to at least one time the diameter for permitting the torque transmission along the whole battery furthermore male insert 5 is provided with centering zones 7 and 8 compatible with respective zones 7 a and 8 a of female insert 6 and can be provided with respective gaskets 9 and 10 the double centering guarantees the perfect alignment among adjacent elements of rods necessary for permitting the correct functioning of the gaskets subject to pressure a unique coupling could anyhow work but it would be much more axially extended and would have therefore a higher realization cost and would require a higher difficulty of insertion during the assembly gasket 10 on the end avoids the leakage of the compacting mixture and at the same time prevents external agents from penetrating as far as internal passage 2 gasket 9 has the function of protecting polygonal coupling 11 and 11 a from the inlet of external agents water soil mixing and so on which could make the disassembly of the rods difficult in inserts 5 and 6 there are spaces 13 and 13 a for the assembly of pins 12 for holding the rods among them in fig5 it may be noticed that pins 12 are assembled with an axis mainly transversal with respect to.