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.
Selection of Soil Strength Values for the Design of Flexible Pavements : Technical Paper
RECENT STUDIES OF QUALITY CONTROL, SOIL AND MATERIAL VARIANCE HAVE SHOWN THAT CONSIDERABLE VARIATION CAN EXIST IN A COMPLETED HIGHWAY PROJECT. THESE STUDIES HAVE SERVED TO POINT OUT THE NEED FOR DEVELOPING DESIGN TECHNIQUES WHICH TAKE INTO ACCOUNT THIS VARIATION. THIS PAPER PRESENTS A METHOD, BASED ON ECONOMICS, FOR SELECTING A DESIGN VALUE FROM AN ARRAY OF SOIL TEST DATA FOR A SOIL AREA. THIS ANALYSIS INDICATED THAT THE OPTIMUM DESIGN STRENGTH VALUE IS GENERALLY LESS THAN THE MEAN TEST VALUE AND THAT IT IS DEPENDENT UPON AMOUNT OF TRAFFIC, ENVIRONMENTAL CONDITIONS, SOIL VARIABILITY, COMPACTION VARAIBILITY AND PAVEMENT COSTS. PROCEDURES WERE DEVELOPED FOR SETTING UP A FIELD SAMPLING PROGRAM TO EVALUATE SOIL STRENGTH ON AN AREA BASIS. THESE CRITERIA ARE BASED ON TRAFFIC THAT WILL USE THE ROAD, ANTICIPATED MOISTURE CONDITIONS IN THE COMPLETED SUBGRADE, AND SOIL VARIANCE. GUIDELINES FOR SOIL SAMPLING AND SELECTION OF SOIL STRENGTH VALUES, BASED ON THE FACTORS OF TRAFFIC, ANTICIPATED DEGREE OF SATURATION AND SOIL VARIANCE, ARE PRESENTED. /AUTHOR/
SPECIAL DEFLECTION STUDIES ON FLEXIBLE PAVEMENT
THE FLEXIBLE PAVEMENT PERFORMANCE STUDY AT THE AASHO ROAD TEST INDICATED THAT THE LIFE OF A PAVEMENT WAS RELATED TO THE LEVEL OF PAVEMENT DEFLECTION AS MEASURED WITH THE BENKELMAN BEAM. IT IS PROBABLE, THEREFORE, THAT PLANNERS OF SATELLITE OR PAVEMENT EVALUATION PROGRAMS WILL INCLUDE MEASUREMENTS OF PAVEMENT DEFLECTION IN THEIR EXPERIMENTAL PROGRAMS. IN ORDER TO INTERPRET THE MEASUREMENTS OF PAVEMENT DEFLECTION TAKEN IN ANY EXPERIMENTAL PROGRAM, SOME KNOWLEDGE OF THE FACTORS THAT AFFECT THE MEASUREMENT IS REQUIRED. AT THE AASHO ROAD TEST SPECIAL PAVEMENT DEFLECTION EXPERIMENTS WERE CONDUCTED TO DETERMINE THE EFFECT OF VARYING THE BENKELMAN BEAM PROCEDURE AND TO CONSIDER AN ALTERNATIVE METHOD OF MEASUREMENT USING AN ELECTRONIC DEVICE. NUMEROUS OTHER SPECIAL EXPERIMENTS WERE CARRIED OUT TO DETERMINE THE RELATIONSHIP OF PAVEMENT DEFLECTION TO WHEEL LOAD, VEHICLE SPEED, TIRE PRESSURE AND PAVEMENT TEMPERATURE. IN ADDITION, A PROGRAM OF PLATE LOAD TESTING MADE POSSIBLE A CORRELATION BETWEEN PLATE LOAD TEST DATA AND BENKELMAN BEAM DATA. TEST DATA ARE SUMMARIZED BY MEANS OF TABLES, GRAPHS AND MATHEMATICAL EQUATIONS. THE EQUATIONS EXPRESS THE PAVEMENT DEFLECTION AS FUNCTIONS OF WHEEL LOAD, VEHICLE SPEED AND PAVEMENT TEMPERATURE. /AUTHOR/