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
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YOU for watching. Let me know what you think.
There's something about Grady's videos that is the perfect combination of calming, entertaining, and informative.
Grady does such a great job communicating.
I love Gradys videos! Really helps me appreciate the work behind civil engineering!
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 ?
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.
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.
And
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
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.
First time seeing these, very nice.