Soakaways and Sustainable Drainage for London Victorian Terraces

A SOAKAWAY IS A SUBSURFACE STRUCTURE THAT RECEIVES RAINWATER FROM ROOFS, DRIVEWAYS, OR PAVED SURFACES AND ALLOWS IT TO PERCOLATE INTO THE SURROUNDING GROUND — RATHER THAN DISCHARGING TO THE SURFACE WATER SEWER OR COMBINED SEWER:

TRADITIONAL SOAKAWAY (RUBBLE-FILLED TRENCH OR PIT): OLDER SOAKAWAYS (PRE-2000) WERE TYPICALLY RUBBLE-FILLED PITS OR TRENCHES DUG INTO THE GROUND AND COVERED. THESE HAVE A VERY SHORT EFFECTIVE LIFE (5-10 YEARS — THE VOIDS BETWEEN RUBBLE FILL PROGRESSIVELY WITH FINE SOIL PARTICLES AND THE PERCOLATION RATE DROPS): - NOT RECOMMENDED FOR NEW INSTALLATIONS - IF AN EXISTING RUBBLE SOAKAWAY HAS FAILED (WATERLOGGED, OVERFLOWING), EXCAVATE AND REPLACE WITH A PLASTIC CRATE SYSTEM

PLASTIC CRATE SOAKAWAY (THE MODERN STANDARD): MODULAR POLYPROPYLENE CRATE STRUCTURES THAT CREATE A LARGE VOID RATIO (TYPICALLY 95% VOID SPACE — VERSUS 30-40% FOR RUBBLE) FOR MAXIMUM WATER STORAGE AND PERCOLATION: - PRODUCTS: POLYSTORM CELL (POLYPIPE — AVAILABLE IN VARIOUS SIZES: 1.2M × 0.6M × 0.52M TALL — EACH CELL HOLDS APPROXIMATELY 358 LITRES); FLOATEX GEOCELL (HYDRO-SCI TECHNOLOGY); ATLANTIS FIRDRAIN Q - CONSTRUCTION: CELLS ARE CONNECTED TO FORM THE REQUIRED VOLUME; WRAPPED IN A GEOTEXTILE MEMBRANE (TERRAM TERRAFIL 700G/M² — NON-WOVEN GEOTEXTILE) TO PREVENT SOIL INGRESS INTO THE CELL VOIDS; COVERED WITH A GEOTEXTILE LID; BACKFILLED WITH PEA GRAVEL OR CLEAN GRANULAR MATERIAL; COVERED WITH TOPSOIL - INSPECTION CHAMBER: INCLUDE AT LEAST ONE INSPECTION CHAMBER (POLYPIPE INSPECTION CHAMBER RISER + COVER TO SUIT LOADING CLASS) TO ACCESS THE SOAKAWAY FOR JETTING AND MAINTENANCE - SEPARATION FROM BUILDING FOUNDATIONS: MINIMUM 5M FROM ANY BUILDING FOUNDATION AND FROM THE BOUNDARY (BS EN ISO 10768 AND CIRIA GUIDANCE)

PERCOLATION TEST (ESSENTIAL BEFORE DESIGNING A SOAKAWAY): A PERCOLATION TEST ESTABLISHES THE RATE AT WHICH WATER SOAKS INTO THE GROUND — ESSENTIAL FOR DESIGNING THE CORRECT SOAKAWAY SIZE: - METHOD (BRE DIGEST 365 PROCEDURE): DIG A TEST PIT (300MM CUBE) AT THE PROPOSED SOAKAWAY DEPTH. FILL WITH WATER AND ALLOW TO DRAIN TWICE (PRE-SATURATION). ON THE THIRD FILL, MEASURE THE TIME FOR THE WATER LEVEL TO FALL FROM 75% TO 25% OF THE CUBE DEPTH. CALCULATE THE PERCOLATION VALUE (VP) IN M/S. - LONDON CLAY: IN MOST OF INNER LONDON (ON LONDON CLAY SUBSOIL), PERCOLATION RATES ARE VERY LOW (LONDON CLAY HAS AN EXTREMELY LOW HYDRAULIC CONDUCTIVITY — 10⁻⁹ TO 10⁻¹¹ M/S). IN THIS CASE, A SOAKAWAY IS NOT FEASIBLE — SURFACE WATER MUST DISCHARGE TO THE SURFACE WATER SEWER, OR ATTENUATION MEASURES (FLOW CONTROL LIMITING DISCHARGE TO PRE-DEVELOPMENT RATES) ARE USED. - GRAVEL AND SAND DEPOSITS: IN PARTS OF EAST LONDON (ON THAMES GRAVEL TERRACES — LEWISHAM, GREENWICH, PARTS OF HACKNEY), PERCOLATION RATES CAN BE HIGH ENOUGH TO SUPPORT A SOAKAWAY. TEST BEFORE ASSUMING.

LONDON SUDS (SUSTAINABLE URBAN DRAINAGE) REQUIREMENTS: THE LONDON PLAN (GLA POLICY SI 13) AND BUILDING REGULATIONS PART H REQUIRE NEW DEVELOPMENTS TO MANAGE SURFACE WATER SUSTAINABLY: - HIERARCHY OF DRAINAGE DISCHARGE (BUILDING REGULATIONS APPROVED DOCUMENT H — TABLE H2): (1) SOAKAWAY/INFILTRATION; (2) WATERCOURSE; (3) SURFACE WATER SEWER; (4) COMBINED SEWER — ONLY IF NO OTHER OPTION IS FEASIBLE - FOR EXTENSIONS AND NEW BUILDINGS: SURFACE WATER RUNOFF FROM NEW IMPERMEABLE SURFACES (ROOFS, DRIVEWAYS) MUST BE ATTENUATED OR DISPOSED OF VIA INFILTRATION WHERE GROUND CONDITIONS ALLOW - LONDON PLAN SuDS REQUIREMENT: FOR MAJOR DEVELOPMENTS (> 10 UNITS — NOT APPLICABLE TO INDIVIDUAL HOUSES), SuDS ARE MANDATORY. FOR INDIVIDUAL HOUSE EXTENSIONS, BUILDING CONTROL WILL CHECK THE DRAINAGE PROPOSAL — INFILTRATION IS PREFERRED, OR ATTENUATION PLUS CONTROLLED DISCHARGE TO SEWER

WHERE A SOAKAWAY IS NOT FEASIBLE (LONDON CLAY, PROXIMITY TO BUILDING, NO SPACE), ALTERNATIVE SUSTAINABLE DRAINAGE MEASURES FOR VICTORIAN TERRACES INCLUDE:

PERMEABLE PAVING: REPLACING IMPERMEABLE SURFACES (CONCRETE, TARMAC, SOLID PAVING SLABS) WITH PERMEABLE ALTERNATIVES REDUCES RUNOFF FROM DRIVEWAYS AND FRONT GARDENS: - TYPES: PERMEABLE BLOCK PAVING (MARSHALLS PRIORA, TOBERMORE HYDROPAVE — JOINTS FILLED WITH FINE GRAVEL OR LEFT OPEN; WATER FLOWS THROUGH THE JOINTS); RESIN-BOUND GRAVEL (RONACRETE, ADDAGRIP TERRAFIRMA — PERMEABLE TO WATER THROUGH THE RESIN MATRIX); GRAVEL (LOOSE GRAVEL WITH A PERMEABLE MEMBRANE UNDERNEATH — THE SIMPLEST AND CHEAPEST OPTION) - SUDS PLANNING BENEFIT: IN LONDON, PAVING A FRONT GARDEN IN AN IMPERMEABLE MATERIAL EXCEEDING 5M² REQUIRES PLANNING PERMISSION — PERMEABLE MATERIALS AVOID THIS REQUIREMENT (SEE FRONT GARDEN PAVING PLANNING GUIDANCE) - BASE COURSE: PERMEABLE PAVING REQUIRES A PERMEABLE SUB-BASE (OPEN-GRADED CRUSHED STONE — TYPE 3 GRANULAR SUB-BASE, NOT COMPACTED TYPE 1 WHICH HAS LESS VOID SPACE)

RAINWATER GARDEN (BIORETENTION CELL): A SHALLOW DEPRESSION PLANTED WITH MOISTURE-TOLERANT PLANTS THAT RECEIVES ROOF WATER FROM A DOWNPIPE AND ALLOWS IT TO SOAK INTO THE GROUND AND EVAPOTRANSPIRE (EVAPORATE THROUGH THE PLANTS): - SIZE: TYPICALLY 5-20% OF THE CONTRIBUTING ROOF AREA - PLANTS: MOISTURE-TOLERANT NATIVE PLANTS (IRIS PSEUDACORUS — YELLOW FLAG IRIS; FILIPENDULA ULMARIA — MEADOWSWEET; MOLINIA CAERULEA — PURPLE MOOR GRASS; CAREX SPP. — SEDGES) - OVERFLOW: MUST HAVE AN OVERFLOW TO THE SEWER OR DRAIN FOR EXTREME RAIN EVENTS

RAINWATER HARVESTING TANK: WHERE A SOAKAWAY IS NOT FEASIBLE, A RAINWATER HARVESTING TANK STORES ROOF WATER FOR GARDEN IRRIGATION OR TOILET FLUSHING — REDUCING POTABLE WATER USE AND MANAGING RUNOFF FROM THE ROOF: - PRODUCTS: KINGSPAN ENDURAMAXX 1500L, GRAF CARAT RAINWATER TANK 3700L (UNDERGROUND) - PAYBACK: PRIMARILY ENVIRONMENTAL (WATER USE REDUCTION) — FINANCIAL PAYBACK IS VERY LONG (15-20 YEARS) WITHOUT WATER METERING

COSTS (LONDON 2025-2026): PERCOLATION TEST (PROFESSIONAL): £200-400 PLASTIC CRATE SOAKAWAY (4M³ POLYSTORM CELLS, GEOTEXTILE WRAP, INSTALL): £1,200-2,500 INSPECTION CHAMBER FOR SOAKAWAY: £250-500 (ADD-ON) PERMEABLE BLOCK PAVING (PER M²): £60-120 (SUPPLY AND FIT) RESIN-BOUND GRAVEL DRIVEWAY (PER M²): £80-150 (SUPPLY AND FIT) RAINWATER HARVESTING TANK (1,500L, ABOVE GROUND — SUPPLY AND FIT): £600-1,200