Expansive soil and soil settlement.
Updated: Apr 17, 2022
Have you ever wonder why do we have foundation problems?
It's all thanks to soil behavior:
There are types of soil that undergo considerable swelling as moisture content is increased and experience shrinking when it is lose the water.
This variation in soil volume leads to the development of cracks during dry season and these cracks would be closed during wet season in addition to rising and falling of ground during wet and dry season respectively.
Lightweight structures are susceptible to damage caused by cyclic shrink-swell movements of clayey soils amounting to significant aggregated global financial loss for repair costs.
Soil plasticity in Texas is high. Soil plasticity index indicates the fineness of the soil and its capacity to change shape without altering its volume. A high plasticity means high shrink-swell potential.
The water also leads to loss of soil strength, strength decrease as water content increase.
Even though expansive soils cause enormous amounts of damage, most of our clients have never heard of them and those that have do not fully understand the magnitude of the issue. This is because the damage from expansive soil occurs slowly over time and cannot be easily attributed to a specific event. The damage from expansive soils is often attributed to poor construction practices or the common misconception that all buildings experience this type of damage as they age. Generally speaking, older construction techniques are inadequate to resist the forces imposed by expansive soils. That does not mean that nothing can be done. Properly engineered site drainage, vegetation management and foundation repairs can significantly improve the performance of older dwellings.
Modern construction techniques used in foundations in expansive soil areas will generally perform far better than older systems. This is one situation where older is not better. The building codes represent the minimum acceptable standard for construction. Unfortunately, most dwellings are built to these minimum code requirements. Expansive soils are a challenging environment. Dwellings constructed to minimum building code requirements will often have issues over time – it is only a matter of how long it may take them to manifest themselves. A more conservative design up front may cost slightly more, but can significantly enhance the performance of the structure over time. With expansive soils, the swell/contract cycle can continue almost indefinitely. This means that damage from expansive soils will become progressively worse over time. As opposed to soil settlement, where the displacement occurs mainly at the earlier stages of the dwellings life and diminishes over time. The failure modes vary by foundation type (slab on grade or raised perimeter) and site vegetation conditions.
Expansive Soil and Slab on Grade Foundations.
The behavior of a slab on grade foundations in expansive soil conditions is significantly different from a raised perimeter foundation. In the earlier stages of the dwellings life, the edges of the slab a pushed upward due to the forces of the expansive soils. This is particularly true in older designs that may be of relatively uniform thickness with little or no perimeter footings. Older post-tensioned slabs may also exhibit significant edge lift due the combination of the upward push of the expansive soils combined with the pull on the edges from the tensioned cables.
Unrestrained slabs will “walk” or move on the soil. Movement can often be seen where the foundation abuts driveways, masonry/concrete stairs, etc. In areas, you may see gaps develop where the foundation and flatwork drift apart. In other areas the driveway may move vertically as the as they move together. As the soil passes through repetitive wet/dry cycles, moisture tends to accumulate at the center of the slab causing the center of the slab to rise. The same wet/dry cycle action occurs at the edges as is found a raised perimeter foundation. That causes the slab perimeter to drop over time. The combination of forces causes the crowning we often see in failed slab foundations. See the figure below.
The soil deformation is a natural phenomenon that will always exist around your home. If the necessary actions and precautions are not taken it will most likely lead to several foundation problems. As the soil absorbs water it will inflate causing sections of your home heave, then when it consolidates it will cause your home to settle down. If this is allowed to happen uncontrolled your home will start suffering the adverse effects of foundation problems. Many of the effects caused by soil deformation are visible and noticeable, but there are many that are more difficult to spot.
By the time we start noticing cracks on the walls we may already have other serious issues. as the soil expands and contracts sewer and water lines can be affected. If the soils get too wet, soil strength will become too low to support the load of the house, the soils will deform and the house will settle
To reduce soil movement and minimize the risks of foundation and plumbing failure a good defense system is needed. First we analize the warning signs and identify the problems. Once we detect the irregularities we can design an adequate preventive/repair plan which could include one or more of the following items: proper grading, surface and/or French drains, root barriers, irrigation system and under pinning. This will ensure your home's foundation remains healthy.