Retaining walls are a useful method of restraining soil when preparing it for a building or creating a separation between landscapes elevations or land and water as in a seawall. The walls are used to control the soil of steeply sloping land and to hold back soil as when building a foundation.
Expanded Use for the Retaining Wall
While holding back soil is the main use for a retaining wall, it can be expanded to include other amenities. The retaining wall hardscapes can include patios, fire features, planters, outdoor kitchens and other amenities. This construction can easily become a multi-purpose setting. Using attractive stones, the builder can create retaining walls that as act as tiered wall seating. The tiers mitigate the grade changes in a landscape. This process of hardscaping can introduce steps and walkways to further add dimensional detail to the land.
Types of Walls that Retain Earth
A wall that retains is built in many different formations. They are built as gravity walls, cantilever walls, piling walls, diaphragm walls and anchored walls. Also providing a similar function are soil nail walls and soil mix walls.
Gravity walls are used as erosion prevention, sea and channel walls, stream channels, revetments, wing walls, bridge abutments, Reverine walls and land slip rehabilitation. These walls use their extreme mass to resist the pressure of the soil it restricts. They a generally made of stacked stone, masonry units, concrete or mortarless stone. For stability, gravity units often lean back against the restricted earth, so the wall appears to be leaning.
These walls are made of steel-reinforced concrete or masonry units and shaped like an upside down “T”. Often the center wall is buttressed in the front and with a counterfort in the back adding strength to the structure. These walls were the most prominent soil retaining structure from the 1920’s to the 1970’s.
• Soldier Pile and Lagging Walls
• Secant Pile Walls
• Tangent Pile Walls
• Sheet Pile Walls
• Combined Sheet Pile Walls
Soldier Pile and Lagging Walls
Soldier pile and lagging walls have been around since the 18th Century. These systems have frequently been used in major metropolitan environments, like New York, Berlin and London. They are most effective in soils that generate constant earth pressure. They are less effective below the groundwater table or in soft clays. One way this structure might be built is with steel H-piles placed vertically 4-10 feet on center supporting a lagging wall of rough sawn timber, pre-cast concrete planks or metal decking to retain the soil.
Secant Pile Walls
A secant pile wall is often installed in difficult ground such as earth with an abundance of cobbles and boulders. The Wall is created by placing rebar reinforced concrete piles in an overlapping configuration.
Tangent Pile Walls
A Tangent pile wall is constructed exactly like a secant pile wall with the concrete pilings placed flush to each other and not overlapping.
Sheet Pile Walls
Pre-fabricated sheet pile sections are driven into the ground in an intersecting pattern to form a solid barrier. This structure can be reused and has a long service life above and below the water table.
Combined Sheet Pile
The walls are formed by combining sheet pile sections with king pile patterns. Most often used in marine applications, the king piles are placed at a greater depth to increase axial capacity. Combined sheet piling is used in both temporary and permanent structures.
Also called a “slurry wall”, these structures are used to retain earth for deep excavations, basements and tunnels. They create foundations and control wall-water. To build a diaphragm wall, the process begins with the excavation of a slurry trench using hydromills, fraise or hydraulic clamshells. These following iconic structures used diaphragm walls in their construction: the World Trade Center, destroyed on 9/11/2001, the Central Artery in Boston, Massachusetts and the Bank of California in San Francisco, California.
Any of the retaining wall structures may be anchored to give the wall extra stability. Anchors are generally made of bored cables extending from the wall into the ground. On the earth end, concrete is injected to generate a bulb that acts as an anchor for the wall.
Soil Nail Walls and Soil Mix Walls
A soil nail wall consists of closely spaced steel bars drilled into the ground as much as 15 feet deep and secured with grout. On the surface a plate of a solid construction holds back the earth. Soil Mix walls combine insitu soils with cement-like substances to form a wall of soft rock. This method of soil retention is common in Japan.
Retaining walls have many uses and many structural forms. Depending on the soil and the location above or below the water table, the structural engineer or landscape designer can select the most effective structure. The function of the wall will decide what type of wall should be built, but the design of the wall can still be made aesthetically pleasing with a wide range of decorative hardscape options. Create retaining structures that combine all your ecological and functional needs and top it off with your appreciation for beauty. Add planters, steps, seating and a whole host of fitting features that accent the newly preserved landscape.