Understanding foundations – what you need to know to keep your building structurally sound

Foundations are more than simply a solid base which a building sits upon. They are a vital element of construction that carries the structural load bearing and weight distribution of a building’s walls, columns and other structural elements.

Factors that influence what type of foundation is best suited to a site are numerous. This can include the weight or scale of the building, drainage requirements and water table, site topography including site slope, construction budget, the type of building materials available, soil composition (geology), local climate, local wind conditions and seismic considerations.

Soil is of particular concern as foundations must be fit for purpose and adaptable to a site’s conditions, which can range from stable, non-reactive rock with no moisture-based ground movement, to highly reactive clay sites that experience significant ground movement following changes in moisture levels. Some sites may be in highly desirable areas with unmissable real estate opportunities but present challenging ground conditions, the building is more likely to be affected by factors such as moisture conditions, reactive soils, landslips, sink holes and voids.

Foundation 101 Whether building, renovating or remediating a property, understanding the different types of foundations will help determine the best path forward. Foundations are broadly classified as ‘shallow’ or ‘deep’ and are differentiated by the depth beneath the ground’s surface. Shallow foundations are placed closer to the surface and are suitable for stable conditions where ground movement is less likely, while deep foundations are recommended for structures that are being built on sites with reactive ground conditions. Deeper foundations mitigate against the impact of seasonal moisture variation and tree root intrusion.

Waffle Raft-Slab foundations are perhaps the most common in modern buildings and are an inexpensive foundation type as the foundation requires minimal excavation. Waffle slabs, also known as waffle pods, are a reinforced concrete footing and slab system created with concrete that is poured around polystyrene forms that sit on top of the soil.

Raft foundations are a type of shallow foundation within the slab family. Usually used as the foundation for new houses and extensions, raft foundations are applied as a solid continuous base across the entire building. They help to evenly distribute weight carried by walls and columns, allowing for loads to be supported by edge and internal beams. The depth and spacing of these beams is determined by the ground reactivity, the construction type and the degree of articulation in the building envelope. Raft foundations are preferred in areas where soil is highly susceptible to moisture changes because of climate and composition.

Strip foundations, or strip footings, form a continuous, linear line of support underneath walls. Strip footings are placed within a shallow trench and reinforced with steel. These footings are popular in Australia and are preferred for light loads, like the interior or exterior walls found in residential properties. The material of the strip footing is typically determined by the construction of the time. Prior to the late 1800s, most strip footings were made of stone. The stone could be dressed blocks or natural stone. From the late 1800s to the early 1900s, bricks were the predominant strip footing type. With increased availability of ready-mix concrete, concrete strip footings replaced brick strip footings from the 1930s. Strip foundation structures are used in combination with stump footings for the internal load bearing.

Stumps, referred to as a pile footing in New Zealand, are the simplest and most familiar systems used for the vertical support and transfer of building loads to the foundation. Stumps are used to support timber-framed houses for which they are typically the most cost effective. The stumps are made from timber, concrete or steel and must have a concrete or timber footing placed underneath the base of the stump.

Pile foundations are a type of deep foundation that comprise of pillars – or ‘piles’ – that provide support for a building by being placed deep into the ground. Pile foundations are preferred when the soil conditions near the surface are too soft or reactive. Such soil conditions can cause loads to shift, making slabs and other shallow foundations less effective. However, pile foundations can prevent differential settlement of foundations by transferring the weight of a building through its columns to a stiffer, deeper soil strata.

Remediation

Foundations are often subject to the surrounding ground conditions, which may change due to changing climate, minor seismic activity or urban influence such as digging, plumbing and tunnelling. Over time, foundations may need maintenance to overcome these factors and continue to perform their design function. In such events, Mainmark can provide guidance and solutions to help re-support foundations to benefit structural performance. For example, Mainmark helped a heritage building in Sydney increase its foundation’s load bearing capacity from 140kPa strength to 200kPa using its Teretek® resin injection solution, which improved and consolidated the soils, preventing the risk of future settlement. In New Zealand, Mainmark used Terefirm® Resin Injection to strengthen the ground and re-level the foundations beneath the Northwood Supa Centa in Christchurch after the shopping centre experienced liquefaction induced ground deformations following the Christchurch earthquakes in 2010 and 2011.

Understanding the characteristics of the different foundations types will put you in the best position to help ensure your building, regardless of its age, location or site conditions, has the ability to stand the test of time.