CAUSE AND ORIGIN OF RETAINING WALL FAILURE

Cause and Origin of Retaining Wall Failure

This past winter we have inspected quite a few retaining walls that have failed. Our assignments were to determine the C&O of the retaining wall failure. This blog provides the essence of the forensic inspections.

A retaining wall is a person-made structure, designed and constructed to hold back a certain amount of soil and to restrain the pressures created by the weight of that soil. The basement foundation wall is in fact a retaining wall constructed hold back the soil around the foundation. Small retaining walls are also used for terracing earth grading to create landscape areas around residential and commercial buildings and properties.

There are basically two main types of retaining walls: gravity walls and cantilever walls. Other types include anchored wall, pilings and counterfort walls. A gravity-type retaining wall is usually made of concrete, concrete block or other heavy construction material and is trapezoidal in shape, i.e., very wide at the base and both faces of the wall taper inward to a smaller width at the top. This type of wall is able to resist the overturning and sliding forces created by horizontal soil pressure because of the wall’s own weight.

A cantilever retaining wall has an inverted T-shape, consisting of a vertical stem to retain the soil and a large footing that is connected to the stem. These walls are typically solid concrete or concrete block which are filled solid with concrete. In essence, a cantilever wall retains the soil behind it because the heel of the large footing extends beyond the face of the wall (and under the soil) and is held down by the same soil trying to push the wall over. The stem of a cantilever retaining wall is supposed to be steel reinforced to resist the lateral forces against it. In addition, the footing and stem must be properly connected with steel bars. Some basement walls are considered braced cantilever walls because the first floor framing is connected to the top of the wall.

The Most Common Reasons of Wall Failure

The most common signs of failure of the wall are a tilting out of plumb or cracking (horizontal, vertical and/or stair-step). The reasons for these types of failures are lack of proper reinforcement, improper drainage behind the wall (luck of weep holes or clogged holes), foundation footing problems, settlement or expansion of the soil, overloading of the wall, construction errors, and/or other design errors.

Figures 1 and 2. Examples of failed retaining walls

Most of the retaining walls that failed consisted of unreinforced concrete block masonry walls. These are the type of walls where most likely no licensed engineer was involved in its design. The end result of an improperly designed and/or constructed wall is gradual, forward tilting, followed by eventual collapse. The latter normally occurs during rainfall or after major snow/ice storms such as the ones we observed this past winter.

The unreinforced concrete block masonry walls generally consist of either 8-inch or 12-inch thick (wide) concrete blocks, laid in mortar joints, sitting on top of a continuous, poured-concrete, footing. If the wall is of hollow core construction, then it is severely limited with regard to long term performance for wall/backfill heights exceeding four feet or so. Yet, the failed walls we observed were seven or eight feet high and without reinforcement! These are some really blatant errors in design and/or construction. If the block wall is left hollow, it simply breaks or separates along the base (bottom) mortar joint and topples or slides forward.

If the wall is filled with concrete and reinforced with steel, it is still severely limited if it is not properly tied to the supporting concrete footing. This can only be accomplished by using properly-sized, steel rebar dowels securely embedded in both the footing and the wall and at the proper depths. Even if the wall is tied to the footing with steel dowels, the wall is still prone to failure for backfill heights exceeding four feet or so, if the concrete footing is only made about two feet wide. Unfortunately, in the residential construction business, most of the footings are built by the developer about two feet wide, irrespective of the height of the wall. What then the developer does is to “assume” that the retaining wall foundation thickness should also be two-feet wide. This is another no-no-no, leading to wall failure. The proper footing with should be about 60 to 75 percent of the wall height.

Drainage conditions also play a large role in success or failure of a retaining wall. If water is allowed to collect behind the wall, the horizontal forces increase substantially. We have observed so many of these drains and/or weep holes to be non-functioning because they were improperly placed or clogged or not of the proper size. Poor drainage conditions are usually the reason most wall failures occur during rainfall. Drainage is improved by backfilling the wall with gravel/sand, installation of a foundation drain, proper terrain, etc.

If the wall is salvageable, typical enhancements/repairs might consist of converting an existing concrete block masonry wall into a “gravity wall”; extend the footing, correct surface drainage problems, replace the backfill material if unsuitable, installing tie-backs or dead men anchors that extend back into the natural soil; reinforce the face of the wall by drilling holes and installing steel dowels, constructing buttresses against the wall, and so on.

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