Tuesday, May 18, 2010

Retaining walls and Landscaping

Landscaping and retaining wall applications are a large market for manufactured concrete block. A number of companies offer various designs for retaining wall and landscaping applications. Many of these designs have been very successful and have established themselves as attractive options for landscapers, homeowners, businesses and municipal customers.

Here is a good discussion of retaining wall systems:
“Gravity walls depend on the weight of their mass (stone, concrete or other heavy material) to resist pressures from behind and will often have a slight 'batter' setback, to improve stability by leaning back into the retained soil. For short landscaping walls, they are often made from mortarless stone or segmental concrete units (masonry units)[6]. Dry-stacked gravity walls are somewhat flexible and do not require a rigid footing in frost areas.

Earlier in the 20th century, taller retaining walls were often gravity walls made from large masses of concrete or stone. Today, taller retaining walls are increasingly built as composite gravity walls such as: geosynthetic or with precast facing; gabions (stacked steel wire baskets filled with rocks); crib walls (cells built up log cabin style from precast concrete or timber and filled with soil); or soil-nailed walls (soil reinforced in place with steel and concrete rods).

Sheet pile retaining walls are usually used in soft soils and tight spaces. Sheet pile walls are made out of steel, vinyl or wood planks which are driven into the ground. For a quick estimate the material is usually driven 1/3 above ground, 2/3 below ground, but this may be altered depending on the environment. Taller sheet pile walls will need a tie-back anchor, or "dead-man" placed in the soil a distance behind the face of the wall, that is tied to the wall, usually by a cable or a rod. Anchors are placed behind the potential failure plane in the soil.

Cantilever wallsCantilevered retaining walls are made from an internal stem of steel-reinforced, cast-in-place concrete or mortared masonry (often in the shape of an inverted T). These walls cantilever loads (like a beam) to a large, structural footing, converting horizontal pressures from behind the wall to vertical pressures on the ground below. Sometimes cantilevered walls are butressed on the front, or include a counterfort on the back, to improve their strength resisting high loads. Buttresses are short wing walls at right angles to the main trend of the wall. These walls require rigid concrete footings below seasonal frost depth. This type of wall uses much less material than a traditional gravity wall.

An anchored retaining wall can be constructed in any of the aforementioned styles but also includes additional strength using cables or other stays anchored in the rock or soil behind it. Usually driven into the material with boring, anchors are then expanded at the end of the cable, either by mechanical means or often by injecting pressurized concrete, which expands to form a bulb in the soil. Technically complex, this method is very useful where high loads are expected, or where the wall itself has to be slender and would otherwise be too weak.”

Retaining walls are commonly used to shore up a steep slope and prevent erosion and hillside creep. Walls are typically built from stacked blocks which lock together by a number of different mechanisms and arrangements; including pins, clips, keys and the like. Usually a geotextile is used to anchor the block into the hillside which it is supporting. The geotextile is a web-like material that gets buried in the earth for some distance behind the retaining wall, to create an effective anchor with a high tensile strength. The top of a retaining wall is commonly capped with a sill or capstone block.

Retaining walls are often assembled with a slight incline from the vertical, so that they are effectively leaning back, or staggered against the earth they are supporting. They can be laid with a curvature in their footprint. This curvature may be both convex and concave, so that a serpentine effect is possible. Most retaining wall systems offer a decent amount of design flexibility.

The block system I’ve been describing makes a great retaining wall. The ability to do a spherical section, like part of a dome, or a ‘bowl’ gives a whole extra design element to the tools of the landscape architect. If the wall curves out, it is like a dome; if the wall curves in, it is like a bowl. The blocks I’ve been describing in this blog can also be woven together with a tensile element, such as steel cable, nylon rope, etc. This system creates a very high strength wall which is simple and easy to install.

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