Pier and Beam Foundation Design
Foundations, the crux of many a failed construction job, are essential to get right, which is why here at Milan we place such an emphasis on this particular detail from the outset. In North Texas we're all familiar with our highly expansive clay soils, and in today's post we're going to discuss one of the best ways to address this detail, the Pier and Beam Foundation.
Prior to deciding upon a foundation it is incumbent upon you to assess the site's soil conditions, which can be done through a soils test conducted by a geotechnical engineering firm. Once contracted, these firms will provide a soils report based on field and laboratory testing to provide recommendations on how to address subsurface soils and foundation design. To understand these reports and the various reasons certain designs are suggested it is vital to have at least a layman's understanding of basic site hydrology. Subsurface water can be separated into two general classifications: the saturation zone and the aeration zone. The saturation zone, commonly referred to as the water table or ground water, is the deepest zone, and is the area at which the soil is normally fully saturated. The aeration zone includes the capillary fringe, intermediate belt, and the soil water belt, which can often be described as the root zone. The soil water belt is the area in which moisture is obtained for the vegetable and plant kingdom, while the intermediate belt contains moisture essentially in dead storage. For the sake of brevity and not to bore you, it is critical to understand that the soil belt is the zone that influences foundation design and stability the most.
When examining these subsurface conditions one of the quickest ways to assess soil movement is to find out the Potential Vertical Rise/Movement (PVR/M). Potential Vertical Rise or PVR for short is expressed in inches and can be defined as the latent or potential ability of a soil material to swell, at a given density, moisture and loading condition, when exposed to capillary or surface water, and thereby increase the elevation of its upper surface, along with anything resting on it (DefinedTerm: Potential Vertical Rise). With highly expansive soils such as the soils we have here in North Texas, the below can occur causing foundation damage:
Image Source can be found here.
Enter the Pier and Beam Foundation. With this foundation design you attain a raised floor that removes the structure from the ground, isolating the living space from ground moisture. Due to the soils report provided by Sam Engineering & Testing, we utilized a pier and beam foundation system on our 5923 Park Lane residence in Dallas, Texas. SE&T advanced three soil test borings at the project site to a depth of twenty feet below the existing grade and from there the following information was provided: The silty clay soils at the upper 6'-9' possess moderate/high movement (heave/swelling) potential, when free water is made available to them. The estimated Potential Vertical Rise/Movement (PVR/M) for these soils is 4.1". The report further continues to recommend that the piers used should be: straight shaft, minimum diameter of 12", founded at a minimum depth of 3' into the gray limestone (stratum II), designed for allowable total bearing capacity of 30,000 psf & increased by 3000 psf below for skin friction. This minimum recommended pier depth may translate to a minimum pier depth of 11'-16' or greater below the existing grade. Armed with this information, our team felt that despite being given alternative foundation design options, we would be best suited to utilize a pier and beam foundation which besides mitigating foundation-related moisture problems, has a plethora of advantages over a traditional slab-on-grade foundation design.
In this photo taken on our 5923 Park Lane job site, you'll notice the formwork panels, bracing, and rebar cage running through the middle of the panels to provide added support to the concrete once poured.
With the pour complete, notice the piers, and the green drainage pipework in the crawlspace to further help relieve any potential moisture-related issues. To achieve such a clean pour it is essential to get the formwork seen in the last image, correct. Additionally, tools such as a screed and immersion vibrator can be used to achieve a level concrete surface and reduce air pockets.
With the perimeter beam poured, and the piers in place, our team pursued an additional step to enhance this home's functionality. Notice in the picture above that the crawlspace is exposed to the ground below. A tell of many pier and beam foundations is a musty smell that is a direct result of leaving this area exposed to the ground by simply pouring gravel over the soil. Over time, moisture and bacteria can accumulate and a faint, yet noticeable smell emanates from below and into the living space. As you can imagine, this is not the an ideal odor for homeowners or their guests. To negate this, we poured concrete in this space (also called a mud slab) to not only eliminate odor-related issues, but also to provide a cleaner surface to work from for maintenance-related issues in the crawlspace.
Here a mud slab is provided for the crawlspace and a bull float is used to flatten and consolidate the slab surface.
Upon completion of the mud slab, Vice President of Operations, Duane Pfannenstiel, coordinates with our HVAC subcontractor to layout the hard ductwork that will be run within the crawlspace. Additionally, our framers begin the process of laying out the wood floor joists and sill plates to prepare for the erection of the superstructure.
Here the foundation wall can be seen with the wood floor joists in place. The black membrane applied to the exterior of this home is a waterproofing application which helps prevent the passage of water into the home.
This drone footage captures the elite craftsmanship provided by our framing crew in laying the floor joists, as well as the HVAC ductwork and plumbing run underneath the floor in the crawlspace.
Earlier I mentioned that a plethora of benefits can be derived from utilizing a pier and beam foundation. The benefits are:
-Less susceptible to poor soil conditions
-Provides a crawlspace to house your mechanical equipment, and repair work doesn't require you to break through a concrete slab
-The raised floor provides an added layer of protection from termite damage since the house is elevated off the ground
-Termite infestation is easily monitored unlike other foundation designs where termite infestation often isn't noticed until it's too late
-A more comfortable first floor walking surface as your floor is set upon wood joists and plywood rather than a concrete pad
With all these advantages it's no surprise that even where optimal soil conditions exist, many homeowners elect to utilize a pier and beam foundation despite the added costs associated with this design. If you'd like to visit this home, or if you're interested in learning more about our company, please contact Sam Allgood at the number provided on our website.
1) Allen, Edward, et al. Fundamentals of Building Construction: Materials and Methods. Wiley, 2014.
2) Brown, Robert Wade. Practical Foundation Engineering Handbook. McGraw-Hill, 2001.