ROANOKE TIMES
Copyright (c) 1996, Roanoke Times
DATE: Sunday, October 13, 1996 TAG: 9610150070
SECTION: HOMES PAGE: D1 EDITION: METRO
COLUMN: hints for homeowners
SOURCE: STEVE ELDER
Masonry retaining walls suffer from a number of physical forces acting upon them individually or in concert at any given time. These forces include gravity, hydrostatic pressure, crystallization (the force of water expanding as it freezes), and those twin banes of construction - the Force of Poor Design and the Force of Bad Construction.
Your retaining wall may exhibit one or more of the following common symptoms: a stepped crack, a bulge, a lateral crack or tilting. Here is some diagnosis and therapy:
A stepped crack (one which follows the mortar joints) usually means an underdesigned footer. Retaining walls should have concrete footers just as house foundations do; however, the tendency is to skimp on the amount of concrete because both the weight of the wall and the forces acting on it are underestimated. Concrete is a relatively cheap building material, and it gets cheaper in larger quantities. If the footer of a retaining wall is built strong enough to support a house wall, the chances of future problems will be greatly reduced.
If a stepped crack occurs because of footer failure, very little can be done about it save re-mortaring the crack. However, stepped cracks can also be caused by the annual freeze-thaw cycle, particularly in a brick or cement block retaining wall built in a convex curve. When the ground freezes, the water in the soil expands; the more heavily saturated the soil is, the greater the force of expansion acting on the wall. The exposed side of the wall is subjected to sub-freezing air temperature; this can freeze the soil behind the wall from top to bottom (not just down to the frost line). The resultant pressure exerted on the wall can cause a stepped crack or a bulge if the wall is not adequately built.
A long horizontal crack between masonry courses is an indication of hydrostatic pressure. The sheer weight of soil saturated with water can cause a masonry wall to buckle at the midpoint if it was not properly reinforced during construction. Sometimes the whole retaining wall will start tilting; this suggests that the wall was adequately bonded, but the footer was too small or the wall was not thick enough. Wall failures can usually be attributed to inadequate design or poor construction technique. Most of them could have been avoided by consulting a professional architect or engineer.
The common element in most retaining-wall problems is water. It is responsible for increasing the effects of freezing soil and soil weight on retaining walls, so it stands to reason that reducing the amount of water behind the wall will mitigate a lot of wall problems. If the damage to your retaining walls has not progressed to the point where rebuilding is necessary, here is some corrective surgery that can be performed to keep the problem from getting worse.
All masonry retaining walls should have weepholes every four to six feet near the base of the wall. Their purpose is to relieve water pressure that invariably builds up behind the wall. However, most weepholes clog up with soil or silt over time and become ineffective. Clean the weepholes out as best you can; and if the wall was originally built without them, put some in. For short walls (under two to three feet), this may be adequate. For higher walls, the preferred therapy is more extensive and requires some elbow grease.
Remove a strip of sod 12 to 16 inches wide behind the wall and set the sod squares aside for reuse. Remove the soil to a depth of at least half the height of the wall or at least two feet. Make sure the bottom of the trench is sloped toward one end. Line the trench with landscape mesh (the stuff you put on a garden to let water through but keep weeds from growing). Next, lay perforated black plastic pipe (standard footer drainpipe) on the bottom of the trench.The drainpipe should run to daylight at the end of the wall; if this is not possible, bring the drainpipe through the face of the wall so that it will drain effectively. Cover the top of the pipe with mesh. The mesh lining will keep silt out of the stone and drain tile so that your system will not get clogged. Then fill the trench with crushed stone to within six inches of the surface and cover the top of the stone with mesh. Finally, add about three inches of topsoil and replace the sod.
This may sound like a lot of work, but it is usually worth it. (Think of how much more work is involved in tearing down the wall and rebuilding it from scratch.) If your retaining wall had been built with a properly designed drainage system, the various problems described above might not have developed. If you are planning to build a new retaining wall, it is easy to install such a system at the time of construction in order to forestall future problems. Since the same physical forces also act on timber retaining walls, a drainage system should also be incorporated in their design as well.
LENGTH: Medium: 83 linesby CNB