With the continuous development of urban construction, a large number of high-rise buildings and municipal works (roads, bridges, subways, etc. ) has appeared, and underground space has been continuously developed and utilized. A large number of deep foundation pit support and construction has become a hot and difficult point in current foundation engineering. Deep foundation pit engineering is a new type of geotechnical engineering with the development of urban construction in China, and it has become one of the main contents of urban geotechnical engineering. With the in-depth understanding of these problems and countermeasures, more and more new technologies have been applied in deep foundation pit engineering.
I. Overview of the Project
A bridge with a span of 400m is divided into two parts. The single bridge is 26.5 m wide, and the substructure is U-shaped abutment, thin-walled vase slab pier and bored pile foundation. The river channel where this project is located is a three-level navigable river channel, and the included angle between the normal and the current is 9.8. The clear navigable height is 12m, the clear width is l20m, and the maximum navigable water level at the bridge site is 4.744rn ... The piers are located in the river with a water depth of over 9m and a pile diameter of 2.3m Each pier has 12 piles, with a pile spacing of 4.6rn, a pile length of 65.5m and a cap size of12.90m×. According to geological data, from top to bottom, it is muddy silt (9.553rr0), muddy clay (7.7In), coarse sand (6.2fro), completely weathered rock zone (32.7 m), strongly weathered rock zone (6.0 m) and weakly weathered rock zone (10.3 m).
Second, the engineering geological and hydrogeological conditions
The bridge is mainly built on the Hanshui River, across Hankou and Hanyang, and its construction site stratum is:
(1) Artificial fill: 0.3——0.5m thick, relatively loose.
(2) Quaternary Holocene alluvium is divided into the following sub-layers from top to bottom: ① cohesive soil layer; ② Muddy soil layer; ③ Sub-clay mixed with clay silt and silt; ④ Silty sand layer; ⑤ Fine sand layer; ⑥ Medium sand layer; ⑦ Round gravel layer. Among them, the total thickness of ①-③ layers is 1 1.24-22.23 m, and the cohesive soil and loam are mostly soft plastic and flowing plastic, and the silt layer is extremely loose; ④ The layer thickness is 4.85-17.22m, which is loose and dense. ⑤ The total thickness of this layer and below is18.66-31.33m, which is dense and dense. Fully weathered and strongly weathered argillaceous sandstone, conglomerate and glutenite with a thickness of 2.85-9.3m.. Clayey silt and loam belong to weak permeable relative water-resisting layer; Silt, fine sand and round gravel are strongly permeable layers; Sandstone and conglomerate are weakly permeable-impermeable layers. The bedrock fissure water has a certain bearing capacity.
The site conditions of the project are relatively tight, and there are roads on both sides of the Hanshui River. The edge of the foundation pit is 14m away from the center of Xining Road in the north, adjacent to a 5-storey house. Distance from the south road center 15m, adjacent to the residential area with 5-7 floors; All roads are densely covered with various pipes.
Third, the construction technology of deep foundation pit
3. 1 process
Engineering and supporting pile construction → Lower bearing well dewatering → Blade excavation to box beam bottom blade → Cap beam construction → Earthwork excavation to the first ring beam bottom → Trestle construction → First ring beam bottom construction → Maintenance → Earthwork excavation to the second ring beam bottom → Second ring beam application → Maintenance → Earthwork excavation to the pit bottom → Trench cleaning → Cushion → Foundation and engineering pile head treatment → Dewatering well dewatering → Floor construction. Backfilling and replacing the supporting belt → dismantling the secondary beam → constructing the underground two-story structure → backfilling the soil between the outer soil and the bored pile → dismantling the first ring beam → constructing the underground one-story structure → backfilling. Precipitation and foundation pit splash monitoring are the whole construction process.
3.2 Foundation pit support
The design of deep foundation pit support in this project must achieve the following objectives: ① to provide enough working face for foundation pit construction; (2) It does not affect the normal operation of the surrounding environment of the foundation pit; (3) Rapid and economical, especially the deformation of the supporting structure, should not cause serious cracking of the soil at the top of the slope, pose no threat to nearby houses and underground pipelines, and do not affect the normal use.
The piling method is interval jumping, and the pile quality is strictly controlled. When pouring concrete underwater, ensure that the quality of pile concrete reaches the design strength. The horizontal ring beam support formwork is made of wood pattern, which is pre-processed in the processing factory and enlarged according to the drawing size to ensure the accurate processing size. The transverse steel pipe of the main keel used for the side formwork of the ring beam is bent and formed according to the radius of the ring beam. When the radius of ring beam is large, the main reinforcement adopts straight reinforcement; The small ring beam has a small radius and the main reinforcement is bent and formed. The closed stirrup at the tangent of the large ring beam and the small ring beam is placed in advance, which avoids the situation that the main reinforcement of the two ring beams cannot be placed after binding. Concrete operation adopts the whole continuous casting of each layer of horizontal support system, leaving no construction joints. After pouring, cover and water conservation in time.
3.3 Foundation pit dewatering
Before foundation pit excavation, the underground water level in the pit should be reduced and eliminated, so that the soil in the pit can reach a certain strength through drainage consolidation during foundation pit excavation, improve the horizontal resistance of the soil in the pit and reduce the deformation of the foundation pit; Enhance the stability of the bottom of the foundation pit and reduce the uplift of the soil at the bottom of the pit. The groundwater in the population structure mainly includes: (1) the upper layer is stagnant water, which is located 3 ~ 4 m below the ground, and the aquifer is artificial fill layer and silt layer with weak permeability; (2) Diving, located 8-9m below the ground. Aquifer is silty clay layer and silty soil layer, with general water permeability; (3) Confined water, located below 12m underground. The aquifer is composed of clay layer, silt layer, medium-coarse sand layer and gravel layer, which has strong water permeability. Dewatering of foundation pit adopts the method of tube well+seepage well, and dewatering begins 20 days before foundation pit excavation. During precipitation, observe and monitor the safety of adjacent buildings and underground pipelines. At the same time, set up a recharge well on the ground outside the pit, and take recharge measures when necessary to ensure the safety of surrounding buildings.
3.4 Foundation pit enclosure construction
Around the foundation pit, the retaining structure of concrete cast-in-place piles with a diameter of 800mm is set, and the pile spacing is 1.0- 1.2m, and the corner parts are strengthened locally. The retaining pile is drilled by rotary drilling rig, and concrete is poured underwater by conduit method. During drilling construction, in order to reduce the interference to adjacent piles and ensure the quality of finished piles, the method of drilling every three piles is adopted (that is, one pile is constructed every three piles). The crown beam connects the retaining piles into a whole bent frame, so that all retaining piles form a * * * stress system to resist the lateral load of external soil or surrounding rock. After the construction of retaining piles is completed, the crown beam should be excavated immediately and the concrete at the top of the piles should be chiseled and cleaned. The main reinforcement of the retaining pile is anchored on the crown beam, and the crown beam is cast-in-situ with the same grade of concrete as the retaining pile, and embedded steel plates are installed at the same time to meet the installation requirements of the lower steel support. After earthwork excavation, shotcrete and anchor rod are used between retaining piles to prevent soil from falling off between piles.
3.5 Lifting, insertion and folding of steel sheet piles
After the steel sheet piles pass the inspection, they will be transported to the dock by two groups of flat cars. Stack up to four layers according to the pile insertion sequence, and the height difference between each layer separated by stow-wood shall not be greater than 10mm, and the center lines of the upper and lower stow-wood shall be on the same vertical line, and the allowable error shall not be greater than 20 mm. For the inserted steel sheet pile, the elevated cableway shall be used for horizontal and vertical transportation, and the steel sheet pile shall be transported to the designated position, and then the steel sheet pile shall be lifted with two hooks to make it vertical, and the small hook shall be removed and moved to the insertion position. Before hoisting, the lock shall be embedded with butter asphalt mixture. The arc clamp used to fasten the steel plate should be loosened one by one when inserted into the lock. Steel sheet piles are inserted into the bottom or the whole cofferdam block by block (group) (rectangular cofferdam can be one side), and then inserted into the cofferdam block by block (group). Generally, the upstream edge of rectangular cofferdam is inserted first, and the downstream is closed. The circular steel sheet pile cofferdam has the following three insertion sequences, as shown in figure 1
As can be seen from the figure, A and B have one less closing point than C, and the cumulative error of B is greater than C. Both A and B may be affected by backflow before closing, which makes the pile foot move out and it is difficult to close, and C is less affected by backflow, so C-shaped insertion is adopted where the flow velocity is high. In the A-type insertion mode, they are inserted symmetrically from both sides to the downstream, and the number of insertions on both sides is roughly equal, with a difference of at most 8 groups. When the verticality of the steel sheet pile is good, the pile is driven to the required depth at one time; When the verticality is not good, it will be driven in two times, that is, all piles will be driven to about half depth first, and then driven to the required depth for the second time.
3.6 pumping plugging
After the steel sheet pile is inserted, it can be excavated with a pump. Design supporting cofferdam, support first and then pump water, and check whether each node is tight and whether the wedge between sheet pile and guide frame is tight, so as to prevent accidents caused by pumping water. The pumping speed should not be too fast, and the change of cofferdam should be observed at any time. When the lock is not tightly locked and leaks, use cotton wool to plug it in, and at the same time, remove a large number of sawdust or chaff at the leaking joint, so that it can be taken to the leaking place by water to block itself. When the pile foot leaks, the pile foot is filled with soil bag and underwater concrete. If the leakage of the pile foot is caused by the water permeability of the river bed, cement mortar is injected into the permeable layer or underwater concrete is used to seal the water.
3.7 Foundation pit excavation and lining construction
The lining is a reinforced concrete ring structure, and the top-down construction method is adopted. Lining thickness varies. The excavation of foundation pit shall be carried out in accordance with the principle of "symmetry, zoning, balance and time limit", and the layering thickness shall be the same as the layering height of lining (3m/ layer). "Island excavation" is adopted, that is, the earthwork in the surrounding area of 6m is excavated symmetrically first, and then the earthwork in the central area is excavated. According to the construction sequence of lining segments, the excavation sequence of each region is determined. Construction of lining system shall be carried out immediately after earthwork excavation around, and earthwork excavation and lining construction in the middle area shall be carried out at the same time. After the strength of the upper lining concrete reaches 80% of the design strength, the next layer of surrounding earthwork will be excavated, and so on, until the basement. The foundation pit is excavated step by step from top to bottom.
In order to make the foundation pit excavation have good dry construction conditions, before each layer of earthwork excavation, start the dewatering well in the pit to drain the groundwater in the pit to ensure that the water level in the pit is 0.5m lower than the excavation surface. In the process of excavation, information construction technology is adopted, and the monitoring data are analyzed by Tongji University in time, and the internal force of supporting structure and equivalent mechanical parameters of soil are obtained. According to the internal force and deformation of the structure, the safety state of the foundation pit is judged. At the same time, according to the equivalent mechanical parameters of soil, the stress and deformation of the structure in the next construction are predicted. In addition, multi-step rolling prediction method can be used to predict the future deformation of foundation pit, and suggestions for adjusting construction parameters are put forward according to the dangerous signals reflected by construction to ensure the safety of foundation pit excavation. Practice has proved that information construction is a powerful means for construction and design units to know the structural safety status in time, adjust the construction technology in time and ensure the safety of the project.
3.8 construction matters needing attention
(1) The construction precipitation should not be too fast, and the monitoring of surrounding buildings, pipelines and surface subsidence should be strengthened during the precipitation process. Earthwork excavation must be carried out under the guidance of water level monitoring.
(2) Pay attention to the water management around the foundation pit during construction, strengthen the observation of pipeline leakage, and cut off the water supply channels around the foundation pit. If there is leakage on the wall of the pay-off pit, find out the reason and avoid blind grouting.
(3) During the construction period, the stacking load around the foundation pit should be strictly controlled. Stacking is strictly prohibited within 2m around the foundation pit, and the stacking load should be controlled within 1.4 times of the pit depth around the foundation pit.
(4) Earthwork excavation must be carried out simultaneously with support erection, and excavation should be carried out in layers according to design requirements. Overexcavation and underexcavation are strictly prohibited. The length of the excavation section must be reasonably determined according to the depth and slope of the foundation pit, and should not be too long. When the foundation pit is dug to the design elevation, the cushion concrete must be poured immediately to further reduce the deformation value of the foundation pit. The concrete of the bottom slab must be completed within 5-7 days, and the construction of corresponding structural layers should be kept up in time, so as to establish a permanent stress balance system and fundamentally control the deformation of the foundation pit.
(5) Strengthen the construction monitoring, grasp the stability, safety and supporting effect of the slope, so as to adjust the design parameters and the construction scheme of the foundation pit at any time, and ensure the safety and reliability of the foundation pit.
4. Summary
In the construction of deep foundation pit of this project, reasonable technical measures and strict construction management are adopted, and construction monitoring is carried out by means of information technology, which has achieved good results in the construction and ensured the smooth completion of deep foundation pit and the safety of surrounding environment. From the analysis of construction situation, compared with other foundation pit support methods, it not only saves the cost greatly, but also has obvious work efficiency. The steel sheet pile support technology is mature, simple in process, short in construction period, low in cost and civilized, and has the characteristics of high strength, tight combination, good water sealing, high speed, economy and rationality.
Quaternary Holocene alluvium is divided into the following sub-layers from top to bottom: ① cohesive soil layer; ② Muddy soil layer; ③ Sub-clay mixed with clay silt and silt; ④ Silty sand layer; ⑤ Fine sand layer; ⑥ Medium sand layer; ⑦ Round gravel layer. Among them, the total thickness of ①-③ layers is 1 1.24-22.23 m, and the cohesive soil and loam are mostly soft plastic and flowing plastic, and the silt layer is extremely loose; ④ The layer thickness is 4.85-17.22m, which is loose and dense. ⑤ The total thickness of this layer and below is18.66-31.33m, which is dense and dense. Fully weathered and strongly weathered argillaceous sandstone, conglomerate and glutenite with a thickness of 2.85-9.3m.. Clayey silt and loam belong to weak permeable relative water-resisting layer; Silt, fine sand and round gravel are strongly permeable layers; Sandstone and conglomerate are weakly permeable-impermeable layers. The bedrock fissure water has a certain bearing capacity.
The detailed information of "Application of Deep Foundation Pit Technology in Bridge Construction" can be found in the link of Zhong Da Consulting and Design, and all relevant building construction information you want is available.
For more information about project/service/procurement bidding, and to improve the winning rate, please click on the bottom of official website Customer Service for free consultation:/#/? source=bdzd