The subject of this article is Swelling Soils, also known as Expansive Soils. In certain areas of metro
Denver these soils "underlie" a home. They can cause severe damage to the foundation and other parts of the structure unless preventive measures are taken during construction.
You can safeguard your investment for years to come by adhering to a few basic maintenance procedures. For now, be aware that many of the precautionary systems incorporated into the design of your home can be defeated. Problems may arise from improper landscaping technique, improper construction in the finishing of basement areas, and neglect in maintenance. Among the most frequently observed detrimental conditions within your control are:
- Neglect in maintaining good drainage through sufficient slopes away from the foundation.
- Failure to keep gutters and downspouts unobstructed, and the removal of down spout extensions which direct water away from back-fill areas.
- Inadvertent changes made to lot drainage patterns through improper placement of fences, patios, retaining walls, gardens, sandboxes, or other obstructions that cause water to pond.
- Selecting the wrong types and species of trees, shrubs, and other vegetation for the location and climate. Planting vegetation with high moisture requirements too close the foundation.
- The location of irrigation equipment too close to the foundation.
- Failing to maintain exterior concrete walks, drives, and patios. Minor cracks may develop in concrete which can readily be sealed with quality exterior acrylic caulking compound or equivalent product. Left unattended, cracks in concrete provide access for water and accelerate the rate of deterioration and saturation of subsoils.
- Improper construction techniques employed in the finishing of basement areas. Most common errors negate the intent of the "floating" basement slab design and special partition voiding in wall framing
WHAT ARE SWELLING SOILS?
Boiled down to essentials, swelling soils are soils containing montmorillonite clay minerals that take on water and expand. Conversely, if water is taken away they shrink. A sample of pure montmorillonite provided sufficient water, increases up to 15 times its original volume! While most soils containing montmorillonite will not expand more than 35 to 50 percent in volume, it is this expansion in volume that causes the pressure and movement that causes foundation, slab, and other swelling soil damage. The amount of volume change that can take place depends on several factors:
- the amount of expansive clay in the soil or shale,
- the type of minerals in the clay,
- soil density,
- the amount of structural loading, and perhaps most importantly,
- the amount of moisture change.
HOW DO YOU KNOW IF YOUR HOME IS BUILT ON THESE SOILS?
This means that soils samples have been taken and subjected to testing in a laboratory under controlled conditions. Recorded data obtained on-site during the soils sampling and the laboratory results have been evaluated by a licensed professional engineer to determine if swelling soils exist, and if they do, the amount of swelling that can be anticipated. The engineers' conclusions and recommendations are the basis for the type of foundation placed beneath your home, as well as other precautionary construction details designed for movement. If swelling soils exist beneath your home, your builder will have provided you with a copy of the soils report and a description of steps taken during the building of your home to prevent damage due to potential swelling.
AN ALTERNATIVE DESIGN DETAIL:
A "floating slab" or a "structural" wood floor system are typically use for basement floors. The "floating" floor system is a concrete floor, the "structural" is a wood floor. (There can be exceptions. There are "floating" wood floor systems, and "structural" concrete floor systems, but these are not generally seen.) Your builder may have decided to employ the structural wood floor system, dependent on recommendations from his engineer pursuant to site and soil conditions. The structural wood floor is by definition, a "structural" element. The floor is suspended from the concrete foundation wall, and supported by a system of beams and floor joists. The floor achieves "isolation" from the normal movement of soil material underneath by means of a voided area directly underneath. If you have such a floor, you may have an access cover that enables you to inspect the area underneath the floor.
BEFORE YOU PLANT OR WATER
If your home is built on potentially expansive soils, you and your builder may have invested thousands of dollars in additional costs for precautions to prevent and minimize potential problems. All of this could be for naught, destroyed by:
- changing the established slope that directs surface water away from your foundation;
- plantings improperly selected and placed in relation to the home; and by
- poorly designed installation of sprinkler systems and watering.
WHAT CAN YOU DO?
First: Check the slope of the soil to be sure it is away from the foundation. If it is not, contact your builder. It is his responsibility initially to establish the grading and one time only during the first year of the limited warranty period to fill settled areas. You will generally be required to remove landscaping affected by the placement of additional fill. Thereafter, it becomes your responsibility to maintain proper grading. You should continue to check the slope frequently during the first few years. Further settlement may occur due to the consolidation of earth that was placed against the foundation after its construction. Fill any depressions that occur. Watch for settlement that reverses the drainage away from the foundation, and tell-tale signs such as puddeling after rains.
Before planting, consider the impact to the land adjacent to the foundation. If one is not initially installed by your builder, consider a sloped bed of decorative rock, gravel or bark mulch, about 3" in depth, installed over geotextile fabric or plastic sheets, and extending out a minimum distance of six (6) feet from the foundation line. This increases surface runoff at the most critical areas immediately adjacent to your foundation. You may utilize a geotextile fabric or equivalent "breathable" membrane, such as "Mirafi 140S". It is important that a good high quality non-woven material be used that is not subjected to rapid deterioration if exposed to ultraviolet radiation. This type of membrane will control weeds and retard water infiltration, but still allow the soil underneath to dry through normal evaporation.
If metal or plastic edging materials are used to define the rock, gravel, or mulch bed and lawn boundary, ensure that they do not block the flow of surface drainage. It is not uncommon to witness a dam effect at this lawn and "dry landscaped" border. Where possible, it is desirable to eliminate the edging entirely. At a minimum, gaps should be left in the edging or holes drilled for drainage. Ensure that gaps or holes are at a level where drainage will occur, and that there are no protruding sharp edges that may be a hazard for people or pets.
At this time also check to make sure your downspout extensions still extend beyond the edging into lawn areas or drainage swales. Where this is not the case, additional extension material which is readily available at hardware stores, is recommended. Under no circumstances ever remove the roof drainage downspout extensions to water flower beds, shrubs, or other vegetation. During torrential rains, a single downspout can discharge at a rate upwards of 500 gallons per hour. Failure to properly direct this water away from your foundation is a prescription for disaster.
ACCEPT THESE RECOMMENDED RULES FOR PLANTING:
Do not plant any shrub, groundcover, perennial or bedding plant less than 4 feet from the foundation, (to the center of the plant). Though it would be preferable not to have any plantings within six (6) feet of foundation line, we recognize for most homeowners that this is not aesthetically acceptable. Moderation then, is advised in the number of plantings, and utilize only those that require minimum moisture. Where rock or decorative gravel covering has been used, plantings must be tolerant to heat and "sunscald" from reflected sunlight. Suggestions include but are not limited to; Yarrow, Sedums, Potentilla, Mountain Common Juniper, and Creeping Juniper. Determine the mature height and spread of trees that you intend to plant, and plant them at least one-half of their mature spread away from your home. Keep lawn and turfgrass areas at least six (6) feet away from the home.
FOLLOW THESE GUIDELINES FOR IRRIGATION:
Do not install any spray heads, rotors, low pressure spray head, bubbler or any similar sprinkler within six (6) feet of foundation. Watch to ensure that no sprinkler, regardless of location, is installed in such a way that the water spray pattern falls within six (6) feet of the foundation. Keep all piping for your sprinkler systems running parallel to building line a minimum of ten (10) feet from the foundation. Those sprinkler heads located between six (6) and ten (10) feet from foundation should be supplied by laterals from the mainline perpendicular to the building line. No valve box should be located within ten (10) feet of the foundation. Valve boxes are a common source of leaks with resultant saturation of soils in the immediate vicinity.
ABSOLUTE MAINTENANCE REQUIREMENTS!
The axiom is to keep water away from your foundation. To do this:
- Check the ground around the foundation during rains, no puddles should exist. Fill in low spots with dirt so that water drains away from your home.
- Keep downspout extensions and/or splash blocks in place.
- Keep your house gutters and downspouts clean and in good repair. Overflows can be dangerous to the health of your home.
- Do not change the grade of the soil away from your foundation by building planters or raised beds.
- Be certain that all paving or patio slabs abutting your home, slope away from the foundation. Check seasonally, that they remain that way.
- Be cautious when planting of trees, shrubs and plants. Six feet and further away from the foundation is ideal unless the plant needs very little water.
- Do not water your foundation.