Basement Retaining Wall Design: Temporary Works, Lateral Earth Pressure, Waterproofing and Drainage

Quick Answer: Basement retaining walls must resist lateral earth pressure calculated to BS EN 1997-1 (Eurocode 7), with reinforced concrete walls typically 200–300mm thick for domestic depths of 2.5–3.5m. Temporary works during excavation require separate structural design under CDM Regulations 2015, and the permanent wall design must integrate waterproofing requirements from BS 8102:2022 from the outset — waterproofing cannot be bolted on afterwards.

Summary

A basement retaining wall carries two distinct structural roles: resisting the lateral earth pressure from the retained soil, and forming part of the waterproofing envelope. These two functions must be designed together, not in sequence. A structurally adequate wall that has poor concrete quality, construction joints in the wrong places, or misplaced wall ties will leak regardless of what waterproofing is applied to the surface.

The distinction between temporary works and permanent works is critical. The sheet piling, king post walls, or contiguous pile walls installed to allow excavation are temporary structures — they require a separate engineer, separate design, and separate checking. Some temporary works systems can be incorporated into the permanent wall design, but this must be explicitly designed for and approved by the permanent works engineer.

For domestic basement conversions under existing houses, the most common approach is traditional mass concrete underpinning followed by reinforced concrete retaining walls cast against the underpinned foundations and the excavated face. This is technically proven but requires careful sequencing and adequate temporary support at every stage.

Key Facts

• Lateral earth pressure — horizontal force exerted by retained soil on a retaining wall; calculated using Rankine or Coulomb earth pressure theories per BS EN 1997-1
• Active earth pressure coefficient (Ka) — for typical UK cohesionless fill or granular soil with friction angle φ = 30°: Ka = tan²(45° - φ/2) ≈ 0.33
• At-rest earth pressure coefficient (K0) — used for rigid walls with no rotation; for normally consolidated soil: K0 = 1 - sin φ ≈ 0.5 for φ = 30°
• Hydrostatic pressure — adds to earth pressure where groundwater is present; 1m water head = 10 kPa additional horizontal pressure
• Surcharge loading — loads from structures, vehicles, or soil above retained height that increase earth pressure; minimum 10 kPa surcharge applied even if no obvious surcharge source
• Reinforced concrete wall thickness — typically 200–250mm for retained heights under 2.5m; 250–300mm for 2.5–4m; structural engineer calculates based on actual loading
• Concrete grade — C32/40 minimum for basement walls; C35/45 for waterproof concrete (Type B construction)
• Construction joints — horizontal and vertical construction joints are waterproofing weak points; location must be specified by the engineer and treated with hydrophilic waterstop bar or crystalline product
• Waterstop bar — PVC or hydrophilic rubber strip cast into construction joints to prevent water migration; must be specified in the correct location (mid-thickness of wall, centred on joint)
• Wall tie holes — penetrations for formwork ties are a common leak source; specify removable ties with cone inserts to leave a tapering void, filled with non-shrink mortar and crystalline slurry after striking formwork
• Thermal movement joints — required in long runs of basement wall (>15m typically) to prevent cracking from temperature and shrinkage
• Underpinning bay sequence — alternate bays (never adjacent bays); 1m maximum bay width for mass concrete; 48-hour cure minimum before adjacent bay
• Propping — temporary props or rakers required during excavation and before permanent slab is cast; remove only after floor slab has achieved adequate strength
• CDM Regulations 2015 — temporary works require a Temporary Works Coordinator and design by a competent person; for complex schemes, a separate Temporary Works Designer and checking engineer

Quick Reference Table

Note: dimensions are indicative only; actual reinforcement must be engineered to specific site conditions.

Detailed Guidance

Temporary Works Systems

Temporary earth retention during basement excavation typically uses one of the following systems:

Sheet piling — steel interlocking sections driven by hydraulic press or vibration; suitable for most soils except cobbles or boulders; relatively quiet press-piling systems are available for residential areas. Sheets can be extracted after construction or left in place (sacrificial). If left in place, they must not be assumed to contribute to the permanent wall design unless explicitly designed for composite action.

King post walls (soldier pile walls) — H-section steel posts installed in pre-bored holes at 1.0–1.5m centres; timber planking (lagging) installed between posts as excavation proceeds. More economical than sheet piling in stable ground; not suitable where groundwater control is required at excavation stage.

Contiguous bored pile walls — bored piles at close centres forming a continuous wall; some water ingress between piles is accepted; can be incorporated into the permanent wall by casting a reinforced concrete facing against them. A genuinely water-retaining solution requires secant piles (alternating hard and soft piles with overlap) rather than contiguous piles.

Jet grouted columns — used in variable fill or where access prevents traditional temporary works; expensive but effective in difficult ground.

Temporary propping — all lateral retention systems require propping unless they are cantilevered. For domestic basements, raking props to a capping beam at ground level are common. Propping loads must be transferred to the permanent structure or relieved before props are removed.

Permanent Retaining Wall Construction

The typical construction sequence for a domestic basement conversion:

1. Setting out and protection — service diversions, protection of adjacent structures, welfare facilities
2. Underpinning — alternate bay sequence, concrete poured and cured before adjacent bay
3. Bulk excavation — to formation level (under propping if required); dispose of spoil
4. Blinding slab — 50mm plain concrete blinding; provides clean working surface and protection to any waterproofing membrane beneath the structural slab
5. Waterproofing membrane (if Type A/C) — laid over blinding with full laps; turned up at edges ready for wall connection
6. Structural slab — reinforced concrete to engineer's specification; includes starter bars for retaining wall connection
7. Retaining wall construction — form, reinforce, and pour in lifts; waterstop in construction joints; no admixtures without engineer approval
8. Curing — minimum 7 days covered cure; cold weather = cover with insulating blankets
9. Wall waterproofing — applied to positive face before backfill, negative face treatment if required
10. Drainage layer — 100mm granular or proprietary drainage board against waterproof face; prevents hydrostatic pressure build-up against the membrane
11. Backfill — granular material in 300mm compacted layers; avoid plant impact loading against green concrete for minimum 28 days

Construction Joint Waterproofing

Construction joints are the most common source of basement water ingress after defective waterproofing membrane installation. Specify:

Hydrophilic rubber waterstop — cast into the centre of the wall thickness at every horizontal and vertical joint; expands 2–3 times on contact with water, sealing the joint. Proprietary products (e.g. Sika Swellstop, Fosroc Supercast SW) must be installed on dry concrete immediately before the next pour.

Crystalline additive — Xypex or Kryton crystalline powder applied as a slurry to the joint face or added to the mix at the joint location; forms insoluble crystals that grow towards water penetration.

Injection hose — polyamide perforated hose cast into the joint at mid-depth; allows post-construction chemical injection (polyurethane or epoxy resin) if a leak develops. Useful as belt-and-braces on WC2/WC3 sites.

Integrating Waterproofing with the Structural Design

The structural and waterproofing design must be coordinated from the outset:

• Type B (integral waterproof concrete) — requires C35/45 concrete with a maximum water-cement ratio of 0.45 and minimum cement content of 340 kg/m³; construction joints treated with waterstop; cover to reinforcement 40mm minimum. Additive packages (crystalline, hydrophobic) improve performance but do not substitute for correct concrete mix design.
• Type A (barrier waterproofing) — applied to the external face before backfill; requires a flat, finished concrete surface free of honeycombing; structural tie holes must be filled before membrane application
• Type C (cavity drain) — installed internally post-construction; wall design still needs to resist earth pressure; drainage channel captures any water passing through the wall and directs to sump

On WC2 and WC3 sites, BS 8102:2022 recommends combining at least two system types for Grade 3 (habitable) use. The structural wall design must accommodate both.

Frequently Asked Questions

Can temporary sheet piles be incorporated into the permanent wall?

Sometimes, but only if explicitly designed for composite action. The temporary works designer and permanent works designer must coordinate. In practice, compositely designed sheet piling is uncommon for domestic basements because the corrosion life and structural section of steel piles are rarely appropriate for the design life required. More commonly, a reinforced concrete facing wall is cast against the inside face of the sheet piles, with the piles left in place as sacrificial formwork.

What is the minimum concrete cover for reinforcement in a basement wall?

Building Regulations and BS EN 1992-1-1 (Eurocode 2) require minimum cover based on exposure class. For a basement wall in contact with soil, exposure class XC2 (wet/dry cycling) to XC4 (cyclic wet/dry with chemical exposure) applies. Minimum cover is typically 40mm + allowance for fixing tolerance (10mm), giving a nominal 50mm cover on the soil face. For waterproof concrete (Type B), 40mm minimum is specified in BS 8102:2022.

Who signs off temporary works?

Under CDM Regulations 2015, the Principal Contractor is responsible for the construction phase plan and ensuring temporary works are designed by a competent person. For schemes with a Temporary Works Coordinator (TWC) — required by BS 5975 for all significant temporary works — the TWC must verify the design before work starts and check that installation matches the design. A TWC does not need to be a structural engineer but must have appropriate experience.

How deep can a domestic basement be without specialist retaining methods?

This depends entirely on soil conditions, adjacent structures, and surcharge loading. In firm, cohesive soils well away from structures, up to 2.5m excavation may be possible with traditional underpinning and minimal temporary earth support. In loose fill, granular, or made ground — particularly adjacent to neighbour foundations — specialist temporary works may be required from the outset. Never assume depth alone determines the need for specialist works; soil type and proximity to adjacent structures matter more.

Regulations & Standards

• BS EN 1997-1 (Eurocode 7) — Geotechnical Design General Rules; governs lateral earth pressure calculation, design approach for retaining walls, and ground investigation scope

• BS EN 1992-1-1 (Eurocode 2) — Design of Concrete Structures; governs reinforced concrete retaining wall design, minimum cover, concrete mix requirements

• BS 8102:2022 — Code of Practice for Protection of Below Ground Structures Against Water; integrated waterproofing requirements for Type B waterproof concrete and construction joint specification

• BS 5975:2019 — Code of Practice for Temporary Works Procedures; Temporary Works Coordinator role, checking requirements, design documentation

• CDM Regulations 2015 (SI 2015/51) — Construction Design and Management; Principal Contractor and Temporary Works Coordinator responsibilities; construction phase plan requirements

• Building Regulations Part A (Structure) — structural adequacy of retaining walls and permanent works; inspections by Building Control

• ICE — Temporary Works: Principles of Design and Construction — industry guidance for temporary works design

• The Concrete Centre — Retaining Wall Design — worked examples for RC retaining wall design to Eurocode 7

• HSE — Temporary Works Safety — CDM 2015 guidance on temporary works responsibilities

• Sika — Structural Waterproofing Handbook — construction joint treatment options and waterproof concrete guidance

• BS 8102:2022 — BSI — primary standard for below-ground waterproofing

• basement conversion Building Regulations overview — the regulatory framework governing structural and waterproofing design

• groundwater risk assessment for basements — the ground investigation that determines hydrostatic pressure loads on retaining walls

• BS 8102:2022 waterproofing system types — Type A, B, and C waterproofing that must integrate with the structural wall

• structural waterproofing design — the overall design process coordinating structure and waterproofing