Expansive soils are present in certain parts of Canada and across the globe and they cause millions of dollars in damage to homes and buildings every year. Expansive soils, typically clay, can lift buildings and crack foundations, floors and walls. When clay minerals in the soil absorb water, they swell; as they dry, they shrink and leave voids in the soil. It is this swell-shrink cycle that causes trouble over time. The greatest amount of damage occurs in situations where the moisture content is often changing.
The damage from expansive soils can be observed within the first few months or years after constructing the home. As subsoil water or water from rainfall migrates underneath the home’s foundation, the soil around the edge of the foundation expands, pushing up on the edges of the foundation. This condition, called edge-lift, can cause cracking in the drywall and in the foundation itself. Over a period of time, after water finds its way to the centre of the slab, centre lift can occur and damage the house even further. Differences in a foundation's performance arise from differences in design and the extent of disturbance of natural conditions.
It is strongly recommended that prior to building any structure, soil testing at the site be done to ensure the soil type present, the bearing capacity it offers and to determine the approximate effect the soil will have on the structure. The information provided in the soil report will lead to foundations being designed to withstand the effects of the existing soil conditions and will help in planning for long-term maintenance of the structure. In some earlier posts, we have explained soil testing and its importance in detail.
However to counter the effects of expansive soils on already existing structures, several solutions are available. One of the most common preventative measures includes proper soil maintenance such as maintaining a uniform and constant moisture level in the soil. In this approach, the soil is moistened continually and uniformly to prevent shrinking. Proper foundation drainage system is another effective measure that can negate the effect of expansive soil on the structure foundation. Homeowners can also address the problem by removing trees and large shrubs that are within 15’ of the structure.
For structures already adversely affected by expansive soils, further damage can be prevented by providing additional strength and support to the foundation. This may include various methods of underpinning to prevent vertical movement or sliding or reinforcing the foundation walls to withstand lateral pressure. We encourage readers to review our previous posts about the details of underpinning, and the processes involved, to have an understanding of how damage can be repaired through foundation stabilization.
When purchasing a new or previously owned home, the buyer should obtain a copy of the soil report, which will indicate the presence, if any, of expansive soils. The buyer should also lookout for cracks in the basement, garage slabs, driveways and sidewalks, and especially at the corners of doors and windows. If soil reports are not available before you purchase, you should hire a soil expert to determine the soil type so that you will know where you stand.
In our experience, the real problem is not the expansive clay itself but the foundation and structural design. Expansive soil isn't always or necessarily going to cause problems but first and foremost you have to know it's there and second, you have to design and build accordingly.