TY - JOUR
AU - Qing-Zhao,, Zhang
AB - Abstract In order to maintain the original appearance of the rocks on a cavern roof and protect the ground environment, a new supporting method for shallow-buried caverns is proposed. This study investigates the design theory and construction process. Based to this method, some crisscross small tunnel sheds are embedded in the overburden layer. Hence a supporting system of interaction between surrounding rocks and supporting structures is formed. By combining the numerical calculation with monitoring measurement, we found that the distribution of calculated deformation generally agreed with the monitoring measurements. The monitoring results revealed that the proportion of rock shelf load-bearing reached 47%. The self-bearing capacity of the surrounding rocks is brought into significantly play. small tunnel shed, shallow tunneling method, underground construction, supporting system, construction monitoring Nomenclature Nomenclature AConcrete cross-sectional area of reinforced concrete. ATunnel cross-sectional area of the tunnel B spacing between various small tunnel sheds ERIR flexural rigidity of embedded support h buried depth of cavern l span of embedded support P0 released load after excavation PR load sustained by supporting structure q uniform load rConcrete unit weight of reinforced concrete u deformation of surrounding rocks u1 released deformation of surrounding rocks u2 deformation before excavation surface reached monitoring section um measured value of deformation. ue elastic deformation up plastic deformation uR deformation under restraint α proportion of load for supporting structure γ unit weight of surrounding rocks δ maximum span settlement caused by excavation θ angle between measuring line and horizontal plane 1. Introduction Over the past decades, the quantities and scales of underground projects have risen rapidly in China, and many projects have been carried out in unfavorable ground conditions. The development of construction techniques has enabled the realization of large-span underground caverns and crossing engineering. However, early underground projects were mostly constructed with opening excavations, which often led to road traffic being severely interfered with and the environment disrupted (Barton et al1974, Loganathan and Poulos 1998). The shallow tunneling method was then studied to solve these problems and bring forth controlled disturbance to the soundings of construction sites. This method can guarantee construction work safety and restrain the land subsidence (Barton 2002). The auxiliary construction methods for shallow tunneling method mainly include the freezing method (Genis et al2007, Zhang and Chuan 2005), the pipe-shed method (Carranza-Torres and Fairhurst 2000), the pipe-roofing method (Xiao et al2005) and the culvert box method. In the freezing method, pipe holes are placed around the tunnel, and then brine with low temperature is kept in circulation. The aquiclude around the pipe holes is frozen and forms a frozen wall with high strength and great sealing conditions (Li et al2004). The excavation and construction work can be performed under the protection of frozen wall. The pipe-shed method is widely applied in subsurface engineering containing rocks or weak soils with a low water level, but it is still hardly workable in large-span subsurface engineering containing saturated weak soils. In addition, pipe sheds usually cause contamination to the environment and significant influence to the landscape (Sari and Pasamehmetoglu 2004). The culvert box method requires the working surface to be stable during the construction process, and this method also leads to high construction costs due to the high level of underground water. The pipe-roofing method is a new type of technique for subsurface engineering based on the mining method. Small pipe roofs can be used as the basis of large span and large section underground structures (Karakus and Fowell 2003). In most auxiliary construction methods, the support system is applied after excavation, or applied with slip casting before construction works (Xu et al2013). However, each construction method mentioned above has its own disadvantages and application limits. In some special projects, the construction methods should be further investigated. For the purposes of supporting the system design of an underground cavern, the excavation and supporting methods should be identified primarily. The shape and size of the working surface as well as the corresponding construction scheme should be estimated with full consideration of all factors including safety, economy and environment (Verruijt and Booker 1996). Generally, the pressure of surrounding rock for a shallow-buried cavern is calculated by using the gravity model. It is assumed that when the burial depth of tunnel is small, the surrounding rocks are not considered as bearing structures since the displacement of rock may extend over the land surface, namely that the self load-bearing capacity of surrounding rocks and the combined action between surrounding rocks and supporting structures are neglected (Serrano et al2014). However, when a shallow-buried cavern has comparatively favorable conditions of surrounding rock, it is worth studying the self-bearing capacity of surrounding rocks to reduce the construction costs for supporting system. The objective of present study is to propose a support method of small tunnel shed. This method can take full advantage of the self load-bearing capacity of surrounding rocks combining with the multi-arch small tunnel shed. Based on this support method, the original facilities and vegetation on the ground surface could be well protected and preserved. 2. Theory and methods 2.1. Supporting methods of shallow-buried cavern In general, excavation methods for underground caverns can be classified into three types, i.e. open-cut method, semi-open-cut method and mining method. For underground caverns constructed by using the mining method, the supporting forms can be often divided into three types based on the support position which include internal support, external support and embedded support (figure 1). When selecting the position of a cavern, the covering depth of the cavern roof should be considered first of all. There should be sufficient fresh rock layers retained above the roof and the roof should be thick enough to impose adequate normal pressure on the joints, so as to form the roof arch (Palmstrom and Stille 2007). In case the thickness above the arch cannot meet the requirement, open excavation method is usually adopted. Anchor bolts, pre-stressed anchors and grouting reinforcement are often used as supplementary means to improve the bearing capacity of surrounding rocks. External support and embedded support are usually applied to shallow-buried caverns to avoid open excavation. Figure 1. Open in new tabDownload slide Supporting styles for mining method. (a) The internal support (b) the external support (c) the built-in support. Figure 1. Open in new tabDownload slide Supporting styles for mining method. (a) The internal support (b) the external support (c) the built-in support. For underground caverns with very small burying depths, the open excavation method is adopted in most cases. However, open excavation has some limitations such as ground environmental damage, occupying large field area, long construction period, high excavation difficulty and high construction costs. The open top and sub-wall method (figure 2) is also used in some cases. The design and construction work of this method is firstly completing the cavern roof with excavating shallowly and grouting, and then completing the lower-part cavern structure with cave excavation after the earth is backfilled. Comparing with open excavation method, the open top and sub-wall method is preferential, since the construction site is smaller due to the decrease of excavation volume, and the excavation difficulty and construction costs are also lower. Figure 2. Open in new tabDownload slide Open top and sub-wall method. Figure 2. Open in new tabDownload slide Open top and sub-wall method. For shallow and bias tunnels with a small span, a loading umbrella arch construction method can be adopted (figure 3). This method can ensure that the tunnel construction is safe and accelerate the progress of construction (Dang 2003). Otherwise, the digging piles curtain method can also be adopted (figure 4). Figure 3. Open in new tabDownload slide Loading umbrella arch method. Figure 3. Open in new tabDownload slide Loading umbrella arch method. Figure 4. Open in new tabDownload slide Digging piles curtain method. Figure 4. Open in new tabDownload slide Digging piles curtain method. 2.2. Theory of small tunnel shed If the stress condition of surrounding rocks exceeds the peak strength of rock mass, a plastic zone will appear around the cavern and plastic deformation will develop. If the plastic deformation is oversized, a supporting system should be adopted (Konishi et al2002). Supporting structures can restrain the deformation of surrounding rocks, and meanwhile the structures are affected by the surrounding rocks. This is the interaction between supporting structures and surrounding rocks. The typical curve of mechanical characteristics for surrounding rocks is given in figure 5. ue is elastic deformation and up is plastic deformation. uR is the deformation under restraint, and PR is the corresponding load. P0 is the released load after excavation, which can be obtained by plasto-elastic theory or through field test. Figure 5. Open in new tabDownload slide Characteristic curve of the surrounding rock. Figure 5. Open in new tabDownload slide Characteristic curve of the surrounding rock. After the excavation of underground cavern, the released load is sustained by surrounding rocks and supporting structures. It is assumed that the load sustained by the supporting structure is PR, and then the proportion of load sustained by the supporting structure can be calculated as in equation (1). α=PR/P01 In figure 5, u is the deformation of the surrounding rocks, and PR decreases with the increase of u. The case u  = max (u) is equivalent to the case without any support (point A), and the surrounding rocks are self-stabilized. For shallow-buried caverns, when the deformation develops, the relaxation load will impose on the supporting structure. The case u=0 is equivalent to the case of rigid support (point C). The released load from the excavation is completely sustained by the supporting structure, and the surrounding rocks do not appear to have any deformations. For the embedded and ground support structure of shallow-buried caverns, it is assumed that both ends of the supporting structure are clamped and bear uniform load q as shown in equation (1). q=γ hB−γATunnel+γConcreteAConcrete2 where γ is the unit weight of the surrounding rocks, h is the buried depth of the cavern, B is the spacing between various small tunnel sheds, ATunnel is the cross-sectional area of the tunnel, rConcrete is the unit weight of the reinforced concrete, and AConcrete is the cross-sectional area of the reinforced concrete. For shallow-buried caverns, surrounding rocks above the embedded support can play partial self load-bearing capacity. The lower surrounding rocks are deemed as having no self-load-bearing capacity, and are hung on the embedded support (figure 5). Figure 6. Open in new tabDownload slide Cross section of inbuilt support. Figure 6. Open in new tabDownload slide Cross section of inbuilt support. For the design and construction work of a shallow-buried cavern, the supporting structure and surrounding rock above the structure are considered as released loads after excavation, i.e. P0  =  q. Flexural rigidity of the embedded support is ERIR, load sustained by surrounding rocks above the embedded support is (1  -  α)q, load sustained by the supporting structure is αq and α is the proportion of load sustained by the embedded support. It can be assumed that both ends of the embedded support are solidly connected (figure 1), i.e. the anchor effect between both ends of the small tunnel shed and the surrounding rocks is pretty good and no shift and rotation between them could happen. The maximum displacement of the mid-span of the embedded support can be calculated as follows: δR=αql4/(384ERIR)3 Namely, α=384ERIRδR/ql44 where l is the span of the embedded support. Through numerical simulation, the characteristic curve of the surrounding rocks can be identified (Zhang et al2004). The deformation of the surrounding rocks at the feature point is determined as u  =  δR, and then the load sustained by the supporting structure can be calculated as shown in equation (5). PR=f(δR)5 By using equations (1), (4) and (5) simultaneously, the proportion of load for supporting structure α and the load sustained by supporting structure PR under different rigidity of embedded support can be obtained. Meanwhile, the deformation during the excavation can be monitored on site, i.e. um=δR ⁠. Hence through calculating the proportion of load sustained by supporting structure and adjusting the supporting parameters in a timely manner, dynamic feedback of design and informational construction can be achieved. Based on the above analysis, a new small tunnel shed support method was proposed. This method can combine the support system of a small tunnel shed and the surrounding rocks in the overburden layer, and take full advantage of the self load-bearing capacity of surrounding rocks. The small tunnel shed structure and surrounding rocks can work together to sustain the load on top and hang on the cavern roof. 3. Engineering overview and construction program 3.1. Engineering geology The Sleeping Buddha Palace project was a new scenic spot which was located in the famous scenic site at the Giant Buddhist Temple of Xinchang, Zhejiang Province, China. The geological formations mainly consisted of the tuff rock masses, which was the main rock type along the small tunnels. Tuff rocks were moderately to highly weathered. The weathered faces of these rocks were brown in color and fresh parts were greyish-green in color. Tuff rocks contained mainly silica mineral and thin layers. In the upper depth of 0.2 m, cultivated soil (Qpd) can be found. Clays mixed with sands and rubble stones were distributed at a depth of 0.6–0.75 m, with brown color and slightly humidity. Highly weathered tuff rock mixed rubble stone was found under the layer of clay, and the thickness was 1.5–2.85 m. Then moderately weathered tuff with horizontal bedding appeared. Moderately weathered tuff rocks were commonly observed and widely distributed. Some other moderately weathered rocks, such as tuffaceous sandstone and tuff breccia, were distributed sparsely. To understand such behavior of the moderately weathered tuff rock, core samples were taken to perform the tests and to estimate the rock mass quality, based on the Chinese Standard for Classification of Engineering Rock Masses (GB50218-94). The average uniaxial compression strength of the rock samples was identified as approximately 25.7 MPa, and the deformation modulus was about 1.4 GPa. The ultrasonic longitudinal wave velocities of intact rock (Vpr) and rock mass (Vpm) were obtained by using both laboratory and field tests, which were 5200 m s-1 and 4700 m s-1, respectively. According to the standard method (GB50218-94), the integrality coefficient Kv can be calculated using the equation Kv  =  (Vpm / Vpr)2. Therefore, the integrality coefficient Kv of the tuff rock was about 0.82, and the integrality grade was estimated as intact. Furthermore, the hardness grade of tuff rock was estimated as middle level, and the quality of tuff rock in the present study was classified as the third grade. One major fissure of longitudinal orientation along the cavern with an inclination of 30 degrees was found right above the cavern. This fissure cut through the ground surface, and cut the central part of the rock column between the big hall and small hall in an inclined way. Since the fissure was almost perpendicular to the cavern roof, the stability of the whole cavern was not affected by this fissure. However, the excavation of the cavern tended to cause overall extroversion, and the stability of arch springs became a little worse. Groundwater activity is an important factor related to the stability of the cavern (Yi 1997). In our study, the groundwater was typical of bedrock fissure water, which was mainly supplied from the surface precipitation. The precipitation can infiltrate into the cavern along the penetrating fissures. Because the water leakage phenomenon was observed during construction, fissure investigation was carried out to find the seepage location. Four seepage fissures were found on the top of the big hall, as shown in figure 7. Fissure A was the biggest seepage fissure, and its dip angle was approximately 90°. Since fissure A almost penetrated the top of the big hall, the water seepage from fissure A was more obvious than other fissures. Fissure B was another main seepage fissure, which crossed fissure A. The dip angle of fissure B was about 35°, and the water seepage of fissure B was less than that for fissure A. The seepage fissures of C and D were shorter than A and B. The dip angles of fissure C was close to 45°. Fissure C was connected to fissure A, and its water seepage became even more obvious. Fissure D was close to fissure B, and its dip angle was close to 40°. Even though water seepage appeared during the construction process, groundwater was not generally abundant. The tuff rocks were relatively intact, so the working face excavated normally did not contain much water, implying that the groundwater activity could hardly reduce the stability of the surrounding rocks. Figure 7. Open in new tabDownload slide Main seepage fissures in the big hall. Figure 7. Open in new tabDownload slide Main seepage fissures in the big hall. 3.2. Design and construction The Sleeping Buddha Palace is composed of a main cavern and several vestibular small caverns (figure 8). The main body is the big hall of the Sleeping Buddha Palace, and the small halls are located outside the big hall. The big hall and small halls are connected by five parallel entries. The big hall has a span of about 18 m and a height of about 12 m. The whole cavern has a hemispheric shape. The main cavern of the the Sleeping Buddha Palace is approximately 43.5 m long, 12 m high and has a span of 19.8 m. The overburden layer of the cavern is about 8–10 m. Figure 8. Open in new tabDownload slide On-site view of the cavern. Figure 8. Open in new tabDownload slide On-site view of the cavern. The design and construction works of the Sleeping Buddha Palace should meet the requirements of keeping the trees on ground surface and the original appearance of rocks on the cavern top (figure 8). Since the covering layer was thin, the space between caverns on the left was small and the anti-horizontal stability was poor. Considering these limitations at the construction site, a longitudinal connective pilot tunnel was firstly excavated in the middle of the overburden layer, and the direction of this pilot tunnel is perpendicular to the span (figure 9(a)). Meanwhile, timely anchor-plate retaining was carried out based on the principle of NATM (figures 9(b) and (c)). Then multiple load-bearing small tunnels were excavated from the longitudinal pilot tunnel, and were along the span direction. Anchor-plate retaining was performed at the same time. Subsequently, secondary lining structures were applied for the pilot tunnel and small tunnels to form a multi-arch small tunnel shed. Finally, the lower part of the large-span cavern was excavated. Figure 9. Open in new tabDownload slide Plan and section view of the supporting structure. (a) Plan view of small tunnel shed, (b) A–A section, (c) B–B section. Figure 9. Open in new tabDownload slide Plan and section view of the supporting structure. (a) Plan view of small tunnel shed, (b) A–A section, (c) B–B section. The construction scheme is demonstrated in figure 10, in which the roman letters indicate the excavation sequence and the Arabic numbers indicate the supporting sequence. The detailed sequence is explained as follows: (I) excavation of longitudinal connect pilot tunnel; (1) early-stage support; (II) excavation of horizontal small tunnel shed; (1) early-stage support; (2) pilot tunnel lining; (3) reinforcement of gallery anchor; (III) gallery excavation; (4) anchor; (IV) small hall excavation; (5) anchor; (V) excavation of the Sleeping Buddha Palace cavern; (6) anchor; (VI) excavation; (7) anchor; (VII) excavation; (8) anchor; (VIII) excavation. Various early-stage supports of surrounding rocks should be completed quickly and the onsite monitoring work improved. Figure 10. Open in new tabDownload slide Construction scheme. Figure 10. Open in new tabDownload slide Construction scheme. The NATM was adopted for the excavation of caverns, and an all-around monitoring and measurement system was established to measure the structural internal force during the construction (figure 11). Based on the onsite monitoring, some judgment criteria was established to evaluate the stability of surrounding rocks and the work status of supporting system (Karakus and Fowell 2005). During construction monitoring, the appearance of data abnormality was taken as the basis for adjusting the supporting parameters and adopting corresponding construction techniques. Measurements such as cavern convergence, fissure convergence, anchor rod axial force, secondary lining reinforced bar axial force and contact pressure, and strain monitoring were carried out. The arch crown settlement of the cavern could not be measured for construction reasons and was calculated reversely by the cavern convergence monitoring data. The fissure investigation was performed during construction to estimate the development of fissures in the excavation of rock mass. In addition, a numerical model was set up to simulate the stages of cavern excavation using the finite element method in the GeoFBA program (figure 12). Figure 11. Open in new tabDownload slide Layout of convergence deformation measuring lines. Figure 11. Open in new tabDownload slide Layout of convergence deformation measuring lines. Figure 12. Open in new tabDownload slide Calculation grid and overall vector diagram. Figure 12. Open in new tabDownload slide Calculation grid and overall vector diagram. 4. Results and discussion During the whole construction process, the disturbance to surrounding rocks was reduced as far as possible. Over-excavation was controlled strictly and under-excavation was eliminated. NATM monitoring measurements were strengthened, and the information was fed back which can be used to adjust the design quickly. For the secondary lining, one-time overall grouting was required, and compactness was ensured (figure 13). The multi-arch small tunnel shed was stretched across the span of the cavern, and a seated length was guaranteed at both ends of the span. Rock bolts were installed to anchor the tunnel shed into surrounding rocks. The longitudinal pilot tunnel was used for connecting the horizontal multi-arch small tunnel shed and providing the entrance for construction of the small tunnel shed. Furthermore, the pilot tunnel could enhance the feasibility of construction work and improve the structural integrity. Figure 13. Open in new tabDownload slide Excavation of the cavern during construction. Figure 13. Open in new tabDownload slide Excavation of the cavern during construction. A few fissures in the rock masses between the great hall and the small halls appeared when the cavern was excavated, but the width of fissures was usually less than 1 mm. Due to construction problems, some monitoring results at the site were disturbed during the cavern excavation. The measured data fluctuated a little, and some convergence curves were not smooth (figure 14). However, the trend of convergence deformation kept stable, and the final convergence deformations were relatively small. The cumulative convergence deformations monitored in the big hall are given in table 1. It can be seen from table 1 that most convergence deformations are less than 2 mm, and the development trend is stable, indicating the fissures usually keep stable, and the stability of the cavern was not affected by the fissures. Therefore, the rock masses in the cavern were stable during and after the construction. Based on the simulation work from different construction stages, the consistency of calculation results and the measured data in magnitude and trend was revealed. Results showed that the distribution of calculated deformation of rock masses was generally agreed with the monitoring measurements. By combining the numerical calculation with monitoring measurements, the dynamic design can be realized (Ng et al2004). Using the dynamic design approach, the small tunnel shed support method could be successfully applied to the construction of Sleeping Buddha Palace. Table 1. Cumulative convergence deformation in the big hall. Location . Measuring line No. . Convergence deformations (mm) . Development trend . Big hall 1 1.56 Steady 2 1.06 Steady 3 2.15 Steady 4 1.45 Steady 5 1.36 Steady 6 0.41 Steady Location . Measuring line No. . Convergence deformations (mm) . Development trend . Big hall 1 1.56 Steady 2 1.06 Steady 3 2.15 Steady 4 1.45 Steady 5 1.36 Steady 6 0.41 Steady Open in new tab Table 1. Cumulative convergence deformation in the big hall. Location . Measuring line No. . Convergence deformations (mm) . Development trend . Big hall 1 1.56 Steady 2 1.06 Steady 3 2.15 Steady 4 1.45 Steady 5 1.36 Steady 6 0.41 Steady Location . Measuring line No. . Convergence deformations (mm) . Development trend . Big hall 1 1.56 Steady 2 1.06 Steady 3 2.15 Steady 4 1.45 Steady 5 1.36 Steady 6 0.41 Steady Open in new tab Figure 14. Open in new tabDownload slide Cumulative convergence displacement of the NO.3 measuring line. Figure 14. Open in new tabDownload slide Cumulative convergence displacement of the NO.3 measuring line. For underground cavern projects, it was impossible to measure close to the work surface right after excavation, so there was already a released deformation of surrounding rocks with measured value u1. In addition, before the excavation surface reached the monitoring section, a deformation with measure value u2 developed. These two values plus the measured value of deformation was the absolute value of deformation of the surrounding rocks, namely, u1  +  u2  +  um, wherein um was the measured value of deformation. Based on the previous research results, u1  +  u2  =  0.5u (Wang 1990), which deemed that convergence measuring line had no displacement on the measuring point of the rock column, then the maximum span settlement caused by excavation of the Sleeping Buddha Palace was δ  =  2 umsinθ  =  3.1 mm, wherein θ was the angle between the measuring line and the horizontal plane. Take longitudinal 4 m as the calculation model (figure 6), the cross section burying depth was 9 m, the interface parameter of the box girder was ER  =  2.85  ×  107 kN m-2, AR  =  3.15 m2, IR  =  2.722 m4, l  =  22 m. We assumed the hanging lower surrounding rocks had no displacement relative to the supporting structure, i.e. δR  =  δ, the uniformed load of the support girder could be calculated through equation (3) as αq  =  384ERIRδR, 4l 4  =  98.55 kN m-1. Based on equation (2), it could be obtained that q  =  184.90 kN m-1, then the proportion of overburden self-weight load of the shallow-buried cavern sustained by the supporting structure was α  = 98.55/184.90  ×  100%  =  53%, the self-bearing proportion of surrounding rocks was 1  -  53%  =  47%. Therefore, it could be seen that the self-bearing capacity of the surrounding rocks played a significant role in the design and construction work for this project. The self-bearing capacity of the surrounding rocks was brought into significantly play. The main load-bearing structure of the cavern, namely the multi-arch small tunnel shed, was arranged in the middle of the overburden layer, so the facilities and vegetation on the ground surface were well protected. The original appearance of the rocks in the internal walls of the caverns was preserved. Since the overburden layer above the cavern was preserved and only small tunnels were excavated, the excavation volume was reduced significantly, and the construction costs also decreased. The load-bearing horizontal small tunnels had a relatively small cross-sectional area and were quickly supported after being excavated, hence the stability of the support system was guaranteed. The cross section area of the small tunnel can be adjusted flexibly. Therefore, this support method is not only suitable for large-span shallow-buried underground caverns, but also suitable for shallow-buried underground caverns with special requirements of maintaining the original conditions on the ground surface and cavern roof. 5. Conclusions In the present study, a new supporting method for shallow-buried caverns based on the mining method was proposed. We successfully applied this method to a specific project to minimize the damage to the surrounding environment. Through this application, the small tunnel shed supporting method can be understood thoroughly. The main conclusions are as follows: The small tunnel shed supporting method is an embedded support method, and the layout can be adjusted based on the project needs. Our study makes an investigation into the design theory and construction method. This supporting method can be combined with numerical calculations and monitoring measurements to realize dynamic design. In traditional design methods, the self load-bearing capacity of surrounding rocks of shallow-buried caverns is hardly considered. The suggested method identified the design parameters of the supporting structure based on the principles of rock mechanics, hence reflecting the partial self load-bearing capacity of the surrounding rocks in a controlled way. For large span underground caverns, the small tunnel shed supporting method has certain superiority in terms of reflecting the self load-bearing capacity of surrounding rocks and reducing costs. This method can be referenced in solving an underground cavern project with the requirement of protecting the ground environment and in controlled construction with slight disturbance, which can effectively avoid heavy excavation and satisfy the special requirements of scenic sites for preserving the original appearance of rocks. Acknowledgment This work is financially supported by National 863 Program (Grant No.2012AA112502) and The National Basic Research Program of China-973 Program (Grant No.2014CB046905 and No. 2011CB013800) and National Natural Science Foundation of China (NO. 51578408). 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TI - A novel support system for shallow buried caverns based on the mining method
JF - Journal of Geophysics and Engineering
DO - 10.1088/1742-2132/13/1/123
DA - 2016-02-01
UR - https://www.deepdyve.com/lp/oxford-university-press/a-novel-support-system-for-shallow-buried-caverns-based-on-the-mining-RrjZir0ujI
SP - 123
VL - 13
IS - 1
DP - DeepDyve
ER -