In a turbulentwall shear flow, we explore, for the first time, the stochastic mechanics of loose boundary particle transport, having variable particle protrusions due to various cohesionless particle packing densities. The mean transport probabilities in contact and detachment modes are obtained. The mean transport probabilities in these modes as a function of Shields number (nondimensional fluid induced shear stress at the boundary) for different relative particle sizes (ratio of boundary roughness height to target particle diameter) and shear Reynolds numbers (ratio of fluid inertia to viscous damping) are presented. The transport probability in contact mode increases with an increase in Shields number attaining a peak and then decreases, while that in detachment mode increases monotonically. For the hydraulically transitional and rough flow regimes, the transport probability curves in contact mode for a given relative particle size of greater than or equal to unity attain their peaks corresponding to the averaged critical Shields numbers, from where the transport probability curves in detachment mode initiate. At an inception of particle transport, the mean probabilities in both the modes increase feebly with an increase in shear Reynolds number. Further, for a given particle size, the mean probability in contact mode increases with a decrease in critical Shields number attaining a critical value and then increases. However, the mean probability in detachment mode increases with a decrease in critical Shields number. © 2017 Author(s).