OK, What's not a Sinkhole? (or the other causes of soil problems)
At first sight, most people tend to think that any soil problem typified by a loss of soil or soil drop is immediately a sinkhole. While this is more of an instinctual reaction, closer inspection may reveal that there is another reason for the problem. These other causes of soil/foundation problems not only occur in Florida, but all over the country. Just a few of the more common causes of soil and/or foundation failure are listed below:
Activity of Shrink-Swell Clay
Certain clay minerals have a high affinity for water and may change dramatically in volume as water is absorbed, or lost through desiccation. Seasonal and annual variations in rainfall affect the amount of water available, and hence these clays are constantly changing in volume. Structures built on these clays may experience significant differential movement, especially if surface drainage collects and wets the clay in a specific area. These clays are commonly found in Florida. Direct tests for shrink-swell clay activity are complicated and costly. Plasticity tests (plastic and liquid limits) are more cost effective and provide a reasonable indication of shrink-swell potential. Higher plasticity is generally an indication of greater shrink/swell potential.
This cause of soil activity or foundation damage is usually caused by the builder of the property or other persons that decide to buried deleterious materials in the yard or under the slab of the property. While sinkholes have mostly have a circular shape to them, buried debris (or trash pits) are usually oblong in form as builders or dumpers tend to excavate a trench in order to dispose of their materials. This type of feature is typically no more than 4 feet in depth.
Many trees and shrubs have considerable root systems that may lift and damage a structure. Careful inspection to the structure must be made to determine whether the damage is actually from sinking or has another part of the structure just been lifted. In addition, these roots may desiccate shrink-swell clays causing settlement as well as swell up when they happen to be saturated with water (i.e.: for instance a water leak in the vicinity of the structure wall and tree). Effects of a large tree root system can be devastating to a structure or foundation.
Settlement Due to Foundation Pressure
Foundation pressure will cause settlement of underlying soils. The magnitude of the settlement depends on the soil type, the soil stiffness, and the magnitude and area of the foundation pressure. Sands settle very quickly, but poorly drained soils containing silt or clay may require many years to fully consolidate. Uniform settlement may limit access to the structure, but usually does not result in structural distress. However, soil stratigraphy is seldom consistent under a structure, and therefore most structures experience some differential settlement and subsequent structural distress. The magnitude of the differential settlement may be as much as one half of the overall settlement.
Organic and Other Deleterious Materials
Organic materials (roots, peat, muck, tree stumps, etc.), and other deleterious materials may decompose over time causing settlement of overlying soils. Organic materials are common in Florida and can commonly be observed in such formations such as Cypress tree domes, swamp areas, old orange groves, and other low lying areas lush with vegetation. Layers of rich organic material
(5% < Organic Content) can be just as devastating to a structure as a sinkhole sometimes.
Change in Groundwater Level
As groundwater levels move downward due to fluctuations in rainfall and/or pumping from wells, as well as tidal influence, internal soil stresses increase due to the loss of buoyancy. This additional stress may cause both uniform and differential settlement. Migration of soils can be common near waterways.
Courtesy of Art Dillman, All Coast Engineering.