Retaining Walls – Drainage & Engineering
Part 1 - The thought process and engineering that goes into building a retaining wall that will withstand the New England winters.
Retaining Walls and Stairs
Retaining walls are supposed to hold what is behind them without moving, bending, bowing or collapsing. I started building retaining walls in my 20’s and I’m nearly 60 now. As far as I know I’ve never had a retaining wall fail whether it be stone or wood. I’m probably guilty of over engineering walls, but that can never be considered a fault if you are serious about the wall lasting several lifetimes and serving the purpose for which it was built.
Retaining walls are functional living, breathing & moving works of art as far as I’m concerned and in New England they have to be. The expanding and contracting of wood, soil, water and even stone makes it necessary to properly engineer a retaining wall. I’m going to primarily discuss one particular type of wall in this article. The wooden wall made of square or rectangular timbers meant to hold back one level gardens, tiered gardens, small ponds & water features, stairways and even driveways. I’ll focus on one style & shape of wood, rebar, spikes and the tools & supplies necessary to engineer a structural masterpiece.
Drainage – Frost – Freezing Temperatures – Expansion & Contraction
Tiered retaining walls & stair understructure
First you need to understand what happens behind the wall you are going to construct in order to understand the engineering that has to go into that wall to keep it intact. In areas that experience freezing temperature where the frost line can go as much as 4-6 feet below the surface depending on the soil conditions. Loose soil that retains a lot of moisture will expand and contract more than well compacted soil. When you carve out the side of a hill to build a retaining wall you are creating a perfect place for water to gravitate to behind the wall. When you build a brand new retaining wall you are going to backfill behind the wall with loose soil that will hold moisture and freeze in winter. Ignoring this simple fact probably causes many retaining wall failures. When you combine it with bad engineering, you have a disaster in the making.
When soil freezes it pushes up, down and out. Anything in its way moves. That is why boulders in New England fields find their way to the surface each Spring. It’s called the New England rock crop. It’s the first harvest of the season. Hence the stone walls that can be viewed anywhere in New England.
You must build your retaining wall with this fact in mind which brings me to drainage under and behind a well-built retaining wall. You can use this information when you build any kind of retaining wall.
Retaining wall - outer base construction. Some drainage already in place.
You want water to run under, through and around the wall. You don’t want it to build up, freeze and create pressure behind the wall. Stone retaining walls built without mortar naturally drain water, but still will break apart, move or collapse in New England because the freezing soil forms a solid block of pressure behind the wall. It doesn’t necessarily move individual stones, it can moves the wall in sections and any weak section in the wall will be exposed. This is what happens behind any retaining wall. The thicker might make it stronger, but it doesn’t necessarily mean it won’t fail.
The solution is to put drainage under a solid retaining wall, through the wall and behind the wall and in some scenarios you might go around the wall. This can be achieved using sand, crushed stone, landscape fabric and corrugated piping. Like when a house is built, you use perimeter drains and surface drains to keep the water from the foundation. Every retaining wall starts with a good foundation and well thought out drainage. You must have a plan as to where the water must drain. Slope and volume have to be considered and drainage designed for worst case scenarios. Observation of your site during high volume rainstorms is a must. Do not build your retaining wall, no matter how big or small it is, before you know the direction of your run-off and the soil conditions of your site. When you build a retaining wall near a foundation you are increasing the chances of trapping water against the foundation walls, but constructing a proper drainage system, paying attention to the slope with your retaining wall project will zero out the chances of leakage into the foundation due to your retaining wall construction. – Written by Steve McRae – Designer, Contractor & Photographer on this project back in 2001-2003.
