INTERNATIONAL JOURNAL OF GEOMATICS AND GEOSCIENCES Volume 1, No 3, 2010

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1 Geologic hazards research and their inducing factors in Xianyang urban area, Northwest China Wu Jianhua, Xi Wenjuan School of Environmental Science and Engineering, Chang an University, No.126 Yanta road, Xi an, Shaanxi, , China ABSTRACT Xianyang urban district is one of most serious geologic hazard zones in Shaanxi province, Northwest China. In this paper, through field investigations, the main types and characteristics of geologic hazards in Xianyang urban district were analyzed, the different developments of geologic hazards were classified, the formation and inducing factors were explained as well as the estimation of harm on economy losses and human beings were figured out. The results show that the main hazard types in this region are collapse, ground subsidence and ground fissures, and the whole area can be divided into two geological disaster prone zones. Geological disasters are mainly developed in the front line of 3rd terrace and in some local areas of the Loess Plateau. The occurrence of geologic hazard in this city is closely related with topography, geology, rock and soil properties, precipitation, and groundwater pumping and human activities. Keywords: Geologic hazard, Xianyang urban area, control measures, disaster prone areas 1. Introduction Since the 1980s, many natural disasters including landslides have drawn unprecedented attention from the international community with the start up of "International Decade for Natural Disaster Reduction (INDR)" plans brought forward by the United Nations. Much international and regional cooperation in natural disaster research has been implemented, which is a strong impetus to natural disasters prediction around the globe. Since the 1950s, many departments, such as the Regional Geological Survey Team of Shaanxi Provincial Bureau of Geology and Mineral Resources, First Team of Hydrogeology and Engineering Geology, the Provincial Geological Environment Monitoring Station, have conducted a large number of regional geological, hydrogeological and environmental geological researches, and accumulated a large amount of valuable information. In addition, Land Resources Bureau of Qindu and Weicheng District carried out a large number of geological surveys since taking up the management of geological disasters prevention, which also have played a positive role for the prevention of geological disasters in the region. However, the geological disasters emergency system in the area is still not perfect, and five level emergency system of geological disasters, namely, city level, district level, street level, community level and person level, need to be set up so that timely and 426

2 effective prevention of geological disasters and restoration before and after the occurrence of disasters can be conducted, and residents and properties in the disaster area can receive timely and proper prevention and treatment, making the losses caused by geological disasters reduce to a minimum. This research was carried out in the hope of providing strong technical support for the prevention of geological disasters in the area. 2. Study area The study area is located in the heart of the Guanzhong Plain where the Xianyang government located. It situated between longitude E and E, latitude N and N. It r covers a total of km 2 with a population of 0.75 million. As the political, economic and cultural center, it s a historical and cultural city and is famous for the ancient cultural heritage and natural and cultural tourism resources. This area belongs to continental monsoon climate region which is featured with four clear seasons with a lot of rains in spring, hot in summer and cold in winter; rapid warming in the spring and rapid cooling in autumn. Distribution of precipitation in the region during the year is uneven. Heavy rains usually appear in July to September and the even rain and the heavy rain always appear in succession. The monthly precipitation is shown in Figure 1. Heavy rain and even rain are the main inducing factors of the collapse, ground fissures, ground subsidence and other geological disasters in the region. 100 Pricipitation mm Month Figure 1: Monthly precipitations in the study area The topographies in the study area are mainly the river alluvial plain and the Loess Plateau (shown in Figure 2). 427

Figure 2: Landforms in Xianyang City From Loess Plateau in the north to the Weihe River in the south, the terrain lowers as a ladder unit by unit, and altitude difference is usually more than 70 m.

3 Figure 2: Landforms in Xianyang City From Loess Plateau in the north to the Weihe River in the south, the terrain lowers as a ladder unit by unit, and altitude difference is usually more than 70 m. According to borehole data, Xian yang urban mainly deposits Quaternary and Tertiary strata formed by lacustrine and fluvial facies alluvial, aeolian or flood borne sediments. Lithology of Quaternary mainly contains loose sand eggs gravel, sand, silt, silty clay and loess, while Tertiary lithology is dominated by mudstone, sandy mudstone and sandstone. According to the age and causes of the soil formation in the area, soil can be divided into the sandy soil, cohesive soil and loess. Loose sand is mainly distributed in the floodplain on both sides of the Weihe River and floodplain at the cross area of the Weihe River and Fenghe River east of Longhai railway; cohesive soil is mainly distributed in the Weihe River's first terrace and the south bank region of the Weihe River; loess is mainly distributed in the second and the third terraces. 3. Distribution of geologic hazards According to the investigation results, the main types of geological disasters in Xianyang urban area are collapse, ground subsidence and ground fissures. Based on field survey, the distribution of geological hazards, geomorphology and geological structure, disasterpoint density, disaster risk as well as the relationship between geological disasters and human activities were precisely investigated. The whole region is divided into 2 428

4 geological disaster prone zones (B1 and B2), 2 safety zones (D1 and D2) with qualitative analytical method. The zonal map is shown in Figure 3. Figure 3: Zonal map of geological disasters Geological disasters are mainly developed in the front line of 3rd terrace and in some local areas of the Loess Plateau, while the flat terrain, such as 1st and 2nd terrace of the Weihe River and the flat part of Loess Plateau, does not face the risk of geological disasters. Most parts of the Loess Plateau Area in the north of Xianyang urban area are broad, the depressions here are wide and shallow, and are not likely to cause landslides, mud rock flow. However, because of the existence of the old sub cellar, tombs and bomb shelters dug before or after 1968 locally, in addition to the soaking rain, ground subsidence is most likely to occur as a hidden danger. 8 hidden ground subsidence disaster points were found during the survey in the Loess Plateau, accounted for 28% of the total number of geological disaster points in the region. In the front of 3rd terrace and posterior marginal of 2nd terrace, in central part of this region, exits a scarp which was corraded consumingly, forming a series of N S direction gullies along the front of 3rd terrace. Over years of human activities, the slope becomes almost vertical. Hence, the area becomes a collapse prone zone. Furthermore, a big fault 429

5 along the north bank of the Weihe River goes through the area along the posterior edge of the 2nd terrace. Many fracture activities, groundwater and geothermal water exploitation and precipitation infiltration have caused many ground fissures. 21 geological disaster points were found in the survey, accounting for 72.4% of the total number of geological disaster points in the region. 4. Evaluation of geological hazards Geological hazard assessment consists of the stability evaluation, seriousness evaluation and economic losses evaluation. 29 hidden geological disasters points were evaluated according to the stability and 9 are worst stability, accounting for 31% of the total number of geological disasters points; 17 are worse stability, accounting for 59%, 3 are bad stability, accounting for 10%. In accordance with the geological disasters seriousness in the region, all the 29 hazard points were evaluated, evaluation results are as follows: 1 point is extremely serious, accounting for 3% of the total number of geological disasters in the area; 5 are serious, accounting for 17%, 21 are medium grievous, accounting for 72%; and 2 are not grievous, only accounting for 7%. The occurrence of geological disasters in the region has resulted in 3342 urban and rural residents affected and 795 houses damaged as well as 11 apartments, 1406 villager housing and 70 caves damaged in varying degrees. Direct economic losses caused by the geological disasters have reached million RMB. The solutions for each disaster point are proposed as well. The results are shown in table1. Table 1: Assessment results of geologic hazards Numbe r Stability Seriousness Solutions 2 Worst Serious Relocation 6 4 Worst Medium Cutting slopes, reinforcing cave, revetment 3 Worst Medium Relocation 3 Bad Serious Relocation 5 Worst Serious Relocation 7 9 Worst Medium Relocation 6 Worst Medium Relocation 430

6 QD QD QD001 2 Worse Serious Slopes cutting, cave reinforcement, cutter repairing Worse Medium Slopes cutting, cave reinforcement, cutter repairing Worse Medium Slopes cutting, cave reinforcement, biological control measures Worst Medium Slopes cutting, cave reinforcement, cutter repairing Worse Worse Medium Medium Slopes cutting, cave reinforcement, biological control measures Slopes cutting, cave reinforcement, biological control measures Worse Medium Slopes cutting, cave reinforcement, cutter repairing Worse Medium Slopes cutting, cave reinforcement, cutter repairing Bad Not grievous Slopes cutting, cave reinforcement, cutter repairing Worst Serious Relocation Worse Worst Medium Extremely serious Slopes cutting, cave reinforcement, biological control measures Relocation Bad Not grievous Relocation 5. Inducing factors analysis Geological disasters are closely related with the geological formation and various internal and external environmental conditions (Zhang 1988; Zhong 2004; Pan and Li 2006; Li 2009). It s necessary and urgent to study the causes and mechanism of its formation for taking effective preventive measures. 431

7 5.1 Landforms Topography condition is a major factor controlling the geological disasters occurrence. If the terrain is broad and flat, collapses, landslides and other geological disasters will not happen even if there are other influencing factors. In contrast, where there are large free face and high elevation gradient or on activity tectonic faults strip, geological disasters may have a relatively larger probability to occur. River terrace edges are common landslide sites. Terraces are often composed of a plane rock surface with fluvial or colluvial sediments above, and then loessic sediments, both primary and reworked to a thickness of 10 m or more (Derbyshire 2001). Main landscape types in the study area are alluvial river terraces and the Loess Plateau (as is shown in Figure 4). Loess Plateau is located in the northern part of the study area, flat, and do not have the conditions to induce collapses and landslides, while in front of the third terrace, south of the Loess Plateau area, due to the formation of scarp with slope angle of 80 0 ~ 85 0, collapse and cracks and other geological disasters are most likely to happen. 5.2 Rock and Soil Figure 4: Profile of the landforms and geology in Xianyang Rock and soil are the material basis of geological disasters, different types of rock form different geological disasters, which depends mainly on rock and soil rigidity, degree of weathering and water resistance and changes in the level of traits after watering (Luo 1997; Liu 2000). Loess Plateau is widely distributed in the north part of the study area. Loess, as a special type of soil, have such properties as weak wind resistance, developed vertical joint, easy to crack when drying, easy to soften with water and low shear strength. These characteristics determine the formation of the Loess Plateau is easy to develop 432

8 ground subsidence geological disasters. The changes of the landform, ecological surroundings, climate and hidden depth also give rise to the spatial variation in the collapsibility of each type of loess (Wang and Zhou 1999). There are also differences between old loess and new loess in characters. The earlier the loess is formed, the smaller the porosity is. The water permeability and collapsibility are also small for the early formed loess. The early formed loess usually has high intensity, so geological disasters are less likely to occur. 5.3 Geological conditions Geological conditions usually refer to geological structure, neotectonic movement and earthquakes. There conditions usually control the development of various geologic disasters.geological structure is an internal factor controlling geological disaster occurrence. Geological disasters will usually happen in the geological structure development zone. The study area is located in the joint areas of North China platform Weihe fault zone and the southern margin of Ordos Sag. The fault developed to the north bank of the Weihe River is an active fault which goes crosses from west to east, forming a terrace scarp with large free face in front of the third terrace, which can cause collapses and cracks as well as other geological disasters. Neotectonic movement affects geological disaster occurrence by the further transformation of the prior structure. The existing hidden fault in the region is an active fault. Since the Quaternary, Xi'an depression, south of the fault, has relatively been declining, resulting in the floodplain of Weihe River being located above the first terrace as a stacked structure. Xian Li fault terrace in the north relatively rise, forming a plateau edge scarp. The fault s activities have increased the height of the slope and have formed many cracks in the soil covered above, making it easy to form collapse, cracks and other geological disasters. The earthquake, a highly destructive disaster, can induce many other geological disasters. Seismic activity is strong in this area because of faults activities in this area. According to the survey statistics, since 131 BC, more than 10 earthquakes above 5 magnitudes have happened around the urban areas, and the most serious one happened in 1568, reaching to 6.75 magnitudes, followed by a 6.25 magnitude earthquake happened in Lintong County. Since regional seismic network was set up in 1970, 7 small earthquakes have happened around the north eastern fringe of the urban area, of which the largest happened on 5 January, 1998 in Jingyang County, reaching 4.8 magnitudes, followed by a 3.8 magnitude earthquake happened in Wugong County in Earthquakes occurred in Jingyang County and Wugong County brought out large damage to the houses and people. 5.4 Hydrological factors Geological disasters are closely related with water, in particular, is closely related with the precipitation and groundwater. Precipitation can cause high soil moisture and soil instability which is usually easy to induce landslides, ground subsidence and ground 433

9 fissures. The runoff will also, of course, erode the ground surface and cause landslides. Many geological disasters usually occur during the storm rain period with large precipitation intensity. It is reported that during the flood season in 2003, when the rain last 76 days with mm precipitation, many collapse occurred in the front line of third terrace, especially, 113 houses and 80 caves collapsed in Weicheng District. In addition, the ground subsidence induced by the direct impact of rainfall in flood season occurred 3 times and cracks occurred twice which resulted in more than 20 households in the village damaged and caused large economic losses. Groundwater type in the southern river valley region is mainly loose alluvial pore water and in the northern Loess Plateau Area is mainly loess pore fissure water. Because of long term exploitation in the urban area, regional groundwater levels continue to decline, resulting in soil compaction in confined aquifer, which causes land subsidence in the pumping cone zone. Land subsidence can usually induce or exacerbate cracks. 5.5 The impacts of human activities Human activities usually can change the topography, imposing impact on geological disasters. Human activities can have great impact on geological disasters in their distribution and development. In the study area, underground water is over exploited, resulting in serious land subsidence which affects or exacerbates the occurrence of ground fissures. At the front line of the third terrace, many residents dug kiln, cut slope and built houses, forming high and steep artificial slope. These activities can cause geological disasters and influence their distributions. In summary, the development of geological disasters is affected by a variety of factors including natural factors and human factors. Natural factors are inherent and can not be eliminated Human factors plays a very important role in the development of geological disasters, so human activities should be normalized to curb human behavior induced geological disasters. 6. Conclusions Through field investigation and indoor assessment, geological disasters types, their distributions, their seriousness were identified and a geologic hazards zonal map was drawn. The inducing factors were also explained in this paper. The following conclusions were reached geologic hazard points were identified. The major hazard types in this region are collapse, ground subsidence and ground fissures. Most of the hazards are distributed along the front line of the third terrace and distributed locally on the loess tableland. 2.The general measure to cope with the hidden dangers of geological disasters is to take relocation. For a small number of points where relocation is not easy to 434

10 implement, cutting slopes, reinforcing cave, revetment or biological control measures are the smartest choice. 3.Landforms, rocks and soil, geological conditions, hydrological factors and human activities are the major factors influencing the occurrence and development of geologic hazards in the study area, among which geological conditions and hydrological factors are the most important and influencing factors. Acknowledgements The authors would like to express their sincere thanks to Senior engineer Luo Tong in Shaanxi Provincial Institute of Regional Geology and Mineral Resources for his support and help in field work. The authors also want to thank Professor Qian Hui in Chang an University for his theoretic help. At last, authors want to thank anonymous reviewers for their useful comments on the manuscript. References 1.Derbyshir E., Geological hazards in loess terrain, with particular reference to the loess regions of China. Earth Science Reviews, 54 (1 3), pp Li P. Y., Analysis of predisposing factors of environmental geologic disasters in Xian Yang City. Environmental Science and Management, 34(S1), pp Liu C. Z., Guide of Geological Hazards Investigation, Geological Publishing House, Peking,China. 4.Luo G. Y., Research Status and Prospection of Geological Hazards. Hydrogeology and Engineering Geology, (2), pp Pan Z. and Li D. F., Geology disaster, Peking University Press, Peking, China. 6.Wang Y. Z. and Zhou L. C., Spatial distribution and mechanism of geological hazards along the oil pipeline planned in western China. Engineering Geology, 51, Zhang X. G., Li Z.Y., Zheng D. H., et al., Specialized engineering geology, Geological Publishing House, Peking, China. 8.Zhong Z. R., Geological environmental protection and prevention of geological disasters, China University of Geosciences Press, Peking, China. 435

Land subsidence due to groundwater withdrawal in Hanoi, Vietnam

Land Subsidence (Proceedings of the Fifth International Symposium on Land Subsidence, The Hague, October 1995). 1AHS Publ. no. 234, 1995. 55 Land subsidence due to groundwater withdrawal in Hanoi, Vietnam