Key Takeaways & Executive Findings
- •• Oscillating normal loads reduce the coefficient of friction of rough rock joints, with the reduction degree increasing at higher shear rates and decreasing at higher oscillation frequencies. • A constant ratio of shear rate to oscillation frequency (v/f) yields a constant friction reduction, indicating a rate-frequency coupled behavior. • A time lag between peak normal load and peak shear load is identified; the lag increases with shear rate and decreases with static COF. • Lower creep rates combined with higher oscillation frequencies may promote instability in creeping fault zones, offering insights for seismic hazard assessment.
Abstract
Dynamic disturbances certainly reduce shear strength of rock joints, yet the mechanism needs deeper explanation. We investigate the shear behavior of a rough basalt joint by conducting laboratory shear experiments. Constant and superimposed oscillating normal loads are applied at the upper block. Meanwhile, the bottom block moves at a constant shear rate. We investigate the shear behavior by: 1) altering the normal load oscillation frequency with a same shear rate, 2) altering the shear rate with a same normal load oscillation frequency, and 3) altering the normal load oscillation frequency and shear rate simultaneously with a constant ratio. The results show that the oscillating normal load reduces the coefficient of friction (COF). The reduce degree of COF increases with higher shear rate, decreases when increasing normal load oscillation frequency, and keeps constant if the special ratio, v/f (shear rate divided by normal oscillation frequency), is constant. Moreover, we identify a time lag between peak normal load and peak shear load. And the lagging proportion increases with higher shear rate, and decreases with larger static COF. Our results imply that a lower creep rate with a higher normal load oscillation frequency easily destabilizes the creeping fault zones.
1. Introduction
The shear behavior of rock discontinuities is an essential element during the rock structure safety evaluation and design. The shear responses of joints subjected to static normal stresses are widely performed [1−6], and several classic shear strength criterions are proposed [7−11]. Oscillating normal loads caused by ocean waves, earth tides and seismic waves are commonly encountered. Sometimes, fault zones are exposed to dynamic load, during which slips along faults experiencing normal load oscillations happen [12−18]. For a deeper understanding of the rock friction laws considering inconstant normal disturbances during shear processes, lab-scaled samples are widely explored [19−44].
Dynamic normal load can be classified into sudden changes and continuous changes based on the period of the dynamic force. Normal loads with step increase, so step decrease and sudden pulse test are systematically considered [21−28] for the sudden changes in normal load. HOBBS and BRADY [21], and OLSSON [22] found that the variations of friction to the step changes in axial stresses present a multi-stage evolution, where measured shear stress is an instantaneous linear alteration first, tracked by an exponential time-dependent development. KILGORE et al [26, 27] investigated the shear behavior under sudden changes of normal loads on dry bare rock surface of granite. They found that sudden changes in normal loads lead to the gradual, nearly exponential variations in shear loads. Moreover, the memory effects of changes in normal load are also identified [23, 28, 29, 37].
Shear resistance responses to continuous (cyclic) changes in normal load have also been widely performed [33, 36, 38−40]. Generally, the cyclic normal loads are controlled by a triangle or sinusoidal wave. BOETTCHER and MARONE [36] studied the impacts of load properties on the frictional resistance and stability of quartz gouge under the application of sine-wave normal force. They changed the normal stress oscillation amplitude, normal stress oscillation period and shear rate. They found that small amplitude, and long period oscillations have little effect on fault friction. In addition, it has been found that the classical criterion of joint shear strength overpredicts the dynamic shear strength [39, 41].
The roles of shear rate on the frictional behavior are widely studied under constant and dynamic normal load conditions [3, 24, 25, 34, 45−47]. When the shear surface is subjected to constant normal load, increasing shear rate does not always increase the shear strength which is dependent on the material types [47]. Generally, shear strength increases with faster shear rate for rigid rocks, and decreases when the shear rate slows down. Previous study [24] pointed out that
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DANG Wen-gang, LI Xing-ling, TAO Kang, FU Jin-yang (2025). Rate-frequency dependent shear behavior of rough rock joint experiencing normal load oscillations. Journal of Central South University. https://doi.org/10.1007/s11771-025-5966-7
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Frequently Asked Questions
How does oscillating normal load affect the shear strength of rock joints?
The study demonstrates that oscillating normal loads reduce the coefficient of friction (COF) of rough rock joints, with the reduction degree increasing at higher shear rates and decreasing at higher oscillation frequencies.
What is the relationship between shear rate and friction reduction under normal load oscillations?
Higher shear rates amplify the reduction in the coefficient of friction, while increasing the normal load oscillation frequency diminishes the reduction effect.
Is there a time lag between peak normal load and peak shear load?
Yes, a time lag exists between peak normal load and peak shear load. The lagging proportion increases with higher shear rate and decreases with larger static coefficient of friction.
What are the implications for fault stability?
The findings imply that a lower creep rate combined with a higher normal load oscillation frequency can more easily destabilize creeping fault zones, suggesting a mechanism for dynamic triggering of fault slip.
Why is the ratio of shear rate to normal oscillation frequency (v/f) important?
When the ratio v/f is kept constant, the degree of friction reduction remains constant, indicating a rate-frequency dependent behavior critical for predicting joint response under dynamic loading.
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