CAUSE AND ORIGIN OF RETAINING WALL FAILURE
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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 (lack 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 un-reinforced 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.
Fortunately, retaining walls are quite forgiving, nearly always displaying telltale signs of trouble and alerting an observer to call for professional help before a collapse. After an evaluation, and determination of the causes, most walls can be saved.
The most common sign of distress is excessive deflection of the wall – tilting out of plumb – caused by a structural overstress and/or foundation problem. Some structural deflection is to be expected and a rule-of-thumb is 1/16th inch for each foot of height, which is equivalent to one-half inch out-of-plumb for an eight foot high wall. More than that is suspect. It’s easy to check with a plumb bob.
Here are six things that can go wrong and signal distress:
Reinforcing not in the right position
If the stem shows sign of trouble (excessive deflection and/or cracking) the size, depth, and spacing of reinforcing should be verified. Testing laboratories have the devices (usually a magnetic field measuring Pachometer) which can locate reinforcing and depth with reasonable accuracy, up to about 4 inches depth. For exact verification you can first locate the reinforcing then chip out to determine its exact depth and bar size. More elaborate devices are also available if needed – check with your testing laboratory, they’ll come to you jobsite for around $100 per hour. Believe it or not, cases have occurred where the reinforcing was placed on the wrong side of the wall, either through a detailing error, or contractor error. When the actual reinforcing size, location, and spacing is determined, and perhaps a core taken to verify strength of stem material, a design can be worked backwards to determine actual design capacity and thereby guide remedial measures.
Saturated backfill
Since retaining walls are generally designed assuming a well drained granular backfill, if surface drainage is allowed to penetrate and accumulate in the backfill, the pressure against the wall can be doubled. Ponding of water behind the wall not only indicates poor grading, but clayey soil impding the downward seepage of water. The surface of the backfill should be graded to direct water away from the wall, or by the use of drainage channels adjacent to the wall to intercept surface water and divert it to disposal. Often surface water problems are attributable to a misdirected or poorly timed irrigation system. Poor backfill material, such as containing clay, can swell and increase wall pressure. One contractor always uses crushed rock for backfill; it’s cheaper than pea gravel, and the elimination of tamping compaction of granular soil offsets the cost of crushed rock, and assures good drainage. Don’t compact backfill by flooding.
Weep holes that don’t weep
The only thing that comes out of most weep holes is weeds – not water. They become clogged when there is no filtering, such as a line of gravel or crushed rock placed along the base to provide a channel for water to find weep holes, or to be coduced by an embedded perforated pipe. Commercial filtering fabric is available. Weep holes in masonry are usually made by omitting mortar at the side joints of every other block (32 inches on center). For concrete walls, 3” diameter pipe sleeves are often used, spaced 4’ – 6’ on center, or as deemed appropriate by the designer. Specifying proper drainage measures (backfill material, surface water control, and base-of-wall drainage) is an important specification task for the EOR (Engineer Of Record).
Design errors
Design errors as the cause of failures are relatively rare when prepared by an experienced designer. However, sometimes the designer is given insufficient or erroneous information. For example, “Design the wall to retain eight feet”, but later examination of the grading plans, or as-built conditions, shows the wall retaining nine feet, an additional foot, thereby increasing the base moment on the stem by nearly fifty-percent. Or there could be surcharge loads, such as an adjacent footing or roadway, of which the designer was unaware. Good data communication between the EOR and his/her client is essential. If software is used as a design aid, it is essential that the designer correctly inputs data and understands the capabilities and limitations of the particular program (Retain Pro advises its users to be licensed civil or structural engineers, or at least have the expertise to design a relatively complex retaining wall by hand calculations).
Detailing errors
This is related to the above, but detailing, particularly of reinforcing, has led to misinterpretation by the contractor. In one case dowels from the footing extended only 6” into the stem, rather than the intended 24”, due to confusing dimensions. Easy-to-read drawings and careful checking by the designer can eliminate these problems.
Foundation problems
When a geotechnical investigation is provided, there will be guidelines for design (allowable soil bearing, friction factors, seismic if applicable) and any caveats based upon site conditions, such as liquefaction potential. Following these recommendations should assure a trouble-free foundation. However, often such an investigation is not provided, calling for special care by the designer. Without such a geotechnical report the soil bearing is limited by code, for example to 1,500 psf, and coefficient of sliding friction of 0.25, and allowable passive pressure of 150 pcf. Regardless of using more conservative values, the designer should be aware of any adverse conditions, such as fill material, compressible soil, water table, or other factors that could cause excessive settlement – or sliding.
And six fixes that could save the wall:
Note that each of the fixes listed below have been successfully used, but it is assumed that the wall is not in such distress that none are viable solutions. Remember too that in some cases, and in conjunction with the below fixes, the wall can be pushed back to near-plumb (an arguable procedure) after some of the backfill has been removed to facilitate the realignment.
Correct surface drainage problems
You can’t economically replace the backfill or get to the base-of-wall drainage system, but you can re-grade at the surface so water does not collect behind the wall. Perhaps a small concrete culvert. Often just shutting off an over active irrigation system will solve the problem. Additional weep holes can also be cored through he wall, although possibly visually objectionable.
Reduce the retained height
If the soil pressure needs to be reduced, investigate whether re-grading of the surface can reduce the height of earth retained. Sometimes a change in landscaping, or a depressed drainage culvert at the back of the wall may reduce the height to an acceptable level based upon the as-built capabilities.
Use tie-backs
If the stem is severely overstressed, an option is to use tie-backs extending back beyond the failure plane. Drill holes through the wall and install conventional tiebacks (also called soil nailing). A downside of this is the appearance of the tie-back anchors on the exposed face of the wall. Or perhaps a tie-back at the surface can be used, with a concrete anchor block, or an added slab-on-grade. Using tie-backs requires re-analyzing the wall moments and shears due to the changed restraints.
Extend the footing
You can extend the toe of the footing and thereby substantially reduce soil pressures. Determine how much you need to extend, then excavate to the bottom of the footing (add deeper for a key if necessary) and place concrete. To transfer shear and moment at the interface, drill holes in the existing footing and epoxy dowels to resist the calculated pullout.
Remove and replace backfill material
This may be the only solution if saturated backfill is the problem and cannot be controlled at the surface. Use crushed rock, and be sure the base-of-wall drainage is functional.
Reinforce the front of the wall
This can be done by forming or pneumatically placing concrete to thicken the base, and tapering to a height where the added strength is no longer needed. This is on the compression side so the only design concern (other than how much thickness to add) is shear transfer at the interface, which can be accomplished by drilled dowel pins.
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