Cracks in the anti-seepage layer of earth-rock dams can lead to landslides or seepage damage, with cracking being the most common seismic failure mode during earthquakes. Accurately assessing whether a dam cracks during an earthquake, estimating the depth of such cracks, and evaluating their impact on the dam's stability are critical challenges in seismic design. Understanding the tensile properties of the soil is essential for addressing these issues, but obtaining accurate tensile data requires testing methods that closely simulate real-world stress conditions. Various techniques have been developed to measure soil tensile properties, and this paper provides an overview of these methods along with key experimental findings.
Currently, several tensile testing methods are widely used both domestically and internationally. The direct tensile test involves applying axial tension to the specimen using clamps or adhesives, often under unconfined or triaxial conditions. However, stress concentrations at the clamped ends may cause premature failure, limiting its accuracy. In contrast, indirect methods like the soil beam bending test, Brazilian test, axial fracturing test, and air pressure split test apply indirect stresses and use theoretical models to estimate tensile strength. Each method has its own advantages and limitations.
For example, the soil beam bending test applies pressure to the top surface of a rectangular beam, inducing tensile stress at the bottom. The Brazilian test uses diametrical compression to induce splitting, while the axial fracturing test applies axial loading to split the sample. The air pressure split test, on the other hand, applies internal pressure to cause tangential fracture. These methods yield different results, with the beam bending test typically showing the highest tensile strength, while the fracturing and air pressure tests give lower values.
Studies have shown that soil tensile strength increases with water content up to an optimal point, after which it decreases. Tensile strain also tends to increase with higher moisture content. Additionally, the ratio of unconfined compressive strength to tensile strength varies depending on soil type and plasticity index. Loading rates significantly affect the measured values, as do repeated loading cycles, which can reduce the tensile strength over time. The Griffith criterion, while useful for some soils, does not universally apply due to discrepancies between theoretical predictions and experimental results.
To better understand the behavior of soil under seismic conditions, two key aspects must be considered: (1) the need for testing under varying stress states, including confining pressures, and (2) the dynamic nature of earthquake loading, which differs from static conditions. This has led to the development of specialized equipment, such as triaxial devices capable of applying axial tension and pressure, dynamic triaxial systems with periodic loading, and acoustic emission sensors to monitor crack propagation.
Acoustic emission detection is particularly valuable for identifying micro-cracks and internal damage during tensile tests. By analyzing the frequency and intensity of acoustic signals, researchers can gain insights into the failure mechanisms of soil samples. For instance, unconfined compression tests show continuous acoustic emissions until failure, whereas radial fracturing tests produce sudden bursts of activity before splitting occurs. These differences highlight the distinct failure modes of shearing and cracking.
In conclusion, understanding soil tensile properties is crucial for improving the seismic resilience of earth-rock dams. Ongoing research and advanced testing methods, including cyclic loading and acoustic monitoring, are helping engineers develop more accurate models for predicting crack initiation and propagation under dynamic conditions.
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