Bridges at the Beach

To a nine-year-old future engineer’s eyes, the beach is a giant construction site. Mounds of sand become arch bridges, soaring castles, and tall spires. Entire cities and civilizations rise and fall between the tides.

When I visited the beach as a kid, I would build from dawn till dusk. I would stake out a good patch of sand not too far from the edge of the water, but not so close that the waves would undo my creations. At a young age I had developed a rough feel for urban planning. I designated patches of sand for cities and areas for the farms and hinterlands in between. They were sand farms, but I imagined that the sand was covered with corn fields, farm houses, and cattle. My sand cities had medieval walls and turrets. The streets were winding and mysterious, and the buildings had carefully crafted shapes formed by my collection of clam shells and plastic shovels.

The cities had to be connected to each other, and they didn’t have sand airports in those days, so I designed and constructed a vast network of expressways. You would think that walled, medieval cities and modern limited-access highways don’t mix, but this historical inaccuracy didn’t bother me at the time. Later I found that the juxtaposition of the two actually existed in real life. This combination is a common feature of the British countryside, where sleek twenty-first century motorways designed for high-speed driving dead end in thatch-covered villages with lanes barely wide enough for cows.

Beyond the city walls and surrounding moat, my sand expressways featured wide, smooth roads for the toy cars. My interchange designs were elaborate. I dug sand arch bridges for the grade separation, and learned roughly how to deal with cut-and-fill sections when connecting the ramps. I wasn’t using a computer or doing any calculations, but playing in the sand gave me plenty of opportunities to iterate and try different layouts, and to get a feel for what worked and what didn’t.

At some point, there was always a big river or other obstacle that the expressway had to cross. I dug out giant pits with my clam shells and left a wall of sand in the middle of the pit. Carefully patting down and trimming the wall, I dug a series of arch shapes through the wall, and it became a bridge crossing the pit. These sand bridges were grand structures. They looked like miniature Roman aqueducts.

After a good ten hours or so of continuous design and construction, my sand civilizations reached their apex. By then I was the color of a lobster, because those were the days when being out in the direct sun was thought to be good, and people actually put cocoa butter on their skin to enhance the burn. As twilight approached, I would sit back and admire my creations. Except for the sounds of crashing waves and crying seagulls, the sand cities were deathly still. But I imagined that they were teaming with life. People streamed on the winding streets, trucks cruised on the expressways, plows tended to the green but sandy farms. In the distance I heard my name being called—a real call, not my imagination—and it was time for me to go. The sun set on my creations, and overnight another civilization was lost to antiquity.

Playing in the sand is thought to be an activity reserved for children. But a sand sculpturer can learn a lot of civil engineering basics while ostensibly playing. Sand is a great building material, although it has its limitations. When compacted and moist, sand can handle compression well. But it can’t resist tension at all. So sand bridges must have arch shapes, because beams made out of sand don’t work very well. I became frustrated with this limitation, and one summer I brought a box of wood blocks along on our summer beach vacation. Then I built beam bridges for my expressways. When that worked, I got some twine, and designed some suspension bridges as well. With sand, wood blocks, and twine I was able to build some nice, long bridges. I had built suspension bridges out of blocks at home, but the bridges were better at the beach. Unlike on my bedroom floor, I could dig out vast rivers and canyons to cross, and use the blocks and sand to build good anchorages.

When building my bridges at the beach, I got a feel for the advantages and disadvantages of working with different materials. The wood blocks were best for all applications, but I had a limited supply, so even though they held up better in compression than the sand, it made sense to use them only as beams and not as columns. This is not unlike the limitations posed by use of concrete and steel. Steel is much stronger than concrete, even in compression, but concrete ends up being the better choice for columns because it is much less expensive and is more constructable than steel. But like sand, concrete can’t handle tension at all. At a minimum, it must be reinforced with steel to handle tension stresses from bending.

Playing in the sand provides an early lesson in soil mechanics. One of the best places to build is the stretch of sand at the ocean’s edge. Some days, instead of avoiding the ocean, I would challenge it. I would build sand castles with protective walls as close as possible to the pounding surf. These were high-maintenance castles because the protective walls had to be built and rebuilt to rebuff the attacking waves. If the tide was going out, the battle eventually would be won, and I would build supplemental walls closer to the surf to up the ante. Tides going in the opposite direction, though, were a problem. After awhile, I couldn’t place enough wet sand on the seawalls to keep out the charging ocean. The walls would be breached, and the castles were doomed.

When digging a hole near the water’s edge, you quickly reach a point not too far down when water fills in the hole continuously. This is the phreatic surface. Later presentations in college about the water table and the principles of effective stress make a lot more sense when you have beach experience under your belt. Playing with the water table and qualitatively experiencing its behavior is a terrific way to understand the analytical characterizations. It may be a good idea for classes studying soil mechanics to take a field trip to the beach and dig in the sand.

An experienced nine-year-old knows that sand behaves differently when it’s wet. If you fill a pail with dry sand, turn it upside down, and then pull the pail, you get a pile of dry sand, not a pail-shaped column. Mix in a bit of water, on the other hand, tamp it down, and then pull the pail, and the mix stands by itself and hardens into a nice architectural shape. Therefore, the mix and construction sequence are important for getting a strong, workable construction material. This is not unlike building with concrete, although beach sand is a lot simpler to work with and has fewer additives.

Sand is an expressive material that is great for sculpture. Some oceanside communities sponsor sand castle competitions. The entries in these contests feature traditional castles, as well as all sorts of other sculptures and creations (Figs. 1 and 2). These entries were constructed by participants a lot older than nine years, so it looks like I’m not the only adult still playing with sand. As a kid I once entered a sand castle contest. I built one of my specialties, a traditional castle with some elaborate sand draw bridges crossing the moat. The judges were impressed. I won a little trophy prize, which was something I treasured growing up. Later I realized that all the kids won a prize, but this didn’t diminish the sense of achievement.

I’m no longer nine years old, but I still like to build castles at the beach and play with blocks. In fact, put me in a room full of blocks at the children’s museum, and I’ll spend an hour or two building giant cantilever structures and different bridge forms. Going to the beach as an adult, it helps to have kids to build along with me. At times I become the chief engineer, managing the design instead of doing it directly by myself. This is a change from when I was the kid master builder. Time and adulthood have led to other changes. In addition to being much taller, married, and having adult things like a car, a job, and my own children, now I see the world from my advanced, mature adult perspective (to a limited extent, some friends would argue). Unlike the nine-year-old who developed an intuitive feel for the design and construction process, now I know, analytically, what I’m doing. I know about stress and strain, and I can prepare analyses to model the material behavior.

Otherwise, it’s pretty much as it was. When I’m at the beach with a crew of kids, the infrastructure of vast civilizations rises from our handiwork. We engage in the process: conceptual design, iteration, implementation. When the sun sets, the junior engineers and I look out over what we’ve built. Shadows form on the turrets and spires. The imaginary people commute on the sand expressways to their waiting homes in their walled cities. In play, we have participated in the act of creation. This really is what engineering is all about.

Brian Brenner is an associate at Fay Spofford Thorndike in Burlington, Massachusetts. He can be contacted by e-mail at [email protected].