Foundations

Foundations transfer the building's weight to the ground. Their form depends on the soil and the load: strip footings under walls, pad footings under columns, raft slabs over the whole footprint, piles taken down to firmer strata, and in older Australian houses, timber or concrete stumps supporting suspended floors. A foundation's behaviour is governed as much by the ground as by the concrete: reactive clays that swell and shrink with moisture, fill, soft alluvium, groundwater, mine workings and earthquakes all move foundations that are themselves sound.

In Australia, sites are classified by soil reactivity under AS 2870, from stable sand and rock through moderately, highly and extremely reactive clays to problem sites; the classification sets how much movement a house footing must tolerate. In Britain, shrinkable clays in the south-east and trees drive most subsidence. In the United States, expansive soils in Texas, Colorado and California, and seismic hazard on the west coast, dominate.

Foundations rarely wear out; they move. What shortens their effective life is a change in the ground around them:

• Trees drawing moisture from clay, which shrinks and lets footings drop, most in dry seasons and droughts.
• Tree removal, which lets clay rewet and swell over years (heave).
• Leaking drains and water pipes, which soften soil and wash fines away.
• Poor site drainage: water ponding against footings, downpipes discharging at the wall base.
• Excavation nearby: neighbouring basements, trenches and dewatering.
• Falling groundwater, which exposes timber piles to decay.
• Mine workings collapsing below the site.

Subsidence from clay shrinkage

Clay soils lose volume as they dry. Tree roots and drought dry the soil beneath part of a building, which then drops relative to the rest. Movement is seasonal at first, opening in summer and closing in winter, and can become progressive as the tree grows. Damage concentrates at the corner or side nearest the tree.

Heave

Clay swells as it wets. Removing large trees, leaking pipes and changed drainage let clay rewet under a building; the affected part rises, sometimes over many years. Heave lifts floors and cracks walls in patterns opposite to subsidence.

Settlement

New loads compress the ground beneath them. Most settlement happens early in a building's life; continuing settlement points to fill that is still consolidating, soft soils, or an extension on a shallower foundation pulling away from the original.

Washout and erosion

Leaking drains and broken stormwater pipes wash fine soil from beneath footings, leaving voids that let them drop locally, often suddenly.

Timber pile and stump decay

Timber piles last as long as they stay saturated. When groundwater falls, through dewatering or changed drainage, the tops dry and rot, and the structure above settles. Timber stumps rot at ground level and are attacked by termites, and the floor sinks unevenly.

Chemical attack

Sulfates in soil and groundwater attack concrete; acid sulfate soils and contaminated land corrode reinforcement and steel piles.

Earthquake and liquefaction

Earthquakes can cause loose saturated sands to liquefy, so foundations lose support. Older foundations not designed for seismic loads can fail to hold buildings together in a quake.

Mine subsidence

Collapse of old coal and mineral workings beneath a site produces settlement troughs and tilt. Mining regions in New South Wales, Britain and parts of the United States carry this risk and have notification or compensation schemes.

Outside

• Tapering diagonal cracks in masonry, usually starting at openings and running through bricks and mortar.
• Distorted window and door heads; gaps between frames and walls.
• Cracks continuing below the damp-proof course into the footing.
• Trees within a distance of the building comparable to their height, especially on clay.
• Paths and paving sloping toward walls; downpipes discharging at the wall base.
• Gaps opening between extensions and the original building.

Inside

• Doors and windows sticking or swinging open; floors sloping noticeably.
• Cracks at the corners of openings and between walls and ceilings.
• Skirting boards pulling away from floors; tiled floors cracking along a line.

In the records

• Soil and site classification; geotechnical reports.
• Previous underpinning or restumping.
• Insurance subsidence claims.
• Crack monitoring data.
• Drain CCTV surveys.
• Mine subsidence or coal mining reports.

Findings are graded by what they demand: safety (can hurt someone now), failure (failed and damaging the building), condition (distress present and progressing), lifecycle (renewal due on age and exposure), deadline (a regulator or ordinance sets the date), check (investigate before sizing works) and nil (cosmetic or stable).

Crack widths map to damage categories under BRE Digest 251 in Britain and Appendix C of AS 2870 in Australia; see the masonry page. A lifecycle grade rarely applies to foundations themselves; it applies to stumps and other replaceable elements.

A structural or geotechnical engineer leads, with an arborist where trees are involved and a drainage contractor for drain surveys.

• Crack mapping and monitoring: record every crack with width and taper, fit tell-tales or gauge studs, and read them monthly for at least a year.
• Level survey of floors and brick courses to show the shape of movement.
• Drain investigation: CCTV and pressure testing of drains and water pipes near the affected area.
• Trial pits and boreholes: foundation type and depth, soil type, moisture content and plasticity, roots.
• Tree survey: species, height, distance and water demand; root identification from samples.
• Groundwater monitoring where timber piles or dewatering are involved.
• Records review: original foundation design, previous repairs, insurance claims, mining reports.

The report should identify the cause of movement with the evidence for it, whether movement is seasonal or progressive, and the remedy, starting with removing the cause.

• Remove the cause first: repair leaking drains and pipes, fix site drainage, manage or remove trees (with heave risk assessed before removal), and stop downpipes discharging at the wall base.
• Monitor after removing the cause; many buildings stabilise and need only crack repair.
• Underpinning where movement continues: mass concrete underpinning, mini-piles, screw piles, or resin injection to stiffen and lift soils, selected for the ground and access.
• Restumping of houses on decayed timber stumps with concrete or steel stumps, with re-levelling of floors.
• Root barriers between trees and buildings where trees must stay.
• Crack repair after stabilisation: stitching with helical bars, repointing, and replacement of damaged bricks.
• Heave precautions where trees are removed from clay sites: phased removal, monitoring and flexible repairs.
• Pile remediation for decayed timber piles: underpinning to new piles, or raising groundwater where possible.

Monitoring and investigation are lump sums spread over a year. Drain repairs are priced by length or per defect. Underpinning is priced per metre of wall or per pile, with access, excavation and reinstatement of floors and finishes inside often doubling the cost. Restumping is priced per stump or per house. Crack repairs are itemised after stabilisation.

Underpinning and restumping are lifecycle or major repair items; drain and drainage works are repairs and maintenance; monitoring is the check line that decides whether underpinning is needed at all. In Britain, subsidence is commonly an insurance matter, and insurers manage the investigation and repair.

• Australia. AS 2870 governs residential slabs and footings, including site classification and damage categories; AS 2159 covers piling; the National Construction Code calls these up. In New South Wales, Subsidence Advisory NSW manages mine subsidence districts and compensation. Restumping and underpinning need building approval.
• United Kingdom. Approved Document A governs structure; BS 8004 and BS EN 1997 cover foundations; NHBC Standards Chapter 4.2 sets building near trees; the Coal Authority provides mining reports. Subsidence is usually handled through buildings insurance, and underpinning needs building regulations approval and often party wall notices.
• United States. The International Building and Residential Codes (Chapter 18 of the IBC) govern foundations, with special provisions for expansive soils; ASCE 7 sets seismic design; the International Existing Building Code governs repairs. Boston's Groundwater Conservation Overlay District protects timber piles from falling groundwater.

• Geotechnical reports and the site or soil classification.
• Foundation drawings and design.
• Crack monitoring records and engineer's reports on movement.
• Underpinning, restumping or previous repair records, with warranties.
• Insurance claims for subsidence or heave.
• Drain CCTV surveys and repair records.
• Tree removal records and arborist reports.
• Mine subsidence or mining reports where relevant.