Subsurface Drainage: French Drains, Curtain Drains, and Foundation Drainage

Not all drainage problems happen on the surface. Saturated soils, high water tables, and groundwater seepage cause wet basements, soggy yards, frost heave, and foundation damage. Subsurface drainage systems intercept and redirect water below the ground surface, keeping soil drained and structures dry.

How Subsurface Drainage Works

Subsurface drains work by providing an easy path for water to flow through soil. Water moves through soil toward any region of lower pressure — and a gravel-filled trench with a perforated pipe creates exactly that. The gravel provides large void spaces (high permeability), and the pipe collects and conveys the drained water to a discharge point.

The fundamental equation governing water flow through soil is Darcy’s Law:

Where:

• Q = flow rate through the soil
• K = hydraulic conductivity (a property of the soil)
• i = hydraulic gradient (head difference / distance)
• A = cross-sectional area of flow

Soils with high K values (sands, gravels) drain quickly and may not need subsurface drainage. Soils with low K values (silts, clays) hold water and often benefit from drainage systems.

Types of Subsurface Drains

French Drains

A French drain is a trench filled with gravel (aggregate) containing a perforated pipe at the bottom. It intercepts shallow groundwater and surface water that has infiltrated into the soil.

Typical construction:

• Trench width: 12–24 inches
• Trench depth: 18–36 inches (residential), up to 6+ feet (civil engineering applications)
• Pipe: 4–6 inch perforated PVC or corrugated HDPE
• Aggregate: Clean, washed stone (3/4” to 1-1/2”)
• Filter fabric: Wraps the aggregate to prevent soil migration into the stone

When to use:

• Intercepting surface water moving across a slope
• Draining wet areas in yards and landscapes
• Protecting retaining walls from hydrostatic pressure
• Lowering localized high water tables

Size a French drain →

Curtain Drains (Interceptor Drains)

A curtain drain is essentially a French drain installed across a slope to intercept groundwater flowing downhill. It “cuts off” the subsurface flow path and redirects water before it reaches the area you want to protect.

Key difference from French drains: Curtain drains are specifically oriented perpendicular to the direction of groundwater flow. They are longer (often 50–200+ feet) and positioned upslope of the problem area.

Common applications:

• Intercepting hillside seepage before it reaches a building or roadway
• Draining saturated slopes to improve stability
• Protecting athletic fields and landscaped areas from upslope groundwater

Foundation Drains (Footing Drains)

Foundation drains are installed around the perimeter of a building foundation to prevent hydrostatic pressure from building up against basement walls and floor slabs. They are one of the most important — and most commonly neglected — components of residential construction.

Typical installation:

• Pipe placed alongside the footing, with the invert at or just below the top of the footing
• Clean aggregate surrounds the pipe and extends up the wall
• Filter fabric wraps the aggregate
• A waterproofing membrane covers the exterior foundation wall
• Pipe drains to daylight, a sump pit, or a storm drain connection

Code requirements: Most building codes (including the IRC) require foundation drains around habitable basements. The specific requirements vary, but the general principle is universal — keep hydrostatic pressure away from foundation walls.

Underdrains

Underdrains are perforated pipes installed beneath pavements, athletic fields, or stormwater BMPs (like rain gardens and bioretention facilities) to drain water from the engineered soil profile.

In stormwater management:

• Rain garden underdrains ensure the facility drains within 24–48 hours, even in poorly draining native soils
• Pavement underdrains prevent water from saturating the base course, which causes frost heave and pavement failure
• Athletic field underdrains keep playing surfaces dry and usable

Design Considerations

Pipe Selection

Perforation placement: Perforated pipe should be installed with the holes facing down (the 4 and 8 o’clock positions). This seems counterintuitive, but water rises into the pipe from below via the holes, while the solid top prevents soil and debris from falling in.

Aggregate Selection

The aggregate (gravel) around the pipe serves two purposes:

1. Provides a highly permeable zone that water flows through easily
2. Structurally supports the pipe and trench walls

Use clean, washed, angular stone — typically 3/4” to 1-1/2” diameter. Never use round (river) gravel, which doesn’t interlock and can shift. Never use stone with fines (crusher run, road base) — the fines fill voids and reduce permeability.

Filter Fabric (Geotextile)

Filter fabric prevents fine soil particles from migrating into the aggregate and clogging it over time. This process, called piping or soil migration, is the most common cause of subsurface drain failure.

Selection criteria:

• Permittivity: The fabric must allow water through faster than the surrounding soil drains. A minimum permittivity of 0.1 sec⁻¹ is typical.
• Apparent opening size (AOS): Must be small enough to block soil particles. For most soils, AOS of 40–80 (US sieve equivalent) works.
• Non-woven vs. woven: Non-woven geotextiles are preferred for most subsurface drainage applications because they resist clogging better than woven fabrics.

Slope

Subsurface drain pipes must slope toward the discharge point. Minimum recommended slopes:

Steeper slopes are fine — they increase flow velocity and reduce the chance of sediment accumulation. Avoid perfectly flat installations; even a slight settlement can create a low spot that traps sediment and water.

Discharge Options

Every subsurface drain needs an outlet. Common discharge points:

• Daylight outlet: The pipe exits the hillside at a point lower than the drain. The most reliable option — gravity does all the work. Protect the outlet with a grate and a small riprap pad.

• Sump pit and pump: When there’s no gravity outlet available (common in flat areas), the pipe drains to a sump pit where a pump discharges the water to grade.

• Storm drain connection: In developed areas, subsurface drains may connect to the municipal storm sewer system. Check local regulations — many jurisdictions restrict or require permits for these connections.

• Dry well: A hole filled with gravel that allows collected water to infiltrate into deeper, more permeable soil layers. Only viable when deeper soils have adequate permeability.

Common Failure Modes

Understanding why subsurface drains fail helps you design better systems:

1. Clogging from soil migration: Fine soil particles wash into the aggregate and fill voids. Prevention: proper filter fabric and clean aggregate.

2. Root intrusion: Tree and shrub roots seek the water inside drain pipes and can completely block them. Prevention: keep drains at least 10 feet from large trees, use root-resistant pipe materials.

3. Crushed pipe: Heavy equipment or excessive backfill depth collapses the pipe. Prevention: adequate cover, proper bedding, and appropriate pipe class for the depth.

4. Inadequate slope: Flat or reverse-sloped sections allow sediment to accumulate and water to stand. Prevention: careful grade control during installation, adequate minimum slope.

5. Disconnected joints: Pipe joints separate, allowing soil to enter. Prevention: properly coupled joints, careful backfilling.

References

1. U.S. Department of Agriculture, Natural Resources Conservation Service. (2001). Engineering field handbook, Chapter 14: Water management (drainage). USDA-NRCS.

2. Powers, J. P. (2007). Construction dewatering and groundwater control (3rd ed.). Wiley.

3. International Code Council. (2021). International Residential Code, Section R405: Foundation drainage. ICC.

4. Cedergren, H. R. (1989). Seepage, drainage, and flow nets (3rd ed.). Wiley.