What is a Foundation
Foundations are designed to have an adequate load capacity with limited settlement – this is usually determined by a geotechnical engineer; the foundation itself is structurally designed by a structural engineer. The primary design concerns are total and differential settlement, and load bearing capacity. When we consider settlement, we have total settlement, the entire structure settling as one total unit, and differential settlement.
Differential settlement is when one part of a foundation settles more than another part. A crack in one corner or part of the footing or foundation would be considered differential settlement. This can cause problems to the structure on which the foundation is bearing or supporting. It is necessary that a foundation is not loaded beyond its bearing capacity or the foundation will “fail.”
In the MD / VA / DC geographical area, we have other design considerations, specifically, scour and frost heave. Scour occurs when flowing water removes supporting or load bearing soil from around a foundation, which may cause undermining, which leads to footing cracks and wall cracks. Frost heave occurs when water in the ground freezes to form ice lenses, which may also cause footing and wall cracks. Frost action and heaving is a phenomenon that occurs in the winter and early springtime in Northern and Northeastern climates. Essentially, all surface soils undergo some frost action, the magnitude of which is dependent upon the climate and precipitation found in that specific geographic location.
Frost Line, Soils, and Water
Capillary action (capillarity, capillary motion, or wicking) is the ability of a substance to draw another substance into it. Capillary action describes the attraction of water molecules to soil particles and describes the wick-like migration of water into the porous walls and floor in the same manner as water is drawn into a sponge. Capillary action is responsible for moving groundwater to the area surrounding your residence and then into your basement.
Therefore, silt and clay supply or hold the water necessary to the formation of ice lenses in the freezing zone. The only soils that can be considered non-frost susceptible are the permeable soils – very clean mixtures of sand and gravel. These soils drain freely by gravity because there is nothing to hold or attract the moisture, and do not create capillary moisture movement. If your home were surrounded by sand and gravel, you might never experience a basement water problem, as long as: 1. The foundation’s footing rested on a properly compacted soil; 2. You do not have a high (seasonal) water table, which is endemic to MD., VA., D.C. area; 3. Your home is or was properly constructed; 4. You have properly installed foundation drain tiles, exterior, interior or both which is best.
In the Maryland, Virginia, and Washington, D.C. area, due to high water tables throughout the region, due to the constant rainfall, and the predominant clay soil, eliminating the supply of water to the soil below the ground is virtually impossible. However, properly installed foundation and sub-soil drainage can partially reduce or minimize the quantity of water available to feed an ice lens and the cause of frost heave. Proper grading, extending downspouts 8 feet from the home does help.
Changes in soil moisture can cause expansive clays (sometimes called marine clay) to shrink and swell around the foundation . This swelling varies due to seasonal changes or the effects of vegetation, surrounding a home or building, removing moisture. This is a particular problem for residential footings and foundation walls in Maryland, Virginia, and Washington, D.C., where wet winters and springs are followed by hot dry summers. The silty, loamy clays, which are predominant in our area, shrink and settle during the summer or dry months, and swell and expand during the fall, winter, and spring, causing walls and footings to crack, and basements to flood – whether the rainfall is torrential or normal.
Excavation, Strata, Capillaries
The foundations of buildings are built on soil. Before homes are built, soil surveys are usually conducted with testing done to determine what soils exist, the water table depth, and whether the soil will bear the foundation weight. In commercial construction, compaction tests are a normal and mandated part of the process of building the foundation. In residential construction, this is not usually done. The contractor simply abides by the architectural blue prints, which specify how the work is to be performed. Excavation is usually nothing more than digging out the ground where the home will be located, forming and then pouring the footings, and then building the foundation walls and the rest of the home, usually as quickly as possible.
What you may or may not be aware of is this – before your home was built, that land probably sat there undisturbed for hundreds if not thousands of years (or hundreds of thousands of years); some communities in P.G. County are built on a landfill or in a flood plain or over top of tributaries – springs, streams, etc. But here is the salient point – the soil upon which your home rests, before it was built, was a conduit for all the watershed activity in the area directly above and adjacent to where your property rests.
Water has a number of absolute characteristics. 1) Water has an excellent memory – if it followed a path once, it will seek that pathway again; 2) Water always seeks the path of least resistance (gravity – it runs downhill); 3) Water will seek its own level (self-balances or equalizes); 4) Capillary water can move upwards or sideways because porous materials will soak or suck the water; 5) because it is the most powerful element on earth, it will go through or move ANYTHING in its path. It does not like to go around – talk with those in Louisiana and Mississippi. Sometimes it takes time before it will move something, but water does not recognize time. It has all the time in the world. This is why homes used to be built to withstand the ravages of time and water for 100 to 200 years. These days, homes are falling down after 30-50 years or less.
In geology and related fields, a stratum (plural: strata) is defined as a layer of rock or soil with internally consistent characteristics that distinguishes it from contiguous layers. Each layer is generally one of a number of parallel layers that lie one upon another, laid down and compressed, and then separated by natural forces. In between these strata are voids through which water migrates as water tables or capillary veins.
They extend over hundreds of thousands of square miles of the Earth’s surface. Strata are typically seen as bands of different colored or differently structured materials, which we see exposed in cliffs and road cuts as we travel through mountain passes or by quarries and riverbanks. When planning civil engineering projects or other large construction projects, the strata of the area where the construction takes place is a significant factor in design decisions. For example if a canal is to be built on a route where the strata are not watertight, the canal will have to be lined with some form of waterproof material.
Situating or building a home on top of a hill is always more prudent than in a valley or at the bottom of a hill. As ground water seeps into the deeper strata of the surrounding soil, it characteristically follows the natural slope or grade of the surrounding area – it moves downhill! First, the topsoil will absorb some of the water. Although water runs on the surface, most water is actually migrating quite slowly through the ground, between the strata. As the water first hits the surface as rainwater, it slowly soaks into the ground, until the ground is saturated which creates ‘seasonal water tables.’
When this happens, we then have flooding conditions. When the ground can no longer absorb a torrential downpour, the rain becomes surface water, and this water runs quickly into the surrounding roadways and streets and enters the municipal storm drains, which are rapidly overwhelmed and subsequently back up. The result is flooding and flash floods. Think about this – all water which falls east of the Appalachian Mountains, finds its way into the Atlantic Ocean by way of surface and sub-surface water ways, rivers, creeks, streams, water tables, and capillary veins. All water which falls west runs to the Mississippi.
The loosely backfilled soil surrounding your home, which probably was not compacted, will always be more absorbent than the virgin soil surrounding it. This means that you will have an artificial or seasonal water table around the home where, in most cases, more water will collect than anywhere in the area. This is sometimes called the “clay bowl effect” by waterproofing companies, since your house in essence, sits in a bowl of dense, impermeable clay.
As moisture and water levels build up in the area around your home, it creates hydrostatic pressure against the floor first, and as it rises above the floor, against the basement and foundation walls. This pressure can lift the floor, damage the walls as moisture presses on and searches for the path of least resistance; or any way to pass through the basement wall and floor cracks as well as through the cove area; or where the basement wall meets the basement floor. This moisture, either in liquid (water) or in vapor, will also wick through porous concrete, mortar, concrete block, and grout. Read the quote that appears throughout our website –
Basements are not designed to be waterproof, only water resistant. When water in the soil is only a few inches above the basement floor, water can find openings and seep or flow into the basement. Water creates such high pressures that sealing cracks will not prevent water from leaking into the house if the soil around the house is saturated. For example, when the soil is saturated to 3 feet above the floor level, the force of the water is more than enough to lift the concrete floor slab. Obtain professional design assistance if you feel you will need a “water-proof” basement.” Iowa State University, “Building Basements in Wet Locations” March 1994.
The Building Block of all Buildings
Foundation is a structure on which a building stands and is supported by it. For residential dwellings and commercial properties, concrete foundation is the most widely used material that gives lasting results and keeps the building intact.
In general, foundation engineering applies the knowledge of geology, soil mechanics, rock mechanics, concrete, steel and structural engineering to the design and construction of foundations for buildings and other structures. The most basic aspect of foundation engineering deals with the type of foundation selected, such as when to use a shallow or deep foundation system. Foundations are commonly divided into two categories: shallow and deep foundations.
Shallow and Deep Foundation
- Combined Footings – reinforced-concrete combined footings are often rectangular or trapezoidal and carry more than one column load.
- Conventional Slab-on-Grade – this type is a continuous reinforced-concrete foundation consisting of load-bearing wall footings and a slab-on-grade. Concrete reinforcement is consisted of steel rebar in the footings and wire mesh in the concrete slab.
- Spread Footings – also called pad footings are often square, are of uniform reinforced concrete thickness, and are used to support a single column load located directly in the center of the footing.
- Strip Footings – (very similar to spread) also called wall footings are often used for load-bearing walls. They are normally long reinforced concrete members of uniform width and shallow depth.
Spread footing foundations are common in residential construction and in many commercial structures that include basements in their design. The following illustration shows a spread footing in homes with basements.
Spread Footing (Footer)
Crawl Space Foundation
Footings are generally made of concrete with steel reinforcement. Most residential footings are between 16″-24″ wide and 8″-12″ thick. The width of the footing helps spread the weight of the house to prevent excessive settlement. Steel reinforcing bar (also called rebar) is run horizontally through the footing for added strength. The rebar is overlapped and tied together. “L” shaped rebar is placed vertically into the footing after the concrete is poured with about 2′ sticking above (see picture below). This rebar strengthens the connection between the foundation walls and the footings.
The footings are usually formed with 2 x 8 or 2 x 10 boards (laid on edge) and held together with stakes on each side and straps or longer stakes across the top. Once the footings are connected, they are set to grade with a laser level or transit. Setting the footings to grade is the process of leveling them. If the footings are not set to the same height, the house will not be level (flat). After the footings are formed and reinforcement placed they are inspected by the building Inspector. The footings are poured and the concrete is allowed to cure – some builders wait a day, some wait 7 days or longer. The forms are then removed and usually reused on another job.
Here we have a home site in P.G. County, Maryland that has been excavated with the footings poured, ready for a cast-in-place wall foundation (which is also pictured further below).
Now it is time to build our walls. Every week or so, we conduct a Foundation Inspection with homes where the walls were constructed of stone, brick (very common in D.C.), or terra-cotta block (pictured below). Modern day construction uses pre-cast or cast-in-place concrete (pictured below under Cast-in-Place), or concrete block, sometimes referred to as cinder block, or concrete masonry units.
|Concrete forms ready for the concrete trucks||Pre-Cast Concrete Walls||Pre-Cast Concrete Wall|
Concrete Block Walls
Concrete block or a Concrete Masonry Unit (CMU) is made from poured concrete. The primary ingredients are Portland cement, gravel and sand, the same ingredients in poured concrete. The difference is the size of the gravel used in either application. Typically, you will see gravel as large as three-quarter inch diameter in poured concrete, whereas the gravel in block is usually pea gravel, no larger than the size of a pea. Most blocks measure to 8″ although the standard units are actually 7-5/8″ in width. This allows for the thickness of the mortar on the finished job.
Examples of Concrete Block