Geotechnical characteristics of weathered granitic gneiss with geo-hazards investigation of pit excavation in Guangzhou, China

Abstract

This paper presents a comprehensive investigation into weathered granitic gneiss in Guangzhou, China, including its formation, grading, classification, and the geotechnical characteristics of different weathered states. The granitic weathering profiles are graded into five weathered states: fresh rock, slightly weathered, moderately weathered, highly weathered, and completely weathered. The fresh rock and the slightly weathered rock have good mechanical behaviour. The highly weathered granitic gneiss was found to possess a relatively higher permeability than the other weathered states, in which the fissure water is usually stored. Geotechnical characteristics of the weathered granitic gneiss that poses geohazards during excavation are identified, including: (1) uneven grain size distribution, (2) water softening of the residual soils, and 3) spherical weathered boulder. The following potential geohazards are identified: (1) quicksand and piping, (2) water–ground settlement or even collapse due to water softening, (3) trenching difficulties in a diaphragm wall, and (4) instability of the lattice columns. Countermeasures for preventing these geo-hazards are proposed, including (1) rate-controlled drilling of dewatering wells, (2) grouting in both sides of retaining walls, and (3) deep hole blasting of large-size residual gravel boulders. These measures has been demonstrated to be useful in construction of Zhongxin Station.