Abolfazl Tarifard

Considering the creep behavior of soft and weak rocks is critical for analyzing the long-term stability of underground constructions. This paper introduces a novel creep constitutive model to characterize the creep behavior of rocks under uniaxial and triaxial stress states. The fractional derivative Abel dashpot was used to improve the Burger model, and a viscoplastic component was added in series with the modified Burgers model to replicate the tertiary phase of rock creep. The effectiveness of the model was verified using creep test data from various soft rocks and monitoring measurements from a tunnel excavated in heavily jointed weak rock masses. Furthermore, a sensitivity analysis was carried out to assess the impact of the model parameters on creep deformation, and a comparative study was performed to evaluate the efficacy of the suggested model in modeling the accelerated stage of rock creep compared with some existing models. The strong agreement observed between the calculated results and both the creep test data and tunnel monitoring measurements underscores the accuracy and validity of the proposed model. The comparative analysis further revealed that the proposed model offers the highest fitting efficiency for describing the tertiary stage of rock creep. These findings suggest that the model effectively captures the creep behavior of rocks and precisely represents the entire creep process.

The creep behavior of rocks has been broadly researched because of its extensive application in geomechanics. Since the time-dependent stability of underground constructions is a critical aspect of geotechnical engineering, a comprehensive understanding of the creep behavior of rocks plays a pivotal role in ensuring the safety of such structures. Various factors, including stress level, temperature, rock damage, water content, rock anisotropy, etc., can influence rocks’ creep characteristics. One of the main topics in the creep analysis of rocks is the constitutive models, which can be categorized into empirical, component, and mechanism-based models. In this research, the previously proposed creep models were reviewed, and their main characteristics were discussed. The effectiveness of the models in simulating the accelerated phase of rock creep was evaluated by comparing their performance with the creep test results of different types of rocks. The application of rock’s creep analysis in different engineering projects and adopting appropriate creep properties for rock mass were also examined. The primary limitation associated with empirical and classical component models lies in their challenges when it comes to modeling the tertiary phase of rock creep. The mechanism-based models have demonstrated success in effectively simulating the complete creep phases; nevertheless, additional validation is crucial to establish their broader applicability. However, further investigation is still required to develop creep models specific to rock mass. In this paper, we attempted to review and discuss the most recent studies in creep analysis of rocks that can be used by researchers conducting creep analysis in geomechanics.

The significance of creep behavior in soft rocks is crucial in rock engineering, particularly when ensuring the time-dependent stability of underground structures. This study proposed a new nonlinear creep constitutive model to represent the soft rock’s creep behavior subjected to uniaxial and triaxial stress conditions. The Burger model was modified by substituting the traditional Newton dashpot with the fractional derivative Abel dashpot, and a viscoplastic body was introduced in series with the improved Burgers model to simulate the accelerating phase of rock creep. The model's efficacy was confirmed by fitting the parameters using creep test data from different soft rocks. The isochronous stress-strain curve approach was employed to calculate the long-term strength of rocks, and a sensitivity analysis was conducted to evaluate how the model parameters affect creep deformation. The high agreement between the predicted outcomes and the actual creep experimental data for salt, shale, and sandstone demonstrates the proposed model's accuracy and logic. These results indicate that the model reliably represents soft rocks' nonlinear creep characteristics and the whole creep process.

The time-dependent stability of tunnels is an important and challenging topic, mainly when the tunnel is excavated in incompetent and weak rocks. The creep property of rock is one of the crucial mechanical properties of weak rock and the main factor affecting the long-term stability of rock masses. Also, water as an important environmental factor influences both the short-term and long-term behavior of rocks and is one of the causes of geotechnical engineering disasters, such as tunnel collapse, slope sliding, surface subsidence, etc. In this research, the effects of rock’s creep behavior and underground water on the long-term stability of the Shibli tunnels were analyzed. Geological maps and reports of Shibli tunnels show a highly jointed condition in the surrounding rocks which have been crushed by two orogenic stages. The Burger-creep visco-plastic model was used to simulate the tunnel host rock creep behavior. The model's parameters were adopted based on the displacement-based direct back analysis technique using a univariate optimization algorithm. In addition, the influence of underground water is assessed under the condition of the varying water table. Support capability diagrams were used to evaluate the loading created on the tunnel’s permanent lining due to the creep behavior of rock mass and underground water. This study suggests that the weak rock's creep behavior and underground water significantly affect the time-dependent stability of tunnels. Results show that the induced stresses due to the rock's creep behavior and underground water are more considerable in the tunnel spring-line. Also, the increasing 20 m in the water table approximately decreases ten years of tunnel lining stability time at the fault zone. Article highlights: Rocks creep behavior and underground water significantly affect the time-dependent stability of tunnels in weak rocks. Displacement-based direct back analysis using a univariate optimization algorithm was used to determine the CVISC model’s properties. Increasing 20 m in the water table approximately decreases ten years of tunnel lining stability time at the fault zone.

The mechanical properties of frozen rocks vary significantly from the properties of the same lithology under ambient temperature. The goal of this paper is to investigate these changes in the physical and mechanical properties of rocks due to saturation and freezing. Besides, the attention was paid on discovering new correlations between the mechanical characteristics. To fulfill these objectives, 36 uniaxial compressive strength tests, 36 Brazilian splitting tests, and 48 point load tests were carried out. The samples were tested in air dry, water saturated, and frozen (− 20 °C) conditions. The measured physical and mechanical parameters were analyzed by using regression analyses. It was found that the average uniaxial compressive strength of frozen samples (21.93 MPa) is 86.4% more than saturated ones (11.76 MPa) but 25.9% less than dry specimens (29.62 MPa). Additionally, high correlations were established between uniaxial compressive strength and I_S(50) under air-dry, saturated, and frozen conditions for the investigated marl samples. Furthermore, it is of particular interest to observe a high correlation with the determination coefficient (R² = 0.95) between the constants of previously published linear regressions of UCS- I_s(50) under dry status.