How Your Foundation was Built

What is a Foundation

A foundation is a structure that transfers loads to the earth. The structure of something is how the parts of it relate to each other, how it was built. The age of your home can sometimes determine how your home was built and whether it even had a foundation to begin with. Is it 200 years old? Then it most certainly was built with stone, as opposed to more modern types of building materials such as terra cotta block in the late 1800's to early 1900's; cinder block, then concrete block, brick, pre-cast or cast-in-place (poured) cement, etc.; and, we find many homes which are 80-200 years old which were built without a footing or foundation upon which the walls rest. That determination is sometimes easy to make, sometimes only when the floor is opened up.

Foundations are designed to have an adequate load capacity with limited settlement - this is usually determined by a geotechnical engineer; the foundation itself is designed structurally by a structural engineer. The primary design concerns are 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 footing crack in one part of the foundation would be considered differential settlement. This can cause problems to the structure the foundation is bearing or supporting. It is necessary that a foundation is not loaded beyond its bearing capacity or the foundation will "fail."

Other design considerations, especially in the MD / VA / DC geographical area, include scour and frost heave. Scour is when flowing water removes supporting soil from around a foundation, which may cause undermining and footing cracks. Frost heave occurs when water in the ground freezes to form ice lenses, which may 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 geographic location.

Frost Line, Soils, and Water

When construction takes place in a geographical area where temperatures reach freezing on a seasonal basis, such as our D.C. Metro area, a home's foundation must be built below the frost line; the frost line - also known as frost depth or freezing depth - is the depth at which the groundwater freezes. This is usually building code or if not, it should be to prevent substandard building practice and differential settlement. For example in Maryland, building code is certainly 30-36 inches below the surface, although the construction of a basement footing is much lower to accommodate ceiling heights of 6-10 feet or greater. If you have a walk-out basement, then the footing where the walk-out is located should also be 30-36 inches deeper than the threshold.

Step Footing

If the site of the home is sloped, maintaining a constant elevation for the footings would either cause the footings to be above the frost line or significantly deeper than the frost line at one end of the building. Therefore, stepping the footings enables them to follow the slope of the site and remain below the frost line. Some soils are more susceptible to the formation of ice lenses than others are because they hold water as opposed to allowing water to drain. Silts or silty clay soils, such as those found throughout Virginia, Maryland and D.C. are considered amongst the most frost susceptible. Silts and clays - because of the extremely small size of their particles, or gradation - permit and encourage the flow of water by capillary action through its pores.

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.

Consequently, silts and clays supply or hold the water necessary to the formation of ice lenses in the freezing zone. The only soils, which can be considered to be non-frost susceptible, are 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 the foundation footing rested on a properly compacted soil, you did not have the high water table, which is endemic to our area, your home was constructed properly, and you had a properly installed foundation drain tile system.

In the Maryland, Virginia, and Washington, D.C. area, eliminating the supply of water to the soil below the ground is virtually impossible, due to high water tables throughout the region, the constant rainfall, and the predominant clay soil. However, good or 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.

Changes in soil moisture can cause expansive clay to swell and shrink. This swelling can vary across the footing due to seasonal changes or the effects of vegetation removing moisture. The variation in swell can cause the soil to distort, cracking the structure over it. This is a particular problem for residential footings 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, usually compaction tests are 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 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), unless it was build on a landfill, which some communities in P.G. County are; that soil upon which your home rests, before it was built, was a conduit for all the watershed in the area directly above and adjacent to where your property rests.

Water has a few absolute characteristics: water has an excellent memory - if it followed a path once, it will seek that pathway again; water always seeks the path of least resistance (gravity - it moves downhill); water will seek its own level (self-balances or equalizes); and 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 stand 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 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 kilometers of the Earth's surface. Strata are typically seen as bands of different colored or differently structured material, 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 a home on top of a hill is always more prudent than in an area which is in a valley. 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.

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.

Because your home is surrounded by looser backfilled soil, which probably was not compacted, it will always be more absorbent than the virgin soil surrounding it. This means that you will have an artificial 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 clay.

As moisture builds up in this 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 or in vapor, will also wick through porous concrete, mortar, concrete block, and grout.

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