Basement, Cellars & Retaining Walls

Basements and cellars are rooms below ground, enclosed by walls that hold back soil and often groundwater. Retaining walls hold back soil at changes in level on a site: driveways, embankments, terraces, and the edges of basement excavations. Both resist earth pressure, and both must manage water, because water behind a wall both adds pressure and finds its way through.

Basement waterproofing follows three broad approaches, set out in Britain as Types A, B and C: a barrier applied to the structure (tanking), a structurally integral watertight concrete, and a drained cavity that accepts some water and pumps it away. Many buildings combine them. Which approach the building has decides how leaks are fixed.

What shortens life:

• Blocked drainage behind retaining walls and basement walls, which raises water pressure.
• Surcharge loads: new structures, vehicles, fill and pools placed behind walls that were not designed for them.
• Tree roots pushing on and through walls.
• Rising groundwater and changed drainage on and around the site.

Water ingress

Water enters basements through cracks, construction joints, tie holes, service penetrations and the junction between walls and floors. Leaks that run in wet weather point to surface water; leaks that run all year point to groundwater. Efflorescence, tide marks and mould follow, and stored goods and services are damaged.

Hydrostatic and earth pressure

A wall designed for drained soil fails when its drainage blocks: water pressure adds to earth pressure and the wall rotates, bulges or cracks. Many retaining wall failures follow heavy rain on walls with blocked weep holes or failed drains.

Rotation and bowing

Cantilever retaining walls rotate forward at the top; masonry cellar walls bow inward under earth pressure where floors do not restrain them. Movement that continues leads to collapse.

Timber sleeper decay

Timber sleepers and posts rot where they are in contact with wet soil, and walls lean and collapse, often after a decade or two. Posts rot below ground first, so a wall can look sound until it fails.

Surcharge

Driveways, car parking, new buildings, pools and fill placed close behind a wall add load it was never designed for.

Anchor and reinforcement corrosion

Ground anchors and soil nails lose capacity as their steel corrodes; reinforcement in walls corrodes where water penetrates cracks.

Radon and ground gases

Basements in radon-affected areas and on contaminated land collect radon or other ground gases through cracks and joints. Radon is a recognised cause of lung cancer; testing is advised in affected areas in Britain and the United States.

On site

• Retaining walls leaning forward, bulging or cracked; fence and handrail lines out of true.
• Weep holes blocked or dry after rain; water seeping through the wall face.
• Timber sleepers soft at the base; posts loose.
• Ground cracking or slumping behind walls; loads placed close behind them.
• Basement walls with damp patches, efflorescence, tide marks and mould.
• Water entering at joints and penetrations; sump pumps running constantly.

In the records

• Waterproofing specification and warranty.
• Retaining wall design and approvals.
• Leak and flood history.
• Monitoring of wall movement.
• Radon test results in affected areas.

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).

A structural or geotechnical engineer leads; a waterproofing specialist joins for basements.

• Wall survey: verticality, bulging and cracking, with fixed survey points for monitoring over time.
• Drainage inspection: weep holes, drains behind walls by CCTV, outlets and surface drainage.
• Leak tracing: mapping wet areas, monitoring in wet and dry periods, and testing at joints and penetrations.
• Timber probing of sleepers and posts at and below ground.
• Load review: what sits behind the wall against its design.
• Design and approval check for retaining walls above the height that needs approval.
• Radon testing in affected areas.

The report should state structural stability, the waterproofing approach and its failures, drainage condition, and remedies.

For basements, testing in both a dry and a prolonged wet period is often needed before the source of water can be confirmed and the right waterproofing repair chosen.

• Restore drainage: clear or replace drains behind walls, open weep holes, fix surface drainage.
• Crack and joint injection with polyurethane or epoxy resins.
• Drained cavity systems on the inside of basements with sumps and pumps where external access is impossible.
• External tanking where walls can be excavated.
• Retaining wall strengthening: buttresses, anchors or soil nails, or rebuilding.
• Sleeper wall replacement with concrete or masonry systems.
• Remove surcharge loads or redesign the wall for them.
• Radon mitigation: sealing and sub-floor depressurisation.

• Joint and penetration seals: hydrophilic strips, injection hoses and puddle flanges at service penetrations, which are the commonest leak points.
• Surface water management: regrading, drainage channels and downpipe connections so rainwater is carried away from basement and retaining walls.

Investigation and monitoring are lump sums. Injection is priced by length of crack or joint; drained cavity systems by area with pumps; external tanking by area with excavation; retaining wall rebuilding by length and height. Drainage repairs by length. Pumps recur on cycle.

Rebuilding and new waterproofing are lifecycle; drainage and injection are repairs and maintenance; unsafe walls are immediate.

Basement leak repairs in occupied car parks and plant rooms often need out-of-hours access and protection of services, which can exceed the cost of the injection itself.

• Australia. AS 4678 governs earth-retaining structures; the National Construction Code covers damp and weatherproofing; councils require approval for retaining walls above set heights, often around a metre, and walls near boundaries can involve neighbours.
• United Kingdom. BS 8102 sets the protection of below-ground structures against water, with grades of environment and Types A, B and C; BS EN 1997 covers geotechnical design; the UK Health Security Agency maps radon-affected areas.
• United States. The International Building Code covers damp-proofing and waterproofing (Section 1805) and retaining walls (Section 1807); the EPA recommends radon testing and mitigation above its action level.

In all three jurisdictions, a retaining wall on or near a boundary can affect a neighbour's land, and responsibility depends on who built it and whose land it supports; failures often lead to disputes, so ownership and approvals should be established early. Strata and condominium buildings usually treat basement structure and waterproofing as common property.

• Waterproofing specification, type and warranty.
• Retaining wall designs and approvals.
• Leak, flood and pump records.
• Movement monitoring data.
• Drainage drawings.
• Radon test results in affected areas.

• Survey plans showing walls in relation to boundaries.
• Geotechnical reports for the site.
• Records of loads or structures added behind walls.
• Correspondence with neighbours about shared walls.