Secant pile walls are formed by interlocking reinforced concrete rotary bored piles. The piles are are constructed by either drilling under mud or with the double rotary method (cased CFA) by using drilling rigs such as our Liebherr LRB 255 and LRB 355. Alternate primary piles, comprising unreinforced concrete are installed first; then secondary, reinforced concrete piles are installed in between the primary piles, once the latter gain sufficient strength. The pile overlap is typically of the order of 100mm to 150mm depending on the depth of excavation and degree of ‘watertightness’ required.