STUDY ON THE FAILURE BY PIPING IN POLDER DIKES FOR LAND RECLAMATION

抄録

Many investigators have studied piping (internal erosion) which was one of the causes of failure in many polder dikes, but most of the formulas proposed for the critical hydraulic gradient for piping were not sufficient in practical application. Namely, Bligh's and Lane's coefficients (they are referred to as C_b and C_w respectively) and C', which is expressed by C'=l/h=5∼10 are not adequate in the case of polder dikes.Mereover, the following coefficient C, which is derived from Terzaghi's theory, cannot fully represent the practical phenomena.C=C'(1-p)(γ_s-γ_w)/γ_w The author studied the piping phenomena with prototype tests and succeeded in deriving a new formula for the critical hydraulic gradient.In this paper, piping phenomena are classified into three types : (1) subsurface erosion, (2) heave and (3) internal erosion. Investigations on the first two types have been carried out and their definitions were established. The author newly defined internal erosion which had not been studied before, and derived an experimental equation from model tests and prototype tests in order to prevent the failure by piping in enclosure banks for land reclamation.The basic equation is given as I_f=i_s{(1-p)(γ_s-γ_w)/γ_w}ⁿ in which I_f is the critical hydraulic gradient, p is the porosity, γ_s and γ_w are the densities of sand and water, respectively, and i_s and n are the coefficients corresponding to soil characteristics and flow condition. As for sandy banks, the values of i_s and n were found to be 0.2 and 2, respectively, and the following equations were derived.I_f=0.2{(1-p)(γ_s-γ_w)/γ_w}²=(1-p)(γ_s-γ_p)/γ_w γ_p=γ_w+χ χ={(γ_s-γ_w)/γ_w}{(γ_w-0.2(1-p)(γ_s-γ_w} in which χ is a factor corresponding to ouopiping.From the above relationships, the safe seepage length L for a critical head H is given by the formula L=C_n·H·F=5{γ_w/(1-p)(γ_s-γ_w)}²·H·F in which F is the safety factor and C_n is a coefficient derived by the author.The theoretical analyses of these equations are also given.The propriety of the above-mentioned relation was suggested in the restoration work of the enclosure bank at Kasaoka Land Reclamation Project in 1956. This has given a possibility of rational design of the polder dikes of Nagasaki Land Reclamation Project and many other projects in Japan.