Geophysical characterization of weathered sedimentary rock mass for surface excavation purpose

Abstract

An accurate and reliable assessment of ground condition is one of the critical aspects in surface excavation work. This issue become more complex when dealing with heterogeneous ground material with various weathering stages. Seismic velocity and electrical resistivity method are among the common tools used to assist the understanding of the subsurface condition. This study aims to investigate the application of the seismic refraction and resistivity method together with the geotechnical assessment for the purpose of surface excavation work. The study was carried out at Iskandar Puteri, Johor namely Legoland (LEGO), SILC Site 1 (SILC 1), SILC Site 2 (SILC 2) and SILC Site 3 (SILC 3). The sites are underlain by a thick residual soil and interbedding of sandstone and shale from various weathering states. The geophysical surveys that were carried out on the same outcrops were then compared. The classification of rock mass was carried out by adopting Rock Mass Rating (RMR) and Q-system. The field results indicate RMR range from 0 to 69 and Q-value of 0 to 16.883 specifies weak to fair rock. Joint spacing was attained with value of 0 – 1.95 m. The laboratory tests were carried out on 144 – 156 samples for dry density, moisture content, point load test and slake durability. Point Load Strength (Is50) for the samples ranges from 0 – 6.889 MPa indicates very weak to strong rock. Laboratory evaluation indicates the rock quality deteriorates with increase of weathering. Trial excavation was carried out on 19 panels using Komatsu PC300 - 6. Four boreholes that were drilled then correlated with five resistivity and four seismic velocity profiles. Resistivity value for residual soil indicates value of less than 1000 Om. Meanwhile for slightly weathered sandstone and shale 1500 Om – 12000 Om, moderately weathered zone ranges from 370 Om – 5000 Om, highly weathered of 100 Om – 3000 Om and completely weathered with 30 Om – 2000Om. Boulder was detected with resistivity value of 5000 Om – 12000 Om. Besides that, seismic velocity for residual soil shows value of less than 750 m/s, slightly weathered zone of 1500 m/s – 3000 m/s, highly weathered zone of 100 m/s – 2000 m/s and completely weathered zone with velocity of 500 m/s – 1500 m/s. Boulder was not able to be detected. Resistivity survey provide more reliable results in sensing lithology and saturated zone. Field assessment quantified that when RMR less than 40 and Q less than 1 is dominated by completely weathered shale is categorized as easy excavation (> 400 m3/h). On the other hand, moderate excavatability (100 m3/h – 400 m3/h) yielded when 40 < RMR < 60 and 1 < Q < 10 which consists of highly/moderately weathered sandstone/shale and completely weathered sandstone while hard excavation (< 100 m3/h) was observed when 60 < RMR < 70 and 10 < Q < 20 which includes slightly weathered sandstone/shale. The result showed that both Q and RMR exhibit a trend of higher value of rating and commensurate with seismic and resistivity value. The findings of this study contributed the development in excavatability assessment by proposing the resistivity and seismic velocity index for interbedded sedimentary rock mass. The proposed scheme of resistivity and velocity index based on tropically weathered sedimentary rock mass with respect to excavation performance is significant advanced compared to existing assessment.