Shotcrete mesh support is a combined support method of anchor rod, steel wire mesh and shotcrete. The self-stabilizing ability of the side wall soil is used to the maximum extent to make the structure in the best stress state, and the supporting parameters can be adjusted at any time according to the monitoring data, which has great flexibility, especially in the case of complex surrounding environment, and has great advantages because of simple equipment and small space required.
1 general situation of the project and engineering geological conditions
The project is located in Pingdingshan City, Henan Province, with a total construction area of 7200m2. There are two main buildings, about 83m high, with 20 floors above ground and 2 floors underground, frame-shear structure and raft foundation, two podium buildings with excavation depth of about 9.0m, 4 floors above ground and 1 floor underground, frame structure and raft foundation. The excavation depth of the foundation pit is about 5.0m, and the surrounding environment of the building is complex. The north side is about 14m away from the proposed fence, and the outside of the fence is a seven-story residential building under construction. The south side is 8.2m away from the proposed fence, the west side is about 4.5m away from the proposed fence and the east side is 4.6m away from the proposed fence. Foundation pit plane (natural ground elevation-1.00m).
According to the detailed investigation report of geotechnical engineering, the project site is divided into two aquifer groups: the first layer is the upper stagnant water, which is buried 4.6 ~ 5.5 m below the natural ground, and the recharge source mainly depends on atmospheric precipitation and underground pipeline seepage; The second floor is diving, with a slightly lower bearing capacity, and the water level is buried under the natural ground11m. See table 1 for the physical and mechanical properties of each layer of soil in the site.
2 Selection of support scheme
2. 1 site evaluation
The foundation pit site is mainly composed of silty clay and clay layer within the excavation depth, with high soil strength and high self-supporting height. At the same time, the water level in the foundation pit is deeply buried and is not affected by groundwater. After calculation, the critical vertical height Hc=7.07m m. Therefore, in the process of foundation pit excavation, special support is not needed, and slope excavation can be carried out when the site permits, and anchor shotcrete net support can be used for local slope excavation, so the space is not large. Therefore, after slope excavation, plain spraying is used to protect the slope soil from weathering and rain erosion and ensure the original characteristics of the soil. At the same time, it can make the construction site in the foundation pit clean and sanitary, and ensure the smooth progress of foundation construction in rainy season.
2.2 Selection of Support Scheme
According to the field investigation data, the north and south sides of the foundation pit are wide, which can be used for large-scale sloping, while the east and west sides are small. After the materials needed for supporting are reserved for stacking and processing, the slope can be put to the north side according to the ratio of 1: 0.4. The south side is also inclined at the ratio of 1: 0.4; The site on the east and west sides is narrow, and the slope shall be graded according to 1: 0. 1. 1# The east side of the deep foundation pit and the west side of the 2# deep foundation pit are sloped, and the shotcrete anchor net is used for support.
2.3 Determination of bolting and shotcreting support scheme
Design parameter values: soil bulk density γ= 19.8KN/m3, excavation depth H=9m, cohesion C=29KPa, friction angle φ = 13, safety factor K= 1.3, external load q= 10KN/m2.
3 construction points
3. 1 earthwork excavation and slope repair of foundation pit
Earthwork excavation of foundation pit should be carried out step by step, and the depth of step excavation mainly depends on the vertical capacity of exposed slope. In order to provide good working conditions for the construction of anchor shotcrete net, the excavation depth of each layer is 1.5m ~ 2m, and ultra-deep excavation is not allowed. The excavation length should be determined according to the premise of maintaining slope stability during cross construction. In general, the excavation length on the same axis is 15m ~ 20m. In slope excavation, the disturbance to the supporting soil layer should be minimized, and the slope should be repaired in strict accordance with the regulations to prevent the final shape and size of the foundation pit from being insufficient due to the error of layered excavation.
3.2 Anchoring structure
Bolt construction is first of all bolt hole-forming. According to the geological conditions, use manual drilling or drilling rig to drill holes, arrange holes according to the design, measure and mark lines, and then drill holes according to the hole length, hole depression angle and aperture required by the design. Secondly, the installation of anchor rods: according to the length and diameter of each row of anchor rods specified in the design, qualified anchor rods are processed. In order to keep the anchor rod in the center of the hole, weld a centering bracket every 1.2 ~ 2m, and put the anchor rod into the hole. Finally, grouting is needed, which is the key to ensure the close adhesion between the anchor and the surrounding soil. In the hole where the anchor rod is installed, inject 1: 1 cement mortar at a pressure of not less than 0.4MPa to ensure that the mortar fills between the anchor rod and the hole wall, and add expansion agent to the mortar to reinforce the water reducer as soon as possible. Grouting shall be carried out from the inside out. It is necessary to insert the grouting pipe about 0.5m from the bottom of the hole, and the grouting bag must be bound at the orifice to prevent it.
3.3 shotcrete mesh
After the anchoring construction is completed, φ 10 round steel is used to connect with the anchoring elbow to form a whole. Then quickly spread a layer of Φ 6.5 steel mesh with a grid of 200× on the slope, and tie the mesh with wire, and the mesh should be firmly overlapped. After the above procedures are completed, concrete can be sprayed, that is, the mixture with a certain proportion is transported through the pipeline, sprayed on the spraying surface of the slope at high speed with the help of the ejector and compressed air, and then condensed with the steel mesh to form a thin-walled reinforced concrete slab wall. According to the design requirements, the thickness of concrete is 80 ~ 100 mm and the strength is C30. When spraying concrete, the distance between the spray gun nozzle and the sprayed surface should be kept at 1 ~ 1.2m, so as not to affect the compactness of the sprayed surface concrete if the distance is too large, and the rebound loss of the mixture will be too large if the distance is too small.
3.4 Setting of measuring points
Monitoring points should be set up, and regular observation should be made during construction and before completion. One is the ground subsidence value and the other is the slope displacement value. On-site monitoring and measurement is particularly important for bolt-shotcrete mesh support technology. Through monitoring, the stability and safety degree of slope can be grasped at any time, which can provide information for design and construction, so as to modify the design and construction scheme at any time and realize design and construction optimization.
4 supporting project cost comparison
Foundation pit supporting project is a bidding project with its own scheme. The three construction units that participated in the bidding * * * chose the manual digging steel supporting scheme with the bid price of 6.5438+0.5 million yuan, the manual digging pile prestressed anchor bolt supporting scheme with the bid price of 6.5438+0.82 million yuan, and the shotcrete anchor net supporting scheme with the bid price of only 980,000 yuan.
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