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.
Characterization of Cementitiously Stabilized Layers for Use in Pavement Design and Analysis
This report presents information on the characterization of cementitiously stabilized layers and the properties that influence pavement performance. It also contains recommended performance-related procedures for characterizing these layers and performance-prediction models for incorporation into the mechanistic–empirical pavement analysis methods. Individual chapters highlight pavement distresses of hot-mix asphalt pavements and concrete pavements, laboratory tests and model development, and model calibration. The material contained in the report will be of immediate interest to state materials, pavement, and construction engineers and others involved in the different aspects of pavement design and construction.
THE THEORY OF STRESS AND DISPLACEMENTS IN LAYERED SYSTEMS AND APPLICATIONS TO THE DESIGN OF AIRPORT RUNWAYS
THEORIES OF LAYERED SOIL DEPOSITS ARE REVIEWED. THE THEORY OF STRESSES ARE DISPLACEMENTS IN A TWO-LAYERED SYSTEM WAS DEVELOPED IN ACCORDANCE WITH THE METHODS OF THE MATHEMATICAL THEORY OF ELASTICITY AND IS PRESENTED IN ORDER TO REVEAL SOME OF THE FUNDAMENTAL RELATIONS EXISTING BETWEEN THE PHYSICAL FACTORS, WHICH CONTROL THE LOAD-SETTLEMENT RELATIONS, AND IN ORDER TO PROVIDE A PRACTICAL METHOD OF ANALYSIS FOR THE DESIGN OF AIRPORT RUNWAYS. THE THEORY REVEALS THE CONTROLLING INFLUENCE OF TWO IMPORTANT RATIOS ON THE LOAD-SETTLEMENT CHARACTERISTICS OF THE TWO-LAYER SYSTEM: (1) THE RATIO OF THE RADIUS OF THE BEARING AREA TO THE THICKNESS OF THE REINFORCING OR PAVEMENT LAYER, AND (2) THE RATIO OF THE MODULUS OF THE SUBGRADE TO THAT OF THE PAVEMENT, FOR PRACTICAL DESIGN PURPOSES, THE THEORETICAL RESULTS WERE EVALUATED NUMERICALLY AND EXPRESSED IN BASIC INFLUENCE CURVES, GIVING VALUES OF THE SETTLEMENT COEFFICIENT IN TERMS OF THESE BASIC RATIOS. THE SETTLEMENT COEFFICIENT IS APPLIED AS A SIMPLE MULTIPLYING OR CORRECTION FACTOR TO THE BOUSSINESQ EQUATION FOR SURFACE SETTLEMENT AT THE CENTER OF A CIRCULAR FLEXIBLE BEARING AREA. THE DESIGN FOR AIRPORT RUNWAYS INVOLVES THE SELECTION OF SUITABLE TYPES OF PAVEMENT CONSTRUCTION AND THE DETERMINATION BY THE INFLUENCE CURVES FOR THE TWO-LAYER SYSTEM OF THE THICKNESS REQUIRED TO GIVE ADEQUATE SUPPORT TO AIRPLANE WHEEL LOADS AND REASONABLE LENGTH OF SERVICE. FIELD LOAD TEST CHECKS OF QUALITY AND STRENGTH CAN BE MADE DURING CONSTRUCTION ON: (1) THE NATURAL SUBGRADE TO DETERMINE TENTATIVELY THE THICKNESS OF BASE COURSE AND PAVEMENT REQUIRED, (2) TOP OF THE BASE COURSE TO CHECK THE QUALITY AND STRENGTH AT THIS STAGE OF CONSTRUCTION, AND (3) TOP OF THE FINISHED PAVEMENT TO CHECK THE EFFECTIVE SUPPORTING CAPACITY OF THE WHOLE PAVEMENT CONSTRUCTION.