The aim of this research is to investigate the effect of spheroidal weathering on the physico-mechanical properties of in situ granite boulder in tropical environment. This research was conducted to gain more understanding due to lack of study related to this topic especially on tropical boulder in completely weathered zone. A number of 34 in situ granite boulders that naturally formed in completely weathered zone were selected and investigated from two quarries located in Southern Johor, Malaysia. Classification of weathering grade was conducted on the material surrounding boulder based on discolouration, degree of friability and texture characteristics as suggested by ISRM(1). The colour of the weathered material were classified based on Rock Colour Chart (2). Petrographic analysis and scanning electron microscopy (SEM) were carried out to analyze the mineralogical and microstructure alterations, respectively due to weathering effect. The changes of physico-mechanical properties at the surrounding of boulder due to weathering were also analyzed including dry density, porosity, point load strength, uniaxial compressive strength and permeability. The dry density and porosity were conducted based on ASTM (3) and Brown (4), respectively. The point load uniaxial compressive strength were carried out according to ISRM (1) while permeability test was undertaken based on Head (5). The tropical granite boulder possessing rindlets with significant different of colours, degree of friability and texture. This differences were classified into four primary zones: corestone (C), inner rindlets (IR), outer rindlets (OR), and saprolites (S) with weathering grade of II, III/IV, IV/V and V, respectively. Due to spheroidal weathering, three to six concentric sheets of rindlets with whitish grey to light brown colour were formed at the surrounding of corestone's boulder. Each sheets of rindlets has 2.0 cm to 3.0 cm thick with total thickness up to 80 cm. The microcracks aperture of IR and OR is ranging from 150 to 200 µm and 200 µm to 1 mm, respectively. Analysis revealed that the higher the weathering grade, the higher and wider the fracture opening formed in the rindlets up to 2 mm width. Analysis exibited that the increase of weathering grade from Grade II to IV along C to S were significantly reduced the plagioclase, K-feldspar, and biotite with reduction of 74%, 74.2% and 87.5%, respectively. The alteration of minerals and microstructure due to the weathering were significantly reduced the dry density, point load strength, uniaxial compressive strength and permeability with 32%, 99.5%, 98.6% and 84.8%, respectively.The reaction of spheroidal weathering is significantly affecting the physico-mechanical properties of the tropical granite boulders by alteration of its minerals and microstructure characteristics at the surrounding of the boulder.
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