Evaluation of subgrade soils using selected in-situ devices / Norasmah Laili Mat Daud

One of the reasons of pavement failure is weak subgrade, which causes rutting, alligator cracking, and upheaval. To prevent sub grade failure, adequate compaction as in the specification must be carried out. Proper quality assurance and quality compliance (QA/QC) procedure must be followed to ensure that compaction is carried out adequately. Existing Public Work Department (PWD) Specification for Road Work (JKR7SPJ/88) relating to compaction only require in-situ field density test and moisture content to ensure that compaction is carried out correctly. There is no
specification on the determination of subgrade strength although the strength of subgrade in terms of elastic modulus is important as input in pavement design. Therefore, there is a need to obtain the in-situ subgrade strength to ensure that the pavement is correctly designed. This research details a study on road subgrade using three types of in-situ devices undertaken on six 150-metres length flexible pavement subgrade test sections. The devices used in this study include the in-situ CBR, Dynamic Cone Penetrometer (DC?) and Portable Falling Weight Deflectometer
(PFWD). Tests using these devices were carried out at specified spacing on the test sections to determine the subgrade CBR, DCP-PR in (mm/blow) and elastic modulus values. The results obtained for laboratory CBR (unsoaked and soaked), in-situ CBR, DCP-PR in (mm/blow) and modulus subgrade were analysed and correlated to develop models to relate the values obtained. It was found that there is a reasonable relationship between the in-situ CBR and the laboratory unsoaked CBR values. The relationship between in-situ CBR with the DCP-PR in (mm/blow) values was found to be conclusive as high positive values of correlation were obtained. The relationship between in-situ CBR with the P-FWD modulus values was found to be inconclusive. The relationship obtained between DCP and P-FWD is a positive correlation. It is recommended that DCP test be included in all PWD road construction specification so that the subgrade strength of the pavement could be checked to meet the desired requirement.