Drainage

Foundation drainage systems manage groundwater and surface water to prevent moisture intrusion, hydrostatic pressure buildup, and soil saturation around building foundations. Effective drainage is the primary defense against water infiltration and foundation damage.

Drainage System Objectives

Primary Functions:

• Remove water from foundation perimeter before infiltration occurs
• Reduce hydrostatic pressure on foundation walls
• Lower water table elevation adjacent to foundation
• Prevent soil saturation and associated settlement
• Protect waterproofing and dampproofing systems from prolonged exposure

Performance Criteria:

• Maintain dry conditions at foundation wall exterior surface
• Limit groundwater elevation to minimum 12 inches below basement floor slab
• Provide positive drainage away from building at all times
• Handle peak storm water inflow without system backup
• Function reliably for minimum 50-year design life

Perimeter Drain Systems

Footing Drains

Standard Configuration:

• 4-inch minimum diameter perforated pipe (6-inch preferred for high water table)
• Placed at footing elevation or maximum 12 inches above footing bottom
• Installed on exterior side of footing, never beneath
• Minimum 1/8 inch per foot slope (1% grade) toward discharge point
• Continuous around entire building perimeter

Pipe Specifications:

Installation Requirements:

• Holes oriented downward (4 o’clock and 8 o’clock positions)
• Filter fabric wrap or sock to prevent fines infiltration
• Minimum 6-inch gravel bed beneath pipe
• Minimum 12-inch gravel cover above pipe
• Inspection cleanouts at corners and maximum 100-foot intervals

Interior Perimeter Drains

Application:

• Existing buildings where exterior excavation is impractical
• Supplemental drainage for high water table conditions
• Retrofit installations for basement waterproofing

Configuration:

• Installed at inside perimeter of basement slab
• Connects to sump pit via sloped channels
• Requires breaking floor slab along perimeter
• 12-inch wide trench minimum
• Captures water entering through floor-wall joint

Limitations:

• Does not reduce hydrostatic pressure on foundation walls
• Allows water to enter building envelope before collection
• Requires active sump pump system
• Not a substitute for proper exterior drainage

Drainage Aggregate and Filter Media

Gravel Bed Design

Aggregate Specifications:

• Clean, washed gravel or crushed stone
• 0.75 to 1.5-inch diameter (No. 4 to 1.5-inch screen)
• Maximum 5% fines passing No. 200 sieve
• Free of clay, silt, and organic materials

Dimensional Requirements:

• Minimum 6 inches beneath drain pipe
• Minimum 12 inches above drain pipe (18 inches preferred)
• Extend minimum 12 inches beyond pipe horizontally
• Total gravel envelope depth: 24 to 30 inches minimum
• Width: 18 to 24 inches minimum

Hydraulic Properties:

• Permeability: minimum 100 times greater than surrounding soil
• Porosity: 35% to 40% void space
• Provides storage volume during peak inflow periods
• Facilitates rapid water movement to drain pipe

Filter Fabric Requirements

Geotextile Selection:

• Non-woven needle-punched fabric (standard applications)
• Woven monofilament (high-flow applications)
• Minimum 4 oz/yd² (standard), 8 oz/yd² (heavy-duty)

Hydraulic Criteria:

Installation:

• Wrap entire gravel bed, sides and top
• Minimum 12-inch overlap at seams
• Extend to grade level or beneath non-free-draining backfill
• Prevents migration of fines into gravel voids
• Maintains long-term permeability

Drainage Boards and Sheet Drains

Drainage Board Systems

Configuration:

• Formed plastic core with geotextile filter fabric facing
• Core depth: 0.25 to 1 inch (creates drainage gap)
• Applied directly to foundation wall waterproofing
• Provides vertical drainage path to footing drain

Performance Advantages:

• Reduces hydrostatic pressure on waterproofing membrane
• Provides backup drainage if membrane is compromised
• Protects waterproofing during backfill operations
• Maintains drainage capacity even with poor backfill soil

Design Criteria:

• Flow capacity: minimum 10 gpm per linear foot at 100 psf load
• Compressive strength: maintain 90% flow capacity at design load
• Extend from footing drain to minimum 6 inches above grade
• Terminate at top with filtered edge or J-bead to prevent soil entry

Sheet Drain Applications

High Water Table Installations:

• Double-drainage system: drainage board on wall plus footing drain
• Reduces reliance on backfill permeability
• Critical for clay soils or areas with poor drainage
• Required for occupied spaces below grade in high water table conditions

Sump Systems

Sump Pit Design

Dimensional Requirements:

• Minimum 24-inch diameter (30-inch preferred)
• Depth: extend minimum 12 inches below floor slab
• Perforated sides to accept groundwater
• Solid bottom with gravel base
• Cover: gas-tight lid with O-ring seal (radon areas)

Capacity Calculation: Q = A × I × 1.25 (safety factor)

Where:

• Q = required pump capacity (gpm)
• A = tributary area (square feet)
• I = infiltration rate (inches per hour) / 96.3

Sump Pump Specifications

Sizing Criteria:

Installation Requirements:

• Pump activated by float switch at 6 to 8 inches above sump bottom
• Discharge pipe: 1.5-inch minimum diameter
• Check valve in discharge line to prevent backflow
• Isolated from building structure to prevent vibration transmission

Redundancy:

• Battery backup pump system for critical applications
• Alarm system to indicate pump failure or high water
• Dual pump configuration for large tributary areas
• Emergency overflow to floor drain or secondary discharge

Discharge Routing

Preferred Methods:

1. Gravity discharge to daylight (slope away from building)
2. Connection to storm sewer system (if permitted by code)
3. Discharge to dry well minimum 20 feet from building
4. Connection to rain garden or infiltration system

Prohibited Discharges:

• Direct connection to sanitary sewer (code violation in most jurisdictions)
• Discharge toward building or adjacent structures
• Discharge onto impervious surfaces without positive drainage
• Discharge within 10 feet of building foundation

Grading and Surface Drainage

Finish Grade Requirements

Slope Specifications:

• Minimum 6-inch drop in first 10 feet from foundation (6% grade)
• Maintain 2% to 5% slope for additional 10 feet beyond
• Provide positive drainage for minimum 20 feet from building perimeter
• Where lot constraints prevent adequate slope, use swales or area drains

Grading Materials:

• Topsoil: maximum 6-inch depth over free-draining subgrade
• Subgrade: compacted clay or low-permeability soil acceptable (acts as barrier)
• Avoid high-permeability materials directly against foundation above footing drain

Surface Water Management

Gutter and Downspout Systems:

• Size gutters and downspouts for peak rainfall intensity
• Discharge minimum 6 feet from foundation (10 feet preferred)
• Connect to underground drain if surface discharge not feasible
• Provide splash blocks or drainage extensions

Area Drains and Swales:

• Place area drains at low points and grade transitions
• Minimum 4-inch diameter discharge pipe
• Connect to storm system or discharge away from building
• Swales: minimum 2% longitudinal slope, 10:1 side slopes

Water Table Considerations

High Water Table Design

Site Assessment:

• Determine seasonal high water table elevation through soil borings
• Conduct percolation tests to establish soil drainage characteristics
• Review historical flood data and groundwater monitoring records

Design Responses:

When Water Table Above Floor Slab:

• Sump system required
• Interior and exterior perimeter drains
• Underslab drainage layer (4-inch minimum gravel with collection pipes)
• Waterproofing membrane (dampproofing insufficient)
• Structural slab design for uplift forces

When Water Table Above Footings:

• Enhanced drainage capacity (6-inch perimeter drains minimum)
• Drainage board on foundation walls
• Possible dewatering system during construction
• Geotechnical engineer review of foundation design
• Consider structural waterproofing systems

Dewatering Strategies

Temporary Construction Dewatering:

• Wellpoint systems for shallow water table (15 to 20 feet)
• Deep well systems for water table below 20 feet
• Required to maintain dry excavation during construction
• Permits may be required for discharge

Permanent Dewatering (Rare):

• Used only where other drainage methods inadequate
• Continuous pumping from perimeter wells
• High energy cost and maintenance burden
• Requires redundant pumps and emergency power
• May cause settlement of adjacent structures

Design Integration

Coordination with Waterproofing

• Install footing drains before waterproofing application
• Protect waterproofing membrane during drainage installation
• Drainage board must not damage membrane during attachment
• Filter fabric prevents backfill from clogging drainage paths

Construction Sequencing

1. Excavate to required depth with proper shoring
2. Install and proof test footing drains
3. Apply waterproofing or dampproofing to foundation walls
4. Install drainage board (if specified)
5. Place filter fabric and drainage aggregate
6. Backfill with compacted fill in 12-inch lifts
7. Establish finish grade and surface drainage
8. Install and test sump system
9. Verify all drainage discharges function correctly

Inspection and Testing

Critical Inspection Points:

• Verify pipe slope with laser level before backfilling
• Confirm continuous grade from all points to discharge
• Document pipe material, perforation pattern, and connections
• Test footing drains by flushing with water
• Inspect filter fabric placement and overlap
• Verify sump pump operation and discharge routing

Maintenance Requirements

Periodic Inspections:

• Annual check of sump pump operation
• Clean area drains and verify surface grades after major storms
• Inspect discharge points for proper flow and erosion
• Test battery backup systems quarterly

Long-Term Maintenance:

• Cleanout access points allow periodic flushing of perimeter drains
• Replace sump pumps every 7 to 10 years
• Monitor for settlement that alters surface drainage patterns
• Re-grade as needed to maintain positive drainage