A bearing capacity approach to the design of low-volume traffic roads
Oloo, Simon Yamo
Pavement design methods based on the elastic layer theory idealize the pavement structure as consisting of linear elastic layers and utilize the theory of elasticity to predict limiting stresses and strains. The assumption of elastic behavior may be valid for relatively stiff pavement materials. In unpaved roads, consisting of unbound granular bases overlying cohesive subgrades, the assumption of elastic behavior is unlikely to be valid. The behavior of such pavements under traffic stresses is markedly nonlinear. Pavement design methods based on the ultimate strength approach assume shear failure of the pavement structure at sufficiently high traffic stresses. Pavement material behavior is assumed to be plastic rather than elastic. The assumption of plastic response is more realistic for unpaved roads in which traffic stresses exceed the elastic range of the pavement materials. The determination of the ultimate wheel load that a pavement structure can sustain is the most important component of a design process based on bearing capacity theory. Existing solutions are restricted to a narrow range of material properties and are also deficient in the manner in which they determine ultimate wheel loads. General and accurate solutions for the determination of the bearing capacity of pavement structures are required. The incorporation of climatic factors in the pavement design process is another important component of design based on bearing capacity theory. Existing methods assume full saturation of the subgrade. Experience has shown that in many regions of the world full saturation rarely occurs and the assumption of full saturation leads to overdesign. There is a need to incorporate the influence of matric suction in the determination of ultimate wheel loads. A limit equilibrium solution, which can handle any combination of pavement material properties, is proposed for the determination of bearing capacity in a 2-layer pavement system. To enable the incorporation of climatic factors in the determination of ultimate wheel loads, limit equilibrium solutions are proposed for the determination of the effects of positive pore-water pressures and matric suction on bearing capacity. The solution developed for the influence of matric suction on bearing capacity is verified in the laboratory using model footing tests in homogeneous soils equilibrated under constant levels of matric suction. A simple method of testing compacted soils in the direct shear apparatus as well as a method of analyzing the test results in terms of the stress state variables is proposed. The method of testing and analysis is shown to give results which are comparable to the results of the modified direct shear test. The method is considered to be a simple and viable alternative for the characterization of shear strength of compacted unsaturated soils. Finally, a method based on bearing capacity theory is proposed for designing unpaved roads whose structure consists of a base layer overlying a subgrade. The method can handle any combination of shear strength parameters as well as constant levels of matric suction in the pavement layers.