Geological and geotechnical characterisation of aggregate source rocks from selected sites in Viti Levu, Fiji
Degree GrantorUniversity of Canterbury
Degree NameMasters of Science
The Nasinu, Semo and Vuda quarries are the three major suppliers of aggregates for roading construction in Fiji. Production at the Nasinu quarry commenced in 1955, and production at the Vuda and Semo quarries started in the early 1990s. Aggregate products from Nasinu and Vuda are mainly used for roading purposes whereas the Vuda quarry also supplies aggregates for concrete manufacturing. One of the main shortfalls in the aggregate industry in Fiji is the lack of data on the characteristics of source rocks and aggregates. This study focuses on the characterisation of source rocks and aggregate products from the three quarries for roading purposes.
Field investigation involved geotechnical logging of quarry faces and sampling of source rocks and aggregates from the quarries. Geotechnical logging was conducted to identify the distribution of the quarry rock, weathering characteristics and other geological structures present in the quarry. Olivine basalt flows are quarried at Nasinu and Vuda, while the quarry rock at Semo is a lens-shaped microgabbro intrusion. Alteration at Nasinu is responsible for the reddish-brown colour of the rubbly lava layers. Weathering only affects the near-surface exposures of gabbro at Semo, while its effect on the basalt at Vuda is minimal.
Laboratory testing of source rocks showed that the Nasinu 1, Vuda and Semo materials all have high strength values with means of 221 MPa, 192 MPa and 252 MPa respectively, while the Nasinu 2 sample has a mean UCS value of 38 MPa. The high strength rocks also have high densities (2863kg/m3 -2938kg/m3) and low porosities (0.8% - 1.5%). The Nasinu 2 sample has a high porosity (12.5%) and low density (2237kg/m3). All the four materials tested have very high slake durability indices (lc12) (98.8% - 99.8%).
Results of aggregate testing showed that basecourse samples from Nasinu have a crushing resistance of 5.14%, a weathering quality index of AB, a value of 80% for the CBR, and a sand equivalent value of 43. These properties of the Nasinu basecourse aggregates complied with TNZ M/4: 1995 specification for basecourse materials. Sealing chips from Nasinu have a marginally acceptable crushing resistance value of 11.05, according to TNZ M/6: 1993 which requires a maximum value of 10%, polished stone value of 56 and cleanness value of 82. The Vuda basecourse aggregates have a high crushing resistance (2.59%), high weathering quality index (AA), high CBR (90) and a sand equivalent value of 46. Sealing chips from Vuda has a low PSV (49) and high cleanness value (45). These data shows that the Vuda aggregates complied with requirements of TNZ M/4:1995 and TNZ M/6: 1993 for basecourse and sealing chips materials.
Petrographical analysis of the source rocks showed that all the four rock types have similar mineralogy in that more than 80% of the rock is made up of augite and plagioclase feldspar, with a slight difference in the Semo sample which does not contain olivine and contains a few quartz crystals. It was also found that the mean grain size is the single most important factor influencing the strength of the rock materials. The Vuda sample has the largest mean grain size (0.34mm) and the lowest UCS value (191.6MPa). The texture coefficients calculated for each rock type also correlates well with the UCS of the source rocks having an R2 value of 0.85.
Based on the test results, the Vuda aggregates and Nasinu generally comply with TNZ /M4: 1995 and TNZ M/6: 1993 specifications for basecourse and sealing chips. The crushing resistance of the Nasinu composite sample is marginally acceptable to be used as sealing chips. Improvements to the quality of aggregates produced at Nasinu could be achieved by separating the two products and only using aggregates from the Nasinu 1 source rock for sealing chips. The low quality materials from the Nasinu 2 sample can be used as basecourse and sub-base materials. Recommendation on improvements to the current Fiji Standards for roading aggregates include the inclusion of the crushing resistance test and weathering quality index test for both basecourse and sealing chips, together with the polished stone value test, and the cleanness value test for sealing chips only. Further research should look into aggregate properties for concrete manufacturing and the role played by the different aggregate types currently used in Fiji on the properties of concrete.