Friday, June 22, 2012

Retaining Wall Basics - Part 1

My last post explained how not to use precast concrete retaining wall blocks, so I’m going to explore how to properly use wall blocks with this and my next post.  As a quick review, retaining wall blocks are for retaining soil – period.  Before we explore using concrete blocks for retaining walls, let’s look at the basics of retaining wall design and engineering.

A retaining wall compensates for a difference in vertical elevation (also called grade) between two points where a slope is impractical or undesirable.  The retaining wall must resist two major forces: the weight of the soil mass being retained and hydrostatic pressure from subsurface water. 

The weight of the soil mass being retained varies based on the type of soil and its angle of repose.  The angle of repose is the steepest slope the soil can naturally maintain without sliding downward.  This angle varies depending on a soil’s sand, silt, and clay content.  The angle of repose is also affected by the amount of moisture contained in soil.  A retaining wall must be strong enough to resist the force of all soil filled beyond the angle of repose.
This cross-section illustrates a basic retaining wall. 
The wall is retaining all soil above the angle of repose.


Gravity forces the soil to its natural angle of repose without the retaining wall.


Hydrostatic pressure is exerted on a retaining wall as subsurface water builds behind it.  As rain water percolates into the ground, water pressure builds behind a retaining wall as the retained soil becomes more and more saturated.  The wall will eventually fail without some means of relieving this pressure.
This cross-section illustrates how hydrostatic pressure builds behind a retaining wall.

If too much hydrostatic pressure is exerted on the wall, it will fail.


A drainage system installed behind the wall relieves hydrostatic pressure.

The basic means of providing resistance against these forces are one or a combination of the following: mass of the wall, structural reinforcement, depth and design of the footing, deadmen, wall batter, and drainage systems.  My next post will detail how different retaining wall materials use these methods and the advantages and disadvantages of each type of wall system.

Friday, June 8, 2012

Retaining Walls Blocks: For Retaining Soil – Period.

Precast concrete retaining wall blocks are widely available in a variety of shapes, colors, and styles and are relatively easy to transport and install.  As a result, these products have infiltrated residential landscapes for uses well beyond their intended purpose – that purpose being retaining grade.  Bed line and tree mulch ring edging are the most common culprits for unnecessary wall block use.  Hopefully this post will convince you that although you can do a variety of things with retaining wall blocks, it doesn’t mean you should.

I am not a fan of using bricks, stone, and especially retaining wall blocks for defining planting bed lines.  The focus of plantings should be the plants, not a boldly defined hard-edged bed line.  A clean-cut spade edge is all you need, in my opinion.  If you don’t want to deal with the hassle of re-cutting bed edges every year, then I suggest using steel or aluminum edging that, when installed properly, is not visually obtrusive, maintains a permanent line, and prevents turf grass from creeping into the planting bed.  From a maintenance standpoint, lining the edge of a planting bed with retaining wall blocks creates a vertical edge that can’t be cleanly mown along, so constant trimming is necessary.  To me, it seems much easier to zip over a spade-cut bed line with the outer edge of the mower deck and be done rather than have to come back with a string trimmer.

This owner spent time and money laying wall blocks that are unnecessary.  The wall blocks aren’t retaining soil, they are just used to define the edge of the bed.  The focus of the planting bed should be the plants, but attention is instead drawn to this hard-defined block line.

Here is the photo shown above after being altered to illustrate how the bed would look without retaining wall blocks.  The edge is clean and defined, but is not as in-your-face so visual focus is drawn to where it should be – the plants.
My beef with using retaining wall blocks for elevated mulch rings at the base of trees goes beyond aesthetic and maintenance issues – it can create horticultural issues when installed around established trees.  The zone at the base of a tree where the trunk transitions to root mass is called the root collar.  This is a sensitive region of the plant that is acclimated to its established relationship to the surrounding soil elevation – not to high, not too low, but just right.  Elevating the grade around the base of a tree by building up retaining wall blocks and back-filling with soil buries the root collar, severely altering the oxygen and moisture exposure to which zone has adapted.  This makes the plant susceptible to rot, pests, root dieback, and premature death.  Mulching around the base of a tree is good, but keep the mulch ring at the established ground level and cut a spade edge for a clean, easy-to-maintain ring instead of using retaining wall blocks.

These wall blocks were used to define the mulch ring at the base of this tree.  Aside from creating a maintenance hassle (notice the grass along the bottom of the blocks that needs trimmed?), the mulch is piled above the tree’s root collar and could invite a number of horticultural problems.
Retaining wall blocks do have their place in the landscaping trade, and I will explore them and other retaining wall alternatives in my next post discussing their intended purpose – retaining soil.  If you aren’t building a retaining wall, save yourself the time, money, and possibly your plants by passing on the unnecessary addition of retaining wall blocks in your landscape.

This is a good example of retaining wall blocks being used for the primary function of retaining soil.  This retaining wall also provides the secondary benefit of being a seat wall.