Study on creep characteristics of fractured rock masses in caved zones with different roof strengths

Objective The caved zone in goaf areas is filled with fractured rock masses, and the strength of these fractured rock masses after collapse serves as a key factor causing differential long-term creep characteristics of the goaf. This study aims to comprehensively explore the creep characteristics of fractured rock masses with varying strengths. Method Grounded in the similarity theory, we selected analogous models of fractured rock masses representing three strength categories: soft rock, moderately hard rock, and hard rock. By integrating indoor step-loading creep tests with theoretical analysis, a systematic comparison of creep characteristics of fractured rock masses with varying strengths was carried out. Results The creep test results of three types of fractured rock masses reveal distinct deformation characteristics. Soft rock masses exhibit “sudden axial strain increments” under loads of 3 kN and 4 kN, while moderately hard rock masses display this phenomenon under a broader range of 3–5 kN. Evolution patterns of instantaneous, creep, and total strains show marked differences as the creep stress increases: soft rock masses demonstrate progressive decreases before stabilizing (instantaneous strain dropping from 0.084% at 1 kN to stable values beyond 4 kN); moderately hard rock masses exhibit an initial increase followed by subsequent decrease with peak strains occurring at different stresses (instantaneous at 2 kN, creep at 3 kN, total at 2 kN); and hard rock masses present more complex characteristics with instantaneous strain initially declining (0.033% to 0.020% in the 1–3 kN range) before rebounding slightly, while creep strain shows a triphasic pattern (increase–decrease–increase) peaking at 0.009% (3 kN) and total strain exhibits an overall trend of initial decrease followed by subsequent increase. Notably, all rock types share decreasing initial and steady-state creep rates with increasing stress. Compared with soft and hard rock masses, moderately hard rock masses experience more significant particle breakage and rearrangement during the creep process. This leads to an initial peak creep rate of 0.176 h−1 at 2 kN, exhibiting distinct peak characteristics. In contrast, hard rock masses exhibit rapid decay in creep rate and quick stabilization, reflecting their dense internal structure and strong interparticle contacts. These characteristics endow hard rock masses with rapid and stable mechanical response properties during the load-bearing process. The response differences of rock masses with varying strengths under the same load show positive correlation between rock strength and initial creep rate but an inverse relationship with steady-state rate. This indicates that while high-strength rocks respond more vigorously initially, they stabilize faster than low-strength rocks which sustain a longer duration of deformation. Conclusion Under incremenal loading, all three types of fractured rock masses exhibit decreasing trends in both initial and steady-state creep rates with increasing load, with particularly pronounced creep rate variations in the initial stage. Significance This study reveals creep characteristics of fractured rock masses of varying strength, providing a theoretical foundation for predicting long-term deformation of caved zones in goaf areas and developing geohazard prevention and control strategies.