To provide a method for preventing the decay of wood when the wood is placed and installed as a foundation on a ground surface before the construction of a structure in case where the structure or earth fill is constructed on soft ground, and a method which dispenses with chemical treatment, heat treatment, air blocking processing of a head of the wood, etc., which makes the wood positioned above a groundwater level in the installation of the wood, which facilitates work and which makes the whole upper portion of the wood sink below the groundwater level as time passes.

A relationship between a void ratio e of the ground and a placement load P applied by a structure created by a consolidation test of soil of the soft ground is determined; a void ratio ef corresponding to increased placement pressure assumed from this Fig. is determined; and assumed settlement ΔH is determined. On the other hand, the groundwater level ELw is measured; and the wood is placed or installed in such a manner that an upper end ELt of the wood is positioned above the groundwater level ELw within the settlement ΔH.