In cold regions, saturated fractured rock is prone to crack initiation, extension, and branching along the original fracture end under the effect of freeze-thaw (F-T) cycles and cyclic fatigue loadings. The resulting strength deterioration is accompanied by large amounts of pore growth, which results in localized damage and fracturing along the crack end and even causes overall failure. To study the fatigue damage behaviors of fractured rocks under F-T cycles and cyclic loadings, single-joint quasi-sandstone specimens with joint angles of 45° and 90° were prepared. Subsequently, F-T damage tests with 0, 10, and 20 cycles and cyclic loading tests with different stress levels were performed. The F-T damage features are discussed based on the binarization image of localized damage along the joint ends and their wave velocity variations. It is found that the frost damage of single-joint quasi-sandstone tends toward a strip with localized fatigue characteristics. Moreover, the changes of strain compliance and dissipated energy are studied under the effect of loading-unloading fatigue. Some of the interesting phenomena observed are as follows: (i) during the early stage of cyclic loading, the joint can be compacted; nevertheless, it tends to expand along the fracture direction once it passes the elastic stage, and the irreversible plastic deformation is stable at this stage. (ii) The cracks caused by F-T damage in the 45° single-joint samples deflect along the fracture direction, in contrast to the 90° single-joint specimens. Moreover, the samples with a 45° single joint are more susceptible to original fracture at the early stage of the failure process, which results in a different failure mode from that of the 90° single-joint samples. (iii) The F-T cycles and cyclic loadings have a coupling effect on the single-joint sandstone. The strain compliance and hysteresis energy keep increasing uniformly after the F-T cycles and cyclic loadings.

Multi-gradient drilling is a new offshore drilling method. The accurate calculation of the related wellbore temperature is of great significance for the prediction of the gas hydrate formation area and the precise control of the wellbore pressure. In this study, a new heat transfer model is proposed by which the variable mass flow is properly taken into account. Using this model, the effects of the main factors influencing the wellbore temperature are analyzed. The results indicate that at the position where the separation injection device is installed, the temperature increase of the fluid in the drill pipe is mitigated due to the inflow/outflow of hollow spheres, and the temperature drop of the fluid in the annulus also decreases. In addition, a lower separation efficiency of the device, a shallower installation depth and a smaller circulating displacement tend to increase the temperature near the bottom of the annulus, thereby helping to reduce the hydrate generation area and playing a positive role in the prevention and control of hydrates in deepwater drilling.

The traditional Coulomb’s earth pressure theory does not consider the effect of local surcharge on the lateral earth pressure and its critical failure angle. However, in practice, local surcharges commonly act on the surface of frozen backfill that is affected by freeze‐thaw actions in cold regions and tend to affect the active thrust and its position. In paper, analytical solutions for estimating the active thrust, critical wedge failure angle, and action position subject to a local surcharge in cold regions are proposed. Herein, the simplified equivalent moment of surcharge is adopted on the premise of maintaining Coulomb’s earth pressure assumptions. The formula derivation is provided as a typical example to obtain the active thrust, critical wedge failure angle, and its position under a strip surcharge. Compared with previous approaches, the proposed solutions lead to easier evaluation of all indexes associated with Coulomb’s active earth pressure. Meanwhile, the expressions of Coulomb’s earth pressure under other types of nonuniform loading acting on the wall are discussed. In addition, sensitivity is performed to assess the effect of some main parameters. The results indicate that the dip angle of retaining wall‐back and the friction angle of frozen backfill soil are two most significant indexes that influence the active thrust and its position.