One of the most common and telling failures of retaining walls is the inevitable tilting, cracking and bowing of brick, timber and concrete block retaining walls built by homeowners, well-meaning builders, and landscapers. After the wall is sized, each wall member is checked for adequate strength and steel reinforcing is determined. This includes checks for wall overturning, base sliding, and soil-bearing capacity failures. Once the lateral earth pressures are known, the wall is checked for stability. Cohesion and plasticity indices (for clays).To determine the lateral earth pressure, several soil parameters must be known in order for a qualified engineer to assess a particular wall design and its overall stability. WHAT IS LATERAL EARTH PRESSURE?ĭesigning any retaining wall requires knowlegde and understanding of the force generated by the pressure of the backfill against the retaining wall, called lateral earth pressure. Expansion joints should be located at intervals up to 90 feet. Greased steel dowels are often cast horizontally into the wall to tie adjacent sections together. These joints may be filled with flexible joint fillers. Contraction joints are usually about 0.25 inches wide and about 1/2 to 3/4 inch deep, and are provided at intervals of not exceeding 30 feet.Įxpansion Joints: Vertical expansion joints are incorporated into the wall to account for expansion due to temperature changes. The base is formed first, and the stem constructed afterwards.Ĭontraction Joints: These are vertical joints or grooves formed or cut into the wall that allows the concrete to shrink without noticeable harm. Keys are almost always formed in the base to give the stem added sliding resistance. If keys are not used, the surface of the first pour is cleaned and roughened before the next placement of concrete. Keys are used to increase the shear resistance at the joint. The footing key is generally an extension of the stem and extends below the bottom of the base.Ĭast concrete retaining walls may be constructed with any or all of the following joints:Ĭonstruction Joints: These are vertical or horizontal joints that are used between two successive pours of concrete. In some cases, a footing key is included to increase resistance to sliding. Counterfort retaining walls have counterforts spaced at center-to-center distances of about 30% to 70% of the total wall height. The length of the base slab is usually about 50% to 70% of the total height of the wall (bottom of base to top of stem).įor cantilever and counterfort walls, the stem thickness at the base is often about 10% of the total wall height, as is the base slab thickness. However, it should always be below the seasonal frost line, and that often is much deeper in northern climates. The depth to the bottom of the base slab should be kept at a minimum of two feet. In general, the top of the stem of any cast concrete retaining wall should not be less than 12 inches for the proper placement of concrete. This can add several inches to the wall's thickness dimensions, and can vary depending on the severity of exposure, soil type and reactivity, etc. In addition to basic structural requirements, wall dimensions (generally member thickness) are also affected by the required minimum reinforcement cover. How thick should a concrete retaining wall be? Construction-friendly proportions facilitate proper concrete placement and provide sufficient room for structural reinforcement. Proper proportioning of a retaining wall is just as important to its construction as its structural design.
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