that the aggregate is allowed to stay in the formed cavity and contacting the cavity wall by the raising movement of the mandrel in contrast passive restriction of upward flow of aggregate is achieved during driving of the mandrel 1 as a result of engagement between aggregate and restrictors 16 the number of passive flow restrictors 16 will vary depending on the length of the bottom portion 2 further as previously noted the flow restrictors 16 will extend into the center of the bottom portion 2 an amount sufficient to restrict upward flow of aggregate during tamping but without substantially preventing the aggregate from remaining at the bottom of the cavity upon raising of the mandrel 1 in all other aspects the embodiment of fig1 17 is otherwise typically the same as the embodiment of fig1 10 in the operation of the embodiment of fig1 17 as before the mandrel 1 is driven to the design depth if the sacrificial plate 3 is used the hopper 5 is again also filled with aggregate after driving to the design depth alternatively as in the case of the embodiment of fig1 10 the aggregate may be partially or fully filled inside the mandrel 1 and bottom tamper head 2 prior to driving and the aggregate is engaged by the passive flow restrictors 16 to form a temporary aggregate plug in the bottom portion 2 of the mandrel 1 so that soil does not enter the inside of the mandrel 1 during driving once the mandrel 1 reaches the design depth and the mandrel 1 is raised slightly the sacrificial plate 3 or the temporary.