Wallace Design Collective

As a structural engineer, I am frequently called upon to investigate and develop repairs for failed or failing earth retaining wall structures. The call often comes from a concerned homeowner or an attorney representing him in civil litigation.

Retaining wall failures are almost always attributable to poor design or poor construction.  Common deficiencies include:

– Insufficient footing width resulting in rotation or settlement of the wall.
– Insufficient stem width resulting in failure or excessive rotation of the stem.
– Inadequate or improperly placed reinforcing steel resulting in failure of the stem.
– Use of inappropriate fill materials including clay or organic materials resulting in settlement of the backfill or failure of the wall.
– Use of inappropriate methods for placement of backfill resulting in settlement of backfill or excessive rotation and failure of the wall.
– Inadequate provision for drainage of the backfill material resulting in excessive rotation or failure of the wall.

In open areas a retaining wall failure may go unnoticed or may not be a concern to the owner; however, when the wall is located adjacent to other structures the failure of the wall usually results in distress to the adjacent structure whether it is a pool, a patio or a building structure.

The installing contractor – who usually is not a registered design professional – frequently prepares the design for the retaining wall that he installs.  His design is usually based on code prescriptions or his experience, which may be limited, particularly with wall heights in excess of 48 inches or where the site presents unusual grading conditions adjacent to the wall.  This practice is considered acceptable for walls retaining less than 48 inches of soil; however, the International Residential Code – adopted by the City of Tulsa – requires that walls supporting more than 48 inches of unbalanced backfill be designed in accordance with accepted engineering practice.  Design of retaining walls over 48 inches tall must be performed by a qualified registered engineer.  A registered engineer should be engaged to design retaining walls of any height that are part of a terraced slope or that include sloping grades either behind the wall or in front of it or walls that are adjacent to other building structures.

Below I describe the process for design of a simple retaining wall and some of the issues that should be considered.  This is by no means a complete step-by-step list of instructions but it will provide the reader with a basic understanding of what goes into the design effort and what to expect from it.

The design begins with the determination of appropriate soil properties. The wall designer – with the consent of the owner – may base his design on soil-engineering properties listed in the building code or he may ask that a geotechnical investigation be performed to determine the appropriate design parameters.  The geotechnical investigation will reduce the owner’s risk and often it will result in use of less conservative design parameters thus reducing the cost of the wall system. The investigation becomes necessary when considering unusual wall configurations or unfamiliar soils.

After determining the design parameters, the designer will establish a trial geometry for the wall based on rules of thumb and his own experience.  Rules of thumb commonly used by designers to establish the geometry of the wall include (refer to diagram):

• Base width = 1/2 to 1/3 of the height of the wall.
• Base thickness = 1/8 of the height of the wall but not less than 12 inches.
• Stem thickness = 6 inches + ¼ inch for each foot of wall height.
• Stem located on base so that 1/3 of total base width projects forward from the face of the stem.

Using the selected trial geometry and the soil parameters provided to him the designer will check the ability of the wall to resist overturning and sliding and will check the bearing pressure beneath the footing.  He will make any adjustments to the geometry necessary to ensure the stability of the wall.  He may find that he needs to add a key at the bottom of the footing to provide sufficient resistance to sliding.

Once he has satisfied the requirements for stability and allowable soil bearing pressure the designer will determine the requirements for reinforcing steel in the stem and in the base.  As a minimum one should expect to find vertical and horizontal reinforcing steel near the back face of the stem (fill side), and in the top and bottom of the base.  There may be a mat of reinforcing steel near the front face of the stem as well.  The vertical reinforcing in the stem should extend and be hooked into the base.

The designer should also consider whether jointing of the wall is required.  Contraction joints are used to control temperature and shrinkage cracking.  The spacing of these joints is usually 1 to 1 ½ times the wall height but not more than 20 feet.  Expansion joints may be needed on unusually long walls or where walls abut another structure.  Construction joints may be required for unusually tall or long walls.

The designer should specify the requirements for the backfill and for the drainage of the backfill.  The type of soil to be used and the required compaction should be specified.  Free-draining aggregate fill placed in shallow lifts and compacted with a vibratory plate compactor will induce the least pressure on the wall and will most likely produce the most economical design for taller walls.  Sometimes it will be desirable to place a cap of clayey soil above the aggregate fill.  The backfill should be drained with perforated pipe wrapped in filter fabric or with weep holes installed at regular intervals through the face of the wall.

The initial cost of a properly designed retaining wall may be from $300 to $1000 per foot depending on the height of the wall and the finished appearance.  The cost of a replacement wall could be on the order of  $1,000 to $2,000 per foot plus the cost to repair any structure or paving adjacent to the wall. The cost of proper engineering for a retaining wall is inconsequential when compared with the cost of repair and replacement of a deficient wall and the collateral damage that may result from it’s failure.  As a wise man once said, “It is far better to do something right the first time than fix it later”.