While shrinkage and settlement of foundation soils are issues commonly associated with reactive clay soils, their potential to swell and expand often gets overlooked. This is primarily due to the fact that the heaving of clay does not occur as frequently or as easily as its shrinkage and settling. In this article, we’ll investigate foundation heave in more detail, discussing its triggers and solutions.
Need to know more about reactive clays first? Head over here for more information.
What is Reactive Soil Expansion?
Soils with a high clay content and plasticity index tend to have very high expansion potential. The amount and type of clay particles in the soil, combined with the availability of moisture, directly influence the behaviour and expansion rate of these soils.
Moisture sensitivity is a key characteristic of clays. When exposed to water, clay soils will typically expand; however, when dried out, they shrink. The expansion of the clay soil causes the soil to swell. The swelling of the clay soil can impose significant pressure on the structures it is supporting, typically footings, which subsequently causes the footings to heave.
Soil that has a “very high” capacity for swelling (or shrinking) will exhibit more extreme changes than those with a “very low” potential. Australian Standard AS2870 – 2011 ‘Residential slabs and footings’ provides five distinct classifications for reactive clays from slightly reactive (i.e., Class S) to extremely reactive (i.e., Class E). Moderately (Class M) and highly reactive (Class H1 and H2) soils reside in between these two extremes. These variations in potential result in differing degrees of swelling or shrinking.
Most Common Causes of Reactive Soil Expansion
Reactive soil can expand drastically when the clay absorbs water. This heaving phenomenon depends on several conditions, including how dry the clay soil was before, how much water has been added, and at what rate it was introduced. Generally speaking, there are a few major causes of water absorption that can lead to the swelling of clay soils:
Significantly Dry or Desiccated Clays
During summer or after a period of drought, the level of water near the ground is usually at its lowest point. This suggests a state of extreme drying of the clay soil. Microcracks will manifest in the clay as the moisture content falls below the shrinkage limit. The more cracks the clay has, the simpler it is for the water to get into the soil and cause the clay to swell faster. At low water content, the suction pressure draws the water into the clay, leading to rapid swelling.
“Very dry clays with natural moisture content below 15 percent usually indicate danger. Such clays will easily absorb moisture to as high as 35 percent with resultant damaging expansion to structures.” – Foundations on Expansive Soils by F.H. Chen
When temperatures are soaring, and the moisture content of the ground is exceptionally low, structures built in such conditions become especially vulnerable to damage. In this situation, when near-surface clay has been severely parched, its water uptake capability will be greatly amplified, creating heaving under the structure. This has been experienced with many family homes built after the extreme drought period in Victoria from 1997 to the end of 2010.
Read our Case Study on a house built over desiccated clay after drought here.
The figures below demonstrate the cycle of the moisture changes of dry soils and subsequent deformation of the footing, particularly the new structures and slabs. Initially, the underlying clay soil tends to have a uniform moisture content when the footing slab is laid. Over time, the outer edge of the slab, being more exposed to moisture from rain and irrigation, begins to heave due to the clay soil expansion. During warmer months, however, the edge of the slab starts to dry out. The drying effect increases significantly if there are trees planted near the building. This causes the area beneath the centre of the structure to be much cooler than the edges of the slab. When this happens, water at a higher temperature will move into the soil towards the cooler area, i.e., the centre of the slab, in order to equalise the thermal energy of the two areas. By doing this, the moisture level increases, resulting in the swelling of reactive soils and the subsequent heaving at the centre of the slab.
In recent years in some suburban regions of Melbourne, such as those in the west, homeowners have been forced to contend with these common issues: edge heave and centre lift of concrete slab footing.
Plumbing Deficiency and Leaks
Undetected leaks, particularly pressurised ones such as leaks in water supply pipes, are a common source in providing clay soil with the water it needs to expand. As the clay absorbs the water from the leaking pipe, it swells. Contrarily to desiccated clays, which swell more evenly as their moisture content increases, the expansion of the clay soil due to a leak is almost always localised to the area in the immediate vicinity of the leak, and as such, the damage to the structure (if any) is also localised to the area of the leak.
Natural Disasters such as Floods
Torrential downpours accompanied by floods are another factor that can greatly contribute to the swelling of reactive clay soils. When this happens, it is typically the light structures with the least load capacity – like pavements and lightly loaded foundations – that suffer damage from the expansive nature of reactive soil. Since these structures typically only support their weight, they can be highly susceptible to damage from expanding soil. The expansion of the clay soil creates upward pressure, which tends to produce a distinct crack pattern known as a “spider” or X-type crack pattern.
Read our case study on a backyard spa damaged by soil heave due to flooding.
Seasonal Variations (Cyclic Heave and Shrinkage)
Lastly, there is the phenomenon of cyclic heaving and shrinking. Almost all structures constructed on clay soils experience cyclic heaving and shrinking, largely caused by seasonal fluctuations. For example, a concrete slab footing will heave along its perimeter during the rainy season and then settle again when the summer comes and thus drying out the clay. These repeated up-and-down movements can cause cracks and damage to the building structure.
Types of Movements Associated with Soil Expansion
When clay absorbs water, it can expand both laterally and vertically. Lateral movement can be particularly destructive to vertical structures such as retaining and basement walls, as the seepage of water into the backfilled clay behind the walls can generate substantial horizontal pressure against the structure.
For the footing structures (i.e., concrete slabs, strip and stumps, etc.), the vertical movement of the expanded clay causes the most damage. This is because the vertical movement of the expanded clay soil creates a significant upward pressure on the footings, which consequently causes the footing to heave.
How to Assess Damage Caused by Soil Expansion/Footing Heave?
Clients often turn to Forensic structural condition assessments when assessing a property for damages due to clay soil expansion. The scope, level of detail, and comprehensiveness of these assessments can vary greatly depending on the situation. Among non-destructive methods, Floor level survey is one that is widely used to identify differences in elevation of a floor structure. For instance, when clay soils expand, it may cause the center of the slab to heave upwards – something that a floor level survey would detect.
Monitoring cracks is another non-destructive technique for ascertaining whether damage has been caused due to the expansion of the clay soil. For example, the width of a crack that occurred due to footing heave is likely to change when the source of moisture is eliminated, and the clay soil has gradually lost the excess moisture and shrunk. Monitoring of cracks is particularly important for Forensic Engineers who are involved with insurance claim cases and sometimes find it difficult to conclude without a doubt whether a single leak or flood event has done the damage or other ongoing factors, such as seasonal variations, are at play.
Destructive testing such as geotechnical investigation, subsurface exploration, and laboratory testing can also be conducted depending on the project’s objective. For example, one way to gain insight is by conducting a Moisture content test, which can supply essential data regarding the moisture content of the soil at the footing level.
How to Repair Damages Caused by Footing Heave?
In order to determine a feasible approach for rectifying damage to a building caused by footing heave, it is essential that Forensic Structural Engineers first identify the root cause of expansion of the clay soil. In the case of a leak, it must be addressed immediately by fixing the leak. In the case where edge heaving is occurring, and it is related to an open and permeable ground around the perimeter or garden beds close to the house – then these should be replaced by an impermeable pavement to minimise moisture fluctuations brought on by seasonal changes; like hot summers and wet winters.
Remember that once the soil has been expanded, it will take considerable time for its moisture level to return to normal. This can create movement in the soil, which could cause further cracks and damage. Thus, attempting repairs before the soil has fully stabilised would be futile. Depending on the reactivity of the clay soil, it can take up to two years for the expanded soil to stabilise and return to its previous state.
With the clay soil stabilised and restored to its original moisture levels, a Forensic Engineer can then formulate an appropriate plan to repair and reinstate any damage triggered by the footing heave. Depending on the magnitude of damage, such repairs could range from complete wall replacement to basic patching and painting of cracks.
Foundation Resilience: Lessons from Reactive Soil Expansion
- Reactive clay soils can expand significantly when absorbing water, putting pressure on home foundations.
- Key causes of soil expansion include previously dry conditions, plumbing leaks, floods, and seasonal moisture changes.
- Soil expansion can lead to both vertical and lateral movement, affecting different parts of a building’s structure.
- Different foundation types respond differently to soil expansion, with lighter structures often more vulnerable to damage.
- Assessing soil expansion damage may involve floor level surveys, crack monitoring, and soil moisture testing.
- Addressing the root cause of moisture changes is crucial before attempting repairs, as soil stabilisation can take up to two years.
