What happens when you have purchased a piece of property or started on a project, and the soil becomes unsuitable for the foundations and the construction of the project?
Unfortunately this occurs many more times than expected, due to the lack of understanding that most builders, home owners as well as ” so called ” professionals have regarding soil that is unsuitable, or becomes unsuitable as the project advances.
This brief article is meant to explain the basic elements of this situation. It is not meant as a guide to professional solutions to the issue, or as a complete education into soils and how they react to pressure, water, vibrations, etc. Use it as a summary of information on inadequate soils.
- The basic characteristic of a good supportive soil, is its ability to withstand the pressures exerted on it from a structure, that is constructed over it and bears down on this soil.
- In most instances, the point of contact between the soil and the structure is the foundation, starting with the footing.
- Foundations are designed to distribute the load from the structure, both dead and live, down on the soil that ultimately supports this pressure.
- Engineers will design the foundation to be the proper width, length and depth to adequately distribute the loading onto the soil, depending on the strength and characteristics of this soil.
- What are some of the characteristics that make a soil a good supportive medium to support the structure?
- a.) In most instances, the soil should be free draining. This means that water will drain from the soil and will allow the soil to dry out. The particle type and size of the soil make up, will determine the ability of the soil to drain.
- b.) The particles that make up the soil should be stable and able to withstand pressure without deforming. Pieces of sharp sand and rock will make a good strong soil. Introduction of clay and / or silt to the mixture will weaken the composite makeup of the soil.
- c.) Lack of any type of organic materials, such as leaves, grass, wood, compost material, etc. The presence of any organic materials will weaken the soil substantially and render the soil unsuitable to build upon.
- d.) Stability during the duration of exposure to the elements. There are some soils that will be structurally adequate when dry, but will become totally inadequate if water is introduced. Due to the fact that the majority of projects are subject to the elements, water will be introduced into the soil mix at some point. A good soil will withstand the influx of moisture and water without deteriorating and losing structural strength.
What could occur, if the soil is not structurally adequate for the loads being imposed on this soil?
- Settlement. One of the most critical structural elements for a structure, is the ability of the structure to be supported by the foundation without any movement or settlement.
- Foundation cracking. If there is settlement, then there could be foundation cracking. The footings are normally the last building element, prior to the surface of the soil. If the soil settles or fails, then in most instances, the foundation will crack or fail in the same area as the soil failed.
- Foundation will lift or be pumped up by either ice or water pressure. If the soil is not free draining and has a tendency to become fluid in a saturated condition, substantial issues, such as jacking or pumping could occur. This will lift the structure up and cause failures within the upper elements of the structure.
- Erosion failures, or simply the structure was “washed away “. If the soils retain water and the influx of water is in a flowing fashion, there is the possibility of a washout or an erosion failure due to the movement of the soil by the flow of water.
How do we reduce the occurrence of poor soil conditions and eliminate any structural issues that could be encountered by this issue?
Employ a professional structural engineer, together with a
professional soils engineer.
The use of both disciplines, in coordination with each other, will offer the best and most comprehensive structural and material evaluation of your site and situation.
The professional structural engineer will have the capability of evaluating the structures loads on the foundation and ultimately the soil. The soils engineer will identify the soil characteristics and its structural capacity for supporting the structure.
Both professionals can work together to design the proper foundation system, based on the existing soil, to properly support your structure.
The KEY is to have both professional work together to ensure that the structural engineer understands the limitations of the soil, as described by the soils engineer.
If you find yourself in a situation where the existing soil is inadequate to stabilize your structure due to failure by the design professionals, or simply a change in the seasons, causing saturation to the existing soils that were originally adequate for the resultant loading, there are techniques, methods or modifications that can be performed on the existing soils to restore their ability to properly support the structure.
Once again, in most instances, the employment of a professional soils and or structural engineer to identify the proper correction to the existing soil is recommended.
What could you do?
- If the situation is simply water, a drainage system that is capable of eliminating this water would work. This could be as simple as perimeter foundation drain, or a stone sub-base under the slab on grade.
- Mix crushed concrete with the existing soil, increasing the permeability of the soil, adding to its strength and providing additional structural elements to the problem material.
- Mix stone into the soil to increase the permeability of the soil, adding to the strength as well as providing additional structural elements to the problem material.
- Mix cement into the soil and allow it to set up. This technique also dries out the soil allowing the moisture content to decrease, thereby allowing compaction of the soil.
- Revise the design of the foundations to spread out the load over a larger area of soil. If the problem is the structural ability of the soil, then a larger, more spread out foundation will reduce the actual square foot loading on the soil, and might be enough to adequately support the structure.
- Revise the slab on grade, if there is one, to a reinforced slab on grade that will distribute all of the structures load over the entire footprint of the building. This is commonly done in areas of high water table or in situations where the soil, or lack of soil is incapable of supporting any type of structure.
- Drive piles. This is probably the most expensive last resort, type of solution. If piles are required, you would have hoped that the experts during the design phase would have already anticipate this issue.
The most important aspect of ” bad soil” is determining this, prior to the commencement of any construction. Modern engineering and new soil technology can solve almost all soil inadequacies, prior to the actual construction, however, once you have started, it becomes much more difficult, as well as costly to solve.
DO NOT IGNORE BAD SOIL OR HOPE THAT IT WILL REMAIN DRY – DEAL WITH IT DURING DESIGN, NOT CONSTRUCTION !