The drainage hole is usually indispensable to seepage control for hydraulic engineering

which influences greatly the safety and stability of structures. Thus optimization design of drainage holes is very important and it is necessary to simulate drainage holes correctly in convenience. Various schemes of drainage holes should be studied for optimization design

which contains multiple factors

while the preprocessing work of seepage analysis is troublesome and time consuming because of the existance of drainage holes. The orthogonal design method is adopted to reduce the times of seepage analysis for fewer schemes of drainage holes. The composite element method(CEM) is adopted for the convenience of preprocess

by which the drainage holes needn’t be meshed out and optimization design needs only one set mesh for different schemes of drainage holes. The optimization design of drainage holes based on orthogonal experiments and CEM contains the following steps: determination of testing indexes

factors and levels; selection of drainage hole designs by the corresponding orthogonal table; seepage analysis by CEM; range analysis and selection of the optimized scheme. The optimized design of drainage holes can be obtained by the method

which is valuable for engineering application. The feasibility and effectiveness of the method are verified by practical project cases.