1 Introduction
With the rapid development of urban construction and reconstruction in China, the technology of foundation pit support is also improving. Deep foundation pit engineering is often located in prosperous cities, construction land shortage, old city reconstruction and expansion, high-rise buildings and other areas. Although the foundation pit support is a temporary project, its technology is far more complicated than the permanent infrastructure or superstructure. A little carelessness will not only endanger the safety of the foundation pit itself, but also cause harm to nearby buildings, roads and bridges. It involves enclosure engineering, earthwork excavation and supporting engineering. In recent years, the technology of deep cement mixing pile combined with steel pipe shotcrete and bolt support has been greatly developed. This method is safe, reliable and time-saving, and is suitable for foundation pit support of engineering construction projects.
2 working principle
2. 1 working principle of deep cement mixing pile
Deep cement mixing pile is a widely used method for foundation reinforcement and foundation pit support. It takes cement and other materials as curing agent, and the soft soil and cement slurry are forced to stir in situ by a special mixer, so that the soft soil is hardened into cement plus solid with integrity, water stability and certain strength, thus improving the strength and physical and mechanical properties of foundation soil and increasing the deformation modulus. It is suitable for treating normal consolidated silt, muddy soil, saturated loess, cohesive soil, plain fill and saturated loose sand without flowing groundwater. In foundation pit support, deep cement mixing pile is also used as the advanced support structure of foundation pit.
According to the exposure of engineering geology, if the stratum contains sand layer, a certain proportion of cement powder can be put in while mixing the cement slurry to increase the consistency and sealing effect of the cement slurry.
2.2 Technical Principle of Steel Flower Pipe Support
Steel flower pipe support is a method of soil nailing wall support structure. Its basic principle is to transfer the tensile force generated by the reinforced material in the active area (sliding area) to the resistance area, increase the vertical stress on the sliding surface, and then improve the shear strength of the soil layer. The reinforced material on the sliding surface generates shear force and bending moment to resist the sliding of the active area with the help of the passive earth pressure provided by the soil layer, thus achieving the purpose of stabilizing the excavation surface. In other words, the unbalanced earth pressure in the active area caused by the excavation of foundation pit soil passes through the concrete surface and the steel pipe anchor, and is finally borne by the steel pipe anchor. The uplift and overall stability of the excavated slope are also realized by reinforcing the soil with steel pipes.
Before the earthwork excavation of foundation pit, the construction of retaining ditch at the top of slope shall be carried out first, and then excavation shall be carried out in layers; After the first layer of earthwork excavation, the first layer of concrete is sprayed on the soil surface.
The steel flower tube anchorage support structure consists of two parts: steel flower tube anchorage and sprayed reinforced concrete surface. Steel pipes with a certain diameter are made into filter tubes (flower tubes), the buried ends are processed into pile tips, the filter holes are opposite, and steel bars or angle steel plates are welded at the front ends of the holes (tube tips) to form barbs and protection blocks in front of the holes. Then use impact hammer or soil nail to drive the steel pipe into the soil according to the designed angle and position, and then carry out high-pressure grouting. Spraying steel mesh on the slope, setting steel bars and welding them with the anchor head at the same time, and then spraying the second layer of concrete.
After the anchor injection of the upper steel flower pipe is completed for a certain time, the next layer of earthwork excavation will be carried out, and this layer will be supported by shotcrete. After the shotcrete work is completed, the drainage ditch and collecting well at the bottom of the foundation pit shall be constructed in time.
The design of steel flower pipe bolt support technology is mainly based on Technical Specification for Building Foundation Pit Support (JGJ 120-99).
3 engineering application examples
3. 1 project overview
A project in Hunan is located in the city center. The project is a frame structure with eighteen floors above ground and one underground. The foundation pit in the basement is 52.0m×92.0m, the excavation depth is 5.0 m ~ 5.5 m, the circumference of the foundation pit is about 288.00m, and the supporting area of the foundation pit is about1.580m2.. The supporting structure of foundation pit mainly adopts deep mixing pile as waterproof curtain, and the steel flower pipe anchorage supporting structure is divided into four supporting sections.
3.2 Engineering Geology
According to the geotechnical investigation report, the terrain of the site is flat, and the geotechnical layers of the site are filled soil, alluvium, eluvial layer and granite respectively. See table 1 for details.
The groundwater level of the site is about 1.50m, and the groundwater is rich in water content. Groundwater is mainly upper stagnant water and confined water.
3.3 Analysis of Construction Difficulties
The project site is narrow, with buildings on three sides, residential buildings with 3-4 floors of natural foundation in the east, municipal roads in the west and four-storey overlay facilities in the north (see figure1); Only the south side of the foundation pit slope can be greater than 1: 1, and other foundation pit slopes are close to vertical. During the construction period, the safety of adjacent buildings, municipal roads and municipal pipelines must be ensured.
Because of the high groundwater level and relatively poor soil quality, dewatering, water-stopping measures and the quality of foundation pit support have become the key points of engineering construction. The construction period is just the rainy season in the south, and the water stop of foundation pit excavation and the stability of soil slope are the guarantee of safe construction.
3.4 Main technical measures
3.4. 1 Deep cement mixing pile enclosure and water stop
The deep cement mixing pile is used to reinforce the soil around the foundation pit, prevent the surrounding groundwater from infiltrating into the foundation pit, and at the same time, as the advanced supporting structure of the foundation pit, increase the stability of the slope, as shown in figure 1.
Construction design of deep cement mixing pile;
(1) The mixing pile adopts a row of φ 600 @ 400 mm, using 32.5R ordinary portland early strength cement, with water cement ratio of 0.50~0.55, cement penetration of 45kg/m and mud powder 10kg/m, as shown in Figure 2;
(2) The mixing pile adopts the construction technology of four mixing and four spraying, the control depth of the pile end is about 7.0m, and the pile end enters the stable water-resisting layer;
(3) The pile position deviation of mixing pile shall not exceed 50mm, and the verticality deviation of mixing pile shall not exceed 65438 0%;
(4) The lap quality between piles must be guaranteed, and the interval between adjacent piles shall not exceed 24h；;
(5) When using stirred pulp machine-made pulp, the stirring time shall not be less than 65438±00min after each feeding, and the prepared cement slurry shall not stand for more than 2 hours;
(6) Borehole gunite shall be stirred to the designed pile bottom elevation, and gunite shall be stirred in situ for 30s；;
3.4.2 Design of Steel Flower Pipe Bolt Support
(1) According to the geotechnical investigation report of the site, the physical and mechanical indexes of the soil layer are shown in Table 2.
(2) According to the slope conditions, the surrounding area of the foundation pit can be divided into three areas: area A, area B and area C, in which area A is on the east side of the foundation pit, area B is on the west side and north side of the foundation pit, accounting for half of the supporting length of the foundation pit, and area C is on the south side of the foundation pit; A and B areas are supported by soil nailing walls, and C areas are protected by natural slope mixing piles.
According to the structural design of the building basement, the foundation pit support adopts the steel flower pipe anchor support structure (soil nailing wall), the importance coefficient of the foundation pit side wall is 1.0, the foundation pit depth is 5.5m, the groundwater level is-1.5m, and the wall slope angle is 90O；; See Table 3 for the selection of grading parameters in each region.
(3) Four rows of steel flower tubes are designed for the support profiles in Area A and Area B, and the selection of design parameters is shown in Table 4. Drawing design: See the sectional views of supports in Area A, Area B and Area C (Figures 3, 4 and 5).
Specific design requirements:
The common anchor adopts φ48 steel flower pipe with a length of 6m~9m and an inclination of 15. The pressure grouting cement is 32.5R ordinary portland cement slurry, the bottom is grouted once, the grouting pressure reaches 0.6MPa~0.8MPa, and the pressure is stabilized for 3min~5min after the slurry is filled. The concrete near the grouting hole should have the function of resisting the pressure generated by grouting. ② Welding length of steel bar: single side 10d, double side 5d; If lap joint is adopted, the lap joint length is 40d；; (3) The slope shall be sprayed with steel mesh, the steel mesh shall be φ 6 @ 200× 200, and the skeleton reinforcement shall be φ 16, which shall be arranged along the anchor position. The total thickness of sprayed concrete is 100, and the strength grade is C20, as shown in Figures 6 and 7. (4) The strength of the anchor body is greater than 70% of the design strength, and the next layer of earthwork excavation can be carried out; ⑤ After the anchor grouting of the upper steel flower pipe is completed for a certain period of time, the next layer of earthwork excavation will be carried out, and the shotcrete anchor support will be carried out in this layer. After the shotcreting work is completed, the drainage ditch and collecting well at the bottom of the foundation pit shall be constructed in time.
3.5 Construction process flow
3.5. 1 Construction of deep cement mixing pile
1) process flow: measurement and positioning → deep mixing pile driver in place → ready-mixed drilling → gunite mixing and lifting → repeated mixing drilling → repeated mixing and lifting → to the orifice → turn off the mixer → pile driver in place;
2) Construction technical measures
① Measurement requirements: according to the axis point and control point of the pile position, accurately survey the pile position with a deviation of ≤ 50 mm;
② Pile-machine alignment: during construction, the drill bit is strictly aligned with the pile position (the error is less than or equal to 20mm); Adjust the pile driver to ensure the stability of the lifting equipment and the verticality of the guide frame (the verticality deviation shall not exceed 1%), and the pile position deviation shall not exceed 50mm；;
③ Mud preparation: Mud is prepared in strict accordance with the water cement ratio of 0.50~0.55, and the cement mixture ratio is 10%~ 12%. The average specific gravity of soil layer is about 1.6T/m3, and the pile diameter is 600mm, which is 0.28m3/m× 1.6t/m3× 600. According to the requirement of one pool slurry for each pile, it is prepared and used at one time;
④ inspection of drill bit: check the drill bit once before each shift starts, and the diameter of the drill bit is
⑤ The mixing pile adopts the "four mixing and four spraying" process, that is, the spraying process is carried out in the order of "two times and two times", and the lowering and lifting speed meets the construction design requirements;
⑥ 32.5R ordinary portland cement shall be adopted, and each batch of cement shall have a certificate of approval and a material performance test report. Different brands and batches of cement shall not be used at the same time.
3.5.2 Steel Flower Pipe Bolt Support Construction
1) Slope support construction process: ① manual slope repair →→ ② shotcrete → ③ driving steel pipes → ④ high-pressure grouting → ⑤ hanging nets, welding steel bars and anchoring →⑤shotcrete;
2) Construction technology of steel pipe anchor:
① Pipeline processing: filter tube (flower pipe) is made of φ 48 steel pipe, and the buried end is processed into the shape of pile tip. The relative spacing of the water filtering holes is 500mm, and the front ends of the holes (pipe-making tip side) are welded with steel bars or angle steel blocks to form barbs and protection blocks in front of the holes, as shown in Figure 8.
(2) Nailing: align the steel pipes according to the designed angle and position with an impact hammer or nailing machine, and drive the steel flower pipes into the soil to the designed length.
(3) Pressure grouting: grouting pump is adopted, which is connected to the steel pipe head through a high-pressure grouting pipe. Low-pressure slow grouting technology is adopted, cement slurry is pressed in, 32.5R cement is adopted, cement slurry is prepared according to the water-cement ratio of 0.50~0.55, and early strength agent is added, and the grouting pressure reaches 0.6MPa~0.8MPa, and the pressure is stabilized for 3min~5min, then the grouting can be stopped.
3) Construction technology of bolting and shotcreting support:
(1) Excavate the working face and trim the slope according to the design requirements; According to the soil conditions, the surface concrete is sprayed once or twice;
(2) smashing steel pipes;
(3) Tie the steel mesh, reserve lap reinforcement (the length of the upper and lower mesh reinforcement is 300mm), the error of the steel mesh shall not be greater than 20mm, weld the steel bars, and strengthen the welding of bones and muscles with steel pipe heads;
(4) Fine stone concrete is sprayed to the designed thickness, and the thickness can be controlled by anchor head or pile insertion, and an accelerator must be added to the concrete.
⑤ After the upper layer is sprayed with concrete for 3 days, the next layer can be excavated and sprayed with concrete, and so on until the pit bottom elevation. ⑥ The concrete on the sprayed concrete surface shall be cured within 7d after the sprayed concrete is completed.
4 Evaluation and summary of construction effect
This project adopts the combination of deep cement mixing pile and steel pipe anchor, which gives full play to the advantages of the two methods. Through on-site excavation monitoring and displacement observation, the maximum displacement of area A is 6mm, and that of area B is 5mm.
In this foundation pit support project, before determining the method of deep cement mixing pile combined with steel pipe shotcrete and anchor rod support, other support structures, such as deep cement mixing pile combined with row pile support, are demonstrated in detail, which can save 35% of the project cost. In loose sand layer and silty sand layer, when deep cement mixing pile is used for water stop, cement powder and cement are properly infiltrated and mixed, which has good water stop effect and reduces cost. The effective construction period of this foundation pit supporting project is 45d, which crosses with earthwork excavation, and the construction period is short, thus gaining time for construction.
In the design of foundation pit support, the influence of earthwork excavation and excavation method and sequence on the deformation of horizontal support structure system should be considered, and the dynamic detection of engineering and the timely analysis and design research of its results should be strengthened, which has a guiding role for foundation pit support. Practice has proved that the application of deep cement mixing pile and steel pipe shotcrete and anchor support in foundation pit support engineering described in this paper is successful, with good effect, low cost and strong pertinence, which is worth popularizing.
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