Occurrence Conditions of and Process Analysis on Fluidization
of Valley type Landslide
Fluidization of landslide indicates the action and process of an alive landslide motion of converting from sliding state to fluidal disposition due to vibration and watering function occurred in process of landslide’s medium and high velocity movement. Research on landslide fluidization is a major subject in the field of landslide and debris flow studies, which has a significance of deepening people’s understanding of landslide disaster and rules of debris flow occurrence. Taking for example, Sichuan Nanjiang Baimeiya large-scale landslide on Sept. 14, 1974 happened after a large quantity of rainfall precipitation resulting in a 7×106m3 large-scale landslide, which eventually converted to debris flow with an estimated maximum velocity of 60m/s and a length of movement of about 5km, and 195 people killed; an other example is the 2×105 m3 medium-sized landslide occurred in Shijiapo, Da’an Town, Ningqiang County, Shanxi Province at 10 pm on Aug. 24, 1981 after a steady rainfall, which finally converted to debris flow, with an approximately moving distance of 700m and14 people killed as well as big loss of property.
For the present, the research on occurrence mechanism of landslide fluidization is stilled focused on model test and theory research at home and abroad. According to CB, a lot of debris flow that is considered as the consequence of rainfall is actually developed from shallow landslide which is caused by reduction of shear strength of soils due to increase in pressure of pore water; HW considered the fluidization of landslide occurring after storm forms in five stages; RD proposed three steps in which landslide converts to debris flow based upon model experiment, field observation, lab experiment and theoretic analysis; Li Zhuofen proposed four failure mechanisms of landslide and debris flow occurring after storm according to analysis on formation cause of landslide and debris flow; Li Yongyi simulated the process in which landslide converts to debris flow through model experiment based upon field survey; while Sun Jiming preliminarily classified the fluidization of landslide into slope-type fluidization and gully-type fluidization.
According to research information at home and abroad, the main achievements of research on fluidization of landslide can be seen from: ① a lot of data has been obtained from field survey and some parameters has been measured; ② great performances has been achieved in research on formation mechanism for fluidization of landslide by means of new experiment and method as well as field and indoor experiments; ③ some characteristics of fluidization of landslide has been found through theoretic research of various aspects. However, there might be some insufficience in those researches, such as no systematical theoretic research and corresponding filed and lab experiments have been carried out on formation conditions of fluidization of landslide, or detailed classification and discussion been made on occurrence and development of it.
Taking the heavy rainfall in Qiaongshangou area for example, the rainfall and slope runoff in Qiaongshangou area led to sliding of soils on sharp slope and further to fluidization, therefore, a large quantity of sliding substance entered into gullys, where it was converted to debris flow immediately by infiltration and scouring of floods in gullys. It is a typical landslide-debris flow formed by fluidization of landslide. According to the related investigation, the reason that fluidization of landslide causes formation of debris flow mainly lies in the conditions favorable for fluidization of landslide such as landforms, slope structure and lithologic characters, etc. Specifically, it is the combined action of sharp slope structure, loose macadam stratum, continuous rainfall, runoff infiltration and the vibration caused by sliding that leads to fluidization of landslide.
General Introduction and Disaster Features
邛山沟(又叫切山沟)为大渡河上游大金川的一条支沟，当地称为水卡子，流域面积84.90km2，海拔为3000m。邛山沟汇水面积约31km2，主沟道长约9.6 km，沟道平均坡度约250‰，是泥石流的主要源区。邛山沟的中游广泛分布有大面积的古老泥石流台地。台地上居住着邛山二村等几个村寨的村民。该流域位于岷山邛崃山和大雪山的交界处，高山耸峙, 河谷深切, 沟壑纵横, 系高山峡谷地貌。沟内出露地层为志留系茂县群厚层石英岩、二云英片岩夹薄至厚层大理岩，基岩为变质花岗岩、闪长片麻岩和少量的变质大理岩。基岩表面风化形成30～60cm厚的残坡积堆积物。
Qiongshangou (also named Qieshangou) is a tributary gully (locally called Shuikazi) of Dajinchuan River on the upper reach of Dadu River, with a drainage area of 84.90km2, an altitude of 3,000m above sea level and a catchment area of 31km2. The main gully is about 9.6km and the average slope grade of gully is about 250‰. It is the main formation area of debris flow. On the middle reach of Qiongshangou spreads a large area of ancient debris flow platform, where several villages including No. 2 Qiongshan Village are located. The drainage area is located within the bordering area of Minshan Mountain, Qionglai Mountain and Snow Mountain, where high mountain, deep river valley and a lot of gullys spread, with a the mountain-valley landform. Outcropping strata in gully consists of thick quartz rock and the thin-to-thick marble contained two-greisen schist, of Maoxian Group in Silurian System. Bedrock consists of metamorphic granite, diorite gneissose and a few of metamorphic marble, and its surface layer contains a 30～60cm thick residual slope deposits formed by weathering.
受地形和气候的影响，流域内降水量呈现明显时空差异：由低海拔的干热河谷到亚高山带逐渐增加，在高山带又降低。年均降雨量在700mm左右，年际变化较大，月、旬降雨分配极不均匀，年降水量呈“双峰型”,即 6 月为最高峰，9月为次高峰，其降雨特征总体表现为雨季迟、降水日数少、夜雨多，最多雨日一般出现在 6、7月份。
Subject to the local landform and climate, there is a large temporal and spatial variation in rainfall of the area: the rainfall increases gradually from dry-hot valley at low altitude to subalpine zone, and decreases again at alpine zone. The average annual rainfall is about 700mm and changes greatly every year. The monthly and ten-day rainfall varies greatly. Annual rainfall distributes in a “double hump” shape, namely the max. peak in June and the second peak in September. In general, rainfall of the area is characterized by late rainy period, a few rainy days, more-ikely night rain and the period with the max. rainfall in June or July.
Landslide occurred on the right bank of Qiongshangou gully. Before it occurred, rainfall lasted for a few days in Qiongshangou area, leading to increase in water content of residual slope deposits and deformation of slope. The slope deformation was aggravated later by heavy rainfall in the area. When water content of landslide increased to 16.1%, the debris soil on surface of slope reached to the saturated status, therefore the deformation was further aggravated and the slope began to disintegrate, which leaded to sliding of the whole slope and occurrence of landslide in large area. Meanwhile, the front part of landslide began to flow in high speed toward Qiangshangou gully and accumulated there, and then converted immediately to debris flow when scoured by floods. The landslide is about 140m long, about 21m wide and 3.5m thick in average. It is estimated that the total volume is about 10,000m3 and the sliding direction is 1450.
The debris flow formed by fluidization of landslide has a max. capacity of 2.0g/cm3 and a max. flow rate of 9.4m/s. The peak flow rate and flow at outlet of gully is 4,836m3/s and 8.5m/s respectively. The max. depth of debris flow is 8.1m in average. In the disaster, 51 people were killed or missing, about 73,136 m2 area of forest, farmland and residential area on deposited area are destroyed, the whole Shuikazi village was almost inundated, and S211 Provincial Highway was damaged by a total length of over 500m.
Analysis on Occurrence Conditions
Qiongshangou belongs to a deep mountain-valley landform. The outlet of gully is about 2，036m above sea level and the landslide occurred 2,809m above sea level. The gully is covered by abundant vegetation while the slope grade varies greatly. The average slope grade of the left bank and the right bank is 550 and 350 respectively, therefore the gully appears in an asymmetric “V” shape. Landslide occurred on the right bank of Qiongshangou gully where the slope grade is 390. The front edge of landslide is near to gully bottom and there is a small area of shallow and low land at the top of landslide, and above the low land is sharp peaks. Thus, a favorable catchment conditions form at the back part of Qiongshangou, which promotes collection of surface water and infiltration of it into slope during rainfall, and provides favorable conditions for formation of landslide.
Qiongshangou gully is shallow, 3m wide in average. During rainfall, a great amount of surface water enters into gully at a large flow rate, providing conditions for supplementing water to accumulated mass immediately. In addition, when landslide slides into gully and accumulates immediately, the gully will be blocked and water level increased, which provides conditions for infiltration of floods into accumulated mass in a large quantity. If more surface water enters into gully, more floods will flow across surface of accumulated mass, therefore a large part of accumulated mass will be carried away and gradually disintegrated, and finally converted to debris flow.
In Qiongshangou area, the outcropping strata in gully consists of thick quartz rock and the thin-to-thick marble contained two-greisen schist, of Maoxian Group in Silurian System, and bedrock consists of metamorphic granite, diorite gneissose and a few of metamorphic marble, and its surface layer contains a 30～60cm thick residual slope deposits formed by weathering. Among slope deposits, silt and clay account for 42.4%, sand 31.1%, and gravel and macadam 26.3%, and its porosity is higher than 34%. Landslide deposits consists of a large quantity of macadam and a few of block stone. It is loose in structure and has a high porosity up to 38%. Therefore, the lithologic conditions are quite favorable for infiltration of surface water.
Sand-soil liquefaction: when sandy soil and silty soil become saturated with water, and at the same time the pressure of pore water increases due to vibration, the pressure transferred by contact between soil particles will be decreased, if the effective pressure disappears completely, the shear strength and bearing capacity of soil layer will loss completely and the soil will become liquid.
According to analysis on composition of landslide, 31% is sand and 22.4% is silt, which is a favorable substance condition for occurrence of landslide fluidization. On the other hand, the vibration caused by sliding of landslide is the dynamic condition for liquefaction of saturated sandy soil and silty soil in landslide. However, the liquefaction of sandy soil and silty soil is just the direct factor leading to fluidization of landslide.