Petrography and engineering properties of the PHU Sanao weathered granites, Amphoe Muang Changwat Loei

Abstract

During the last decade, granitic rocks become more important materials for engineering works. They are used as construction aggregates, dimension stones, and even foundation. However, the granitic rocks in Thailand are found mostly as weathered rocks while the petrographic and engineering properties of the weathered granites are poorly understood. Therefore, the granitic rocks of Phu Sanao batholith within Loei-Chiang Khan area are intensely investigated for both petrographic and engineering properties. This batholith is formed by magma differentiation during early Triassic. Consequently, the rocks in this area are grouped into four subunits; Phu Sanao, Phu Lek, Ban Kok Du and Ban Na Khaem. The sequence of crystallization appears to start from hornblende-biotite onzogranite of the Ban Na Khaem subunit grading respectively to biotite granite or less mafic content syenogranite of the Phu Sanao and Phu Lek subunits or less mafic content syenogranite of the Phu Sanao and Phu Lek subunits and finally to alkali-feldspar phase with no mafic content of the Ban Kok Du subunit. The Phu Sanao subunit comprises of fresh rocks at the ridge and weathered rocks of grade V to VI at the lower surrounding area. The Phu Lek subunit contains complete weathering profile ranging from grad I to grad VI. Whereas the Ban Kok Du shows fresh corestones occurring as both continuous and discontinuous frame work which is the typical weathering profile of grade III and grade IV. The Ban Na Khaem rocks are highly weathered at grade V to grade VI only The engineering properties of the rocks from each subunit except Ban Na Khaem are determined. These properties are correlated to petrographic property mathematically. The unconfined compressive strength of the rocks falls in the rang of 35-60, 70-260, 120-270 MN/sq.m for the weathering grade III, II, and I respectively. The empirical models for rock strength estimation are proposed from the high correlation of linear regression and exponential curve fitting with point load strength index, porosity and weathering decomposition values. These strength models are UCS = 5.8 + 19.4 (Is50) or UCS = (5.19) (-0.23)ⁿ or UCS = (5.1) (-2.49)ᵡᵈ