150 mm in length and 230 mm in width the keyholes 320 are located such that once the slab has been fractionated each resulting block 324 326 328 and 330 is provided with keyholes 321 which will be useful in the case of using the retaining members the block 324 in the present embodiment may be provided with a fractionating groove 336 while block 326 is provided with a fractionating groove 338 fractionating groove 336 extends from the end wall 318 to the side wall 312 at an obtuse angle to the longitudinal axis and in fact can be seen to form a right angle triangle between side walls 312 end wall 318 and the base of the triangle formed by a fractional groove 336 the block would not normally be separated at fractionating groove 336 unless it is required to form a curved radius in the retaining wall in which case a number of blocks would be fractionated on site along a fractional line such as fractional groove 336 in order to provide an end face with an angle so that when merged with other blocks a radius or curve can be defined the block modules 326 and 328 could be fractionated along lines 338 and 340 respectively as part of the mold cycle thus.