Advanced Soakaway Sizing Engine (ASSE)

Methodology Summary (BRE Digest 365 & CIRIA C753)

This tool performs soakaway design calculations in accordance with BRE Digest 365 (2016) and CIRIA C753. The design objective is to ensure the soakaway has sufficient volume to store stormwater runoff from a specific storm event (typically 1-in-10 or 1-in-25 year return periods) minus the volume lost to infiltration during the storm.

Key Principles
1. Mass Balance ( Inflow (I) - Outflow (O) = Required Storage (S) )
2. Inflow Calculated using the Rational Method ( I = A × Rainfall )
3. Outflow Calculated via soil infiltration ( O = as50 × f × Duration × 60 )
4. Half-Empty Check The structure must discharge half its volume within 24 hours to be ready for subsequent storms.
5. Cover & Clearance Minimum 1.0m cover above structure and 1.0m clearance to groundwater (BRE Digest 365).

Project Information

Current Soakaway Configuration

Duration (min)
Intensity (mm/h)

Climate Change Considerations

Climate Change Impact Assessment

Selected Scenario: No climate change adjustment applied

Rainfall Intensity Adjustment: +0%

Design Life Consideration: Current climate conditions

Recommendation: No additional climate resilience measures required.

Representative Concentration Pathways (RCPs)

  • RCP 4.5: Intermediate scenario with emissions peaking around 2040 then declining
  • RCP 8.5: High emissions scenario with continued growth throughout the 21st century

Note: Climate factors are regionally averaged. For critical projects, consult local climate studies.

Scenario 2030 2050 2080 2100 Confidence
RCP 4.5 (Low) +5% +10% +15% +18% Medium
RCP 4.5 (Medium) +8% +15% +22% +25% Medium-High
RCP 8.5 (Medium) +10% +20% +30% +35% High
RCP 8.5 (High) +12% +25% +40% +50% Medium

Geotechnical Considerations

USDA Soil Texture Triangle

USDA Soil Texture Triangle

Use this chart to determine your soil type based on particle size distribution

Recommended Infiltration Rate: 3.3e-5 m/s
Based on selected soil type and typical values from BRE Digest 365

Determine Soil Type from Particle Size Distribution

Enter the percentage of each particle size in your soil sample to determine the soil type:

Particles > 0.05 mm
Particles 0.002-0.05 mm
Particles < 0.002 mm
Particle Size Classification Guide:
  • Gravel: > 2.0 mm
  • Sand: 0.05 - 2.0 mm
  • Silt: 0.002 - 0.05 mm
  • Clay: < 0.002 mm

Note: Ensure the total percentage adds up to 100%

Percolation Test Data

Enter field percolation test results to calculate actual infiltration rate:

Percolation Test Method:
  1. Dig test hole to proposed soakaway depth
  2. Fill with water and allow to saturate for 4+ hours
  3. Refill and measure drop in water level over time
  4. Calculate infiltration rate using formula: f = V / (A × t)

Typical Soil Properties

Soil Type Typical Infiltration Rate (m/s) Particle Size Drainage Characteristics Suitability for Soakaways
Gravel 1×10⁻³ to 1×10⁻⁴ > 2 mm Excellent Excellent
Sand 1×10⁻⁴ to 1×10⁻⁵ 0.05-2 mm Very Good Very Good
Loamy Sand 1×10⁻⁵ to 5×10⁻⁶ Mixed Good Good
Sandy Loam 5×10⁻⁶ to 1×10⁻⁶ Mixed Moderate Moderate
Loam 1×10⁻⁶ to 5×10⁻⁷ Mixed Fair Fair
Silt Loam 5×10⁻⁷ to 1×10⁻⁷ 0.002-0.05 mm Poor Poor
Clay Loam 1×10⁻⁷ to 1×10⁻⁸ < 0.002 mm Very Poor Very Poor
Clay < 1×10⁻⁸ < 0.002 mm Impermeable Unsuitable
Design Recommendations:
  • Soakaways generally require infiltration rates > 1×10⁻⁶ m/s
  • For rates < 1×10⁻⁷ m/s, consider alternative drainage methods
  • Always conduct field percolation tests for critical projects
  • Consider seasonal variations in groundwater levels
Based on soil type selection or percolation test
Account for seasonal groundwater fluctuations
Surface Type Area (m²) C (Runoff Coeff.) Action

Dimensions & Material

Note: 0.3 for stone, 0.95 for plastic cells.

Cover & Clearance Requirements

BRE Digest 365: Minimum 1.0m cover required
BRE Digest 365: Minimum 1.0m clearance to groundwater

Basic Cost Estimation (USD)

Advanced Cost Estimation (USD)

Cost Categories

Quantity Control

Check "Auto" boxes to calculate quantities automatically. Uncheck to enter manually.

Item Unit Quantity Auto Unit Cost ($) Total Cost ($) Include
Excavation m³
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0.00
Filter Fabric m²
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0.00
Granular Fill m³
🔒
0.00
Inspection Chamber unit
🔒
0.00
Geotextile Wrap m²
🔒
0.00
Pipework (Inlet/Outlet) m
🔒
0.00
Labor hours
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0.00
Testing & Permits lump sum
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0.00
Other (Sundries) lump sum
🔒
0.00
Subtotal: 0.00
Regional Multiplier: 1.00x
Total Estimated Cost: 0.00

Cost Breakdown Summary

Cost details will appear here after calculation.

Project Designs Summary

Add calculated designs to this list to generate a combined report.

Ref ID Type Return Period Dimensions Req. Vol Status Cost Actions
No designs added yet. Configure above and click "Add to Project List".

Generate Professional Report

Multiple Soakaways Cost Estimation Summary (USD)

Soakaway Ref Type Dimensions Volume (m³) Excavation Materials Labor Other Costs Subtotal Regional Multiplier Total Cost
No soakaways designed yet. Add designs to see cost summary.
Project Totals: $0.00 $0.00 $0.00 $0.00 $0.00 1.00x $0.00

Detailed Cost Breakdowns

Detailed cost breakdowns will appear here when soakaways are added.

Cost Optimization Recommendations

Add multiple soakaway designs to see optimization suggestions.