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Three Reasons Why Retaining Walls Fail
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 opportunities for creative landscape architects to use retaining walls as a design element.
Different types of common retaining walls.
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.”
Retaining Wall Description
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 are minimal for a wall this size, design principles can prevent three common reasons for failed retaining walls.
Retaining walls are great when they 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 holds back a surcharge is 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 avoiding 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.
Earthquake Strengthening Your Foundation
Homes constructed more than two decades ago may need earthquake strengthening upgrades to their foundations. Here’s how to resist seismic forces.
If you live in an earthquake country, which includes at least part of most states, a few relatively simple earthquake strengthening upgrades may be all you need to keep your house from sliding off its foundation in a quake. You can tackle the work yourself for as little as $500, or hire a contractor for about $2,000. Either way, you stand to avoid repairs that might cost tens of thousands of dollars—if repairs are even possible.
That’s because damage from an earthquake can be so extensive—in some cases, the only remedy is to completely rebuild. Earthquakes can exacerbate existing home foundation problems. If you don’t have earthquake insurance, the financial consequences can be dire. Even if you do carry earthquake insurance, simple earthquake strengthening measures could still save you tens of thousands of dollars, since earthquake insurance usually carries a hefty deductible. You also gain peace of mind, and it may make your house easier to sell. At least one state (California) requires sellers to fill out a checklist specifying whether earthquake strengthening measures have been installed. If you buy earthquake insurance, which can range from a couple of hundred dollars to several thousand dollars a year, retrofitting may save you 5% each year on premiums.
Check with your local building department
Houses built to today’s building codes should be strong enough to keep people safe during an earthquake. But older houses, even those built a few decades ago, might need strengthening. Your local building department can tell you what’s required in your area, and whether you need design help from a structural engineer.
If you need just simple upgrades, the department might have free plans you can use. Whether you do the work yourself or hire a contractor, get a permit. If you’re planning to insulate or finish the basement, tackle earthquake strengthening first before you close off access to walls.
Poured perimeter foundations
If your house sits on a wall of poured concrete, with perhaps a few posts in the center under beams, there may be nothing but a few nails and gravity to hold the house in place.
To check whether your house needs earthquake strengthening, go into the crawl space or basement and look for thick bolts along the top of the sill plate, and for steel anchor plates that tie an edge of the sill plate to the side of the foundation. If you find neither, they are easy to install.
If you have enough room to use an electric hammer drill, drill straight down through the sill plate and 4 inches into the concrete—you can rent a hammer drill for about $20 per day from a tool rental center. You may need one ½-inch-diameter wedge anchor or bolt with epoxy every 32 inches to 6 feet, depending on the recommendations of your local building department.
On top of the sill plate, add hefty square washers (often called bearing plates) and nuts. Avoid standard round washers because they may fold and split the sill plate during a quake. If there isn’t enough room to drill straight down, use ties that fasten into the foundation from the side. You’ll find them at a local building-supply company. If the first floor sits several feet higher than the perimeter foundation, the short “cripple” walls on top of the sill plate may need strengthening. To check, look between the studs—if you see diagonal boards or plywood on the outside of the studs, the cripple wall will be braced properly.
Add bracing by nailing plywood to the interior side of each wall. The bracing blocks access to the sill plate, so be sure your house is bolted down first. Your local building department can tell you the specifics about the type of plywood and the nailing pattern.
Unreinforced masonry foundations
If your house sits on a perimeter foundation made with concrete blocks that are filled with rebar and concrete, retrofit it as if the foundation was made of solid poured concrete (above). But if the blocks are hollow or if the foundation is unreinforced brick or stone, you’ll need a structural engineer’s advice. You might learn that your foundation is sturdy enough and you just need a creative way to fasten down your house. Or, you might learn that the foundation is at risk of collapsing in a quake. Expect to pay $500-$700 for an evaluation and recommendation from a structural engineer.
If your foundation consists of more than three rows of concrete blocks and is in good shape, you might be able to fasten the sill plate to the foundation by drilling slightly oversize holes into hollow parts of the blocks and then inserting mesh sleeves, epoxy, and threaded bolts. Tightening the bolts causes the epoxy to squeeze through the mesh and mushroom out inside the hollow cavity, holding the bolts much like drywall anchors work to hold screws in walls.
This relatively new option costs as little as $5,000 for a one- or two-story house with a footprint of 1,000 square feet. That’s about half of what it would cost for the more traditional method of cutting into the blocks and installing rods that tie the sill plate to the foundation footing.
If the foundation consists of only a couple of courses of blocks, or if the walls aren’t in good shape or are made of brick or stone, you might need a new foundation. A new foundation costs about $40,000—more for a house with a basement.
Short block walls are more vulnerable to collapsing in an earthquake than walls of four or more courses because short walls have fewer mortar joints. Fewer joints mean more stress concentrated in each joint. In a taller wall, there are more joints to share the seismic forces. “I know it’s counterintuitive,” says Leif Jackson, owner of Sound Seismic, a retrofit contractor in Seattle, “but that’s what the engineers say.”
For a house that rests directly on a concrete slab, metal straps or bolts should tie the sill plate to the concrete. If you have an unfinished garage, you can check sill plates there and assume the house is built the same way. If the garage walls are closed in, check by removing a section of drywall or siding in an unobtrusive spot.
Those who are not inclined to open up a wall can take comfort in the fact that a slab house probably won’t collapse since it doesn’t have far to fall. Of course, if you are removing siding or drywall for another purpose, that’s the perfect time to check for straps or bolts and add them if they’re missing.
Some houses are supported by upright posts that rest on concrete blocks or piers. During an earthquake, these support posts are especially vulnerable to back-and-forth seismic movement and may collapse. Repairing collapsed posts starts at around $20,000 if the house can be salvaged. The preventative solution may be as easy as bracing the posts at a cost of about $1,000, or as expensive as adding a new foundation for approximately $25,000. Between these extremes, you may be able to pour short L-shape concrete foundations around each corner and securely attach them to the floor framing. Seek the advice of a structural engineer.
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