With the rapid development of my country's economic construction, large-scale and super-high-rise buildings have emerged in various cities. So far, there are more than 20 super high-rise buildings with a height of more than 200 meters across the country. Foundation pit projects are characterized by tightness (the site is compact), close (the project distance is close), deep (getting deeper and deeper), and large (large scale and size). At present, the Fuzhou New Century Building, which has the deepest basement of a domestic high-rise building, has six floors underground, with a depth of -26.2m. The National Center for the Performing Arts, which is about to be built, has three floors of basement and a foundation pit depth of -32.5m. Deep foundation construction is a major part of the construction of large and high-rise buildings. It is an extremely important link in the construction of deep foundation pits, and the supporting structure technology of deep foundation pits is undoubtedly the key to ensuring the smooth construction of deep foundations.
For the measurement standard of deep foundation pits, some foreign countries regard the depth of 20ft (about 6.1m) as the limit of deep foundation pits. my country's construction and acceptance specifications do not clearly define deep foundation pits. According to Shandong Province's "Construction Site Management Standards", deep foundation pits refer to projects where the excavation depth exceeds 5m or the basement has more than three floors, or the depth does not exceed 5m, but the geological conditions, surrounding environment and underground pipelines are extremely complex.
The design and construction of foundation pit support must comprehensively consider engineering geological and hydrological conditions, foundation type, foundation pit excavation depth, drainage conditions, surrounding environment, loads around the foundation pit, construction season, and supporting structures. factors such as lifespan. The key to the control of foundation pit support construction is the stability of the foundation pit, the control of ground deformation and groundwater, the prevention of dangers such as foundation pit uplift, piping, and quicksand, and the support plan must be adjusted in a timely manner according to changes in geological and environmental factors.
Basic requirements for deep foundation pit support: (1) Advanced technology, simple structure, and reliable stress to ensure that the foundation pit enclosure system can retain soil and keep the slopes around the foundation pit stable. . (2) Ensure the safety of adjacent buildings (structures), underground pipelines, roads, etc. around the foundation pit. During the excavation of the foundation pit and the construction of underground projects, they will not be affected by the deformation, subsidence, collapse or displacement of the soil. harm. (3) Through drainage, precipitation, water interception and other measures, the foundation construction should be carried out above the groundwater level. (4) Economically reasonable, protecting the environment and ensuring construction safety.
Basic requirements for deep foundation pit support: (1) Advanced technology, simple structure, and reliable stress to ensure that the foundation pit enclosure system can retain soil and keep the slopes around the foundation pit stable. . (2) Ensure the safety of adjacent buildings (structures), underground pipelines, roads, etc. around the foundation pit. During the excavation of the foundation pit and the construction of underground projects, no damage will be caused due to deformation, subsidence, collapse or displacement of the soil. harm. (3) Through drainage, precipitation, water interception and other measures, the foundation construction should be carried out above the groundwater level. (4) Economically reasonable, protecting the environment and ensuring construction safety.
The main function of the deep foundation pit support structure is to retain soil, so that the entire process of excavation and foundation construction of the foundation pit can be carried out safely and smoothly, and to ensure that there is no harm to adjacent buildings and the surrounding environment. . At present, domestic deep foundation pit support technologies include: underground diaphragm wall column support, cement mixing columns, soil nail walls and composite soil nail walls, shotcrete anchor net support, reverse construction and semi-reverse construction, annular support structures, etc. wait. In practice, an economical and reasonable plan is selected based on soil conditions, foundation pit depth, groundwater conditions, etc., and combined with the advantages and disadvantages of different support methods.
Shotcrete anchor net support is a support method commonly used in deep foundation pit support projects. It is the abbreviation of combined support of shotcrete, anchor rods and steel mesh. As an advanced support and reinforcement technology, it has been carried out at home and abroad in geotechnical high slopes and large-span underground projects, especially under adverse geological conditions. widely and successfully applied. Shotcrete anchor net support is to construct a certain length and distribution of anchor rods in the rock and soil body, and interact with the rock and soil mass to form a complex, which can make up for the lack of strength of the rock and soil mass and exert the anchor pull effect, so that the rock and soil mass can be strengthened. The potential of its own structural strength is fully utilized to ensure the stability of the slope. The reinforced mesh sprayed concrete is installed on the slope surface to restrain the deformation of the slope surface and make the entire slope surface form a whole. The construction process is: excavation of earth and stone, slope repair → drilling → installation of anchor rods (cables) → pressure grouting → hanging of steel mesh → welding of reinforcing bars → sprayed concrete → (prestressed tensioning and anchoring of anchor cables) →Excavate the lower layer.
For unstable soil layers, after excavation and slope repair, the first concrete injection should be added.
In order to achieve timely support and effectively maintain the strength of the soil, the construction of shotcrete anchor net support must "follow the excavation closely and support as the excavation progresses." The excavation height of each layer will depend on the geological conditions. It depends, generally 1.5m~2.5m.
The main features of using shotcrete anchor net support are: simple structure, high bearing capacity, safe and reliable: can be used in a variety of soil layers, strong adaptability; simple construction machinery and tools, flexible construction, low pollution and noise It is low and has little impact on the surrounding environment; it can be carried out simultaneously with earth excavation and does not occupy the absolute construction period: it does not require piling itself and the support cost is relatively low.
1 Project Example
A project supervised by our company consists of three super high-rise buildings (58 floors) A, B and C and a podium, with a construction area of ??220,000? O, three floors underground, each floor has an area of ??17,000? O, the depth of the foundation pit is 16m, and the perimeter of the foundation pit is about 550m long. The construction base is located on the south side of the city's main road. There are existing buildings on the east, south and west sides, and the grading space of the site is limited. The geotechnical properties of foundation pit slopes from top to bottom are basically fill layer, silty clay, residual soil, fully weathered rock, strongly weathered rock, and moderated rock. The buried depth of the stable water level is 1.35m~2.45m. Based on the geotechnical survey data, calculations and experience in similar projects, the construction unit chose the shotcrete anchor net support method. There are 7 rows of anchors from top to bottom, with a hole diameter of 110mm. The 2nd, 3rd, and 4th layer anchors use prestressed anchor cables 2①s15.24, and the rest are 25-diameter rebar. The anchors use grouting bags to seal the holes. Grouting. There are transverse reinforcing bars at the tail of the anchor rod, and the diameter of the layered steel mesh is 6.5250. The average thickness of shotcrete C20 is 10cm. For the vertical excavation part, cement mixing piles are driven before excavation, and the bottom of the piles is driven to the weathered rock.
2 Supervision basis
Unified standard for construction quality acceptance of construction projects GB503002001; specifications for construction quality acceptance of building foundation projects GB50202-2002; technical regulations for anchor shotcrete support GB50086-2001; Technical specifications for building slope engineering GB50330-2002; Technical specifications for building foundation pit support JGJ120-99; Technical specifications for box and raft foundations of high-rise buildings JGJ6-99.
3 Key points of supervision and control
3.1 We must attach great importance to geological survey work
Supervision engineers must carefully read the geological survey report of the project, understand the topography, landforms and geological characteristics of the location where the foundation pit is excavated, and analyze the possible causes of slope landslides various factors, and be well aware of the key areas, important strata and soil quality indicators that affect slope stability. Since the geological survey data is not necessarily very detailed and may differ from the actual situation, the supervision engineer must often compare the on-site geological conditions during foundation pit excavation. If there is a big difference with the geological report, the construction unit must be notified in time, and the construction unit shall notify Survey and design units to see if the plan needs to be adjusted.
3.2 The design plan must undergo technical demonstration
The design of the building is generally the responsibility of the formal design unit, and the support project is often considered to be part of the construction measures and is not included in the construction drawing design Within this period, the support construction unit with design qualifications shall design or the construction unit shall entrust other units to design. Since foundation pit support is a very complex technology, if the foundation pit designers are inexperienced, it is easy for the design to be poorly considered. Therefore, the construction unit is required to hire experts with rich experience to review the design and construction plans, so as to effectively reduce the risks of foundation pit support and prevent the occurrence of safety accidents.