A thorough investigation of the distress and deformation properties of subgrade materials is under way at the National Danish Road Laboratory. Thus far, the program has concentrated on establishing sound laboratory relationships for cohesive subgrades with respect to resilient and permanent strain characteristics through the use of triaxial testing equipment. In the dynamic testing phase alone, more than 75 million repeated loads have been applied on a series of intact samples representing 12 test sites from 6 countries, and 4 new sites are being added. Although the results of the program thus far must be regarded as inconclusive from a predictive performance point of view, some basic relationships have emerged that are both sound and useful. First, the analysis of the data collected has indicated that the concept of equivalent axle loads is both misleading and irrelevant with respect to directly related subgrade-distress causes. A criterion based on a realistic requirement aimed at limiting the amount of permanent deformation in the soil itself has an extremely high axle-load exponent (mean value greater than 15). Second, the use of a constitutive relationship that can describe the progress of permanent subgrade strain is possible, but because of other uncertainties associated with natural variations, inhomogeneities, and difficulties inherent in the determination of material-characterization constants, the use of a permissible subgrade deviator stress is preferred; the suggested one is a function of a reference resilient modulus. As a result of the high load exponent hereby implied, this permissible stress is only slightly dependent on the number of load repetitions. On the other hand, the concept of a permissible resilient strain is shown to be poorly correlated with permanent strain and the number of repeated loads because of the various forms and degrees of nonlinear elastic response observed among the investigated subgrades. A fictive strain can however, be used in lieu of the permissible-stress approach, although this will result in overly conservative designs if linear-elastic material behavior is assumed in the design technique. /Author/

Media Info

  • Media Type: Print
  • Features: Figures; References;
  • Pagination: pp 84-91
  • Monograph Title: Analysis of pavement systems
  • Serial:

Subject/Index Terms

Filing Info

  • Accession Number: 00193385
  • Record Type: Publication
  • Files: TRIS, TRB
  • Created Date: May 11 1979 12:00AM