THE ENGINEERING PROPERTIES OF CORAL LIMESTONE USED AS SUB BASE IN DAR-BAGAMOYO ROAD PROJECT

This paper presents and discusses the unusual poor engineering properties of coral limestone from Mpiji Borrow area in Bagamoyo District - Tanzania used as subbase materials during the construction of Wazo Hill – Bagamoyo Road. The discussion is based on the study and assessment of road subbase made of coral limestone at the project site in the coast of Dar Es Salaam and Coastal region, which involved about 148,000 square metres of pavement from chainage 28+900 to 42+800. The road length is 42.8 Kilometres starting at Wazo Hill Junction (Chainage 0+000) ending at Bagamoyo town (Chainage 42+800). The deterioration and damage of pavement started while the road construction was ongoing. The pavement failure warranted an immediate forensic investigation to establish the probable cause of the pavement failure. The geo-forensic investigation established that the primary cause of pavement failure was due to softening and swelling of coralline subbase caused by the imbibition of water supplied to the subbase layer during crushed base placing and compaction using ‘slushing’ compaction procedures.