2007 Volume 60 Issue 2 Pages 13-24
In Japan, small wooden dams were built to prevent soil erosion and control sediment discharge. However, the wooden materials deteriorate under the actions of fungi that are controlled by temperature, oxygen and humidity, which cause the durability (service time) of wooden dam shorter than that of concrete and steel crib dam. Therefore, it is very important to evaluate the durability of dam in order to serve effectively for designing and maintenance works. The purpose of this study is to develop a new design method with considering the deterioration rate that allows evaluating the durability of crib dam. We defined a new term of the deterioration rate and have conducted the field measurement of actual deterioration rates of Cryptomeria japonica D.DoN and Chamaecyparis obtusa ENDL. at four wooden dams in Kyoto Prefecture. The actual deterioration rates of Cryptomeria japonica D.DoN and Chamaecyparis obtusa ENDL. were obtained. The diameter and cross sectional area of the sound part of deteriorated log were calculated using the deterioration rate and the time after completion of dam. Then the maximum bending, shear and tensile stresses acting on deteriorated log were calculated using the diameter and the cross sectional area of the sound part. We proposed three formulas for evaluating the durability considering the maximum stresses equal to allowable stresses of wood. The parameters in three formulas showed that the durability of small wooden crib dam depended upon the deterioration rate, log diameter, log length, dam height, and overflow depth. To increase the durability, the log diameter needs to be increased, and the dam height, overflow depth, as well as the log length need to be decreased. In an application of the new design method, a new diagram was also developed to show the durability of crib dam built with Cryptomeria japonica D.DoN.