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?