# Long-Term Creep Mechanical and Acoustic Emission Characteristics of Water-Immersed Coal Pillar Dam
## 3 Results and Discussion  
### Time-Dependent Axial Strain  
#### 3.1.1 Water-Immersed Sample  
The time-axial strain curve of the long-term immersed specimen is illustrated in Figure 3(a), with a total loading duration of 174 days. The test results indicate that the creep curve of water-immersed coal-rock comprises three distinct stages:  
Stage I: Transient and primary creep stage (0-40 d). The specimen rapidly entered the primary creep stage followed by immediate deformation stabilization. This transient creep stage was exceptionally brief (\(\approx\)1 d). During early steady creep (0-30 d), the strain decreased progressively, meaning that the strain at 5.927 d is 0.491%, while at 34.074 d it is 0.4668%. The strain variation rate is calculated as \(-\)0.00086%/d.  
Stage II: Steady-state creep stage (40-130 d). After Stage I creep completion, the strain-time curves transitioned from minor fluctuations to gradual growth. The overall curve exhibited steady creep deformation characteristics. The average strain rate is \(3.04\times 10^{-6}\)/d, indicating stable development of the sample.  
Stage III: Accelerated creep stage (130-173 d). Following Stage II, the strain-time curves showed a sustained increase after initial rapid growth. The average creep strain rate is 0.00234%/d. At 171.5 days, axial load was increased incrementally at about 5% UCS \(\approx\) 0.8 MPa per step every 8 h to show the accelerating creep process until rock failure (Figure 3(b)).  
Figure 4: Long-term creep deformation characteristics of coal under water-immersed conditions.  
Analyzing the unique phenomenon of the long-term creep deformation process of coal under water immersion conditions, negative creep phenomenon occurred in the first stage of coal-rock. Negative creep is a property in which the axial strain of a material decreases with time. Negative creep is more common in metal and alloys under high-temperature and low-stress conditions [22; 23; 24; 25]. The cause of this phenomenon is that the internal energy reduction in the material is greater than the external stress and is closely related to the initial strain and composition of the material [26]. In terms of rock materials, [PERSON] [27] and [PERSON] [28] et al. have observed the negative creep phenomenon in rock. It is believed that the negative creep phenomenon is induced by the fact that only part of the pores and voids of the structural rock are compacted under the low-stress condition during creep, and the other parts of the pores and voids are gradually compacted during the subsequent creep process, which causes the non-uniform deformation of the rock mass. The stress release phenomenon during this period is equivalent to the stress reduction, and the skeleton of rock produced elastic rebound, which is an abnormal phenomenon.  
As Stage I creep approached completion (35-40 d), the coal sample exhibited a rapid increase in time-dependent deformation, indicating partial creep deformation compensation. During this period, uniform creep deformation occurred, with continuous growth transitioning into Stage II. At the onset of Stage III creep deformation, a minor accelerated deformation emerged, attributed to fragmented creep damage. Existing research demonstrates that rock failure under uniaxial compression primarily occurs through end-zone compression-induced tensile cracking [29]. Following prolonged creep-water immersion weakening, the localized disintegration of end-zone coal-rock material may occur [30], causing abrupt strain acceleration and propelling the specimen toward the accelerating creep stage.