|
|
|
|
Retaining Walls Many of us live in parts of the country that are not so level. The geography and topography can range from slightly rolling to mountainous. Those of us who do live in these regions are quite familiar with retaining walls. These walls allow us to create steps or level areas on a sloped surface. However, if constructed improperly, the forces of nature (gravity, water, etc.) will topple a retaining wall in short order. Retaining walls have been in use for thousands of years. The Romans used retaining walls to aid in the construction of their famous roads. Many of the castles in Europe incorporated retaining walls into their design. Hillside rice paddies in Asia have depended on retaining walls for hundreds of years. Trial and error construction methods of the past and advancements in engineering knowledge have indicated that four basic types of retaining walls seem to perform quite well. Each design has its limitations however. The four basic types of retaining walls are: gravity wall; cantilever wall; counterfort wall; and buttressed wall. Gravity Walls A gravity retaining wall is usually a low height (less than 3 to 4 feet) wall which depends on its own weight or mass to hold back the earth behind it. This goal is achieved by constructing the wall with a volume of material so that when stacked together, the weight and friction of the interlocking material exceeds the forces of the earth behind it. The wall is thicker at the base than at the top. Also, note that as the front of the wall gets taller it slants backwards. This is often referred to as 'battering'. This battering effect creates a visual message of strength, as over time, the wall will probably succumb to the forces of gravity and begin to tilt outward. By battering the wall backwards, you extend the visual life of the wall. Retaining walls that appear to be tipping over, tend to indicate faulty construction and impending failure. Walls that are battered send a visual message that the wall is 'working' and that it is continuing to beat the force of gravity. Gravity walls become very cumbersome to construct as they get higher, because they require vast quantities of materials. The thickness of a gravity wall at its base should be one half to three fourths its height. So, if you intend to build a wall 4 feet high, the base should be 2 to 3 feet wide. As the wall gets higher, it begins to get thinner. Cantilever Walls A cantilever retaining wall is one that consists of a uniform thickness wall which is tied to a footing. A cantilever wall usually is asked to hold back a significant amount of earth, so it is a good practice to have these walls engineered. A simple example of a cantilever retaining wall is a typical basement wall of a house. The width of the footing for a cantilever wall is very important. The footing is designed to resist tipping or sliding forces which the earth exerts upon the wall. Also, the wide area of the footer allows the weight of the earth to actually keep the wall from tipping in some instances. These walls require significant steel reinforcing in both the footer and the wall structures. The steel also has to extend from within the footer up into the wall so that the two pieces actually become one integral unit. As you can see, this is why these walls need to be designed by structural engineers. If you try to guess yourself at the amount, size and placement of structural steel in this type of wall, you are gambling. Also, the thickness of both the footer and the wall is extremely critical. Don't be a fool and try to become a weekend engineer. Spend several hundred dollars and get it right the first time! Counterfort Retaining Walls A counterfort retaining wall is very similar to a cantilever wall, except that it has one additional feature. This wall has a triangular shaped wall which connects the top of the wall to the back of the footer. This added support wall is hidden within the earthen or gravel backfill of the wall. The footer, retaining wall and support wall must be tied to one another with reinforcing steel. If the structure is poured concrete, often the retaining wall section and the support walls are poured as one unit at the same time. The support walls add a great deal of strength to the retaining wall. The supports make it virtually impossible for the wall to become detached from the footer. As with cantilever walls, a counterfort wall should be designed by a competent structural engineer. If you decide to attempt to construct this type of wall without approved plans, you are making a huge mistake. If the wall fails, the cost to remove the failed wall, construct the new one, etc. could be ten times or one hundred or more times the cost of engineering services. Remember, engineers have to eat just like you and me! Buttressed Retaining Walls A buttressed retaining wall is basically identical to a counterfort wall except for one thing. The support wall is on the outside of the retaining wall. They are visible. The buttresses add incredible strength to the wall system. For the retaining wall to fail or tip over, the buttresses would have to be crushed. The buttress concept was widely used in the construction of many cathedrals in Europe. Because of the height of the cathedral walls, the buttresses helped to stabilize them. They do the exact same thing in a retaining wall. Once again, if you intend to build one of these walls, you must give serious consideration to hiring an engineer. Situations which demand this type of wall usually have tremendous loads which bear against the walls. The buttresses can often be designed to be decorative in nature and covered with stone or some other material. Depending upon the overall length of the wall, you may have several buttresses. They can be spaced to create rooms, parking spaces, handball courts or any other functional space. Use your imagination!
|