SELECTION OF DESIGN STRENGTHS OF UNTREATED SOIL SUBGRADES AND SUBGRADES TREATED WITH CEMENT AND HYDRATED LIME
Selection of design strengths of soil subgrades and subgrades treated with cement or hydrated lime is a problem in pavement design analysis and construction. Different types of soils may exist in a highway corridor and different strengths may exist after the soils are compacted to form the pavement subgrade. The selected subgrade strength will largely affect the pavement thickness obtained from the design analysis, future pavement performance, and the overall bearing capacities of the subgrade during construction and the pavement structure after construction. In developing the proposed selection scheme, a newly developed mathematical model, based on limit equilibrium, is used. Relationships among undrained shear strength [or California bearing ratio (CBR)] and tire contact stresses are developed for factors of safety 1.0 and 1.5. The minimum subgrade strength required to sustain anticipated construction tire contact stresses during construction is determined. A criterion is proposed for determining when subgrade stabilization is needed and methods of selecting the design subgrade strength are examined. A least-cost analysis appears to be an appropriate approach as shown by analysis of a case study involving pavement failures. Two case studies show that soaked laboratory strengths appear to be fairly representative of long-term field subgrade strengths. Hence, using soaked laboratory strengths and least-cost analysis appears to be a reasonable means for selecting the design strength of subgrades for pavement analysis. To avoid failures of chemically stabilized layers, relationships among thicknesses of chemically treated layers and the CBR values of the untreated subgrade for a factor of safety of 1.5 are presented.
Bearing Capacity Analysis and Design of Highway Base Materials Reinforced with Geofabrics
The primary objective of this study was to develop and implement mathematical bearing capacity models originally proposed by Hopkins (1988, 1991) and Slepak and Hopkins (1993, 1995). These advanced models, which are based on limit equilibrium and are operated together, can be used to analyze the bearing capacity, or stability, of early construction of loads on a single layer of material, two-layered problems involving a layer of base aggregate and subgrade, and a foundation involving multiple layers of different materials, such as flexible asphalt pavement. A Prandlt-type shear surface is used in the model analyses of layered foundations. In this report, the models are extended to analyzing flexible pavements reinforced with tensile elements. Although the current model does not account for strain compatibility, the strength of the tensile elements may be input for assumed strain levels. Any number of tensile elements may be analyzed in a given problem. In the limit equilibrium approach, shear strengths, the angle of internal friction, phi, and cohesion, c, are entered for each layer of material. Triaxial testing of the asphalt material is performed in a manner that the shear strength parameters, phi and c, are developed as a function of temperature. Hence, if the temperature of the asphalt layer is known (or assumed) at a site, then values of phi and c may be calculated from the relationships between the shear strength parameters and temperature. Moreover, to facilitate and provide an efficient means of analyzing early construction cases and flexible pavements reinforced with geosynthetics, "Windows" software was developed. In the case of the asphalt layer, the entire layer is divided into finite layers because phi and c varies with depth of asphalt. When the surface temperature of the asphalt is known (or assumed), a temperature distribution model is used to estimate the temperature at any depth below the asphalt layer surface. Consequently, the shear strength parameters are known at any depth (of each finite layer) below the surface. To establish the validity and reasonableness of the newly developed limit equilibrium models, bearing capacity factors are derived from the limit equilibrium methods and compared to classical bearing capacity factors, N sub c and N sub q, developed by Prandlt and Reissner. Differences range from 1 to 10 and 1 to 3%, respectively. The Slepak-Hopkins model yields values of N sub y that are 12 to 38 larger than values published by Caquot and Kerisel. However, values of N sub y from the Slepak-Hopkins model are only 3 to 11% larger than backcalculated values obtained by Debeer and Ladanyi from experimental footing tests. The Slepak-Hopkins model was also used to analyzed 237 flexible pavement sections of the 1959-1960 AASHO Road Test. Factors of safety from the model analyses showed that very reasonable results were obtained and were in line with failures recorded at the test site. Finally, actual analyses of a stretch of roadway where failures occurred were analyzed. Three sections involved tensile elements.
Compressive and flexural properties of hemp fiber reinforced concrete
The compressive and flexural properties of hemp fiber reinforced concretes (FRC) were examined in this paper. Natural hemp fiber was mixed using dry and wet mixing methods to fabricate the FRC. Mechanical properties of the FRC were investigated. The main factors affecting compressive and flexural properties of the FRC materials were evaluated with an orthogonal test design. Fiber content by weight has the largest effect. The method for casting hemp FRC has been optimised. Under the optimum conditions, compressive strength increased by 4 %, flexural strength increased by 9 %, flexural toughness increased by 144 %, and flexural toughness index increased by 214 %.