Abstract: The Great Wenchuan Earthquake which measured 8.0 Ms on the Richter scale occurred on May 12, 2008 in Sichuan province of China. Seismic activities were concentrated in the mid-fracture of the Longmenshan fault (known as Yingxiu-Beichuan fracture). Starting from the epicenter at Wenchuan, the rupture propagated at an average speed of 3.1 km/s, 49° toward the northeast, rupturing a total of about 300 km. Very disastrous damages resulted. As of May 27, 2008, 34 large quake lakes had formed in nine earthquake-affected counties, the most precarious being that located in the extremely difficult terrain of Mount Tangjiashan. Faculty and graduate students from the State Key Laboratory for Hydroscience and Engineering in Tsinghua University not only took part in rescue activities but also conducted site surveys in the Xiaojiaqiao and Tangjiashan lake areas, collecting first-hand data on site. Quake lakes present challenging questions of mechanics. First, how are dams formed during the processes of massive landslides, rockfills and debris flows, and secondly how strong are they?. Dams may be regarded as a loose packing of granular materials on a huge scale, exhibiting quite different properties from common materials, which is exactly the key issue in the study of granular mechanics.