The Dangers of Riding On Clay

I had so much fun riding the antenna hills trails that I returned last Sunday morning to sneak in a ride before flying out for work. I started later than usual, a little before 9:00AM, but it was cold and overcast, and the trail seemed solid, mostly frozen, if not rock-hard. Climbing the Shoreline trail from City Creek I zipped up the switchbacks, and onto the long rolling stretches linking up to the antenna towers road. Flurries blew in repeatedly, and the trail was covered with a think dusting of snow. Visibility was poor, and though the trail is just above downtown, the lack of visibility and other people made it feel as though I was in the middle of nowhere.

Keeping an eye on the time, I returned the same way, and soon began the descent back down the switchbacks into City Creek, and almost instantly knew I’d screwed up. The tires started to slide and throw huge chunks of clay-y mud up onto the bike’s frame, drive-train and me. The mud glommed onto the tires, making the wheels suddenly 2 or 3 times as heavy, and clogged in my brake-calipers, threatening to stop rotation altogether. I cursed and struggled to keep the bike on track, knowing that if I had to walk it, I might miss my flight.

Tangent: You know when I get crankiest? When I realize I’ve done exactly the kind of stupid thing that I’m disdainful of when others do it. I’ve actually blogged about people who ride muddy trails when unfrozen previously, and it pained me to think of another vampire-biker seeing my tracks/ruts and shaking his/hear head.

The most frustrating thing about clay trails is how dramatically a little (unfrozen) water can transform them. Dry or frozen they’re smooth, firm and high-traction- a joy to ride.

But add a little water and they’re sticky, soupy and slippery all at the same time. Wet clay sticks to tires, brakes and other components like nothing else, quickly increasing the bike’s weight by as much as 100% or more. Though it hasn’t (yet) happened to me, I’ve heard stories of bikes abandoned in mud to be retrieved days later, after things have dried out.

I held it together and rode through, clear down to the trailhead and my car. Wasting no time, I heaved the mud-caked bike (weighing probably 40 pounds) atop my vehicle, and drove straight to the do-it-yourself carwash. Foothill-clay-mud is almost impossible to wash off with an ordinary garden hose; I needed the power and warm water of the carwash blaster-nozzle.

Tangent: Yes, yes I know: taking your mtn bike to the carwash is a huge no-no. Water gets in seals, messes up bearings, yada, yada. But like many mtn bikers, I am an occasional closeted carwash visitor. It’s just so much easier and time-efficient than any other conceivable method to remove clay from your bike. I always feel sort of embarrassed, guilty and yet resigned all at the same time when I do it, sort of like a high-school kid who’s just completed an abstinence-only sex ed class: I know the safest thing is not to do it, but simultaneously am aware that given the combination of need and opportunity, I almost certainly will.

So I blasted away, spending the most time on the tires, and eventually the clay relented, loosened its grip and fell away. And as it did so I wondered the same questions I’ve wondered on so many previous carwash visits over the 15 years I’ve lived in Utah: What is clay, and when it gets wet, why is it so sticky, why is it so hard to wash off, and yet why is it so slippery at the same time?

Tangent: Utah mud is a surprise to transplants like me. In other parts of the country, mud is no big deal. People ride through it all the time. In fact mountain bike magazine reviews of tires routinely include descriptions of “mud-shedding” capability. In Utah there’s no such thing. No bike tire sheds mud. A “mud-shedding tire” is an oxymoron.

Utah clay-mud is an even bigger concern for backcountry drivers. Countless unpaved roads in Southern and Western Utah are completely impassable when wet. When I drive deep solo into the backcountry, I generally pack along a mountain bike as an escape pod, meaning that if I break down or get stuck, I can reasonably bike 50, 60, 80 miles or more to pavement and assistance. But in mud, my escape pod is as immobile as my vehicle; there’s nothing to do but wait, either for sun (to dry) or cold (to freeze.)

Ranchers and other folks who routinely drive around the Utah backcountry in fall/spring/winter typically bring along tire-chains, and if you look closely at deep, dried-out tire-ruts on many desert-clay roads, you’ll noticed that they’re cross-hatched with chain-marks.

All About Clay

When off-road and not on solid rock or pure sand, we tend to think of what we are traveling over as “dirt”. Dirt, as it turns out is way, way complicated. A cubic meter of soil contains literally trillions of living things, and constitutes an incredibly complex and sophisticated ecosystem. Soil is fascinating in that it is the fundamental connecting layer between the organic and inorganic worlds, and it’s worth a whole blog in its own right. But in this post we’re going to focus on the fundamental inorganic components of soils- their mineral particles.

Mineral particles are characterized as sand, silt or clay, and all are basically teensy-weensy chips of rock. Rocks are constantly being eroded and broken down, and these smaller particles are products of this process.

Extra Detail: Factors in rock erosion include of course wind, sun and water. You may think you already “get” the erosive effect of water on rocks: waves, streams and raindrops repeatedly pound on exposed rock, and when frozen, water-ice expands in volume, expanding and propagating cracks and crevices. But water- specifically rain- also has a corrosive chemical effect on rock. Raindrops (H2O) combine with Carbon dioxide(CO2) in the atmosphere to create Carbonic acid (H2C03). Over time, the positively charged hydrogen atoms in the acid molecules replace other positively-charged elements in rocks like limestone and granite, causing them to break into smaller pieces.

Lichens also play an important role in the creation of mineral particles and ultimately soil. Lichens, which cover 8% of the Earth’s surface and are extremely long-lived, produce an array of acids which eat into and break down rock surfaces. Mosses also contribute to soil formation, not by breaking down rock directly (they don’t produce corrosive acids) but by sheltering tiny fungi and other organisms that do produce corrosive acids.

The primary difference between sand, silt and clay particles is size. A grain of sand ranges from 0.05mm to 2mm in diameter, while a grain of silt is anything between 0.05mm and 0.002mm. Clay particles are those under 0.002 mm in diameter.

Note: No, I don’t have an electron microscope. The electron microscopy shots in this post were pulled from this extremely informative slide deck on the Washington State University Dept. of Crop and Soil Sciences website.

To put this in perspective, individual “grains” of clay are smaller than typical bacteria. They’re so small that you can’t even see them under an optical microscope; you need an electron microscope to see them. If a “typical” grain of sand were the size of the Earth, a grain of clay would only be about 16 miles across*.

*My calculation, could be wrong.

Up close, clay particles are generally flattish flakes, which means that have more surface area for their size. They’re typically negatively charged and the combination of negative charge and large surface area means that they bond easily to water molecules. Saturated sand is about 20% water. Saturated clay is about 48% water.

When you walk out of the water onto a sandy beach, wet sand sticks to your bare feet. But if you lie on the dry sand for a few minutes, the sun and wind will dry the water molecules holing the sand grains together and to your skin, and the sand will easily brush off. If you’re impatient, you can walk up to one of those beachside showers or hoses and quickly rinse off. The sprayed water drops will easily penetrate the large gaps between the roundish sand-grains, pushing them apart and away.

But if you walk barefoot in wet clay, and then lay back in the sun, your feet will take much longer to dry. The much smaller spaces between flakes are more difficult for sun and wind to penetrate. And when, eventually, the clay does dry, it’ll still take more effort to dislodge than dry sand; the tiny flattish flakes make almost a little concrete later on your skin. More frustratingly, if you walk over to a shower or hose, it’s way, way harder to wash the clay off. The sprayed water drops have great difficulty penetrating the much smaller gaps between clay-flakes, which are already filled/clogged with adhering water molecules.

This is one of the interesting things about saturated clay; it’s virtually impenetrable to water, creating special concerns for hydrologists, builders and… gardeners. Clay soils hold water well, but they drain poorly. Saturated clay soil in a garden can deprive plant roots of access to oxygen, compromising the health of plants. Clay soils are also subject to expansion, contraction, cracking and crusting as they gain and lose water, which can fracture plant roots or prevent seed germination. Gardeners in heavy clay areas typically need to mix local soils with larger particle things like compost and peat moss.

The prevalence of clay in foothill soils is not only a factor for mountain-bikers, gardeners and builders, but also for wild plants. Clay soils (particularly saturated clays soils) take longer to warm up in the spring, and it’s likely that the heavy clay content in the soil of the Wasatch foothills delays spring blooms by a week or two.

Well, that all makes sense. The small flat particles of clay make it sticky, hard to wash off and generally problematic for mtn bikers. But if it’s so sticky, why is it so slippery? To understand this one, we have to know something about the composition and structure of clay.

The Structure of Clay

Clay is a sheet silicate, meaning a silicon-based substance that forms into 2-dimensional crystalline sheets. Silicon is a common element in rocks, and hence clay. Silicon, atomic #14, has 4 open spots in its 3^rd electron shell, and so “wants” to make 4 electron bonds. Another abundant element is of course Oxygen, atomic #8, which has 2 open spots in its 2^nd electron shell. Together, Silicon and Oxygen comprise Silicon Tetraoxide, SiO4, in which the single Silicon atom binds to each of 4 Oxygen atoms, which each have 1 bond with the Silicon atom, and another bond with a fellow Oxygen atom. Together the 5 atoms form a tetrahedron*, with an Oxygen atom at each corner and the Silicon atom in the center.

*A tetrahedron is 4-sided, with a triangle on each face. It looks, at first glance, sort of like a pyramid, but it’s not. A pyramid is 5-sided, with triangles on 4 sides, and a square on the 5^th/base.

The base of each tetrahedron is an equilateral triangle, and the triangles of adjacent SiO4 tetrahedrons fit together to form a 2-dimensional hexagonal matrix, or sheet. With me so far?

OK, now the pointy-“up” corner of the tetrahedron is connected to an Aluminum molecule, together with other tetra-vertices and bonded hydroxyls (OH) to form AlO6, an octahedron (8-sided). So the tetrahedral Silica layer lies adjacent to an octahedral Alumina layer. At a molecular level then, clay is basically a bunch of stacked, alternating sheets, which is why the flakes are flattish under the electron microscope.

That’s the basics. From here things start to get complicated, and diverge into different clay types. The biggest difference is between so-called 1:1 clays, which alternate tetrahedral and octahedral layers, and 2:1 clays, which consist of octahedral layers sandwiched between 2 tetrahedral layers. The other big differentiator is something called isomorphic substitution, which is a fancy term for the replacement of a Silicon or Aluminum atom by something else, such Aluminum (in the tetrahedral sheet) or Magnesium (in the octahedral layer).

Depending on the exact structure of the clay in question, the clays can be neutrally or negatively charged. If negatively charged the various layers of 1:1 or 2:1 clays are linked with an interlayer of positively charged metallic ions (cations), such as Magnesium or Potassium.

Now here’s the thing: in water-saturated clay, positively-charged water molecules can easily take the place of metallic cations, and when they do, the interlayer becomes fluid and slippery. So when you slip on wet clay, the 2:1 or 1:1 individual layers are sliding across each other.

This effect is most pronounced in many of the 2:1 clays, one of the most common of which is a group called the smectite clays. Smectites are expansive clays, hazardous to build on, as they expand when wet and contract when dry. Later in the summer you’ll see cracks and fissures develop along smectite-rich foothill trails as they dry out.

March is month that rewards flexibility. It might be a good day to bike, or it might be a clay-disaster. It might be a great day to ski, or the snow may turn to sun-baked mashed potatoes. It’s challenging and frustrating to plan more than a day in advance. It’s a weird month, sort of a winding, uncertain bridge to Spring. You know where it’s going to end up, but not how it’s going to get there.