Foundations for Building Alterations and Extensions

What a foundation has to do

A foundation spreads load from the structure into ground that can carry it, at a depth where the ground behaves predictably. Two things must be satisfied: the bearing pressure must be within the capacity of the soil, and the resulting settlement must be small enough, and even enough, that the building is not damaged.

Differential settlement — one part of a building moving relative to another — causes far more damage than uniform settlement. That is why the interface between new and existing construction receives so much attention on extensions.

Ground conditions and why they dominate

For small extensions the ground is usually assessed by trial pit and by inference from local experience and geological mapping. For larger works, a site investigation with boreholes and laboratory testing is appropriate, and it is far cheaper than discovering the problem in the ground works package.

• —Clay — moderate to good bearing capacity, but shrinks and swells with moisture content; the reason for deep foundations near trees
• —Sand and gravel — generally good bearing, settles quickly, sensitive to water ingress and to being loosened during excavation
• —Silt and soft alluvium — low capacity and slow, continuing settlement
• —Peat and organic soils — very poor, normally requiring piles or ground improvement
• —Made ground — fill of unknown origin and compaction; almost never suitable for founding on directly
• —Rock and weathered rock — excellent, but excavation cost and the risk of hard digs increase
• —Mining, quarrying and landfill legacy — common in parts of the North West and requires specific investigation

Trees, clay and foundation depth

In shrinkable clay, trees remove moisture from the soil, causing it to shrink; when a tree is removed, the soil recovers moisture and swells, causing heave. Both mechanisms can move a shallow foundation enough to damage a building.

Foundation depth near trees is therefore set by the species, mature height and distance of the tree, and by the shrinkage potential of the clay. NHBC Standards Chapter 4.2 provides the tables most commonly used in UK housing, and depths of 1.5 m to 3.0 m are routine in these situations, far deeper than load alone would require.

Where deep foundations are needed in clay affected by trees, heave protection is usually specified as well — compressible material against the foundation face and a void former beneath ground beams and slabs — so that swelling soil can move without lifting the structure.

Common foundation types on alterations and extensions

• —Traditional strip — mass concrete strip under a masonry wall, typically 450 mm to 600 mm wide, at depths from 900 mm
• —Trench fill — a narrow trench filled almost to the surface with concrete; fast, safe and the default for deeper domestic foundations
• —Reinforced strip or ground beam — used where the foundation must span soft spots, drains or service runs
• —Pad foundations — for individual columns, common on steel-framed extensions and garden rooms
• —Raft — a stiffened slab spreading load over a wide area; used on poor or variable ground and over mining or fill
• —Mini piles with ground beams — used near trees, on poor ground, in restricted access, and where excavation next to an existing building would be unsafe

Connecting a new extension to an existing building

New foundations should be taken at least to the depth of the existing foundation at the junction, and often deeper if the ground or trees dictate it. Founding an extension shallower than the existing building is a reliable way to produce cracking at the join.

The two structures will still settle differently, because the existing building has already consolidated its ground and the new one has not. This is normally accommodated with a movement joint at the junction, with the new masonry tied to the existing with proprietary wall starter ties that allow vertical movement.

Excavating next to an existing foundation also has to be done carefully and in short lengths, so the existing building is not undermined. That is a temporary works consideration as much as a foundation design one.

Underpinning: what it is and when it is used

Underpinning extends an existing foundation deeper or wider, either to remedy movement or to allow a new, deeper construction alongside. Traditional mass concrete underpinning is carried out in short, alternating bays — typically no more than one metre at a time — so the wall above is never left unsupported over a long length.

Underpinning is disruptive and comparatively expensive, and it is not automatically the right answer to cracking. Where movement is caused by a leaking drain, a nearby tree, or seasonal clay shrinkage, dealing with the cause is usually preferable to underpinning the symptom.

• —Mass concrete bay underpinning — the traditional and most common method
• —Beam and base, or piled underpinning — where loads are high or access below is limited
• —Mini piles and needle beams — used in restricted spaces and where deep ground improvement is needed
• —Resin injection — proprietary ground improvement, appropriate in some soils and not in others

Foundations for internal alterations

Internal alterations change foundations even though nothing is dug. Removing a wall replaces a distributed load with two concentrated reactions at the beam bearings, and the existing foundation beneath those points was designed for the old, spread arrangement.

Whether that matters depends on the reaction, the existing footing width, and the ground. Sometimes the existing foundation is comfortably adequate; sometimes a local pad, a spread footing or a small underpin is required. Establishing the answer requires knowing the existing foundation, which typically means a trial pit.

Drains, services and neighbouring structures

• —Foundations must not impose load onto a drain; where a foundation crosses a drain run, a lintel or reinforced foundation bridges it
• —Building over or near a public sewer requires a build-over agreement with the water authority
• —Excavating within three metres of a neighbouring structure, and deeper than its foundations, engages the Party Wall etc. Act 1996
• —Existing services, soakaways and old wells frequently appear during excavation and change the design
• —Building control will normally inspect the excavation before concrete is placed

Managing the uncertainty

Foundations are the one element of a domestic project where a provisional sum is genuinely justified. The design can only be confirmed once the excavation is open and the engineer or building control inspector has seen the ground and the existing foundation.

The most cost-effective step available at the start of a project is a trial pit: a small excavation next to the existing wall, recording foundation depth, type and the soil beneath. It usually costs very little and removes the largest single unknown from the programme.

Related guidance

• BES-TG-003Understanding Cracks in BuildingsRead →
• BES-TG-009Understanding Structural Surveys and AppraisalsRead →
• BES-TG-002Removing a Load-Bearing Wall: A Structural Engineer's GuideRead →
• FAQWhat causes cracking in buildings?Read →
• FAQDo I need Building Regulations approval?Read →
• FAQWhen do I need structural calculations?Read →