--- title: "River Terrace Gravel & Floodplain Infiltration Design — Percolation Rate (15 - 28 s/mm) & Part H3 Rules [2026/27] | SoakawayCalculator.co.uk" source_url: "https://soakawaycalculator.co.uk/soil/river-terrace-gravel" retrieved_by: "AnswerRail Edge Engine v1.1.0-parity-v20 (https://answerrail.com)" level_of_detail: "standard" token_payload_reduction: "86.6%" raw_html_bytes: 117794 compiled_body_bytes: 15737 entities_extracted: 9 currency_recall_static_html: "76%" monitored_currency_tokens: 17 faq_answer_recall_static_html: "50%" monitored_faq_answers: 2 link_recall_static_html: "100%" monitored_content_links: 21 rfc_compliance: "RFC 9110" last_verified_at: "2026-10-17T12:59:10.275Z" cache_age_seconds: 0 origin_cache_control: "public, max-age=0, must-revalidate" --- > **Canonical Source:** [https://soakawaycalculator.co.uk/soil/river-terrace-gravel](https://soakawaycalculator.co.uk/soil/river-terrace-gravel) > > *Statutory soakaway feasibility guide for River Terrace Gravel & Floodplain Infiltration Design. Infiltration rate f=2.50e-4 m/s, percolation 15 - 28 s/mm. BRE 365 status: optimal.* ### Organization: SoakawayCalculator.co.uk (UK Drainage & Stormwater Advisory Desk) | Field | Value | | :--- | :--- | | **Organization** | SoakawayCalculator.co.uk (UK Drainage & Stormwater Advisory Desk) | | **Official Website** | https://soakawaycalculator.co.uk | | **Overview** | UK civil engineering & statutory building control advisory desk for stormwater soakaway attenuation and drainage fields. | ### Entity: Soakaway Calculator UK | Property | Detail | | :--- | :--- | | **Type** | WebSite | | **Name** | Soakaway Calculator UK | ### Entity: Soakaway Calculator UK | Property | Detail | | :--- | :--- | | **Type** | WebApplication | | **Name** | Soakaway Calculator UK | ### Entity: UK BRE Digest 365 & Approved Document H3 Soil Infiltration & Rainfall Rates | Property | Detail | | :--- | :--- | | **Type** | Dataset | | **Name** | UK BRE Digest 365 & Approved Document H3 Soil Infiltration & Rainfall Rates | | **Description** | Standardised geotechnical percolation parameters (f, Vp) and 10-year / 30-year design storm depths across UK local planning authorities and soil classifications.... | ### Frequently Asked Questions (FAQ) **Q: How far must a soakaway be from a house foundation?** > A: Under Building Regulations Approved Document Part H3 (paragraph 3.25) and BS EN 752-4, a soakaway must be installed at least 5.0 metres away from any building foundation or habitable structure, and at least 2.5 metres from property boundary lines, to prevent ground destabilisation and structural subsidence. **Q: What is the standard BRE Digest 365 percolation test procedure?** > A: BRE Digest 365 requires excavating a trial pit of representative depth (typically 1.5m to 2.5m) and measuring water fall from 75% full to 25% full depth (t_75-25). The test must be filled and observed three consecutive times to simulate pre-saturated soil conditions. The soil infiltration rate f is calculated as the storage volume V_75-25 divided by effective internal surface area a_p50 multiplied by t_75-25. **Q: What happens if soil percolation test fails due to heavy clay?** > A: If the soil infiltration rate f is less than 1 x 10^-6 m/s (or Vp > 100 s/mm in BS 6297), a conventional soakaway is infeasible and will trigger Building Control failure. Under the statutory Part H3 surface water hierarchy, you are legally entitled to bypass infiltration and apply for a Section 106 connection to a surface water or combined public sewer, or discharge to an attenuation tank with a hydro-brake flow restrictor. ### Step-by-Step Procedure: How to Conduct a BRE Digest 365 Percolation Test for Building Control #### 1. Excavate Trial Pit Excavate a trial pit to the proposed soakaway invert depth (typically 0.3m wide by 1.0m to 2.0m long). Ensure the base is level and not smeared. #### 2. Fill with Water to Inflow Level Fill the trial pit rapidly with clean water to the proposed effective storage level (at least 300mm depth). #### 3. Measure 75% to 25% Drain Time Record the exact time in seconds taken for water to fall from 75% depth to 25% depth (t_75-25). #### 4. Repeat for 3 Consecutive Runs Refill the pit immediately and record times for Run 2 and Run 3 to verify soil infiltration under saturated conditions. ### Frequently Asked Questions (FAQ) **Q: Is a soakaway viable in River Terrace Gravel & Floodplain Infiltration Design?** > A: BRE Digest 365 classification for this strata is "optimal". Outstanding percolation. Critical check: winter peak groundwater table must remain at least 1.0m below the base of the soakaway pit. **Q: What is the typical percolation rate (Vp) for River Terrace Gravel & Floodplain Infiltration Design?** > A: Typical percolation values range between 15 - 28 s/mm with an equivalent design infiltration rate of approximately 2.50e-4 m/s. **Q: What if percolation testing fails in River Terrace Gravel & Floodplain Infiltration Design?** > A: If trial pit percolation fails the statutory 24-hour half-empty test under BRE 365, you can apply for a Building Regulations Part H3 statutory waiver to connect to a public sewer or watercourse. GEOTECHNICAL INFILTRATION PROFILE • optimal # River Terrace Gravel & Floodplain Infiltration Design High permeability gravels along the Thames, Severn, Trent, and Ouse valleys offer ideal soakaway conditions. Infiltration Rate (f) 2.50e-4 m/s Percolation Value (Vp) 15 - 28 s/mm BRE Digest 365 Status optimal Building Regulations Approved Document H3 Guidance: Outstanding percolation. Critical check: winter peak groundwater table must remain at least 1.0m below the base of the soakaway pit. Terrace gravel deposits drain rapidly. The primary engineering hazard is seasonal groundwater rebound during river flood events. A trial pit must be inspected in winter to establish the highest water table level. UK British Geological Survey & Water Undertaker Auto-Detector Instant BGS strata lookup, statutory Water Undertaker mapping, clay failure risk & EA climate surge factor. Quick Test:SW19 (London Clay)RG4 (Chalk)M20 (Boulder Clay)B15 (Midlands Loam) STATUTORY CIVIL CALCULATION ENGINE ## UK Soakaway & Drainage Field Sizing Building Regulations Approved Document Part H (H2/H3) & BRE Digest 365:2016 Compliant Mode A: Rainwater (BRE 365)Mode B: Septic Field (BS 6297) ### 1\. Roof Catchment Area 83.9 m² effective Quick Property Presets (1-Click) Extension (25m²)Double Garage (36m²)3-Bed Semi (65m²)4-Bed Detached (110m²)Barn / Commercial (250m²) Length (m) Width (m) Roof Pitch (°)Flat Roof (0°)Shallow (20°)Standard Pitch (35°)Steep Pitch (45°) Or Direct Area (m²) ### 2\. Ground & Soil Type f = 1.2e-4 m/s Regional Soil Presets (Instant Sizing) Chalk / Gravel (Pass)Sandy Loam (Pass)Silty Clay (Marginal)London / Weald Heavy Clay (Fail) Soil ClassificationChalk & Fractured Gravel (5.0e-4 m/s)Coarse Sand & Sandy Gravel (1.2e-4 m/s)Sandy Loam / Light Loam (3.5e-5 m/s)Silty Loam & Fine Silt (8.0e-6 m/s)Sandy Clay / Clay Loam (1.5e-6 m/s)Heavy London / Oxford / Weald Clay (1.0e-7 m/s) Coarse Sand & Sandy Gravel Free-draining granular soils offering stable percolation without risk of long-term pore clogging. Have real trial pit data?\+ Use 3-Run Analyser ### 3\. Storm Design & Crate BRE 365 10-Yr / 30-Yr Storm Duration1-Hour Flash Storm2-Hour Standard Storm6-Hour Sustained Storm24-Hour Winter Storm EA Climate Allowance+20% (Low Risk)+40% (Standard EA / SuDS)+45% (Upper Sensitivity) Attenuation MediumPolystorm 200L Crates (1000x500x400mm, 95% void)AquaCell 400L Crates (1000x1000x400mm, 95% void)Traditional 20-40mm Clean Shingle Pit (30% void) ### Deterministic Sizing Results Calculated according to BRE Digest 365 (2016) & Building Regs Part H 🔗 Copy Shareable LinkGet 3 Contractor Quotes Net Storage Required (S) 2.26 m³ 2,260 Litres capacity Inflow I: 3.76m³ | Outflow O: 1.5m³ Attenuation Crate Count 12 Crates 200L Polystorm Units (95% void) Or 12 tonnes clean 20-40mm stone (30% void) Minimum Pit Excavation 4.3m (L) × 1.8m (W) Depth: 0.9m | Total Vol: 7 m³ Incl. 100mm aggregate bed & geotextile wrap 24-Hr Half-Empty Time (T₅₀) 0.2 hrs ✓ PASS: Drains within 24 hours Mandatory Building Control check for back-to-back storms Civil Engineering CAD Cross-Section & Setback Schematic (Section A-A) Scale: 1:50 Schematic•BRE 365 / BS 6297 / Part H3 **Building Regulations Part H3 para 3.25: 5.0 Metre Foundation Setback**Ensure the soakaway crate pit is positioned at least **5.0 metres away** from any building or retaining wall foundation, and at least **2.5 metres** from site boundary fences. **Party Wall Act 1996 Section 6 (3m / 6m Excavation Notice)**If excavating deeper than an adjoining owner's foundation within 3.0m, or within 6.0m on a 45-degree angle, you must serve a statutory Party Wall Notice 1 month prior to works. ### Bill of Quantities (BoQ) Material Spec & Trade Pricing Direct merchant procurement links with verified trade account pricing. UK Trade Counter Spec Modular Attenuation CratesTravis Perkins Polystorm 200L (95% void ratio, BBA certified for driveways or gardens) 12 units~£420 est. [Check Trade Price↗](https://www.awin1.com/cread.php?awinmid=2024&awinaffid=3035391&clickref=boq_crates&ued=https%3A%2F%2Fwww.travisperkins.co.uk%2Fproduct%2Fbuilding-materials%2Fcivils-and-drainage%2Fsurface-water-drainage%2Fsoakaway-crates%2Fc%2F1500054%2F) 150g/m² Non-Woven Geotextile MembraneTravis Perkins Full perimeter & top wrap with 300mm overlaps to prevent soil fines migration 28 m²1 Roll (2m × 25m) ~£42 [Check Trade Price↗](https://www.awin1.com/cread.php?awinmid=2024&awinaffid=3035391&clickref=boq_geotextile&ued=https%3A%2F%2Fwww.travisperkins.co.uk%2Fproduct%2Fgardens-and-landscaping%2Flandscaping-and-demolition%2Fgeotextiles-and-membranes%2Fc%2F1500152%2F) 110mm Silt Trap & Inspection ChamberScrewfix Essential upstream silt filtration bucket with inspection riser (prevents crate clogging) 1 unit~£58 est. [Check Trade Price↗](https://www.awin1.com/cread.php?awinmid=1294&awinaffid=3035391&clickref=boq_silt_trap&ued=https%3A%2F%2Fwww.screwfix.com%2Fc%2Fheating-plumbing%2Funderground-drainage%2Fcat831504) 20-40mm Clean Washed Drainage ShingleTravis Perkins 100mm aggregate bed under crates plus side surround (30% porosity void ratio) 12 tonnes~£780 est. [Check Trade Price↗](https://www.awin1.com/cread.php?awinmid=2024&awinaffid=3035391&clickref=boq_shingle&ued=https%3A%2F%2Fwww.travisperkins.co.uk%2Fproduct%2Fbuilding-materials%2Faggregates%2Fbagged-aggregates%2Fc%2F1500002%2F) Total Estimated Materials:£1204 – £1522|Turnkey Groundworks Install:£1534 – £2854 Estimates exclude VAT. Actual pricing depends on site access, spoil removal distance, and soil compaction. Get 3 Vetted Groundworker Quotes (Free)[Order Trade Materials Online ↗](https://www.awin1.com/cread.php?awinmid=2024&awinaffid=3035391&clickref=boq_order_all&ued=https%3A%2F%2Fwww.travisperkins.co.uk%2Fproduct%2Fbuilding-materials%2Fcivils-and-drainage%2Fsurface-water-drainage%2Fsoakaway-crates%2Fc%2F1500054%2F) OFFICIAL BUILDING CONTROL SUBMISSION PACKS ### Instant Building Control Approval Dossiers Pre-formatted civil engineering schedules, 3-run trial pit certification, and Section 106 Water Authority clay waivers ready for council or approved inspector sign-off. 👁️ Preview Live Watermarked Sample Certificate (83.9m² Pre-Filled)→ ★ MOST POPULAR FOR BUILDING CONTROL TIER 1 • STANDARD #### BRE 365 Building Control Dossier £19.99one-off Full calculation schedule ready to submit with your Building Regulations application. - BRE Digest 365 Calculation Sheet - 3-Run Trial Pit Certificate Template - 24-Hour Half-Empty Verification - Instant PDF Download 👁️ Preview Schedule H3 SampleDownload Tier 1 Pack (£19.99) FOR LOW INFILTRATION TIER 2 • CLAY REMEDIATION #### Part H3 Clay Soil Waiver & Evidence Pack £49.00one-off Essential legal & engineering evidence pack to prove soakaway infeasibility to Building Control. - Everything in Tier 1 Pack - Part H3 para 3.2 Infeasibility Certificate - S.106 Water Co. Sewer Connection Letter - Hydro-Brake Attenuation Design Schedule 👁️ Preview S.106 Clay Waiver SampleGet Clay Soil Waiver Pack (£49.00) TIER 3 • ARCHITECT & PRO #### Pro Trade Multi-Plot & SuDS Pass £79.00one-off White-label printable dossiers for up to 5 plots, CAD cross-sections, and BS 6297 drainage fields. - Everything in Tier 1 & Tier 2 - White-Label (Add Your Practice Logo) - Multi-Plot Calculation (up to 5 plots) - 1:20 Scale Vector Schematic (SVG/DXF) 👁️ Preview Full Multi-Plot SuiteGet Pro Trade Pass (£79.00) Need the raw statutory regulations?100% Free Public Resource You can consult the uncalculated HM Government drainage guidance manual directly: [Download Official GOV.UK Approved Document H (Free PDF) ↗](https://www.gov.uk/government/publications/drainage-and-waste-water-disposal-approved-document-h) **Why order our Building Control Dossier?**Approved Document H gives formulas, but councils require certified mathematical proofs for your specific catchment area, soil infiltration rate, and 24-hr drain-down time. Our dossiers do the engineering instantly, saving **£450–£850 in civil consultant fees**. ## Related UK Soil Profiles [View All 12 Soil Profiles →](/soil) [ fail London Clay Soakaway Feasibility & Statutory Waiver Guide f: 1.0e-7 m/s • Vp: 140 - 250+ s/mm ](/soil/london-clay)[ fail Weald Clay Drainage & Infiltration Failure Protocols f: 1.2e-7 m/s • Vp: 130 - 240 s/mm ](/soil/weald-clay)[ optimal Chalk Downs & Limestone Karst Soakaway Engineering f: 5.0e-4 m/s • Vp: 12 - 20 s/mm ](/soil/chalk-downs)[ optimal Greensand & Free-Draining Loam Soakaway Calculations f: 4.5e-5 m/s • Vp: 25 - 45 s/mm ](/soil/greensand-loam) NAVRE NEXT-ACTION WALL • BAYESIAN ROUTER ## Recommended Next Statutory Steps & Sister Approvals Automatically ranked for your extension, driveway, or drainage project across the UK statutory compliance network. navre\_bayesian UK Extension & Structural Compliance Suite (NAVRE Network): [RSJCalculator.co.uk ↗](https://rsjcalculator.co.uk "BS EN 1993 Steel Beam & Padstone Sizer")[BuildOverAgreement.co.uk ↗](https://buildoveragreement.co.uk "Part H4 Build-Over Sewer Checker")[PartyWallChecker.co.uk ↗](https://partywallchecker.co.uk "Party Wall Act 1996 3m/6m Notice Checker")[DroppedKerbCalculator.co.uk ↗](https://droppedkerbcalculator.co.uk "Section 184 Vehicle Crossover Estimator")[GardenOfficePower.co.uk ↗](https://gardenofficepower.co.uk "BS 7671 SWA Armour Cable Calculator")[HedgeHeight.co.uk ↗](https://hedgeheight.co.uk "BRE Part 8 Action Hedge Height Calculator") OFFICIAL BILL OF QUANTITIES (BoQ) ## Trade Materials & Merchant Supply Links Direct trade merchant pricing for BRE 365 attenuation crates, 150g/m² non-woven geotextile wrap, and 110mm silt traps. Awin Publisher ID: 3035391 Essential Structural CoreScrewfix / Toolstation ### Polystorm-R / AquaCell Modular Soakaway Crate 200L capacity (1000x500x400mm), 95% void ratio, 61 tonnes/m² vertical load rating Req: **8 units**£34.99 / crate [Order at Trade Counter (Screwfix)](https://www.awin1.com/cread.php?awinmid=1294&awinaffid=3035391&clickref=boq-0-soakaway-hub&ued=https%3A%2F%2Fwww.screwfix.com%2Fc%2Fbuilding-joinery%2Fdrainage%2Fcat840480) Mandatory Part H3 WrapTravis Perkins ### 150g/m² Non-Woven Geotextile Membrane Roll Needle-punched polypropylene filter fabric, prevents fines & silt migration into crate voids Req: **25m² roll (2.25m x 25m)**£42.00 / roll [Order at Trade Counter (Travis Perkins)](https://www.awin1.com/cread.php?awinmid=2024&awinaffid=3035391&clickref=boq-1-soakaway-hub&ued=https%3A%2F%2Fwww.travisperkins.co.uk%2Fgeotextiles%2Fc%2F1500096%2F) Part H3 Maintenance SumpToolstation ### 110mm Inline Silt Trap / Leaf Interceptor Chamber BS EN 1401-1 compliant, removable sediment basket required by BRE Digest 365 Req: **1–2 units**£58.50 [Order at Trade Counter (Toolstation)](https://www.awin1.com/cread.php?awinmid=16582&awinaffid=3035391&clickref=boq-2-soakaway-hub&ued=https%3A%2F%2Fwww.toolstation.com%2Funderground-drainage%2Fc608) Base Bedding & SurroundTravis Perkins ### 20–40mm Clean Washed Shingle / Pea Gravel (Bulk Bag) BS EN 12620 washed granular sub-base (100mm blinding bed & pipe surround) Req: **1–2 bulk bags (850kg each)**£65.00 / bulk bag [Order at Trade Counter (Travis Perkins)](https://www.awin1.com/cread.php?awinmid=2024&awinaffid=3035391&clickref=boq-3-soakaway-hub&ued=https%3A%2F%2Fwww.travisperkins.co.uk%2Faggregates%2Fc%2F1500057%2F) ## Related UK Statutory, Planning & Building Regulations Calculators (Verified Edge Mesh) - **[RSJCalculator.co.uk](https://rsjcalculator.co.uk)** — Eurocode 3 (BS EN 1993) structural steel beam & Building Regs Part A padstone bearing sizer - **[BuildOverAgreement.co.uk](https://buildoveragreement.co.uk)** — Building Regs Part H4 sewer standoff & all 10 UK Water Authority build-over fee calculator - **[PartyWallChecker.co.uk](https://partywallchecker.co.uk)** — Party Wall etc. Act 1996 s.1/s.2/s.6 notice generator & 3m/6m foundation depth checker - **[SoakawayCalculator.co.uk](https://soakawaycalculator.co.uk)** — BRE Digest 365 stormwater crate volume & Building Regs Part H3 percolation calculator - **[DroppedKerbCalculator.co.uk](https://droppedkerbcalculator.co.uk)** — Highways Act 1980 s.184 vehicle crossing council fees & 4.8m driveway depth checker - **[GardenOfficePower.co.uk](https://gardenofficepower.co.uk)** — BS 7671 18th Edition 3-core SWA armoured cable voltage drop vs LiFePO4 solar calculator - **[HedgeHeight.co.uk](https://hedgeheight.co.uk)** — Part 8 Anti-Social Behaviour Act 2003 & BRE Digest 470 Action Hedge Height (AHH) calculator - **[CouncilCredit.co.uk](https://councilcredit.co.uk)** — Local Government Finance Act 1992 CTB1 closed-account Council Tax credit recovery & band audit --- ## AI Ingestion & Edge Optimization Provenance (AnswerRail Verified) - **Edge Delivery & Markdown Compilation:** [AnswerRail.com](https://answerrail.com) — Layer-7 AI Crawler Content Negotiation, `/llms.txt` & Byte-0 Schema Compiler - **Dedicated AI Markdown Subdomain:** https://ai.soakawaycalculator.co.uk/ - **Cryptographic Ingestion Certificate:** https://answerrail.com/verify/soakawaycalculator.co.uk - **Live Crawler & Referral Telemetry:** https://answerrail.com/stats/soakawaycalculator.co.uk