Microscopic Characteristics of Fractured Sandstone after Cyclic Freezing-Thawing and Triaxial Unloading Tests

Xiaohui Ni,Xiaomei Shen,Zhende Zhu

doi:10.1155/2019/6512461

Xiaohui Ni, Xiaomei Shen + Show 1 more

Open Access

https://doi.org/10.1155/2019/6512461

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Abstract

The understanding of tunneling rock failure characteristics under unloading conditions in the cold region is critical for the proper design of rock tunneling support and mining safety operation. Given this understanding is currently limited, this study aimed to investigate the characteristics of fractured sandstone samples in the microscale after cyclic freezing-thawing and triaxial unloading tests. The samples were first subjected to different cycles of freezing and thawing, followed by the triaxial unloading test and scanning electron microscopy imaging. The peak strength and damage dilatancy stress were measured from the stress-strain curves. The microcrack characteristics (number, length, and width) were obtained through the image analysis. The results show that the decrease in peak strength and damage dilatancy stress was more significant by the first 20 freezing-thawing cycles when the pore pressure gradient is maximum compared to the later freezing-thawing cycles. The mechanical properties also significantly deteriorated when the severe freezing-thawing treatments were performed. The fracture section mainly had the morphology of honeycomb-like microstructure, stripped microstructure, and flocculent microstructure. The cracking extent was mainly influenced by the freezing-thawing rather than the triaxial unloading test, but the azimuthal angle of microcracks was significantly altered by the triaxial unloading. To properly design rock tunneling support and safe operation of mining in the cold region, both impact of cyclic freezing-thawing and the excavation operation direction should be considered.

Highlights

During the excavation of deep tunnels under high-stress conditions, the stress concentration near the excavation surface reduces the radial stress component and increases the tangential stress component, leading to nonviolent or violent surface failure, i.e., spalling and strain burst, respectively [1,2,3,4]. ese two significant geohazards could affect the safety of personnel and infrastructure in excavations and deep underground mines
E results show that the peak strength and the damage dilatancy stress of the samples both significantly decreased with increasing freezing-thawing cycles
After the first 20 freezing-thawing cycles, the peak strength decreased by 10.7% and the damage dilatancy decreased by 5.9%, compared to that of samples without experiencing freezingthawing treatment

Summary

Introduction

During the excavation of deep tunnels under high-stress conditions, the stress concentration near the excavation surface reduces the radial stress component and increases the tangential stress component, leading to nonviolent or violent surface failure, i.e., spalling and strain burst, respectively [1,2,3,4]. ese two significant geohazards could affect the safety of personnel and infrastructure in excavations and deep underground mines. Note that the unloading-induced failures of rocks (i.e., spalling and strain burst) are different in characteristics from those caused by the loading on rocks. There has been a growing interest in the understanding of the freezing-thawing rock failure behaviour under unloading conditions as it is critical for the proper design of structures such as tunneling and underground mining in freezing-thawing zones. This understanding is still limited as currently most unloading rock failure studies focus on the rock failure behaviour without freezing-thawing damage [7,8,9]

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