Sand Castle Holds Up A Car! – Mechanically Stabilized Earth

Sand Castle Holds Up A Car! – Mechanically Stabilized Earth


One of the most important considerations when
you build anything is the material. It impacts everything from the look and feel
to the more fundamental characteristics like weight and strength. But maybe the most significant aspect of a
construction material is its cost. Infrastructure is not made to be glamorous
and it’s often paid for by you and me through taxes, so we like to keep the costs down. And there’s one construction material that’s
cheaper than just about anything else out there: dirt. Hey I’m Grady. Today on Practical Engineering, we’re talking
about reinforced earth. You’ve probably don’t think much about
the strength of the soil beneath your feet, but some of us have dirtier minds than others. Just about every structure out there sits
on the ground. And anyone who’s ever built a sand castle
knows that soil is not all that strong. So if we want our buildings and bridges and
pipelines and roads and anything else that sits on the ground to keep standing, and especially
if we want to use earth itself as a construction material, we’re going to need some geotechnical
engineering. Soils are frictional materials. Rather than being held together by molecular
bonding like steel or by a binder like the cement in concrete, their strength almost
completely depends on internal friction between the soil particles themselves. If we want to avoid sliding, the frictional
force can be considered the shear strength. The more friction, the more strength against
shearing. Just like the simple block on the plane, the
shear strength of soil depends on the internal forces too. But unlike that example, soils have an infinite
number of potential sliding planes all at once. Let’s look at a sample of soil and apply
a vertical force. If you analyze a horizontal failure plane,
our force is completely perpendicular, or normal, so it is increasing the shear strength
of the soil. But if we look at an angled plane, things
change. On this plane, the force is partly acting
normal, increasing the strength, but it’s also partly acting in parallel to the plane
increasing the shear stress. The steeper the angle of the failure plane,
the more the vertical force contributes to shear stress and the less it adds to the shear
strength. If the shear stress exceeds the strength,
sliding occurs and we say that the material has failed. This is why granular materials generally can’t
stand vertically. The weight of the material itself is enough
to cause a shear failure along an angled plane. Pour out some sand on a table, and you’ll
notice that the pile forms a slope. The angle of this slope is called the angle
of repose which is the steepest angle at which a soil can naturally rest. In other words, this is the slope at which
the shear stresses within the soil due to its own weight are exactly equal to the shear
strength caused by internal friction. Any steeper and the soil will slide. Let’s look back at our sample of soil. If we put the sample back in the ground, now
it’s surrounded by additional soil that can apply horizontal pressure. This is called the confining pressure, and
it helps to balance out vertical forces like the weight of the soil itself. This confining pressure is the reason that
a granular material can be stable at a slope, but usually won’t be stable vertically. This can be a problem if you’re trying to
build an earthen structure for two reasons. First, it takes about twice as much material
than if you’re using something that can stand vertically. And second, is space. In crowded cities, space is at a premium. If you’re building an earthen structure,
every foot that you go up in height, you have to go out that far as well, or even further. So what’s a geotechnical engineer to do? What if there was a way to add confining pressure
to the soil, without having to build on a slope. Behold, reinforced earth. Just like rebar in concrete, you can create
an incredibly strong composite material with soil just by adding reinforcing elements. A wall created in this way is called mechanically
stabilized earth, or MSE. And if you look closely, MSE walls are everywhere. Here’s a quick demonstration of how this
works. I cut up some circles of paper towel and layer
them into the sand in this cup. Without any reinforcement, the wet sand can
stand up vertically, but as soon as you apply a load, you get failure. Even just a few discs of paper towel to reinforce
the soil allow the sand to hold up this 15 pound weight. So what’s happening here? The tension in the reinforcement is generating
confining pressure in the soil. This pressure acts perpendicularly to the
failure planes, increasing the shear strength of the sand. Building an MSE wall in real life works exactly
the same way, and they are primarily used in highway projects, especially on the approaches
to elevated roadways. Compacted soil is added in layers with reinforcing
elements in between each layer. Most MSE walls have a facing of interlocked
concrete panels usually with some kind of decorative pattern, and these facing systems
are what make them so recognizable. Any time you see a vertical wall of tessellated
concrete panels, you can almost be sure that there’s reinforced earth behind it. By the way, the only purpose of these panels
is to look nice, and keep the soil on the edges of the wall from raveling. The wall would be completely stable without
the concrete facing, just not as pretty. So how strong is an MSE wall? The simple answer is stronger than you would
think. Let’s try one more demo. I built an 8” cube out of plywood. Just like the cup, I layered in sand and squares
of reinforcement. For this first test, I used scraps cut from
an old t-shirt. This part is not to demonstrate the strength
of MSE, but rather to show that the soil really is transferring load into the reinforcement. The t-shirt material is stretchy, so when
the vertical load is transferred into the reinforcement, it spreads out and deforms. I wanted to demonstrate this because it may
not be obvious in the next example. For the next test, I used pieces of fiberglass
window screen as reinforcement. This is a much stiffer material that does
not deform under load. Under about 70 pounds it didn’t budge. Under my weight, even bouncing up and down,
it didn’t budge. Let’s try something heavier. For the sake of science, we should probably
have a control test with no reinforcement. But this is an engineering channel, not a
science channel, and we all know what would happen to a block of dry sand under a car
wheel. This is probably on the order of 600 pounds
and you can barely even see movement as the weight of the car is transferred to the cube. Unfortunately, I don’t have a hydraulic
press, so my Mazda grocery hauler is about the heaviest thing I could think of to test
the homemade MSE cube, so I moved on to dynamic loading. I dropped this 25 pound barbell from
about 6 feet up to simulate what would happen if you drop a 25 lb weight on the cube from
6 feet up. Almost no damage. In fact if I had a facing system to keep the
edges intact, you probably wouldn’t have known the difference. Dirt was probably your first construction
material. We are born geotechnical engineers, trying
to build taller and stronger earthen structures from probably even before we could talk. With a little bit of reinforcement, we’ve
transformed this dirty propensity into a simple, inexpensive, construction material that you
probably drive over every day. Thanks for watching, and let me know what
you think.