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All Walls, Great and Small

All Walls Great and Small

Article written by Jack Carr, P.E., R.S., LEED AP, Criterium Engineers
Published in Condo Media, March 2012

Here in hilly New England, an often-overlooked asset in our maintenance plan is the condo site's retaining walls. After a while they blend into the landscape and they just disappear. Yet they are everywhere. They are in the city, as well as the countryside.
 
They can be made of many different materials. They can be poured concrete, steel or timber. They can be concrete block or stone or brick. They can be very tall or very short. Whatever they are, if they fail, they can be dangerous and costly to repair. So a prudent grounds committee should schedule regular inspections and maintain a log of repairs. But what should be looked for?
 
It is recommended that retaining walls be inspected four times a year, before or after each season, as heavy rains or soil movement due to frost heaving can have a significant effect. Someone should perform an annual inspection and compare those findings with the year before.

Ground Effects

With age, retaining walls deteriorate, primarily due to the effects of gravity. This can be due to excessive soil loading behind the wall or the weight of the wall overloading the soil conditions beneath. The increase of forces behind the wall is often the result of water buildup due to surface water above the slope percolating into the ground or a rising groundwater table. Weep holes are designed to reduce this increase in hydrostatic forces, but they can be blocked by debris or overwhelmed by the quantity of water.
 
Therefore, one of the first steps in the inspection process is to observe the conditions of the ground surfaces upslope of the retaining wall. Some walls have trench or other types of drains to intercept the water, while some ground surfaces are designed to quickly drain water over or around the wall before it can enter the ground behind it. If ponding above the wall is discovered, this problem must be corrected. Ponding will occur if the ground settles or is removed due to erosion. If the surface was paved, cracking in the surface can allow unintended water to enter the soil. 

Surface Inspection

The surface of the retaining wall is the next and perhaps easiest element to inspect. Wall movement should be noted. Walls should lean back toward the slope uphill. If they lean forward or have signs of bulging in any location, this could indicate excess back pressure or failure in the underlying soils. As this could be serious, the first time this is noted it should be brought to the attention of the facility manager or your engineer.
 
Timber retaining walls can move or crack due to both earth movement and insect attack. Timber wall elements can rot with age. Any type of
wall can crack and allow vegetation matter to grow from either side. All roots, weeds or other growth should be removed when discovered, as they can apply considerable pressure to the crack and create localized weakening of the wall. Wall cracks in concrete surfaces can allow moisture to reach the reinforcing steel, causing excess corrosion that creates not only internal pressures from expanding rust, but also provide a passage for freezing water to create further deterioration from expanding freeze/thaw cycles.
 
Missing wall stones or bricks can also be an indicator of severe problems developing. These missing elements not only will weaken the structural integrity of the retaining wall, they may also be an indication of excessive soil pressures that forecast future wall bulging, including bowing or bellying, either vertically or horizontally.

Changes in Proximity

As the original retaining wall was designed for specific loading conditions, any changes in the general proximity to it should be considered. This
could include the placement of a structure or other heavy mass near the surface at the top ofthe wall or changes in land use by your upslope neighbor. This could include a roadway, parking lot or the use of road salts or de-icers near the wall. It also includes any changes to the pattern of surface water drainage, such as switching from a grassy surface to a mulched surface.
 
One of the most subtle wall failures is movement of the slope itself. In some circumstances, it is possible for the hillside to shift, carrying the entire retaining wall with it without actually damaging the wall itself. For this reason, it is important to document not only the wall, but the peripheral areas to ensure any movement out of the ordinary is noted and reported. Especially note any ground movement at the foot of the retaining wall.

 
© 2012 CRITERIUM ENGINEERS