This past week, we took the very rare chance to get out of the concrete jungle of Washington DC and out into the sleepy and near-endless suburbs beyond. We had the chance to take a close look at a new concrete foundation for a contemporary basic suburban house. This house will be built in a quick and simple kind of way with construction elements and finishes that look appealing to the average prospective home buyer but will actually be almost as basic and cheap as you can get.
However, for us, it’s a good opportunity to show our clients and readers some of the fundamental principles of foundation construction. Even though we work in the historic parts of Washington DC, some of the same principles apply between modern foundation construction and historic masonry construction. Although the materials might be different and some of the methodologies have changed significantly, many of the fundamental principles remain the same.
The layout of this particular foundation is simple yet includes all the contemporary amenities that we might expect in a new home. There is a two-car garage, a bedroom and bathroom planned for the basement and two means of external egress and one external means of ingress to and from the basement.
The site preparation required for the foundation construction process involves a series of tasks aimed at ensuring the site is suitable and stable for the proposed structure. Typical soil testing checks some of the subsoil’s properties and characteristics, including its bearing capacity, compressibility, and potential for settlement or expansion. This data helps in determining the appropriate foundation design and depth.
In this particular case, since the basement wall is built with a cast in place concrete, the entirety of that wall is a part of the foundation which allows the foundation to reach a bearing point that is both below the frost line, and in this case below the point of the needed subsoil structural bearing capacity. The design height of the building can make a difference in the required size and thickness of the foundation and foundation stem wall and footing, and also dictate or influence the required depth of excavation.
Grading involves shaping and leveling the site to achieve the desired elevation and slope, facilitating proper drainage and minimizing the risk of soil erosion or water accumulation. From one site to the next there are different constraints and binding points, but overall, in most cases, the goal is to provide a positive grade away from the building foundation. In other words, in most cases, the ground and dirt or yard around the building foundation will start out higher at the edge of the building foundation and then go slightly downward from there out to the perimeters of the property. In some cases there may be drastic grades or hills on the property and in some cases retaining walls can be added to reduce and excessive slope away from the foundation but it’s almost always better to have grade away from the foundation instead of grade towards the foundation.
Compaction, on the other hand, is the process of increasing the density of the soil by mechanical means, such as rollers or vibratory compactors. Proper compaction enhances the soil’s load-bearing capacity and reduces the potential for differential settlement, which can compromise the structural integrity of the foundation. Back home here in Washington DC, by comparison, we have much more confined and limited spaces without large open areas on most sites. We also have a higher degree of constraints to work around in terms of grade. In the past century, somewhere around the middle of the century, around the halfway point of the life, to-date, for many of the historic buildings here in Washington dc, additions were added to buildings and row homes throughout the city. Many of these additions were the enclosure of the rear sleeping porches to turn those sleeping porches into interior building space. In many cases, we find that the structural capacity of the soil at the footings for those additions is entirely inadequate. We also commonly find that the areas with slabs built on grade are very poorly compacted. Poor compaction under footings and slabs is definitely a problem, but it’s almost not as bad of a problem as when many contractors around the middle of the 20th century built wood framed decks below grade but without sufficient space to separate the joist from the dirt below. We’ve seen this condition in many different buildings and it’s extremely problematic because in most cases the joists are rotten and or destroyed by being eaten by termites.
Understanding and working with the principles of soil compaction, slabs can be built directly on grade, in some cases with a downturn footing or a separate footing or stem wall, but when built properly, they can adequately support multi-level parts of the building above.
Soil mechanics, a branch of analysis that focuses on the study of soil behavior and its interaction with structures, helps with the foundation design, as it provides insights into the soil’s ability to support the loads imposed by the structure. The bearing capacity of the determines the type and dimensions of the foundation required.
Different types of soil exhibit varying properties that influence their bearing capacity. For instance, cohesive soils, such as clays, derive their strength from the cohesive forces between soil particles, while the strength of granular soils, like sands and gravels, is primarily derived from internal friction and interlocking of particles. Factors such as soil density, moisture content, and the presence of groundwater also play a significant role in determining the soil’s bearing capacity. The best combination is generally a balance because soils that are either to high in clay or too high in soil are problematic. Soils that are overly high in clay can shift with changes in ground moisture and soils that are overly high in sand lack some of the characteristics of this long-term stability.
In this coming week, we’ll look at some of the other details of the construction of this building foundation.
We can Help
Our company focuses on historic restoration more than modern building upkeep, maintenance, and construction, but our company understands both types of construction very well and a full picture well-rounded approach is needed in any niche in the construction industry. Although we focus on historic restoration, repointing, tuckpointing and historic brick repair, our company also has technical knowledge and competencies in the areas of modern and contemporary construction as well as we become one of the leaders in that area of the market today. Understanding both historic and modern or contemporary construction is useful because both aspects help understand the challenges and potential solutions for challenges in building science and construction.
We can help with a variety of historic masonry restoration needs and upkeep, from modest tuckpointing and or repointing to complicated and extensive historic masonry restoration. Infinity Design Solutions is a historic restoration specialist contractor specializing in both historic masonry restoration such as tuckpointing our repointing, and brick repair. If you have questions about the architectural details or facade of your historic building in Washington DC, reach out and say hello and if we can help we’ll be glad to assist you. You can email us or call us on the telephone at the following link: contact us here.
