Tangshan city has a long history of ground subsidence. As early as the 1920s, there was a serious ground collapse accident due to karst water inrush in coal mines, which caused heavy losses. After the founding of New China, serious karst collapse occurred in two stages. The first stage is after 1976 Tangshan earthquake. More than 120 collapsed. It is mainly distributed in the west foot of Fenghuang Mountain in the urban area, that is, the present residential area of 17 and the suburbs of Zhaogezhuang Middle School, Linxi, Heiya, Dazhuangtuo, Trust Tun, Xiaotun and Shahe. Its collapse scale is different, and the diameter of large collapse pit is larger than 100m. The second stage is 1983 ~ 1988. Its scale is large, with about 19 harmful collapses (table 13- 1). Most pits are irregular and round, with a diameter of 5 ~ 10m and a maximum of 30 ~ 50m. The general depth is 1 ~ 3m, and the maximum depth is 5 ~ 7m.

Karst collapse is mainly distributed in western lubei district and northern lunan district, and concentrated in the following two areas: First, Zhanggezhuang (No.6 residential area)-Stadium-Brewery area in the west, with the central Stadium-Xinglong area being the most serious. The main collapses are the collapse of the stadium, the collapse of the heating company, the collapse of the Beijing Opera Troupe, the collapse of Fuqiang Building on Weiguo Road, and the collapse of the housing management office. These collapses occurred around 1988. Second, Dacheng Mountain-Phoenix Mountain-People's Park in the east, and Northwest Well-Earthquake Exhibition Hall in the middle is the most serious. The main collapses are the collapse of Tangshan No.10 Middle School and the collapse of the earthquake exhibition hall. These collapses mainly occurred near 1984 ~ 1986 and 1988. In addition to the above two intensive subsidence areas, Longhua Community and WangXie Zhuang Street also suffered sporadic subsidence.

Second, the formation conditions and distribution law of karst collapse

(A) the formation conditions of karst collapse

Karst collapse in Tangshan city is mainly controlled by geological structure, the nature and thickness of Quaternary overburden, the nature of soluble rocks and the intensity of groundwater exploitation. These factors determine the distribution law and activity intensity of karst collapse.

1. Relationship between geological structure and karst collapse

The evaluation area is located at the junction of the northern part of Luji depression belt and Yanshan fold belt. This is an area where tectonic activity is extremely active. The subsidence area is located in the west wing of Kaiping syncline and the uplift zone between Douhe fault and Tangshan fault. The Quaternary sedimentary thickness in this area is small and the bedrock is shallow. Due to the repeated action of multi-stage tectonic stress field, faults and joint structures in Tangshan uplift zone are very developed. The distribution directions of main faults and fractures are NNE, S-N, NW and EW ... The hydraulic connectivity test results of bedrock show that Ordovician limestone has good connectivity.

Table 13- 1 Overview of Karst Collapse in Tangshan City

The above-mentioned structural characteristics of the evaluation area determine that the bedrock aquifer in this area has good permeability and hydraulic conductivity, which shows the dynamic consistency of the groundwater level change of bedrock in the whole area.

2. The relationship between soluble rock properties and karst collapse.

Soluble rock is a necessary condition for karst collapse. There are two sets of carbonate rock series in the evaluation area: one is dolomite of Wumishan Formation, dolomite limestone of Jingeryu Formation and leopard limestone of Fujunshan Formation; The second category is oolitic limestone of Zhangxia Formation, marl and bamboo-leaf limestone of Gushan Formation and Changshan Formation, limestone of Fengshan Formation, bamboo-leaf limestone of Yeli Formation, dolomite and dolomite of Liangjiashan Formation, dolomite and shale of Majiagou Formation. These soluble rocks have different degrees of karst development because of their different purity, thickness and distribution (Table 13-2). Therefore, the effect on karst collapse is different.

Table 13-2 Table of Karst Development Degree of Soluble Rock in Tangshan City

3. Relationship between rock and soil properties, overburden thickness and karst collapse.

Karst collapse is the collapse phenomenon of covering soil under the action of various collapse factors. Therefore, understanding the lithology, structural characteristics, thickness distribution and mechanical properties of Quaternary caprock is the basis of studying karst collapse.

The Quaternary in the evaluation area is mainly composed of gravelly residual clay, loam, clayey silt, silty sand, fine sand, medium sand, coarse sand and gravel. From the analysis of collapsibility conditions, with the coarsening of soil particles, its collapsibility becomes worse; With the increase of sand content, its collapse strength decreases. Clay with uniform structure has the best anti-collapse effect. For example, the residual clay layer ubiquitous at the bottom of the cap rock plays a vital role in the stability of rock collapse in the central region (Figure 13- 1), and there is a window where the residual clay layer is missing in the central region of Li Nan. The fine sand layer there directly covered the Ordovician limestone, resulting in a large collapse pit in the central area.

Figure 13- 1 Zoning Map of Residual Clay Layer Thickness in Karst Subsidence Area of Tangshan City (according to Wang Hongtao)

1-the area with clay layer thickness < < 1m; 2- The area with clay layer thickness of 1 ~ 2m; 3- the area where the thickness of clay layer is 3 ~ 4m; 4- area with clay layer thickness > 4m.

In addition to the properties of overburden, the thickness of overburden also has an important influence on karst collapse. Practice has proved that the thinner the covering layer, the easier it will collapse. The critical thickness of collapse is generally less than 30m. Therefore, according to the different thickness and types, according to the actual data of collapse in some areas of China, and according to the formation conditions of collapse, the stability conditions of surface overburden are divided into four grades: ⅰ extremely unstable; Ⅱ unstable; Ⅲ local instability; ⅳ basically stable (table 13-3, table 13-4).

Table 13-3 Statistics of Collapse Distribution of Different Thickness and Different Types of Soils

Table 13-4 Division Table of Surface Coverage Stability Conditions

4. The relationship between karst water exploitation intensity and collapse.

Overexploitation of karst water is the main factor causing collapse. According to the data analysis, before 1970s, the water supply in Tangshan was mainly shallow water, and karst water was not exploited in large quantities, and karst water was under pressure in most areas. Although there was karst collapse in this period, it was small in scale and low in frequency. The high-frequency collapse in the central area of Tangshan appeared after 1978, and its distribution gradually expanded outward from Fenghuangshan Park, which was consistent with the expansion of the unconfined zone of karst water and was the result of over-exploitation of karst water. Figure 13-2 shows the corresponding relationship among karst water production, groundwater level change and karst collapse. As can be seen from the figure, with the increase of exploitation, the water level of karst groundwater drops from +5m to -—29.0m, and after a period of sharp decline, there is a peak of collapse.

(2) The formation mechanism of karst collapse in Tangshan.

Although many scholars have studied the formation mechanism of karst collapse in Tangshan, there are still great differences so far. Of course, no one can rule out the damage caused by human factors, that is, over-exploitation of karst water. However, the differences focus on the role of the 1976 earthquake. How big is the role of this earthquake, and how is its direction and influence reflected? The main reasons for this view are: almost all rock collapses occur after earthquakes; The distribution of rock collapse is directly related to active faults (Table 13-5). In the spatial analysis of the relationship between Tangshan active fault and rock collapse, through the distance query of GIS system, it is found that about half of the collapse points occur within 50m from the active fault, and about 70% occur within 150m from the active fault.

We believe that the mechanism of rock collapse in Tangshan should be analyzed from both macro and micro aspects:

Table 13-5 Overview of Six Main Subsidence Points in Tangshan City

Figure 13-2 Relationship between Karst Water Exploitation, Water Level and Collapse Distribution in Tangshan City

1. Overall analysis

Tangshan earthquake is mainly manifested in the macro function, mainly in the following three aspects. First, it destroyed the relatively stable karst caves. Due to the influence of strong earthquake force, the original cavity filler loosens or even moves. This provides a good channel for later exploitation of karst water. Secondly, the stable and well-consolidated clay layer above the cave is destroyed. This layer of clay is about 1 ~ 6m thick, which is a good water-resisting layer between the upper Quaternary phreatic water and the lower karst water. Disturbed and deformed after the earthquake, the karst water failed to be stopped by layers in the later stage of exploitation, which led to the increasing exploitation intensity of karst water and the formation of skylight, which provided power and space for rock collapse. Thirdly, the earthquake force also destroyed the structure of overlying loose soil, and the formation of fractured ground fissures provided a channel for the connection between surface water and groundwater, and the formation of collapse pits was beneficial to water collection and infiltration.

2. Microscopic analysis

As we all know, among the three conditions of rock collapse, the change of groundwater dynamic conditions (karst water level decline speed, karst hydrodynamic state and hydraulic gradient) plays a vital role. When karst water drops sharply, the uplift pressure of overburden decreases, resulting in the loss of support and load; At the same time, due to the increase of slope, the lateral seepage pressure increases, which leads to seepage deformation and gradually forms soil holes of different sizes. When the upper earth pressure increases, the rock collapses.

When Liu Qiren studied the mechanism of rock collapse in Tangshan, he made a simulation test. The test shows that the critical failure speed of cohesive soil is 0.65 cm/s; The critical speed of sand damage is 0. 146 cm/s; /s； The negative pressure of karst cave is 7.86kPa.

To sum up, it can be considered that its formation mechanism is the result of the comprehensive action of many mechanisms, among which infiltration and undercurrent are the main ones, followed by surface water soaking, softening, loading and vacuum adsorption.

(3) Distribution law of karst collapse in Tangshan city.

1. Land subsidence is concentrated in the soluble rock development area.

The thick limestone, dolomite limestone and dolomite of Majiagou Formation of Middle Ordovician are the most developed karst strata in Tangshan, followed by bamboo leaf limestone, oolitic limestone, leopard limestone and Jixian dolomite of Cambrian. The existing land subsidence occurs in these three types of rock development areas.

2. The karst collapse near the fault zone is the most serious.

The fault structure zone is not only a karst development zone, but also a strong groundwater runoff zone, so the karst is particularly developed and the land subsidence activity is particularly serious. The two distribution zones of urban land subsidence are formed along two important fault zones. Zhanggezhuang-Stadium-Brewery collapse zone is distributed in the southeast wing of F0 fault in Douhe fault zone in a northeast direction. Dachengshan-Fenghuangshan-People's Park subsidence zone is also distributed in ne direction in the northwest wing of F 1 fault in Tangshan fault zone. The composite action of fault structure also has obvious control effect on ground collapse. Many collapse points, such as the second training hall of the stadium and the nine collapse points of the thermal power company, are distributed in F0 and F 1, the composite parts of two main faults and other faults, or the turning parts of the secondary syncline associated with faults.

Affected by geological structure and hydrogeological conditions, karst is particularly developed in the subsidence area. For example, according to the data of six exploration holes around the collapse pit of the second training hall of the stadium; The thickness of the Upper Quaternary is 39.0 1 ~ 43. 19m, and the underlying bedrock is Cambrian gray-white dolomite and calcareous dolomite, including flint banded dolomite. All the soluble rock sections exposed by drilling have different degrees of dissolution, especially in the shallow soluble rock about 65 m deep from the roof, the average drilling rate is 12.9%, and the height of the caves drilled is generally 0.6~5.8m, and the highest is 1 1. 17m. ..

3. Land subsidence mainly occurs in areas where Quaternary overburden is thin and buried soil caves are developed.

The thickness of Quaternary system in Tangshan changed greatly. F concentrates two karst collapse zones, and the thickness of the overburden here is less than 50m on the projection of the NE extension between the fault and F 1 fault. Its lithology is clay, loam, silty fine sand mixed with medium coarse sand and gravel layer. The structure is loose, the collapse resistance is poor, and soil holes are developed. At the same time, the Quaternary pore water is closely related to the lower karst water, forming a unified double-layer medium water-bearing system. Therefore, when the groundwater level rises and falls sharply and the dynamic conditions of groundwater change frequently, there will be strong infiltration erosion, which will lead to ground collapse.

4. Karst collapse is concentrated in the groundwater drop funnel area.

In addition to seismic activity, the inducing factors of karst collapse in Tangshan are the continuous over-strength exploitation of groundwater by urban water supply and mine drainage. Tangshan is a city that is seriously short of water. Since the late 1960s, groundwater overexploitation has become increasingly serious. By 1985, the underground water in urban and suburban areas was 4.76× 108m3, the mining intensity was 50×104m3/km2, and the over-exploitation rate was 100. Since 1970, the groundwater level has been decreasing at the rate of 1.5 ~ 6.0m per year, and it has decreased by more than 30m by 1985, forming a large area of groundwater falling funnel area. The funnel center of Tangshan City is located in the coking plant of Tangshan Iron and Steel Company, Qixin Cement Plant and Tangshan Coal Research Institute. The buried depth of Quaternary pore water level in the central area is 29 ~ 43m, and the pore water has been exhausted or nearly exhausted. The buried depth of karst water is 48 ~ 58m. Urban land subsidence is concentrated in the center or adjacent area of groundwater drop funnel. The reason is that the hydraulic gradient in the funnel area where the groundwater level drops is too high, and the velocity of groundwater increases. Groundwater strengthens the potential erosion and transportation of Quaternary loose sediments and soluble rock karst fillings, which leads to ground collapse or uneven settlement. This dynamic activity is particularly strong in the direction of groundwater inflow and near the structural fracture zone in the funnel area, so the ground collapse is also the most concentrated.