Numerous homeowners have told me about instances that happened in the middle of the night. The stories are all very similar. They are awakened by a loud "crack" or "pop". Yes, part of their house cracked like a dry piece of kindling wood. However, often the part that cracked is the foundation!
Other stories speak to doors and windows that work perfectly one day and stick the next day. Sometimes the sticking is seasonal. That is, the doors and windows work fine for three to four months and then trouble begins. Magically, the doors and windows work fine four to six months later.
In all these cases, the common denominator is some form of major structural movement. The movement can be within the house (lumber swelling and shrinkage) or it can involve the entire house (settlement or some other force which is stressing the entire structure.)
All too often, however, the term 'settlement' is used to describe any movement. This can be misleading, as settlement is really just one form of movement which can affect the way the inside and outside of your house looks. Cracks can develop in your house from other forces such as landslides, heaving (frost or soil swell), soil shrinkage, erosion of soil from beneath your foundation, earthquakes, construction blasting, soil creep, etc. You see, lots of things can be happening! Sometimes, two or more at once.
Common Causes
It is not uncommon for a house to be built on fill dirt or on a hillside. Have you seen huge earthmoving machines working on a new subdivision? They scrape dirt from the high spots and deposit it on the low areas. The dirt that is used for fill is supposed to be compacted. However, it may not always be. Gravity and water entering the soil over time compacts the loose fill. If the compaction is not the same under the entire foundation, your foundation may fracture.
Hillside construction is a simple matter of high school physics. Gravity is constantly pulling the soil down the hill. I learned this in my first geology class. This soil creep, as it is called, takes place at a faster rate the closer one is to the surface of the ground. So, houses dug into a hillside basically have their backsides exposed! The part of the foundation that is shallow and is near the surface is subject to movement, while the remainder of the foundation is quite stable where it is dug deeply into the hillside. Perhaps you have seen foundation failures like this.
Hillsides also pose another problem. The soil creeping down the hillside can exert huge forces on the uphill part of the foundation. These walls can crack or tilt inward from the force of this pressure.
Water, or the movement of water in soils, can cause foundations or slabs to crack as well. For instance, imagine if a sewer line or water line that runs beneath your house develops a leak. It erodes soil from beneath your house and floor. Eventually the foundation footer, wall, and/or floor cracks in response to the absence of the support. Remember, your foundation was designed to work with adequate support beneath it. Remove this support and.......CRACK!
Certain parts of the nation have clay soils. Some of these clay soils shrink and swell (like an inexpensive sponge) in response to the amount of water they contain. This movement can be dramatic. As the soil beneath your foundation dries out, your foundation drops. In wet weather the clay swells and lifts your foundation. This is no problem if the movement is the same at every point along your foundation. In more cases than not the movement is not equal. Stress builds and your foundation cracks.
Solutions
If your foundation develops a crack or a fracture, it usually can be stabilized. If the crack is vertical or diagonal, it may require a support from beneath that cradles the footer or foundation. If your foundation develops a horizontal crack, it can be stabilized in several ways as well.
In all instances, it would be wise to consult with a licensed structural engineer who specializes in residential problems. If you try to solve the problem yourself, or merely trust the workmen, you may have a problem occur at a later time. Some of the solutions can actually transmit the stress of the problem to another portion of your foundation. These cracks may happen months later. The contractor will generally say that those are not his fault, when, in fact, they may be! Have a structural engineer develop the solution. Then hire a contractor to perform the work.
By: Tim Builder
http://www.askthebuilder.com/B102_House_Foundation_Settlement.shtml