The Measurement of Applied Pressure at Depth with Two Natural Soil Surfaces at Different Densities

Abstract

To advance the engineering of natural turf sports surfaces it is necessary to characterize the stress states and paths of the loads applied by athletes during activity. Such loads are transitory and dynamic. In order to characterize the pressure distribution in a natural soil sports(?) surface a novel experiment was conducted in the 20 m long, 1.8 m wide, 1.0 m deep soil dynamics laboratory at Cranfield University. Two soil surfaces of 1460 kg m⁻³ and 1590 kg m⁻³ were constructed from a sandy loam soil (66% sand, 17% silt and 17% clay). Hardness (0.5 kg Clegg impact hammer) was 125 and 235 g, and maximum penetration resistance 1200 and 1800 kPa, respectively. Seven subjects (57–85 kg body mass) were asked to run at a constant speed of 4 m s⁻¹ (±5%) over each surface, three times, in three different types of footwear used in soccer. Loading and unloading of the soil surface was measured using a ceramic membrane pressure transducer of 19 mm diameter, aligned to the vertical and buried at 100, 200 and 350 mm below the surface. Pressure data were recorded at 5 kHz and processed to determine peak pressure and loading and unloading behaviour of the soil surface. ANOVA determined maximum pressure for the two surfaces was significantly lower at 350 mm (7–15 kPa) and 200 mm (2–3 kPa) than at 100 mm (52–61 kPa) depth (p=0.05) but that there was no significant difference between the two surfaces at any particular depth. Maximum pressure at 100 mm depth was linearly correlated with subject weight (for Subjects 3–7). Loading and unloading behaviour of the soil showed a pattern of bimodality, caused by heel strike and push-off, similar to biomechanics running experiments conducted with force plates. These results suggest that in soil surfaces, pressure distribution at and below 200 mm is independent of surface density or subject, but that mechanical properties such as density and stiffness must be considered in the top 100 mm of a surface. This research also demonstrates the applicability of in-surface pressure transducers in integrated soil mechanics and biomechanics testing.