Soakaway installation | Mainline Groundworks
Soakaways dispose of surface water by infiltration.
Soakaways dispose of surface water by infiltration. They are often used where connection to the main drain is not possible or preferred. We install to design with the correct size and connections.
How soakaway installation is engineered and delivered
A soakaway is an infiltration asset sized around runoff, tested ground permeability, groundwater separation and available plan area. Installation quality depends on protecting formation, using the specified storage and geotextile arrangement, coordinating upstream silt control and leaving a maintainable inspection route.
Engineering workflow
Stage 1
Drainage design and evidence review
Confirm contributing area, design storage, infiltration-test results, groundwater information, exceedance route and planning or approval conditions.
Output: Agreed location, level, construction detail and connection schedule.
Stage 2
Excavation and formation acceptance
Excavate without smearing or damaging the infiltration surface, manage temporary water and verify dimensions and ground condition before installation.
Output: Recorded formation and approval to install the storage system.
Stage 3
Storage, geotextile and connections
Install crates or the designed medium, specified membrane or geotextile, inlet, inspection access and upstream silt-management components.
Output: Photographic close-up record and connection checks.
Stage 4
Backfill, testing and maintenance handover
Backfill with approved material, protect the system from construction traffic and provide commissioning, as-built and maintenance information.
Output: As-built location, levels, test record and maintenance requirements.
Planning, constraints and ground risk
Planning considerations
- Infiltration feasibility should be evidenced before the drainage design is fixed.
- The soakaway needs suitable separation from structures, boundaries, services and groundwater as defined by the design.
- Surface-water exceedance must have a safe route if inflow exceeds the design event or infiltration reduces.
- Silt control and inspection access are necessary for long-term performance.
Site constraints
- Insufficient plan area or conflict with foundations and services
- Deep excavation, unstable sides or groundwater ingress
- Restricted excavator access and limited clean backfill storage
- Protecting the system from plant and construction traffic
Ground conditions
- Low-permeability clay may make infiltration unsuitable or require a different drainage strategy.
- Variable made ground can make test results and excavation behaviour inconsistent.
- High groundwater reduces usable infiltration depth and storage feasibility.
- Fine soils and construction silt can clog the system if upstream control is omitted.
Procurement and project preparation
Do not tender a soakaway from drained area alone. Provide the drainage design, infiltration evidence, storage volume, levels, product detail, membrane specification, upstream treatment, traffic loading, testing and reinstatement requirements.
Tender checklist
- Infiltration-test report and groundwater information
- Drainage calculations, layout, invert levels and exceedance route
- Crate or storage specification plus membrane/geotextile detail
- Silt traps, inspection points and maintenance access
- Backfill, testing, as-built and reinstatement requirements
Prepare before mobilisation
- Complete infiltration testing and issue the approved drainage design.
- Set out services, foundations and the proposed exceedance route.
- Confirm excavation support, temporary water and material-storage arrangements.
- Protect the installation area from sediment-laden runoff and construction traffic.
Scope clarity and handover
Commercial deliverables
- Installation sequence and inspection plan
- Formation, component and connection photographs
- Product and material information
- Commissioning or test records required by the design
- As-built location, levels and maintenance handover
Typical exclusions to resolve
- Drainage design and infiltration testing unless included
- Planning, water-company or other approval fees
- Contaminated groundwater or unsuitable-soil remediation
- Upstream drainage alterations and landscaping outside the scheduled works
Project examples
Representative scenarios with a direct method or package connection to this service family.
Attenuation Tank Installation Package
This scenario covers an attenuation tank installation package in Kent, storing stormwater to the SuDS design before controlled discharge. A typical package would excavate, bed, install and connect the system so cover loads and drainage ties remain reliable.
Adoptable Drainage (S104) Package
This scenario covers an adoptable drainage (S104) package in Kent, building sewers to undertaker standards for eventual adoption. A typical package would focus on line, level, materials and records so technical approval and testing can progress cleanly.
Related commercial resources
Typical costs
Small domestic soakaway
£1,500–£4,000 + VAT
Typical crate or rubble install to design.
Larger residential / light commercial
£4,000–£12,000 + VAT
Bigger drained area, deeper dig, more connections.
Attenuation / SuDS-linked scope
£12,000+ + VAT
Storage, flow control and adoption requirements.
Ready to install surface water disposal?
Get a clear quote for soakaway installation with commissioning and documentation for building control or adoption.
Related local services
Soakaway installation FAQs
Can a soakaway be priced before infiltration testing?
Only as a provisional allowance. Testing and design determine whether infiltration is viable, the required storage and the installation depth; those inputs materially affect excavation, product and drainage scope.
What causes new soakaways to underperform?
Common causes include unsuitable ground, damaged or smeared formation, incorrect storage volume, high groundwater, missing silt control, poor connection levels and construction sediment entering before handover.
What should a soakaway handover include?
The handover should identify its location and levels, installed components, inspection access, relevant testing or commissioning information and the maintenance needed to control silt and preserve performance.
Is a crate soakaway always the right solution?
No. The drainage designer must assess infiltration, storage, loading, groundwater, available space and approvals. A crate system is one construction option, not proof that infiltration is appropriate.
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