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.
Use of Waste Materials for Control of Acid Mine Drainage and Subsidence
This paper presents the results of a study of the use of fluidized-bed combustion ash in an underground coal mine for control of acid mine drainage and subsidence. Fluidized-bed combustion ash (FBC ash) is a waste by-product of advanced coal combustion. Presently, most of the FBC ash produced in power generation is disposed in landfills. Subsidence and acid mine drainage are two major problems found in coal mining areas. The problems of disposal of FBC ash, mine subsidence, and acid mine drainage can be reduced by placing the ash in abandoned coal mines. The study included laboratory experiments, computer modeling, and a field application of FBC ash in an abandoned underground mine. Potential subsidence values under different backfill configurations were computed by using the finite element method. Strength requirements for the grout were determined on the basis of site-specific geologic information and a stress analysis. The FBC ash was placed in a mine cavity by pumping it in the form of a grout to explore the viability of hydraulic backfilling. Field observations were made on the field strength and flow characteristics of the FBC ash grout. The paper contains information on strength requirements of the grout, flow characteristics of the grout, expected reductions in potential subsidence, and acid mine drainage around the grouted mine. The results show that a grout made of fluidized-bed combustion ash can be successfully placed in a mine cavity by hydraulic backfilling in order to reduce the problems caused by subsidence and acid mine drainage.
A FIELD EXPERIMENT OF ASPHALT-TREATED BASES IN COLORADO
THICKNESS REQUIREMENTS FOR SEVERAL DIFFERENT ASPHALT- TREATED BASES WERE EVALUATED AND THEIR PERFORMANCE COMPARED TO OTHER BASES CONSTRUCTED BOTH AT THE TEST SITE AND AT THE AASHO ROAD TEST. THE EXPERIMENT CONSISTS OF TWENTY 550-FOOT LONG TEST SECTIONS CONSTRUCTED IN 1965 NEAR PUEBLO, COLORADO . IT INCLUDES TWO COMPLETE INTERRELATED FACTORIAL EXPERIMENTS. THE FIRST IS COMPRISED OF 3 THICKNESSES /5.5-, 7.0- AND 8.5-INCHES/ OF 3 BASE COURSE TYPES /ASPHALT CONCRETE, LOW-STABILITY SAND-ASPHALT AND HIGH-STABILITY SAND -ASPHALT/ ON A-7-6 SOIL. THE SECOND IS COMPRISED OF 2 THICKNESSES /5.5- AND 7.0-INCHES/ OF 2 OF THE SAME BASE COURSE TYPES /ASPHALT CONCRETE AND LOW-STABILITY SAND- ASPHALT/ ON A-6 SOIL. FOR FURTHER COMPARISON, TEST SECTIONS COMPRISED OF THE STATES STANDARD SECTION FOR THE GIVEN CONDITIONS, WERE INCLUDED ON EACH SOIL. A 2-INCH ASPHALT CONCRETE SURFACE COURSE IS COMMON TO ALL SECTIONS. THE TEST SECTIONS WERE RANDOMIZED AND SEVERAL WERE REPLICATED. LABORATORY TESTS WERE CONDUCTED ON SAMPLES OF THE VARIOUS PAVEMENT SYSTEM LAYERS. TEMPERATURES WERE OBTAINED FROM THERMOCOUPLE INSTALLATIONS AT VARIOUS DEPTHS IN THE PAVEMENT STRUCTURE. BENKELMAN BEAM AND DEHLEN CURVATURE METER MEASUREMENTS WERE MADE. AS-BUILT SURFACE COURSE AND BASE THICKNESS WERE MEASURED. PRESENT SERVICEABILITY INDEX VALUES /PSI/ WERE DETERMINED FOR EACH SECTION FROM BOTH CHLOE PROFILOMETER AND BPR ROUGHMETER MEASUREMENTS. BENKELMAN BEAM DEFLECTIONS WERE USED WITH LAYERED SYSTEM THEORY TO DETERMINE ELASTIC MODULI FOR THE VARIOUS ASPHALT BASE TYPES. THE MODULI ARE IN GOOD AGREEMENT WITH THOSE DETERMINED IN THE LABORATORY FOR SIMILAR TEMPERATURE CONDITIONS AND A LOADING FREQUENCY OF ONE CYCLE PER SECOND. THEORY AND INFERRED ASPHALT BASE MODULI WERE USED TO DERIVE THICKNESS- DEFLECTION RELATIONS FOR AVERAGE MEASURED SURFACE COURSE AND BASE TEMPERATURES. SUCH RELATIONS WILL PROVIDE A MEANS OF OBTAINING EQUIVALENT THICKNESSES OF THE DIFFERENT MATERIALS WHEN CRITICAL DEFLECTION VALUES BECOME AVAILABLE.