What is the supporting structure of foundation pit?
In recent years, due to the rapid development of urban subway engineering, subway stations, open excavation in some areas, etc. It also involves a large number of foundation pit projects. In the subway station with double-track crossing, the depth of foundation pit reaches 20-30m.
Water conservancy and electric power also have the problem of foundation pit excavation of underground powerhouse and underground pump house.
Both high-rise buildings and subway deep foundation pit projects are excavated in cities, and there are usually various structures around the foundation pit, such as traffic arteries, built buildings or pipelines, which involves a very important content of foundation pit excavation and needs to protect the safe use of surrounding structures.
The general foundation pit support is mostly temporary structure, which is easy to cause waste if the investment is too large, but the unsafe support structure will inevitably lead to engineering accidents.
Therefore, according to the characteristics of foundation pit engineering, how to choose the appropriate supporting structure safely and reasonably and carry out scientific design is the main content to be solved in foundation pit engineering.
The following briefly introduces the common types of supporting structures in foundation pit engineering, and the selection principles of supporting structures in foundation pit engineering under different soil conditions.
1 Types, characteristics and scope of application of foundation pit support
1. 1 slope excavation
It is suitable for the surrounding areas where the site is open and there are no important buildings around. It only requires five strict requirements of stability and displacement control, and the price is the cheapest, and the amount of backfill earthwork is large.
1.2 Deep mixing cement-soil retaining wall
Deep mixing cement-soil retaining wall is a kind of continuous overlapping cement-soil column and solid retaining wall. The deep mixing machine is used to forcibly mix the soil and input cement slurry on site.
Advantages of cement-soil retaining wall: because there is no support in the general pit, it is convenient for mechanized rapid excavation; It has dual functions of retaining soil and stopping water; Generally more economical; There is no vibration, no noise, little pollution and slight soil squeezing during construction, so it has more advantages in downtown construction.
Disadvantages of cement-soil retaining wall: first, the displacement is relatively large, especially when the length of foundation pit is long, measures such as adding pier in the middle and arching can be taken to limit the excessive displacement; Secondly, the thickness is large, and it can only be used when the red line position and surrounding environment permit. Attention should be paid to prevent the surrounding environment from being affected during the construction of cement-soil mixing piles.
1.3 high-pressure jet grouting pile
High-pressure jet grouting pile is also made of cement slurry, which is sprayed into the soil layer by rotating nozzle at high pressure, mixed with soil to form cement soil and solid, and superimposed with each other to form a pile for retaining soil and stopping water.
The construction cost of high-pressure jet grouting pile is higher than that of deep mixing cement-soil pile, but its construction equipment has compact structure, small volume, strong maneuverability, less land occupation, little vibration of construction machines and tools and low noise, and will not affect the surrounding area.
Buildings bring vibration and noise, and small spaces can be used, but a large amount of mud is discharged during construction, which is easy to cause pollution.
This method is not suitable for strata with excessive groundwater velocity, frozen soil in karst areas without filler and soil with serious cement corrosion, because the sprayed slurry cannot solidify around grouting pipe.
1.4 channel steel sheet pile
This is a simple steel sheet pile enclosure wall, which consists of front and rear buckles with overlapping or side-by-side channel steel.
The channel steel is 6 ~ 8m long, and the model is determined by calculation.
Its characteristics are: channel steel has good durability and can be pulled out for recycling after foundation pit backfilling; Convenient construction and short construction period; The water and fine particles in the soil can't be blocked, and water isolation or precipitation measures should be taken in areas with high groundwater level; The bending capacity is weak, and it is mostly used for shallow foundation pits or grooves with a depth of ≤4m, and the top should be provided with supports or anchor rods; The supporting rigidity is small, and the deformation after excavation is large.
1.5 reinforced concrete sheet pile
Reinforced concrete sheet pile is widely used in foundation pit engineering because of its simple construction and short field operation period. However, due to the hammering method, the vibration and noise of reinforced concrete sheet piles are large, and the soil squeezing is also serious when driving piles, which is limited in urban engineering.
In addition, its production is generally prefabricated in the factory and then transported to the construction site, and the cost is slightly higher than that of cast-in-place piles.
However, due to its reasonable cross-sectional shape and reinforcement, it can be designed as required. At present, sheet piles with large thickness (for example, more than 500mm) can be built, and there are also hydraulic static pile sinking equipment, so it is still a form of supporting sheet wall in foundation pit engineering.
1.6 bored pile
Bored pile retaining wall is one of the most widely used pile types and has been widely used in China.
It is mostly used in foundation pit engineering with pit depth of 7 ~ 15m, and there are 8 ~ 9m cantilever pile retaining walls in areas with good soil quality in northern China.
The characteristics of bored pile retaining wall are: no environmental pollution such as vibration and noise during construction, no soil squeezing phenomenon, and little impact on the surrounding environment; The wall has high strength, high rigidity, good support stability and small deformation; while
When the engineering pile is also a cast-in-place pile, it can be constructed at the same time, which is beneficial to organization, convenience and short construction period; The gap between piles is easy to cause soil erosion, especially in soft clay areas with high water level. Construction measures such as grouting, cement mixing piles and jet grouting piles should be taken according to engineering conditions to solve the water retaining problem. It is suitable for soft clay and sandy soil areas, but it should be used with caution when gravel layer and pebble layer are difficult to construct. Pile-to-pile is mainly connected into a whole by pile top crown beam and purlin, which is relatively poor in integrity, so it needs special care when applied to important areas, special projects and deep foundation pits.
Author: linfan842006-3-520:2 1 Reply to this statement.
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2 Introduction to the types and design principles of foundation pit supporting structure (reproduced)
1.7 Diaphragm Wall
Usually, the thickness of continuous wall is 600mm, 800mm, 1000mm, and some are as thick as 1200mm, but it is rarely used.
Diaphragm wall is the strongest supporting type in supporting structure because of its high rigidity and good water sealing effect. It is suitable for the foundation pit with poor and complicated geological conditions, deep foundation pit and high requirements of surrounding environment, but the cost is high and special equipment is needed for construction.
1.8 Soil nailing wall
Soil nailing wall is a kind of slope stability support, which is different from passive retaining wall. It plays an active role in embedding, increasing slope stability and maintaining slope stability after foundation pit excavation.
Soil nailing wall is mainly used in areas with good soil quality and is widely used in North China and Northeast China. At present, it is also used in southern China, and some of it has been used in foundation pits with pit depth exceeding 10m. Stability and reliability, simple construction, short construction period, good effect and good economy should be actively promoted in areas with good soil quality.
1.9SMW construction method
SMW construction method, also known as rigid cement-soil mixing pile method, is to insert H-beam into cement-soil piles (mostly H-beam, but also Larsen steel sheet piles and steel pipes) to combine load bearing with seepage prevention and water retaining, making it a retaining wall with both stress and seepage prevention functions.
The characteristics of SMW support structure are: basically no noise during construction, and little impact on the surrounding environment; The structural strength is reliable, which is suitable for all occasions of cement-soil mixing piles, especially in soft strata dominated by clay and silt; Good water retention and seepage prevention performance, no need to set up another water retaining curtain; Suitable for deep foundation pit with multi-point support; This method can replace the diaphragm wall as the underground enclosure structure under certain conditions. If certain construction measures can be taken, tensile materials such as H-beam can be successfully recovered; It is much lower than the diaphragm wall, so it has great development prospects.
1. 10 summary of foundation pit support selection
Reasonable selection of foundation pit support type is the primary work of foundation pit support design, which should be comprehensively determined according to geological conditions, surrounding environment requirements, characteristics of different support types and cost.
Generally, when the geological conditions are good and the surrounding environment is relaxed, flexible support, such as soil nailing wall, can be used; When the requirements of the surrounding environment are high, the horizontal displacement should be controlled by rigid support types, such as row piles or underground continuous walls.
Similarly, for the support type, when the surrounding environment is demanding and the geological conditions are poor, the anchor rod is easy to cause the disturbance of the surrounding soil and affect the safety of the surrounding environment, so the internal support type should be adopted; When the geological conditions are particularly poor, the depth of the foundation pit is deep and the requirements for the surrounding environment are high, the underground continuous wall plus top-down method can be used as the strongest support type.
The most important thing of foundation pit support is to ensure the safety of the surrounding environment.
2. Design requirements of foundation pit support
As a structural system, foundation pit support should meet the requirements of stability and deformation, that is, the requirements of two limit States mentioned in the usual specifications, namely, the bearing capacity limit state and the normal use limit state.
The so-called ultimate state of bearing capacity means that the supporting structure is destroyed, toppled, slipped or the surrounding environment is destroyed, resulting in large-scale instability.
The general design requirements do not allow such a limit state of the supporting structure.
The limit state of normal use refers to the deformation of supporting structure caused by foundation pit excavation or excessive deformation of surrounding soil, which affects normal use, but does not cause structural instability.
Therefore, the design of foundation pit support should have enough safety factor relative to the limit state of bearing capacity, so as not to cause the instability of support, and control the displacement under the condition of ensuring no instability, so as not to affect the safe use of surrounding buildings.
Therefore, as a design calculation theory, it is necessary to calculate not only the stability of the supporting structure, but also its deformation, and control the deformation within a certain range according to the surrounding environmental conditions.
Generally speaking, the displacement control of supporting structure is mainly horizontal displacement, which is intuitive and easy to monitor.
Horizontal displacement control is related to the requirements of the surrounding environment, that is, the division of the so-called safety level of foundation pit in the usual specifications. If there are important structures around the foundation pit that need to be protected, small deformation should be controlled, which is the usual displacement requirement of the first-class foundation pit; For the surrounding open space, there is no need to protect any structures, and the displacement can be large, as long as it is theoretically stable, which is the displacement requirement of the third-level foundation pit; Between the first level and the third level, it is the displacement requirement of the second level foundation pit.
Generally, the maximum horizontal displacement of first-class foundation pit should not be greater than 30mm, and it should be less than 0.3%H, h for deep foundation pit, where h is the excavation depth.
For general foundation pit, the maximum horizontal displacement should be no more than 50 mm
Generally, there will be no obvious cracks on the ground with the maximum horizontal displacement within 30mm, and there will be obvious cracks on the ground with the maximum horizontal displacement within 40-50 mm. Therefore, the maximum horizontal displacement of foundation pit should not be greater than 50 mm, otherwise there will be obvious ground cracks and settlement, resulting in insecurity.
Generally, rigid supporting structures, such as retaining piles, diaphragm walls and internal support systems, have small displacements, which can be controlled within 30mm; For soil nailing, the geological conditions are good, and the smaller displacement, generally greater than 30 mm, can be controlled by adopting reinforcement measures such as advanced support and prestressed anchor. ..
Conclusion: Foundation pit support is a special structural mode with multiple functions.
Different supporting structures adapt to different hydrogeological conditions. Therefore, the economic and applicable supporting structure should be selected according to the specific analysis of specific problems.