Monday, March 10, 2014

Tips On Building A Dry Stack Stone Wall #4: Why They Work

In the last post I did for this series on tips for building a dry stack stone wall 
I showed how I go about building a dry stack wall.
This time I want to drive home a few of the principles that I think explain 
why these walls work the way they do and are fundamental to the success of a wall.

The posts to date in this continuing series geared for the owner builder are:

To start let's look at the serpentine form that I use in my walls.

There are no straight lines here and actually I avoid them whenever possible.
As I mentioned in a previous post there is nothing inherent in stone that requires a straight line,
unlike lumber, plywood, sheetrock, and even cement block that are designed for a lineal layout.
That is one of the things that makes stone so attractive to people
 and makes it one of the most natural of building materials.

This is a form that Thomas Jefferson explored in a very symmetrical way in the brick walls 
  he designed at Montecillo and on the grounds of the University of Virginia,
 as he realized that the serpentine form would  be stronger than a straight wall,
particularly in freestanding walls.

Think of a piece of corrugated metal roofing. 
With the corrugation the metal is quite strong compared to a piece of flat metal,
which can be very flimsy and will not hold it's shape well.
The serpentine wall acts much like the corrugation in metal roofing.

An even more important factor for the longevity of these walls is the batter 
or the way the wall leans into the embankment that it is built against.

This creates a tension between the earth that the wall is holding back 
and gravity that is pushing the wall back into the embankment.
This creates an equilibrium that will hopefully be sustained far past our time.

As I mentioned in the last post on building the walls, I don't think you can have too much batter.

The wall above is 2'+/- high and leans in about 7" or 8" 
which works out to 3" or 4" batter per foot of rise.

A minimum of 1" to 2" per foot of rise would be required at the very least.
You may notice that the amount of batter I use varies from place to place in the wall,
as I tend to go for a rather fluid form.

In the first photo of this post the wall shown is 3' high and leans in from 10" to 14".

Now let's look into the wall (above) and see what is in there and why.

As I showed in the last post the top of each face stone slopes down as it extends into the wall 
ensuring that the next stone on top will have a tendency to slide into the wall and not out.
Over the lifespan of this wall, the wall will tighten into itself making it stronger as time goes on.

This is a fundamental principle for the dry stack stone wall!

In contrast to the hardness of stone, the stone wall is flexible 
and is designed for subtle movement that we will never notice,
 imperceptible to any of our senses.

In another section of wall above we see the same thing:
the face stones are selected and set so they will want to slide into the wall.

Up on the top of the wall smaller stones have been worked in behind the face stones 
to create a tight fit and to keep debris and mud from working its way into the wall.

Water will pass through freely. 

In these last photos let's look at how the wall is designed to withstand hydrostatic forces,
that is the power of water and water saturated earth.

Have you ever noticed a cement block, concrete, or mortared brick or stone retaining wall
 that has been  pushed out (to a negative batter) or knocked over by some unseen force?
Oftentimes that force is the pressure from the buildup of water or wet frozen ground 
hidden behind the wall and resulting from blocked or nonexistent weep holes 
that gave the water nowhere to go.

The dry stack stone wall will never have that problem as it is designed for the free flow of water.
The soil behind is unlikely to ever be saturated enough to push the wall out
 or heave in wet freezing temperatures.

Looking inside the wall in these photos, I think it is easy to see why.

These walls can even be incorporated with french drains to deal with areas 
that have water and drainage problems.

When landscaping around dry stack walls keep in mind that the soil adjacent to the wall 
will be the first to dry out, making it excellent for plants and trees requiring well drained conditions.

We will end with the same photo we started this post with
 to show the finished wall that I am working on in these last 3 photos.

Next post we will look at some of the different styles I used in the walls shown here.

The posts to date in this continuing series are: