Over the years in shoring design, it has been common practice to use 3x12 wood lagging in conjunction with steel soldier beams for excavation shoring. This lagging has always carried the specification of "..pressure treated with a minimum retention of preservative of 0.4 pcf approved for ground contact", which follows the AWPA guidelines. Whether used as backwall formwork or not, this lagging is typically left buried. Recently, some clients have objected to this. Does anyone out there have any adverse experience with this practice, or can cite any studies on the matter?
We are designers and installers of sheeting and underpinning in Washington, D.C. and get asked this question about once a year. Millions of square feet of untreated timber lagging have been installed in D.C. over the last 50 years with no evidence of any problem. We think the specification for treated timbers is being misapplied when used for temporary sheeting projects. Its intended use is for timber retaining walls, a permanent or semi-permanent earth-retaining structure. In the case where the sheeting will be out of service once the new concrete structure has been constructed, of what use is treatment of timbers? Also, the requirement to remove the timbers (except the top few feet) if not treated is just plain silly.