Downslope Weakening of Soil Revealed by a Rapid Robotic Rheometer

Moving down a hillslope from ridge to valley, soil develops and becomes increasingly weathered. Downslope variation in clay content, organic matter, and porosity should produce concomitant changes in soil strength that influence slope stability and erosion. This has yet to be demonstrated, however, because in situ measurements of soil rheology are challenging and rare. Here we employ a robotic leg as a mechanically sensitive and time-efficient penetrometer to map soil strength along a canonical temperate hillslope profile. We observe a systematic downslope weakening, and increasing heterogeneity, of soil strength associated with a transition from sand-rich ridge materials to cohesive valley bottom soil aggregates. Weathering-induced changes in soil composition lead to physically distinct mechanical behaviors in cohesive soils that depart from the behavior observed for sand. We also demonstrate the promise that legged robots may use their limbs to sense and improve mobility in complex environments, with implications for planetary exploration. Our infrastructure and livelihoods are literally built on soil. The strength of soil governs its stability under disturbance-with impacts on agriculture, ecosystems and natural hazards. Soil strength is very sensitive to variations in composition including clay, organic material, and water content. This means that strength must be directly measured in the field, but such measurements are rare because existing techniques are expensive, time consuming, and require specialized equipment. We develop a robotic soil strength tester that mimics intrusion behavior of plants and animals, and that allows rapid and reliable measurements. We use this device to demonstrate that soil gradually softens along a natural hillside, as it becomes progressively enriched with clay and organic matter downslope. Our device is based on a robot leg, and represents a significant step toward the development of legged robots that are capable of mapping soil strength by walking. Soil strength controls hillslope stability and erosion, but is difficult to measureA novel robotic soil rheometer reveals downslope soil weakeningWeakening results from increasing porosity associated with soil weathering