SOME NOTES ON BASE COURSE STABILITY

INVESTIGATIONS INTO THE CAUSES OF EXTENSIVE PAVEMENT DISTRESS ON A SECTION OF THE TRANS CANADA HIGHWAY NEAR GOLDEN HAVE SHOWN THAT, IN SPITE OF AN ADEQUATE THICKNESS OF PAVEMENT STRUCTURE, THE BASE GRAVEL HAD INSUFFICIENT STABILITY DURING THE SPRING THAW PERIOD TO SUPPORT TRAFFIC. EXTENSIVE RUTTING AND ALLIGATORING OF THE PAVEMENT OCCURRED AND SOME SECTIONS COMPLETELY DISINTEGRATED. BENKELMAN BEAM SURVEYS WERE CARRIED OUT WHICH SHOWED THAT VERY HIGH DEFLECTION OCCURRED DURING THE PERIOD OF GREATEST WEAKNESS BUT THE DURATION OF THIS PERIOD WAS SHORT. REBOUND READINGS TAKEN A FEW DAYS AFTER THE WEAKEST PERIOD WERE LESS THAN .020 INCHES. IT WAS NOT POSSIBLE TO PREDICT WHERE HIGH READINGS WOULD OCCUR. DENSITY TESTS ON THE BASE GRAVEL SHOWED THAT IN WEAK AREAS DENSITY WAS HIGHER AND VOIDS LOWER THAN IN STRONG AREAS. SPECIAL FREEZING TESTS IN WHICH PORE PRESSURES WERE MEASURED SHOWED THAT FREEZING GRADUALLY SATURATED THE GRAVEL AND PORE PRESSURES WERE GENERATED UNDER APPLIED LOADS. UNCONSOLIDATED UNDRAINED TRIAXIAL TESTS ALSO SHOWED A HIGH DEGREE OF PORE PRESSURE RESPONSE UNDER LOAD. THE BASE GRAVEL HAD A PERMEABILITY OF ABOUT 1X10 EXPONENT-5 CM/SEC. AND WAS DILATANT WHEN ALMOST SATURATED. THE PASSING ?200 CONTENT VARIED FROM 8% TO 15%. IT WAS DEDUCED THAT FAILURE WAS CAUSED BY THE HIGH DEFORMATION WHICH OCCURRED UNDER WHEEL LOADS WHEN THE EFFECTIVE STRESSES WERE REDUCED BY THE LOAD INDUCED PORE-PRESSURES AND THAT THESE PORE- PRESSURES WERE INDUCED BECAUSE THE PERMEABILITY OF THE GRAVEL WAS TOO LOW TO ALLOW RAPID ENOUGH DRAINAGE. THE AMOUNT OF ADDITIONAL MOISTURE REQUIRED TO PRODUCE THIS CONDITION WAS VERY SMALL (ABOUT 0.5%). THIS AMOUNT OF MOISTURE WAS DRAWN INTO THE GRAVEL BY FREEZING EVEN THROUGH THE GRAVEL WAS NOT CONSIDERED FROST SUSCEPTIBLE BY NORMAL STANDARDS. THE ANGLE OF SHEARING RESISTANCE OF THE GRAVEL WAS OVER 40 DEGREES, NORMALLY SUFFICIENT FOR STABILITY. IT IS CONSIDERED THAT OTHER PAVEMENTS IN THE PROVINCE MAY BE FAILING BY A SIMILAR MECHANISM RATHER THAN BECAUSE OF INSUFFICIENT PAVEMENT THICKNESS. FURTHER TESTING IS UNDER WAY TO INVESTIGATE THIS. TENTATIVE RESULTS SHOW THAT SOME GRAVELS USED FOR BASES EITHER BECAME CONTAMINATED WITH FINES DURING CONSTRUCTION AND SUBSEQUENT TRAFFICKING OR THAT DEGRADATION OF AGGREGATE OCCURS DURING CRUSHING AND LAYING OPERATION. IT IS RECOMMENDED THAT ENGINEERS EXAMINE SPECIFICATIONS TO ENSURE THAT BASE GRAVELS HAVE A LOW FINES CONTENT, A HIGH PERMEABILITY AND DO NOT TEND TO DEGRADE DURING CRUSHING OPERATIONS. DURING CONSTRUCTION GRAVELS SHOULD BE CHECKED TO SEE THAT THE FINES CONTENT IS NOT INCREASING AND THAT PERMEABILITY REMAINS HIGH. /AUTHOR/

• Supplemental Notes:
  • Pp279-294, 11 FIG, 5 PHOT, 5 REF
• Authors:
  • Readshaw, E E
• Publication Date: 1967-9

Media Info

• Serial:
  • Canadian Good Roads Association Proc

Subject/Index Terms

• TRT Terms: Alligator cracking; Base course (Pavements); Benkelman beam; Contaminants; Defects; Deflection; Deformation; Density; Dilatancy; Drainage; Failure; Fines (Materials); Forecasting; Freezing; Gravel; Loads; Measurement; Pavement distress; Pavements; Percent saturation; Permeability; Plastic deformation; Pore pressure; Rebound; Shear strength; Stability (Mechanics); Testing; Thickness; Triaxial shear tests
• Uncontrolled Terms: Contamination; Pavement thickness
• Old TRIS Terms: Alligatoring; Base failure; Degradation (Aggregate); Density measurement
• Subject Areas: Highways; Hydraulics and Hydrology; Pavements;

Filing Info

• Accession Number: 00206577
• Record Type: Publication
• Source Agency: CANADIAN GOOD ROADS ASSOCIATION
• Files: TRIS
• Created Date: Jan 23 1994 12:00AM