Why Retaining Walls Collapse

Why Retaining Walls Fail In March of 2021, a long-running construction  project on a New Jersey highway interchange   ground to halt when one of the retaining  walls along the roadway collapsed.   This project in Camden County, called the Direct  Connection, was already 4 years behind schedule,   and this failure set it back even further. As of  this writing, the cause of the collapse is still   under investigation, but the event brought  into the spotlight a seemingly innocuous   part of the constructed environment. I love  innocuous parts of the constructed environment,   and I promise by the end of this video  you’ll pay attention to infrastructure   that you’ve never even noticed before.  Why do we build walls to hold back soil,   what are the different ways to do it, and  why do they sometimes fall down? I’m Grady   and this is Practical Engineering. In today’s  episode, we’re talking about retaining walls. This video is sponsored by HelloFresh,  America's #1 meal kit. More on that later. 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. Don’t tell them I said this, but 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. When you see a retaining wall in the wild,  the job of holding back soil looks effortless.   But that’s usually only true because much of the  wall’s structure is hidden from view. 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. Estimating all these loads and designing a wall  to withstand them can be a real challenge for a   civil engineer. Unlike most structures  where loads are vertical from gravity,   most of the forces on a retaining wall are  horizontal. There are a lot of different   types of walls that have been developed to  withstand these staggering sideways forces.   Let’s walk through a few different designs,  using this demonstration I built in my garage.   These dowels act like soil particles so we can  easily see how different types of retaining   walls are able to withstand such tremendous  stress, and what happens when they can’t. The most basic retaining walls rely  on gravity for their stability,   often employing a footing along the base. The  footing is a horizontal member that serves as a   base to distribute the forces of the wall into  the ground. 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. One common type of retaining wall involves tying  a mass of soil together to act as its own wall,   retaining the unreinforced soil beyond and  this was actually the subject of one of the   very first engineering videos on my channel.  It’s accomplished during the fill operation by   including reinforcement elements between layers of  soil, a technique called mechanically stabilized   earth. The reinforcing elements can be steel  strips or fabric made from plastic fibers called   geotextile or geogrid. It is remarkable how well  this kind of reinforcement can hold soil together.   In this demonstration, I’m just using  layers of sandpaper between the dowels,   but in that previous video, I built a cube  entirely of sand with layers of window screen,   and it held up one of the tires of  my car. Go check that out after this   if you want some more engineering  details on this kind of structure. Gravity walls and mechanically stabilized earth  are effective retaining walls when you’re building   up or out. In other words, they’re constructed  from the ground up. But, excavated slopes often   need to be retained as well. Maybe you’re cutting  out a path for a roadway through a hillside   or constructing a building in a dense urban area  starting at the basement level. In these cases,   you need to install a retaining wall before  or during excavation from the top down,   and there are several ways to go about it. Just  like reinforcements hold a soil mass together in   mechanically stabilized earth, you can also stitch  together earth from the outside using a technique   called soil nailing. First, an angled hole is  drilled in the face of the unstable slope. Then   a steel bar is inserted into the hole, usually  with plastic devices called spiders to keep it   centered. Cement grout is added to the hole to  bond the soil nail to the surrounding earth. Both mechanically stabilized earth and soil  nails are commonly used on roadway projects,   so it’s easy to spot them if you’re  a regular driver. But don’t examine   too closely until you are safely stopped.  These walls are often faced with concrete,   but the facings are rarely supporting  much of the load. Instead, their job is   to protect the exposed soil from erosion due  to wind or water. In temporary situations,   the facing sometimes consists of shotcrete, a  type of concrete that can be sprayed from a hose   using compressed air. For permanent  installations, engineers often use   interlocking concrete panels with a decorative  pattern. These panels not only look pretty,   but they also allow for some movement over  time and for water to drain through the joints. One disadvantage of soil nails is that the soil  has to settle a little bit before the strength   of each one kicks in. The nails also have to  be spaced closely together, requiring a lot of   drilling. In some cases it makes more sense to  use an active solution, usually called anchors   or tiebacks. Just like soil nails, anchors are  installed in drilled holes at regular spacing,   but you usually need a lot fewer of them. Also  unlike soil nails, they aren’t grouted along   their entire length. Instead, part of the anchor  is installed inside a sleeve filled with grease,   so you end up with a bonded length and an unbonded  length. That’s because, once the grout cures,   a hydraulic jack is used to tension each one. The  unbonded length of the anchor acts like a rubber   band to store that tension force. Once the anchor  is locked off, usually using a nut combined with a   wedge-shaped washer, the tension in the unbonded  length applies a force to the face of the wall,   holding the soil back. Anchored walls  often have plates, bearing blocks,   or beams called walers to distribute the  tension force across the length of the wall. One final type of retaining wall uses piles.  These are vertical members driven or drilled   into the ground. Concrete shafts are installed  with gigantic drill rigs like massive fence posts.   When they are placed in a row touching  each other, they’re called tangent piles.   Sometimes they are overlapped, called secant  piles, to make them more watertight. In this case,   the primary piles are installed without steel  reinforcement, and before they cure too hard,   secondary piles are drilled partially  through the primary ones. The secondary   piles have reinforcing steel to provide most of  the resistance to earth pressure. Alternatively,   you can use interlocking steel shapes called  sheet piling. These are driven into the earth   using humongous hammers or vibratory rigs. Pile  walls depend on the resistance from the soil   below to cantilever up vertically and resist  the lateral earth pressure. The deeper you go,   the more resistance you can achieve. Pile walls  are often used for temporary excavations during   construction projects because the wall can  be installed first before digging begins,   ensuring that the excavated faces have  support for the entirety of construction. 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. As I mentioned, soil loads aren’t the only forces  applying to walls. Some fail when unanticipated   surcharge loads are introduced like larger  buildings or heavy vehicles driving too close to   the edge. If you’re ever putting something heavy  near a retaining wall, whether it’s building a new   swimming pool or operating a crane, it’s usually  best to have an engineer review beforehand. 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. Finally, soil can shear behind the wall, even  completely bypassing the wall altogether.   For tall retaining walls with poor soils,  multiple tiers, or lots of groundwater,   engineers perform a global stability  analysis as a part of design. This   involves using computer software that  can compare the loads and strengths along   a huge number of potential shearing planes  to make sure that a wall won’t collapse. Look around and you’ll see retaining  walls everywhere holding back   slopes so we all have a little more  space in our constructed environments.   They might just look like a pretty  concrete face on the outside,   but now you know the important job they do and  some of the engineering that makes it possible. It’s time for everyone’s favorite segment of   me trying to cook while my wife  tries to capture that on video. The holidays in our house get a little bit   chaotic. That’s why we’re thankful for  HelloFresh, the sponsor of this video,   for converting cooking from a chore into  our favorite thing to do on date night. This time our little helper got to open  the box, but he went to bed so we could   goof around in the kitchen and cook dinner  for ourselves. Most HelloFresh meals take   about 30 minutes to make, so they’re perfect  for the window between his bedtime and ours. HelloFresh has a bunch of options to choose  from, depending on your tastes. We get the   vegetarian meals, and the recipes are excellent:  dishes we would never think to make ourselves. The pre-portioned ingredients mean  there’s less prep and less food waste,   and the packaging is already recycled  content and most of it is recyclable as well. Go try it yourself at HelloFresh.com. If you use  code PRACTICAL14, you’ll get 14 free meals and 3   free gifts. Supporting our sponsors helps support  this channel. That’s HelloFresh.com and use my   code PRACTICAL14. Thanks, HelloFresh, and thank  YOU for watching. Let me know what you think.

Channel: Practical Engineering

Views: 1,381,964

Keywords: retaining wall, New Jersey highway, Direct Connection, slope, lateral earth pressure, surcharge load, gravity walls, cantilever walls, mechanically stabilized earth, geotextile, geogrid, soil nailing, shotcrete, Anchored walls, tangent piles, piles, secant piles, sheet piling, soil mechanics, weepholes, shear, shearing plane, grady, engineering, practical engineering

Id: --DKkzWVh-E

Length: 12min 50sec (770 seconds)

Published: Tue Dec 07 2021

Reddit Comments

[u/Eccentrica_Gallumbit]

There's something about Grady's videos that is the perfect combination of calming, entertaining, and informative.

[u/going-for-gusto]

Grady does such a great job communicating.

[u/SCEngineer]

I love Gradys videos! Really helps me appreciate the work behind civil engineering!

[u/Jmazoso]

Trying to get the stupid pool guys to understand that when I told them not to dig their hole closer than x feet from the base of the existing MSE wall I really did mean it. Now they are pissed cause I wouldn’t sign off of the hole they dug x/2 from the wall until I run full global stability. No the fact that you didn’t listen doesn’t move you to the top of my list when people have been waiting in line for a month for me to get to the top of my pile.

Extra credit: just because you told us you were cutting 35 feet doesn’t mean you can do it straight up. And no figuring out how to make it work is not an overnight answer. Guess who’s getting CD triax run on the fat clays ?

[u/antiduh]

That was a cool video, that rotating/shearing kind of failure was one I didn't even know could happen, but it makes perfect sense after seeing it.

[u/GlockAF]

Solid technical exclamations in the video, but he failed to mention the two main factors leading to retaining wall failures, both of which are temporal in nature.

1. Back when this was built, it met the minimum standard in use at the time

And

1. They knew this was potentially an issue, but they didn’t want to spend the money at the time

In the modern era, few (if any) structural / geo engineering collapses are the result of novel or unexpected failure modes.

Instead, they can almost always be traced back to one or more design decisions where known engineering principles were compromised in order to save money during construction

[u/rawbface]

Oh god, this is my hometown... I recognized it from the thumbnail.

Happened in March, and it's still there, untouched. Also, "four years behind schedule" is beyond an understatement. That highway interchange has been constantly under construction for 30 years.

[u/TheKingOfRandom3]

First time seeing these, very nice.