There are a few factors related to the geology and the climate in Saint Augustine that make it less than ideal for foundation stability and longevity. We’ll discuss the most common underlying causes of damages below.
Poorly-Drained, Sandy Soil
Florida’s Atlantic Coast is well-known for its abundance of beautiful beaches, but the gradual deterioration of seashells, oceanic rock, and other organic materials that created the beaches also contributes to very sandy soil throughout Eastern Florida.
The most abundant dirt in the area is called Myakka soil, which has a high ratio of sand to other soil particles. Sandy soil typically drains rapidly, but the total lack of elevation above sea level and minimal topographical relief leave the area poorly drained. Poorly-drained, sandy soil is detrimental to foundations for a few reasons.
First, Saint Augustine’s soil is prone to underground erosion because the sand particles are so large and loosely packed. Runoff moving through the ground often pulls the soil with it, contributing to instability under your home.
Over time, the soil can sink at varying rates depending on how rainwater drains through it, leaving sections of your foundation without level support. The process of uneven movement is called differential settling, and it can easily cause structural damage to your property.
Much of the area is not only at or slightly above sea level but is also poorly drained, meaning the groundwater is located very near to the surface. When rain falls on the city and soaks into the ground, it has minimal space to drain before getting backed up to the surface.
The frequently saturated soil can leave your foundation exposed to moisture for long periods, allowing it to wick through cracks or pores in your concrete slab. The result is prevalent water intrusion and moisture accumulation under your first floor, which can lead to damaging and hazardous mold growth, rotting of construction material, and decreased indoor air quality.
Concentrated Rainfall and Flooding
The risk of underground erosion of Saint Augustine’s sandy soil and elevated groundwater is made more severe by rainfall. While the city receives an average amount of rain, the precipitation is concentrated from June to October during hurricane season, when hurricanes and tropical storms bring more than double the monthly rainfall the rest of the year.
The heavy rainfall during those months increases the rate of underground erosion and the resulting differential settling beneath your house. The poor drainage in the area not only exposes your concrete foundation to moisture for long periods during these months, but it also contributes to multiple types of flooding in St. Augustine.
The majority of the city is situated in a flood plain, with most areas being prone to tidal flooding from the ocean and rivers. Flash floods are also common because of the shallow groundwater and slowly draining soil.
Flooding contributes to underground erosion as an abundance of water drains through the soil. It also increases the likelihood of water intrusion and moisture accumulation under your home.
Myakka soil is known to be acidic, as are most of the other soil types in the region. This is primarily because of the deterioration of organic materials in the ground, like seashells. Acidic soil is detrimental to foundations, as it can eat away at the concrete over time, gradually weakening it.
The process of foundation corrosion occurs over many years, but eventually, you could be left with a structurally compromised concrete slab.
Lastly, Florida’s bedrock is mostly limestone and dolomite, two minerals that can be supportive in the right conditions. However, they also dissolve in the presence of water, meaning that groundwater and draining runoff will erode them over time and leave subterranean caverns and channels behind.
These voids are the reason that sinkholes are such a widespread issue in Florida. The loose, sandy soil above these caverns and waterways can eventually collapse, creating instability in the ground above. Differential settling is a significant risk above these areas of eroded bedrock. If the voids are large enough, sinkholes can form and cause massive damage to your foundation and your home, often necessitating structural repair.