The Haba Snow Mountain Tunnel, located in Yunnan Province, China, has become the focus of intense scrutiny following the alarming occurrence of severe deformation attributed to foliated metamorphic basalt. This specific rock type is characterized by high levels of metamorphism, making it prone to weathering and exhibiting low strength, posing significant risks to the tunnel's structural integrity. Researchers aim to shed light on the rock's microscopic characteristics and mechanical properties to devise strategies for ensuring tunnel stability under similar geological challenges.
Research conducted by Z.S. Tan and colleagues sought to understand the reasons behind the tunnel's deformation and the properties of the surrounding rock formations. Despite the tunnel's depth of over 800 meters, which typically increases ground stress, the observed deformation was unprecedented. "The situation of severe deformation has raised concerns about underground construction risks," explained the researchers. Their objective was to examine the geological and mechanical behavior of the foliated metamorphic basalt, offering technical references for future engineering endeavors.
The basalt displayed significant metamorphism as revealed during the excavation phase, complicatin the stability of the tunnel. The rock's mineral composition included actinolite, albite, chlorite, and epidote, which were categorized and analyzed using advanced techniques such as thin section identification, X-ray diffraction (XRD), and scanning electron microscopy. These methods comprised the core framework utilized to assess the strength of the rock. A combination of uniaxial compression tests (UCT), point load tests (PLT), and Schmidt hammer rebound tests (SHRT) was employed, ensuring comprehensive measurements.
By utilizing these varied testing methods, the study uncovered several notable correlations among rock strength parameters. For example, the relationship between point load index (PLI) and uniaxial compressive strength (UCS) was found to be linear, expressed as y = 10.97x, with a high correlation coefficient (R² = 0.93). This significant finding suggests the efficiency of using point load tests as viable alternatives for determining rock strength when traditional methods may not be feasible. "To determine the strength of weak surrounding rocks, a combined approach using PLTs and UCTs is recommended," noted the authors.
Results from the comprehensive analysis indicated the average UCS of the foliated basalt was approximately 33.33 MPa, falling short of the strength documented for other geological formations. The challenges inherent to extracting intact rock cores became evident throughout the tests, limiting the efficacy of direct measurement, especially as softer and more weathered samples often crumbled under pressure. The study also disclosed the SHRT results, indicating lower rebound values compared to conventional concrete. While SHRTs provide rapid evaluations, they are less effective for assessing low-strength rocks, emphasizing the necessity of employing multiple testing techniques for thorough assessments.
Following extensive testing, the researchers conducted statistical analyses of the data gathered, yielding most reliable results from UCS tests. They noted a clear need to combine methods to mitigate each approach's limitations, as strength testing for metamorphic basalt presents unique challenges due to its geological properties. The inclusion of PLI testing has proven invaluable, offering adaptability for irregularly shaped and fragmentary samples inherent to the study's rock type.
The findings of this research not only highlight the mechanical properties of foliated metamorphic basalt but also contribute broadly to the body of knowledge on tunnel design under complex geological conditions. "The results highlight the complementary nature of these methods," stated the authors, reinforcing the importance of diversifying testing approaches to cater to the challenges posed by geological diversity. This collective mindset will be indispensable for future projects confronting similar underground construction risks.
Conclusively, this study elucidates the characteristics of the foliated metamorphic basalt evidenced during the Haba Snow Mountain Tunnel's construction, asserting its significant role as one of the principal causes of severe deformation. Enhanced knowledge of these geological materials aids engineers and researchers to address engineering measures effectively within architectures affected by similar geological instabilities. The analysis of mineral composition, microstructure, and the relationships between different strengths presents invaluable insights for future tunnel stability assessments. The authors assert, "These findings are pivotal for comprehending rock characteristics and informing future tunnel designs in analogous geological contexts."