Underpinning Methods: Mass Concrete, Mini-Pile and Beam-and-Base — When Each Is Used

Summary

Underpinning is the process of strengthening or deepening the foundations of an existing building. It is carried out when foundations are failing (cracking due to subsidence, clay shrinkage, or tree root damage), when a new basement is being added below the existing foundation level, when an adjacent excavation threatens the stability of the existing foundations, or when load increases (additional storey, change of use) exceed what the existing foundations can carry.

It is one of the most technically demanding and inherently risky operations in construction. Working in close proximity to existing loaded foundations, in confined excavations, with the building structure bearing above — the margin for error is small. Every underpinning project requires a structural engineer's design, a Construction Phase Plan under CDM, and building control approval at every stage.

Understanding underpinning methods helps groundworkers assess what they are being asked to build, and helps building owners understand why the process is slow, staged, and expensive.

Key Facts

• Structural engineer mandatory — underpinning cannot be designed from tables; every project needs site-specific structural calculations
• Building control notification — underpinning is notifiable under Building Regulations; staged inspections required
• CDM Regulations — almost all underpinning projects meet the notifiable threshold; Construction Phase Plan required
• Mass concrete underpinning — most common domestic method; sequential bay excavation, typically 1.0–1.5m bay lengths
• Bay sequence — never excavate adjacent bays consecutively; work in 1st, 3rd, 5th... sequence, then 2nd, 4th, 6th...; prevents undermining loaded sections
• Mini-pile underpinning — steel or concrete piles bored or driven to bearing stratum; good for deep or inaccessible underpinning
• Beam-and-base — reinforced concrete beam spans between mass concrete bases; used where continuous strip impractical
• Resin injection — non-excavation method for modest heave/settlement; injects expanding resin below foundation to lift and stabilise; limited to certain soil conditions
• Ground anchors — tension anchors used to resist lateral or uplift forces in retaining walls; specialist application
• Temporary propping — before underpinning commences, existing structure may need temporary propping or monitoring
• Settlement monitoring — survey pins installed in building before work; check crack widths and levels daily during excavation
• Pinning/grouting gap — the gap between top of underpinned concrete and underside of existing foundation must be filled with dry-pack mortar or non-shrink grout before the new section bears load

Quick Reference Table: Underpinning Method Comparison

Detailed Guidance

Mass Concrete Underpinning: Step-by-Step

Mass concrete (or 'traditional') underpinning is the most widely used method for domestic buildings. It involves excavating beneath the existing foundation in short bays, extending the foundation downward to a deeper bearing stratum, and pouring concrete to fill the bay.

Process:

1. Engineer's design — specifies bay lengths, pin depths, concrete grade, and bay sequence
2. Settlement monitoring pins installed throughout the building; baseline survey taken
3. Temporary propping of structure if required (cracks, weak walls, openings)
4. Bay 1 — excavate beneath existing foundation for the first bay (typically 1.0m wide); shore trench walls; clean up base to bearing level
5. Inspect — building control or engineer inspects the excavated bay
6. Pour concrete — fill bay to within 75mm of underside of existing footing
7. Allow to cure (typically 24–48 hours minimum before adjacent bay)
8. Pin and pack — when concrete has gained adequate strength, dry-pack mortar or non-shrink grout is rammed into the 75mm gap between new concrete and existing footing; this transfers the load
9. Backfill any remaining gap above pin level
10. Repeat in the specified sequence — skip bays, then fill interleaved bays

Bay sequence: The sequence is critical. Excavating adjacent bays simultaneously undermines the loaded strip between them and can cause collapse. The standard sequence for 6 bays:

• Phase 1: Bays 1, 3, 5
• Phase 2 (after Phase 1 concrete has cured): Bays 2, 4, 6

For longer runs, a 1-in-3 or 1-in-4 sequence is sometimes used. The engineer specifies the maximum number of bays that can be open simultaneously.

Concrete specification: Minimum C25/30 concrete; often C30/37 specified. Mix must have adequate workability to flow into the bay without leaving voids; typically S3/S4 slump class. RC concrete with reinforcement is specified where the underpinned section is cantilevered or subject to lateral load.

Mini-Pile Underpinning

Mini-piles (also called micro-piles or needle piles) are small-diameter (typically 100–300mm) bored or driven piles that transfer load from the existing foundation down to a competent bearing stratum. They are used when:

• Adequate bearing stratum is too deep for economical mass concrete (>3m)
• Access is restricted (low headroom in basement, narrow passage, internal rooms)
• Ground conditions are too variable or contaminated for mass concrete excavation

Installation methods:

• Bored mini-piles — continuous flight auger (CFA) or rotary cored through the existing slab/floor; auger drills down to bearing stratum; reinforcement cage inserted; concrete pumped in; typically 100–200mm diameter
• Driven steel pile — steel H-pile, tube, or cased pile driven through the existing floor/foundation into bearing; requires probing to confirm depth; lower noise/vibration with hydraulic hammer

Connection to existing foundation: A reinforced concrete needle beam or plate is cast between the pile heads to transfer the existing foundation load to the piles. This is the most complex part of the design.

Headroom: Modern mini-pile rigs can work in headroom as low as 1.5–2.0m, making them viable in existing basements and garages.

Beam-and-Base Underpinning

Beam-and-base uses a reinforced concrete beam at the level of the existing foundation, spanning between pockets of mass concrete (the 'bases') at intervals. The bases bear on a deeper stratum; the beam transfers the load between them.

This method is used where:

• A continuous strip of mass concrete would require too many small bays (weak soils requiring very close spacing)
• The existing foundation is at irregular depths
• The required underpinning depth varies across the run

The beam is designed by the structural engineer to span between the bases, and the bases are designed to the required bearing area.

Resin Injection Underpinning

Resin injection is a relatively modern, non-excavation method:

• Small-diameter probes (typically 12–18mm diameter) are driven into the ground through the floor or externally at angles
• Expanding polyurethane resin is injected under pressure at the base of the probe
• The resin expands, filling voids, compacting loose soils, and lifting the structure slightly
• Process is monitored with precision levelling in real time; injection stops when target level or resistance is reached

When resin injection works well:

• Shallow subsidence in granular soils
• Void-filling below foundations (drainage leak has washed out fine material)
• Minor lifting of settled elements (steps, paths, floors)

When it doesn't work:

• Deep clay subsidence (resin won't compact expansive clay meaningfully)
• Organic soils (peat, soft silt)
• Very heavy loads requiring large bearing improvements

Resin injection is typically carried out by specialist contractors. It is faster, less disruptive, and cheaper than excavation-based methods, but is limited to specific ground conditions and load levels.

Settlement Monitoring During Underpinning

Before work starts, survey pins (typically painted or screwed markers) are fixed to the building fabric at regular intervals — corners, above openings, at each underpinning bay. Crack monitors are fixed across any visible cracks.

Daily monitoring during excavation phase:

• Level survey of all pins (usually by dumpy level or digital level; record to nearest 0.1mm)
• Crack monitor readings
• Visual inspection for new cracking

Any movement beyond the trigger values in the monitoring plan (typically >1mm movement or >0.5mm crack increase per day) triggers a stop and review. The engineer must be notified of any trigger exceedance before work continues.

Frequently Asked Questions

How long does underpinning take?

For a typical terraced house with 6–8 underpinning bays, the excavation, pour, and cure phase for each bay takes 3–5 working days per sequence pass. With two sequence passes, the active construction is 6–10 days. Add time for preparation, monitoring, and inspection, and a typical job takes 3–6 weeks. Deeper underpinning or larger buildings take proportionally longer.

Does the building need to be vacated during underpinning?

Not always. For most domestic underpinning, the building can remain occupied with appropriate precautions. Dust, noise, and vibration are significant disruptions. If the structural engineer identifies a risk of significant movement, temporary vacation may be recommended for specific phases. Party wall agreements with neighbours should be served well in advance (see Party Wall etc. Act 1996).

What triggers the need for underpinning on a standard domestic extension?

The most common triggers for domestic underpinning are: clay shrinkage cracking from nearby tree roots (the tree must usually also be removed or managed), leaking drain washing out fines below the foundation, and adjacent excavations threatening an existing foundation. Many insurance claims for subsidence result in underpinning works.

Can I over-dig and pour concrete without formal underpinning design?

No. Digging beneath an existing foundation without design and engineering oversight is extremely dangerous and a criminal breach of Building Regulations. The existing loaded structure above can collapse without warning if the foundation is undermined beyond safe limits. Building control will inspect, and any underpinned section poured without approval may have to be broken out.

Regulations & Standards

• Building Regulations 2010 Approved Document A — structural stability; underpinning is a structural alteration requiring approval

• CDM Regulations 2015 — notifiable project in most cases; Construction Phase Plan required

• BS 8004:2015 — Code of practice for foundations; underpinning design guidance

• Party Wall etc. Act 1996 — underpinning adjacent to party wall requires party wall notice

• HSG185 — Health and Safety in Excavations; trench safety during underpinning

• BS 8004:2015 via BSI — foundations code of practice including underpinning

• HSE HSG185 — Excavations — trench safety

• NHBC Chapter 4.4 — Underpinning — NHBC guidance for warranty-covered work

• strip foundation design — existing foundation types being underpinned

• excavation safety trench support — trench safety during underpinning excavation

• cdm regulations groundworks — CDM obligations for underpinning projects

• soil investigation trial pits — ground investigation to determine underpinning depth