench at the start of each shift and after any rainfall event
Use mechanical aids to lower pipes and distribute aggregate in the trench
Check drainage falls and levels with a laser or spirit level during installation
Wrap drainage aggregate in geotextile to prevent soil clogging as specified
Obtain discharge consent where drains connect to watercourses or the public system
Provide safe access and egress from the trench at regular intervals
Dewater the trench if groundwater ingress makes the working area unsafe
Record the as-built drain location and depth for future reference

DON'T enter unsupported trenches deeper than 1.2 metres under any circumstances
DON'T excavate without checking for buried services along the drainage route
DON'T work in a trench showing signs of wall cracking, bulging, or water seepage
DON'T carry heavy bags of aggregate down into the trench manually if avoidable
DON'T connect to watercourses or public drains without the required discharge consent
DON'T backfill drainage trenches until the pipe level and falls are verified
DON'T omit geotextile wrapping if specified — it prevents long-term drain clogging
DON'T stockpile spoil at the trench edge where it loads the excavation sides
DON'T leave open drainage trenches unguarded when the workforce is not present
DON'T skip the as-built survey — buried drainage is impossible to locate later without it

See also: Below Ground Drainage Installation | Trench Collapse Prevention

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