Different Methods of Underpinning Explained

Underpinning is a crucial process in construction and civil engineering used to strengthen and stabilize the foundations of existing structures. It becomes necessary when the original foundation is no longer strong or stable enough to support the building, often due to changes in soil conditions, structural alterations, or damage caused by natural events. Understanding the different methods of underpinning can help property owners, engineers, and contractors choose the most appropriate technique for their specific project needs.

In this article, we will explore the various methods of underpinning, their applications, advantages, and considerations.

What is Underpinning?

Underpinning is the process of strengthening and stabilizing an existing foundation by extending its depth or breadth so it rests on a more stable soil layer or distributes loads more effectively. This can prevent further settlement, correct subsidence issues, and increase the load-bearing capacity of foundations. Underpinning is often performed in residential buildings, commercial structures, bridges, and retaining walls.

Common reasons for underpinning include:

• Weak or unstable soil conditions
• Changes in soil moisture content (drying out or saturation)
• Damage from nearby excavation or construction activities
• Structural modifications increasing load requirements
• Subsidence caused by natural factors like tree roots or water leaks

Traditional Mass Concrete Underpinning

Overview

Traditional mass concrete underpinning is one of the oldest and most widely used methods. It involves excavating small sections beneath an existing foundation in a staged manner and then filling these sections with mass concrete to create a new support base.

Process

1. The area beneath the foundation is divided into small bays or pits.
2. One pit is excavated first to a required depth below the existing foundation.
3. Mass concrete is poured into the pit to form an underpinning base.
4. Once the concrete has cured and gained strength, excavation proceeds to the adjacent pit.
5. This process continues until the entire foundation length has been underpinned.

Advantages

• Simple and cost-effective for many applications.
• Does not require specialized equipment.
• Suitable for shallow foundations on firm soils.

Limitations

• Time-consuming due to staged excavation and curing times.
• Not suitable for very deep foundations or where access is limited.
• May cause temporary movement or settlement during excavation.

Beam and Base Underpinning

Overview

Beam and base underpinning is an alternative to traditional mass concrete underpinning that uses reinforced concrete beams supported on mass concrete bases or piles. This method transfers loads from weak soil areas to zones of greater strength.

Process

1. Reinforced concrete beams are designed to span between mass concrete bases or piles beneath the existing foundation.
2. The beams are constructed either below or alongside the existing foundation.
3. The bases or piles provide additional bearing capacity by transferring loads deeper into stable soil layers.

Advantages

• Suitable for sites with poor soil conditions where deep support is needed.
• Provides increased load distribution.
• Can be adapted for use where traditional underpinning may be difficult.

Limitations

• More complex design and construction compared to mass concrete underpinning.
• Requires careful planning to avoid damage to the existing structure.
• Higher costs due to reinforcement and additional materials.

Mini-Piled Underpinning

Overview

Mini-piled underpinning involves installing small-diameter piles (mini-piles) beneath an existing foundation to transfer loads into deep, stable soils. It’s especially useful when soil conditions are poor at shallow depths or access constraints exist.

Process

1. Mini-piles are installed through drilling machinery in close proximity to the existing foundation.
2. The piles extend downwards until they reach stable strata.
3. The piles are connected via reinforced concrete caps or beams that support the original structure’s loads.

Advantages

• Can be used where traditional underpinning isn’t feasible due to ground conditions or restricted access.
• Minimal disturbance to surrounding structures and environment.
• Allows underpinning at significant depths without extensive excavation.

Limitations

• Requires specialized equipment and expertise.
• Higher cost than traditional methods.
• Design complexity increases with pile layout requirements.

Chemical Grouting Underpinning

Overview

Chemical grouting involves injecting chemical solutions into soils beneath foundations to improve their strength and reduce permeability. This method stabilizes soils without extensive excavation.

Process

1. Injection holes are drilled beneath or adjacent to foundations.
2. Chemical grout (such as polyurethane resin) is injected under pressure into soil voids.
3. The grout solidifies within pores, binding soil particles together and increasing bearing capacity.

Advantages

• Minimally invasive with little excavation required.
• Rapid setting times allow quick project completion.
• Effective at filling voids caused by soil erosion or animal activity.

Limitations

• Less effective for large-scale underpinning requiring significant load transfer.
• Chemical materials must be chosen carefully based on soil type and environmental considerations.
• Cost can be high depending on chemical usage volume.

Screw Pile Underpinning

Overview

Screw pile underpinning employs helical screw piles driven into the ground beneath foundations to provide deep support through load transfer into stronger soils.

Process

1. Helical screw piles are rotated into position using hydraulic machinery next to existing foundations.
2. The piles are installed at precise depths and locations based on structural requirements.
3. Piles are connected with reinforced concrete beams which carry building loads.

Advantages

• Rapid installation with minimal site disruption.
• Suitable for weak soils where deep support is required.
• Piles can be removed or adjusted if necessary.

Limitations

• Not suitable where underground obstructions exist such as rock layers or utilities.
• Requires skilled operators and equipment.
• Design must accommodate pile spacing for effective load distribution.

Jet Grouting Underpinning

Overview

Jet grouting creates in-situ cemented columns by injecting high-pressure grout mixes into native soils using specialized jets from boreholes. These columns strengthen soft ground beneath foundations enabling load transfer.

Process

1. A rotating drill rig injects grout under high pressure while withdrawing vertically through boreholes around or beneath foundations.
2. The grout mixes with surrounding soil forming stiff cemented columns of improved bearing capacity.
3. Columns can be arranged in patterns supporting existing footings above.

Advantages

• Very effective at improving weak soils without excavation.
• Can be used in confined urban environments with limited access.
• Provides both strengthening and sealing properties (reduces groundwater flows).

Limitations

• High initial equipment investment and technical complexity.
• Disruption caused by drilling noise and vibration may impact occupants/neighbors.
• Requires detailed site investigation for design accuracy.

Conclusion

Underpinning offers a range of methods tailored to address diverse challenges associated with foundation instability or weakness. The choice depends on factors such as:

• Soil conditions
• Depth of required support
• Access limitations
• Load requirements
• Budgetary constraints
• Environmental impacts

Traditional mass concrete underpinning remains widely used for straightforward cases involving shallow foundations on firm soils. However, advanced techniques such as mini-piled underpinning, jet grouting, chemical grouting, screw piles, and beam-and-base underpinning provide solutions for more complex scenarios involving poor soils, restricted access, or heavy structural demands.

Engaging experienced geotechnical engineers and foundation specialists early can ensure that appropriate investigations guide method selection toward safe, efficient, and cost-effective underpinning solutions that protect structural integrity for years to come.