Fence Posts & MSE Walls

Discussion by Joseph Kowalski, May 2014

A Mechanically Stabilized Earth (MSE) wall often requires a fence or railing along the top of the wall.  Of course, the purpose of a fence or railing is to reduce or eliminate the potential of accidental falls off the wall.  Wind loads on a fence or human loading on a fence or handrail can be unpredictable.

Because of this, most building codes require a 42-inch high railing that can safely resist a 200 pound concentrated load.  The moment generated (42 inches x 200 pounds) is equal to 700 ft-lbs.  To resist the overturning generated by a 700 ft-lb moment requires a lot of mass.  Very large retaining wall blocks (those weighing a thousand pounds each) are well-suited to providing the required mass but the smaller wall blocks (those weighing less than 100 pounds each) are not.

The smaller wall blocks are not well-suited to providing the mass because each individual block is too light.  Because the blocks are segmental (i.e., they are not mechanically connected to one another vertically or horizontally) they cannot distribute the load effectively to other adjacent blocks.

Using smaller wall blocks requires that several courses of wall blocks be joined together into a single coherent mass.  If mortar or grout is used as a bonding agent, then the mortar would have to extend vertically and horizontally within the cores of the blocks to create an area large enough to provide a large mass.  Steel reinforcement of some type would be required across the blocks (horizontally and vertically) to join the blocks together.  Additionally, the mortar would have to have the same expansion and contraction properties as the wall blocks or the wall blocks will crack.

Careful review of the method to use small wall blocks as supports for fence posts and handrails exposes several potential flaws.  Empirical evidence also has shown this to be a system prone to problems.

We have observed the following problems when fence posts or railings were anchored directly to small wall blocks:

  • Overturning of the fence or railing occurs (due to insufficient resisting weight);
  • Cracking and breaking of the caps occurs (due to the geometry change at the core hole);
  • Cracking and breaking of wall blocks occurs (due to the mortar and the wall blocks expanding and contracting differently).

There are several ways to support fences and handrails behind MSE wall blocks.  Each site is different, and not all of these are applicable to every site.  We have experienced successful fence and railing support using the following methods.

  1. During retaining wall construction, install hollow cylinders (cardboard or plastic tubes) at the planned fence post locations.  The depth and diameter should be determined by the structural engineer.  Install concrete and posts into the cylinder after wall completion.
  2. During retaining wall construction, install a manufactured product (e.g., Sleeve-It) that is designed to work integrally with MSE wall blocks and provide fence support close to the back of the wall blocks.  Install concrete and posts into the product after wall completion.
  3. After wall construction, auger holes for the fence posts, install the fence posts and fill those holes with concrete.  It has been our experience that these holes can be augered through the uppermost layer of geogrid without displacing the wall face.  The depth and diameter should be determined by the structural engineer.  However, the edge of the augered hole should be no closer than about one to two feet from the back of the wall block.  We recommend a trial-and-error process to ensure the wall blocks are not being displaced.  To reduce risk, increase the distance between the fence and the back of the wall.
  4. After wall construction, hydraulically push (or drive) the fence posts into the ground. Small-diameter fence posts and some steel I-beam posts can installed this way but large-diameter wooden posts cannot.

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