CAUSE AND ORIGIN OF RETAINING WALL FAILURE
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.
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.
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 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).
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.
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.
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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