Foundation protection method on karst underground river in low-lying area around reservoir

Abstract

The application discloses a foundation protection method on a karst underground river in a low-lying area around a reservoir, and the method can reduce the pressure head of the karst confined water in the field area, reduce the floating force of the clay layer on the top of the karst underground river, and avoid the damage of the foundation caused by the sudden gushing of the karst underground river; when the reservoir water level suddenly drops, the underground water level in the karst underground river system suddenly drops, and at this time, a part of the karst cave under the foundation is in a vacuum state, causing the engineering geological problems of ground karst collapse and uneven settlement of the foundation; at this time, the air supplied through the pressure relief well can solve the above engineering geological problems.

Description

Technical Field

[0001] This invention belongs to the field of geological exploration technology for water conservancy and hydropower engineering, specifically relating to a method for protecting the foundation of a karst underground river in a low-lying area around a reservoir. Background Technology

[0002] Karst is widely distributed in my country, covering an area of ​​approximately 3.44 million square kilometers, accounting for one-third of the country's land area. Karst landforms, such as caves, solution depressions, and underground rivers, are particularly well-developed in the southwest region. When building foundations are located in karst areas, engineering geological problems such as karst collapse, uneven foundation settlement, foundation buoyancy, karst water backflow, and site submersion exist, affecting the stability of building foundations and site safety.

[0003] Currently, in karst areas, methods such as filling, reinforcement, and pile foundations are mainly used to treat the foundations of buildings, which can solve problems such as uneven settlement and karst collapse caused by dissolution fissures and caves. However, for karst underground rivers, these sealing measures are extremely difficult to implement, and engineering geological problems such as foundation buoyancy, karst water backflow, and site submersion cannot be solved.

[0004] Therefore, a new method for protecting the foundation of karst underground rivers in low-lying areas around reservoirs is needed to solve the above-mentioned technical problems. Summary of the Invention

[0005] This invention provides a method for protecting the foundation of karst underground rivers in low-lying areas around reservoirs. It can solve engineering geological problems such as karst collapse, uneven settlement of foundation, foundation buoyancy, karst water backflow, and site immersion in karst areas, while ensuring the economical and efficient completion of foundation protection work in karst areas.

[0006] To solve the above problems, the technical solution provided by the present invention is as follows:

[0007] This invention provides a method for protecting the foundation of a karst underground river in a low-lying area around a reservoir, comprising:

[0008] Step S1: Conduct a karst survey of the project area to determine the distribution and characteristics of karst underground rivers at the foundation of buildings, as well as the changes in groundwater level of the karst underground river system under the foundation of buildings before and after reservoir impoundment.

[0009] Step S2: Install multiple pressure relief wells at the karst underground river beneath the building foundations in the project area. The number of pressure relief wells is determined based on the identified flow rate of the karst channel.

[0010] Step S3: Arrange a ground surface channel around the pressure relief well. The height of the pressure relief well opening is higher than the bottom of the ground surface channel to prevent water from flowing back into the pressure relief well through the ground surface channel. The ground surface channel is equipped with a preset longitudinal slope to facilitate the overflow of water from the karst underground river under pressure and discharge it into the surface river through the ground surface channel.

[0011] In step S4, after the reservoir connected to the karst underground river is filled with water, the pressure head of the karst underground river is reduced through the pressure relief well to prevent it from penetrating the clay layer at the top of the karst underground river and causing a sudden surge. At the same time, the pulsating pressure caused by the change in reservoir water level is reduced, which plays a role in regulating groundwater pressure. When the reservoir water level drops suddenly, air is supplied to the karst caves under the foundation of the building through the pressure relief well to prevent the vacuum from causing uneven settlement or ground collapse of the clay layer at the top of the karst underground river, which would damage the foundation. This achieves the protection of the foundation of the karst underground river.

[0012] Step S5: Construct earth and rock dams around the project area to prevent the pressure head of the karst underground river from backflowing and submerging the project area during periods of high water level after the reservoir is filled.

[0013] According to an optional embodiment of the present invention, step S4 specifically includes: when the karst underground river system receives infiltration replenishment from atmospheric precipitation in the mountain depression and discharges into the surface river, when the reservoir is filled to a depth H1 = 61m, the pressure head at the foundation of the building is higher than the bottom surface of the clay layer. At this time, the pressure head exerts a buoyancy force on the foundation of the building, causing the clay layer at the top of the karst underground river in the project area to be breached and a sudden surge to form, causing the foundation to bulge and crack, resulting in damage. In this case, by arranging a pressure-reducing drainage well at the karst underground river under the foundation of the building, the groundwater of the karst underground river system overflows in the form of gravity flow and is then discharged through the surface channel. This can reduce the pressure of the karst pressure head in the site area, reduce its buoyancy force on the clay layer at the top of the karst underground river, and avoid the situation where a sudden surge damages the foundation.

[0014] According to an optional embodiment of the present invention, step S4 specifically includes: when the reservoir water level drops sharply to depth H2 = 40m, the groundwater level in the karst underground river system drops sharply. At this time, part of the karst cave under the foundation of the building is in a vacuum state. Under the vacuum compression, the foundation of the building experiences uneven settlement of the clay layer above the karst underground river. Moreover, the reservoir water level is constantly rising and falling during operation. The groundwater level will generate alternating pulsating pressure on the foundation, causing foundation buoyancy and karst collapse. Under these engineering geological conditions, air is supplied to the karst cave under the foundation of the building through a decompression well to avoid uneven settlement or ground collapse of the top clay layer of the karst underground river caused by vacuuming, which would damage the foundation and achieve protection of the foundation of the karst underground river.

[0015] According to an optional embodiment of the present invention, step S2 uses an impact drilling process to prepare a decompression well and drills through the top plate of the karst cave to ensure that the well location is on the karst underground river system; wherein, the decompression well includes a wellhead part and a through hole part, one end of the through hole part is connected to the wellhead part, and the other end passes through the clay layer and is connected to the karst cave of the karst underground river; the diameter of the through hole part located in the clay layer is 80cm, and a steel pipe with a wall thickness of 1cm is used for wall protection, and the diameter of the through hole part located in the karst cave is 60cm; the cross-sectional shape of the wellhead part is U-shaped, the width of the two sides of the wellhead part is 210cm, the height is 160cm, the sidewall thickness of the wellhead part is 30cm, the bottom thickness is 20cm, and a 10cm thick sand cushion layer is also provided at the bottom of the wellhead part.

[0016] According to an optional embodiment of the present invention, the number of decompression wells in step S2 is 14.

[0017] According to an optional embodiment of the present invention, the cross-sectional dimensions of the surface channel arranged around the pressure relief well in step S3 are set to a width-to-height ratio of 1:1, with a preset longitudinal slope of 0.2%. C20 concrete is used for in-situ casting, and a P-type rubber waterstop is installed every 5m to ensure that the wellhead of the pressure relief well is 30cm higher than the bottom of the drainage channel to prevent backflow into the pressure relief well. In this way, the groundwater in the karst underground river system flows under pressure and is discharged through the surface channel to reduce the pressure head of the karst underground river and reduce the damage of buoyancy to the foundation.

[0018] According to an optional embodiment of the present invention, step S5 specifically includes: arranging earth-rock dams and dikes around the building to protect the overall safety of the site. The top of the earth-rock dams and dikes is 3-5m above the high water level. The slope ratio of the waterproof side and the backwater side of the earth-rock dams and dikes is set to 1:2. The dikes are filled with clay with a compaction degree ≥0.9. Every 30cm of filling is compacted in layers. The foundation bearing capacity is 150kPa to prevent the karst confined water from backflowing and submerging the project area after the reservoir is filled.

[0019] Beneficial Effects: This invention provides a method for protecting the foundation of karst underground rivers in low-lying areas surrounding a reservoir. After the reservoir is filled, the groundwater level of the karst underground rivers rises, causing an overall increase in the groundwater level beneath the building foundations. This leads to engineering geological problems such as karst collapse, uneven foundation settlement, foundation buoyancy, karst water backflow, and site submersion. By arranging pressure-reducing drainage wells at the karst underground rivers beneath the building foundations, the groundwater in the karst underground river system overflows by gravity and is then drained away through surface channels. This reduces the pressure head of the karst confined water in the site, decreasing its buoyancy on the clay layer at the top of the karst underground rivers and preventing sudden surges that could damage the foundation. When the reservoir water level drops sharply, the groundwater level within the karst underground river system also drops sharply. At this time, part of the karst caves beneath the foundation are in a vacuum state, causing engineering geological problems such as surface karst collapse and uneven foundation settlement. Replenishing air through pressure-reducing wells can solve these engineering geological problems. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments or prior art, the drawings used in the description of the embodiments or prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 This is a schematic diagram of the groundwater level distribution in the karst underground river system beneath the foundation of a building after the reservoir water level rises, as provided in an embodiment of this application.

[0022] Figure 2 This is a schematic diagram of the groundwater level distribution in the karst underground river system beneath the foundation of a building when the reservoir water level drops suddenly, as provided in an embodiment of this application.

[0023] Figure 3 This is a schematic diagram of a pressure relief well structure provided in an embodiment of this application.

[0024] Figure 4 A cross-sectional view of a decompression well provided for an embodiment of this application.

[0025] Figure 5 This is a schematic diagram showing the distribution of groundwater level in a karst underground river beneath the foundation of a building after drainage from a pressure-reducing well, as provided in an embodiment of this application.

[0026] Figure 6 A cross-sectional view of an earth-rock dam provided in an embodiment of this application. Detailed Implementation

[0027] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the scope of protection of this application.

[0028] To address the problems existing in the prior art, this invention provides a design method for protective engineering in karst underground river systems in low-lying areas surrounding reservoirs. The implementation process of this invention is as follows: After reservoir impoundment, the groundwater level of the karst underground river rises, causing an overall increase in the groundwater level beneath the building foundations. This leads to engineering geological problems such as karst collapse, uneven foundation settlement, foundation buoyancy, karst water backflow, and site submersion. By arranging pressure-reducing drainage wells at the karst underground river beneath the building foundations, the groundwater from the karst underground river system overflows by gravity and is then drained away through surface channels. This reduces the karst confined water head pressure in the site area, decreasing its buoyancy force on the clay layer at the top of the karst underground river, thus preventing sudden surges that could damage the foundation. When the reservoir water level drops sharply, the groundwater level within the karst underground river system also drops sharply. At this time, part of the karst caves beneath the foundation are in a vacuum state, causing engineering geological problems such as surface karst collapse and uneven foundation settlement. Replenishing air through pressure-reducing wells can solve these engineering geological problems.

[0029] like Figure 1 As shown in the figure, this embodiment of the invention provides a method for protecting the foundation of a karst underground river in a low-lying area around a reservoir. The specific method includes the following steps:

[0030] Step S1: Conduct a karst survey of the project area to determine the distribution and characteristics of karst underground rivers at the foundation of buildings, as well as the changes in groundwater level of the karst underground river system under the foundation of buildings before and after reservoir impoundment.

[0031] Step S2: Set up pressure relief wells at the karst underground river beneath the building foundations in the project area. The number of pressure relief wells is determined based on the identified flow rate of the karst channel.

[0032] Step S3: Arrange a ground surface channel around the pressure relief well. The height of the pressure relief well opening is higher than the bottom of the ground surface channel to prevent backflow into the pressure relief well. The ground surface channel is equipped with a preset longitudinal slope to facilitate the overflow of pressure water from the karst underground river into the surface river through the ground surface channel.

[0033] In step S4, after the reservoir connected to the karst underground river is filled with water, the pressure head of the karst underground river is reduced through the pressure relief well to prevent it from penetrating the clay layer at the top of the karst underground river and causing a sudden surge. At the same time, the pulsating pressure caused by the change in reservoir water level is reduced, which plays a role in regulating groundwater pressure. When the reservoir water level drops suddenly, air is supplied to the karst caves under the foundation of the building through the pressure relief well to prevent the vacuum from causing uneven settlement or ground collapse of the clay layer at the top of the karst underground river, which would damage the foundation. This achieves the protection of the foundation of the karst underground river.

[0034] Step S5: Construct earth and rock dams around the project area to prevent the pressure head of the karst underground river from backflowing and submerging the project area during periods of high water level after the reservoir is filled.

[0035] The technical solution of the present invention will now be described in detail with reference to the accompanying drawings and specific examples.

[0036] like Figure 1 As shown, step S4 specifically includes: when the karst underground river system receives atmospheric precipitation S1 infiltration replenishment in the mountain depression and discharges into the surface river, the groundwater level S2 rises. When the reservoir is filled to a depth H1 = 61m, the pressure head at the foundation of building 1 is higher than the bottom surface of the clay layer 2. At this time, the pressure head exerts a buoyancy force S3 on the foundation of the building, causing the clay layer 2 at the top of the karst underground river 3 in the project area to break through and form a sudden surge, causing the foundation to bulge and crack, thus causing damage. In this case, by arranging a pressure-reducing drainage well at the karst underground river 3 under the foundation of building 1, the groundwater of the karst underground river system overflows in the form of gravity flow and is then discharged through the surface channel. This can reduce the pressure of the karst pressure head in the site area and reduce its buoyancy force S3 on the clay layer 2 at the top of the karst underground river 3, thus avoiding the situation where a sudden surge damages the foundation.

[0037] like Figure 2 As shown, step S4 specifically includes: when the karst underground river 3 system receives less atmospheric precipitation S4 in the mountain depression, and when the reservoir water level drops sharply to depth H2 = 40m, the groundwater level S5 in the karst underground river 3 system drops sharply. At this time, part of the karst cave under the foundation is in a vacuum state. Under the vacuum compression, the foundation of the building foundation will experience uneven settlement of the clay layer 2 above the karst underground river 3. Moreover, the reservoir water level is constantly rising and falling during operation. The groundwater level S5 will generate alternating pulsating pressure on the foundation, causing foundation buoyancy and karst collapse. Under these engineering geological conditions, air is supplied to the karst cave under the foundation of building 1 through the decompression well to avoid the vacuum forming uneven settlement or ground collapse of the top clay layer of the karst underground river 3, which would damage the foundation, thus achieving the protection of the foundation of the karst underground river 3.

[0038] like Figure 3As shown, step S2 uses an impact drilling process to prepare the pressure relief well 4 and drills through the top plate of the karst cave 31 to ensure that the pressure relief well 4 is located on the karst underground river 3 system. The pressure relief well 4 includes a wellhead part 41 and a through hole part 42. One end of the through hole part 42 is connected to the wellhead part 41, and the other end passes through the clay layer 2 and is connected to the karst cave 31 of the karst underground river 3. The diameter of the through hole part 42 located in the clay layer 2 is 80cm, and it is protected by a steel pipe with a wall thickness of 1cm. The diameter of the through hole part 42 located in the karst cave is 60cm. The cross-sectional shape of the wellhead part 41 is U-shaped. The width of the two sides of the wellhead part 41 is 210cm, the height is 160cm, the sidewall thickness of the wellhead part 41 is 30cm, the bottom thickness is 20cm, and a 10cm thick sand cushion layer 43 is also provided at the bottom of the wellhead part 41.

[0039] like Figure 4 and Figure 5 As shown, the number of pressure relief wells is determined based on the flow rate of the karst channels as determined by exploration. In this embodiment, 14 pressure relief wells 4 are set. A surface channel 5 is arranged around each pressure relief well 4. The cross-sectional dimensions of the surface channel 5 have a width-to-height ratio of 1:1 and a longitudinal slope of 0.2%. It is constructed using C20 concrete and is cast-in-place. A P-type rubber waterstop is installed every 5m to ensure that the wellhead of the pressure relief well 4 is 30cm above the bottom of the surface channel 5 to prevent backflow into the well. In this way, the groundwater in the karst underground river system flows under pressure and is discharged through the surface channel 5, reducing the pressure head of the karst underground river 3 and minimizing the damage to the foundation caused by buoyancy.

[0040] like Figure 5 and Figure 6 As shown, step S5 specifically includes: constructing an earth-rock dam 6 around the building 1 to protect the overall safety of the site. The top of the earth-rock dam 6 is 3-5m above the high water level. The slope ratio between the waterproof and backwater sides of the earth-rock dam 6 is set at 1:2. The dam is constructed with clay filling, with a compaction degree ≥0.9. Every 30cm of filling is layered and compacted. The foundation bearing capacity is 150kPa, preventing backflow of karst confined water into the project area after the reservoir is filled. In this embodiment, the earth-rock dam 6 includes a base 61 and a protrusion 62 located on the base 61.

[0041] The above are merely specific embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any modifications, equivalent substitutions, and improvements made by those skilled in the art within the scope of the technology disclosed in the present invention and within the spirit and principles of the present invention should be included within the scope of protection of the present invention. Those skilled in the art can make various modifications and refinements without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention shall be determined by the scope defined in the claims.

Claims

1. A method for protecting a foundation on a karst underground river in a low-lying area around a reservoir, characterized by, The method comprises the following steps: Step S1, karst investigation is carried out on the engineering area to find out the distribution position and characteristics of the karst underground river at the building foundation and the change of the underground water level of the karst underground river system before and after the reservoir is filled with water; Step S2, a plurality of pressure relief wells are arranged at the karst underground river under the building foundation in the engineering area, and the number of the pressure relief wells is determined according to the flow of the karst channel; Step S3, a surface drainage ditch is arranged around the pressure relief well, the wellhead of the pressure relief well is higher than the bottom of the surface drainage ditch, so as to avoid the water from flowing back into the pressure relief well from the surface drainage ditch, and a preset longitudinal slope is arranged on the surface drainage ditch, so that the confined water head of the karst underground river can be overflowed and discharged into the surface river through the surface drainage ditch; Step S4, after the reservoir is filled with water and is connected with the karst underground river, the pressure relief well is used to reduce the confined water head of the karst underground river, so as to avoid the clay layer at the top of the karst underground river from being broken due to the sudden gushing, and the pulsating pressure generated by the change of the water level of the reservoir is reduced, so as to adjust the underground water pressure; when the water level of the reservoir is suddenly lowered, the pressure relief well is used to supply air to the karst cave under the building foundation, so as to avoid the uneven settlement or ground subsidence of the clay layer at the top of the karst underground river due to the vacuum extraction, and the building foundation is damaged, so as to protect the building foundation of the karst underground river; Step S5, an earth-rock dam is arranged around the engineering area, so as to prevent the confined water head of the karst underground river from flowing back and submerging the engineering area during the high water level period after the reservoir is filled with water; Step S4 specifically comprises: when the underground water in the karst underground river system is discharged into the surface river after accepting the atmospheric precipitation infiltration in the mountain depression, and the reservoir connected with the karst underground river system is filled with water to a depth H1=61m, the confined water head at the building foundation is higher than the bottom surface of the clay layer, at this time, the pressure head acts on the building foundation to generate a floating force, which causes the clay layer at the top of the karst underground river to be broken to form a sudden gushing, and the building foundation is damaged due to the bulging and cracking of the foundation, so that the pressure relief well is arranged at the karst underground river under the building foundation to overflow the underground water of the karst underground river system in a self-flowing manner, and then the underground water is discharged through the surface drainage ditch, so that the pressure of the karst confined water head in the engineering area is reduced, the floating force of the karst confined water head on the clay layer at the top of the karst underground river is reduced, and the damage of the building foundation due to the sudden gushing is avoided; Step S4 specifically comprises: when the water level of the reservoir connected with the karst underground river system is suddenly lowered to a depth H2=40m, the underground water level in the karst underground river system is suddenly lowered, at this time, part of the karst cave under the building foundation is in a vacuum state, the building foundation is compressed under the action of the vacuum, the clay layer at the top of the karst underground river is unevenly settled, and the water level of the reservoir is frequently raised and lowered during the operation of the reservoir, the underground water level generates an alternating pulsating pressure on the foundation, which causes the engineering geological conditions of the floating force of the foundation and the karst collapse, so that the pressure relief well is used to supply air to the karst cave under the building foundation in a vacuum state, so as to avoid the uneven settlement or ground subsidence of the clay layer at the top of the karst underground river due to the vacuum extraction, and the building foundation is damaged, so as to protect the building foundation of the karst underground river.

2. The method for protecting the foundation of a reservoir on a karst underground river in a low-lying area around the reservoir according to claim 1, characterized in that, The step S2 adopts a percussion drilling hole process to prepare the pressure relief well and punch through the karst cave roof, and ensure that the well position is located on the karst underground river system; wherein, the pressure relief well comprises a wellhead part and a through-hole part, one end of the through-hole part communicates with the wellhead part, and the other end communicates with the karst cave of the karst underground river through the clay layer; the diameter of the through-hole part located in the clay layer is 80 cm, which is protected by a steel pipe casing with a wall thickness of 1 cm, and the diameter of the through-hole part located in the karst cave is 60 cm; the cross-sectional shape of the wellhead part is U-shaped, the width of the wellhead part on both sides is 210 cm, the height is 160 cm, the side wall thickness of the wellhead part is 30 cm, the bottom thickness is 20 cm, and the bottom of the wellhead part is further provided with a 10 cm thick sand cushion layer.

3. The method for protecting the foundation of a building on a karst underground river in a low-lying area around a reservoir according to claim 2, characterized in that, The number of the pressure relief wells in the step S2 is 14.

4. The method for protecting the foundation of a reservoir on a karst underground river in a low-lying area around the reservoir according to claim 1, characterized in that, The channel cross-sectional size width-height ratio of the surface open channel arranged around the pressure relief well in the step S3 is set to 1:1, the preset longitudinal slope is 0.2%, the C20 concrete is cast in situ, and a P-shaped rubber water stop is arranged every 5 m to ensure that the wellhead of the pressure relief well is 30 cm higher than the bottom of the drainage channel, so as to prevent backflow into the pressure relief well, so that the groundwater pressure flow of the karst underground river system passes through the surface open channel to be discharged, so as to reduce the pressure head of the karst underground river and reduce the damage of the float force to the foundation.

5. The method for protecting the foundation of a building on a karst underground river in a low-lying area around a reservoir according to claim 1, characterized in that, The step S5 specifically comprises: arranging the earth-rock dam embankment around the building for protecting the overall safety of the site area, the top of the earth-rock dam embankment is 3-5 m higher than the high water level, the slope ratio of the waterproof surface and the backwater surface of the earth-rock dam embankment is set to 1:2, the embankment is filled with clay, the compaction degree is ≥0.9, each filling layer is 30 cm, and the ground bearing capacity is 150 kPa, which prevents the karst confined water from backflowing and flooding the engineering area after the reservoir is impounded.

Images