Foundation pit supporting It is the support, reinforcement and protection measures for the side wall of the foundation pit and the surrounding environment to ensure the safety of the underground structure construction and the surrounding environment of the foundation pit.

The definition of foundation pit support in Technical Regulations for Building Foundation Pit Support in the industry standard of the People's Republic of China is as follows: In order to protect the construction of the underground main structure and the safety of the surrounding environment of the foundation pit, temporary retaining, reinforcement, protection and groundwater control measures are adopted for the foundation pit.

1. Common forms of foundation pit supporting engineering are as follows:

(a)Slope excavation:

Advantages: lowest cost, fast construction progress.

Disadvantages: backfill earthwork large, rainy season soak easy partial collapse.

apply:The site is open, the soil layer is good, there are no important buildings and underground pipelines around.

(2)Soil nailing wall:

Advantages: stable and reliable, good economy, good effect.

Disadvantages: It is difficult to use the area with poor soil quality, which requires earthwork to cooperate with layered excavation, and more equipment needs to be invested in the construction site.

Application: Mainly used in the area of good soil quality, excavation of shallow foundation pit.

(3)Composite soil nail wall:

Advantages: composite soil nailing wall has the dual function of retaining soil and stopping water; Generally, there is no support in the pit, which is convenient for mechanized excavation.

Disadvantages: Long construction period.

Application: Soft soil, backfill or vertical excavation area limited by site.

(4)Steel sheet pile:

Advantages: good durability, high secondary utilization; The construction is convenient and the construction period is short.

Disadvantages: can not block the small particles in the soil and water, water isolation or precipitation measures should be taken in the area of high groundwater level; The cantilever has weak bending resistance and large deformation after excavation.

Application: cantilever support is suitable for less than4mThe foundation pit.

(5) cast-in-place pile+Anchor:

Advantages: High wall strength, high stiffness, good support stability, small deformation. According to the requirements of soil layer and construction period, the hole forming equipment can be more selective.

Disadvantages: High cost, long construction period. The gap between piles is easy to cause soil and water loss, especially in the area of high water level sand layer.

Application: Mainly used for2Anchor cables are used to control the deformation of the foundation pit designed for floor and above basement support. Deep pit8～20m Foundation pit engineering, Suitable for poor soil layers.

(6)Gravity soil-cement retaining wall:

Advantages: pollution-free construction, simple construction, due to the gravity structure, can not use anchor or support, easy to dig foundation pit earthwork construction, good permeability, retaining wall, water curtain double effect, relatively low cost.

Disadvantages: the construction speed is slow, because the mixing pile can not be excavated until it reaches a certain age. If the foundation pit is deepened, the width of the retaining wall will be widened, and the cost will increase greatly. When the mixing pile cannot penetrate the thick soft soil area, the foundation pit deformation will be relatively large.

Application: thick backfill soil, silt, silt soil area.

(7)Underground diaphragm wall:

Advantages: high stiffness, good sealing effect.

Disadvantages: high cost, high construction site requirements, construction requirements for special equipment.

Application: Poor geological conditions, complex, deep foundation pit, high surrounding environment requirements.

(8)SMWMethod:

Advantages: the construction is basically noiseless, has little impact on the surrounding environment, reliable structural strength, good waterproof and impermeable performance, deep foundation pit can be combined with multiple anchor cables or supports,H Section steel can be recycled to reduce costs.

Disadvantages: the construction site of the soft pile machine walking is not stable, or the guide groove excavation is deep, the groove wall is not stable, leading to the tilt of the mixing pile machine, affecting the vertical degree of the mixing pile; Encountered more underground obstacles can not be constructed; If the construction in the sand layer above medium densityMore than18mIt is easy to happen drilling accident, and the steel pile can not be inserted into the design depth by its own weight.

Application: Can be used in soil, silt, clay, sand, sand, gravel, pebble and other soil layers.

(9) Enclosure pile+Concrete support:

Advantages: give full play to the advantages of materials; Speed up earthwork excavation and transportation; Reduce engineering costs; Not limited by the lack of surrounding sites.

Disadvantages: heavy weight, harmful to the environment, slightly higher cost than steel inner support, more difficult to dismantle, more engineering waste.

(10) Enclosure pile+Fabricated steel support:

Advantages: no site cast-in-place node, faster installation speed, construction quality is easier to ensure; Steel structure is ductile material, better seismic performance; The steel structure has light weight and low basic cost; Steel structure for recyclable materials, green environmental protection; Precast buildings with carefully designed steel structures are more economical than precast concrete buildings. The beam-column section is smaller, and more usable area can be obtained.

Disadvantages: Exterior wall systems are different and complex compared to precast concrete structures, and steel structures can be more expensive than traditional concrete structures if not designed properly.

Two, the characteristics of foundation pit support:

1Foundation pit supporting project is a temporary project with different geological conditions in different areas.

2Foundation pit supporting engineering has high cost, large amount of construction, complex technology, wide scope, many changing factors and frequent accidents.

3,Foundation pit supporting engineering to large depth, large area development, some length width exceeds100Meters, depth of more than20meter

4Excavation under complex site conditions such as soft soil and high groundwater level is easy to cause soil sliding, foundation pit instability, pile displacement, heave at the bottom of the pit, serious water leakage of the supporting structure and other problems, which pose a great threat to the safety of surrounding buildings, underground structures and pipelines.

5Many links of foundation pit supporting engineering are closely related, such as retaining soil, supporting, waterproof, precipitation and excavation. The failure of one link will lead to the failure of the whole project.

6The foundation pit supporting project has high cost and is a temporary project. Once an accident occurs, it is difficult to deal with, resulting in serious economic losses and social impact.

7, foundation pit supporting engineering construction cycle is long.

Three, foundation pit supporting design requirements

(1) Design service life

In most cases, the foundation pit support is a temporary measure adopted in the construction of the underground part of the main structure. After the construction of the underground structure is completed, the foundation pit support also completes its function. The construction conditions of foundation pit engineering are generally complex and easily affected by environmental and meteorological factors, so the construction period should not be too long. Therefore, the validity of support design is generally not more than two years, but not less than one year.

For most construction projects, one year support period can meet the requirements of the construction cycle of the main underground structure. For the project with special construction period, the support period should be extended according to the actual situation, and the design conditions such as load should be considered.

Support structure and main structure according to the requirements of the main structure design period"The joining together of two walls"Service life of the structure design.

(2)Foundation pit supportingSecurity level

1Foundation pit design grade

GB50007The foundation pit engineering is regarded as a part of the foundation, divided into A, B, C three design levels.

The differences in principle of foundation pit engineering with different design levels are mainly reflected in the requirements for deformation control and groundwater control. The design grade isAIn addition to the deformation of the supporting structure of the foundation pit, the ground settlement around the foundation pit and the deformation of the surrounding protected objects should also be calculated. When the hydrogeological conditions of the site are complex, the design levelAThe FOUNDATION pit of grade I should carry out special design of groundwater control, the main purpose is to reduce the damage of surrounding buildings (structures) and underground pipelines due to improper groundwater treatment on the basis of fully mastering the groundwater law of the site.

2.Safety grade and importance factor of support structure

specificationGB50007It IS SUGGESTED THAT IN THE DESIGN OF FOUNDATION PIT engineering, the safety grade of SUPPORTING structure should be determined according to the severity of the damage of supporting structure. The bracing structure of foundation pit may encounter unpredictable load conditions during construction or use. Therefore, the structural importance coefficient used in the design of foundation pit retaining structureγThe value of must not be less than or equal to 1.0.

disciplineJGJ 120 The safety grade of the supporting structure is divided into one, two and three, and the importance coefficient of the structure is respectivelyγShall not be less than1.1,1.0,0.9; In the analysis of various stability, the value of stability safety factor is also related to the safety level of the supporting structure. The higher the safety level of the support structure, the higher the value of the stability safety factor.

Select support structure security rank principle is: the surrounding homes, public buildings, roads or underground pipelines, or due to the complicated geological conditions, the lack of similar geological conditions similar to the depth of foundation pit experience, damage, the instability of foundation pit support structure or excessive deformation of human life, economic, social, or environmental impact is very big, the security level to level. When the support structure damage, excessive deformation of foundation pit will not endanger human life, slight economic loss, and the impact on society or environment is small, the safety level can be divided into three levels. For most foundation pits, the safety level should be divided into two.

For the internal bracing structure, when the unstable damage of one side of the foundation pit will affect the continuous collapse of the other side of the foundation pit due to force changes, the supporting structures on both sides of the foundation pit that interact with each other should reach the same safety level.

(three)Foundation pit supportingBasic design requirements

1Foundation pit engineering design shall include the following contents:

(1) Support structure system scheme and technical and economic comparison;

(2) stability checking calculation of foundation pit supporting system;

(3) calculate the bearing capacity, stability and deformation of the supporting structure;

(4) groundwater control design;

(5) control design for the influence of the surrounding environment;

(6) earthwork excavation scheme for foundation pit;

(7) Monitoring requirements for foundation pit engineering.

2The design of foundation pit supporting structure shall comply with the following provisions:

(1All support structures shall be designed to meet the requirements of bearing capacity, deformation calculation and soil stability checking

①The support structure, including the support system or bolt structure, shall meet the requirements of the strength and stability design of the component material.

②The ground deformation caused by foundation pit excavation should not exceed the allowable value of buildings and underground facilities around the foundation pit, nor should it affect the safety of foundation piles or the construction of underground structures.

③Stability requirement refers to the stability of the soil around the foundation pit, that is, no soil sliding failure, no seepage caused by flow sand, flow soil, piping, support structure and support system instability.

(2) For the foundation pit of grade A and B, the soil deformation inside and outside the foundation pit caused by earthwork excavation and dewatering should be calculated.

(3) Foundation pits with grade A design for high groundwater level areas should be specially designed for groundwater control.

3, foundation pit supporting design requirements

(1) The design should set the horizontal displacement control value of the supporting structure and the settlement control value of the building around the foundation pit, and put forward the clear use requirements of the foundation pit, such as the load limit around the foundation pit, groundwater and surface water control. These design conditions and foundation pit using requirements shall be as the important content in the design documents clearly reflected in the design documents, the total structure design plan and section should be accurate tags, matters needing attention of construction is noted in the introduction to design, strictly prevent the support structure during the construction and use of the actual situation of more than the design conditions.

(2Sufficient construction working space should be left between the supporting structure and the main underground structure, and the bolts, waist beams and inner supports should not interfere with the construction of the main underground structure and waterproof construction.

(3When the supporting structure is reduced to a plane structure model, the design conditions along the vertical plane around the foundation pit will often be different. In addition to the depth of the foundation pit, the surrounding conditions and the additional load may be different, the variation of geological conditions is a very important particularity that makes the support structure different from the superstructure. The distribution and thickness of naturally formed soil layer in the scale of foundation pit are often quite different. When the depth of foundation pit, surrounding environment and geological conditions are different, the influence of these differences on the soil pressure load of the support structure can not be ignored. Therefore, the calculation part should be divided according to the excavation depth of each part of the foundation pit, the surrounding environmental conditions, geological conditions and other factors. Each calculation section shall be calculated on the basis of the most unfavourable conditions.

(4The calculation of support structure should be carried out according to the actual situation of foundation pit excavation and support structure, and the working conditions of design and calculation should be consistent with the actual construction conditions. The design document shall indicate the construction sequence of each component of the supporting structure and the corresponding excavation depth of the foundation pit, so as to prevent the uncompleted excavation of the foundation pit from not excavating according to the plan, which may lead to engineering accidents caused by excessive excavation.