a plurality of longitudinally extending transversely spaced grooves 67 which are situated generally in the central region of wall 66 a the top surface of wall 66 a is further provided with longitudinal channels 68 which are positioned outwardly from the grooves 67 and inwardly from the outer lateral extremities 66 g and 66 h of the flooring section 66 the flooring section 66 is further provided with a plurality of transversely spaced vertically extending intermediate walls 66 b that project downwardly from the top wall 66 a to define a number of laterally adjacent cells the flooring section 66 has basically two types of cells which are defined by the walls 66 b i the cells 66 c which are generally situated in the central region of the section 66 and inboard of the longitudinal channels 68 and the cells 66 d which are located beneath the channels 68 at the lower end of each of the walls 66 b rib like foot elements 66 e and 66 f are provided as may be observed by referring to fig6 the foot elements 66 e and 66 f have somewhat different cross sections and the elements 66 f may differ in their directional orientation relative to one another however the elements 66 e and 66 f function similarly to the foot elements 58 of flooring section 56.