Experimental study on the shear creep characteristics of joints under wetting-drying cycles

Highlights

• Multi-stage shear creep experiments are firstly conducted on the joints subjected to different wetting-drying cycles.

• A classification of the shear creep process of joints containing four stages is firstly proposed.

• The presence of asperities is the physical basis of strengthening and cutting stages.

• A new fractional-order damage creep model of joints considering wetting-drying cycles is proposed.

Abstract

To better understand the shear creep characteristics of joints under wetting-drying cycles, multi-stage shear creep experiments are firstly conducted on the joints subjected to different wetting-drying cycles. Then, the shear creep damage characteristics of joints are investigated from the microscopic and macroscopic perspectives through wave velocity tests, scanning electron microscope observations, and failure morphologies. The investigation demonstrates that the increasing wetting-drying cycles lead to a porous and disintegrated microstructure of joints, which further results in increases in the shear deformation and steady-creep rate, and decreases in the failure strength and long-term strength. With increasing wetting-drying cycles, the wear rate of joints decreases and the damaged region of joints becomes smaller and invisible. Furthermore, a classification of the shear creep process of joints containing four stages is firstly proposed based on the test results. It is found that the presence of asperities on the rough joints is the physical basis of the existence of strengthening and cutting stages of the shear creep process. Finally, based on the experimental results, a new fractional-order damage creep model of joints considering wetting-drying cycles is proposed.

Introduction

Landslides triggered by reservoir water-level fluctuations account for casualties and property losses. The Vajont landslide on October 9, 1963 is one of the most catastrophic slope failures. A sliding mass with 275 × 10⁶ m³ slipped down the hillside at a peak velocity of 30 m/s^1,2. Some 2000 fatalities were caused.³ Studies found that the weakening of rock over time, impounding, and the brittle fracture play essential roles in the failure of the Vajont slide.^4,5 Many other landslides associated with water-level fluctuations and fractures have caused serious consequences.6, 7, 8, 9, 10 Li et al.¹¹ reported that water-level fluctuations resulted in more than thousands of landslides in the Three Gorges Reservoir area, China, leading to casualties and economic losses. Besides, Jones et al.¹² reported that more than 400 slope movements were induced by the water-level fluctuation of the Grand Coulee reservoir, USA, from 1941 to 1953. During the operation period, the bedrock and hydro-fluctuation belt of the reservoir slope will experience periodic rise and fall of water level. Besides, engineering practice shows that failure often happened without warning because earlier movements have passed unnoticed.¹³ Consequently, studying the long-term stability of rock mass under cyclic wetting-drying is of paramount importance.

In recent years, investigations into mechanical properties of rock influenced by cyclic wetting-drying have been mainly focused on the uniaxial/triaxial compressive strength,^14,15 shear strength^16,17 and dynamic behaviors.¹⁸ The results show that the compressive and shear strength decreases with increasing wetting-drying cycles, especially for the rock containing expansive clay minerals. Besides, with increasing wetting-drying cycles, the fracture toughness decreases, and the crack propagation speed increases.¹⁹ In-situ observations revealed that failure in rock engineering does not happen immediately but progresses over time.^20,21 Therefore, investigations of the long-term behaviors of rocks are essential for the long-term stability of geotechnical engineering projects. The effects of cyclic wetting-drying on rockfill creep were studied by laboratory experiments²² and numerical methods.²³ The results showed that the strain induced in the first wetting-drying cycle was larger than that in the second wetting-drying cycle. Zhang et al.²⁴ conducted multi-stage shear creep tests on the granite subjected to different wetting-drying cycles. Based on the test results, a variable-parameter creep damage model was proposed and the sensitivity of creep parameters to the wetting-drying cycles was revealed.

The above studies focused on intact rocks. Actually, the joints in rock mass show an essential influence on the mechanical properties of the rock mass.25, 26, 27 Landslide typically slides along a failure surface that coincides with pre-existing discontinuities in the form of joints, faults, and bedding.^28,29 Therefore, discontinuities are one of the most important factors that govern the long-term stability of landslides. During the past decades, considerable studies have been conducted to investigate the long-term deformation behavior of joints. The shear creep tests were conducted on joints of salt rocks,³⁰ shale,³¹ granite,³² marble,³³ and sandstone.³⁴ Furthermore, the creep behavior in the discontinuity infillings (crushed lava, crushed quartzite, and natural gouge) was investigated.³⁵ The results showed that the creep behavior of discontinuities was influenced by normal stress, shear stress, shear displacement velocity, and joint roughness. In addition, careful observation of the joint surfaces showed that the asperities on the joints play essential roles during the shear creep process.36, 37, 38 It is worth noting that more asperities provide additional space for creep and thus, higher joint roughness coefficients produce more creep.39, 40, 41

Investigation into the influence of water-rock interaction on the shear behavior of rock joints is essential for the stability of the reservoir slope. The influence of water on the shear behavior of joints42, 43, 44, 45 was studied. The results show that the joint's shear strength, shear stiffness, shear strain and deformation mechanisms were affected by water and the weakening mechanism of joints induced by the physical-chemical reaction was revealed. Moreover, the influence of immersion time in water on the morphology of sandstone joints was investigated by Tang et al.⁴⁶ It demonstrated that the water-rock interaction leads to roughness degradation and reduction of the damaged area of rock surfaces. This can be attributed to water lubrication.⁴⁷ Unfortunately, previous studies only considered the influence of a single factor (either time or water-rock interaction) on the deformation mechanism of joints. The investigation into the influence of water level fluctuation on the time-dependent behavior of joints is far from enough to meet the engineering demand.

To investigate the shear creep characteristics of joints under periodic water level fluctuation, multi-stage shear creep experiments are conducted on the joints subjected to different wetting-drying cycles. The effects of wetting-drying cycles on the shear deformation, steady-state creep rate, long-term strength, and failure strength of joints are studied. Besides, through wave velocity testers, scanning electron microscope (SEM) observations, and failure morphologies, the shear creep damage characteristics of joints under wetting-drying cycles are revealed from the macro and micro perspectives. Then, the shear creep process containing four stages is firstly proposed and the effects of asperities on the shear creep process are analyzed. Furthermore, a fractional-order damage creep model considering wetting-drying cycles is proposed and the relationships between the creep parameters and wetting-drying cycles are obtained.