A method for excavating a large-section tunnel by full-sand layer underground excavation

Abstract

This invention relates to a method for excavating a large-section, all-sand-layer tunnel using a cut-and-cover method, specifically a method for excavating a large-section, all-sand-layer tunnel using a cut-and-cover method, comprising the following steps: dividing the tunnel cross-section horizontally into three parts: a left section, a middle section, and a right section; then dividing these three parts vertically into upper and lower sections, resulting in a total of six areas; excavating pilot tunnels in each area; and excavating and supporting the other pilot tunnels (except for the lower pilot tunnel in the middle section) in two separate steps; measuring and setting out the outline of the excavation cross-section and the excavation range of each pilot tunnel according to the designed tunnel cross-section; this invention further refines the six excavation cross-sections of the double-side-wall pilot tunnel method, dividing the other pilot tunnels (except for the lower pilot tunnel in the middle section) into two separate steps for excavation and support, reducing the cross-sectional dimensions of each pilot tunnel excavation and support step, shortening the construction time, reducing the exposure time of the excavation face, and improving the safety of tunnel construction.

Description

Technical Field

[0001] This invention relates to the field of tunnel construction, specifically to a method for excavating a large-section, cut-and-cover tunnel in a sand layer. Background Technology

[0002] With the continuous advancement of urbanization in my country and the expansion of urban rail transit construction, the number of large-section shallow-buried tunnels laid under complex geological conditions is also increasing. Most of these tunnels pass under existing urban buildings or roads, making it crucial to control tunnel convergence and ground settlement during tunnel construction. The total length of the tunnel section for the post-station distribution line in this project is 162.909m, with a maximum excavation cross-section of 12.4*9.688m. Located under a main urban road, it is 25m away from a 32-story commercial and residential building to the west. Water supply, natural gas, and stormwater / sewage pipelines are laid above the tunnel. The entire tunnel cross-section is located in a medium sand layer, with drainage located in the middle of the tunnel. During construction, the large excavation cross-section, poor geological conditions in the construction area, and complex surrounding environment pose significant safety risks, including excavation face collapse, settlement and deformation of nearby buildings and overhead pipelines, if the excavation face is not sealed in a timely manner. Summary of the Invention

[0003] To address the aforementioned problems, this invention provides a method for excavating a large-section, full-sand-layer tunnel using a cut-and-cover method that is safe and reliable in construction and significantly reduces tunnel deformation and convergence.

[0004] This invention discloses a method for excavating a large-section, cut-and-cover tunnel in a sand layer, comprising the following steps: Step 1: Divide the tunnel cross section into three parts horizontally: left, middle, and right. Then divide these three parts vertically into upper and lower sections, for a total of 6 areas. Excavate pilot tunnels in each area. Excavate and support the other pilot tunnels, except for the lower pilot tunnel in the middle section, in two separate steps. Step 2: Measurement and layout. Based on the designed tunnel cross-section, measure and lay out the outline of the excavation cross-section and the excavation range of each pilot tunnel. Step 3: Advanced support construction is carried out using large pipe sheds and single-row and double-row advanced small pipe supports; Step 4: Manually excavate the upper guide tunnel of the left area, with an excavation advance of 0.5-0.6m. The upper guide tunnel of the left area is excavated in two layers. The first excavation uses the step method to excavate the upper guide tunnel of the left area located in the upper layer. After the core soil is excavated, install the vertical temporary support, the grid steel arch frame, and install the locking anchor rods at the arch foot and the bottom of the temporary support. Shotcrete is then applied. After the upper part of the left zone is excavated for 3-5m, the lower part of the left zone is excavated. This step is completed in one excavation, and the installation of the vertical temporary support, temporary invert arch, and grid arch frame is completed. Locking anchor rods are installed at the arch foot and the bottom of the vertical temporary support, and shotcrete is sprayed to complete the construction of the upper part of the left zone. After entering the upper pilot tunnel in the left area, soil dumping holes are left every 3 to 5 meters on the temporary invert arch. The excavated soil is dumped from the soil dumping holes onto the battery-powered vehicles in the lower pilot tunnel and transported out of the tunnel.

[0005] Step 5: After the pilot tunnel in the upper part of the left zone is excavated to a depth of 3-5m, the lower part of the pilot tunnel in the left zone is excavated manually or mechanically. The lower part of the pilot tunnel in the left zone is excavated in two layers using the step method. After the upper part of the lower part of the pilot tunnel in the left zone is excavated, vertical temporary supports and grid arch frames are installed. Locking anchors are installed at the arch foot and the bottom of the vertical temporary supports, and shotcrete is applied. The lower part of the pilot tunnel in the left zone is excavated after the upper part of the lower part of the left zone is excavated to a depth of 3-5m. After excavation, vertical temporary supports and grid arch frames are installed in a timely manner, and shotcrete is applied. This completes the construction of the lower part of the pilot tunnel in the left zone, thus completing the construction of the left side of the tunnel. Step 6: After excavating the lower part of the left zone 3-5m into the lower pilot tunnel, manually excavate the upper part of the right zone. The upper part of the right zone pilot tunnel is excavated in two layers. The first excavation uses the step method to open the upper part of the right zone pilot tunnel. After the core soil is excavated, install the vertical temporary support and the grid steel arch frame. Install the anchor bolts at the arch foot and the bottom of the vertical temporary support, and spray concrete. After the upper part of the right zone is excavated for 3-5m, the lower part of the right zone is excavated. This step is completed in one excavation, and the installation of the vertical temporary support, temporary invert arch, and grid arch frame is completed. Locking anchor rods are installed at the arch foot and the bottom of the vertical temporary support, and shotcrete is sprayed to complete the construction of the upper part of the right zone. During the construction of the pilot tunnel in the upper part of the right area, soil dropping holes were left every 3 to 5 meters on the temporary invert arch to drop the excavated soil from the pilot tunnel to the lower pilot tunnel and transport it out by battery truck. Step 7: After the pilot tunnel in the upper part of the right zone is excavated to a depth of 3-5m, the lower part of the pilot tunnel in the right zone is excavated manually or mechanically. The lower part of the pilot tunnel in the right zone is excavated in two layers using the step method. After the upper part of the lower part of the right zone is excavated, vertical temporary supports and grid arch frames are installed. Locking anchors are installed at the arch feet and the bottom of the temporary supports, and shotcrete is applied. The lower part of the pilot tunnel in the right zone is excavated after the upper part of the lower part of the right zone is excavated to a depth of 2-5m. After excavation, vertical temporary supports and grid arch frames are installed in a timely manner, and shotcrete is applied. This completes the construction of the lower part of the pilot tunnel in the right zone, and the construction of the right side of the tunnel is thus completed. Step 8: After the lower section of the right zone has been excavated to a depth of 10-15m, the upper section of the central zone will be manually excavated. The upper section of the central zone will be excavated in two layers using the step method. The upper section of the central zone will retain core soil and be excavated along the tunnel outline. A grid arch frame will be installed to connect the grid arch frame installed in the upper section of the left zone and the grid arch frame installed in the upper section of the right zone. The arch will be sealed with shotcrete. The lower section of the central zone will be excavated after the upper section of the central zone has been excavated to a depth of 3-5m. A central partition will be installed between the upper section of the central zone and the lower section of the central zone. The excavated soil from the upper section of the central zone will be manually transferred to the lower section of the central zone through the pre-reserved opening at the central partition. Step 9: After the pilot tunnel in the upper part of the middle zone is excavated to a depth of 3-5m, the pilot tunnel in the lower part of the middle zone is excavated manually or mechanically. The pilot tunnel in the lower part of the middle zone is excavated in one go. After the excavation is completed, the inverted arch grid arch frame is installed to connect the grid arch frame installed in the lower pilot tunnel of the left zone and the grid arch frame installed in the lower pilot tunnel of the right zone. Shotcrete is sprayed to close the tunnel into a ring.

[0006] Preferably, the large pipe shed uses seamless steel pipes with a diameter of 108mm and a wall thickness of 8mm, with a length of 15 meters, a circumferential spacing of 400mm, and a laying angle of 1 to 2°. If the working space is limited, the large pipe shed is constructed in sections of 2m to 3m each. During construction, each section is rammed and connected to the pipe. Each pipe section is connected using a steel pipe joint with an inner diameter of 90mm and a length of 0.3m.

[0007] Preferably, the grouting of the large pipe shed uses cement mortar, wherein the cement is ordinary Portland cement of grade 42.5 or above, the water-cement ratio is 0.8 to 1.2, the grouting pressure is 0.5 to 0.8 MPa, the grouting is stopped after stabilizing for 10 minutes, and the supplementary grouting is carried out 3 hours later.

[0008] Preferably, the pre-conduit pipes are installed in a single layer in the large pipe shed section, and in a double layer at the top of the tunnel in other sections. The upper layer of the double-layered pre-conduit pipes is arranged within a 150° range of the arch top, and the lower layer is arranged within a 180° range of the arch top. The pre-conduit pipes are seamless steel pipes with a diameter of 42mm, a wall thickness of 3.5mm, and a length of 3.0m. The pre-conduit pipes are prefabricated into a cone shape, with the cone tip being 10cm long. The pipe wall is drilled with 6-8mm grout drainage holes at 30cm intervals in a staggered pattern, and no holes are drilled at the end 0.6m. The pipes are arranged in a double layer with a circumferential spacing of 0.4m and a longitudinal spacing of 1.5m, with a drilling angle of 10°. The pipes are injected with a 1:1 cement-water glass double grout. A single layer of pre-conduit pipes is arranged within a 150° range of the arch top at the large pipe shed.

[0009] Preferably, the grid arch frame includes four arc-shaped main ribs, two of which are positioned directly above the other two main ribs. Two adjacent main reinforcement bars in the horizontal direction are fixedly connected by a U-shaped steel bar located between the two main reinforcement bars; Two adjacent main bars in the vertical direction are fixedly connected by a Z-shaped steel bar located between the two main bars; One end of the main reinforcement bar is welded to a connecting angle steel; bolt connection holes are reserved on the connecting angle steel; The cross-section of the lattice arch is a rectangular frame; A single grid arch frame is divided into 10 grid units in the circumferential direction. Each grid unit is connected in the circumferential direction by bolts passing through the bolt connection holes on the connecting angle steel. Two grid arch frames are connected by welding 25mm diameter connecting steel bars along the tunnel direction. The spacing of the connecting steel bars is 30-50cm.

[0010] Preferably, temporary supports are made of I20a I-beams, with one every 50cm along the tunnel direction. They are fabricated in advance outside the tunnel as needed for construction. The two ends are welded with 414mm×240mm×10mm steel plates and connected to the corresponding steel plates on the arch frame with M22×70mm bolts. Longitudinal connecting bars with a diameter of 25mm are provided, with a spacing of 1m and staggered arrangement. The connection is made by single-sided welding, and the welding length is not less than 25cm. Two anchor bolts are installed at each arch foot and temporary support node. The anchor bolts at the grid arch are 3.0m long and the anchor bolts at the temporary support points are 2.5m long. They are all made of steel pipes with a diameter of 42mm and a length of 3.5mm and are installed at an angle of 45°.

[0011] Preferably, the shotcrete is C25 early-strength concrete with a thickness of 350mm.

[0012] Beneficial effects of this invention: 1. Reduce the excavation cross-section, shorten the initial support construction time, and reduce safety risks. This method further refines the six excavation sections of the double-sided wall pilot tunnel method, dividing the pilot tunnels except for the lower pilot tunnel in the middle zone into two steps for excavation and support, reducing the cross-sectional dimensions of each pilot tunnel for excavation and support, shortening the construction time, reducing the exposure time of the excavation face, and improving the safety of tunnel construction.

[0013] 2. It can effectively control tunnel deformation and convergence. By reducing the cross-sectional dimensions of tunnel excavation and support, and increasing the number of anchor bolts, initial support can be completed in a timely manner, and the surrounding rock can be sealed off promptly.

[0014] 3. The segmented processing of the grid arch frame can reduce the processing size of the grid arch frame, which facilitates the transportation and installation of components in the tunnel and reduces construction safety risks. Attached Figure Description

[0015] Figure 1 This is a cross-sectional diagram of the tunnel support and excavation sequence.

[0016] Figure 2 This is a schematic diagram of the tunnel excavation construction sequence.

[0017] Attached reference numerals: 1-Large pipe shed, 2-Advanced small pipe, 3-Grid arch frame, 4-Anchor bolt, 5-Temporary inverted arch, 6-Vertical temporary support, 7-Concrete. Detailed Implementation

[0018] This invention discloses a method for excavating a large-section, cut-and-cover tunnel in a sand layer, comprising the following steps: Step 1: Divide the tunnel cross section into three parts horizontally: left, middle, and right. Then divide these three parts vertically into upper and lower sections, for a total of 6 areas. Excavate pilot tunnels in each area. Excavate and support the other pilot tunnels, except for the lower pilot tunnel in the middle section, in two separate steps. Step 2: Measurement and layout. Based on the designed tunnel cross-section, measure and lay out the outline of the excavation cross-section and the excavation range of each pilot tunnel. Step 3: Advanced support construction is carried out using large pipe shed 1, single-row advanced small pipe 2 and double-row advanced small pipe 2; Step 4: Manually excavate the upper guide tunnel of the left area, with an excavation advance of 0.5-0.6m. The upper guide tunnel of the left area is excavated in two layers. The first excavation adopts the step method to excavate the upper guide tunnel of the left area located in the upper layer. After the core soil is excavated, install the vertical temporary support 6, the grid steel arch frame, and install the locking anchor rod 4 at the bottom of the arch foot and the temporary support. Shotcrete 7. After the upper part of the left zone is excavated for 3-5m, the lower part of the left zone is excavated. This step is completed in one excavation. The vertical temporary support 6, temporary invert arch 5, and grid arch frame 3 are installed. Locking anchor rods 4 are installed at the bottom of the arch foot and the vertical temporary support 6. Shot concrete 7 is sprayed, and the construction of the upper part of the left zone is completed. After entering the upper pilot tunnel in the left area, soil dumping holes are left every 3 to 5 meters on the temporary invert arch 5. The excavated soil is dumped from the soil dumping holes onto the battery-powered vehicle in the lower pilot tunnel and transported out of the tunnel.

[0019] Step 5: After the pilot tunnel in the upper part of the left area is excavated to a depth of 3-5m, the lower part of the pilot tunnel in the left area is excavated manually or mechanically. The lower part of the pilot tunnel in the left area is excavated in two layers using the step method. After the upper part of the lower part of the pilot tunnel in the left area is excavated, the vertical temporary support 6 and the grid arch frame 3 are installed. Locking anchor rods 4 are installed at the bottom of the arch foot and the vertical temporary support 6, and shotcrete 7 is applied. The lower part of the pilot tunnel in the left area is excavated after the pilot tunnel in the lower part of the left area is excavated to a depth of 3-5m. After excavation, the vertical temporary support 6 and the grid arch frame 3 are installed in time, and shotcrete 7 is applied. This completes the construction of the lower part of the pilot tunnel in the left area, and the construction of the left side of the tunnel is completed. Step 6: After excavating the lower part of the left zone 3-5m into the lower guide tunnel, manually excavate the upper part of the right zone. The upper part of the right zone guide tunnel is excavated in two layers. The first excavation uses the step method to open the upper part of the right zone guide tunnel. After the core soil is excavated, install the vertical temporary support 6 and the grid steel arch frame. Install the locking anchor rod 4 at the bottom of the arch foot and the vertical temporary support 6, and spray concrete 7. After the upper part of the right zone is excavated for 3-5m, the lower part of the right zone is excavated. This step is completed in one excavation. The vertical temporary support 6, temporary invert arch 5, and grid arch frame 3 are installed. Locking anchor rods 4 are installed at the bottom of the arch foot and the vertical temporary support 6. Shot concrete 7 is sprayed to complete the construction of the upper part of the right zone. During the construction of the pilot tunnel in the upper part of the right area, soil dropping holes were left every 3 to 5 meters on the temporary invert arch 5 to drop the excavated soil from the pilot tunnel to the lower pilot tunnel and transport it out by battery truck. Step 7: After the pilot tunnel in the upper part of the right zone is excavated to a depth of 3-5m, the lower part of the pilot tunnel in the right zone is excavated manually or mechanically. The lower part of the pilot tunnel in the right zone is excavated in two layers using the step method. After the upper part of the lower part of the right zone is excavated, the vertical temporary support 6 and the grid arch frame 3 are installed. Locking anchor rods 4 are installed at the arch foot and the bottom of the temporary support, and shotcrete 7 is applied. The lower part of the pilot tunnel in the right zone is excavated after the pilot tunnel in the upper part of the lower zone is excavated to a depth of 2-5m. After excavation, the vertical temporary support 6 and the grid arch frame 3 are installed in time, and shotcrete 7 is applied. This completes the construction of the lower part of the pilot tunnel in the right zone, and the construction of the right side of the tunnel is completed. Step 8: After the lower part of the right zone's lower pilot tunnel has been excavated to a depth of 10-15m, the upper part of the central zone's pilot tunnel is manually excavated. The upper part of the central zone's pilot tunnel is excavated in two layers using the step method. The upper part of the central zone's upper pilot tunnel leaves core soil and is excavated along the tunnel outline. A grid arch frame 3 is installed to connect the grid arch frame 3 installed in the upper part of the left zone's pilot tunnel and the grid arch frame 3 installed in the upper part of the right zone's pilot tunnel. Shotcrete 7 is used to seal the arch crown. The lower part of the central zone's upper pilot tunnel is excavated after the upper part of the central zone's upper pilot tunnel has been excavated to a depth of 3-5m. A central partition is installed between the upper part of the central zone's upper pilot tunnel and the lower part of the central zone's upper pilot tunnel. The excavated soil from the upper part of the central zone's upper pilot tunnel is manually transferred to the lower part of the central zone's upper pilot tunnel through the pre-reserved opening at the central partition. Step 9: After the pilot tunnel in the upper part of the middle zone is excavated to a depth of 3-5m, the pilot tunnel in the lower part of the middle zone is excavated manually or mechanically. The pilot tunnel in the lower part of the middle zone is excavated in one go. After the excavation is completed, the inverted arch grid arch frame 3 is installed to connect the grid arch frame 3 installed in the lower part of the left zone pilot tunnel and the grid arch frame 3 installed in the lower part of the right zone pilot tunnel. Shotcrete 7 is sprayed to close the tunnel into a ring.

[0020] The large pipe shed 1 uses seamless steel pipes with a diameter of 108mm and a wall thickness of 8mm, with a length of 15 meters, a circumferential spacing of 400mm, and an installation angle of 1 to 2°. If the working space is limited, the large pipe shed is constructed in sections of 2m to 3m each. During construction, each section is rammed and connected to the pipe. Each pipe section is connected with a steel pipe joint with an inner diameter of 90mm and a length of 0.3m.

[0021] The grouting of the large pipe shed 1 uses cement mortar, in which the cement is ordinary Portland cement of grade 42.5 or above, the water-cement ratio is 0.8 to 1.2, the grouting pressure is 0.5 to 0.8 MPa, the grouting is stopped after stabilizing for 10 minutes, and the supplementary grouting is carried out 3 hours later.

[0022] The advanced small guide pipe 2 is installed in a single layer in section 1 of the large pipe shed, and in a double layer in the tunnel top of other sections. The upper layer of the double-layered advanced small guide pipe 2 is arranged within a 150° range of the arch top, and the lower layer is arranged within a 180° range of the arch top. The advanced small guide pipe 2 is a seamless steel pipe with a diameter of 42mm, a wall thickness of 3.5mm, and a length of 3.0m. The advanced small guide pipe 2 is prefabricated into a cone shape, with the cone tip being 10cm long. The pipe wall is drilled with 6-8mm grout discharge holes at 30cm intervals in a staggered pattern, and no holes are drilled at the end 0.6m. The small guide pipes are arranged in a double layer with a circumferential spacing of 0.4m and a longitudinal spacing of 1.5m, with a drilling angle of 10°. The small guide pipes are injected with a 1:1 cement-water glass double grout. A single layer of advanced small guide pipe 2 is arranged within a 150° range of the arch top at section 1 of the large pipe shed.

[0023] The grid arch frame 3 includes four arc-shaped main bars, two of which are positioned directly above the other two main bars. Two adjacent main reinforcement bars in the horizontal direction are fixedly connected by a U-shaped steel bar located between the two main reinforcement bars; Two adjacent main bars in the vertical direction are fixedly connected by a Z-shaped steel bar located between the two main bars; One end of the main reinforcement bar is welded to a Q235 connecting angle steel; a 24mm diameter bolt hole is reserved on the Q235 connecting angle steel; The cross-section of the lattice arch 3 is a rectangular frame; One grid arch frame 3 is divided into 10 grid units in the circumferential direction. Each grid unit is connected in the circumferential direction by bolts passing through the bolt connection holes on the connecting angle steel. Two grid arch frames 3 are connected by welding 25mm diameter connecting steel bars along the tunnel direction. The spacing of the connecting steel bars is 30-50cm.

[0024] Temporary supports are made of I20a I-beams, with one every 50cm along the tunnel direction. They are fabricated outside the tunnel in advance as needed for construction. The two ends are welded with 414mm×240mm×10mm steel plates and connected to the corresponding steel plates on the arch frame with M22×70mm bolts. Longitudinal connecting bars with a diameter of 25mm are provided, with a spacing of 1m and staggered arrangement. The connection is made by single-sided welding, and the welding length is not less than 25cm. Two anchor rods 4 are installed at each arch foot and temporary support node. The anchor rods 4 at the grid arch frame 3 are 3.0m long and the anchor rods at the temporary support points are 2.5m long. They are all made of steel pipes with a diameter of 42mm × 3.5mm and installed at an angle of 45°.

[0025] The shotcrete 7 uses C25 early-strength concrete 7 with a thickness of 350mm.

[0026] Implementation Examples With attachment Figure 1 and attached Figure 2 This method will be described as follows: Step 1: Divide the tunnel cross section into three parts: left, middle, and right, and then into two parts: upper and lower, for a total of 6 pilot tunnels. Then, excavate and support the pilot tunnels I to V in two separate steps.

[0027] Step 2: Measurement and layout. Based on the designed tunnel cross-section, measure and lay out the outline of the excavation cross-section and the excavation range of each pilot tunnel. Step 3: Advanced support construction. Advanced support adopts the method of large pipe roof 1 + single row of advanced small pipe 2 and double row of advanced small pipe 2 for support. The large pipe shed 1 uses seamless steel pipes with a diameter of 108mm and a wall thickness of 8mm, with a length of 15 meters, a circumferential spacing of 400mm, and a driving angle of 1-2°. If the working space is limited, the pipe shed can be constructed in sections of 2m-3m each. During construction, each section is rammed and connected, and each section is connected using a steel pipe joint with an inner diameter of 90mm and a length of 0.3m. The grout used for pipe spraying is a 1:1 cement-water glass dual-liquid grout, where the cement is ordinary Portland cement of grade 42.5 or higher. The grouting pressure is 0.5-0.8Mpa, and grouting is stopped after stabilization for 10 minutes. Supplementary grouting can be carried out after 3 hours. The advanced small guide pipe 2 is installed in a single layer in section 1 of the large pipe shed, and in a double layer at the top of the tunnel in other sections. The upper layer of the double-layered advanced small guide pipe 2 is arranged within a 150° range of the arch crown, and the lower layer within a 180° range of the arch crown. The small guide pipes are seamless steel pipes with a diameter of 42×3.5mm and a length of 3.0m. The small guide pipes are prefabricated into a conical shape, with a cone tip length of 10cm. Grouting holes of 6-8mm are drilled in a staggered pattern at 30cm intervals on the pipe wall, with no drilling at the end 0.6m. The small guide pipes are arranged in a double layer with a circumferential spacing of 0.4m and a longitudinal spacing of 1.5m, at a drilling angle of 10°. The small guide pipes are filled with a 1:1 cement-water glass grout. A single layer of small guide pipes is installed within a 150° range of the arch crown at section 1 of the large pipe shed. Step 4: Manually excavate Pilot Tunnel No. 1, with an excavation advance of 0.5m. Pilot Tunnel No. 1 is excavated in two stages. The first stage uses the bench method to excavate I-1, leaving core soil for excavation. After excavation, install vertical temporary supports 6, a grid steel arch frame, and install anchor bolts 4 at the arch foot and the bottom of the temporary supports. Shotcrete 7 is then applied. After excavating 3-5m of I-1, excavation I-2 begins. This stage is completed in one excavation, completing the installation of vertical temporary supports 6, temporary invert arch 5, and grid arch frame 3. Anchor bolts 4 are installed at the arch foot and the bottom of the temporary supports, and shotcrete 7 is applied, completing the construction of Pilot Tunnel No. 1. After entering Pilot Tunnel No. 1, leave soil dumping holes every 5m on the temporary invert arch 5. The excavated soil is dumped from these holes onto a battery-powered truck in the lower pilot tunnel for transport out of the tunnel. Step 5: After excavating 3-5m of I-2, manually or mechanically excavate Pilot Tunnel II. Pilot Tunnel II is excavated in two layers using the step method. After excavating II-1, install the vertical temporary support 6 and the grid arch frame 3, and install the locking anchor rod 4 at the arch foot and the bottom of the temporary support. Shotcrete 7 is then applied. II-2 is excavated after excavating 3-5m of II-1. After excavation, promptly install the vertical temporary support 6 and the grid arch frame 3, and apply shotcrete 7 to complete the left side of the tunnel. Step Six: After excavating 3-5m of Pilot Tunnel II, Pilot Tunnel III is manually excavated. Pilot Tunnel III is excavated in two stages. The first stage uses the step method to excavate III-1, leaving core soil for excavation. After excavation, vertical temporary supports 6 and lattice steel arch frames are installed. Locking anchor bolts 4 are installed at the arch foot and the bottom of the temporary supports, and shotcrete 7 is applied. After excavating 3-5m of III-1, III-2 is excavated. This step is completed in one excavation, completing the installation of vertical temporary supports 6, temporary invert arch 5, and lattice arch frames 3. Locking anchor bolts 4 are installed at the arch foot and the bottom of the temporary supports, and shotcrete 7 is applied, completing the construction of Pilot Tunnel III. During the construction of Pilot Tunnel III, soil drop holes are left every 5m on the temporary invert arch 5 to lower the excavated soil from the pilot tunnel to the lower pilot tunnel, where it is transported out by battery-powered vehicle. Step 7: After the excavation of III-2 reaches 3-5m, the No. IV pilot tunnel is excavated manually or mechanically. The No. IV pilot tunnel is excavated in two layers using the step method. After the excavation of IV-1, the vertical temporary support 6 and the grid arch frame 3 are installed. Locking anchor rods 4 are installed at the arch foot and the bottom of the temporary support, and shotcrete 7 is applied. IV-2 is excavated after the excavation of IV-1 reaches 2-5m. After the excavation, the vertical temporary support 6 and the grid arch frame 3 are installed in time, and shotcrete 7 is applied to complete the right side of the tunnel. Step 8: After excavating 15m in section IV-2, manually excavate pilot tunnel V. Pilot tunnel V is excavated in two layers using the step method. In section V-1, core soil is left and excavated along the tunnel outline. Install grid arch frame 3 to connect the arch frames of pilot tunnels I and III, and seal the arch top with shotcrete 7. In section V-2, excavation begins after V-1 has advanced 3 to 5m. The excavated soil is manually transferred to the lower pilot tunnel through the reserved opening at the central diaphragm. Step 9: After V-2 has been excavated to a depth of 3-5m, the No. VI pilot tunnel is excavated manually or mechanically. The No. VI pilot tunnel is excavated in one go. After the excavation is completed, the inverted arch grid frame 3 is installed to connect the arch frames of the No. II and No. IV pilot tunnels, and the shotcrete 7 is applied to close the tunnel into a ring.

[0028] The grating arch 3 is welded together from four 25mm diameter main reinforcing bars, 12mm diameter stirrups, 8mm diameter closed stirrups, and Q235 type connecting plates. Each grating is divided into 10 units, connected by M24 bolts. The main reinforcing bars of every two arches are connected at the connecting plate using 25mm diameter steel bars. L125×80×10mm angle steel is used between the sections, with 24mm diameter bolt holes drilled in the angle steel. Bolts are used for installation inside the holes, and the sections are then welded together.

[0029] The temporary supports are made of I20a I-beams, spaced 50cm apart. They are prefabricated outside the tunnel as needed for construction. 414×240×10mm steel plates are welded to both ends and connected to the corresponding steel plates on the arch frame using M22×70mm bolts. Longitudinal connecting bars with a diameter of 25mm are provided, spaced 1m apart, staggered, and connected using single-sided welding with a weld length of not less than 25cm.

[0030] Two anchor bolts 4 are installed at each arch foot and temporary support node. The anchor bolts 4 at the grid arch frame 3 are 3.0m long, and the anchor bolts at the temporary support points are 2.5m long. All anchor bolts are made of steel pipes with a diameter of 42mm × 3.5mm and are installed at an angle of 45°. The shotcrete 7 is C25 early-strength concrete 7 with a thickness of 350mm.

Claims

1. A method for excavating a large-section, cut-and-cover tunnel in a sand layer, characterized in that, Includes the following steps: Step 1: Divide the tunnel cross section into three parts horizontally: left, middle, and right. Then divide these three parts vertically into upper and lower sections, for a total of 6 areas. Excavate pilot tunnels in each area. Excavate and support the other pilot tunnels, except for the lower pilot tunnel in the middle section, in two separate steps. Step 2: Measurement and layout. Based on the designed tunnel cross-section, measure and lay out the outline of the excavation cross-section and the excavation range of each pilot tunnel. Step 3: Advanced support construction is carried out using large pipe sheds and single-row and double-row advanced small pipe supports; Step 4: Manually excavate the upper guide tunnel of the left area, with an excavation advance of 0.5-0.6m. The upper guide tunnel of the left area is excavated in two layers. The first excavation adopts the step method to excavate the upper guide tunnel of the left area located in the upper layer. After the core soil is excavated, the vertical temporary support and the grid steel arch frame are installed. Locking anchor rods are installed at the arch foot and the bottom of the vertical temporary support, and shotcrete is applied. After the upper part of the left zone is excavated for 3-5m, the lower part of the left zone is excavated. This step is completed in one excavation, and the installation of the vertical temporary support, temporary invert arch, and grid arch frame is completed. Locking anchor rods are installed at the arch foot and the bottom of the vertical temporary support, and shotcrete is sprayed to complete the construction of the upper part of the left zone. After entering the upper pilot tunnel in the left area, soil dumping holes are left every 3 to 5 meters on the temporary invert arch. The excavated soil is dumped from the soil dumping holes onto the battery-powered vehicle in the lower pilot tunnel and transported out of the tunnel. Step 5: After the pilot tunnel in the upper part of the left zone is excavated to a depth of 3-5m, the lower part of the pilot tunnel in the left zone is excavated manually or mechanically. The lower part of the pilot tunnel in the left zone is excavated in two layers using the step method. After the upper part of the lower part of the pilot tunnel in the left zone is excavated, vertical temporary supports and grid arch frames are installed. Locking anchor bolts are installed at the arch foot and the bottom of the temporary supports, and shotcrete is applied. The lower part of the pilot tunnel in the left zone is excavated after the upper part of the lower part of the left zone is excavated to a depth of 3-5m. After excavation, vertical temporary supports and grid arch frames are installed in a timely manner, and shotcrete is applied. This completes the construction of the lower part of the pilot tunnel in the left zone, thus completing the construction of the left side of the tunnel. Step 6: After excavating the lower part of the left zone 3-5m into the lower pilot tunnel, manually excavate the upper part of the right zone. The upper part of the right zone pilot tunnel is excavated in two layers. The first excavation uses the step method to open the upper part of the right zone pilot tunnel. After the core soil is excavated, install the vertical temporary support and the grid steel arch frame. Install the anchor bolts at the arch foot and the bottom of the vertical temporary support, and spray concrete. After the upper part of the right zone is excavated for 3-5m, the lower part of the right zone is excavated. This step is completed in one excavation, and the installation of the vertical temporary support, temporary invert arch, and grid arch frame is completed. Locking anchor rods are installed at the arch foot and the bottom of the vertical temporary support, and shotcrete is sprayed to complete the construction of the upper part of the right zone. During the construction of the pilot tunnel in the upper part of the right area, soil dropping holes were left every 3 to 5 meters on the temporary invert arch to drop the excavated soil from the pilot tunnel to the lower pilot tunnel and transport it out by battery truck. Step 7: After the pilot tunnel in the upper part of the right zone is excavated to a depth of 3-5m, the pilot tunnel in the lower part of the right zone is excavated manually or mechanically. The pilot tunnel in the lower part of the right zone is excavated in two layers using the step method. After the pilot tunnel in the lower part of the right zone is excavated, the vertical temporary support and grid arch frame are installed. Locking anchor rods are installed at the arch foot and the bottom of the vertical temporary support, and shotcrete is applied. The lower part of the right zone, located in the lower layer, is excavated after the lower part of the right zone, located in the upper layer, has advanced 2 to 5 meters. After excavation, vertical temporary supports and grid arch frames are installed in a timely manner, and shotcrete is applied to complete the construction of the lower part of the right zone, thus completing the construction of the right side of the tunnel. Step 8: After the lower section of the right zone has been excavated to a depth of 10-15m, the upper section of the central zone will be manually excavated. The upper section of the central zone will be excavated in two layers using the step method. The upper section of the central zone will retain core soil and be excavated along the tunnel outline. A grid arch frame will be installed to connect the grid arch frame installed in the upper section of the left zone and the grid arch frame installed in the upper section of the right zone. The arch will be sealed with shotcrete. The lower section of the central zone will be excavated after the upper section of the central zone has been excavated to a depth of 3-5m. A central partition will be installed between the upper section of the central zone and the lower section of the central zone. The excavated soil from the upper section of the central zone will be manually transferred to the lower section of the central zone through the pre-reserved opening at the central partition. Step 9: After the pilot tunnel in the upper part of the middle zone is excavated to a depth of 3-5m, the pilot tunnel in the lower part of the middle zone is excavated manually or mechanically. The pilot tunnel in the lower part of the middle zone is excavated in one go. After the excavation is completed, the inverted arch grid arch frame is installed to connect the grid arch frame installed in the lower pilot tunnel of the left zone and the grid arch frame installed in the lower pilot tunnel of the right zone. Shotcrete is sprayed to close the tunnel into a ring.

2. The method for excavating a large-section, full-sand tunnel using a cut-and-cover method as described in claim 1, characterized in that, The large pipe shed uses seamless steel pipes with a diameter of 108mm and a wall thickness of 8mm, with a length of 15 meters, a circumferential spacing of 400mm, and an installation angle of 1 to 2°. If the working space is limited, the large pipe shed is constructed in sections of 2m to 3m each. During construction, each section is rammed and connected to the pipe. Each pipe section is connected using a steel pipe joint with an inner diameter of 90mm and a length of 0.3m.

3. The method for excavating a large-section, full-sand tunnel using a cut-and-cover method as described in claim 2, characterized in that, The grouting of the large pipe shed uses cement mortar, in which the cement is ordinary Portland cement of grade 42.5 or above, the water-cement ratio is 0.8 to 1.2, the grouting pressure is 0.5 to 0.8 MPa, the grouting is stopped after stabilizing for 10 minutes, and the grouting is supplemented 3 hours later.

4. The method for excavating a large-section, full-sand tunnel using a cut-and-cover method as described in claim 3, characterized in that, The advanced small guide pipes are installed in a single layer in the large pipe shed section and in a double layer at the top of the tunnel in other sections. The upper layer of the double-layered advanced small guide pipes is arranged within a 150° range of the arch top, and the lower layer is arranged within a 180° range of the arch top. The advanced small guide pipes are seamless steel pipes with a diameter of 42mm, a wall thickness of 3.5mm, and a length of 3.0m. The advanced small guide pipes are prefabricated into a cone shape, with the cone tip being 10cm long. The pipe wall is drilled with 6-8mm grout drainage holes at 30cm intervals in a staggered pattern, and no holes are drilled at the end 0.6m. The small guide pipes are arranged in a double layer with a circumferential spacing of 0.4m and a longitudinal spacing of 1.5m, with a drilling angle of 10°. The small guide pipes are injected with a 1:1 cement-water glass double grout. A single layer of advanced small guide pipes is arranged within a 150° range of the arch top at the large pipe shed.

5. The method for excavating a large-section, full-sand tunnel using a cut-and-cover method as described in claim 4, characterized in that, The grid arch frame includes four arc-shaped main ribs, two of which are positioned directly above the other two main ribs. Two adjacent main reinforcement bars in the horizontal direction are fixedly connected by a U-shaped steel bar located between the two main reinforcement bars; Two adjacent main bars in the vertical direction are fixedly connected by a Z-shaped steel bar located between the two main bars; One end of the main reinforcement bar is welded to a connecting angle steel; bolt connection holes are reserved on the connecting angle steel; The cross-section of the lattice arch is a rectangular frame; A single grid arch frame is divided into 10 grid units in the circumferential direction. Each grid unit is connected in the circumferential direction by bolts passing through the bolt connection holes on the connecting angle steel. Two grid arch frames are connected by welding 25mm diameter connecting steel bars along the tunnel direction. The spacing of the connecting steel bars is 30-50cm.

6. The method for excavating a large-section, cut-and-cover tunnel in a sand layer as described in claim 5, characterized in that, The temporary supports are made of I20a I-beams, with one every 50cm along the tunnel direction. They are fabricated outside the tunnel in advance as needed for construction. The two ends are welded with 414mm×240mm×10mm steel plates and connected to the corresponding steel plates on the arch frame with M22×70mm bolts. Longitudinal connecting bars with a diameter of 25mm are provided, with a spacing of 1m and staggered arrangement. The connection is made by single-sided welding, and the welding length is not less than 25cm. Two anchor bolts are installed at each arch foot and temporary support node. The anchor bolts at the grid arch are 3.0m long and the anchor bolts at the temporary support points are 2.5m long. They are all made of steel pipes with a diameter of 42mm and a length of 3.5mm and are installed at an angle of 45°.

7. The method for excavating a large-section, full-sand tunnel using a cut-and-cover method as described in claim 6, characterized in that, The shotcrete used is C25 early-strength concrete with a thickness of 350mm.

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