Causes of failure
The uneven distribution of loading from the superstructure can induce uneven stresses at different locations of the foundation. This can cause differential settlement at locations where vertical structural elements, such as columns and walls, directly transfer the superstructure loads to the foundation. Differential settlement can eventually lead to cracks at the foundation.
Overloading from the superstructure can also create foundation failure. Foundations can fail by cracking when the design moment and/or shear is above its moment and/or shear capacity. Failure can also occur when there are large concentrated or point loads, which can induce large punching shear onto the foundation, and when there is over designing of bearing pressure.
Different properties of soil at the foundation interface
Different parts of the foundation can rest on different properties of soil. For example, one part of the foundation can sit on clay, while another part of the foundation can sit on rock. When all design checks are adequate for one part of the foundation due to that part resting on good soil and when checks fail for another part of the foundation due to bad soil properties at the other part of the foundation, the whole foundation can fail.
Ground investigation will need to be used to determine these different soil properties. The foundation structure will need to be designed in consideration of the different soil stiffness and soil properties.
The soil fill underneath the foundation might not be compacted properly and to its required degree of compaction. Since the soil is not compacted properly, air voids can be created within the soil, in which soil and water can displace in and out of. There will then be movement within the soil, which causes swelling and contracting. The swelling and contraction of the soil can cause pressure to the foundation that the soil supports.
Air voids within the soil can cause loose soil or soil with low density, which lacks adequate strength to support the foundation. Poor compaction equipment can also lead to foundation failure.
Therefore, it is best to compact the soil beneath the foundation to its required compaction degree before concrete placement of the foundation, in order to reduce soil displacement, to increase subgrade reaction and soil density, and to reduce differential and overall settlement of the foundation.
Similar to the above scenario, uneven moisture levels of the soil can cause soil swelling and contraction at specific parts of the foundation. This can lead to stress at intersecting locations where the soil is swelling and contracting and where the soil is not.
Changes in moisture levels of soil beneath the foundation
Moisture levels of the soil can change due to varying humidity levels, rainy weather, or poor drainage conditions, which can cause soil to swell (or heave) and contract, therefore leading to cracks. Similar to insufficient soil compaction, the voids within the soil can be filled up by water or other fluids, which can create pressure onto the soil particles from the fluid.
However, when there are dry periods, the water evaporates from the soil and leaves from the voids within the soil. This can cause soil shrinkage.
Moreover, when there are cracks within the foundation, water seepage can also occur.
Vibration from adjacent construction
Vibration from nearby construction can displace soil particles underneath the foundation. This can then create air voids within the soil, which can loosen up the soil and lower the soil density. The lower the soil density, the lower the soil strength for the support of the foundation. This will then cause foundation failure.
If there are trees adjacent to the foundation, the trees can evaporate the water from the soil into its roots and into the atmosphere. This can cause changes in the moisture level of the soil.
Next, we will explore the different remediation and prevention techniques.
But before we do that, are there any other causes of foundation failures you have encountered? Any examples that you experienced? Be sure to let us know in the comment section below!