The foundation is the initial element of a house or building to be constructed, creating a support structure for the remainder of the building. However, if the foundation is improperly constructed, built with sub-par materials, or damaged by external forces, problems can arise. An adept investigator may find that the evidence indicates construction defect or property owner neglect regarding landscaping and water control. Cracks, leaning, nearby earth movement, untenable weight loads, bowing, sagging, and interior damage can all give clues as to the timing, sequence and cause of the problem.
Foundations can be placed into two categories: shallow foundations and deep foundations. Shallow foundations include slabs-on-grade with turn-down footings, mat foundations, footings with piers, and footings with retaining walls. A deep foundation, such as piles and caissons, provides support well below grade. The type of foundation chosen for a building project will depend on the type of building, terrain, soil type, costs and many other factors. Houses and light structures are typically constructed with slabs-on-grade or footings with piers and perimeter walls. A slab-on-grade foundation is a concrete slab placed directly on the grade where the turn-down footing provides support for the bearing walls in the structure. Crawlspace and basement walls are typically constructed of concrete masonry units (CMUs), cast-in-place concrete, or insulated concrete panels.
Failure Modes
Foundation failures occur for a number of reasons. Excess moisture, drought conditions, fill material consolidation, and settlement due to differing foundation types are the most common causes. The root causes of the different failure modes will determine the difference between a covered loss and a non-covered loss.
WATER — Water, in the form of excess moisture in the soil matrix, is the foremost cause of foundation displacement and damage. Excess moisture may originate from several sources, including: rooftop runoff, surface drainage, groundwater, leaks and flooding. Some soils, such as expansive clays, are more affected by changes in moisture content than others. Settlement can occur due to the presence of periodic excess moisture in the soils supporting a foundation.
The key to loading earth with a foundation is to control the moisture in the soil. Well compacted and maintained soil has two physical properties which allow it to hold together: cohesion and shear resistance. As the soil moisture increases to a point where the buoyancy overcomes the interlocking shape of the particles, the soil loses its ability to carry weight. In other words, as the moisture in the soil increases, the soil's ability to carry weight decreases. Cracks resulting from excess moisture in the soil that supports that foundation are typically wider at the top than at the bottom.
The slope of the terrain around a building must be sufficient to effectively direct surface water, and thus groundwater, away from the foundation. A slope of at least 5% is recommended by good construction practices and most building codes. Gutter downspouts should discharge into extensions that conduct water away from the foundation. Grading and rooftop runoff collection are usually related to the original construction and are typically long-term issues. Investigation of the root cause can determine who is responsible for resulting damage.
Leaks can occur in the water supply or drainage plumbing. If a leak is present in the drainage plumbing, it will occur only when water and/or contents pass the leak site. If a leak is present in a water supply line, the leak will be continuous. The damage caused by a water supply line leak will typically be greater than the damage caused by a drainage line leak due to the ever-present water in the supply line. Localized damage is indicative of a point-source of water.
Flood events can be divided into two categories, rising floodwater and moving floodwater. Rising floodwater damages structures by wetting the building and soils that support them. Typically, structural elements exposed to a single water event will dry and retain their structural integrity; whereas, repeated or long-term exposure to moisture can cause deterioration over time. The surrounding soil is not as forgiving. The effect of water in soil can cause differential settlement of the foundation system even with a single exposure, depending on the grade of the land, soil type, volume of water, and other conditions. The differential results from a portion of the foundation being exposed to the water while other areas remain out of harm's way. These distresses can take on the form of settlement-type cracks or movement of the foundation itself.
Moving flood waters are very damaging even at low speeds. They impart lateral and vertical forces on surfaces and structures. The forces associated with moving floodwater are hydrostatic, buoyant, hydrodynamic and frictional. Hydrostatic and buoyant forces are in direct proportion to the weight of water and the material properties of the affected object. These forces apply horizontal and uplift forces onto the structure. Hydrodynamic forces are associated with the water's momentum as it moves against structural elements. Frictional forces along the ground surface cause the soil to erode and possibly undermine foundations. Any of these forces can cause localized or widespread failure in a building. Any substantial movement of a foundation will require removal and replacement, while foundations that exhibit little to no movement as a result of moving water can sometimes be salvaged.
DROUGHT — In the same way that excess moisture can influence the soil matrix, extremely dry conditions can adversely affect the ability of a soil to adequately support a foundation and its load. During an extended drought, the soils deep in the ground will begin to dry out. When this happens, clays and silts will lose water and shrink. In general, settlement due to the drying of the soil usually occurs irregularly. The corners of buildings are attacked from two sides and are apt to settle more than the long stretches of the foundation. The moisture is pulled mostly from the exterior soil due to the exposure to sun and wind. This causes the footing to settle toward the exterior and hold toward the interior, creating a tilt outward. When the corner shifts, vertical cracks in the walls will form on either side of the corner.
Drought-induced settlement often affects those structures with foundations that vary in depth. Houses built on hillsides typically have foundations that are located several feet into the soil on one side of the structure but are relatively close to the surface on the other. A drought will therefore affect the shallow portions of the foundation but not those buried more deeply. As the shallow foundation settles, the deeper foundation does not, creating cracks in the structure.
Construction Errors — A foundation system can incur damage as a result of the consolidation of fill material beneath the foundation. Fill material is used to level lots with sloping topography or to raise the elevation of the ground in a low-lying area. Most materials must be compacted in increments (lifts) to achieve proper soil densities. If the lifts are too thick or the fill material is not compacted until the end of placement, the fill material will self-compact over time. On occasion, organic materials or a "trash pit" are placed within the fill, with subsequent consolidation occurring upon decomposition of the material. Moisture variations in the soil exacerbate consolidation of the fill material and decomposition of organic materials. Most consolidation of fill material is considered to be construction-related. If significant downward displacement of the supporting soil is observed, the soil is consolidating either due to lack of sufficient compaction or due to voids from the decomposition of the organic materials.
Differential settlement sometimes occurs when adjoining portions of the structure are built on different foundations. When the downward displacement is not equal, brittle components like masonry and drywall can be easily damaged. The root cause of the settlement must be determined before proper action is taken.
Improper construction can cause nearly immediate or long-term problems for a foundation system. Depending on its height and the amount of earth to be retained, a foundation wall may require steel reinforcement. Reinforcing steel and grout placed in the hollows of a concrete masonry unit wall will aid the wall's ability to properly function. The placement of grout without the reinforcing steel will not provide sufficient resistance to a bending failure. The amount of reinforcement is dependent upon the pressures imposed on the wall, which are proportional to the unbalanced fill that the wall retains. If a wall is not adequately sized, it can fail. A lack of sufficient steel reinforcement, improperly sized walls and/or footings, and improper attachment of a foundation wall to the footing are the most common construction issues. Cracks from lateral soil pressure in concrete block walls almost always take the form of horizontal cracks in the mortar joints that are open on the inside of the walls because of the inward bowing. As the horizontal crack approaches a support, like a corner, the crack normally transforms into a stair-step crack.
Common Misconceptions
Homeowners and tenants sometimes claim that their foundation problems are the result of sinkhole activity. Holes in the ground may be classified into four categories or causes: rotted vegetation, animal, man-made and natural (geological). Of special importance in some areas of the country are the effects of Karst Terrain. Limestone can be dissolved by eons of water percolating through the soil, leaving a void. The presence of sinkhole activity should be verified by a specialist, such as a professional trained in the geological or geotechnical fields.
Property owners also express concern about damage to their property after experiencing ground vibrations from earthquakes, blasting events, or heavy construction activities. Humans can perceive vibration levels much lower than what is necessary to damage a structure. Ground vibrations affect the more brittle components of a structure, such as drywall, before damaging a foundation. The foundation is intended to move with the earth that supports it. Even the poorest quality concrete, brick, block, and mortar are much stronger than drywall. Unless extensive damage is present to the brittle components, foundation damage due to vibrations is improbable.
One thing is certain: In an imperfect world, foundations will continue to fail. Whether due to extreme variations in the moisture content of the soil, consolidation of the soil, construction defect, or faulty landscaping, damage to foundation systems is common. The key to properly diagnosing the failure is understanding different types of foundation construction and the ways a foundation reacts to different issues.
Matthew G. Richardson, P.E., is a Senior Forensic Engineer with
Donan Engineering Co. in Knoxville, Tenn. He can be reached at
mrichardson@donan.com.