Ask the builder
Dry-stone walls last for generations, require some engineering
DEAR TIM: While viewing the colorful autumn leaves in New England, I saw more than one rock wall. I was so impressed with them and wonder whether rock retaining walls really hold up over time. What are some of the best practices used when building these natural wonders, which seem as strong as they are beautiful? - Beverly W., Erie, Pa.
DEAR BEVERLY: Dry-stone wall construction has been a standard practice in New England and other parts of the world for centuries. I've been told that the original settlers used tree stumps at first to create fences that kept livestock from wandering. They then replaced these temporary walls with rock and stone as they started to clear their fields. There are stone walls in New England that date back hundreds of years. Built correctly, they'll last many generations.
Thousands of miles of rock and stone walls exist in New England. As recently as the 1850s, about 80 percent of the land area of New Hampshire was cleared of most trees. The land was used for grazing sheep. The wool from the sheep kept the textile mills, one of New England's primary industries in the 1800s, at full capacity as the fiber was transformed into garments.
The farmers needed to control where the sheep grazed, and discovered that rock walls were the way to go. I'm convinced this technology came with the settlers from across the Atlantic, as you can find stone walls on farms all over the United Kingdom.
There are two types of stone walls. One is free-standing and creates a fence or boundary. The other is a stone retaining wall. The method of construction for each is somewhat similar, but a stone retaining wall requires some engineering if you want it to last hundreds of years.
A free-standing stone wall can be made by stacking a single row of stones that interlock with one another. You'll also find walls that are much wider and have multiple rows of interlocking rock. Both will work, but the thicker walls often will last longer, especially if they are wider at the bottom than at the top.
Rocks that are more angular than rounded stack more easily and are more stable. Angular rocks resist the movement induced by gravity and frost heaving. The severe frost heaving, which causes the stone walls to move up in the winter then back down in spring when the ground thaws, often doesn't cause damage to dry-stone wall construction (walls that don't use mortar or cement to bond the rocks together). Friction and gravity are all that keep the stones from tumbling down.
One good practice to follow is to use larger stones as often as possible for the lower courses. As you build higher on the wall, use progressively smaller stones.
Retaining walls higher than three feet should tilt backwards into the hillside. This is commonly referred to as battering. It's not uncommon for you to see six inches of backward tilt for every four vertical feet of height. Some structural engineers call for even more tilt.
Drainage is also very important. Fortunately, stone walls that are laid dry allow water to pass through them. But you need to be sure the backside of the rock wall is filled with smaller stones, rounded gravel or crushed gravel that allows groundwater to flow easily and not get trapped behind the wall. Trapped water adds to the overall weight of the soil the retaining wall is trying to hold up.
You can lift heavy stones any number of ways. Many tool-rental businesses offer excavators that have a thumb as part of the bucket. This extra feature works like your own thumbs. allowing it to grab and hold onto a large stone while it's being carefully placed. Before hydraulic equipment was used, I assume that stones were put in place using the simplest machines of all, the inclined plane and lever.
Tim Carter is a columnist for Tribune Media Services. He can be contacted through his Web site, www.askthebuilder.com.