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replace retaining wall

hi we have an existing timber retaining wall which needs replacing,its 1.4-1.5m Hx19m L, i have 4.2mx 200mm posts and have 150x50 retaining timber the posts will go into solid ground 2m as section below has a 700mm raised garden above existing lawn height which makes wall 1.4-1.5m H posts need to be cemented into ground, there are 15x posts with 1357mm spacing"s, the soil behind wall will need to be dug out down to 1.5m and back filled with pumice. what sort of cost will this cost if posts and timber are supplied? section has easy access and is flat