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Retaining walls & privacy walls: this article series describes types of privacy walls, retaining walls and retaining wall guard railing requirements, guard railing construction and building codes, and critical safe-construction details for retaining wall guardrails. We include definitions of important retaining wall terms such as wall surcharge, and we provide diagnostic descriptions & photographs of types of damage to retaining walls & privacy walls. Our page top photo shows a traditional dry stone laid retaining wall abutting a newly-constructed poured concrete retaining wall that sports a formed-concrete faux-stone face.
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Our photo (left) shows an antique segmental gravity retaining wall built of cut bluestone dry-stacked beginning at grade level. At its uppermost end this wall was a bit over five feet in height. Not every retaining wall in every community requires a guard railing, as this Poughkeepsie New York retaining wall photo illustrates.
The city of Folsom CA building department provides some assistance in understanding the basic elements of retaining wall design, from which we quote, paraphrase, and expand: 
In this article we include descriptions of problems & solutions for both retaining walls and privacy or barrier walls.
Definition of a retaining wall
A retaining wall is any wall designed to resist lateral earth and/or fluid pressures, including any surcharge, in accordance with accepted engineering practice. This definition may also apply to freestanding [swimming] pool walls.
Cantilevered Retaining Walls
A cantilevered retaining wall (white arrow in photo at left) is typically constructed of reinforced concrete masonry units or a reinforced concrete stem wall supported on a reinforced concrete footing, typically 12" thick.
"Reinforced" means that wire mesh or re-bar are inserted in CMU cores or through the wall vertically to help resist wall bending, and the wall bottom is pinned to the supporting footing. In a freezing climate, the reinforced masonry footing for a cantilever retaining wall may be buried below the frost line.
A cantilevered retaining wall is usually constructed vertically, that is, not leaning back into the hill (compared with the segmental gravity wall described next). [Sketch of a Cantilevered Retaining Wall- Folsom CA]
Privacy walls or barrier walls compared to retaining walls
A privacy wall as we use the term here is providing a visual or physical barrier between area outside and area inside of a property. Unlike a retaining wall, neither side of a privacy wall is expected to be retaining or holding back earth.
However some walls may combine both a retaining wall feature and a privacy wall feature if the retaining wall height extends above ground level on both sides of the wall.
Our photographs above illustrate a nice above-ground barrier or privacy wall in a serpentine brick design. This wall is on the Vassar College Campus in Poughkeepsie, NY. In a freezing climate exterior masonry is exposed to significant water and frost stress and such walls can become damaged or even collapse. Key in preserving this wall is maintenance of its top cap to reduce water from penetrating the wall interior.
Segmental Gravity Retaining Walls
A segmented gravity wall typically does not have a poured reinforced concrete footing, though one or more courses of modular concrete block units may be buried below grade level at the wall bottom.
Most modern segmental gravity wall retaining walls that we've seen have been constructed in a stair-stepped or angled design that leans back into the hill or soil that it is supporting, as you can see in our photo. [Sketch of a Segmental Retaining Wall - Folsom CA] 
Wooden Retaining Walls Using Landscape Ties or Railroad Ties
Anchored landscape tie or railroad tie wooden wall (photo at left) is typically constructed of 6x6" treated wood placed horizontally, sometimes stair-stepped back against a hillside, with additional horizontal "ties" or members placed at right angles to the wall face and extending back into fill in the hillside.
Our red arrow points to the end of a landscape tie placed orthogonally to the wall face and extending back into the upper soil area to serve as a wall anchor.
The anchoring tie member helps resist earth loading pressures behind the wooden retaining wall.
The life expectancy of treated wood landscape tie retaining walls varies widely depending on the quality and extent of wood treatment, wood species, and soil conditions.
Details about retaining wall damage types, diagnosis & recommendations are now found at RETAINING WALL DAMAGE
Definitions of retaining wall, retained wall height, exposed retaining wall height and retaining wall surcharge loading
Definitions of key terms used to describe retaining walls such as retaining wall surcharge, structural types, and structural components are now found at RETAINING WALL DEFINITIONS.
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