Answer: The answer to your question depends on a number of factors, including both the quality and dimensions of the timbers and the condition of the ties when you install them as well as the amount of annual weathering and soil moisture. Construction technique such as the provisions made for drainage behind the wall can also make a big difference. Certainly they will last longer when used in a location where the soil is not saturated constantly. Used railroad ties continue to deteriorate, but they are so large that the damage can usually be tolerated for quite some time. New ties will last longer but in my opinion they tend to stink on warm days for a number of years, so they may not be that desirable in some locations. Treated timbers (for ground contact) usually have an estimated lifetime which may vary depending on how they have been prepared. Naturally rot resistant wood such as cedar or redwood will also last for a good number of years, but again the length of time will depend on the dimensions and the amount of weathering and soil moisture they are exposed to.
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