Tianjin Economic and Technological Development Zone sewage treatment plant is to improve the investment environment and the establishment of environmental protection infrastructure projects, the sewage pump room was built in the former is known as salt Wang Zi sea sedimentation tank, about 2m below the surface of the earth fill for soil, and then All of them are sediment-shaped flow-like silt for many years. The underground pumping station of the pump house is 13m in depth with a convex shape. Due to the arrangement of substations, dehumidifier rooms and mud cake storage pits nearby, it is impossible to adopt the open-cut foundation pit or deep foundation maintenance structure and design it as a polygonal caisson structure. It is very difficult to control the speed and direction of caisson sinking, so it has not yet been solved so far Construction problems. In accordance with the design intent, the three stations of the 18th Bureau of China Railway Construction contracting for this project are to prefabricate the DJM piles at the foot of the sink shaft to form a continuous diaphragm pylon underground continuous wall. For the shaft wall of the sunk shaft, Supporting the wall, the process of sunk well sinking in the silt becomes a controllable process and successfully solves this construction problem.
1 construction preparation
1.1 excavation fill, reduce the initial Shen elevation
According to the geology of the caverns, in order to ensure the balanced sinking of the caisson in the initial stage, excavation of the artificial fill layer, setting the caisson precasting and the initial settlement elevation to 0.48m, can create two favorable conditions: First, due to the initial sediment silt, its moisture content and bearing capacity uniform, easy to calm; Second, the total subsidence of caisson to reduce 2.5m, the upper third 0.5m caisson can not be used as caisson construction, and After pouring in the caisson back cover, so that not only reduces the caisson weight (still able to meet the sinking weight requirements), but also shorten the sinking depth, serve two purposes.
1.2 powder spray pile construction
Dusted powder injection pile, reinforcing the soft soil at the foot of the caisson so that the caisson sinks under the support of the continuous diaphragm wall formed by continuous powder injection pile. DJM construction should pay attention to the following points:
â‘ The position should be accurate. The outer edge of pile should be tangent to the outer edge of the shaft wall, and should not be exposed to avoid the cement soil from squeezing to the outside of the shaft wall when the caisson sinks and loses its supporting function.
â‘¡ pile bottom should be deep caisson edge design elevation 16m.
â‘¢ outer ring pile at the end of the foot and the pile top 1.0m range of cement 10% (according to the weight of the pile), the remaining pile 7%.
â‘£ The inner circle pile cement volume is 10%.
⑤ inner and outer two rows of piles spacing 10cm, in order to ensure excavation of the inner side of the pile does not hurt outside the pile.
2 caisson prefabrication
2.1 Prefabricated site layout
As the shaft wall is built together pump wall pool, so choose the pump pool position for caisson prefabricated position. In order to facilitate the construction and consider the need of concrete pouring in the caisson support wall, the surface of the soft soil in the site is compacted with 15cm and 8% lime soil, and the bottom of the support wall is made another 10 limestone platform.
2.2 prefabricated way
The original design of the caisson is three sections. After the topsoil is dug, the elevation of the ground is exactly the top elevation of the caisson of the second section. Therefore, the third section of the caisson is no longer regarded as the caisson structure, while the two caisson sinks are in place Then pick up. In order to reduce the weight of the first pouring concrete, to avoid excessive sinking and uneven settlement caused by concrete cracking, the first section of concrete pouring completed in two, the first casting edge (pouring 1.0m) until the concrete strength to design strength Over 70%, pour the remaining part and construct the second section in turn. Two sinkholes concrete all completed, a sink in place.
2.3 blade foot mold and support the bottom mold
According to the preliminary design, two rows of DJM piles are used to reinforce the soft soil under the blade foot. The original ground is muddy clay, the allowable bearing capacity is 80kPa, the cement content is 10%, the unconfined compressive strength is up to 600kPa for 7d, and the compressive strength is up to 800 ~ 1000kPa for 28d. The total amount of caisson concrete is 444m3, according to the capacity of 2.5t / m3, the total weight is 3610t. The sinkhole bottom area is 44.88m2. Therefore, when the sole surface of the blade edge is used as the support surface, the bearing load is 804 kPa.
According to the above calculation, and considering the error of DJM construction, the sinking edge is placed on the top of DJM pile and the bearing capacity can not be fully satisfied
However, during the prefabrication of a caisson, the edge of the blade edge has not been carried yet, so a supporting bottom mold is added at the bottom of the supporting wall to share the weight of part of the caisson.
3 caisson sinking
3.1 Preparation
The caisson must begin to sink after the concrete strength has reached the design strength, and the following preparatory work before sinking:
â‘ Wells wall painting observation signs, set the standard observation points in the four corners of the caisson to observe the amount of subsidence and balance; set at the central axis of the vertical line to observe the caisson displacement and balance.
â‘¡ remove the module.
â‘¢ dig the top layer of dirt
After the bottom wall of the supporting wall is removed, the caisson sinks slightly, but then the side of the edge of the blade resists the load and the caisson sinks.
3.2 sinking coefficient calculation
Subsidence factor formula:
K = Q / (f · h · L)> 1 (1)
Where? Q? - Caisson gravity gravity
? f? - coefficient of friction, soft soil 9.8 ~ 11.76 kN / m2
h? - Maximum sinking depth
L? - perimeter of caisson
Friction coefficient to take the maximum of soft soil, the general structure of caisson from the gravity subsidence coefficient can still reach 3.0, not to mention the silt, no stagnation problem. The problem is that when the sinking depth reaches the requirement, it will still sink more than that, so control measures must be taken.
3.3 DJM pile wall control sinking mechanism
â‘ guide and prevent sudden sink, influx
According to the preliminary design concept, under the edge of the wellbore and under the blade edge, pre-punching two rows of DJM piles to reinforce the formation is a comprehensive measure to prevent the sunk sinking sinking, settling speed too fast and the infiltration of soil. The principle of action is as follows: First, the DJM pile forms a continuous wall of cement land, which is a bearing wall enclosed in the silt for the caisson. The entire caisson sinking process is the excavation process of the bearing wall. This caisson sinking speed and smoothness can be completely removed by manual dredging method to control. Second, the implementation of open caisson in silty soft soil, in the process of digging soil subsidence prone to flooding, that is, flow-shaped plastic silo outside the well due to large differences in soil surface when the instability and slippery , Sliding arc edge into the hole at the foot of the foothole, so that the hole is only unearthed but the soil elevation does not drop, and the soil elevation subsidence, easily lead to uneven soil pressure, resulting caisson inclination. Pre-beating the blade at the foot of the first two lance spray pile, forming a 1.2m thick cement soil wall to prevent the formation of soil outside the wellbore sliding wall sliding inrush.
â‘¡ control sunk sinking design depth
When the caisson sinks to the design elevation and the soil below the bottom of blade edge is still silt, it can be known from the subsidence coefficient that the carrying capacity of the silt is not enough and will sink more than that of the underground continuous wall , Which can make the bottom surface of sunken shaft edge sink to the designed elevation and fall on the top surface of the pile of underground continuous wall of DJM, which solves the problem of controlling sunk shaft sinking depth. When sinking need sinking, only need to chisel pile head, sink well to overcome the friction and support strength of the soil with its own weight and sink, once the blade edge to the pile head to stop sinking. According to the above inference, when the caisson is about to reach the design elevation (with a difference of 50 ~ 70cm), the DJM will be pre-cut at the designed elevation to control the caisson accurately and stop sinking.
3.4 Excavation method
Surface limed hard crust artificial excavation, the rest of the mud using high pressure water gun mud, a layer of 50cm, layer by layer stripping down. In order to prevent the support wall and the ground beam to bear the bending moment and catch the mud, the soil should be washed into the pot bottom type (middle low, four weeks high). At the bottom of the sinkhole, each exposed layer of powder jetting pile is exposed. The inner layer of powder jetting pile is dug and cut along the same height first, and the outer layer of powder jetting pile is dredged intermittently so that the remaining powder jetting pile is crushed by a caisson and manually removed .
3.5 caisson correction
Due to the guiding effect of DJM, the caisson generally does not have a large deflection. To ensure that the caisson in-position error is within the allowable range of the standard, the following control measures are taken:
a. With the sinking sinking level and the central axis monitoring, at any time to adjust the position and height of dredging pile.
b. When the sinkhole sinks close to the place, stop the chiseling, dig the soil for 24h and observe the settlement. If there is no obvious settlement, it can sink once and stop taking other measures to stop the sinking. If there is obvious settlement, the reason should be found and increased Stop the measures.
c. When the caisson is close to its place, if the axial displacement or inclination exceeds the allowable range, unilateral compaction and filling, unilateral excavation and deloading, counterweight and other means can be used to correct it.
4 sinkhole back cover
Caisson sinking is completed, the deviation should be consistent with the norms:
â‘ axis displacement is not greater than 1% well depth;
â‘¡ elevation: + 40mm, -60mm;
③ inclination ≯ 0.7% well depth.
Caisson in place 2 ~ 3d, the edge has been a stable fall in the top powder spray pile, you can carry out the caisson backfill. In order to avoid the buildup of groundwater to form a large buoyancy, the top-sealing concrete can be divided into 2 to 3 holes on the bottom plate, and the seepage pipe buried in the well. After the pump tank structure is completed, the wellhead is blocked.
5 Conclusion
Due to the poor bearing capacity of the strata and the small friction coefficient, it is very easy to appear sudden sinking, infilling, rapid sinking and over-sinking displacements and tilting in the sinking silt strata. And other phenomena, it is difficult to control. The design and construction of the caisson, make full use of the characteristics of cement soil, pre-fired at the foot of sinking play two rows of powder spray pile in the soft soil layer to form an appropriate strength of the continuous bearing wall body, sinking process In the form of a reliable guide. Through the sub-section, sub-site chiseling powder pile head to adjust the supporting force, accurate control of caisson attitude and sinking speed, depth.
Through the above construction process can be seen in similar soil caisson design and construction, you can change the edge area and the length of the powder injection pile, diameter, strength (by adjusting the amount of dust to achieve) and other means to adjust the bearing capacity, Various methods, simple process, low cost, is a successful construction process.
1 construction preparation
1.1 excavation fill, reduce the initial Shen elevation
According to the geology of the caverns, in order to ensure the balanced sinking of the caisson in the initial stage, excavation of the artificial fill layer, setting the caisson precasting and the initial settlement elevation to 0.48m, can create two favorable conditions: First, due to the initial sediment silt, its moisture content and bearing capacity uniform, easy to calm; Second, the total subsidence of caisson to reduce 2.5m, the upper third 0.5m caisson can not be used as caisson construction, and After pouring in the caisson back cover, so that not only reduces the caisson weight (still able to meet the sinking weight requirements), but also shorten the sinking depth, serve two purposes.
1.2 powder spray pile construction
Dusted powder injection pile, reinforcing the soft soil at the foot of the caisson so that the caisson sinks under the support of the continuous diaphragm wall formed by continuous powder injection pile. DJM construction should pay attention to the following points:
â‘ The position should be accurate. The outer edge of pile should be tangent to the outer edge of the shaft wall, and should not be exposed to avoid the cement soil from squeezing to the outside of the shaft wall when the caisson sinks and loses its supporting function.
â‘¡ pile bottom should be deep caisson edge design elevation 16m.
â‘¢ outer ring pile at the end of the foot and the pile top 1.0m range of cement 10% (according to the weight of the pile), the remaining pile 7%.
â‘£ The inner circle pile cement volume is 10%.
⑤ inner and outer two rows of piles spacing 10cm, in order to ensure excavation of the inner side of the pile does not hurt outside the pile.
2 caisson prefabrication
2.1 Prefabricated site layout
As the shaft wall is built together pump wall pool, so choose the pump pool position for caisson prefabricated position. In order to facilitate the construction and consider the need of concrete pouring in the caisson support wall, the surface of the soft soil in the site is compacted with 15cm and 8% lime soil, and the bottom of the support wall is made another 10 limestone platform.
2.2 prefabricated way
The original design of the caisson is three sections. After the topsoil is dug, the elevation of the ground is exactly the top elevation of the caisson of the second section. Therefore, the third section of the caisson is no longer regarded as the caisson structure, while the two caisson sinks are in place Then pick up. In order to reduce the weight of the first pouring concrete, to avoid excessive sinking and uneven settlement caused by concrete cracking, the first section of concrete pouring completed in two, the first casting edge (pouring 1.0m) until the concrete strength to design strength Over 70%, pour the remaining part and construct the second section in turn. Two sinkholes concrete all completed, a sink in place.
2.3 blade foot mold and support the bottom mold
According to the preliminary design, two rows of DJM piles are used to reinforce the soft soil under the blade foot. The original ground is muddy clay, the allowable bearing capacity is 80kPa, the cement content is 10%, the unconfined compressive strength is up to 600kPa for 7d, and the compressive strength is up to 800 ~ 1000kPa for 28d. The total amount of caisson concrete is 444m3, according to the capacity of 2.5t / m3, the total weight is 3610t. The sinkhole bottom area is 44.88m2. Therefore, when the sole surface of the blade edge is used as the support surface, the bearing load is 804 kPa.
According to the above calculation, and considering the error of DJM construction, the sinking edge is placed on the top of DJM pile and the bearing capacity can not be fully satisfied
However, during the prefabrication of a caisson, the edge of the blade edge has not been carried yet, so a supporting bottom mold is added at the bottom of the supporting wall to share the weight of part of the caisson.
3 caisson sinking
3.1 Preparation
The caisson must begin to sink after the concrete strength has reached the design strength, and the following preparatory work before sinking:
â‘ Wells wall painting observation signs, set the standard observation points in the four corners of the caisson to observe the amount of subsidence and balance; set at the central axis of the vertical line to observe the caisson displacement and balance.
â‘¡ remove the module.
â‘¢ dig the top layer of dirt
After the bottom wall of the supporting wall is removed, the caisson sinks slightly, but then the side of the edge of the blade resists the load and the caisson sinks.
3.2 sinking coefficient calculation
Subsidence factor formula:
K = Q / (f · h · L)> 1 (1)
Where? Q? - Caisson gravity gravity
? f? - coefficient of friction, soft soil 9.8 ~ 11.76 kN / m2
h? - Maximum sinking depth
L? - perimeter of caisson
Friction coefficient to take the maximum of soft soil, the general structure of caisson from the gravity subsidence coefficient can still reach 3.0, not to mention the silt, no stagnation problem. The problem is that when the sinking depth reaches the requirement, it will still sink more than that, so control measures must be taken.
3.3 DJM pile wall control sinking mechanism
â‘ guide and prevent sudden sink, influx
According to the preliminary design concept, under the edge of the wellbore and under the blade edge, pre-punching two rows of DJM piles to reinforce the formation is a comprehensive measure to prevent the sunk sinking sinking, settling speed too fast and the infiltration of soil. The principle of action is as follows: First, the DJM pile forms a continuous wall of cement land, which is a bearing wall enclosed in the silt for the caisson. The entire caisson sinking process is the excavation process of the bearing wall. This caisson sinking speed and smoothness can be completely removed by manual dredging method to control. Second, the implementation of open caisson in silty soft soil, in the process of digging soil subsidence prone to flooding, that is, flow-shaped plastic silo outside the well due to large differences in soil surface when the instability and slippery , Sliding arc edge into the hole at the foot of the foothole, so that the hole is only unearthed but the soil elevation does not drop, and the soil elevation subsidence, easily lead to uneven soil pressure, resulting caisson inclination. Pre-beating the blade at the foot of the first two lance spray pile, forming a 1.2m thick cement soil wall to prevent the formation of soil outside the wellbore sliding wall sliding inrush.
â‘¡ control sunk sinking design depth
When the caisson sinks to the design elevation and the soil below the bottom of blade edge is still silt, it can be known from the subsidence coefficient that the carrying capacity of the silt is not enough and will sink more than that of the underground continuous wall , Which can make the bottom surface of sunken shaft edge sink to the designed elevation and fall on the top surface of the pile of underground continuous wall of DJM, which solves the problem of controlling sunk shaft sinking depth. When sinking need sinking, only need to chisel pile head, sink well to overcome the friction and support strength of the soil with its own weight and sink, once the blade edge to the pile head to stop sinking. According to the above inference, when the caisson is about to reach the design elevation (with a difference of 50 ~ 70cm), the DJM will be pre-cut at the designed elevation to control the caisson accurately and stop sinking.
3.4 Excavation method
Surface limed hard crust artificial excavation, the rest of the mud using high pressure water gun mud, a layer of 50cm, layer by layer stripping down. In order to prevent the support wall and the ground beam to bear the bending moment and catch the mud, the soil should be washed into the pot bottom type (middle low, four weeks high). At the bottom of the sinkhole, each exposed layer of powder jetting pile is exposed. The inner layer of powder jetting pile is dug and cut along the same height first, and the outer layer of powder jetting pile is dredged intermittently so that the remaining powder jetting pile is crushed by a caisson and manually removed .
3.5 caisson correction
Due to the guiding effect of DJM, the caisson generally does not have a large deflection. To ensure that the caisson in-position error is within the allowable range of the standard, the following control measures are taken:
a. With the sinking sinking level and the central axis monitoring, at any time to adjust the position and height of dredging pile.
b. When the sinkhole sinks close to the place, stop the chiseling, dig the soil for 24h and observe the settlement. If there is no obvious settlement, it can sink once and stop taking other measures to stop the sinking. If there is obvious settlement, the reason should be found and increased Stop the measures.
c. When the caisson is close to its place, if the axial displacement or inclination exceeds the allowable range, unilateral compaction and filling, unilateral excavation and deloading, counterweight and other means can be used to correct it.
4 sinkhole back cover
Caisson sinking is completed, the deviation should be consistent with the norms:
â‘ axis displacement is not greater than 1% well depth;
â‘¡ elevation: + 40mm, -60mm;
③ inclination ≯ 0.7% well depth.
Caisson in place 2 ~ 3d, the edge has been a stable fall in the top powder spray pile, you can carry out the caisson backfill. In order to avoid the buildup of groundwater to form a large buoyancy, the top-sealing concrete can be divided into 2 to 3 holes on the bottom plate, and the seepage pipe buried in the well. After the pump tank structure is completed, the wellhead is blocked.
5 Conclusion
Due to the poor bearing capacity of the strata and the small friction coefficient, it is very easy to appear sudden sinking, infilling, rapid sinking and over-sinking displacements and tilting in the sinking silt strata. And other phenomena, it is difficult to control. The design and construction of the caisson, make full use of the characteristics of cement soil, pre-fired at the foot of sinking play two rows of powder spray pile in the soft soil layer to form an appropriate strength of the continuous bearing wall body, sinking process In the form of a reliable guide. Through the sub-section, sub-site chiseling powder pile head to adjust the supporting force, accurate control of caisson attitude and sinking speed, depth.
Through the above construction process can be seen in similar soil caisson design and construction, you can change the edge area and the length of the powder injection pile, diameter, strength (by adjusting the amount of dust to achieve) and other means to adjust the bearing capacity, Various methods, simple process, low cost, is a successful construction process.
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