House Lifting and Underpinning Alternatives in London: When and Why to Consider It

LONDON VICTORIAN TERRACES WERE BUILT ON SHALLOW FOUNDATIONS — TYPICALLY BRICK FOOTINGS 450-600MM DEEP SEATED ON THE LONDON CLAY SUBSOIL. THESE FOUNDATIONS WERE ADEQUATE WHEN BUILT (LOADS WERE LOWER; VEGETATION WAS MANAGED), BUT SEVERAL FACTORS NOW CAUSE PROBLEMS:

CLAY SHRINKAGE AND TREE ROOT DAMAGE: LONDON IS BUILT LARGELY ON LONDON CLAY — A HIGHLY SHRINKABLE CLAY THAT SWELLS WHEN WET AND SHRINKS WHEN DRY. SEASONAL MOVEMENT OF LONDON CLAY IS NORMAL, BUT STRUCTURAL DAMAGE (SUBSIDENCE) OCCURS WHEN: - TREES OR LARGE SHRUBS WITHIN 10-20M OF THE PROPERTY EXTRACT MOISTURE FROM THE CLAY DURING DRY SUMMERS — PARTICULARLY TREES WITH LARGE ROOT SYSTEMS (POPLAR, ELM, WILLOW, OAK, ASH). LONDON\'S DRY SUMMERS (INCREASING WITH CLIMATE CHANGE) INTENSIFY CLAY DESICCATION. - THE ROOT ZONE OF TREES HAS BEEN REMOVED (TREE FELLING OR MAJOR PRUNING) — THE CLAY THEN SWELLS AS MOISTURE RETURNS, CAUSING HEAVE (UPWARD MOVEMENT) RATHER THAN SETTLEMENT (DOWNWARD MOVEMENT). HEAVE CAN BE MORE DAMAGING THAN SETTLEMENT TO SHALLOW VICTORIAN FOUNDATIONS. - INSURANCE: SUBSIDENCE DAMAGE TO LONDON PROPERTIES IS TYPICALLY COVERED BY BUILDINGS INSURANCE — THE INSURER WILL ARRANGE A STRUCTURAL ENGINEER SURVEY AND MONITOR THE MOVEMENT OVER ONE OR MORE SEASONS BEFORE DECIDING ON REMEDIATION. UNDERPINNING IS USUALLY A LAST RESORT.

DRAINAGE LEAKS: LEAKING DRAIN RUNS UNDER OR ADJACENT TO THE FOUNDATIONS CAN SOFTEN AND WASH OUT THE CLAY OR MADE GROUND BELOW THE FOOTINGS — LEADING TO SETTLEMENT. CCTV DRAIN SURVEY (DYNO-ROD, LOCAL DRAINAGE SPECIALIST) IS A STANDARD FIRST STEP WHEN CRACKS APPEAR.

ORIGINAL INADEQUATE FOUNDATIONS: SOME VICTORIAN TERRACES — ESPECIALLY THOSE BUILT QUICKLY ON RELATIVELY POOR MADE GROUND (FILLED GROUND, OLD WASTE GROUND, MARSHY AREAS) — HAVE FOUNDATIONS THAT WERE NEVER ADEQUATE. IN AREAS OF SOUTH AND EAST LONDON IN PARTICULAR (LEWISHAM, BERMONDSEY, STRATFORD), GROUND CONDITIONS CAN BE COMPLEX.

NEW EXTENSION LOADING: ADDING A REAR EXTENSION OR CHANGING THE USE OF THE LOFT (ADDING DEAD LOADS) CAN INCREASE THE BEARING STRESS ON THE EXISTING FOUNDATIONS. A STRUCTURAL ENGINEER\'S FOUNDATION CHECK IS GOOD PRACTICE BEFORE SIGNIFICANT EXTENSIONS ON OLDER PROPERTIES.

WHERE FOUNDATION REMEDIATION IS REQUIRED, THE OPTIONS DEPEND ON THE CAUSE, THE EXTENT OF DAMAGE, AND THE GROUND CONDITIONS. HOUSE LIFTING (DESCRIBED LAST) IS THE MOST EXTREME OPTION.

TRADITIONAL BEAM-AND-BASE UNDERPINNING (MASS CONCRETE): THE MOST COMMON UNDERPINNING METHOD IN UK RESIDENTIAL — A SEQUENTIAL METHOD WHERE SECTIONS OF GROUND BELOW THE EXISTING FOUNDATION ARE EXCAVATED AND FILLED WITH MASS CONCRETE IN SEQUENCE (NEVER MORE THAN 1/3 OF THE WALL LENGTH AT ONE TIME): - A SERIES OF BAYS (TYPICALLY 900MM-1,200MM WIDE) ARE EXCAVATED UNDER THE EXISTING FOUNDATION IN AN ALTERNATING SEQUENCE - EACH BAY IS EXCAVATED DOWN TO STABLE GROUND (FIRM CLAY OR GRAVEL — TYPICALLY 1.5-3.0M BELOW EXISTING FOUNDATION LEVEL IN LONDON) - THE BAY IS FILLED WITH MASS CONCRETE (C30 MIX), COMPACTED, AND LEFT TO CURE BEFORE ADJACENT BAYS ARE EXCAVATED - THE NEW CONCRETE FORMS AN ENLARGED BASE SITTING ON DEEPER, STABLE STRATA - SEQUENTIAL PATTERN: BAYS ARE NUMBERED; ODD BAYS FIRST, EVEN BAYS SECOND — THIS PREVENTS CONTINUOUS EXCAVATION RUNS UNDER THE WALL - TYPICAL DEPTH OF UNDERPINNING IN LONDON CLAY: 1.5-3.0M - COST (SUPPLY AND FIT, LONDON): £1,000-2,000 PER LINEAR METRE OF UNDERPINNED WALL

MINI-PILE UNDERPINNING: WHERE DEPTH TO STABLE GROUND IS GREATER THAN 3.0M, OR WHERE THE GROUND IS VARIABLE (MADE GROUND, COMPLEX STRATIGRAPHY), MINI-PILE UNDERPINNING IS MORE APPROPRIATE: - SMALL-DIAMETER PILES (TYPICALLY 150-250MM DIAMETER) DRILLED FROM GROUND LEVEL THROUGH THE MADE GROUND TO FIRM STRATA OR BEDROCK - PILE HEADS CONNECTED BY A REINFORCED CONCRETE GROUND BEAM (CAPPING BEAM) CAST AGAINST THE EXISTING FOUNDATION - ADVANTAGE OVER BEAM-AND-BASE: MINIMAL EXCAVATION; FASTER (DRILLING RATHER THAN HAND-DIG); SMALLER SPOIL VOLUMES; SUITABLE FOR RESTRICTED ACCESS (MINI-RIGS CAN ACCESS THROUGH A STANDARD DOOR) - LONDON MINI-PILE CONTRACTORS: MAINMARK UK, GROUNDFORCE SHORCO, GEOBEAR (POLYURETHANE RESIN INJECTION — A NON-INVASIVE ALTERNATIVE FOR MINOR SETTLEMENT) - COST (LONDON): £1,500-3,000 PER PILE (PLUS CAPPING BEAM); TOTAL PROJECT COST TYPICALLY £15,000-50,000+ DEPENDING ON EXTENT

GEOBEAR RESIN INJECTION (NON-INVASIVE): FOR MINOR SETTLEMENT (LESS THAN 25MM, STABLE CLAY, NO ACTIVE DRAIN LEAKS, NO TREE ROOT ISSUE), POLYURETHANE RESIN INJECTION IS A FASTER AND LESS INVASIVE ALTERNATIVE: - SMALL-DIAMETER TUBES (14MM) INSERTED THROUGH THE FLOOR SLAB OR GROUND ADJACENT TO THE FOUNDATION - POLYURETHANE RESIN INJECTED UNDER CONTROLLED PRESSURE — EXPANDS AND DENSIFIES THE GROUND, LIFTING AND STABILISING THE FOUNDATION - ADVANTAGE: NO EXCAVATION; MINIMAL DISRUPTION; BUILDING REMAINS OCCUPIED; PROCESS TAKES 1-2 DAYS - LIMITATION: ONLY SUITABLE FOR MINOR SETTLEMENT ON STABLE CLAY GROUND; NOT APPROPRIATE WHERE ACTIVE DRAIN LEAKS OR TREE ROOT ISSUES ARE PRESENT - COST: TYPICALLY £3,000-10,000 FOR A RESIDENTIAL PROJECT

HOUSE LIFTING (STRUCTURAL RAISING): HOUSE LIFTING — PLACING HYDRAULIC JACKS UNDER THE STRUCTURE AND PHYSICALLY RAISING THE BUILDING — IS OCCASIONALLY USED IN THE UK FOR: - FLOOD RISK MITIGATION: RAISING A PROPERTY ABOVE THE PREDICTED FLOOD LEVEL - PROVIDING ADDITIONAL HEAD HEIGHT IN A BASEMENT OR LOWER GROUND FLOOR - ACCESSING AND REPLACING THE ENTIRE FOUNDATION SYSTEM

IN LONDON, HOUSE LIFTING IS EXTREMELY RARE FOR VICTORIAN TERRACES — THE PARTY WALL CONSTRAINTS (THE PROPERTY IS STRUCTURALLY CONNECTED TO NEIGHBOURS ON BOTH SIDES) MAKE INDEPENDENT LIFTING PRACTICALLY IMPOSSIBLE WITHOUT MAJOR STRUCTURAL INTERVENTION ON THE PARTY WALLS. ALTERNATIVE APPROACHES (BEAM-AND-BASE UNDERPINNING, MINI-PILES) ALMOST ALWAYS ACHIEVE THE SAME RESULT AT LOWER COST AND DISRUPTION.