Today’s topic residential foundations and basements built in Texas. My guest today is Tom Werling with North Texas Basements.
Why don’t most builders in Texas, build basements? The answer is liability.
Our soils in Texas are expansive and create problems not found in the northern states where basements are a common item. Colorado and Texas--what do they have in common? Unfortunately, they're the biggest two problem states in the country when it comes to expansive soil. Expansive soil is usually clay or shale (or a combination of both) that swells when wet and contracts when dry. Build a house on it with the wrong foundation or poor drainage and you're asking for trouble. Expansive soils cause cracked foundations, damage to basement and interior walls and ceilings, even plumbing pipe breaks. Over time, the repeated swelling and shrinkage can make a house un-livable. Could something be moving in that dark, quiet basement? Of everything that could lurk down there, the sneakiest and creepiest is water. Boogeyman aside, the last thing a customer wants in a basement is musty smells, damp carpet, damaged finishes, or standing water. Moisture in the basement can cause permanent damage to both the home and the builder’s reputation.
Water should never have a chance to enter a basement, so the best move against it is to be proactive. That takes a plan and the gumption to follow it through. Set a course of action for moisture control and make sure those methods are used on the construction site. If customers call back to question the quality of workmanship, prepared builders will be able to defend themselves by explaining their approach. So, what’s a good plan for below-grade moisture control? First, it’s important to understand the ways water can move into the basement. Gravity, splash back, and hydrostatic pressure are frequent ways that water enters over or through foundation walls. Capillary action is another more subtle way it moves though the wall and slab. A solid plan for moisture control should have protection strategies from each of these types of movement.
You can build basements in Texas if you follow a good plan of action. The basements here may cost more than building one up north but if you scrimp on the process or leave out necessary techniques, you’re asking for trouble.
IMPACT OF MOISTURE CHANGES ON
Most problems resulting from expansive soils involve swelling or shrinking as evidenced by upward or downward movement of the foundation producing distress to the structure. The difference between the water content at the time of construction and the equilibrium water content is an important consideration. Potential swell increases with lower initial moisture content, while potential shrinkage increases with higher initial moisture content. Moisture contents and shrink/swell movements may vary seasonally even after equilibrium is reached. Precipitation and evaporation control soil moisture and groundwater levels. A slab will greatly reduce the evaporation rate beneath the slab and partially reduces the inflow due to precipitation or irrigation because of groundwater's ability to migrate laterally. Therefore, soils beneath a slab are frequently wetter than soils at the same depth away from the slab. However, a wet season may result in wetter conditions away from the slab than under the slab. With time and normal precipitation patterns, the soil moisture profile will return to its normal condition. Seasonal variations in soil moisture away from the slab will generally occur fairly quickly. Seasonal variations in soil moisture beneath the slab will be slower. In addition roots from trees and large vegetation will seasonally remove moisture from nearby soils. Wetting of expansive soils beneath slabs can occur as a result of lateral migration or seepage of water from the outside. It can be aggravated by pooled water resulting from poor drainage around the slab or landscape watering. Leaking utility lines and excessive watering of soil adjacent to the structure can also result in foundation heave. Foundations can experience downward movement as the result of the drying influence of nearby trees. As trees and large bushes grow, they withdraw greater amounts of water from the soil causing downward foundation movement. The area near trees removed shortly before construction may be drier and subject to localized heave.
Some construction and maintenance issues include the following:
a. In general, set top of concrete at least eight inches above final adjacent soil grade for damp proofing.
b. For adjacent ground exposed or vegetative areas, provide adequate drainage away from the foundation (minimum five percent slope in the first ten feet and minimum two percent slope elsewhere). The bottom of any drainage swale should not be located within four feet of the foundation. Pervious planting beds should slope away from the foundation at least two inches per foot. Planting bed edging shall allow water to drain out of the beds.
c. Gutters or extended roof eaves are recommended, especially under all roof valleys. For adjacent ground exposed or vegetative areas, all extended eaves or gutter down spouts should extend at least two feet away from the foundation and past any adjacent planting beds.
d. Avoid placement of trees and large vegetation near foundations (taking into account the water demands of specific trees and vegetation).
IMPACT OF FILL ON FOUNDATIONS
Fill is frequently a factor in residential foundation construction. Fill may be placed on a site at various times. If the fill has been placed prior to the geotechnical investigation, the geotechnical engineer should note fill in the report. Fill may exist between borings or be undetected during the geotechnical investigation for a variety of reasons. The investigation becomes more accurate if the borings are more closely spaced. Occasionally, fill is placed after the geotechnical investigation is completed, and it may not be detected until foundation excavation is started. If uncontrolled fill (see discussion below) is discovered later in the construction process, for instance, by the Inspector after the slab is completely set up and awaiting concrete, great expense may be incurred by having to remove reinforcing and forms to provide penetration through the fill. Therefore, it is important to identify such materials and develop a strategy for dealing with them early on in the construction process. Fill can generally be divided into three types: engineered fill, forming fill, and uncontrolled fill. These three types of fill are discussed below.
Engineered fill is that which has been designed by an engineer to act as a structural element of a constructed work and has been placed under engineering inspection, usually with density testing. Engineered fill may be of at least two types. One type is “embankment fill,” which is composed of the material randomly found on the site, or imported to no particular specification, other than that it be free of debris and trash. Embankment fill can be used for a number of situations if properly placed and compacted. “Select fill” is the second type of engineered fill. The term “select” simply means that the material meets some specification as to gradation and P.I., and possibly some other material specifications. Normally, it is placed under controlled compaction with engineer inspection. Examples of select fill could be crushed limestone, specified sand, or crusher fines, which meet the gradation requirements. Select underslab fill is frequently used under shallow foundations for purposes of providing additional support and stiffness to the foundation, and replacing a thickness of expansive soil. Engineered fill should meet specifications prepared by a qualified engineer for a specific project, and includes requirements for placement, geometry, material, compaction and quality control.
Forming fill is that which is typically used under residential foundation slabs and is variously known as sandy loam, river loam or fill dirt. Forming fill is normally not expected to be heavily compacted, and a designer should not rely on this material for support. The only requirements are that this material is non-expansive, clean, and that it works easily and stands when cut. If forming fill happened to be properly compacted and inspected in accordance with an engineering specification it could be engineered fill.
Uncontrolled fill is fill that has been determined to be unsuitable (or has not been proven suitable) to support a slab-on-ground foundation. Any fill that has not been approved by a qualified geotechnical engineer in writing shall be considered uncontrolled fill. Uncontrolled fill may contain undesirable materials and/or has not been placed under compaction control. Some problems resulting from uncontrolled fill include gradual settlement, sudden collapse, attraction of wood ants and termites, corrosion of metallic plumbing pipes, and in some rare cases, site contamination with toxic or hazardous wastes.
Building on Non-Engineered (Forming Or Uncontrolled) Fill
Foundations shall not be supported by non-engineered fill. To establish soil-supported foundations on non-engineered fill, the typical grid beam stiffened slab foundation is required to penetrate the non-engineered fill with the perimeter and interior beam bottoms forming footings. Penetration will take the load supporting elements of the foundation below the unreliable fill. Penetration could be accomplished by deepened beams, spread footings or piers depending on the depth and the economics of the situation. Generally, piers are most cost effective once the fill to be penetrated exceeds about three feet, but this depends on the foundation engineer’s judgment and local practice. Floor systems shall be designed to span between structurally supported foundation elements. Pre-existing fill may be classified as engineered fill after investigation by the geotechnical engineer. The approval may depend on the fill thickness, existence of trash and debris, the age of the fill, and the results of testing and proof rolling. The geotechnical engineer must be able to expressly state after investigation that the fill is capable of supporting a residential slab- on-ground foundation.
Design and structural considerations
Structurally, for houses, the basement walls typically form the foundation. In warmer climates, houses sometimes do not have basements because they are not necessary (although many still prefer them.) In colder climates, the foundation must be below the frost line. Unless constructed in very cold climates, the frost line is not so deep as to justify an entire level below the ground, although it is usually deep enough that a basement is the assumed standard. In places with odd stratified soil substrata or high water tables, such as most of Texas, Oklahoma, Arkansas, Tennessee, Mississippi, Alabama, Georgia, Louisiana, and Florida, basements are usually not financially feasible unless the building is a large apartment or commercial structure.
Some designs elect to simply leave a crawl space under the house, rather than a full basement. Most other designs justify further excavations to create a full height basement, sufficient for another level of living space. Even so, basements in Canada and the northern United States were typically only 7 feet 10 inches (2.39 m) in height, rather than the standard full 8 feet (2.44 m) of the main floors . Older homes may have even lower basement heights as the basement walls were concrete block and thus, could be customized to any height. Modern builders offer higher basements as an option. The cost of the additional depth of excavation is usually quite expensive. Thus, houses almost certainly never have multi-story basements though 9' basements heights are a frequent choice among new homebuyers. For large office or apartment buildings in prime locations, the cost of land may justify multi-story basement parking garages.
The concrete floor in most basements is structurally not part of the foundation; only the basement walls are. If there are posts supporting a main floor beam to form a post and beam system, these posts typically go right through the basement floor to a footing underneath the basement floor. It is the footing that supports the post and the footing is part of the house foundation. Load-bearing wood-stud walls will rest directly on the concrete floor. Under the concrete floor is typically gravel or crushed stone to facilitate draining. The floor is typically four inches (100 mm) thick and rest on top of the foundation footings. The floor itself is typically sloped towards a drain point, in case of leaks.
Since warm air rises, basements are typically cooler than the rest of the house. In summer, this makes basements damp, due to the higher relative humidity. Dehumidifiers are recommended. In winter, additional heating, such as a fireplace or baseboard heaters may be required. A well-defined central heating system may minimize this requirement. Heating ducts typically run in the ceiling of the basement (since there is not an empty floor below to run the ducts). Ducts extending from the ceiling down to the floor help heat the cold floors of the basement. Older or cheaper systems may simply have the heating vent in the ceiling of the basement.
The finished floor is typically raised off the concrete basement floor though modern laminate flooring is typically placed on concrete floors in Canada with a thin foam underlay. Radiant heating systems may be embedded right within the concrete floor. Even if unfinished and unoccupied, basements are heated in order to ensure relative warmth of the floor above, and to prevent water supply pipes, drains, etc. from freezing and bursting in winter. It is recommended that the basement walls be insulated to the frost line. In Canada, the walls of finished basements typically are insulated to the floor with vapor barrier(s) to prevent moisture transmission.