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Retaining Walls - The Hidden Cost

 

For many condo properties, an often overlooked asset is the 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 prudence suggests regular inspections and maintaining a log of repairs. 

Before discussing how walls fail and what to look for, let’s review the various types of walls and their purpose.  Basically, retaining walls are designed to hold back soil when creating a level site for construction, holding back a natural or man-made slope, retaining a driveway, or for landscaping between properties or on an individual property.  Retaining walls are most commonly built with treated wood or timbers, boulders, cut stone, or segmental concrete blocks.  The type selected is usually based on the goals for the project, costs, and aesthetics.

We recommend that retaining walls be inspected four times a year; that is, before or after each season as heavy rains or soil movement due to frost heaving can have a significant effect on a retaining wall in a short time.  At the very least, someone should perform an annual inspection and compare those findings with the year before.  But what should be looked for?

Like all of us, with age retaining walls deteriorate primarily due to the effects of gravity.  This can be due to the excessive soil loading behind the wall or the weight of the wall overloading the soil conditions beneath the wall.  The increase of forces behind the wall is often the result of water build up due to surface water above the slope percolating into the ground or a raising ground water table.  As irrigation piping is notorious for leaking, pay attention to any piping near the top of the retaining wall.  Weep holes are designed to reduce this increase in hydrostatic forces but weep holes 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 has the opportunity to enter the ground behind the wall.  If water 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 asphalt or concrete surface can allow unintended water to enter the soil.

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 locations 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.  Prior to the change in chemicals put in timber today walls used to last 30 years, today we are lucky to get 20 years life.   Any type of wall can crack and allow vegetation matter to grow from either side of the wall.  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 creating not only internal pressures from expanding rust but also provide a passage for freezing wall 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.

As the original retaining wall was designed for specific loading conditions, any changes in the general proximity to the wall should be considered.  This could include the placement of a structure or other heavy mass near the ground surface at the top of the wall or changes in use of the land by your upslope neighbor.  This could include a roadway, parking lot, or the use of road salts or deicers 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 the actual movement of the slope itself.  In some circumstances it is possible for the hillside to shift or move 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 in the vicinity to ensure any movement out of the ordinary is noted and reported.  Especially note any ground movement at the foot of the retaining wall.  Good observation and site management will go a long way to avoid the hidden cost of retaining wall failures. 

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Article written by Jack Carr, P.E., R.S., LEED AP, Criterium Engineers

Published in Condo Media, July, 2017

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