Why Do Concrete Sidewalks Sink?

There are several reasons why concrete sidewalks sink and crack, some of which can be mitigated, while others are a natural by-product of time. Understanding why these happen can be useful not only for preventive maintenance but pinpointing when issues start to develop. 

Largely, all issues with concrete sidewalks begin and end with the soil supporting the slabs. 

• Soil Erosion 

Erosion can lead to concrete sidewalk sinking. This is due to water gradually washing away the supporting soil beneath the slab. Eventually, this creates a void which will lead to the slab collapsing or cracking. 

Even small voids can be an issue, as animals and insects will see these pre-made burrows as a perfect home. Some may expand these voids as well. 

Intense rainfall and flooding are triggers for greater erosion events. The excess of water soaks and loosens the soil. From here, the soil may lose structural integrity, allowing the weight of the slab or pressure from water currents to erode the earth. The same can happen to soil that is rapidly flooded with water after an extended dry spell. 

• Soil Expansion 

Soil expands as it absorbs moisture. Soil compositions that include clay particles will see even more expansion. It may not seem like it, but soil that goes through this swelling and shrinking process, which will happen as it dries, will upset the sidewalk. Over time, this process allows more water and debris to get beneath the slab. Causing the expansion’s impacts to grow. 

Soil also expands as the water particles within it freeze during colder weather. The freeze-thaw cycle can cause the slab to shift over time, resulting in a comparable situation outlined above. 

• Soil Settlement 

Concrete sidewalks are laid directly on top of compacted soil. As this soil naturally shifts over time, the slabs may lift or sink. In most cases, this is unavoidable. However, by properly compacting the soil in preparation for a sidewalk the impact of these shifts is minimized. At times, soil isn’t correctly compacted, or other influences change the composition of the soil, like extreme flooding or extended drought. 

Soil disturbed by construction crews and landscapers needs to be compacted in preparation for supporting structures. When this isn’t done correctly, the weight of the concrete slab compresses the soil to a greater degree than it would otherwise. 

Soil settlement is a primary cause for sidewalk sinking.