Sand is typically prone to crumble, but it can still be made into various beautiful sandcastles as usually seen on beaches. In 2021, the remarkable 21.6-meter (69 feet and 5 inches) sandcastle was recognized by the Guinness World Records as the tallest sandcastle structure. Achieving such stability requires the optimal blend of sand, water, and air to defy gravity and endure the elements.
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Science of Sandcastle: Understanding Composition and Water's Role in Building Strong Sandcastles
Joseph Scalia, a geotechnical engineer at Colorado State University, explained in an article that sand is composed of mineral particles ranging in diameter from 0.003 to 0.187 inches (0.075 to 4.75 millimeters). Conversely, particles of a smaller size are referred to as clay or silt, whereas gravel particles are larger in scale.
While sand provides the structural framework for sandcastles, water is essential for their cohesion. When water is added to dry sand, it adheres to the grains' surfaces, forming small water bridges where grains meet, explains sedimentologist Jim Best from the University of Illinois Urbana-Champaign.
These bridges gain strength from the surface tension of water, a property that compels water droplets to form compact shapes. In the presence of air-filled pores between grains, water bridges can form, effectively holding the sand together through suction, as elucidated by Best.
Achieving the optimal water balance is crucial for building a robust sandcastle. Insufficient water leads to crumbling due to inadequate bridging, while excess water eliminates pores and bridges, turning the sand into a slurry.
Experiments conducted by sedimentologist Matthew Bennett and colleagues in 2004 determined that the ideal sandcastle mixture consists of one part water to every eight parts dry sand, typically found near the high-tide line on beaches during low tide when the sand is visibly moist.
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The Science Behind Optimal Sand Selection and Construction Techniques
To bolster the resilience of sandcastles, opting for angular grains with sharp edges can be beneficial. According to Best, envisioning sand grains as perfectly spherical spheres diminishes their stability compared to more angular grains.
Moreover, the size of sand particles dictates the sand's appearance and texture, ranging from fine powder to lentil-sized grains. While any sand can serve for building sandcastles, optimal sand possesses diverse grain sizes and angular or rough edges to enhance structural integrity.
The snug fit between angular sand grains amplifies the influence of water surface tension while interlocking angular grains offer increased resistance to collapsing. This preferred sand is often sourced from riverbeds rather than coastal beaches, where erosion hasn't yet dulled the grains' edges.
Professional sandcastle builders often blend a fraction of clay with sand to enhance durability. This addition allows smaller clay particles to fill gaps between larger sand grains, facilitating the formation of more water bridges for robust construction.
Furthermore, compacting wet sand contributes to sturdiness by fostering interlocking among angular grains. While seawater salt crystals can aid in stabilizing sandcastles as they evaporate, resulting structures remain fragile and prone to cracking.
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