Tuesday, July 17, 2012

Retaining Wall Basics - Part 2

My last post discussed the basic forces a wall must resist when retaining grade: the weight of soil beyond the angle of repose and hydrostatic pressure.  Different means of resisting these forces are required based on the material used to construct a retaining wall.

Wood Timber Walls
Wood Timber Wall Section
Wood timber retaining walls achieve structural integrity partly from the spikes used to tie each course of timber together. However, spikes alone are typically not enough to keep a wood timber wall from failing. The addition of deadmen is what provides true stability to the wall. A deadman is a wood timber that is tied perpendicular into the face of the wall and runs horizontally back into the retained grade, then has another timber tied to the opposite end that is positioned parallel to the wall. The deadman must extend a sufficient distance into soil within the angle of repose in order to maintain a stable “grip.”  Wood timbers are a relatively affordable building material and don’t necessarily require professional installation.  However, proper design of these walls is critical to determine the necessary length of deadmen.  Another disadvantage to wood timber walls is their susceptibility to rot, even if constructed with treated wood.
Precast Concrete Block Walls
Precast Concrete Block Wall Section
Precast concrete block walls, also called segmental retaining walls, get their structural integrity from structural reinforcement and geogrid.  The most common forms of reinforcement lips cast into the blocks or metal pins that tie each course of block together, depending on the preference of the block manufacturer.  When installed properly, these lips or pins establish a batter to the wall for structural  support.  This batter may be all that is needed for shorter walls.  However, for walls that are taller or have a surcharge (additional live load) placed on them, additional support is provided by geogrid.  Geogrid essentially performs like a deadman in a wood timber wall: it connects to the vertical wall and extends horizontally through the retained grade into stable soils below the angle of repose to form an anchor.  The advantages of segmental retaining walls are a relative low cost when compared to other wall systems and ease of installation.  Disadvantages of concrete block walls are their modular man-made appearance; however, many manufacturers are developing units with more convincing natural shapes and textures.
Stone Boulder Walls
Stone Boulder Wall Section
Stone boulder walls retain soils by means of the sheer mass of the stones.  The structural integrity and height of a stone boulder wall is directly proportional to how large of stones used to construct it.  These walls provide a natural and “solid” appearance and will last a lifetime or more if designed properly.  However, they require heavy machinery to install a wall that can achieve a significant height, and calculating design requirements can be complicated due to wide variability of stone sizes, shapes, and availability.
Dry-Laid Stone Walls
Dry-Laid Stone Wall Section
Dry-laid stone walls typically use smaller, flat, linear stones stacked in courses as opposed to randomly stacked rounded or irregularly shaped boulder walls.  Since the stones may not have sufficient mass to support retained grade, additional reinforcement can be provided by using exterior grade construction adhesive to “glue” each course together.  These walls are only recommended for shorter applications (less than 3’ tall) with no additional surcharge since they can only achieve structural integrity from wall batter formed during installation and the construction adhesive holding each course together.  Much like stone boulder walls, the advantages of dry laid stone walls include the natural appearance of the material and longevity of the wall when properly installed.  The disadvantages of this wall type include the cost of the construction materials and high level of skill required to properly install.
Cast-In-Place Concrete Walls

Cast-In-Place Concrete Wall
Cast-in-place concrete retaining wall systems maintain structural support through the combined strength of the concrete and steel reinforcing bars (rebar) used to form the wall as well as the depth and design of the wall’s footing. Most concrete walls incorporate a spread footing, which is a wide, flat slab connected to the wall’s vertical stem. The spread footing needs to be at least as deep as frost-depth for the regional climate, but often times even deeper in order to have sufficient stabilized soil cover to prevent over-turning of the wall. These walls can be made decorative with brick or stone veneer, but these facing materials are strictly decorative - they provide no structural support for the wall.  The advantages of cast-in-place concrete walls are their longevity, geometric capabilities (curves, angles, slopes – you name it – can be formed with these walls), and aesthetic options available when cladding with brick, stone, or tile veneer.  The drawbacks of cast-in-place concrete walls are the cost of materials and complexity of design and installation.
As you can see from the all the graphics above, a drainage system comprised of a column of free-draining stone wrapped in filter fabric and perforated drain pipe is recommended for all wall system types in order to relieve hydrostatic pressure.
Finally, required lengths of deadmen and geogrid, depths of footings, and maximum heights walls will vary based on the type of wall, type of soil being retained, additional surcharges being placed on the wall, and other possible factors.  A licensed structural engineer should be consulted on the design of any wall taller than four feet.