Settlement, Heave & Creep: Types of Foundation Movement January 1, 2016
The general types of movement which can contribute to foundation distress and building damage are settlement, heave and creep. Another type of phenomena is shear failure of a footing. Shear failure is unusual in residential or light commercial structures because of light loadings. The vast majority of problems which we see related to foundation distress in single and multi-family residences and light commercial buildings have to do with either settlement, heave or creep, or combinations of these.
Settlement is what most people think of first when they see obvious foundation movement. This is a conditioned reflex. However, in most foundations around Central Texas settlement is probably less than 25% of the causes of foundation distress. Most of the soils around Central Texas are hard and strong and do not settle under typical residential or light commercial foundation loadings. The major exception is non-engineered and uncontrolled fill, of which Central Texas (and other parts of the world for that matter) have their share. Uncontrolled fill is becoming more common as more of the good building sites are used up and sites are used which require fill.
Fill can take several forms. It can be filling of a ravine to produce a level building area in a development, or it can be cut and fill on a slope. Cut and fill on slope is commonly used on rocky outcroppings. Even so, this type of fill, which is placed over rock and composed primarily of rock, can settle as well as slide down the slope.
Cut and fill can be used to level the building pad by cutting earth or rock from the uphill side and placing it on the downhill side. Often this placement is done without controlled compaction or quality control of the materials or techniques used. Many things can be found in such fills, including boulders, construction trash, brush, automobiles, and occasionally toxic wastes. Such fills should never be expected to support a foundation and the only solution is to rework them under the proper controlled conditions or construct foundations that have their bearing elements penetrate completely through these fills to stable conditions.
Occasionally, soft ground, load induced settlement occurs in Texas. Soil conditions conducive to this type of settlement are found along the coast, around lakes and in river bottom alluvial deposits. Settlement in these soils is caused by squeezing out water from the soil by adding structural loads, resulting in a volume reduction. The greater the added load, the more settlement could occur. Highly loaded footings can also undergo elastic strain settlement. For most residential and light commercial construction. These types of phenomena rarely cause problems.
Heave refers to the spontaneous movement of the soil under a foundation caused by clay expanding due to increase in the soil moisture. Moisture can change under foundations due to environmental factors, poor drainage, or plumbing leaks. This is a common situation in many parts of Texas, and indeed, many parts of the world. Most foundation problems in Texas (both in frequency and economic loss) are due to this type of movement. It is especially troublesome for lightly loaded foundations such as residential or light commercial structures, because the heavier the load of the structure, the more the swelling potential is restrained. Heave can take several forms. A center heave or doming effect is frequently seen where the soil swells more in the center than around the edges of the house (or dries and shrinks around the edges more than in the center). A reverse effect can also be seen with the edges curling up around the outside edge, because a wetting front has moved in from the outside. Either type of movement can produce excessive deflection of the foundation and damage to the superstructure.
Downhill creep is a phenomena typically associated with expansive clays which are on slopes. The steeper the slope and the higher the clay reactivity, the more creep occurs. Downhill creep consists of very small movements of the clay soil in the downhill direction, such movements typically being on the order of fractions of an inch per year. The movements occur in spurts, usually accelerating during wet weather and stopping during dry weather. This downhill creep is not a landslide or catastrophic type movement of the earth, but simply a slow creep of the near surface soil which is reflected in tensile cracking of pavements, tilting of retaining walls, and lateral movement of foundations.
All these type of soil movements can contribute their share to foundation distress and even failure. All types of foundation distress can be remediated to some extent, depending on the amount of money available. It is far less expensive to have the site properly investigated by a geotechnical study and an adequate foundation design executed in the first place.