Undrained mechanical behavior of unsaturated completely weathered granite: Experimental investigation and constitutive modeling

Abstract

The undrained mechanical behavior of unsaturated completely weathered granite (CWG) is highly susceptible to alterations in the hydraulic environment, particularly under uniaxial loading conditions, due to the unique nature of this soil type. In this study, a series of unconfined compression tests were carried out on unsaturated CWG soil in an underground engineering site, and the effects of varying the environmental variables on the main undrained mechanical properties were analyzed. Based on the experimental results, a novel constitutive model was then established using the damage mechanics theory and the undetermined coefficient method. The results demonstrate that the curves of remolded CWG specimens with different moisture contents and dry densities exhibited diverse characteristics, including brittleness, significant softening, and ductility. As a typical indicator, the unconfined compression strength of soil specimens initially increased with an increase in moisture content and then decreased. Meanwhile, an optimal moisture content of approximately 10.5% could be observed, while a critical moisture content value of 13.0% was identified, beyond which the strength of the specimen decreases sharply. Moreover, the deformation and fracture of CWG specimens were predominantly caused by shear failure, and the ultimate failure modes were primarily influenced by moisture content rather than dry density. Furthermore, by comparing several similar models and the experimental data, the proposed model could accurately replicate the undrained mechanical characteristics of unsaturated CWG soil, and quantitatively describe the key mechanical indexes. These findings offer a valuable reference point for understanding the underlying mechanisms, anticipating potential risks, and implementing effective control measures in similar underground engineering projects.

1. Introduction

Granite is a kind of igneous rock widely distributed in the earth crust [1−3]. Under the joint action of weathering, erosion, and geological tectonic stress, etc., it will form rock and soil bodies with different weathering grades [4−8], such as residual soil (Grades Ⅳ and higher), completely weathered (Grade Ⅴ), highly weathered (Grade Ⅳ), moderately weathered (Grade Ⅲ), and slightly weathered (Grade Ⅱ). Among them, although the appearance of completely weathered granite (CWG) retains the structure of rock, the overall structure is loose and broken, and its particles consist mainly of powder and sand grains, often containing special minerals and chemical components, so it is a familiar unfavorable geological formation [9, 10]. In particular, CWG in different hydraulic environments show significant differences in physical and mechanical properties and engineering characteristics. And it is susceptible to construction disturbances and erosion of the natural environment during the construction, potentially increasing the technical difficulties of construction, thus resulting in frequent engineering accidents [11−14]. In general, CWG is extensively and deeply distributed in subtropical, tropical, and humid temperature zones, and is frequently exposed directly due to its geological location. As a result, it has become a prevalent primary geotechnical material in granite regional engineering construction.

In view of the mechanical peculiarities of the CWG formation and the technical difficulties of construction, several researchers and engineers have carried out valuable explorations under drained and undrained conditions. Considering the water inrush disasters originating from CWG, LIU et al [6] conducted a study on the mass transfer and flow properties of water inrush in CWG with varying particle size distributions, selected from a weathered granite with a depth of 100 m in Guangxi, China.