Introduction
Underpinning is a structural technique used to strengthen and stabilise existing foundations that have weakened due to subsidence, ground movement, changes in load, or design deficits. It involves extending the foundation depth or redistributing loads to a more stable foundation to restore a building’s structural integrity.
Whether caused by shifting soils, leaking drains, nearby excavation, or increased load demands, foundation movement can cause severe cracking and uneven settlement. The appropriate underpinning method depends on soil type, site access, depth to competent strata, and the level of acceptable construction disturbance.
This article outlines the five most widely used underpinning foundation methods, explaining how each one works, where it is most effective, and the advantages and limitations you should consider before selecting a solution for your project.
1. Mass Concrete Underpinning (Pit Method)
Mass concrete underpinning, also called the pit method, is the oldest and most widely used technique for shallow foundations. It involves excavating sections beneath the existing footing in short controlled bays, filling them with concrete, and allowing each section to cure before moving to the next. The process gradually replaces and deepens the old foundation while keeping the structure stable.
Best for: Shallow foundations on firm, dry ground where deep bearing is not required.
Advantages: Simple design, low cost, minimal vibration.
Limitations: Labour-intensive and unsuitable for high groundwater levels or restricted sites.
2. Beam and Base Underpinning
In this method, reinforced concrete beams are cast under or alongside the existing walls to transfer loads onto strategically positioned concrete bases. The beams span between new bases to support rough or variable ground conditions, distributing loads more evenly.
Best for: Buildings where differential settlement occurs or where spanning between stronger bearing points is necessary.
Advantages: Provides structural continuity and control over load transfer.
Limitations: Requires accurate design, shuttering, and sequencing.
3. Pile and Beam (Mini-Piled Underpinning)
Mini-piled underpinning is a modern solution for deeper or weaker ground. Small-diameter piles are drilled through unstable soil layers until they reach firm strata. Reinforced beams or pile caps connect these piles and transfer the structure’s load directly to deeper ground.
Best for: Sites with barren soil, high water tables, or restricted access.
Advantages: Minimal excavation, adaptable to most site constraints, and high load capacity.
Limitations: Higher cost and specialist piling equipment required.
4. Resin Injection Underpinning
Resin injection uses expanding polymer resins injected underneath foundations to fill voids and compact surrounding soil. As the resin expands, it stabilises the ground and can gently lift settled areas. The method requires minimal excavation and is quick to execute.
Best for: Localised settlement in lightweight structures or slabs.
Advantages: Fast installation, minimal mess, and negligible disruption to occupants.
Limitations: Not suitable for heavy loads or deep foundation correction.
5. Jet Grouting
Jet grouting entails injecting a high-pressure cementitious slurry into the ground to create soil-cement columns. These columns improve the bearing capacity and stiffness of weak soils, building a solid base for existing foundations.
Best for: Deep or very weak soils, high groundwater conditions, or confined urban sites.
Advantages: High strength and flexibility in shape and depth.
Limitations: Specialist equipment is required, and careful control of spoil and pressure is needed.
At A Glance: Choosing A Method
Method | Best for | Typical depth | Access | Vibration | Notes |
Mass concrete (pit) | Shallow improvements on competent strata | Shallow to moderate | Good access required | Low | Traditional, economical, staged bays |
Beam and base | Uneven loads, spanning poor spots | Moderate | Moderate | Low | RC beams span between new bases |
Pile and beam (mini-piled) | Deep bearing, weak/variable soils | Deep | Tight access possible | Low–moderate | Small-diameter piles + RC beam/pile cap |
Resin injection | Localised settlement, light loads | Shallow | Minimal | Very low | Fast, minimal disruption, limited capacity |
Jet grouting | Very weak soils/high water table | Deep | Moderate | Moderate | Creates soil-cement columns in situ |
Bottom Line
Each underpinning method serves a discrete purpose based on site conditions and structural requirements. Traditional mass concrete underpinning remains effective for shallow, accessible sites, while modern solutions such as mini-piles or jet grouting offer precision and strength in more challenging environments. Selecting the right approach requires expert assessment of soil properties, load demands, and access constraints — ensuring lasting stability and safety for the structure.
