ORIGINAL ARTICLE Dynamic behaviour of soils used for natural turf sports surfaces Igor Guisasola • Iain James • Victoria Stiles • Sharon Dixon Published online: 12 March 2010 Ó International Sports Engineering Association 2010 Abstract The modulus and damping properties of soils in compression are a function of soil type, water content, stress history and loading rate. To model human–surface inter- action with natural turf sports surfaces, stiffness and damping properties must be determined at dynamic loading rates. Two contrasting soil types, a Sand and a Clay Loam, commonly used in sports surfaces were loaded uniaxially to 2 kN at loading rates between 0.6 and 6 kN s -1 in modified dynamic soil testing apparatus. Soils were compacted prior to loading but initial cycles resulted in viscoplastic defor- mation, with strain accumulation with repeated cycles of loading. Ultimately a resilient, viscoelastic steady-state equilibrium with loading was established. Resilient modu- lus and damping ratio varied with soil type, water content, stress history and increased significantly with loading rate. The resilient modulus of the Sand soil, typical of modern free-draining sand construction natural turf sports surfaces, was significantly greater than that of a Clay Loam soil more characteristic of traditional natural turf surfaces; reducing water content caused an increase in modulus and a decrease in damping ratio in the Clay Loam soil. Determination of these properties provides initial data for the modelling natural turf surface behaviour in terms of both ball and human interactions, with further research required to determine the effect of both grass roots and leaves on mechanical behaviour. Keywords Sports surface  Natural turf  Sand construction  Soil  Soil mechanics  Stiffness  Resilient modulus  Damping  Soil dynamics 1 Introduction The dynamic behaviour of a sports surface is critical in determining the dynamics of humans/animals interacting with that surface and the performance of the sport being played. The human–surface interaction when running comprises three key components: the human; the footwear and the surface. The dynamics of human movement in sport have been described extensively in the literature [1–4]. The dynamic stiffness and damping of different footwear, in isolation, and in combination with different synthetic sports surfaces, have also been described [5, 6]. Dynamic studies of the elastic behaviour of synthetic sports surfaces including polymeric athletics tracks; indoor sur- faces and synthetic turf surfaces have been conducted using the Artificial Athlete Berlin [7, 8]. Alternative equipment such as the Clegg Impact Hammer [9–11] and the ASTM F355 device [12] have been used to characterise similar properties in natural turf. The dynamics of sports surfaces, in particular stiffness and damping characteristics, are important in player per- formance, player comfort and stability, ball–surface inter- actions and player energy consumption [1–4]. Running speed has been shown to be maximised at an optimum track stiffness in experiments using wooden running tracks with different stiffness values (100 and 195 kN m -1 ) to validate mechanical model predictions [1]. Where a very I. Guisasola  I. James (&) Centre for Sports Surface Technology, School of Applied Sciences, Cranfield University, Building 42, Cranfield MK43 0AL, UK e-mail: i.t.james@cranfield.ac.uk URL: http://www.cranfield.ac.uk/sas/sst V. Stiles  S. Dixon School of Sport and Health Sciences, University of Exeter, Exeter EX1 2LU, UK Sports Eng (2010) 12:111–122 DOI 10.1007/s12283-010-0036-1