Ground fissure is a geological phenomenon that the surface rock and soil body cracks under the action of natural or man-made factors, forming cracks with a certain length and width on the ground. When this phenomenon occurs in areas with human activities, it may become a geological disaster.
Second, what harm will ground fissures cause?
Ground fissure is a unique urban geological disaster. Since the late 1950s, it has been found that the activity of the 1976 Tangshan earthquake has been obviously strengthened, especially since the 1980s, excessive pumping of confined water has caused uneven ground settlement on both sides of the ground fissures, which further aggravated the activity of the ground fissures. All kinds of ground fissures pass through houses, factories, mines, farmland, roads, water pipes and various public facilities, causing building damage, farmland damage, road deformation, pipelines and so on.
3. What are the types of ground fissures?
The reasons for the formation of ground fissures are complex and diverse. Crustal activity, water action and some human activities are the main reasons for ground cracking. According to the causes of ground cracks, they are often divided into the following categories: (1) earthquake cracks.
All kinds of earthquakes cause strong ground vibration, which can produce such cracks.
(2) Active cracks in basement fracture
Due to the long-term creep of basement fracture, rock mass or soil layer gradually cracks and exposes the surface.
(3) Open cracks in hidden cracks
In the soil with hidden cracks, the substances in the cracks are taken away by water under the scouring and undercurrent of surface water or groundwater, and the cracks open and penetrate upwards.
(4) Hidden corrosion cracks in loose soil
Due to the scouring, undercurrent, softening and liquefaction of surface water or groundwater, some particles in loose soil are lost with water and the soil cracks.
(5) Collapsible cracks in loess
Because the loess stratum is soaked by surface water or groundwater, it sinks.
(6) Expansion and contraction cracks
Due to the dry and wet changes of climate, expansive soil or muddy soft soil undergoes expansion and contraction deformation and develops.
(7) ground subsidence cracks
It is formed by the cracking of rock and soil in the process of land subsidence caused by various types of land subsidence, groundwater overexploitation and underground mining.
(8) Landslide cracks
It is formed by surface cracking caused by slope sliding.
In addition, according to the dynamic reasons for the formation of ground fissures, that is, internal forces and external forces in the crust, ground fissures are usually divided into three categories: structural ground fissures, non-structural ground fissures and mixed ground fissures. The above (1) and (2) belong to structural ground fissures, (4), (5), (6), (7) and (8) belong to non-structural ground fissures, and (3) belongs to mixed ground fissures.
These are just the main types, and there are other types, so I won't go into details here.
Ground fissure disaster is also one of the main geological disasters that seriously affect people's life, production and construction in China. Widely distributed throughout the country, there are thousands of cracks. According to the incomplete statistics of Hebei, Henan, Shaanxi, Shanxi, Shandong, Jiangsu, Anhui and other provinces, 746 ground fissures have been found in 200 counties (cities, districts). In cities, ground fissures have appeared in Xi, Datong, Handan, Baoding, Shijiazhuang, Tianjin and Zibo, among which Xi 'an is the most typical and serious. Xi 'an ground fissures are distributed from Hesui River in the west, Textile City in the east, Sanyao Village in the south and Inoue Village in the north, covering an area of about 155 square kilometers. It is developed on the basis of the special landform of loess liangwa, and has the characteristics of zonal development, parallel and equal spacing, NNE distribution, and the main ground fissures are all inclined south.
Fourth, the causes of ground fissures
In addition to tectonic activity, the factors that cause strong ground fissures are mainly related to uneven ground settlement on both sides of the ground fissures caused by over-exploitation of confined water (mainly taking Xi city, where the ground fissures are the most serious, as an example).
Tectonic activity. Xi 'an fault depression has been sinking, and Chang 'an-Lintong fault is also moving south and falling north. Xi 'an 1 1 ground fissure zones are located in the descending north wall, and their distribution and activities are controlled by Chang 'an-Lintong fault. The leveling data across faults for many years show that the average vertical activity rate of Chang 'an-Lintong fault is 3.98 mm/year, accounting for about 7.3% of the average vertical activity of cracks.
Overexploitation of confined water is the main reason for the intensification of ground fissure activity. The data of drilling, seismic geophysical exploration and trench exploration confirm that the Quaternary system in Xi 'an area is an interbedded structure with unequal thickness, consisting of cohesive soil layer and sandy pebble layer. The cohesive soil layer in the southern suburb of Xi 'an Andong is thick, and the sand layer gradually increases from southeast to northwest, and the thickness increases. The thickness of cohesive soil layer in Huangtuliang depression is obviously different, and the thickness of cohesive soil layer in the depression is greater than that in Huangtuliang area. In addition, due to the activity of Chang 'an-Lintong fault, a certain width of soil fracture loose zone is produced along the ground fissure and its vicinity, and the upper wall of Xi 'an ground fissure is looser than the lower wall.
Over the years, due to the excessive pumping of groundwater in the suburbs of Xi 'an, the confined water level has dropped greatly, resulting in the tight ground release pressure and large-scale land subsidence. Because the thickness difference of cohesive soil layer on both sides of ground fissure is different from the degree of loose cracking of soil, the degree of deformation and settlement of ground fissure is also different, which further intensifies the vertical activity of ground fissure. According to the monitoring data, 70%~90% of the vertical activities of ground fissures in different sections and with different activity rates are caused by differential settlement.
V observation and general survey of ground fissures
Among all kinds of ground fissures developed in China, apart from earthquake fissures (which are often studied together with earthquakes), the active fissures of basement faults are the largest in scale and harm. Generally distributed in active tectonic units, such as Wei Fen Graben, it has obvious directionality and displacement in both horizontal and vertical directions, especially in Xi 'an and Datong. Hidden cracks and open cracks have certain directionality in distribution, and the scale is not large. Jingyang in Shaanxi, Wanrong in Shanxi, Handan in Hebei and Zhengding are the most typical places. The ground subsidence cracks are mostly annular, and various mining areas, karst subsidence areas and ground subsidence areas are developed. Other types of cracks are small in scale and widely distributed, and generally do not have regular directionality. The potential corrosion cracks of loose soil are most typical in the Yellow River flood area of Henan and Hebei, Shandong and other places. In loess area, expansive soil and southern muddy soft soil area, the landslide zone is loess collapse crack, expansive crack and landslide crack respectively. Earthquake ground fissures often occur at the same time as earthquake activities, and a large number of such ground fissures have been produced in various earthquake areas in China, such as Tangshan, Lancang-Gengma and Luhuo.
The monitoring of ground fissures mainly adopts the following methods: ① YDD, an audio-frequency geoelectric field instrument, is used to survey bedrock mountainous areas with complex topographic and geological structures and covered layers, and to determine the depth and scope of ground fissures, which is superior to other geophysical methods. If it is used in Lianziya, a dangerous (fractured) rock mass on the bank slope of the Three Gorges Valley of the Yangtze River, the effect will be better. ② Shallow high-resolution longitudinal wave reflection method. In the study of Xi 'an ground fissures, it is proved that this method can infer the faults in the Quaternary unconsolidated layer according to the faults on the seismic profile and the absence of reflection horizons. This is a means to study neotectonic activities. Due to the shallow seismic reflection method, the existence of Quaternary faults under the surface ground fissures was quickly found out, which provided valuable information for the genesis of Xi ground fissures and the ground fissure-fault model. ③ Fracture displacement observation, which is a conventional method. That is to say, displacement changes are observed at the fixed points on both sides of the surface crack.
Among all kinds of ground fissures, except ground fissures and basement fracture active cracks, other types can be controlled and defended artificially, and even avoided and eradicated. However, the current technical means are still difficult to resist earthquake cracks and active cracks in basement faults. Improving human activities and some governance measures can only play a certain mitigation role. Under the current technical level and understanding, it is the most effective disaster reduction measure for all kinds of engineering buildings to bypass and avoid such cracks. For example, Xi 'an, a city with serious ground fissures, has formulated the Interim Provisions on the Survey and Design of Ground Fissures Construction Land, which stipulates that all kinds of buildings should avoid a certain distance on both sides of ground fissures according to their types and importance, which has played an important role in alleviating ground fissures in Xi 'an.