Upper foundation pit and lower tunnel combined construction structure and construction method in saturated soft soil layer

Abstract

A kind of saturated soft soil stratum upper foundation pit-under tunnel combined construction structure and construction method, structure includes foundation pit, the bottom of foundation pit is connected with tunnel by shaft, and geogrid is arranged above the bottom of foundation pit;The outside of foundation pit is provided with steel sheet pile from inside to outside, pit interior dewatering well, bored pile, rotary jetting pile water-stop curtain and pit exterior dewatering well;First, bored pile and rotary jetting pile are constructed in site, intercept groundwater outside foundation pit, pit exterior dewatering is carried out by driving pit exterior dewatering well in water-stop curtain periphery, after completion of pit exterior dewatering, foundation pit is excavated in stages, steel sheet pile is driven to support after completion of excavation of section I, dewatering well is driven to carry out internal dewatering of foundation pit;Excavate section II to predetermined depth, geogrid is reinforced at the bottom of foundation pit, excavate shaft in foundation pit to the same depth of lower tunnel, excavate horizontal passage to tunnel interior to tunnel body, and carry out excavation support;Effectively control the deformation of tunnel section, reduce the influence on foundation pit, and then ensure construction safety, reduce the influence between foundation pit and tunnel.

Description

Technical Field

[0001] The present invention belongs to the technical field of tunnel construction, and in particular relates to an upper foundation pit-lower tunnel combined construction structure and a construction method in a saturated soft loess stratum. Background Art

[0002] During dewatering construction of saturated soft loess, in addition to the effect of water pressure in the water and soil pressure on the support structure, the most important problem is that the foundation pit is unstable due to water and sand gushing, seepage damage (sand flow, sudden gushing, and piping), and soil erosion, resulting in ground collapse. Drainage consolidation also produces large compression deformation, which can easily lead to large additional settlement of the surface and surrounding buildings, and the rate of occurrence is relatively fast. At the same time, tunnel excavation at the bottom of the foundation pit will further cause surrounding ground settlement and surrounding rock instability, leading to damage to the upper foundation pit and tunnel support structure. During the construction process, improper control of the water head difference between the inside and outside of the foundation pit can easily cause foundation pit water gushing, collapse, and base uplift.

[0003] Currently, the main prevention and control measures are: through the selection of reasonable groundwater control to prevent the adverse effects of groundwater on the safety of the foundation pit and the surrounding environment, to ensure the normal construction of the foundation pit and underground engineering, and at the same time, to strengthen the tunnel support structure of the lower excavation to ensure safety during construction while reducing the impact on the foundation pit and the surrounding surface. In the case of upper foundation pit and lower tunnel, the tunnel and foundation pit have higher deformation requirements. Therefore, the tunnel excavation and support should be constructed using methods that have less impact on the upper foundation pit. At the same time, the tunnel support structure of the lower excavation should be advanced to ensure safety during construction while reducing the impact on the foundation pit and the surrounding surface.

[0004] Common dewatering methods currently include: ① Open ditch with catchment well. During high water levels, this method is often used as a supplementary drainage measure to other dewatering methods, primarily draining groundwater, construction water, and rainwater. ② Light wellpoints, which can be arranged in single, double, or circular patterns. ③ Jet wellpoints, which dewater to a greater depth. ④ Electroosmosis wellpoints, which effectively extract water from fine-grained soils, require integration with light or jet wellpoints, but are more cumbersome. ⑤ Tube wellpoints, suitable for gravel layers with high permeability and abundant groundwater. ⑥ Deep wellpoints, offering high drainage capacity and a wide dewatering depth and range, can be deployed both within and outside the foundation pit, or combined with other wellpoint systems. ⑦ Combined wellpoints, used in certain hydrogeological conditions or projects with special requirements where a single wellpoint dewatering method is insufficient to achieve satisfactory results. Prior to dewatering, a waterstop curtain can be constructed to block groundwater infiltration, combined with dewatering wells to ensure effective dewatering. Existing technologies often consider foundation pit construction or tunnel construction separately, failing to achieve a coordinated foundation pit-tunnel approach. This leads to deficiencies in construction cost control, construction safety assurance, and construction organization and implementation. Summary of the Invention

[0005] In order to overcome the deficiencies of the above-mentioned prior art, the purpose of the present invention is to propose a combined construction structure and construction method of an upper foundation pit and a lower tunnel in a saturated soft loess stratum, which can ensure that in a saturated soft loess stratum, when there is a foundation pit above and a tunnel is excavated below, the surface settlement and the head difference between the inside and outside of the foundation pit bottom can be controlled, the impact on surrounding buildings can be reduced, the deformation and internal force of the tunnel support structure can be reduced, the mutual influence between the foundation pit and the tunnel during construction can be reduced, the deformation and internal force of the two can be reduced as much as possible to avoid damage to the support structure, and the safety and reliability of the foundation pit and tunnel support structures can be improved.

[0006] In order to solve the above technical problems, the technical solutions adopted by the present invention are as follows:

[0007] A combined upper foundation pit and lower tunnel construction structure in a saturated soft loess stratum includes a foundation pit 17, the bottom of which is connected to a tunnel 11 via a vertical shaft 7, and a geogrid 6 is provided above the bottom of the foundation pit 17; the outer side of the foundation pit 17 is provided with steel sheet piles 3, a dewatering well 5 in the pit, cast-in-place piles 2, a jet-jet pile water-stop curtain 1 and a dewatering well 4 outside the pit from the inside to the outside.

[0008] The inner side of the foundation pit 17 is a slope structure, which is arranged into section I slope and section II slope according to depth. A slope reduction platform is provided between section I slope and section II slope. The steel sheet piles 3 and the drainage wells 5 in the pit are provided on the slope reduction platform between section II slope and section I slope. The cast-in-place piles 2, the rotary jet pile water-stop curtain 1 and the drainage wells 4 outside the pit are provided at the starting point of section I slope.

[0009] An initial support 8 is provided above the bottom of the tunnel 11 along the circumference of the tunnel, and a transverse channel 9 is distributed between the initial support 8 and the bottom of the tunnel 11 .

[0010] A combined construction method of an upper foundation pit and a lower tunnel in a saturated soft loess stratum comprises the following steps:

[0011] Step 1: Prepare for construction

[0012] 1.1. Conduct site survey and leveling and select appropriate construction machinery and tools;

[0013] 1.2. Before construction, test piles shall be carried out for technical purposes, with the number of test piles being no less than three groups, with four piles in each group;

[0014] Step 2: Carry out cast-in-place pile construction;

[0015] 2.1. Measure and stake out pile positions according to the pile position plan;

[0016] 2.2. Make the casing and set stiffening ribs at the upper and lower ends and the outer side of the middle part of the casing;

[0017] 2.3. Use the drill bucket to dig out the upper soil layer of the predetermined pile hole and place the casing;

[0018] 2.4. Drilling rig in place;

[0019] 2.4.1. Slowly place the drill bit into the casing;

[0020] 2.4.2. Lift the drill head and place wooden blocks under the drill head base;

[0021] 2.4.3. Align the drill rig with the hole and install the drill disc;

[0022] 2.5. Drill holes inside the casing with a diameter of 1100 to 1200 mm;

[0023] 2.6. Clean the hole and place the steel cage;

[0024] 2.7. For underwater concrete inside the pile hole, the concrete surface rises at a rate of 2 to 2.2 m / min;

[0025] 2.8. Clean up the site and complete the support pile construction;

[0026] Step 3: Waterstop curtain construction

[0027] 3.1. Drilling rig in place;

[0028] 3.1.1. Drill and lower the pipe to the predetermined position;

[0029] 3.1.2. After the grouting pipe reaches the predetermined depth, a clean water pressure test is carried out. During the pressure test, the pressure gradually increases to reduce friction resistance and prevent nozzle clogging;

[0030] 3.1.3. After the pressure test is completed, start mixing the cement slurry with equal water-cement ratio and cement dosage of 160-200 kg / m;

[0031] 3.1.4. Carry out high-pressure grouting operation, with the grouting pressure set at 30-35 MPa;

[0032] 3.1.5. High-pressure slurry injection is carried out from bottom to top, with the nozzle lifting speed not exceeding 15 cm / min, and repeated 1 to 2 times;

[0033] 3.1.6. After spraying from bottom to top to the designed height, pull out the spraying pipe in time and finish spraying;

[0034] 3.1.7 After the spraying is completed, clean the high-pressure pump, slurry delivery pipeline, grouting pipe and nozzle;

[0035] Step 4: Conduct precipitation outside the pit

[0036] 4.1. Measure and lay out the well points and construct enclosures according to the well location plan;

[0037] 4.2. Bury the mouth guard pipe, insert the mouth guard pipe into the original soil layer, use clay soil to compact the outside of the pipe to prevent slurry from flowing outside the pipe, and the upper part of the mouth guard pipe should be 0.1 to 0.3 meters above the ground;

[0038] 4.3. Install the drilling rig. The drilling rig platform should be installed firmly and levelly, and aligned with the center of the hole;

[0039] 4.4. Drilling: After reaching the predetermined depth, drill an additional 0.3 to 0.5 meters. During the drilling process, ensure that the drilling rig is level, the verticality of the borehole is guaranteed, and the mud density is controlled at 1.10 to 1.15;

[0040] 4.5. When drilling to the top of the aquifer, add clean water to mix the slurry, drill to the designed elevation, and before lifting the drill, lift the drill rod to 0.5 to 1.0 m from the bottom of the hole to punch the hole, and gradually adjust the slurry density to 1.00 to 1.05;

[0041] 4.6. When lowering the well pipe, a aligner should be installed to ensure that the filter pipe is centered. The well pipe welding should be firm, vertical, and watertight. After lowering to the predetermined depth, the wellhead should be fixed in the center.

[0042] 4.7 Before filling with gravel, the depth inside and outside the well pipe should be measured with a measuring rope. The difference should not exceed the length of the sedimentation pipe. The filling process should not be stopped midway until it reaches the predetermined position. The material input should not be less than 95% of the designed amount.

[0043] 4.8. Use clay to seal the wellbore. Crush the clay before filling it in. Slow down and seal it around the wellbore.

[0044] 4.9. After the well is completed, immediately lower the deep well submersible pump to the bottom of the well and lay the drainage pipe. Once the pumping and drainage systems are installed, start pumping water;

[0045] Step 5: Excavation of foundation pit 17

[0046] 5.1. Clear obstacles in the excavation area and conduct surveying and setting out;

[0047] 5.2. Use large-scale slope cutting from top to bottom, with an excavation slope ratio of 1:1 to 1:2, and excavate the top of the slope to the slope platform in stages;

[0048] 5.3. During the excavation process, check the width of the slope platform. If the width is not enough, trim it in time. Trim the edge every 1 to 2 meters to the designed elevation. After the excavation is completed, trim the slope and check the width and elevation.

[0049] 5.4. After checking that everything is correct, drive steel sheet piles into the footing of the slope platform as support;

[0050] 5.5. Remove the pile heads, insert the lower part of the steel frame into the piles, and weld it to the main reinforcement of the piles. After completion, pour concrete.

[0051] 5.6. Excavate the slope platform to the bottom of the slope. Check the width of the slope bottom during the excavation process. If the width is not enough, trim it in time and trim it to the elevation. After the excavation is completed, perform a unified slope trimming and check the width and elevation.

[0052] Step 6: Reinforce the bottom of the foundation pit 17 with grids

[0053] 6.1. After cleaning the debris on the working surface, level the site with a height difference of no more than 5cm;

[0054] 6.2. When laying geogrid, it should be laid flat and straight;

[0055] 6.3. Use overlap to connect two adjacent geogrids with an overlap width of not less than 10 cm. Fix them and set joints in non-main load bearing directions.

[0056] 6.4. After the geogrid is laid, spread the crushed stone cushion layer, with an interval of no more than 48 hours, and repeatedly roll it to ensure compaction. After compaction, level it and the height difference should not exceed 5cm;

[0057] Step 7: Dewatering the pit

[0058] 7.1. Measure and lay out the well points and construct enclosures according to the well location plan;

[0059] 7.2. Bury the mouth guard pipe, insert the mouth guard pipe into the original soil layer, use clay soil to compact the outside of the pipe to prevent slurry from flowing outside the pipe, and the upper part of the mouth guard pipe should be 0.1 to 0.3 meters above the ground;

[0060] 7.3. When installing the drilling rig, the drilling rig platform should be installed firmly and levelly, and aligned with the center of the hole;

[0061] 7.4. Drilling: After reaching the predetermined depth, drill an additional 0.3 to 0.5 m. During the drilling process, ensure that the drilling rig is level, the verticality of the borehole is guaranteed, and the mud density is controlled at 1.10 to 1.15;

[0062] 7.5. When drilling to the top of the aquifer, add clean water to mix the slurry. Drill to the designed elevation. Before lifting the drill, lift the drill rod to 0.5 to 1.00 m from the bottom of the hole to punch the hole, and gradually adjust the slurry density to 1.0 to 1.05.

[0063] 7.6. When lowering the well pipe, a aligner should be installed to ensure that the filter pipe is centered. The well pipe welding should be firm, vertical, and watertight. After lowering to the predetermined depth, the wellhead should be fixed in the center.

[0064] 7.7 Before filling with gravel, the depth inside and outside the well pipe should be measured with a measuring rope. The difference should not exceed the length of the sedimentation pipe. The filling process should not be stopped midway until it reaches the predetermined position. The material input should not be less than 95% of the designed amount.

[0065] 7.8. Use clay to seal the wellbore. Crush the clay before filling it in. Slow down and seal it around the wellbore.

[0066] 7.9. After the well is completed, immediately lower the deep well submersible pump to the bottom of the well and lay the drainage pipe. Once the pumping and drainage systems are installed, start pumping water;

[0067] Step 8: Construction of vertical shaft and excavation of horizontal passage in foundation pit 17

[0068] 8.1. Measure and lay out the shaft points according to the plan layout and construct enclosures;

[0069] 8.2. Use short-section excavation and masonry to construct the lock section. Temporary support is provided by driving anchor rods, setting up steel mesh, and spraying concrete to form a "sprayed anchor mesh" support. The steel bars are composed of 6mm to 10mm diameter steel bars, and the mesh size is 20×20cm to 25×25cm. The nozzle is kept perpendicular to the spraying surface, and the spraying pressure is maintained at 0.12 to 0.15MPa.

[0070] 8.3. After completion, tie the steel bars, support the formwork, and pour the concrete;

[0071] 8.4. After the shotcrete reaches the required strength, temporary horizontal and angled supports shall be installed. The angled supports shall be arranged at intervals of horizontal steel grids, and the angles between the angled supports and the shaft wall shall be 45-60 degrees.

[0072] 8.5. Repeat the above steps to excavate to the predetermined elevation, spray concrete 25mm-30mm first, and after final setting, lay steel mesh and set up grid steel frame. The grid spacing inside the well bottom is 800 to 1000mm, and the grids are connected by welding. Finally, spray concrete to seal the bottom.

[0073] 8.6. After the vertical shaft construction is completed, the construction of the intersection of the horizontal channel and the vertical shaft is carried out. The shaft wall at the connecting channel is broken, and glass fiber anchor rods are driven beyond the excavation outline for advanced support. The excavated section is sprayed with initial concrete. After completion, the steel mesh is laid, the grid steel frame is erected, and the grid is welded to the steel mesh. After the welding is completed, the steel mesh and grid steel frame are sprayed with concrete again.

[0074] 8.7. Repeat the above steps until the connection between the cross passage 11 and the tunnel main line is completed;

[0075] Step 9: Excavate the tunnel mainline

[0076] 9.1. Excavate the upper step 16 of the transverse passage 11 and turn into the first right pilot pit ② and the second right pilot pit ④ of the tunnel section 10 on the right side of the main line;

[0077] 9.2. Carry out initial support construction immediately after excavation is completed;

[0078] 9.3. Drive glass fiber anchor rods beyond the excavation contour surface, and spray concrete on the excavated section. Set up the grid steel frame and steel mesh and weld them together. After completion, spray concrete again to form a "sprayed anchor mesh" support.

[0079] 9.4. Continue excavating the first right pilot pit ② and the second right pilot pit ④ of tunnel section 10 to the designed advance and simultaneously implement initial support;

[0080] 9.5. At the designed advance, excavate the first core rock mass ⑦ and the second core rock mass ⑧ in the middle of the tunnel section 10 to the left to form a connecting passage 12. Simultaneously, use fiberglass anchors to advance support the right pilot pit in the direction of the short mileage and excavate.

[0081] 9.6. After excavation is completed, repeat step 9.3 to complete the initial support;

[0082] 9.7. Use fiberglass anchors to pre-support the left pilot pit of tunnel section 10 on the left side of connecting passage 12. Excavate the first left pilot pit ① and the second left pilot pit ③ of tunnel section 10.

[0083] 9.8. On the right side of the connecting passage, use fiberglass anchors to advance support the first right pilot pit ②, the second right pilot pit ④, and the third right pilot pit ⑥ of tunnel section 10 and excavate them;

[0084] 9.9. Carry out initial support construction immediately after excavation is completed;

[0085] 9.10. Simultaneously excavate the lower steps of the left and right pilot pits and apply initial support, following the same excavation sequence;

[0086] 9.11. After the left and right pilot tunnels are excavated, fiberglass anchor bolts are driven into the left and right directions of the tunnel from the connecting channel 12. Fiberglass anchor bolts are driven outside the excavation contour surface. Initial concrete spraying is carried out on the excavated section. Grid steel frames and steel mesh are installed and welded together. After completion, concrete is sprayed again to form a "sprayed anchor mesh" support.

[0087] Step 10: Backfill the foundation pit 17.

[0088] In step 2.7 of step 2, the underwater concrete inside the pile hole is poured using the mud-submerged vertical pipe method.

[0089] In step 3.1.3 of step 3, the water-cement ratio of the cement slurry used is 1:1.

[0090] Step 5: Trim as described in Steps 5.3 and 5.6, trimming one side to the designed elevation every 1m.

[0091] Compared with the prior art, the present invention has the following beneficial effects:

[0092] 1. The construction method of the present invention first constructs cast-in-place piles 2 and rotary jet piles on site, and forms a rotary jet pile water-stop curtain 1 through the rotary jet piles to intercept the groundwater outside the foundation pit 17 and separate the groundwater inside and outside the foundation pit 17, thereby ensuring the reliability and sustainability of subsequent precipitation; the water-stop curtain is composed of rotary jet piles, which are divided into two rows, and the adjacent piles are interlocked with each other by 100 to 120 mm. The purpose is to reduce water infiltration through the interlocking of the rotary jet piles, thereby enhancing the water-stopping effect, and at the same time, in conjunction with the precipitation wells inside and outside the pit, the groundwater can be effectively controlled.

[0093] 2. After the construction of the water-stop curtain is completed, an outside-pit dewatering well 4 is driven outside the water-stop curtain to carry out outside-pit dewatering. After the outside-pit dewatering is completed, the foundation pit 17 is excavated in stages. After each stage of excavation, support and reinforcement are carried out. The bottom of the foundation pit 17 is reinforced with geogrids to cope with base uplift and water and sand gushing, etc., and the reinforcement and dewatering effects are good.

[0094] 3. After the reinforcement is completed, a vertical shaft is excavated inside the foundation pit 17 to the same depth as the tunnel below, and a cross passage is excavated into the tunnel body to the inside of the tunnel. The tunnel main line is turned into the vertical shaft, and the core rock and soil are retained. Pilot pits on both sides are excavated first, and glass fiber anchor rods are used for advance support. After the excavation and support on both sides are completed, the core rock and soil are excavated and supported. This can effectively control the deformation of the tunnel section 10, reduce the impact on the foundation pit 17, and thus ensure construction safety and reduce the impact between the foundation pit 17 and the tunnel. BRIEF DESCRIPTION OF THE DRAWINGS

[0095] Figure 1 It is a cross-sectional view of the upper foundation pit-lower tunnel combined construction structure of the present invention.

[0096] Figure 2 It is a plan view of the upper foundation pit-lower tunnel combined construction structure of the present invention.

[0097] Figure 3 This is the division diagram of tunnel section 10.

[0098] Figure 4a 、 4b 4c and 4d are the overall schematic diagrams of the tunnel-cross passage and the schematic diagrams of excavation at different stages respectively; Figure 4a This is the overall schematic diagram of the tunnel-cross passage; Figure 4b Schematic diagram of the excavation of the first left pilot pit ①, the first right pilot pit ②, the second left pilot pit ③, the second right pilot pit ④ and the connecting passage of the tunnel section 10; Figure 4cSchematic diagram of the excavation of the first left pilot pit ① of the tunnel section 10, the first right pilot pit ②, the second left pilot pit ③, the second right pilot pit ④, the third right pilot pit ⑥ and the lower step of the connecting passage; Figure 4d This is a schematic diagram of the partial excavation of the third left pilot pit ⑤, the third right pilot pit ⑥, the first core rock and soil mass ⑦, the second core rock and soil mass ⑧, and the third core rock and soil mass ⑨ of tunnel section 10.

[0099] In the figure: 1 jet jet water-stop curtain; 2 cast-in-place piles; 3 steel sheet piles; 4 dewatering well outside the pit; 5 dewatering well inside the pit; 6 grid reinforcement; 7 vertical shaft; 8 initial support; 9 transverse passage; 10 tunnel section 10; 11 transverse passage; 12 connecting passage; 13 left pilot pit; 14 right pilot pit; 15 core rock and soil; 16 step on the transverse passage; 17 foundation pit.

[0100] The present invention will be further described in detail below with reference to the accompanying drawings

[0101] See also Figure 1 A combined construction structure of an upper foundation pit and a lower tunnel in a saturated soft loess stratum includes a foundation pit 17, the bottom of which is connected to a tunnel 11 through a vertical shaft 7, and a geogrid 6 is provided above the bottom of the foundation pit 17; the outside of the foundation pit 17 is provided with steel sheet piles 3, a dewatering well 5 in the pit, cast-in-place piles 2, a rotary jet pile water-stop curtain 1 and a dewatering well 4 outside the pit from the inside to the outside.

[0102] The inner side of the foundation pit 17 is a slope structure, which is arranged into section I slope and section II slope according to depth. A slope reduction platform is provided between section I slope and section II slope. The steel sheet piles 3 and the drainage wells 5 in the pit are provided on the slope reduction platform between section II slope and section I slope. The cast-in-place piles 2, the rotary jet pile water-stop curtain 1 and the drainage wells 4 outside the pit are provided at the starting point of section I slope.

[0103] See also Figure 3 An initial support 8 is provided above the bottom of the tunnel 11 along the circumference of the tunnel, and a transverse channel 9 is distributed between the initial support 8 and the bottom of the tunnel 11.

[0104] See also Figure 4a 、 4b , 4c, 4d, the tunnel section 10 is divided into a left pilot pit 13, a right pilot pit 14, and a core rock and soil body 15.

[0105] A combined construction method of an upper foundation pit and a lower tunnel in a saturated soft loess stratum comprises the following steps:

[0106] Step 1: Prepare for construction

[0107] 1.3. Conduct site survey and leveling and select appropriate construction machinery and tools;

[0108] 1.4. Before the construction of the high-pressure rotary jet pile water-stop curtain, process test piles shall be carried out, with the number of test piles being no less than three groups, with four piles in each group.

[0109] Step 2: Carry out cast-in-place pile construction

[0110] 2.1. Measure and stake out pile positions according to the pile position plan;

[0111] 2.2. Make the casing and weld a stiffening rib at the upper and lower ends and the outer side of the middle part of the casing;

[0112] 2.3. Use a special drill to dig out the upper soil layer of the predetermined pile hole and place the casing;

[0113] 2.4. Drilling rig in place;

[0114] 2.4.1. Slowly place the drill bit into the casing;

[0115] 2.4.2. Start the winch to lift the drill plate and place wooden blocks under the drill plate base;

[0116] 2.4.3. Align the drill rig with the hole and install the drill disc after completion;

[0117] 2.5. Drill holes inside the casing using a rotary drilling rig, with an inner diameter of 1100 to 1200 mm;

[0118] 2.6. Clean the hole and place the steel cage;

[0119] 2.7. Use the underwater concrete pouring method inside the pile hole using the vertical pipe method, with the concrete surface rising at a speed of 2 to 2.2 m / min;

[0120] 2.8. Clean up the site and complete the support pile construction;

[0121] Step 3: Waterstop curtain construction

[0122] 3.1. Drilling rig in place;

[0123] 3.1.1. Drill and lower the pipe to the predetermined position;

[0124] 3.1.2. After the grouting pipe reaches the predetermined depth, a clean water pressure test is carried out. During the pressure test, the pressure gradually increases to reduce friction resistance and prevent nozzle clogging;

[0125] 3.1.3. After the pressure test is completed, start mixing the cement slurry with a water-cement ratio of 1:1 and a cement dosage of 160-200 kg / m;

[0126] 3.1.4. Carry out high-pressure grouting operation, with the grouting pressure set at 30-35 MPa;

[0127] 3.1.5, High pressure shotcrete is applied from bottom to top, the nozzle lifting speed is not more than 15 cm / min, and the re-spraying is 1-2 times;

[0128] 3.1.6, After spraying from bottom to top to the designed height, the shotcrete nozzle is pulled out in time and the shotcreting is ended;

[0129] 3.1.7, After the shotcreting is ended, the high pressure pump, the shotcrete pipeline, the nozzle and the shotcrete head are cleaned;

[0130] Step four, pit outside dewatering

[0131] 4.1, According to the well site layout, the well point is measured and placed, and the enclosure is constructed;

[0132] 4.2, The mouth protection pipe is buried, the pipe is inserted into the original soil layer, the outside of the pipe is filled with cohesive soil to prevent back slurry, and the upper part of the mouth protection pipe is 0.1-0.3 m above the ground;

[0133] 4.3, The drilling machine is installed, the drilling machine platform should be installed stably and horizontally, and is aligned with the hole center;

[0134] 4.4, The hole is drilled, the hole diameter is 800 mm, after drilling to the predetermined depth, 0.3-0.5 m is drilled more, the drilling machine is ensured to be horizontal during the drilling process, the verticality of the hole is ensured, and the mud density is controlled to be 1.10-1.15;

[0135] 4.5, When the hole is drilled to the top plate of the water-bearing layer, the slurry is adjusted by adding clean water, before the drilling rod is lifted to 0.5 m from the hole bottom for punching, and the mud density is gradually adjusted to 1.05;

[0136] 4.6, When the well pipe is placed, the aligner should be installed to ensure that the filter pipe can be centered, the well pipe welding should be firm, vertical and water-tight, after being placed to the predetermined depth, the well mouth is fixed and centered;

[0137] 4.7, Before filling the sand and gravel, the depth inside and outside the well pipe should be measured by the measuring rope, the difference should not exceed the length of the sedimentation pipe, the filling process should not be terminated in the middle, until the predetermined position is reached, and the amount of the material should not be less than 95% of the designed amount;

[0138] 4.8, The well hole is closed by using cohesive soil, the cohesive soil is crushed before being filled, and the filling is carried out slowly along the well pipe;

[0139] 4.9, After the well is formed, the deep well submersible pump is immediately lowered to the bottom of the well, and the drainage pipeline is laid, after the pumping and drainage system is installed, the water pumping is started;

[0140] Step five, foundation pit 17 excavation, i.e. graded slope excavation and setting excavation platform

[0141] 5.1, The obstacles in the excavation area are removed and the measurement and line laying are carried out;

[0142] 5.2. Use large-scale slope cutting from top to bottom, with an excavation slope ratio of 1:1 to 1:2, and excavate the top of the slope to the slope platform in two stages;

[0143] 5.3. During the excavation process, check the width of the slope platform. If the width is not enough, trim it in time. Trim every 1m to the designed elevation. After the excavation is completed, perform a unified slope trimming and check the width and elevation.

[0144] 5.4. After checking that everything is correct, drive steel sheet piles into the footing of the slope platform as support;

[0145] 5.5. Remove the pile heads, insert the lower part of the steel frame into the piles, and weld it to the main reinforcement of the piles. After completion, pour concrete.

[0146] 5.6. Excavate the slope platform to the bottom of the slope. Check the width of the slope bottom during the excavation process. If the width is not enough, trim it in time. Trim every 1m to the designed elevation. After the excavation is completed, perform a unified slope trimming and check the width and elevation.

[0147] Step 6: Reinforce the bottom of the foundation pit 17 with grids

[0148] 6.1. After cleaning the debris on the working surface, level the site with a height difference of no more than 5cm;

[0149] 6.2. When laying geogrid, it should be laid flat and straight;

[0150] 6.3. Use overlap to connect two adjacent geogrids with an overlap width of not less than 10cm and fix them with "U" shaped nails. Set joints in the non-main force direction and ensure that the binding spacing is within 1m.

[0151] 6.4. After the geogrid is laid, spread the crushed stone cushion layer, with an interval of no more than 48 hours, and repeatedly roll it to ensure compaction. After compaction, level it and the height difference should not exceed 5cm;

[0152] Step 7: Dewatering the pit

[0153] 7.1. Measure and lay out the well points and construct enclosures according to the well location plan;

[0154] 7.2. Bury the mouth guard pipe, insert the mouth guard pipe into the original soil layer, use clay soil to compact the outside of the pipe to prevent slurry from flowing outside the pipe, and the upper part of the mouth guard pipe should be 0.1 to 0.3 meters above the ground;

[0155] 7.3. When installing the drilling rig, the drilling rig platform should be installed firmly and levelly, and aligned with the center of the hole;

[0156] 7.4. Drilling: The hole diameter is 800mm. After drilling to the predetermined depth, drill 0.3-0.5m more. During the drilling process, ensure that the drilling rig is level, ensure the verticality of the borehole, and control the mud density between 1.10 and 1.15;

[0157] 7.5. When drilling to the top of the aquifer, add clean water to mix the slurry. Before drilling to the designed elevation, lift the drill rod to 0.5m from the bottom of the hole to punch the hole, and gradually adjust the slurry density to 1.05.

[0158] 7.6. When lowering the well pipe, a aligner should be installed to ensure that the filter pipe is centered. The well pipe welding should be firm, vertical, and watertight. After lowering to the predetermined depth, the wellhead should be fixed in the center.

[0159] 7.7 Before filling with gravel, the depth inside and outside the well pipe should be measured with a measuring rope. The difference should not exceed the length of the sedimentation pipe. The filling process should not be stopped midway until it reaches the predetermined position. The material input should not be less than 95% of the designed amount.

[0160] 7.8. Use clay to seal the wellbore. Crush the clay before filling it in. Slow down and seal it around the wellbore.

[0161] 7.9. After the well is completed, immediately lower the deep well submersible pump to the bottom of the well and lay the drainage pipe. Once the pumping and drainage systems are installed, start pumping water;

[0162] Step 8: Construction of vertical shaft and excavation of horizontal passage in foundation pit 17

[0163] 8.1. Measure and lay out the shaft points according to the plan layout and construct enclosures;

[0164] 8.2. Use short-section excavation and masonry to construct the lock section. Temporary support is provided by driving anchor rods, setting up steel mesh, and C25 shotcrete to form a "sprayed anchor mesh" support. The steel mesh is overlapped by 20 cm, and the steel bars are composed of Φ6 to Φ10 steel bars. The mesh size is 20×20 cm to 25×25 cm. The nozzle is kept perpendicular to the spray surface, and the spray pressure is maintained at 0.12 to 0.15 MPa.

[0165] 8.3. After completion, tie the steel bars, support the formwork, and pour the concrete;

[0166] 8.4. After the shotcrete reaches the required strength, install temporary horizontal and diagonal supports. Set horizontal steel grids at intervals between the diagonal supports.

[0167] 8.5. Repeat the above steps to excavate to the predetermined elevation, spray concrete first, and after final setting, lay steel mesh and set up grid steel frame. The grid spacing inside the well bottom is 1000mm, and the grids are connected by welding. Finally, spray concrete to seal the bottom.

[0168] 8.6. After the vertical shaft construction is completed, the construction of the intersection of the horizontal channel and the vertical shaft is carried out. The shaft wall at the connecting channel is broken, and glass fiber anchor rods are driven beyond the excavation outline for advanced support. The excavated section is sprayed with initial concrete. After completion, the steel mesh is laid, the grid steel frame is erected, and the grid is welded to the steel mesh. After the welding is completed, the steel mesh and grid steel frame are sprayed with concrete again.

[0169] 8.7. Repeat the above steps until the cross passage is connected to the tunnel main line;

[0170] Step 9: Excavate the tunnel mainline

[0171] 9.1. Excavate the upper step 16 of the transverse passage 11 and turn into the first right pilot pit ② and the second right pilot pit ④ of the tunnel section 10 on the right side of the main line;

[0172] 9.2. Carry out initial support construction immediately after excavation is completed;

[0173] 9.3. Drive glass fiber anchor rods beyond the excavation contour surface, and spray concrete on the excavated section. Set up the grid steel frame and steel mesh and weld them together. After completion, spray concrete again to form a "sprayed anchor mesh" support.

[0174] 9.4. Excavate the first right pilot pit ② and the second right pilot pit ④ of tunnel section 10 to the designed depth and simultaneously implement initial support;

[0175] 9.5. Excavate the first core rock mass ⑦ and the second core rock mass ⑧ in the middle of the tunnel section 10 to the left to form a connecting passage. At the same time, use glass fiber anchors to advance support the small mileage direction of the right pilot pit and excavate.

[0176] 9.6. After excavation is completed, repeat step 9.3 to complete the initial support;

[0177] 9.7. Use fiberglass anchors to advance support the left pilot pit of tunnel section 10 on the left side of the connecting passage. Parts of the first left pilot pit ① and the second left pilot pit ③ of tunnel section 10 are excavated simultaneously.

[0178] 9.8. Use fiberglass anchors to pre-support the right pilot pit of tunnel section 10 on the right side of the connecting passage. The first right pilot pit ② and the second right pilot pit ④ are excavated.

[0179] 9.9. Carry out initial support construction immediately after excavation is completed;

[0180] 9.10. Simultaneously excavate the lower steps of the left and right pilot pits and apply initial support, following the same excavation sequence;

[0181] 9.11. After the excavation of both the left and right pilot tunnels is completed, fiberglass anchor bolts are driven from the connecting passage toward the tunnel's larger and smaller sections. Fiberglass anchor bolts are driven beyond the excavation contour surface. Initial concrete spraying is then carried out on the excavated section. Grid steel frames and steel mesh are then installed and welded together. After completion, concrete is sprayed again to form a "sprayed anchor mesh" support.

[0182] Step 10: Backfill the foundation pit 17.

[0183] The present invention is well adapted to the conditions of a foundation pit and tunnel on saturated soft loess. The upper foundation pit 17 combines a water-stop curtain with drainage wells inside and outside the pit. The water-stop curtain is composed of jet grouting piles, which are arranged in two rows, with one row outside the support piles and one row between the support piles. Adjacent jet grouting piles are interlocked, which can better intercept groundwater. At the same time, combined with the drainage wells inside and outside the pit, groundwater can be effectively controlled. The foundation pit 17 is excavated in stages, and support reinforcement is carried out after each stage of excavation. The bottom of the foundation pit 17 is reinforced with geogrids to cope with base uplift and water and sand gushing, achieving good reinforcement and drainage effects. The tunnel main line is transferred through the vertical shaft, and the core rock and soil are retained. The pilot pits on both sides are excavated first, and glass fiber anchor rods are used for advance support. After the excavation and support on both sides are completed, the core rock and soil are excavated and supported. This can effectively control the deformation of the tunnel section 10, reduce the impact on the foundation pit 17, and thus ensure construction safety and reduce the mutual impact between the foundation pit 17 and the tunnel.

Claims

1. A combined construction structure of an upper foundation pit and a lower tunnel in a saturated soft loess layer, comprising a foundation pit (17), characterized in that: The bottom of the foundation pit (17) is connected to the tunnel (11) through a vertical shaft (7), and a geogrid (6) is provided above the bottom of the foundation pit (17); the outside of the foundation pit (17) is provided with steel sheet piles (3), a dewatering well (5) in the pit, cast-in-place piles (2), a jet-jet pile water-stop curtain (1) and a dewatering well (4) outside the pit; the inside of the foundation pit (17) is a slope structure, and the slope structure is arranged according to the depth into section I slope, section II slope, section I slope and section II slope. A slope reduction platform is provided between the slope of section II and the slope of section I; the steel sheet piles (3) and the dewatering wells (5) in the pit are provided on the slope reduction platform between the slope of section II and the slope of section I; the cast-in-place piles (2), the jet-jet pile water-stop curtain (1) and the dewatering wells (4) outside the pit are provided at the beginning of the slope of section I; an initial support (8) is provided above the bottom of the tunnel (11) along the circumference of the tunnel (11), and a transverse passage (9) is distributed between the initial support (8) and the bottom of the tunnel (11).

2. The construction method of the upper foundation pit-lower tunnel combined construction structure in saturated soft loess strata according to claim 1 is characterized in that it specifically comprises the following steps: Step 1: Prepare for construction Conduct site survey and leveling and select appropriate construction machinery and tools; Before the construction of the high-pressure jet grouting water-stop curtain, a process test pile should be carried out, with the number of test piles not less than three groups, with four piles in each group; Step 2: Construction of cast-in-place piles (2) 2.

1. Measure and stake out pile positions according to the pile position plan; 2.

2. Make the casing and set stiffening ribs at the upper and lower ends and the outer side of the middle part of the casing; 2.

3. Use the drill bucket to dig out the upper soil layer of the predetermined pile hole and place the casing; 2.

4. Drilling rig in place; 2.4.

1. Slowly place the drill bit into the casing; 2.4.

2. Lift the drill head and place wooden blocks under the drill head base; 2.4.

3. Align the drill rig with the hole and install the drill disc; 2.

5. Drill holes inside the casing with a diameter of 1100 to 1200 mm; 2.

6. Clean the hole and place the steel cage; 2.

7. Underwater concrete inside the pile hole; 2.

8. Clean up the site and complete the support pile construction; Step 3: Waterstop curtain construction 3.

1. Drilling rig in place; 3.1.

1. Drill and lower the pipe to the predetermined position; 3.1.

2. After the grouting pipe reaches the predetermined depth, a clean water pressure test is carried out. During the pressure test, the pressure gradually increases to reduce friction resistance and prevent the grouting pipe nozzle from being blocked; 3.1.

3. After the pressure test is completed, start mixing the cement slurry with equal water-cement ratio; 3.1.

4. Carry out high-pressure grouting operation, with the grouting pressure set at 30~35MPa; 3.1.

5. High-pressure grouting is carried out from bottom to top, with the nozzle lifting speed not exceeding 15 cm / min, and repeated 1 to 2 times; 3.1.

6. After spraying from bottom to top to the designed height, pull out the spraying pipe in time and finish spraying; 3.1.7 After the spraying is completed, clean the high-pressure pump, slurry delivery pipeline, grouting pipe and nozzle; Step 4: Conduct precipitation outside the pit; 4.

1. Conduct well point survey and layout according to the well location plan, i.e. measure and lay out the lines, and construct enclosures; 4.

2. Bury the mouth guard pipe, insert the mouth guard pipe into the original soil layer, use clay soil to compact the outside of the pipe to prevent slurry from flowing out of the pipe, and the upper part of the mouth guard pipe should be 0.1~0.3m above the ground; 4.

3. Install the drilling rig. The drilling rig platform should be installed firmly and levelly, and aligned with the center of the hole; 4.

4. After drilling to the predetermined depth, drill an additional 0.3-0.5m. During the drilling process, ensure that the drilling rig is level, ensure the verticality of the borehole, and control the mud density between 1.10 and 1.15; 4.

5. When drilling to the top of the aquifer, add clean water to mix the slurry and drill to the designed elevation. Before lifting the drill, lift the drill rod to 0.5 to 1.0 m from the bottom of the hole to punch the hole; 4.

6. When lowering the well pipe, a aligner should be installed to ensure that the filter pipe is centered. The well pipe welding should be firm, vertical, and watertight. After lowering to the predetermined depth, the wellhead should be fixed in the center. 4.

7. When filling gravel, the filling process shall not be stopped midway until it reaches the predetermined position, and the material input shall not be less than 95% of the designed amount; 4.

8. Use clay to seal the wellbore. Crush the clay before filling it in, and seal it around the wellbore. 4.

9. After the well is completed, immediately lower the deep well submersible pump to the bottom of the well and lay the drainage pipe. Once the pumping and drainage systems are installed, start pumping water; Step 5: Excavation of foundation pit (17) 5.

1. Clear obstacles in the excavation area and conduct surveying and setting out; 5.

2. Use large-scale slope cutting from top to bottom, with an excavation slope ratio of 1:1 to 1:2, and excavate the top of the slope to the slope platform in stages; 5.

3. During the excavation process, check the width of the slope platform. If the width is not enough, trim it in time. Trim the edge every 1 to 2 meters to the designed elevation. After the excavation is completed, trim the slope and check the width and elevation. 5.

4. After checking that everything is correct, drive steel sheet piles (3) into the footing of the slope platform as support; 5.

5. Remove the pile head of the bored pile (2), insert the lower part of the steel frame into the bored pile (2), and weld it to the main reinforcement of the bored pile (2). After completion, pour concrete; 5.

6. Excavate the slope platform to the bottom of the slope. Check the width of the slope bottom during the excavation process. If the width is not enough, trim it in time and trim it to the elevation. After the excavation is completed, perform a unified slope trimming and check the width and elevation. Step 6: Reinforce the bottom of the foundation pit (17) with grids 6.

1. After cleaning the debris on the working surface, level the site with a height difference of no more than 5cm; 6.

2. When laying geogrid, it should be laid flat and straight; 6.

3. Use overlap to connect two adjacent geogrids with an overlap width of not less than 10cm and fix them; 6.

4. After the geogrid is laid, spread the crushed stone cushion layer, with an interval of no more than 48 hours, and repeatedly roll it to ensure compaction. After compaction, level it and the height difference should not exceed 5cm; Step 7: Dewatering the pit 7.

1. Measure and lay out the well points and construct enclosures according to the well location plan; 7.

2. Bury the mouth guard pipe, insert the mouth guard pipe into the original soil layer, use clay soil to compact the outside of the pipe to prevent slurry from flowing out of the pipe, and the upper part of the mouth guard pipe should be 0.1~0.3m above the ground; 7.

3. When installing the drilling rig, the drilling platform should be installed firmly and levelly, and aligned with the center of the hole; 7.

4. After drilling to the predetermined depth, drill an additional 0.3~0.5m. During the drilling process, ensure that the drilling rig is level and the verticality of the drill hole is guaranteed; 7.

5. When drilling to the top of the aquifer, add clean water to mix the slurry and drill to the designed elevation. Before lifting the drill, lift the drill rod to 0.5 to 1.00 m from the bottom of the hole to punch the hole; 7.

6. When lowering the well pipe, a aligner should be installed to ensure that the filter pipe is centered. The well pipe welding should be firm, vertical, and watertight. After lowering to the predetermined depth, the wellhead should be fixed in the center. 7.

7. When filling gravel, the filling process shall not be stopped midway until it reaches the predetermined position, and the material input shall not be less than 95% of the designed amount; 7.

8. Use clay to seal the wellbore. Crush the clay before filling it in. Slow down and seal it around the wellbore. 7.

9. After the well is completed, immediately lower the deep well submersible pump to the bottom of the well and lay the drainage pipe. Once the pumping and drainage systems are installed, start pumping water; Step 8: Construction of the vertical shaft (7) and excavation of the horizontal passage (9) in the foundation pit (17) 8.

1. Measure and lay out the vertical shaft (7) points according to the plan layout and construct enclosures; 8.

2. Temporary support is provided by driving anchor rods, setting up steel mesh, and spraying concrete to form a "sprayed anchor mesh" support. The steel bars are composed of Φ6~Φ10mm steel bars, and the mesh size is 20×20cm~25×25cm. The nozzle is kept perpendicular to the spraying surface, and the spraying pressure is maintained at 0.12~0.15MPa; 8.

3. After completion, tie the steel bars, support the formwork, and pour the concrete; 8.

4. After the shotcrete reaches the required strength, temporary horizontal and angled supports shall be installed. The angled supports shall be arranged at intervals of horizontal steel grids, and the angles between the angled supports and the shaft wall shall be 45-60 degrees. 8.

5. Repeat the above steps to excavate to the predetermined elevation, spray concrete 25mm-30mm first, and after final setting, lay steel mesh and set up grid steel frame. The grid spacing inside the well bottom is 800 to 1000mm, and the grids are connected by welding. Finally, spray concrete to seal the bottom. 8.

6. After the construction of the vertical shaft (7) is completed, the construction of the intersection of the horizontal channel (9) and the vertical shaft (7) is carried out, the shaft wall at the connecting channel is broken, and glass fiber anchor rods are driven into the area beyond the excavation outline for advanced support. The excavated section is sprayed with initial concrete, and after completion, the steel mesh is laid, the grid steel frame is erected, and the grid is welded to the steel mesh. After the welding is completed, the steel mesh and the grid steel frame are sprayed with concrete again; 8.

7. Repeat the above steps until the connection between the cross passage (9) and the tunnel (11) is completed; Step 9: Excavation of the tunnel (11) 9.

1. Excavate the upper step (16) of the transverse passage (9) and turn into the first right pilot pit and the second right pilot pit of the tunnel section (10) on the right side of the main line; 9.

2. Immediately after excavation is completed, initial support (8) construction shall be carried out; 9.

3. Drive glass fiber anchor bolts beyond the excavation contour surface, and spray concrete on the excavated section. Set up the grid steel frame and steel mesh and weld them together. After completion, spray concrete again to form the "sprayed anchor mesh" support. 9.

4. Continue excavating the first right pilot tunnel and the second right pilot tunnel of the tunnel section (10) to the designed advance and simultaneously implement the initial support (8); 9.

5. At the designed advance, excavate the first core rock mass and the second core rock mass in the middle part to the left of the tunnel section (10) to the left pilot pit and form a connecting passage (12). At the same time, use glass fiber anchors to advance support the right pilot pit in the direction of small mileage and excavate; 9.

6. After excavation is completed, repeat step 9.3 to complete the initial support (8); 9.

7. Use fiberglass anchors to advance support the left pilot pit (13), the first left pilot pit, and the second left pilot pit of the tunnel section (10) on the left side of the connecting passage (12) and excavate them; 9.

8. Use fiberglass anchors to advance support the right pilot pit (14), the first right pilot pit, the second right pilot pit, and the third right pilot pit of the tunnel section (10) on the right side of the connecting passage (12) and excavate them; 9.

9. Immediately after excavation is completed, initial support (8) construction shall be carried out; 9.

10. Simultaneously excavate the lower steps of the left and right pilot pits and apply initial support (8). The excavation sequence is the same. 9.

11. After the excavation of the left and right pilot pits is completed, fiberglass anchor rods are driven into the left and right directions of the tunnel (11) from the connecting channel (12). The fiberglass anchor rods are driven into the outside of the excavation contour surface. The excavated section is sprayed with initial concrete. The grid steel frame and steel mesh are set up and welded together. After completion, the concrete is sprayed again to form a "sprayed anchor mesh" support. Step 10: Backfill the foundation pit (17).

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