Composite treatment method for covering layer karst surface collapse

Abstract

The invention belongs to the technical field of karst surface collapse treatment, and discloses a covering layer karst surface collapse composite treatment method which comprises the following steps: S1, excavating to the upper surface of a bedrock layer to form a foundation pit, then leveling and cleaning the bottom surface of the foundation pit, and marking a karst hole opening; s2, a gabion is placed in the trend of the karst hole gap, the gabion is filled with stone, and geogrids and geotechnical cloth are sequentially laid on the other outer side faces, except for the bottom side, of the gabion from inside to outside; s3, the foundation pit is backfilled with cohesive soil for the first time, a gravel inverted filter space is reserved over the gabions, then ventilation pipes are erected in a bilateral symmetry mode, the bottom ends of the ventilation pipes stretch into the gabions, and the top ends of the ventilation pipes stretch out of the original ground; and S4, the gravel inverted filter layer is laid in the reserved space of the gravel inverted filter layer, cohesive soil is backfilled in the remaining area of the foundation pit again, and a planting soil layer is laid. The method has the advantages that composite treatment is achieved, rigidity and softness are combined, the overall stability is achieved, and the method is all-around, three-dimensional, free of dead corners, high in attractiveness and the like.

Description

Technical Field

[0001] The invention belongs to the technical field of karst ground collapse control, and in particular relates to a composite control method for karst ground collapse in an overburden layer. Background Art

[0002] The collapse of the karst ground in the overburden layer is a common geological disaster in karst areas. The fundamental reason for the collapse of the overburden layer caused by engineering construction lies in the change of hydrogeological conditions: on the one hand, groundwater is lost rapidly along the favorable discharge points formed by underground engineering construction, and on the other hand, due to surface water and soil erosion, precipitation and surface water can directly replenish groundwater. The contrast of the strength of replenishment and discharge in time leads to frequent and drastic rise and fall of groundwater. The seepage and vacuum erosion caused by the drop in water level continuously erode the overburden soil layer, and the high-pressure expansion caused by the rise in water level destroys the originally stable soil arch structure, resulting in continuous loss of the overburden soil and eventually ground collapse.

[0003] At present, the commonly used measures for controlling collapse in karst areas are mainly: beam-slab spanning method, grouting sealing method, backfilling method and pressure unloading method.

[0004] The beam-slab spanning method uses a beam-slab structure to span the openings of karst caves to protect the upper structure from collapse. It is an engineering measure aimed at avoiding disasters and does not interfere with the occurrence and development of collapse. The collapse will continue to evolve until it stabilizes naturally, resulting in permanent soil erosion and destruction of the natural landscape.

[0005] The grouting plugging method has good short-term treatment effects by plugging the pores of the karst caves with grouting, but it lacks consideration of the cracks in the bedrock and is a treatment method that only takes poison to quench thirst. The essential characteristic of rock mass is cracks. There is no rock mass without cracks. Karst phenomena occur with cracks, and there are no separate karst caves. The closed plugging measures not only block the original holes, but also lead water and gas to the surrounding karst caves or cracks for discharge, causing them to develop faster and induce new collapses in the surrounding areas.

[0006] The backfill method mainly uses sand and gravel or geotextiles to construct an anti-filtration layer to prevent seepage damage. It can not only retain soil, but also has a certain air permeability. The backfill method uses sand and gravel with particle sizes from large to small to fill the collapse in layers from bottom to top. Vertical seepage damage is not easy to occur, but the stone layer with large particle sizes at the bottom has large pores, and contact soil flow is prone to occur around it. Over time, there is still a risk of re-collapse and later settlement.

[0007] The pressure relief method uses pipes to connect karst caves and the outside atmosphere. Its main function is ventilation, solving the problem of internal and external air pressure balance. It can effectively relieve vacuum erosion and high-pressure expansion and avoid soil erosion. This method has a single function and mainly plays a role in preventing collapse. It does not belong to the management structure for existing collapse.

[0008] At present, the methods for controlling karst ground collapse generally lack the concept and measures of water blocking, and ignore the fundamental reason of changed groundwater recharge conditions. As a result, various surface waters can still infiltrate and recharge freely, and the frequent rise and fall of groundwater cannot be fundamentally alleviated, resulting in collapse control often being ineffective. Summary of the invention

[0009] The present invention aims to provide a composite treatment method for karst ground collapse in the covering layer, which takes multiple measures such as air permeability, soil conservation and water blocking, and solves the problem that the current karst area collapse treatment measures cannot completely prevent soil and water loss.

[0010] To this end, the technical solution adopted by the present invention is: a composite treatment method for karst ground collapse in the overburden layer, comprising the following steps:

[0011] Step S1, excavating around the karst cave to the upper surface of the bedrock layer to form a foundation pit, then leveling and cleaning the bottom of the foundation pit, and measuring and marking the karst cave openings;

[0012] Step S2, placing gabions in a horizontal direction closely to the front and back of the karst cave, filling the gabions with stones, and laying geogrids and geotextiles in sequence from the inside to the outside along the outer sides of the gabions except the bottom side;

[0013] Step S3, using clay soil to initially backfill the foundation pit by layered compaction, reserving a filter layer space just above the gabion, and then symmetrically setting up ventilation pipes, with the bottom end of the ventilation pipe extending into the gabion, the top end extending vertically out of the original ground and then bending horizontally;

[0014] Step S4, laying a sand and gravel filter layer in the reserved space of the sand and gravel filter layer, then backfilling with clay soil again, and finally laying a planting soil layer and planting plants for greening.

[0015] As a preferred embodiment of the above scheme, in step S1, the ground excavation range is determined according to the geological survey data and the actual situation on site, based on the range of the pit bottom working surface and the stable slope of the covering layer, and the slope method is used to excavate to the upper surface of the bedrock layer to ensure the stability of the foundation pit slope.

[0016] Further preferably, in step S2, the gabion includes a gabion frame formed by binding horizontal and vertical steel bars and a gabion mesh surface installed on the gabion frame, thereby forming two front-to-back symmetrical stone placement cells, and the gabion mesh surface is made of a double-twisted hexagonal mesh surface made of steel wire coated with polyvinyl chloride organic coating.

[0017] It is further preferred that, in step S2, the center of the gabion structure is aligned with the center of the karst cave, and the lengths of the supports on both sides are the same and sufficient, so that the stress state of the gabion structure is reasonable, thereby ensuring the stability of the overall structure.

[0018] It is further preferred that in step S2, the stones are loaded in an artificial layered stacking manner, and the particle size of the lower layer of stones is larger than that of the upper layer of stones. The stones are made of weathering-resistant hard rock with an average particle size of 12 cm to 16 cm, and more than 85% of the stone particles by mass have a particle size greater than 12 cm, and the material selection is reasonable.

[0019] More preferably, in step S2, the geogrid is installed on the outer side of the gabion by tying, and the installation is stable. The geotextile is a whole surface that fits all the geogrids from top to bottom, and the bottom is fixed to the bottom of the foundation pit by U-shaped steel nails. The geotextile is made of polyester filament spunbond needle-punched non-woven geotextile, with an equivalent pore size of 0.05 mm and a permeability coefficient of 1×10 -3 cm / s, reasonable material selection, no joints, stable installation, good integrity and strong anti-filtration ability.

[0020] It is further preferred that, in the step S3, the initial backfill height of the clay soil is not less than 60 cm above the gabion, thereby ensuring that the reserved filter layer space is not less than 60 cm.

[0021] It is further preferred that in step S3, a right-angle elbow with an outlet facing the horizontal direction is provided on the top of the ventilation pipe, the top horizontal section is more than 20 cm higher than the top of the green plant, and the length of the horizontal section is 20 cm to 24 cm, which effectively prevents debris from falling into the ventilation pipe and clogging the pipe. The ventilation pipe adopts a hard UPVC pipe with a diameter of 5 cm to 10 cm, which has a reasonable size to ensure the ventilation effect.

[0022] Further preferably, in step S4, the sand and gravel filter layer includes a medium-fine sand and gravel layer and a fine sand and gravel layer horizontally laid in sequence from bottom to top, forming a step-by-step filter to ensure the filter effect.

[0023] It is further preferred that in step S4, the thickness of the backfilled clay soil is not less than 50 cm, ensuring that the permeability coefficient of the compacted clay soil layer is less than 0.01 m / d, thereby ensuring the effect of delaying the infiltration of surface water. The planting soil layer thickness is greater than 30 cm, and the size is reasonable to ensure the survival rate of plant planting.

[0024] More preferably, in step S4, the greening plants are local dominant species and low shrubs with roots shorter than 30 cm.

[0025] Beneficial effects of the present invention:

[0026] (1) At present, the measures for the treatment of karst subsidence cannot completely prevent soil erosion. The conventional filter layer backfill method only focuses on soil conservation, ignoring water blocking and air permeability, and cannot alleviate the frequent rise and fall of groundwater and changes in air pressure. This scheme adopts a composite governance structure to construct an all-round three-dimensional filter barrier to relieve the changes in air pressure in the karst pipeline and slow down groundwater recharge. It integrates soil conservation, air permeability, water blocking, bearing capacity, and greening functions to carry out comprehensive treatment of karst subsidence, inhibiting soil erosion and ground subsidence from the root. It is novel in conception and ingenious in design.

[0027] (2) Gabions are built across the karst caves on the leveled bedrock layer. The bedrock layer has a high compressive strength and the stress state of the spanning structure is reasonable, making the overall structure highly stable. The steel skeleton makes the gabions rigid to support the loads of the overlying structures and avoid excessive deformation that causes damage to the overlying sand and gravel filter layer and significant ground settlement. The gabion structure itself is flexible. When the karst cave expands, it can adapt to and block the new openings through its own deformation to avoid brittle failure of the structure. At the same time, the stones placed in the gabions can maintain smooth air circulation due to the presence of a large number of interconnected pores.

[0028] (3) Compared with the existing anti-filtration layer laid layer by layer from bottom to top, this scheme forms an all-round anti-filtration structure by wrapping the geotextile outside the gabion and laying the sand and gravel anti-filtration layer just above the gabion to achieve three-dimensional soil conservation. It can not only prevent soil erosion from above, but also prevent lateral infiltration damage such as clay soil. The geotextile has a good soil conservation effect, and the gabion also provides support for the geotextile, effectively preventing the geotextile from deformation and damage due to excessive load, and the layout is reasonable.

[0029] (4) Use clay soil to fill the remaining space in the foundation pit, and use the clay soil’s own strong water-locking ability to effectively reduce the permeability coefficient of the composite structure, prevent surface water from running off and replenishing the underground, and slow down or eliminate the frequent rise and fall of groundwater.

[0030] (5) Use ventilation pipes to connect the karst caves and the atmosphere to maintain air circulation in the karst caves, balance the internal and external air pressure, effectively reduce the air pressure load on the gabion structure, and slow down or eliminate vacuum erosion and high-pressure expansion.

[0031] (6) Planting plants in the planting soil layer for greening can further block water through the plant root system and maintain the consistency of the ground environment landscape, thereby restoring the human living environment and natural landscape damaged by soil erosion and improving the overall aesthetics.

[0032] To sum up, this treatment method takes comprehensive measures such as soil conservation, water blocking, and ventilation in response to collapse factors such as changes in groundwater recharge conditions, frequent rise and fall of groundwater, and changes in air pressure in caves. It has the advantages of composite treatment, combination of rigidity and flexibility, overall stability, all-round three-dimensional without dead ends, and strong aesthetics. It can effectively prevent the risk of collapse and later settlement after treatment. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 This is a front view of the composite governance structure set up by the present invention.

[0034] Figure 2 This is a schematic diagram of the structure of the gabion (the gabion mesh is not fully shown and the gabion mesh on the top is not installed).

[0035] Figure 3 This is a diagram showing the composite governance structure set up for the present invention. DETAILED DESCRIPTION

[0036] The present invention will be further described below by way of embodiments and in conjunction with the accompanying drawings:

[0037] Combination Figure 1 — Figure 3 As shown, a composite treatment method for karst ground collapse in the overburden layer is implemented in the following specific steps:

[0038] The thickness of the covering layer of the karst ground collapse is 2.5m to 5m, and the diameter of the vertical karst cave opening is within 30cm.

[0039] Step S1, excavating around the karst cave 1 to the upper surface of the bedrock layer 2 to form a foundation pit 3, then leveling and cleaning the bottom surface of the foundation pit 3, and measuring and marking the opening of the karst cave 1;

[0040] In step S1, the ground excavation range is determined according to the geological survey data and the actual situation on site, based on the range of the pit bottom working surface and the stable slope of the covering layer, and the slope method is used to excavate to the upper surface of the bedrock layer 2.

[0041] Step S2, placing a gabion 4 in a horizontal direction closely to the front and back of the karst cave 1, filling the gabion 4 with stones 5, and laying a geogrid 6 and a geotextile 7 in sequence from the inside to the outside along the outer side of the gabion 4 except the bottom side;

[0042] In step S2, the gabion 4 is mainly composed of a gabion frame 41 formed by binding horizontal and vertical steel bars and a gabion mesh surface 42 installed on the gabion frame 41, thereby forming two front-to-back symmetrical stone placement cells.

[0043] The gabion frame 41 has a length of 2m, a width of 1m, and a height of 1m. The gabion frame 41 is made of φ16 ribbed steel bars sprayed with epoxy resin coating by bending and tying. Four longitudinal steel bars are arranged at intervals of 33cm on the left and right of the bottom of the gabion frame 41, and two longitudinal steel bars are arranged at intervals of 1m on the top. The square stirrups at the front and rear ends and in the center are bent at an angle of 135° and a hook length of 20cm. The contact position of the steel bars is connected by tying, and the cut steel bar ends are brushed with anti-corrosion coating.

[0044] The gabion mesh surface 42 is made of double-twisted hexagonal mesh surface with polyvinyl chloride organic coating on steel wire.

[0045] A continuous mesh is used to cut out continuous bottom plate, panel and end plate meshes. The bottom plate mesh is laid along the gabion frame 41 and tied to fix. The front and rear panel meshes and the end plate meshes on both sides are laid vertically along the steel frame and fixed by twisting the edges. A square mesh with a side length of 1m is cut and the partition mesh is laid vertically along the steel frame and fixed by twisting the edges. A rectangular mesh with a length of 2m and a width of 1m is cut as the cover mesh.

[0046] When the mesh is installed, the bottom plate mesh is located above the bottom plate reinforcement and is fixed to the bottom longitudinal reinforcement and transverse square stirrups of the skeleton by binding; the panel mesh is fixed to the top longitudinal reinforcement of the skeleton by twisting the edge, and the side wires are fixed to the vertical part of the transverse square stirrups at both ends of the gabion unit, and the mesh is fixed to the vertical part of the transverse square stirrups in the middle by binding; the end plate mesh is fixed to the top of the transverse square stirrups at both ends by twisting the edge, and the end wires are fixed to the vertical part of the transverse square stirrups at both ends; the partition mesh is erected along the middle transverse stirrups and fixed to the square stirrups by twisting the edge, and fixed to the vertical stirrups by binding. When the stone is filled and covered, the end wires of the cover plate mesh are fixed to the top longitudinal reinforcement by twisting the edge, and the side wires are fixed to the top of the transverse square stirrups at both ends.

[0047] When the gabion mesh surface is twisted, single and double turns are twisted alternately at intervals of 10cm to 15cm. The material and mechanical performance indicators of the twisted edge steel wire are consistent with those of the mesh wire.

[0048] The center of the gabion 4 is aligned with the center of the opening of the karst cave 1.

[0049] In step S2, the stones 5 are preferably loaded in an artificial layered stacking manner, and the particle size of the lower layer of stones 5 is larger than that of the upper layer of stones 5. The stones 5 are preferably made of weathering-resistant hard rock with an average particle size of 12 cm to 16 cm, and more than 85% of the stones 5 by mass have a particle size greater than 12 cm.

[0050] In step S2, the geotextile 7 is applied with a whole surface to fit all the geogrids 6 from top to bottom, and the bottom is fixed to the bottom surface of the foundation pit 3 by U-shaped steel nails.

[0051] The remaining width of the geotextile 7 at the bottom is more than 10 cm, and the edges are laid close to the bedrock at the bottom of the pit. The excess parts at the four corners are folded and attached to each surface, and the edges are fixed to the bedrock at the bottom of the pit with U-shaped steel nails. The geogrid 6 and geotextile 7 corresponding to the upper surface of the gabion 4 are reserved with perforations for ventilation pipes 8.

[0052] Geotextile 7 is made of polyester filament spunbonded needle-punched nonwoven geotextile with an equivalent pore size of 0.05 mm and a permeability coefficient of 1×10 - 3 cm / s, the geogrid 6 is installed on the outer side of the gabion 4 by binding.

[0053] Step S3, using clay soil to initially backfill the foundation pit 3 by layered compaction, reserving a sand and gravel filter layer space just above the gabion 4, and then symmetrically setting up ventilation pipes 8, wherein the bottom end of the ventilation pipes 8 extends into the gabion 4, and the top end extends vertically out of the original ground to communicate with the atmosphere;

[0054] In step S3, the initial backfill height of the clay soil is preferably not less than 60 cm above the gabion 4, and the layered compaction thickness of the clay soil is preferably 150 mm to 250 mm, and the permeability coefficient is less than 0.01 m / d, ensuring that the permeability coefficient of the clay soil layer after compaction is less than 0.01 m / d.

[0055] In step S3, a right-angle elbow with an outlet facing the horizontal direction is provided at the top of the ventilation pipe 8, the top horizontal section is more than 20 cm higher than the top of the green plant, and the length of the horizontal section is preferably 20 cm to 24 cm.

[0056] The ventilation pipe 8 is preferably a hard UPVC pipe with a diameter of 5 cm to 10 cm.

[0057] Step S4, lay the gravel filter layer 9 in the reserved space of the gravel filter layer, then backfill the clay soil in the remaining area of ​​the foundation pit 3 again by layers and compaction, and finally lay the planting soil layer 10 and plant plants for greening.

[0058] In step S4, the sand and gravel filter layer 9 is composed of a medium-fine sand and gravel layer 91 and a fine sand and gravel layer 92 horizontally laid in sequence from bottom to top.

[0059] The gradation of the fine sand and gravel layer is set according to the gradation of the protected soil, i.e. the overlying clay layer, and must meet the Terzaghi filter soil criterion, i.e. according to D 15 / d 85 ≤4 and D 15 / d 15 ≥4 Determine the particle size D of the fine sand and gravel layer 15 Range, which is defined as the soil weight smaller than this particle size in the fine sand and gravel layer accounts for 15% of the total soil weight; d 85 The particle size of the protected soil, i.e. the overlying clay layer, is defined as the particle size smaller than which the weight of the clay soil accounts for 85% of the total soil weight; d 15The particle size of the protected soil, i.e. the overlying clay layer, is defined as the particle size smaller than which the weight of the soil in the clay soil accounts for 15% of the total soil weight.

[0060] The gradation of the medium-fine sand and gravel layer should be set according to the gradation of the protected soil, that is, the fine sand and gravel filter layer, and must meet the Terzaghi filter soil criterion, that is, according to D 15 / d 85 ≤4 and D 15 / d 15 ≥4 Determine the particle size D of the medium and fine sand and gravel layer 15 Range, which is defined as the soil weight smaller than this particle size in the medium-fine sand and gravel layer accounts for 15% of the total soil weight; d 85 is the particle size of the protected soil, i.e. the fine sand and gravel filter layer, and is defined as the weight of soil smaller than this particle size in the fine sand and gravel filter layer accounting for 85% of the total soil weight; d 15 It is the particle size of the protected soil, i.e. the fine sand and gravel filter layer, and is defined as the weight of soil in the fine sand and gravel filter layer that is smaller than this particle size accounting for 15% of the total soil weight.

[0061] In step S4, the thickness of the clay backfilled again is not less than 50 cm, and the thickness of the planting soil layer is greater than 30 cm.

[0062] When backfilling clay soil, it should be compacted layer by layer. The thickness of each compacted soil layer should be 150-250mm. The layers should be tightly combined to ensure that the permeability coefficient of the clay soil layer after compaction is less than 0.01m / d.

[0063] In step S4, the greening plants are local dominant species and low shrubs with roots shorter than 30 cm.

Claims

1. A composite treatment method for karst ground collapse in the overburden layer, characterized in that: The following steps are involved: Step S1, excavating around the karst cave (1) to the upper surface of the bedrock layer (2) to form a foundation pit (3), then leveling and cleaning the bottom surface of the foundation pit (3), and measuring and marking the opening of the karst cave (1); Step S2, placing a gabion (4) in a horizontal direction closely to the front and back of the karst cave (1), filling the gabion (4) with stones (5), and laying a geogrid (6) and a geotextile (7) in sequence from the inside to the outside along the outer side of the gabion (4) except the bottom side; Step S3, using clay soil to initially compact and backfill the foundation pit (3), reserving a sand and gravel filter layer space just above the gabion (4), and then symmetrically setting up ventilation pipes (8), wherein the bottom end of the ventilation pipes (8) extends into the gabion (4), the top end extends vertically out of the original ground and then is bent horizontally; Step S4, paving a sand and gravel filter layer (9) in the reserved space of the sand and gravel filter layer, and then backfilling the clay soil in layers and compacting again, and finally laying a planting soil layer (10) and planting plants for greening.

2. A composite treatment method for karst ground collapse in the overburden layer according to claim 1, characterized in that: In step S1, the ground excavation range is determined according to the geological survey data and the actual situation on site, based on the range of the pit bottom working surface and the stable slope of the covering layer, and the slope method is used to excavate to the surface of the bedrock layer (2).

3. The composite treatment method for karst ground collapse in the overburden layer according to claim 1 is characterized by: In step S2, the gabion (4) comprises a gabion frame (41) formed by binding horizontal and vertical steel bars and a gabion mesh surface (42) installed on the gabion frame (41), thereby forming two front-to-back symmetrical stone placement cells, and the gabion mesh surface (42) is made of a double-twisted hexagonal mesh surface made of steel wire coated with polyvinyl chloride organic coating.

4. The composite treatment method for karst ground collapse in the overburden layer according to claim 1 is characterized by: In step S2, the center of the gabion (4) is aligned with the center of the karst cave (1), the stones (5) are filled in an artificial layered stacking manner, and the particle size of the lower layer of stones (5) is larger than that of the upper layer of stones (5), and the stones (5) are made of weathering-resistant hard rocks.

5. The composite treatment method for karst ground collapse in the overburden layer according to claim 1 is characterized by: In step S2, the geogrid (6) is installed on the outer side of the gabion (4) by binding, the geotextile (7) is laminated to fit all the geogrids (6) from top to bottom, and the bottom is fixed to the bottom of the foundation pit (3) by U-shaped steel nails, and the geotextile (7) is made of polyester filament spunbonded needle-punched non-woven geotextile.

6. The composite treatment method for karst ground collapse in the overburden layer according to claim 1 is characterized by: In step S3, the initial backfill height of the clay soil is not less than 60 cm above the gabion (4), and the layered compaction thickness is 150 mm to 250 mm.

7. A composite treatment method for karst ground collapse in the overburden layer according to claim 3, characterized in that: In step S3, a right-angle elbow with an outlet facing the horizontal direction is provided at the top of the ventilation pipe (8), the top horizontal section is more than 20 cm higher than the top of the green plant, and the length of the horizontal section is 20 cm to 24 cm. The ventilation pipe (8) is a hard UPVC pipe with a diameter of 5 cm to 10 cm.

8. The composite treatment method for karst ground collapse in the overburden layer according to claim 1 is characterized by: In the step S4, the sand and gravel filter layer (9) comprises a medium-fine sand and gravel layer (91) and a fine sand and gravel layer (92) which are horizontally laid in sequence from bottom to top.

9. The composite treatment method for karst ground collapse in the overburden layer according to claim 1 is characterized by: In the step S4, the thickness of the clay backfilled again is not less than 50 cm, and the layered compaction thickness is 150 mm to 250 mm, and the thickness of the planting soil layer (10) is greater than 30 cm.

10. The composite treatment method for karst ground collapse in the overburden layer according to claim 1 is characterized by: In step S4, the greening plants are local dominant species and low shrubs with root systems shorter than 30 cm.

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