- Platform: Practical Engineering - Real Title: Why Retaining Walls Collapse ## Highlights ### id262791393 engineers flatten building sites for long-term stability > The natural landscape is never ideally suited to construction as it stands. The earth is just too uneven. Before things get built, we almost always have to raise or lower areas of the ground first. We flatten building sites, we smooth paths for roads and railways, and we build ramps up to bridges and grade-separated interchanges. You might notice that these cuts and fills usually connect to the existing ground on a slope. Loose soil won’t stand on its own vertically. That’s just the nature of granular materials. The stability of a slope can vary significantly depending on the type of soil and the loading it needs to withstand. You can get many types of earth to hold a vertical slope temporarily, and it’s done all the time during construction, but over time the internal stresses will cause them to slump and settle into a more stable configuration. For long-term stability, engineers rarely trust anything steeper than 25 degrees. That means any time you want to raise or lower the earth, you need a slope that is twice as wide as it is tall, which can be a problem. - [View Highlight](https://read.readwise.io/read/01fqy75nk7b2yq7vzhsd6s5m0q) ### id262791528 retaining walls are less wasteful of space than slopes > slopes are kind of a waste of space. Depending on the steepness, it’s either inconvenient, or entirely impossible to use sloped areas for building things, walking, driving, or even as open spaces like parks. In dense urban areas, real estate comes at a premium, so it doesn’t make sense to waste valuable land on slopes. Where space is limited, it often makes sense to avoid this disadvantage by using a retaining wall to support soil vertically. - [View Highlight](https://read.readwise.io/read/01fqy76cdnrnr13taa2nph3dzm) ### id262791604 retaining walls are basically soil dams > A retaining wall is essentially a dam, except instead of water, it holds back earth. Soil doesn’t flow as easily as water, but it is twice as heavy. The force exerted on a retaining wall from that soil, called the lateral earth pressure, can be enormous. But that’s just from the weight of the soil itself. Include the fact that we often apply additional forces from buildings, vehicles, or other structures, on top of the backfill behind the wall. We call these surcharge loads, and they can increase the forces on a retaining wall even further. Finally, water can flow through or even freeze in the soil behind a retaining wall, applying even more pressure to its face. - [View Highlight](https://read.readwise.io/read/01fqy76kzvd3rxkt0vpypwxdn9) ### id262791609 difference between gravity wells and cantilever wells > Your first inclination might be to extend the footing on the outside of the wall to extend the lever arm like an outrigger on a crane. However, it’s actually more beneficial for the footing to extend inward into the retained soil. That’s because the earth behind the wall sits atop the footing, which acts as a lever to keep the wall upright against lateral forces. Retaining walls that rely only on their own weight and the weight of the soil above them to remain stable are called gravity walls (for obvious reasons), and the ones that use a footing like this are called cantilever walls. - [View Highlight](https://read.readwise.io/read/01fqy7777bzgftdt45jyxspmrf) ### id262791666 reasons retaining walls fail > All these types of retaining walls perform perfectly if designed correctly, but retaining walls do fail, and there are a few reasons why. One reason is just under designing for lateral earth pressure. It’s not intuitive how much force soil can apply to a wall, especially because the slope is often holding itself up during construction. Earth pressure behind a wall can build gradually such that failure doesn’t even start until many years later. Lots of retaining walls are built without any involvement from an engineer, and it's easy to underestimate the loads if you’re not familiar with soil mechanics. Most cities require that anything taller than around 4 feet or 1.5 meters be designed by a professional engineer. - [View Highlight](https://read.readwise.io/read/01fqy78n9zrpzzrm4saespc5rs) ### id262791725 water causes structural damage > Water is another challenge with retaining walls. Not only does water pressure add to the earth pressure, in some climates it can freeze. When water freezes, it expands with a force that is nearly impossible to restrain, and you don’t want that happening to the face of a wall. Most large walls are built with drainage systems to prevent water from building up. Keep an eye out for holes through the face of the wall that can let water out, called weepholes, or pipes that collect and carry the water away. - [View Highlight](https://read.readwise.io/read/01fqy7b8pefd7m09bb7h97xr7f)