a round cross section timber once again typically of the saligna variety in this case the lower end of the member 50 is inserted into the operatively upper end 20 of the sleeve and bears against the steel plate 36 both the plunger 48 and member 50 are snug fits in the respective ends of the sleeve 12 the prop 40 forms part of a mine support indicated generally with the numeral 52 in addition to the prop 40 the mine support 52 includes a preload device indicated diagrammatically by the numeral 54 this preload device may for instance be a hydraulically operated preload unit such as that described in south african patent 95 9113 the disclosure of which is incorporated herein alternatively it may include a headboard and a preload bag which is placed on top of the headboard and which is charged with a settable grout in the latter case the preload device may for instance be of the type described in any one of south african patents 91 4190 92 9685 93 1433 and 94 7397 the disclosures of which are also incorporated herein the present invention is not directly concerned with the nature of the preload device which may in fact have any appropriate design capable of applying an axial preload force of predetermined magnitude to the prop 40 it will be appreciated that in each case the preload device serves to apply a predetermined preload force to the installed prop 40 in some applications of the present invention there may be no preload device at all in such.