ENGINEERING GEOLOGICAL ANALYSIS OF CALCAREOUS SOIL IN TIMOR-LESTE COAST REEF

Abstract

The calcareous soil of coast reef is limited in distribution, and is seldom used in engineering and lacks engineering geological research. In order to find out the spatial distribution and physical and mechanical characteristics of the calcareous soil on the coast reef, and service design, we referred to the results of previous studies on calcareous soils and collected the existing geological data in Timor-leste, and carried out field geological survey, drilling, standard penetration test and laboratory physical and mechanical test. We have studied the landform, spatial distribution, mineral chemical composition, grain size distribution, specific gravity, permeability coefficient and shear strength of calcareous soil in Timor-leste coast reef. We have summarized the physical and mechanical characteristics of calcareous soil in coast reef under the landform of sand dam and lagoon. Our study results are as following:(1)The landform of Timor-leste coast reef has its own characteristics. The land is low hill landform, the coast area is bay landform. The bay has developed sand bar, sand beach and lagoon. (2)The study area has developed six unit layers from top to bottom in the stratigraphic space:Unit 1 modern calcareous gravelly sand on the surface of sand bar, Unit 2 very soft to soft clay/silt at the lagoon surface, Unit 3 thick layers of calcareous gravelly sand in the upper part, Unit 4 silty/clayey gravelly sand/grave at the middle part, Unit 5 sitly/clay gravelly sand/grave at the lower part and Unit 6 highly to moderately weathered phyllite at the bottom. (3)The calcareous soil in Timor-leste is composed of pebbled sand lithologically and gravel locally, containing about 19% fine-grained soil. The mineral composition is mainly aragonite and calcite, followed by mica and quartz. The sediment is mainly derived from marine coral clasts and partly from terrestrial sediments. Calcareous soils, similar to atolls and barrier reefs, are also characterized by the presence of land-based sediments. (4)The special gravity of calcareous soil on coast reef is 2.70,which is larger than quartz sand, reflecting the unique special gravity formed by different mineral composition. (5)Coast-reef calcareous soil is mainly medium sand, coarse sand and gravel, but the permeability coefficient is lower than the conventional value due to the presence of more fine grains. Attention should be paid when filling. (6)The cohesion of calcareous gravelly sand of coast reef is 0, and the internal friction Angle is 33 degrees in loose state, 36 degrees in dense state, and 41 degrees in dense state, which is similar to the mature empirical evaluation of quartz gravelly sand. Our studies show that the calcareous soil in Timor-leste coast reef contains some feldspar, quartz. The mechanical strength of rock and soil is similar to that of conventional quartz sand. The empirical relationship of rock and soil parameters evaluated by the quartz sand in situ test can be applied to the calcareous soil of east Timor shore reef with reliable results. The determination of the physical and mechanical properties of calcareous soil in timor-leste coast reef provides a reference for similar types of calcareous soil geotechnical engineering applications.