Do you know the three ways restraining walls fail?
Retaining walls are useful engineering solutions landscape architects use to create useable space on sloping or hilly landscapes.California’s unique terrain creates lots of opportunity for creative landscape architects to use retaining walls as a design element.
Expect a couple of questions about retaining walls on the California Supplemental Exam
According to Wikipedia, “A retaining wall is a structure designed and constructed to resist the lateral pressure of soil when there is a desired change in ground elevation that exceeds the angle of repose of the soil.”
Landscape architects are limited to designing walls that have up to three feet (3′) of exposed wall height. While the level and complexity of engineering is minimal for a wall this size, design principles can prevent three common reasons for failed retaining walls.
Retaining walls are great when the work as designed. However, the forces of nature and laws of physics can work against even the best engineered retaining walls. These forces can cause catastrophic failure.
3 Ways Retaining Walls Fail
- Settlement of the foundation
Overturning is caused by the force of the soil pushing a wall over when the thrust exceeds the wall’s weight. Overturning can be prevented by increasing the wall mass, enlarging the foundation, or using a cantilevered foundation. Retaining Walsh that hold back a surcharge are more likely to fail than walls that do not retain a surcharge.
Sliding is also caused by the thrust of the soil. Instead of the wall overturning, it slides horizontally as an entire unit. Larger foundations, more massive walls, or cantilevered foundations are some solutions that can reduce the chances of a wall sliding. Ensure that there is a drainage system installed behind the wall to reduce hydrostatic pressure.
Settlement of the Foundation
Settlement of the foundation happens when the weight of the wall causes the soil to compress or subside. When this happens, the height of the wall above grade shrinks. Settlement can encourage other weaknesses as well.
However, good design and construction practices can help prevent settlement. These things can ensure a strong wall:
- Undisturbed sub-grade
- Strengthen sub-grade with gravel
Compaction—Landscape contractors will compact the soil before laying the foundation. Compacting soil reduces the pore space between soil particles and creates a denser substrate for a retaining wall foundation. Double-check your details and specifications to ensure that they specify that the soil under the footing will be compacted during construction.
Undisturbed Sub-grade—Landscape architects are limited by state law and regulation as to the height and engineering features of retaining walls. Any retaining wall that landscape architects design should have a footing in du disturbed sub-grade. That means avoid building walls on fill soil. Even the best engineered fill soils lack the strength of average undisturbed soil. Only civil or structural engineers can design a retaining wall footing for fill soil conditions. As landscape architects, we will keep our footings in undisturbed soil.
Strengthen Sub-Grade With Gravel—Weak or expansive soils can be strengthened by placing several inches of gravel, aggregate, or crushed stone under the retaining wall foundation. Gravel is dimensionally stable and can withstand the compressive forces of the wall structure. Be sure to note that the gravel base should be compacted.
More About Retaining Walls
Landscape Architectural Graphic Standards is an excellent resource for every landscape architect. Some of the information in this article was inspired by the soil mechanics section.
Read more about retaining walls in “Pass the California Supplemental Exam for Landscape Architects” and “Practice Problems for the California Supplemental Exam”.
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