CN112813985A - Shallow gas protection method for foundation pit - Google Patents
Shallow gas protection method for foundation pit Download PDFInfo
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- CN112813985A CN112813985A CN202110003890.1A CN202110003890A CN112813985A CN 112813985 A CN112813985 A CN 112813985A CN 202110003890 A CN202110003890 A CN 202110003890A CN 112813985 A CN112813985 A CN 112813985A
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- foundation pit
- wall
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- shallow
- grouting pipe
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- E—FIXED CONSTRUCTIONS
- E02—HYDRAULIC ENGINEERING; FOUNDATIONS; SOIL SHIFTING
- E02D—FOUNDATIONS; EXCAVATIONS; EMBANKMENTS; UNDERGROUND OR UNDERWATER STRUCTURES
- E02D17/00—Excavations; Bordering of excavations; Making embankments
- E02D17/02—Foundation pits
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- E—FIXED CONSTRUCTIONS
- E02—HYDRAULIC ENGINEERING; FOUNDATIONS; SOIL SHIFTING
- E02D—FOUNDATIONS; EXCAVATIONS; EMBANKMENTS; UNDERGROUND OR UNDERWATER STRUCTURES
- E02D17/00—Excavations; Bordering of excavations; Making embankments
- E02D17/02—Foundation pits
- E02D17/04—Bordering surfacing or stiffening the sides of foundation pits
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- E—FIXED CONSTRUCTIONS
- E02—HYDRAULIC ENGINEERING; FOUNDATIONS; SOIL SHIFTING
- E02D—FOUNDATIONS; EXCAVATIONS; EMBANKMENTS; UNDERGROUND OR UNDERWATER STRUCTURES
- E02D19/00—Keeping dry foundation sites or other areas in the ground
- E02D19/06—Restraining of underground water
- E02D19/10—Restraining of underground water by lowering level of ground water
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- E—FIXED CONSTRUCTIONS
- E02—HYDRAULIC ENGINEERING; FOUNDATIONS; SOIL SHIFTING
- E02D—FOUNDATIONS; EXCAVATIONS; EMBANKMENTS; UNDERGROUND OR UNDERWATER STRUCTURES
- E02D29/00—Independent underground or underwater structures; Retaining walls
- E02D29/16—Arrangement or construction of joints in foundation structures
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- E—FIXED CONSTRUCTIONS
- E02—HYDRAULIC ENGINEERING; FOUNDATIONS; SOIL SHIFTING
- E02D—FOUNDATIONS; EXCAVATIONS; EMBANKMENTS; UNDERGROUND OR UNDERWATER STRUCTURES
- E02D31/00—Protective arrangements for foundations or foundation structures; Ground foundation measures for protecting the soil or the subsoil water, e.g. preventing or counteracting oil pollution
- E02D31/02—Protective arrangements for foundations or foundation structures; Ground foundation measures for protecting the soil or the subsoil water, e.g. preventing or counteracting oil pollution against ground humidity or ground water
Abstract
The invention provides a shallow gas protection method for a foundation pit. The method comprises the following steps: arranging a temporary precipitation well in an area where a groove for accommodating the underground diaphragm wall is located, and mounting a precipitation well pipe in the temporary precipitation well; excavating soil bodies and the dewatering well casing pipes in the area where the grooves are located to form grooves for containing underground continuous walls; constructing in the groove to form an underground continuous wall, wherein the area formed by enclosing the underground continuous wall is the area where the foundation pit to be excavated is located; reinforcing the bottom of the foundation pit to prevent residual shallow layer gas in the shallow gas layer from entering the foundation pit; and excavating soil contained in the area where the foundation pit is located to form the foundation pit. Most of shallow gas is released through the temporary dewatering well, so that the influence of the shallow gas on the quality of the underground diaphragm wall is reduced; the bottom of the foundation pit is reinforced, so that methane entering the foundation pit is reduced, and the potential safety hazards of fire and confined water inrush in the foundation pit are eliminated.
Description
Technical Field
The invention relates to the technical field of foundation pit engineering construction, in particular to a shallow gas protection method for a foundation pit.
Background
With the rapid development of urban infrastructure construction, more and more traffic major roads and important pipelines are developing underground. There are various methods for underground construction, among which the shield method and the pipe-jacking method have the advantages of little influence on the ground and little environmental restriction, and are widely used. The shield method and the pipe jacking method usually need to construct small deep foundation pits which are usually open with the opening area smaller than 1000m and are used as construction structures such as an originating working well, a receiving working well or an intermediate air shaft2And excavating a foundation pit with the depth larger than 15 meters.
In the soft soil area near the coast in China, seabed shallow gas is provided, the shallow gas mainly belongs to a biological cause, namely in multiple sea-entering and sea-exiting processes, organic matters in a sludge layer are decomposed and fermented under the biochemical action of anaerobic bacteria, gas is generated along with certain temperature and pressure, and when the generated gas cannot be diffused into the atmosphere, the gas is moved and accumulated into the pores of the surrounding stratum from time to time and is stored in the surrounding stratum, so that a shallow gas reservoir is formed. The main component of shallow gas is methane, in soft soil areas, particularly clay areas, the shallow gas distribution is mainly in a lump shape or a bag shape and has the characteristics of moderate air pressure, weak connectivity, slow supply, long duration and the like, and the shallow gas is difficult to completely release through a limited number of shallow gas exploration holes.
When a deep foundation pit is dug in an area where shallow gas is located, in the existing deep foundation pit construction technology, a large deep foundation pit has enough emergency disposal conditions due to large open area, and the harm of the shallow gas can be ignored. However, for a small deep foundation pit, a shallow gas-soil layer may be located in an excavation layer of the deep foundation pit or a pit bottom lying layer, and if a conventional foundation pit construction method is adopted, the following problems may exist:
(1) when the underground continuous wall is constructed in a groove forming mode, the groove section penetrates through the shallow gas layer to trigger the release of shallow gas pressure relief in the shallow gas layer, so that the groove wall collapses, and the potential safety hazard of the quality of the underground continuous wall is formed. In the concrete pouring process of the underground diaphragm wall, shallow air is slowly released to cause that underwater concrete cannot be solidified, and weak links such as wall joint of the underground diaphragm wall cause a communication channel, so that safety risks such as leakage of an underground enclosure wall in the process of excavation of a foundation pit are caused.
(2) When small-size deep basal pit excavated to the bottom, the soil body of foundation ditch bottom is owing to lose original state soil load, and the shallow layer gas that the bed contains easily leaks into the foundation ditch through weak passageways such as underground continuous wall and soil body border, pile foundation and soil body border, soil body self, because small-size deep basal pit's open area is little and the degree of depth is dark, and air circulation is not smooth, easily ignites marsh gas when carrying out fire-moving operations such as reinforcement in the foundation ditch, causes safety hazard.
(3) When the foundation pit is excavated to the bottom, shallow gas of a horizontal layer at the bottom of the pit leaks, the lower part of the shallow gas layer is often provided with a pressure-bearing water layer, the shallow gas leaks to create a channel for pressure-bearing water inrush, and the pressure-bearing water inrush can cause irreparable damage to the small-sized deep foundation pit in a short time.
Disclosure of Invention
The invention provides a shallow gas protection method for a foundation pit, which aims to solve the technical problems of unqualified underground continuous wall quality, fire ignition in the foundation pit and confined water inrush caused by shallow gas.
In order to solve the technical problem, the invention provides a shallow gas protection method for a foundation pit, which comprises the following steps:
step S1, arranging a temporary dewatering well in an area where a groove for containing the underground diaphragm wall is located, and installing a dewatering well pipe in the temporary dewatering well, wherein the upper end of the temporary dewatering well is communicated with the atmosphere, the lower end of the temporary dewatering well is communicated with the shallow gas layer, and the dewatering well pipe is used for extracting water in the shallow gas layer and releasing shallow gas in the shallow gas layer;
step S2, excavating soil bodies and the dewatering well pipe in the area where the groove is located to form a groove for containing an underground continuous wall;
s3, constructing in the groove to form an underground continuous wall, wherein the area formed by enclosing the underground continuous wall is the area where the foundation pit to be excavated is located;
s4, reinforcing the bottom of the foundation pit to prevent residual shallow layer gas in the shallow gas layer from entering the foundation pit;
and S5, excavating soil bodies contained in the area where the foundation pit is located to form the foundation pit.
Optionally, before the step S1, the method further includes the following steps: the method comprises the steps that a guide wall is constructed at an opening position of an area where a groove for containing the underground continuous wall is located, the guide wall is arranged on soil bodies on two sides of the opening position of the groove, and the guide wall is used for reinforcing the opening of the groove.
Optionally, the step S5 is followed by the following steps: detecting the content of shallow layer gas in the foundation pit by using a detection instrument, and if the content of the shallow layer gas is less than a preset threshold value, indicating that the foundation pit meets the safety requirement; and if the content of the shallow layer gas is greater than or equal to the preset threshold value, checking whether a gap with gas leakage exists between the underground continuous wall and the bottom of the foundation pit or not, and plugging the gap.
Optionally, the underground continuous wall is composed of a plurality of unit walls, and a wall gap is formed between two adjacent unit walls; the following steps are also included between the step S3 and the step S4:
and step S34, sealing and reinforcing the wall joints of the underground continuous wall after the underground continuous wall reaches the maintenance age.
Optionally, the dewatering well casing is a hard water pipe made of plastic, and the step S2 specifically includes: and (3) damaging and grabbing the hard water pipe and the soil body in the groove along the direction guided by the guide wall by using a grooving machine.
Optionally, the side wall of the dewatering well pipe is uniformly provided with a plurality of through holes for releasing the shallow gas, and the outer side of the dewatering well pipe is wrapped with a layer of nylon net for preventing blockage.
Optionally, the step S3 specifically includes the following steps:
s31, manufacturing a steel structure of the unit wall, wherein the steel structure is a cuboid and comprises a first plane and a second plane, and the first plane and the second plane are planes where the wall seams are located;
step S32, arranging a pattern hole water-stop grouting pipe on the first plane and the second plane respectively, wherein a plurality of pattern holes for grouting are uniformly formed in the side wall of the pattern hole water-stop grouting pipe, a film for preventing blocking is wrapped on the outer side of the pattern hole water-stop grouting pipe, the pattern hole water-stop grouting pipe is vertically arranged downwards, the upper end of the pattern hole water-stop grouting pipe is communicated with the atmosphere, and the lower end of the pattern hole water-stop grouting pipe is communicated with the shallow air layer;
step S33, placing the steel structure into the groove and pouring concrete to form the unit wall;
step S34, repeating the steps S31 to S33 until the underground continuous wall is formed;
the step S5 specifically includes: in the process of excavation the foundation ditch, when the wall seam appears the seepage, cut through the seepage position through the high pressure water the line hole stagnant water slip casting pipe, toward cut through after the line hole stagnant water slip casting pipe pours into the thick liquid that is used for shutoff seepage position.
Optionally, the steel structure is provided with cross steel plate joints on the first plane and the second plane respectively; the cross steel plate joint comprises a first steel plate and a second steel plate, the first steel plate is welded on the first plane, the second steel plate is welded on the first steel plate, and the second steel plate vertically and equally divides the first steel plate along the vertical direction; the patterned hole water-stop grouting pipe comprises an excavation face patterned hole water-stop grouting pipe and an earth-facing patterned hole water-stop grouting pipe, the excavation face patterned hole water-stop grouting pipe is arranged on an excavation face at the welding position of the first steel plate and the second steel plate, the earth-facing patterned hole water-stop grouting pipe is arranged on the earth-facing surface at the welding position, and a connecting line between the excavation face patterned hole water-stop grouting pipe and the earth-facing patterned hole water-stop grouting pipe is parallel to the width direction of the groove; the cross steel plate joint on the second plane and the cross steel plate joint on the first plane are symmetrically arranged relative to the steel structure, and the pattern hole water-stopping grouting pipe on the second plane and the pattern hole water-stopping grouting pipe on the first plane are symmetrically arranged relative to the steel structure;
the step S5 specifically includes: in the process of excavating the foundation pit, when the wall joint leaks, the water-stopping grouting pipe of the pattern hole of the excavation surface at the leakage position is split through high-pressure water, and then slurry for blocking the leakage position is injected into the split water-stopping grouting pipe of the pattern hole of the excavation surface; and if the leakage position is blocked, ending the blocking process, and if the leakage position is not blocked, splitting the water stop grouting pipe through the soil facing patterned hole at the leakage position by high-pressure water, and then injecting slurry for blocking the leakage position into the split water stop grouting pipe through the soil facing patterned hole.
Optionally, the step S34 specifically includes:
and sealing and reinforcing the soil facing surface of each wall gap by utilizing an omnibearing high-pressure jet construction method, wherein the reinforcement adopts swing jet, the jet direction is aligned to the wall gap, and the reinforcement depth is from the top of the underground continuous wall to the bottom of the shallow air layer.
Optionally, the step S4 specifically includes: and carrying out omnibearing reinforcement on the bottom of the foundation pit by utilizing an omnibearing high-pressure injection construction method to form a full pit bottom reinforcing layer.
According to the shallow gas protection method for the foundation pit, provided by the invention, most of shallow gas is released through the temporary dewatering well, so that the influence of the release of the shallow gas on the quality of the underground continuous wall in the construction process of the underground continuous wall is reduced; by reinforcing the bottom of the foundation pit, methane entering the foundation pit is reduced, and potential safety hazards of fire and confined water inrush in the foundation pit are eliminated; the sealing performance of the wall joint 7 is enhanced by carrying out MJS (Metro Jet System, all-round high-pressure Jet method) reinforcement on the wall joint 7; the water-stopping grouting pipe is pre-buried in the pattern holes, so that a good emergency plugging effect is achieved when the wall gap 7 leaks; the sealing performance of the foundation pit bottom 18 is enhanced by performing MJS reinforcement on the foundation pit bottom 18.
Drawings
Fig. 1 is a flowchart of a shallow gas protection method for a foundation pit according to an embodiment of the present invention;
FIG. 2 is a schematic elevation view of a temporary precipitation well in a trough section of an underground diaphragm wall according to an embodiment of the present invention;
FIG. 3 is a top view corresponding to FIG. 2;
FIG. 4 is a schematic top view of a plurality of temporary dewatering wells disposed in a trough section of an underground diaphragm wall according to an embodiment of the present invention;
FIG. 5 is a schematic diagram of a steel structure of an underground diaphragm wall and a distribution diagram of a water stop grouting pipe with a pattern hole according to an embodiment of the invention;
fig. 6 is a schematic structural diagram of a water stop grouting pipe with a pattern hole according to an embodiment of the present invention;
FIG. 7 is a schematic structural view of a semi-circular MJS reinforcing structure for the soil-facing surface of the wall joint of the underground diaphragm wall according to an embodiment of the present invention;
fig. 8a to 8h are schematic structural diagrams corresponding to a construction flow of an MJS construction method pile-forming provided in an embodiment of the present invention;
fig. 9 is a schematic plan view of a full MJS staggered lap joint reinforcement for the bottom of a foundation pit according to an embodiment of the present invention;
fig. 10 is a schematic cross-sectional view of soil layer distribution and foundation pit methane protection reinforcement provided by an embodiment of the invention.
[ reference numerals are described below ]:
1-a guide wall; 2-groove; 3, a temporary dewatering well; 4-dewatering well casing, plum blossom hole-41; 5-nylon mesh; 6-medium coarse sand; 7-wall joint; 8-steel structure, 81-cross steel plate joint, 811-first steel plate, 822-second steel plate, 82-anti-streaming iron sheet, 83-underground continuous wall distribution rib and 84-underground continuous wall main rib; 9-digging a pattern hole water stop grouting pipe of the surface, and 91-pattern holes; 10-water stop grouting pipe with pattern holes on soil-facing surface; 11-semicircular MJS reinforcing piles; 12-pile foundation; 13-full MJS pile position; 14-pit bottom foundation layer; 15-shallow gas layer; 16-a confined water layer; 17-underground diaphragm wall; 18-the bottom of the foundation pit; 19-full pit bottom reinforcing layer; 20-a fan; 21-bending an air pipe; 22-flame-retardant extension air hose; 23-concrete support; 24-steel support; an outer sleeve-25; a drill rod-26; pile-27.
Detailed Description
In order to make the objects, advantages and features of the present invention clearer, a shallow gas protection method for a foundation pit according to the present invention will be described in further detail with reference to the accompanying drawings. It is to be noted that the drawings are in a very simplified form and are not to precise scale, which is merely for the purpose of facilitating and distinctly claiming the embodiments of the present invention.
The main component of the shallow gas is methane, so the method for protecting the shallow gas of the foundation pit can also be called as a method for protecting the foundation pit from methane. Referring to fig. 1 to 5 and fig. 10, the present embodiment provides a method for protecting a shallow gas in a foundation pit, including the following steps:
step S1, arranging a temporary precipitation well 3 in the area where the groove 2 for accommodating the underground continuous wall 17 is located, and installing a precipitation well pipe 4 in the temporary precipitation well 3, wherein the upper end of the temporary precipitation well 3 is communicated with the atmosphere, the lower end of the temporary precipitation well 3 is communicated with the shallow gas layer, and the precipitation well pipe 4 is used for extracting water in the shallow gas layer and releasing shallow gas in the shallow gas layer. The bottom of the temporary precipitation well pipe 3 and the bottom of the precipitation well pipe 4 can be flush, the top of the precipitation well pipe 3 can be flush with the opening position of the groove 2, namely the top position, and the top of the precipitation well pipe 4 can be higher than the top of the temporary precipitation well pipe 3.
Alternatively, as shown in fig. 4, WN-1, N-2, etc. in fig. 4 indicate the numbers of the sections of the underground diaphragm wall 17, and a temporary precipitation well 3 is provided every 10m in the sections of the underground diaphragm wall 17, i.e., in the groove 2 for receiving the underground diaphragm wall 17 to be constructed. The temporary dewatering well 3 can be constructed by a hole guiding machine with the diameter of 600mm, and the verticality is controlled at 1/300. The dewatering well casing 4 is a hard water pipe made of plastic, for example, a dewatering well casing 4 made of PVC (Polyvinyl chloride) with the diameter of 150mm is arranged in the temporary dewatering well 3, the depth of the temporary dewatering well 3 entering a shallow gas layer is usually not less than 3m, but is usually more than 3m away from a pressure water layer. The side wall of the dewatering well pipe 4 is uniformly provided with a plurality of through holes for releasing the shallow gas, the outer side of the dewatering well pipe 4 is wrapped with a layer of nylon net 5 for preventing blockage, for example, the side wall of the PVC dewatering well pipe 4 is provided with plum blossom holes 41 with uniform spacing of 20cm, the porosity is about 15%, and the outer side is wrapped with a layer of 60-mesh nylon net 5 for preventing the plum blossom holes 41 from being blocked. The temporary dewatering well 3 can be backfilled by medium coarse sand 6, and is washed after backfilling, and then a water pump is put into the dewatering well pipe 4 to pump water. After pumping water, most of shallow gas in the shallow gas layer can be guided to be released in advance through the dewatering well pipe 4, so that a steam pocket with larger pressure or larger methane storage area is released in advance, stratum dewatering is consolidated, and collapse of the groove wall due to the fact that the groove wall penetrates through the shallow gas layer during grooving is avoided. During trenching construction, the PVC precipitation well pipe 4 can be directly crushed and grabbed by the trenching machine, and the influence on trenching caused by residual in the trench can be avoided. The temporary precipitation well 3 is uniformly arranged, so that shallow gas can be released more fully.
In other embodiments, the dewatering well casing 4 may be made of HDPE (High Density Polyethylene) or PPB (Polypropylene Block Copolymer).
And step S2, excavating the soil body contained in the area where the groove 2 is located and the dewatering well casing pipe 4 to form the groove 2 for accommodating the underground continuous wall 17. The grooving of the underground continuous wall is completed, the grooving can be excavated by using a grooving machine, and then the underground continuous wall 17 is formed in the groove 2.
And S3, constructing in the groove 2 to form an underground continuous wall 17, wherein the area formed by enclosing the underground continuous wall 17 is the area where the foundation pit to be excavated is located. The construction can be divided into groove sections by the way of manufacturing the steel structure 8 on site and pouring concrete, and the underground continuous wall 17 can also be formed by the way of hoisting the prefabricated wall body in the groove 2.
And step S4, reinforcing the bottom 18 of the foundation pit to prevent the residual shallow gas in the shallow gas layer from entering the foundation pit. Concrete or asphalt may be poured into the foundation pit bottom 18 to reinforce the foundation pit bottom 18.
And S5, excavating soil bodies contained in the area where the foundation pit is located to form the foundation pit. During excavation of a foundation pit, a ventilation apparatus for supplying air to the inside of the foundation pit is generally provided at an opening position of the foundation pit for safety of a constructor.
During excavation of the foundation pit, an axial flow fan 20 with the diameter of 600mm can be arranged at the opening position of the foundation pit, namely the ground surface position, a 90-degree stainless steel bent air pipe 21 with the diameter of 600mm and the turning radius of 3m is adopted to enable an air port to be opposite to the lower part of the foundation pit, the thickness of each layer of soil is about 3m when the foundation pit is excavated, a section of 3m flame-retardant extension soft air pipe 22 is connected, fresh air is supplied into the foundation pit, and the air supply amount is usually not less than 300 cubic minutes. The fan 20 is typically positioned at an oblique angle to the excavation. The fan 20 is arranged to provide fresh air for workers on the working face and quickly blow away air at the bottom of the pit, so that toxic and harmful gases are difficult to accumulate to form harm. In other embodiments, a fan may be used to supply air into the pit.
According to the method for protecting the shallow layer gas of the foundation pit, most of the shallow layer gas is released through the temporary dewatering well 3, so that the influence of the release of the shallow layer gas on the quality of the underground continuous wall 17 in the construction process of the underground continuous wall 17 is reduced; by reinforcing the bottom 18 of the foundation pit, the methane entering the foundation pit is reduced, and the potential safety hazards of fire and confined water inrush in the foundation pit are eliminated.
Optionally, as shown in fig. 2 and 10, the step S1 further includes the following steps before: the method comprises the steps that a guide wall 1 is constructed and formed at the opening position of the area where a groove 2 for accommodating the underground continuous wall 17 is located, the guide wall 1 is arranged on the soil body on two sides of the opening position of the groove 2, and the guide wall 1 is used for reinforcing the opening of the groove 2. The guide wall 1 is the key for ensuring the accurate position and the grooving quality of the underground continuous wall 17, during construction, the guide wall 1 has the function of bearing loads of a reinforcement cage, a guide pipe for pouring concrete, a drilling machine and the like, and the guide wall 1 is equivalent to a right-angle reinforcing wall arranged at the opening position of the groove 2.
Optionally, the step S5 is followed by the following steps: detecting the content of shallow layer gas in the foundation pit by using a detection instrument, and if the content of the shallow layer gas is less than a preset threshold value, indicating that the foundation pit meets the safety requirement; and if the content of the shallow layer gas is greater than or equal to the preset threshold value, checking whether a gap with gas leakage exists between the underground continuous wall and the bottom of the foundation pit or not, and plugging the gap. The construction manager can adopt a handheld combustible gas detection instrument to perform biogas detection on the underground continuous wall 17 exposed in the foundation pit and the soil body boundary, the pile foundation 12 and the soil body boundary and the pit bottom soil body, wherein the LEL (Lower explosion Limited) of the biogas is 4.95% vol.
Alternatively, as shown in fig. 4 and 10, the underground continuous wall 17 is composed of a plurality of unit walls, and a wall gap 7 is formed between two adjacent unit walls; the following steps are also included between the step S3 and the step S4: and step S34, sealing and reinforcing the wall gap 7 of the underground continuous wall 17 after the underground continuous wall 17 reaches the maintenance age.
The curing age of the underground diaphragm wall 17 is usually about 30 days. And when the underground continuous wall 17 reaches the maintenance age, sealing and reinforcing the wall gap 7 of the underground continuous wall 17, so that shallow air can be prevented from leaking into the foundation pit. Specifically, concrete or glue can be poured into the wall gap 7 to achieve the effect of sealing and reinforcing.
Optionally, as shown in fig. 4 to 6 and 10, the step S3 specifically includes the following steps:
step S31, manufacturing a steel structure 8 of the unit wall, wherein the steel structure 8 is a cuboid, the steel structure 8 comprises a first plane and a second plane, and the first plane and the second plane are both planes where the wall seams 7 are located;
step S32, arranging a pattern hole water-stop grouting pipe on the first plane and the second plane respectively, wherein a plurality of pattern holes 91 for grouting are uniformly formed in the side wall of the pattern hole water-stop grouting pipe, a film for preventing blockage is wrapped on the outer side of the pattern hole water-stop grouting pipe, the pattern hole water-stop grouting pipe is vertically and downwards arranged, the upper end of the pattern hole water-stop grouting pipe is communicated with the atmosphere, and the lower end of the pattern hole water-stop grouting pipe is communicated with the shallow air layer;
step S33, placing the steel structure 8 into the groove 2 and pouring concrete to form the unit wall;
step S34, repeating the steps S31 to S33 until the underground continuous wall is formed;
the step S5 specifically includes: in the process of excavation the foundation ditch, when seepage appears in wall seam 7, cut through the seepage position through the high pressure water the line hole stagnant water slip casting pipe, toward after cutting through the line hole stagnant water slip casting pipe pour into the thick liquid that is used for shutoff seepage position. The slurry can be a mixed liquid composed of cement and water glass.
In the scheme that this embodiment provided, through setting up decorative pattern hole stagnant water slip casting pipe, can be in time when seepage appears at wall seam 7 carry out the shutoff to it.
Optionally, as shown in fig. 4 to 6, the steel structure 8 is provided with cross steel plate joints 81 on the first plane and the second plane respectively; the cross steel plate joint 81 comprises a first steel plate 811 and a second steel plate 822, the first steel plate 811 is welded on the first plane, the second steel plate 822 is welded on the first steel plate 811, and the second steel plate 822 vertically bisects the first steel plate 811 along the vertical direction; the pattern hole water-stop grouting pipe comprises an excavation face pattern hole water-stop grouting pipe 9 and an earth-facing pattern hole water-stop grouting pipe 10, the excavation face pattern hole water-stop grouting pipe 9 is arranged on an excavation face at the welding position of the first steel plate 811 and the second steel plate 822, the earth-facing pattern hole water-stop grouting pipe 10 is arranged on an earth-facing face at the welding position, and a connecting line between the excavation face pattern hole water-stop grouting pipe 9 and the earth-facing pattern hole water-stop grouting pipe 10 is parallel to the width direction of the groove 2; the cross steel plate joint 81 on the second plane and the cross steel plate joint 81 on the first plane are symmetrically arranged around the steel structure 8, and the pattern hole water-stopping grouting pipe on the second plane and the pattern hole water-stopping grouting pipe on the first plane are symmetrically arranged around the steel structure 8; wherein, the left plane in fig. 5 is a first plane, and the right plane is a second plane; the steel structure 8 also comprises an anti-streaming iron sheet 82, an underground continuous wall distribution rib 83 and an underground continuous wall main rib 84 in general;
the step S5 specifically includes: in the process of excavating the foundation pit, when the wall joint 7 leaks, the water-stopping grouting pipe 9 of the pattern hole of the excavation surface at the leakage position is split through high-pressure water, and then slurry for blocking the leakage position is injected into the split water-stopping grouting pipe 9 of the pattern hole of the excavation surface; if the leakage position is blocked, the blocking process is finished, if the leakage position is not blocked, the water-stopping grouting pipe 10 of the pattern hole on the soil facing surface of the leakage position is split through high-pressure water, and then slurry for blocking the leakage position is injected into the split water-stopping grouting pipe 10 of the pattern hole on the soil facing surface.
In the scheme that this embodiment provided, the excavation face indicates the inboard of foundation ditch, and the face of meeting the soil indicates the outside of foundation ditch, divide into excavation face decorative pattern hole stagnant water slip casting pipe 9 and face of meeting the soil decorative pattern hole stagnant water slip casting pipe 10 with decorative pattern hole stagnant water slip casting pipe, can carry out the slip casting to excavation face one side earlier like this, if the wall seam 7 is by the shutoff then end slip casting, can save the grouting volume like this and improve shutoff efficiency.
Optionally, as shown in fig. 7 and 10, the step S34 specifically includes: and (3) sealing and reinforcing the soil facing surface of each wall gap 7 by utilizing an omnibearing high-pressure Jet method (Metro Jet System, MJS), wherein the reinforcement adopts swing Jet, the Jet direction is aligned with the wall gap 7, and the reinforcement depth is from the top of the underground continuous wall 17 to the bottom of the shallow air layer.
Specifically, 180-degree semicircular swinging spraying can be adopted, the semicircular spraying direction is aligned to the wall joint 7, the distance d between a hole leading point and the wall joint 7 of the underground continuous wall is equal to 50cm, a shallow air layer is separated by the depth of a semicircular MJS reinforcing pile 11 formed by swinging spraying, the potential weak point of the wall joint 7 of the underground continuous wall is reinforced by utilizing the characteristic that the MJS forces to arrange soil and reinforce uniformly, the communication between the defect of the underground continuous wall 17 formed by methane and the outside of a pit is avoided when the underground continuous wall 17 is constructed, and the condition that more methane and underground water are leaked and enter during excavation of the foundation pit to cause the harm of the foundation pit is avoided.
The diameter of the MJS pile can be 2200mm, and the construction parameters can be as follows:
(1) slurry pressure: not less than 38 MPa;
(2) air pressure: not less than 0.8 MPa;
(3) air flow rate: 0.8-2.0 Nm/min;
(4) the pressure in the ground: a coefficient of 1.3-1.6 (properly adjusted and controlled according to geological conditions);
(5) pile-forming verticality error: less than or equal to 1/100;
(6) the cement dosage is as follows: about 720kg/m3；
(7) The lifting speed is as follows: 30 min/m;
(8) slurry flow rate: 85-100L/min;
(9) proportioning the slurry: the mass ratio of the water to the cement is 1:1, and the cement with the type of P.O42.5 can be used as the cement.
As shown in fig. 8a to 8h, the construction process of the MJS construction method pile-forming refers to the following steps:
(1) drilling the outer sleeve 25 to a designed elevation;
(2) pulling back the outer sleeve 25 of 6 meters;
(3) an MJS construction method drill rod 26 is arranged in the outer sleeve 25;
(4) forming a pile 27 by using an MJS construction method;
(5) after 6m of construction, pulling out the MJS working method drill rod 26, and pulling back the outer sleeve 25 of 6m again;
(6) placing the MJS method drill rod 26, and completely pulling out the MJS method drill rod 26 after 6m construction;
(7) the outer sleeve 25 is completely pulled out, and an MJS construction method drill rod 26 is placed;
(8) and (5) constructing to the designed pile top elevation by using the MJS construction method, and finishing the construction.
Optionally, as shown in fig. 9 and 10, the step S4 specifically includes: and carrying out omnibearing reinforcement on the bottom 18 of the foundation pit by utilizing an omnibearing high-pressure injection construction method to form a full pit bottom reinforcing layer 19.
The full pit bottom reinforcing layer 19 can improve the firmness and sealing performance of the pit bottom 18. The bottom 18 of the foundation pit can be reinforced by adopting full MJS, the full MJS pile positions 13 are shown in figure 9, after the construction of the underground continuous wall 17 and the construction of the semicircular MJS reinforcing piles 11 are completed, the MJS soil replacement principle is utilized to reinforce all weak channels of the underground continuous wall 17, the soil body boundary, the pile foundation 12, the soil body boundary, the soil body and the like, and a full pit bottom reinforcing layer 19 is formed, so that the reinforcing body is uniform and has no gap, the strength and the tightness for resisting deep methane leakage are realized, and the risk that the channel is formed by methane leakage to cause lower confined water surge can be resisted. The reinforcing depth can be 5m from the bottom surface of foundation pit excavation to the bottom of the pit, the piles with the diameter of 2200mm and the lap joint of 1800mm can be adopted to be arranged in the foundation pit in a left-right support mode in the horizontal direction, and staggered lap joint construction is carried out to ensure that the MJS piles are in gapless lap joint, so that the foundation pit bottom 18 is guaranteed to be uniformly reinforced without weak points.
Referring to fig. 10, as a specific construction case, a foundation pit of a pipeline engineering tunnel a-line shield receiving well of a Natural Gas backbone network chongming island-changxing island-purdon new region five-channel LNG (Liquefied Natural Gas) station tunnel in shanghai city, namely, a chongming island working well, is constructed by an open-cut forward method, wherein a soil layer comprises a pit bottom ground base layer 14, a shallow Gas layer 15 and a pressure-bearing water layer 16, the outer packing size of the foundation pit is 22.6m × 15.6m, and the area of the foundation pit is 266.64m2And an excavation depth of 28.385 m. The enclosure structure is an underground continuous wall 17 with the depth of 62m and the thickness of 1200mm, and a cross steel plate connector 81 is adopted. 8 supports are arranged in the foundation pit, 2 concrete supports 23 and 6 steel supports 24 are added, wherein the 1 st and 6 th concrete supports are concrete supports, and the 2 nd, 3 rd, 4 th, 5 th, 7 th and 8 th concrete supports are concrete supports
And (5) supporting steel. The special biogas investigation report proves that the actual measured biogas pressure of the work well region of the Chongming island is between 15KPa and 50KPa, the corrected pressure is between 55KPa and 100KPa, the lower part of the biogas layer is a pressure-bearing water layer, and the adverse effect of the biogas pressure on the excavation construction of the foundation pit and the stability of the enclosure structure is expected. By adopting the shallow gas protection method for the foundation pit, provided by the invention, the operation safety during deep foundation pit operation of a working well of the Chongming island and shield receiving can be ensured. The defects of the local wall joint 7 caused by the influence of methane can be seen in the process of excavating the wall joint 7 of the deep foundation pit, but the foundation pit does not leak finally under the action of MJS protection and the pre-buried pattern hole water-stop grouting pipe. The foundation pit is dug to the bottom, and construction management personnel need adopt hand-held type combustible gas detecting instrument to carry out marsh gas detection to underground continuous wall 17 and soil body boundary, pile foundation 12 and soil body boundary, the pit bottom soil body that the foundation pit exposes, and local marsh gas leakage department concentration is less than 10% LEL (Lower Explosive Limited), and marsh gas is not surveyed to middle part and upper portion in the foundation pit, and the LEL of marsh gas is 4.95% vol.
The shallow gas protection method for the foundation pit is particularly suitable for the opening area of less than 1000m2The small deep foundation pit with the excavation depth larger than 15 meters can prevent long-term multi-aspect influence caused by methane during construction of the deep foundation pit on the soft soil shallow gas layer, and safety and quality of the underground structure during construction are guaranteed.
In conclusion, according to the shallow gas protection method for the foundation pit, provided by the invention, most of shallow gas is released through the temporary dewatering well 3, so that the influence of the released shallow gas on the quality of the underground continuous wall 17 in the construction process of the underground continuous wall 17 is reduced; by reinforcing the bottom 18 of the foundation pit, methane entering the foundation pit is reduced, and potential safety hazards of fire and confined water inrush in the foundation pit are eliminated; the wall joint 7 is reinforced through MJS, so that the sealing performance of the wall joint 7 is enhanced; the water-stopping grouting pipe is pre-buried in the pattern holes, so that a good emergency plugging effect is achieved when the wall gap 7 leaks; the sealing performance of the foundation pit bottom 18 is enhanced by performing MJS reinforcement on the foundation pit bottom 18.
The above description is only for the purpose of describing the preferred embodiments of the present invention, and is not intended to limit the scope of the present invention, and any variations and modifications made by those skilled in the art based on the above disclosure are within the scope of the claims of the present invention.
Claims (10)
1. A shallow gas protection method for a foundation pit is characterized by comprising the following steps:
step S1, arranging a temporary dewatering well in an area where a groove for containing the underground diaphragm wall is located, and installing a dewatering well pipe in the temporary dewatering well, wherein the upper end of the temporary dewatering well is communicated with the atmosphere, the lower end of the temporary dewatering well is communicated with the shallow gas layer, and the dewatering well pipe is used for extracting water in the shallow gas layer and releasing shallow gas in the shallow gas layer;
step S2, excavating soil bodies and the dewatering well pipe in the area where the groove is located to form a groove for containing an underground continuous wall;
s3, constructing in the groove to form an underground continuous wall, wherein the area formed by enclosing the underground continuous wall is the area where the foundation pit to be excavated is located;
s4, reinforcing the bottom of the foundation pit to prevent residual shallow layer gas in the shallow gas layer from entering the foundation pit;
and S5, excavating soil bodies contained in the area where the foundation pit is located to form the foundation pit.
2. The shallow gas protection method for the foundation pit according to claim 1, wherein the step S1 is preceded by the steps of: the method comprises the steps that a guide wall is constructed at an opening position of an area where a groove for containing the underground continuous wall is located, the guide wall is arranged on soil bodies on two sides of the opening position of the groove, and the guide wall is used for reinforcing the opening of the groove.
3. The shallow gas protection method for the foundation pit according to claim 1, wherein the step S5 is followed by the steps of: detecting the content of shallow layer gas in the foundation pit by using a detection instrument, and if the content of the shallow layer gas is less than a preset threshold value, indicating that the foundation pit meets the safety requirement; and if the content of the shallow layer gas is greater than or equal to the preset threshold value, checking whether a gap with gas leakage exists between the underground continuous wall and the bottom of the foundation pit or not, and plugging the gap.
4. The method for shallow gas protection of a foundation pit according to claim 1, wherein the underground continuous wall is composed of a plurality of unit walls, and a wall gap is formed between two adjacent unit walls; the following steps are also included between the step S3 and the step S4:
and step S34, sealing and reinforcing the wall joints of the underground continuous wall after the underground continuous wall reaches the maintenance age.
5. The shallow gas protection method for foundation pit according to claim 2, wherein the dewatering well pipe is a hard water pipe made of plastic, and the step S2 specifically comprises: and (3) damaging and grabbing the hard water pipe and the soil body in the groove along the direction guided by the guide wall by using a grooving machine.
6. The shallow gas protection method for the foundation pit according to claim 1, wherein a plurality of through holes for releasing the shallow gas are uniformly formed in the side wall of the dewatering well casing, and a nylon net for preventing blockage is wrapped on the outer side of the dewatering well casing.
7. The shallow gas protection method for the foundation pit according to claim 4, wherein the step S3 specifically comprises the following steps:
s31, manufacturing a steel structure of the unit wall, wherein the steel structure is a cuboid and comprises a first plane and a second plane, and the first plane and the second plane are planes where the wall seams are located;
step S32, arranging a pattern hole water-stop grouting pipe on the first plane and the second plane respectively, wherein a plurality of pattern holes for grouting are uniformly formed in the side wall of the pattern hole water-stop grouting pipe, a film for preventing blocking is wrapped on the outer side of the pattern hole water-stop grouting pipe, the pattern hole water-stop grouting pipe is vertically arranged downwards, the upper end of the pattern hole water-stop grouting pipe is communicated with the atmosphere, and the lower end of the pattern hole water-stop grouting pipe is communicated with the shallow air layer;
step S33, placing the steel structure into the groove and pouring concrete to form the unit wall;
step S34, repeating the steps S31 to S33 until the underground continuous wall is formed;
the step S5 specifically includes: in the process of excavation the foundation ditch, when the wall seam appears the seepage, cut through the seepage position through the high pressure water the line hole stagnant water slip casting pipe, toward cut through after the line hole stagnant water slip casting pipe pours into the thick liquid that is used for shutoff seepage position.
8. The shallow gas protection method for the foundation pit according to claim 7, wherein the steel structure is provided with cross steel plate joints on the first plane and the second plane respectively; the cross steel plate joint comprises a first steel plate and a second steel plate, the first steel plate is welded on the first plane, the second steel plate is welded on the first steel plate, and the second steel plate vertically and equally divides the first steel plate along the vertical direction; the patterned hole water-stop grouting pipe comprises an excavation face patterned hole water-stop grouting pipe and an earth-facing patterned hole water-stop grouting pipe, the excavation face patterned hole water-stop grouting pipe is arranged on an excavation face at the welding position of the first steel plate and the second steel plate, the earth-facing patterned hole water-stop grouting pipe is arranged on the earth-facing surface at the welding position, and a connecting line between the excavation face patterned hole water-stop grouting pipe and the earth-facing patterned hole water-stop grouting pipe is parallel to the width direction of the groove; the cross steel plate joint on the second plane and the cross steel plate joint on the first plane are symmetrically arranged relative to the steel structure, and the pattern hole water-stopping grouting pipe on the second plane and the pattern hole water-stopping grouting pipe on the first plane are symmetrically arranged relative to the steel structure;
the step S5 specifically includes: in the process of excavating the foundation pit, when the wall joint leaks, the water-stopping grouting pipe of the pattern hole of the excavation surface at the leakage position is split through high-pressure water, and then slurry for blocking the leakage position is injected into the split water-stopping grouting pipe of the pattern hole of the excavation surface; and if the leakage position is blocked, ending the blocking process, and if the leakage position is not blocked, splitting the water stop grouting pipe through the soil facing patterned hole at the leakage position by high-pressure water, and then injecting slurry for blocking the leakage position into the split water stop grouting pipe through the soil facing patterned hole.
9. The shallow gas protection method for the foundation pit according to claim 4, wherein the step S34 specifically comprises:
and sealing and reinforcing the soil facing surface of each wall gap by utilizing an omnibearing high-pressure jet construction method, wherein the reinforcement adopts swing jet, the jet direction is aligned to the wall gap, and the reinforcement depth is from the top of the underground continuous wall to the bottom of the shallow air layer.
10. The shallow gas protection method for the foundation pit according to claim 1, wherein the step S4 specifically comprises: and carrying out omnibearing reinforcement on the bottom of the foundation pit by utilizing an omnibearing high-pressure injection construction method to form a full pit bottom reinforcing layer.
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