📄 earth_science

In-situ stress characteristics of the Tamusu argillaceous rock candidate area for China’s HLW disposal

This study utilizes hydraulic fracturing data from boreholes TZK-1 and TZK-2 to characterize the in-situ stress field of the Tamusu argillaceous rock candidate area, revealing a predominantly horizontal stress regime with a NNE–NE maximum horizontal stress orientation that supports its suitability for China's high-level radioactive waste disposal.

Original authors: Yuzhen Sun, Meihua Huang, Shuai Liu, Zhenxing Liu, Keyao Sun

Published 2026-08-24

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CC BY 4.0

Original authors: Yuzhen Sun, Meihua Huang, Shuai Liu, Zhenxing Liu, Keyao Sun

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Deep beneath the Earth's surface, the ground is not merely a static pile of rock; it is a material under immense, invisible pressure. This pressure, known as in-situ stress, comes from two main sources: the sheer weight of the rock layers above pushing down, and the slow, powerful forces of the planet's tectonic plates pushing from the sides. For scientists looking to store high-level radioactive waste, understanding this stress is a matter of safety. If the rock is too stressed, it might crack or shift over thousands of years, potentially allowing dangerous material to escape. To build a safe, deep underground vault, engineers need to know exactly how hard the rock is being squeezed and in which direction.

In the vast, flat desert of the Tamusu region in northern China, researchers have identified a potential home for such a vault. The site is made of argillaceous rock, a type of clay-rich stone that is naturally good at sealing itself and holding back fluids. Before this study, scientists had a good idea of the rock's chemical makeup and how it handled water, but they lacked a clear picture of the physical forces pressing on it at the depths required for a repository, which would be between 300 and 700 meters underground. Without knowing the stress field, it is impossible to design a safe tunnel system or predict how the rock will behave over centuries.

To solve this puzzle, a team of geologists drilled two deep holes, named TZK-1 and TZK-2, into the desert floor. They did not just drill; they performed a delicate experiment called hydraulic fracturing. Imagine a long, sealed section of the borehole, isolated from the rest of the rock. The researchers pumped water into this sealed section at high pressure until the rock wall, which was already under tremendous natural stress, finally gave way and cracked. By measuring exactly how much pressure it took to break the rock, and how much pressure was needed to keep that crack open or to close it again, the team could calculate the strength of the forces squeezing the rock from all sides. They repeated this process at various depths, reaching down to 800 meters, and used special tools to take "impressions" of the new cracks to see exactly which direction they opened.

The results revealed a dynamic and complex stress environment. In the upper layers, down to about 400 meters, the rock was being squeezed most strongly from the sides, with the horizontal forces dominating the vertical weight of the ground above. However, as the researchers went deeper, the balance began to shift. While the horizontal pressure remained very high, the vertical pressure from the overlying rock started to catch up. At the deepest points they tested, the rock was being squeezed almost equally from all directions, a state that is generally favorable for stability. The team found that the strongest horizontal force was pushing the rock from the northeast toward the southwest, a direction that aligns perfectly with the massive tectonic forces moving across the Asian continent.

This alignment with the broader tectonic map gives the researchers confidence that their measurements are accurate. The data suggests that the Tamusu region is relatively stable, with a stress structure that supports a type of geological fault movement called strike-slip, where rocks slide past one another horizontally rather than crashing up or down. This is a crucial finding for the safety of a future waste repository, as it implies the rock is less likely to experience sudden, violent vertical shifts. The study confirms that the rock at this site is of high quality, with very few natural cracks, and that the stress levels are moderate enough to allow for safe underground excavation.

Ultimately, this work provides the first concrete map of the invisible forces at play in one of China's most promising sites for nuclear waste disposal. By measuring the rock's response to water pressure deep underground, the team has moved the project from theoretical screening to engineering reality. They have shown that the Tamusu argillaceous rock can withstand the immense pressures of the deep earth, offering a solid foundation for the long-term safety of high-level radioactive waste. The findings serve as a vital guide for future engineers, ensuring that the design of the repository will work with the natural forces of the Earth, rather than fighting against them.

Technical Summary: In-situ stress characteristics of the Tamusu argillaceous rock candidate area for China's HLW disposal

Problem Statement
The selection of deep geological repositories (DGR) for high-level radioactive waste (HLW) in China requires a rigorous evaluation of host rock suitability. While the Tamusu area in the Bayingobi basin has been identified as a key candidate for argillaceous rock repositories, there is a significant lack of in-situ stress data for this specific geological setting. Previous research in China has largely focused on granite candidate areas, leaving the stress field characteristics of argillaceous rocks in the Tamusu region undefined. Understanding the magnitude and orientation of the present-day stress field is critical for engineering design, long-term safety assessment, and site screening, particularly given the complex stresses imposed by overburden weight and tectonic forces at recommended repository depths (300–700 m).

Methodology
This study presents the first in-situ stress measurement campaign in the Tamusu argillaceous rock candidate area, utilizing the hydraulic fracturing method. The research focused on two sub-vertical boreholes, TZK-1 and TZK-2, drilled to depths of up to 800 meters.

Site Selection: Boreholes were positioned in intact argillaceous rock sections, distant from major faults to minimize fault-induced stress anomalies. Rock quality designation (RQD) values indicated good to excellent rock mass integrity (75%–90% and >90%, respectively).
Measurement Technique: A single-loop hydraulic fracturing system was employed following ISRM standards. The process involved isolating test intervals (0.9 m length) using straddle packers, pressurizing the interval to induce fracturing (breakdown pressure, $P_b$ ), and subsequently reopening the fracture to determine the instantaneous shut-in pressure ( $P_s$ ) and reopening pressure ( $P_r$ ).
Data Analysis: Vertical stress ( $S_v$ ) was calculated based on overburden weight (using an average rock density of 2.48 g/cm³). Minimum horizontal stress ( $S_h$ ) was equated to $P_s$ . Maximum horizontal stress ( $S_H$ ) was derived using the formula $S_H = 3P_s - P_r - P_0$ (where $P_0$ is pore pressure). Fracture orientations were determined using impression packers with automatic orientation indicators.

Key Results

Stress Magnitude and Regime: The study reveals that the Tamusu area possesses an intermediate horizontal stress level. The stress regime exhibits a depth-dependent transition:
- At shallow depths (300–400 m), the stress field follows the order $S_v > S_H > S_h$ .
- At depths exceeding 400 m, the regime shifts to $S_H > S_v > S_h$ , indicating that horizontal tectonic stress dominates over the vertical overburden stress.
- Linear regression analysis established the following relationships with depth ( $H$ in meters): $S_H = 0.0390H - 4.853$ and $S_h = 0.0236H - 2.015$ .
Stress Orientation: The maximum horizontal principal stress ( $S_H$ ) is consistently oriented along the NNE–NE direction. This orientation aligns with the regional tectonic stress field of Northwest China.
Structural Implications: The identified stress structure ( $S_H > S_v > S_h$ ) is conducive to strike-slip fault activity. However, the area is historically seismically stable, with no recorded earthquakes exceeding magnitude 5.

Significance and Claims
The authors claim that this study provides the first key in-situ stress data for the Tamusu argillaceous rock candidate area, filling a critical gap in the site screening process for China's HLW disposal.

Technical Support: The results offer essential technical support for the screening of argillaceous rock sites, validating the stability of the host rock for underground excavation due to the relatively small difference between maximum and minimum principal stresses compared to crystalline rocks.
Tectonic Correlation: The findings confirm that the local stress field is driven by the NE-tectonic stress originating from the collision of the Indian and Eurasian plates, consistent with the broader crustal stress environment of Northwest China.
Limitations and Future Work: The paper modestly notes that hydraulic fracturing primarily measures horizontal stresses and that the vertical stress was calculated rather than measured. Furthermore, the study is based on only two boreholes. The authors suggest that future site evaluation requires additional boreholes and complementary measurement methods to fully verify the regional stress field distribution.

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