Distributed optical fiber sensing technology and its application in engineering monitoring

This research was supported by the National Natural Science Foundation (No.:40 172096) and the special science and technology plan of the Ministry of Land and Resources in 2000. This paper was first published in Journal of Engineering Geology, 2003 (04): 354 ~ 406.

Xu Qiang Cheng Qianhong Ding Dongshan Li

(Chengdu University of Technology Geological Disaster Prevention and Geological Environment Protection)

National Professional Laboratory, Chengdu, Sichuan, 6 10059)

In the process of landslide geological disaster investigation, how to accurately identify the characteristics of sliding soil and determine the specific position of sliding surface is a headache for most engineers and technicians. Taking the ancient Xietan landslide in the Three Gorges reservoir area as an example, this paper introduces in detail the structural characteristics of the sliding body of Xietan landslide and the basic characteristics of the sliding soil exposed by various means such as exploring adits, drilling cores and ground exposure. The results show that there are not only obvious scratches, mirror marks and crushed stone marks in the sliding zone soil of Xietan landslide, but also mylonitization in some boreholes. The surface outcrop can also see the traces of the sliding zone soil being crushed and kneaded in the process of groundwater and high-speed decline, and the original structure being re-cemented after being destroyed. At the same time, there is a special landslide influence zone below the slip zone. The characteristics of the soil in the sliding zone of Xietan landslide are very typical and representative, which can be used as a reference for colleagues to determine the sliding surface in practical work.

Xietan landslide; Sliding belt; Soil sliding surface; Landslide influence zone

1 Introduction

Xietan ancient landslide is located on the left bank of the Yangtze River in Xietan Township, Zigui County, Hubei Province, and is one of the typical landslides in the Three Gorges reservoir area. Through the in-depth and detailed study of Xietan landslide, the author found that the landslide not only has very standard and typical landslide characteristics in topography, but also is representative and typical in many other aspects (such as landslide geological structure, hydrogeological structure, slip zone characteristics, genetic mechanism, etc.), especially in slip zone characteristics. In the actual process of landslide survey, how to quickly and accurately identify and identify the typical characteristics of the slip zone soil according to the specific conditions of the investigation, so as to accurately determine the location of the slip zone, is often the most headache problem for field engineers and technicians. This paper briefly introduces the characteristics of the sliding zone of Xietan landslide, which can be used as a reference for engineering geologists in the process of landslide investigation.

2. Basic characteristics of landslides

Xietan landslide is in the shape of an armchair, and the front edge of the landslide protrudes out of the river. The landform of the ancient landslide is very obvious. Xietan landslide has a long tongue shape on the plane, which is wide at the front and narrow at the back. The overall terrain slope of the landslide is 20 ~ 35, and the slope of the rear edge is about 43. The longitudinal length is about 780m, the maximum width of the leading edge is 460m, the width of the trailing edge is about 140m, and the area is about 0.2km2 The thickness of the lower half of the upper sliding body is relatively large, with the maximum thickness of 45m；. The upper part is relatively thin, generally more than 20 meters, with an average thickness of about 30 meters and a total volume of about 8.94 million m3 (figure 1).

The exposed strata in the landslide area are: landslide accumulation layer (Qde 1): light yellow and light red broken rock and soil with loose structure and good water permeability, and the ratio of rock and soil is 2 ∶ 1 ~ 1 ∶ 2. Gravel is mainly composed of sand shale, and the size of gravel is generally 40 ~ 60 cm, and its content gradually increases from the middle to both sides and the rear edge. Niejiashan Formation (J2n) of Middle Jurassic: purplish red and grayish green medium-thick layered siltstone, feldspathic sandstone mixed with purplish red mudstone and silty mudstone. Mainly distributed at the rear edge and above the sliding body. Lower Jurassic Tongzhuyuan Formation (J 1t): grayish green and grayish yellow silty mudstone, fine sandstone mixed with shale, mainly distributed in the middle and upper parts of sliding bodies. Upper Triassic Shazhenxi Formation (T3s): grayish green and grayish yellow silty mudstone, fine sandstone mixed with shale and coal lines, mainly distributed in the middle and lower members of the sliding body. Middle Triassic Badong Formation (T2b): purplish red sandstone and siltstone mixed with mudstone, mainly exposed in the riverside area at the front of landslide. The investigation results show that after the Xietan ancient landslide occurred, the front edge of the landslide slipped through the bedrock (T2b) of Badong Formation again, forming an obvious trailing platform-secondary platform.

Structural characteristics and material composition of sliding body 3

On the profile, Xietan landslide is divided into five layers from top to bottom in terms of material composition and structural characteristics: slope deposit, landslide deposit, slip zone, landslide influence zone and bedrock (Figure 1 and Figure 2).

Figure 1 Representative Section of Xietan Landslide

Slope deposit (Qdl): This layer is mainly yellowish brown, grayish yellow and purplish red silty clay mixed with gravel. The soil-rock ratio is generally 15: 1 ~ 8: 1, and the silty clay is in a plastic state. Gravel is mainly composed of sandstone and siltstone, angular to subangular, with the diameter ranging from10 ~ 300 mm. The part of this layer where buildings and structures are built is replaced by artificial fill. The thickness of diluvium varies from 0.5m to 4.4m, with a general thickness of 2 ~ 3m.

Landslide deposit (Qdel): This layer is mainly grayish green, light yellow and purplish red gravel soil, and the rock-soil ratio varies greatly in different parts, ranging from 6: 1 to 1: 5. Some are mainly clay and some are mainly gravel. The soil is plastic clay and silty clay, and the gravel is mainly composed of siltstone, sandstone and silty mudstone, which is angular to subangular, with a diameter of 20 ~ 220 mm and a maximum of 300 ~ 400 mm, and is scattered and accumulated in different sizes. Debris of feldspathic sandstone of Niejiashan Formation (J2n) can be seen on the secondary platform and above, and the debris of purplish red argillaceous siltstone of Badong Formation (T2b) is mainly found in the front of landslide. The maximum thickness of landslide deposits is 45m, and the average thickness is about 30m. Generally speaking, landslide deposits have loose structure and good water permeability.

Fig. 2 Schematic diagram of slope section structure

Slip zone: The soil in the slip zone of Xietan landslide is mainly purplish red, grayish black, grayish white clay and silty clay mixed with gravel, and the ratio of soil to rock is about 7:3. The structure is compact, and mirrors, scratches and gravel marks can be seen in many sliding zones. The local soil mortar in the sliding zone is columnar, with a thickness of 0.7m ~ 2.7m The specific characteristics of the soil in the sliding zone will be detailed below.

Landslide-affected zone: under the slide zone of Xietan landslide, there is still a broken zone with a certain thickness under the influence of landslide dynamics during the high-speed sliding of the original Xietan ancient landslide. The crush zone is similar to the fault-affected zone in structural characteristics, so it is called the landslide-affected zone. Through the exploration of adit and borehole coring, it can be observed that the sandstone in the landslide-affected zone is obviously broken into blocks, gravel and even granular cuttings and rock powder, and its structure is loose, broken or loose (Figure 3). In the adit, near the sliding zone, you can grab sandstone cuttings and rock powder directly in this zone by hand, and gradually transfer the loose structure and fracture structure composed of blocks and gravels inward to the normal layered bedrock.

The characteristics of the landslide influence zone seen in the core are as follows: the influence zone is mainly gravel soil, and the soil-rock ratio is about1:65,438+0 ~1:3. Gravel is severely crushed by extrusion and twisted into sand by hand, and the composition of gravel is the same as that of bedrock at the corresponding position. Because the rock mass in the landslide-affected area of Borehole No.9 is too broken, it is impossible to take out the core directly during drilling, but only the debris similar to river sand left after water erosion (Figure 4). The thickness of landslide affected zone is generally 3 ~ 5m.

Fig. 3 Landslide influence zone exposed by exploration adit.

Fig. 4 No.9 Borehole Landslide Affected Area

4 slip zone soil characteristics

The structural characteristics of the soil in the sliding zone of Xietan landslide are very typical and representative. Sliding zone soil with obvious characteristics can be seen from exploration adit, drilling coring and surface outcrop at landslide front.

Fig. 5 shows the material composition and structural characteristics of rock and soil near the sliding zone of Xietan landslide revealed by the exploration adit. From the normal landslide deposition above the slip zone to the landslide influence zone below the slip zone, * * * is divided into seven layers, and the specific characteristics of each layer are as follows:

Fig. 5 Characteristics of rock and soil near the slide zone of Xietan landslide revealed by 97m exploration adit.

① The first layer (numbered as (1) in the drawing): dark brown gravel mixed with clay, mainly composed of light yellow-green argillaceous siltstone, mixed with a small amount of gray-black carbonaceous shale, and the earth-rock ratio is about 3∶7. Gravel is angular, with an average particle size of 10cm and a maximum of 30cm. Poor cementation, loose structure and strong water permeability. This layer is actually a normal landslide deposit.

② Second layer (No.2): dark brown gravel mixed with clay. Gravel is mainly composed of light yellow-green argillaceous siltstone, with a small amount of gray-black carbonaceous shale, and the ratio of soil to rock is about 4:6. Gravel is angular, but there are obvious broken traces, with an average particle size of 2 ~ 5 cm and a maximum of 10cm. Poor cementation, loose structure and strong water permeability (but weaker than the first layer). This layer is actually formed by the landslide deposits near the sliding zone being crushed during the sliding process.

③ The third layer (No.3): purplish red and reddish brown clay, containing a small amount of gravel, mainly composed of light yellow-green, grayish white sandstone and siltstone, and the earth-rock ratio is about 8: 2 ~ 9: 1. Influenced by groundwater (this layer is a relatively water-resisting layer, and obvious groundwater seepage can be seen at the contact interface with (2)), the clay is fluid plastic ~ plastic. The average particle size of gravel is 0.5 ~ 1 cm, which is sub-angular and once round, without obvious directional arrangement characteristics. The overall structure is relatively dense, with medium cementation and poor water permeability. This layer should be the upper layer of the main slip zone.

④ The fourth layer (No.4): gray-black clay, with a small amount of gravel, and the ratio of soil to rock is about 9: 1. Gravel composition is mainly light yellow green, grayish white sandstone and siltstone. The average particle size of gravel is 0.5 ~ 1 cm, which is subangular-subrounded. Gravel is characterized by directional arrangement, and its plane is parallel to the sliding surface. After the gravel is broken, obvious gravel marks can be left in the clay. Clay is plastic. The whole structure is very compact, with good cementation and poor water permeability. This layer should be the central layer of the main slip zone, and the carbonaceous components in it may be caused by carbonization during high-speed sliding, or the coal line in T3s may be brought here during high-speed sliding.

⑤ The fifth layer (No.5): purplish red clay, with a small amount of gravel, and the ratio of soil to rock is about 9: 1. Gravel composition is mainly light yellow green, grayish white sandstone and siltstone. The average particle size of gravel is 0.5 ~ 1 cm, which is subangular-subcircular. You can see the directional arrangement of gravel, and its plane is parallel to the sliding surface. Clay is plastic, and gravel marks can be seen. The whole structure is very compact, with good cementation and poor water permeability. This layer should be the lower layer of the main slip zone.

⑥ Layer 6 (No.6): grayish white and light yellow gravel mixed with cuttings and stone powder, with the content of stone powder being 70% ~ 80%. Gravel composition is mainly light yellow-green, gray-white sandstone or feldspar sandstone. The average particle size of gravel is 1 ~ 3 cm, which is subangular and shows obvious crushing characteristics. Generally speaking, the structure is loose, the cementation is poor and the water permeability is good, but the groundwater is only wet (the groundwater has been separated by the main slip zone). This layer should be the landslide influence zone near the bottom of the slip zone.

⑦ The seventh layer (No.7): yellowish brown granulated rock debris powder mixed with gravel. Gravel composition is mainly grayish white sandstone or feldspar sandstone. The average particle size of gravel is 1 ~ 3 cm, which is subangular and shows obvious crushing characteristics. Generally speaking, the structure is loose, the cementation is poor and the water permeability is good, but the groundwater state is only wet. This layer should be the main body of the landslide influence zone.

From the above characteristics, it can be seen that the layered characteristics of the original Xietan ancient landslide sliding zone revealed by the exploration adit basically correspond to the surface outcrop of the borehole core and the sliding zone at the front of the landslide. Generally speaking, the rock and soil near the slip zone has a three-layer structure, including the slip zone, the landslide deposits broken in the sliding process at the upper part of the slip zone and the landslide-affected zone at the lower part of the slip zone affected by landslide dynamics. Among them, the sliding belt can be subdivided into three layers (namely (3), (4) and (5) in Figure 5). However, the three-layer structure of the slip zone is not common, but it is relatively complete in individual boreholes (such as ZK 1, ZK7, ZK8, etc.). ), generally only one or two layers can be seen in the sliding belt.

In the drilled core, we can observe the characteristics similar to the sliding zone soil exposed in the adit: the structure is compact, the profile of the core taken out is relatively complete (compared with the landslide affected zone below it and the adjacent landslide deposits), and the heart rate is high (just the opposite of the difficulty in obtaining the sliding zone soil core in general landslides). After breaking the core by hand, you can generally see very obvious mirrors, scratches and gravel marks (Figure 6). Mylonization of soil in sliding zone can also be seen in the core of No.5 borehole (Figure 7). This is difficult to see in the sliding zone of general landslides.

Other characteristics of the sliding zone soil of Xietan landslide can be seen from the surface outcrop of the sliding zone soil at the front of the landslide (Figure 8). Fig. 8 shows that the soil in the sliding zone here is mainly light yellow calcareous clay mixed with gravel, with a soil-rock ratio of about 5:5, which is compact in structure and well cemented. Gravel is sub-angular-sub-circular, and the clay in the sliding zone shows the traces of rolling and friction when the original landslide slides at high speed, and the original structure is re-cemented after being destroyed.

Figure 6 Clear mirrors and scratches can be seen in the core of ZK6 sliding zone.

Fig. 7 Mylonization of soil in ZK5 sliding zone can be seen in the core of sliding zone.

Fig. 8 The slip zone of the surface outcrop left traces of re-cementation after crushing.

5 conclusion

Through the above research on the soil characteristics of the sliding zone of Xietan ancient landslide, the results show that:

(1) Xietan landslide has a five-layer structure in profile: slope deposit, landslide deposit, slip zone, landslide influence zone and bedrock. Compared with the general landslide, the particularity of Xietan landslide is that there is a certain thickness of landslide influence zone below the sliding zone. The landslide-affected zone is a broken zone formed by the high-strength dynamic compression of the bedrock of the sliding bed in the reverse slope during the high-speed sliding of the slope. This kind of landslide influence zone should be difficult to see in bedding landslides and low-speed landslides.

(2) Typical sliding zone soil can be found in the exploration adit, drilling and coring and ground head of Xietan landslide. The main characteristics of the soil in the sliding zone of Xietan landslide are: the material composition is clay, containing a small amount of gravel, and the gravel is sub-prismatic-sub-circular, and the directional arrangement is visible. Compact structure, medium cementation and poor water permeability. Slip zone soil has obvious scratches, mirror surface and gravel marks. In some boreholes, it can be seen that the slip zone soil is mylonitized, and in the exposed head, it can also be seen that the slip zone soil is crushed and rubbed in the process of groundwater and high-speed decline, which destroys the original structure and then re-cemented. It can be considered that the sliding zone soil of Xietan landslide contains many typical characteristics of high-speed landslide sliding zone.

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