Ways to develop methods of determining permeability of nonsaturated soils

Abstract

During the design and construction of hydraulic structures it is often necessary to estimate the seepage properties of the earth materials of the foundation and abutments, which during surveys are unsaturated. Whereas methods of estimating the seepage properties of saturated soils have been developed to a sufficient degree, for unsaturated materials a number of problems require special investigations. The permeability of unsaturated materials can be estimated by calculation and direct (field) methods, which can be used depending on the required accuracy and reliability of the problems being solved.Calculation Methods of Determining Permeability of Rocks and Soils. The seepage and piping properties of earth masses (rock and soil) are determined by the size, shape, and relative number of water-conducting channels, and also by the characteristics of the physicochemical interaction of solid soil particles with the seepage flow.To solve integral problems in the area of seepage (determination of seepage losses or mean gradients of the head), we can confine ourselves to a homogeneous model, and for estimating the local seepage strength we should use a piecewise homogeneous model. Numerous (direct or indirect) measurements of the water-conductlng channels are used for constructing the piecewlse homogeneous model of an earth mass. The number of channels, being minimum in jointed rock masses, increases in fragmental and reaches a maximum in clay soils. With an increase in the number of channels the effect on permeability of their shape and also orientation relative to the boundaries of the mass decreases. This makes it possible for each variety of earth masses to establish the most important characteristic determining the seepage and piping properties with consideration of secondary characteristics in the form of correction factors.Seepage ~n rocks was investigated by Lomize, Ratz, Chernyshev, Ivanova, Nasberg, and others. As a result of these investigations, regularities of the flow of water in single, equally rough joints were established, which are described by a family of gradlent-veloclty characteristics in a wide range of variation of velocities and gradients (Fig. i). An important feature of these regularities is the transition of a laminar flow regime to a turbulent regime, whereupon condition (I) is fulfilled for the laminar regime and condition (2) for the turbulent regime: is/g~]ug;where iZ, it, uz, u t are the gradients and velocity of the flow in a Joint, respectively, for the laminar and turbulent seepage flow regimes; g, acceleration of gravity; ~, open width of the joints (the average distance between its walls); v, kinematic viscosity coefficient of water; A and B, roughness parameters determined experimentally.The equation of the critical line demarcating the flow regimes has the form S --4 uc= 7where u c and uiare respectively the critical velocity and gradients of the flow.