Every building needs a solid foundation to support it. Any problems with the foundation soil can therefore have a negative impact on the performance of the footings and the building as a whole. There are many conditions that can cause damage to foundations, but some of the more common ones include settlement, reactive soil heave/expansion, slope movement, and deterioration of footing material. In this blog, we’ll take a closer look at footing settlement: its causes and potential solutions. Need help determining what are footing and foundation? Read different types of footings commonly used in Australia
What is footing settlement?
The settlement of the footings is always related to the downward displacement or differential movement of the supporting foundation soil resulting in building damage or collapse. This can happen when the soil is too loose, too wet, too dry or when there is too much weight on the foundation.
Most common causes of footing settlement
Settlements can generally be classified into two types: those caused by the weight of the structure shortly after construction and those caused by secondary factors long after the completion of the structure. The former occurs when the foundation soil compress or settle under the weight of the building. The latter type of settlement is usually due to factors such as consolidation of uncontrolled fill, shrinkage of reactive clay soil, sinkholes, or natural disasters like earthquakes or floods.
Settlement caused by the weight of the structure
Different types of soils have different bearing capacities, which must be taken into account when designing and constructing a building. Foundation soil must have adequate bearing capacity to support the weight of a building. Bearing capacity is determined by a Dynamic Cone Penetrometer (DCP) test during the initial geotechnical investigation of a site. Stiff clay or sandy soil rarely experiences bearing failure, but initial settlement under load during the early years of a building’s life (usually 10 years or less after construction) is normal and should be expected. This initial settlement is relatively uniform across an entire site and does not affect a structure’s integrity.
Settlement caused by secondary factors
- Uncontrolled Fill – Bearing failure is a common problem when buildings are constructed on top of uncontrolled fill material. Uncontrolled fills are fills that were not compacted as per Australian Standard AS 3798 “Guidelines on earthworks for commercial and residential developments“. Sometimes, fill material is simply dumped on site or only lightly compacted with hand tampers, which can cause the fill to settle significantly once the building’s weight is added. The settlement can become much worse if the fill material becomes wet due to a leak, which leads to the consolidation of the fill material. Pool owners in Victoria often have to deal with consolidation of fill material behind their pools. This is because backfilled soil behind pool structures is not always compacted properly, and over time this can lead to settling and extensive damage to a pool’s shell and any surrounding pavers.
- Shrinkage of reactive clay soil – The expansion and contraction of clay soil due to changes in moisture content is a major factor in settlement of buildings. Dry clay soils can become very hard and cracked, with deep furrows forming as the soil shrinks. This type of settlement is common in many parts of Victoria, where the foundation soil is reactive to changes in moisture levels. Trees and vegetation , long periods of drought , and abnormally hot summers are among the most common contributing factors to the shrinkage of clay soil and the consequent settlement of buildings.
- Sinkholes – Sinkholes are typically found in areas with limestone bedrock and karst topography. They can range in size from a few feet across to hundreds of feet, and their depth can also vary considerably. Although they are not common on residential properties in Victoria, Australia, sinkholes can occur when large trees are removed, and their root systems no longer provide support for the soil. This can result in the failure of light structures with shallow footings, such as brick fences.
- Natural disasters such as earthquakes or floods – The accepted mechanism for footings to settle (downward displacement) as a result of an earthquake is when a phenomenon known as “liquefaction” of the ground occurs. This only occurs in certain geological areas and with large magnitude earthquake. Liquefaction occurs when loose sand deposits are shaken by ground movement, causing an increase in pore water pressure. This may turn the sand into a liquid-like state, making it unable to support buildings or other structures on top of it.
Settlement can also occur due to bearing failure of the foundation soil during sever flood events.
How much settlement is considered allowable?
When it comes to most structures, especially residential buildings, what’s deemed acceptable in terms of settlements is usually determined by aesthetic and serviceability requirements rather than structural ones. Problems like cracks, jammed doors and windows, uneven floors and so on, will start cropping up long before there’s any danger to a structure’s integrity.
Different structures have different levels of tolerance for settlement. The amount of settling a structure can tolerate depends on various factors, such as the followings:
- The type of construction: Those structures that are more flexible, such as timber framed buildings, or have more rigid footings , such as stiffened raft slabs, can sustain larger values of total settlement and differential movements.
- The use of the structure: The amount of settlement that is considered acceptable can vary greatly, based on the structure’s intended purpose. For instance, even small cracks in a house might be considered unacceptable by some, whereas much larger cracks in an industrial building might not even be noticed.
- The type of wall and floor finishes: For example, floor tiles or cementitious rendering on external walls are much less tolerant of movements and can crack easily.
How to assess damage caused by settlement?
Forensic Structural Engineers often use floor level surveys and contour plans to help assess damage to foundations subjected to settlement. This data can provide valuable insights into the pattern of foundation displacement and corresponding building cracking. For example, figure 4 below shows floor level survey results and contour plan of a house built on a concrete waffle slab. The contour depicts clear evidence of settlement towards the rear side and front right-hand corner of the house. External and internal wall cracks are associated with the foundation settlement, as shown in figures 5 and 6.
The damage caused by settlement is often most evident through cracks that are wider at the top than at the bottom. This is due to the downward rotation of the settling portion of the wall.
With this information, the forensic engineer can study different hypotheses regarding the possible causes of foundation settlement and come to a reliable engineering opinion.
How to repair footing settlement?
As discussed, there can be many reasons why a foundation might settle. It is, therefore, crucial to identify the problem correctly before starting any repairs. The solution will differ depending on the cause, so it is essential to figure out what is causing the foundation settlement in order to choose the best course of action.
A Forensic Structural Engineer can recommend various remedial measures based on the cause and extent of settlement. These measures can range from something as simple as removing site factors that are causing the settlement, to more extensive solutions such as structural modifications or total demolition and rebuilding. More complex options such as major drainage improvement, deep underpinning, or resin injection ground engineering work may be considered only after careful consideration and analysis by a competent Forensic Structural Engineer.