以世界上在建第二高拱坝——小湾坝基现场开挖工程为依托，对大规模岩体开挖卸荷现象的特征和表现出的规律，及其卸荷力学机制进行了探讨。通过现象描述、工程地质分析和力学分析相结合的手段，对小湾坝基开挖现象进行分析，归纳出了卸荷现象的表现规律，并提出相应力学模式合理解释了小湾坝基开挖的卸荷现象。最终得到如下结论：①小湾坝基开挖卸荷现象表现出多样性，并且在地貌特征和发生位置上具有明显的规律性；②结合河谷应力场特征，对小湾坝基开挖卸荷现象进行分析，并相应地提出了剪胀、纵弯曲张裂、错动板裂和上拱张裂等力学模式，合理地解释了小湾坝基开挖产生的卸荷现象；③板裂现象是高应力区发现的一种比较特殊的卸荷现象，它的产生需要比较苛刻的地质和力学条件。

The Xiaowan Power Station is the second highest arch dam in the world under construction. The height of the dam is about 292 m. The maximum excavated depth of dam foundation is 130 m. Large-scale excavation in the dam foundation of Xiaowan Power Station has brought intensive unloading phenomenon. The research purpose of this paper is to understand the rule and get mechanics model of unloading phenomenon. After excavation, the regularities can be found on landform characteristics and locations. In this paper the unloading phenomenon of Xiaowan is concluded into five types: step type, bend type, debark onion type, plate fracture type and floor heave type. Then the five types are analyzed by the methods of engineering geology and mechanics. Finally, four mechanical models are got ,and the unloading phenomena of Xiaowan can be explained reasonably by those models.