The Grid That Doubles the Strength of the Ground

[00:00] The Port of Long Beach in Southern California  is the second-busiest container seaport in the   United States, a major gateway for goods flowing  to and from the west coast. Together with its  

[00:13] next door neighbor, the Port of Los Angeles,  roughly half a TRILLION dollars worth of trade   moves through the port every year. To keep up  with the staggering growth of shipping demand,   the port is in a nearly constant  state of improvement and expansion.

[00:29] In the early 2000s, Long Beach was expanding  the container storage yard at the Pier T Marine   Terminal, but engineers hit a problem. The  area they were expanding into was an old,  

[00:41] disused dry dock, much lower than the rest  of the yard. They were going to need a lot   of fill to bring it up. There aren’t many  more difficult paving challenges than a   container terminal. You have constant heavy  traffic of cranes, forklifts, and yard trucks,  

[00:58] and the local fill materials that are available  are junk: soft, water-logged silt scraped   from the bottom of the ocean that has the  structural integrity of a cake in the rain. Traditionally, you would have to dig out all  the muck, then haul in a literal mountain of  

[01:14] expensive backfill, a process that would cost  millions of dollars and take years. Instead,   the engineers turned to a solution that  looks more like a giant plastic accordion   than a structural foundation. This 3D network  did more than just hold the soil in place;  

[01:31] it physically transformed those mushy dredged  spoils into a high-capacity platform capable of   supporting 100-ton machines and rows  and rows of heavy container stacks.

[01:43] The prefix “geo” gets a lot of work in the  world of engineering. Just tack it on something   semi-technical-sounding, and there’s a good chance  there’s a product out there. Geotextile, geogrid,  

[01:57] geofoam, and more. Today I want to show you how  geocells work, and of course I built a little   model in the garage so you can see it in action.  I’m Grady and this is Practical Engineering.

[02:18] When you think about engineering, it’s easy  to conjure the typical materials we use:   steel, concrete, bolts, gears,  electrical components, pipes,   beams, and so on. The first thing to  come to mind typically isn’t dirt.  

[02:33] Most geotechnical engineers prefer the alternative  four-letter words of “rock” or “soil” to describe   their medium of choice. But whatever they call  it, they don’t really get to choose what it is.  

[02:46] Where most engineers work with materials that  adhere to stringent specifications, geotechnical   engineers work with the materials nature gives  them, which, by the way, are often terrible.  

[02:58] So much of our built environment completely  depends on our ability to predict and improve   the behavior of soil and rock below our feet.  And those materials are so entirely different  

[03:10] from really anything else that engineers  work with, particularly in how they fail. When you walk on the ground, you don’t really  even consider that it can quote-unquote “fail”   under load. But just like a bridge has a maximum  weight limit, every patch of dirt, sand, clay,  

[03:28] or loam has a specific amount of pressure it can  take before it physically gives up. Of course,   most soils can squish a little bit, what we  normally call settlement. But it gets worse  

[03:40] than that. Soil can’t just disappear under a  vertical load, it doesn’t crush like concrete   or bend like a steel beam. Instead, it shears. The  friction between the soil particles isn’t enough  

[03:53] to resist the pressure in the soil, so they  slide along each other in a plane. Generally,   it looks like this: the soil moves down, then out,  then up, causing anything on top to sink or tip.

[04:06] This is called a bearing capacity failure. It’s  one of the most important failure modes of soil,   and it has to be considered when  designing just about any type of   structure (since everything sits on the  ground). Engineers for buildings, dams,  

[04:21] retaining walls, and of course, roadways  have to contend with this limitation. If you’re wearing high heels on a muddy lawn,   you sink; if you put down a wide  piece of plywood and stand on that,  

[04:36] you don't. In engineering, this means building  wide concrete footings to distribute the weight   over a larger area of the ground. The problem is  that concrete is expensive. For many structures,  

[04:48] spread footings make a lot of sense. But there’s  a reason not every roadway is paved with it. Another option is just to replace the subgrade. If  the dirt is mushy, you dig it all out and haul it  

[05:00] away. Then you bring in so-called, “select-fill”  an ambiguous term that really just means whatever   material the engineer has determined will  be able to support the load. For roadways,  

[05:12] it’s usually high-quality, angular crushed stone.  But you don’t want just large chunks of gravel;   you need some finer particles to help lock  everything together. That’s generally called  

[05:25] “road base,” and it does most of the heavy lifting  when it comes to both paved and unpaved roads. But   how much of it you need is an important decision.  The stress from a load spreads out with depth,  

[05:38] dissipating as it reaches downward. The layer  of road base has to be sufficiently thick that   the remaining pressure from that “stress cone”  is low enough for the native soil to support   it without failing. The weaker the subgrade, the  thicker the base. Sometimes that can be many feet,  

[05:55] which requires an enormous amount of  digging, hauling, placing and compacting. Footing and subgrade replacement are good  solutions in many cases, but there are situations   where it makes more sense to bring outside  reinforcements to the soil we already have.  

[06:12] These are often known as geosynthetics: humanmade  materials meant to make the ground work harder.   Geosynthetic option 1 is geotextiles, industrial  strength fabrics that go down like a big bedsheet.  

[06:28] These are used across the construction industry,  mainly to act as a physical boundary to keep   individual gravel or rock particles from sinking  into the mud underneath. In some situations,  

[06:40] they’re used as filters to let water flow  through without carrying soil with it. But   they don’t have a lot of strength, and more  importantly, they don’t really engage with the   layer above or below. Geotextiles can  move relatively independently of the soil,  

[06:56] meaning if the loads are heavy enough,  the whole thing will still rut and sag. Option 2 is geogrid, which is exactly what  it sounds like: a stiff plastic mesh used to  

[07:08] reinforce layers of soil. And this basically works  in the exact same way as the steel reinforcement   used in concrete. The benefit over geotextile  is pretty clear. Not only is the material much  

[07:20] stronger, but the grid allows it to grab on  to the material above and below. One of the   first videos on this channel was about reinforced  earth retaining walls. I built a cube of sand with  

[07:33] layers of fiberglass window screen, and it  could hold up one of my car’s wheels. A lot   of the retaining walls you see in the world,  especially on highway projects, use technology   just like this. It really is pretty remarkable  what it can do for even weak soils. But, that demo  

[07:50] had multiple layers of it, pointing toward one of  the limitations of geogrid: it’s inherently 2D. It   can grab the soil near it, but the further  away from the grid you get, the less it’s  

[08:02] able to help. If you need to reinforce a thicker  layer, that’s where option 3 comes in: geocells. If geotextile is a sheet and geogrid is  a mesh, geocell is a 3D structure. It’s  

[08:17] basically formed from plastic strips welded  together in a somewhat-honeycomb shape. And   these strips don’t just engage with the backfill  like geogrid; they encapsulate it. I feel like  

[08:30] this is pretty intuitive. If I drop a heavy weight  on this unconfined sand, it sinks in pretty deep.   But if you confine the sand to a small  area, it you get a lot less sinking. Like  

[08:42] I mentioned before, the soil can’t  just disappear under a heavy load,   it has to shear and shift in order to fail, but  when it’s confined, there’s nowhere for it to go. If you put these confinement cells right next  to each other, they start acting like the  

[08:58] foundation footing we talked about earlier.  It’s spreading the load out a lot more so,   even with heavy vertical forces, you get  a lot lower stress on the potentially weak   soils below. And the advantages of this are a  big deal. Road base may just look like dirt,  

[09:14] but it’s actually a fairly expensive and  specialized material. It has to be quarried,   crushed, sorted, and sometimes hauled over a  long distance. Not every place on earth has  

[09:26] a rock or gravel quarry nearby that can produce  the stuff in large quantities. Just through the   added strength alone, geocells can significantly  reduce the thickness of a base layer you need,  

[09:38] but it’s more than that. Cellular confinement  systems are a lot more flexible when it comes   to the material properties of the infill. You’re  no longer relying on that perfect interlocking   of angular crushed rock and fines to distribute  the load. The plastic does a lot of the work,  

[09:55] potentially opening the door to use cheaper base  materials or even using what’s already there This isn’t really a new thing. The technology was  pioneered by the US Army Corps of Engineers in   the 1970s during the Vietnam war when there was a  need to quickly build landing strips and roads on  

[10:13] soft soils. Even NASA has done research on using  similar technology to build stuff on the moon.   One idea is filling sandbags with regolith,  making them into a sort of building block,  

[10:25] but there is research happening right now on  the use of actual geocells. Maybe they would   be lunarcells? When you consider how light this  stuff is, and how compact it can be for transport,  

[10:37] it just makes sense that it might be a good  fit for extraterrestrial infrastructure. Geocells can improve bearing capacity,  but there’s another failure mode where   they also shine. I released a video about  washboarding on unpaved roadways. You’ve  

[10:53] a dirt or gravel road has somehow formed  rhythmic corrugations that jostle your   vehicle and may even cause you to lose traction.  I built this circular track with a motorized arm  

[11:07] and wheel to simulate the phenomenon in real  time. It's really fascinating how this works,   and I definitely encourage you to check that video  out after this. The one thing I didn’t really dig   deep into in that video is potential solutions.  Well it turns out that one of those solutions is  

[11:25] geocells. So I got my trusty contraption  back out. Let’s see if it really works. There’s not a lot of scientific rigor in this  demonstration. I just looked at a few academic   papers where engineers have done these tests in a  lab and tried to recreate the setup they used. So  

[11:41] I wasn’t really sure how much to scale my geocells  down. But I’m pretty proud of this design. I had   them printed in TPU which is a flexible material  kind of like the real thing. In the real world,  

[11:53] the cells can bulge a little bit under load,  engaging the strength of the material and the   passive resistance of all the neighboring cells.  Let’s see how my model geocells do on the track.

[12:05] If you watched the previous video, you know that  washboard formation is a very speed-dependent   process. When the tires are moving slowly, it  just leaves a rut, not a cyclical corrugation. But   there’s kind of a threshold speed where it really  starts to happen. In my model without the geogrid,  

[12:22] that was about 35 rpm. I have a 10-to-1 gearbox  between the motor and arm. So let’s start there. and you can see there’s basically no change  in the profile of the track. Not even a rut.  

[12:38] There’s a little bit of movement at some  of the geocells, but this loose sand is   basically a worst-case scenario when  it comes to an unpaved roadway. There’s   practically no situation where you would  use loose sand for the surface of a road,  

[12:52] except maybe on an actual beach, and there’s a  reason tow trucks make good money in beach towns. Even in this worst-case material, there’s nothing  even approaching the look of a washboard. So,   let’s bump the speed up a bit. This is about  as fast as I trust this homebuilt contraption to go.  

[13:12] You can see that it’s still remarkably  stable. After quite a few rotations,   one of the geocell sections is starting to “float”  a bit, but normally this would be underneath a   layer of cover. I left the geocells at  the top so you could see them better,  

[13:27] and because it’s kind of a worst case scenario for  the test. Compare the high speed footage between   the unreinforced and the geocell setup. You can  see there’s no skip and push of the particles with   the geocells. The sand stays mostly confined in  that grid. I hope this demo doesn’t feel mundane.  

[13:45] It’s a little hard to show at scale, but this is  a tiny amount of plastic. It’s almost negligible,   at least compared to the weight of the sand in  the track. But it completely transformed the  

[13:57] behavior of the system. A small amount  of plastic makes a huge difference. A while back, I made a video on geofoam and  lightweight fills used on roadways projects   to build tall embankments without overloading the  foundation soil underneath. A lot of the feedback  

[14:13] I got on that video was about the environmental  considerations of putting styrofoam in the ground.   People were just uncomfortable with that  idea, and I totally get it. It’s something   we definitely need to be thoughtful about. But  it’s not something I really worry about with  

[14:29] geosynthetics, and especially geocells. Most of the products out there are made of HDPE, the same material that we use for a lot of underground piping and  

[14:41] conduits. It’s a relatively inert material,  especially when it’s not exposed to UV light. And actually, when you look at the totality of  the situation, there are times when geocells   provide a net environmental benefit. They can  substantially reduce the amount of excavation  

[14:58] needed at a site. They expand the options  for materials that can be used as road base.   They reduce haul distance for specialized fills  because you can often use local materials instead.  

[15:10] In some situations, you might even forgo  pavement like asphalt altogether when it   otherwise would have been justified, because now  you can build a road that’s just as strong and   maintenance free for a lower cost. Geocells  can also use cleaner fill with less fines,  

[15:25] making it more permeable compared to  something like an asphalt parking lot,   reducing the amount of impervious cover that  contributes to flooding. Since you're more able to   use the native materials, you also are less likely  to generate truckloads of muck that you need to  

[15:41] ship somewhere as well. And there are so many  applications, from roadways to retaining walls,   protecting steep slopes from wind and rain  erosion, and of course, shipping terminals.

[15:53] Geocells are not magic, and there are plenty of  times where this technique isn’t the right choice,   but they fit in a really interesting niche.  The only reason engineering exists is because,  

[16:05] in the real world, technical decisions aren’t  simple. Engineers have a job because you have   to weigh all these competing factors of  cost, maintenance, material availability,   traffic volume, and so on, and find the right  balance between them. And I’m fascinated by  

[16:22] innovations like this that find a little  area of that multi-dimensional state space   where all the constraints can be met in  a clever way. Geocells are that for all   kinds of geotechnical projects. Sometimes  it’s not all about more massive concrete  

[16:39] slabs or the brute strength of steel, but just  a little bit of clever geometry and some dirt. Building this sand track was a pretty big project.  I had to learn how to wire up stepper motors and  

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