SUBGRADE EVALUATION AND PREPARATION

THE SINGLE PRINCIPLE THAT GOVERNS EVALUATION AND PREPARATION OF SUBGRADES IS THAT SUBGRADES FOR CONCRETE SHOULD BE REASONABLY UNIFORM BUT HIGH STRENGTH IS NOT REQUIRED. THIS PRINCIPLE WAS VERIFIED BY SUBGRADE PRESSURE MEASUREMENTS MADE DURING THE BATES TEST ROAD AND ON FULL SCALE SLABS AT THE PCA LABORATORIES. THE PRINCIPLE OF UNIFORM SLAB SUPPORT WAS ALSO VERIFIED BY SURVEYS OF PAVEMENT PERFORMANCE WHICH SHOW BETTER PERFORMANCE ON LOW STRENGTH UNIFORM SUBGRADES THAN ON HIGHER STRENGTH SUBGRADES THAT LACK 1NIFORMITY. SHRINK AND SWELL OF EXPANSIVE SOILS IS A MAJOR CAUSE OF NON-UNIFORM SUBGRADE SUPPORT. METHODS OF CONTROL OF EXPANSIVE SOILS TO PROVIDE UNIFORM SUPPORT INCLUDE' /1/ CROSS-HAULING AND SELECTIVE GRADING TO REMOVE SURCHARGE PRESSURES IN DEEP CUTS, TO PLACE MORE EXPANSIVE SOILS IN THE LOWER PART OF THE EMBANKMENT AND TO SECURE REASONABLY UNIFORM SOIL CONDITIONS, /2/ COMPACTION CONTROL WITH THE MOISTURE CONTENT SPECIFIED BY AASHO T 99, /3/ USE OF AN INSULATING LAYER TO PREVENT EXPANSIVE SOILS FROM DRYING OUT AFTER THEY HAVE BEEN PROPERLY COMPACTED, AND /4/ USE OF CEMENT TREATMENT TO STABILIZE THE UPPER PART OF EXTREMELY EXPANSIVE SUBGRADES IN AREAS WITH LONG PERIODS OF HOT, DRY WEATHER. A SECOND MAJOR SOURCE OF NON-UNIFORM SUBGRADE SUPPORT IS FROST ACTION. CONTROL METHODS INCLUDE' /1/ SETTING GRADE LINES AND DITCH DEPTHS AT ELEVATIONS THAT WILL PLACE HIGHLY FROST SUSCEPTIBLE SOILS BEYOND CAPILLARY RANGE OF FREE WATER, /2/ CROSS-HAULING AND SELECTIVE GRADING, /3/ REPLACING HIGHLY FROST-SUSCEPTIBLE POCKETED SOILS WITH SOILS LIKE THOSE SURROUNDING THE POCKET, AND /4/ USE OF MIXING AND COMPACTION CONTROLS TO ENSURE UNIFORMITY. THE THIRD SOURCE OF NON-UNIFORM SUPPORT IS MUD-PUMPING WHERE PAVEMENTS ARE PLACED DIRECTLY ON FINE-GRAINED SOILS AND SUBJECTED TO FREQUENT HEAVY AXLE LOADS. THIS IS PREVENTED BY A WIDE VARIETY OF GRANULAR AND CEMENT-TREATED SUBBASES.

  • Supplemental Notes:
    • Proc 1963, PP 48-65, 4 FIG, 21 REF
  • Corporate Authors:

    University of New Mexico, Albuquerque

    1 University of New Mexico
    Albuquerque, NM  USA  87131

    Civil Engineering Department

    ,    
  • Authors:
    • Fordyce, P
  • Publication Date: 0

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Filing Info

  • Accession Number: 00231111
  • Record Type: Publication
  • Files: TRIS
  • Created Date: Aug 15 2004 2:23AM