1) Houston, S. L. and Fredlund, D. G.: Unsaturated soil engineering practice, Geotechnical special publication, No.68, ASCE, pp.146-161, 1997.
2) Hillel, D., Krentos, V., and Stylianou, Y.: Procedure and test of an internal drainage method for measuring soil hydraulic characteristics in situ, Soil Sci., Vol.114, No.5, pp.395-400, 1972.