Reconstruction and Measurement of Irregular Karst Caves Using BLST Along the Shield Metro Line
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applied sciences Article Reconstruction and Measurement of Irregular Karst Caves Using BLST along the Shield Metro Line Shangqu Sun 1, Liping Li 2, Jing Wang 2,* , Shuguang Song 3, Peng He 1,* and Zhongdong Fang 2 1 Shandong Provincial Key Laboratory of Civil Engineering Disaster Prevention and Mitigation, Shandong University of Science and Technology, Qingdao 266590, China; [email protected] 2 School of Qilu Transportation, Shandong University, Jinan 250061, China; [email protected] (L.L.); [email protected] (Z.F.) 3 School of Transportation Engineering, Shandong Jianzhu University, Jinan 250101, China; [email protected] * Correspondence: [email protected] (J.W.); [email protected] (P.H.) Received: 15 December 2019; Accepted: 2 January 2020; Published: 4 January 2020 Featured Application: Accurate exploration of karst caves and the protection of springs. Abstract: This study investigated the application of the borehole laser scanning technology (BLST) method in the detection of both dry and water-filled karst caves. In order to solve the problem of excessive laser attenuation during the detection, we designed a test for the characteristics of multiwavelength laser attenuation in water-filled karst caves and studied the influence exerted by various factors, including different wavelengths, different laser power levels, different suspended media, and effect of turbidity on the attenuation coefficient. During the test, we discovered the existence of a “blue-green window” with low turbidity and a “near infrared window” with high turbidity in karst cave water environments. Based on the general survey results of drilling and comprehensive geophysical prospecting, a quantitative method using targeted drilling was proposed to detect the spatial morphology of karst caves in complex environments. We also investigated the effects of complex environmental factors such as suspended media and high turbidity on the laser detection distance and accuracy in karst caves, and established a quantitative matching model of laser wavelengths, laser power, and complex environmental parameters. Based on this, we obtained the best acquisition mode for detecting lasers in different karst development environments. A high-precision, three-dimensional visualized model of a real karst cave was established to quantitatively obtain the characteristic parameters, such as accurate position, three-dimensional shape, space volume, and cave filling type, which was applied to the detection of karst caves along the Jinan subway line. Keywords: irregular karst cave; measurement; 3D parameters; BLST method; shield metro 1. Introduction As cities develop and urban populations increase rapidly, traffic congestion has become one of the greatest problems facing many cities in China. Metro systems, or subways, which makes full use of underground space and reduce congestion on the ground, have become an important part of urban infrastructure and a popular traffic choice for Chinese people in the 21st century [1–4]. Due to complex urban geologic conditions, a large number of metro tunnels have to pass through underground karst areas, such as the Jinan metro tunnel passing through water-rich hard rock karst cave areas, the Wuhan metro tunnel passing through honeycomb-shaped caves, and the Changsha metro tunnel passing through complicated underwater karst caves. The covertness of typical karst geological bodies such as karst caves increases the permeability of the rock structure and lowers relevant rock mechanical Appl. Sci. 2020, 10, 392; doi:10.3390/app10010392 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 392 2 of 22 parameters, which may result in engineering disasters and hazards, including tunnel water bursting and inrush, water leakage, karst surface collapse, and shield cutterhead drooping, particularly when the caves bear both confined water pressure and shield tunneling disturbance [5,6]. It is no exaggeration that water inrush has become a serious threat to shield tunneling projects [7–9]. With the rapid development of computer technology, photogrammetry technology, and intelligent control technology, the detection of unfavorable geological bodies in tunnels and underground engineering projects utilizes more digital and quantitative methods and equipment with higher accuracy [10]. Currently, the common methods to detect cavity disaster sources, such as caves developed in the shallow karst areas, are drilling and geophysical exploration. The drilling method, as the most conventional and direct geological survey method, is especially suitable for detection in high-risk areas, such as underground karst caves. With a single borehole, one can obtain the approximate position, development height, and roof thickness of a karst cave at the borehole measuring point, while multiple boreholes can determine the horizontal boundary range of the karst cave. However, the drilling method lacks clear direction and is essentially a type of single-point detection method, during which the connection between boreholes can only be determined by empiricism and relevant calculation. This leads to the inevitability of blind areas, high costs, and consumption of time and labor [11]. The commonly used geophysical exploration methods, classified according to the exploration principles, include electrical methods (such as the high density electrical method), electromagnetic wave methods (such as the transient electromagnetic method and ground penetrating radar method), and seismic wave methods (such as the land sonar method) [12]. In the process of geophysical exploration, the detection capacity of geophysical exploration methods is limited by many factors, including the physical characteristics and spatial morphology of the detection medium or object, the geological structure and structural characteristics of the detection site, the hydrogeological conditions of the detection site, the topographic relief of the working face, the distribution of electromagnetic interference sources and interference bodies, the performance parameters of the devices, and the experience levels of the personnel operating the devices outdoors and processing and analyzing data indoors. Therefore, it is almost impossible to avoid fuzziness, multiplicity, and uncertainty in comprehensive geophysical exploration [13]. Consequently, drilling and comprehensive geophysical exploration methods cannot accurately detect karst cave boundaries in complex geological environments, rather can only deliver two-dimensional or even one-dimensional qualitative data with poor visualization effects and cannot provide quantitative parameters of karst caves, such as the accurate position and size information. The rapid development and wide application of contactless drilling laser measurement technology provides a solution for the exploration of karst caves. The biggest advantage of this method is that the microlaser probe can adapt to various kinds of narrow channels and boreholes and can be inserted deeply into a cave to obtain the point cloud coordinate data. This method has been widely employed in many professional instruments for the exploration of poor geological structures, such as the Cavity Monitoring System (CMS) [14] and Cavity Autoscanning Laser System (C-ALS) [15]. Li [16] took the lead in applying borehole scanning technology in goaf detection, stability calculation, monitoring, and early warning. Luo [17] investigated the formation rules of point cloud grids and maximum angle triangles, the dominant vertex segmentation strategy, and the irregular triangle optimization method, and realized the three-dimensional modeling and visualization of scattered point clouds in complex goaf sections. Many scholars have also carried out a lot of research on point cloud data acquisition, reconstruction, and visualization [18,19]. At present, the research and relevant applications are mainly focused on the detection of goaf sections in relatively favorable environments, while the occurrence environment of karst caves along subway lines is extremely complex. Many scholars have carried out studies on laser transmission characteristics in water [20–22], most of which focused on the attenuation characteristics and detection effects of lasers in ocean water and showed that a “blue-green window” exists in ocean water; that is, blue and green lasers experience the least attenuation in ocean water [23]. However, the water environment inside a water-filled cave is much more complex than that in the ocean. Most of the media in a water-filled cave, such as CaCO3, clay, and silt, are suspended. As a Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 22 Appl.the water Sci. 2020 environment, 10, 392 inside a water-filled cave is much more complex than that in the ocean.3 Most of 22 of the media in a water-filled cave, such as CaCO3, clay, and silt, are suspended. As a result, the optical reaction between the above-mentioned suspended media and the laser during its transmission result,is more the complicated, optical reaction and the between attenuation the above-mentioned characteristic of suspendedthe laser is also media more and obvious, the laser which during result its transmissionin the detection is more distance complicated, being ex andtremely the attenuation limited, and characteristic thus not meeting of the laserthe requirements is also more obvious,for karst whichcave detection. result in the Moreover, detection the distance influence being of extremely complex limited,environmental and thus factors, not meeting such as the cave requirements