###### Abstract  
The extent of the plastic zone is critical in determining the stability and extent of damage to the surrounding rock in tunnels, crucial for designing support structures and thermal insulation layers. This study focuses on understanding how rock thermal stress affects the expansion of the plastic zone in deep-buried tunnels subjected to high geothermal temperatures, based on the derivation of the boundary line formula of the plastic zone in a high geothermal tunnel, and combined with the test results of the hydraulic fracturing method in a high geothermal tunnel of the Bulunkou Hydropower Station in Xinjiang. The findings indicate that thermal stress in the rock mass slows the growth of the plastic zone but significantly increases its extent. However, the influence of thermal stress on the shape and size of the plastic zone is less significant compared to the lateral pressure coefficient. In conditions of high geothermal temperature and geostress, rock mass thermal stress induces substantial changes in the morphology and extent of the plastic zone, which cannot be overlooked and can lead to significant errors if not properly considered. The theoretical formulas derived from engineering analysis, along with observed patterns of plastic zone expansion, demonstrate practical applicability. rock thermal stress; high geothermal temperature; plastic zone; morphology; extension pattern; high geostresses tunnel 28 June 2024 19 August 2024 28 August 2024 28 August 2024 28 August 2024 28 June 2024 28 June 2024 19 August 2024 28 August 2024 28 August 2024 28 June  
In high-temperature and high-stress environments, the surrounding rock of deep-buried tunnels undergoes extensive deformation and damage, primarily due to the uneven expansion of the plastic zone [2]. Analyzing the geometry and expansion pattern of the plastic zone has always been crucial to comprehend the damage mechanisms in tunnels and controlling the stability of the surrounding rock. Through extensive global scholarly exploration, the morphology and expansion characteristics of the plastic zone in the surrounding rock of tunnels under non-uniform stress fields have been thoroughly investigated, resulting in continuous enhancement and refinement of plastic zone theory. The size of the plastic zone in rock surrounding tunnels is influenced by key rock mechanical parameters such as cohesion, internal friction angle, support resistance, and ground stress levels. The shape of the plastic zone depends on the distribution of stress within the rock mass. Particularly, high-ground stress is predominantly responsible for the expansion. [PERSON] [3] and colleagues elucidated the extension law of the plastic zone in four types of deep-well peripheral rocks. On the other hand, [PERSON] and co-authors [4] deduced the equation for the boundary line of the plastic zone in the peripheral rocks of a circular roadway, revealing its tongue or sickle-shaped behavior. Recently, [PERSON] [5] and collaborators discovered that apart from circular and elliptical forms, the plastic zone morphology in a roadway under a non-uniform stress field can also manifest as a butterfly shape. To validate the range and morphology of the plastic zone under varying stress conditions, [PERSON] et al. [6; 7] utilized the boundary line equation of the plastic zone in the tunnel surrounding rock under non-uniform stress fields. [PERSON] [8] and colleagues employed FLAC 3D to simulate the plastic zone morphology in the surrounding rock of a circular tunnel, proposing corresponding determination criteria. [PERSON] et al. [9] elucidated the theoretical basis for the development of circular, elliptical, and butterfly-shaped plastic zones in the non-uniform stress field of circular roadway surrounding rock. Furthermore, based on observations in the high-stress soft-rock roadway of Qujiang coal mine, [PERSON] [10] posited that the destabilization of the roadway surrounding rock stems from the pernicious expansion of the plastic zone was induced by localized distortion of the plastic ring.