Construction technology analysis of soil nailing wall in deep foundation pit of building engineering?

What is the technical analysis of soil nailing wall construction in deep foundation pit of building engineering? What role can it play? Please see the information compiled by Zhong Da Consulting Company.

As an in-situ soil reinforcement technology, the action mechanism of soil nailing wall is obviously different from that of pile-wall supporting structure system. Its main function on the stability of soil slope is to change the properties of soil by displacement and seepage control, and to realize the interaction between soil nails and soil by grouting soil nails. This paper mainly analyzes and discusses the function, application and quality control of soil nailing wall technology in deep foundation pit support construction of building engineering.

With the utilization and development of high-rise buildings and underground spaces, there are more and more deep foundation pit projects in China, no matter the technical difficulty or the project scale. Especially in areas with poor or complicated geological conditions, the traditional methods of foundation pit support and expansion can no longer meet the needs of current development. In the composite soil nailing support technology, soil nailing actively supports the soil and works with soil * * * to maintain, utilize and improve the original strength of the soil on the side wall of the foundation pit as much as possible, and transform the disturbed soil which has a load on the supporting structure in the traditional supporting method into a part of the supporting structure, so that it can be effectively applied to the foundation pit support under special geological conditions such as soft soil areas, and has the advantages of simple process, low cost and short construction period. However, the current composite soil nailing technology is not mature and perfect in theoretical analysis method, design theory and engineering practice.

1 the role of soil nailing wall in deep foundation pit support

Soil nailing wall is a supporting method developed from Germany, France and the United States in the 1970s. It is developed on the basis of NATM and according to the physical reinforcement mechanism of soil. In the early 1980s, this method was adopted in mine slope support in China, and then soil nailing wall support method was widely used in foundation pit support. The soil nailing wall is composed of reinforced soil, slender metal strips placed in the original soil and concrete panels attached to the slope, and finally the gravity supporting structure is realized. Soil nails with a certain length and density are placed in the soil, and the soil nails work together with the soil to effectively improve the strength and stiffness of the in-situ soil. This support technology is mainly applied to the excavation depth of foundation pit below12m. If the groundwater level is above the pit bottom, effective drainage and water interception must be carried out according to the actual construction requirements.

1, stress transfer and diffusion

When the load increases to a certain extent, the surface and internal cracks of the slope have developed to a certain width, and the stress at the foot of the slope is the largest. At this time, the part of the lower soil nail extending into the stable soil outside the sliding zone can still provide greater resistance. Through its stress transfer function, the soil nail transfers the internal stress of the sliding surface to the stable soil behind it, and disperses it in a wide range of soil, reducing stress concentration. Under the same load, the strain level in soil reinforced by soil nails is much lower than that in other plain soil slopes, which has the advantage of obviously hindering the formation and development of cracking areas.

2, hoop skeleton function

Soil nailing and soil * * * work together, which is determined by the stiffness and strength of soil nailing itself and its distribution space in soil. It can limit the deformation of soil and make the composite soil form a whole structure.

3. Slope deformation constraints

The reinforced concrete face slab connected with soil nails on the slope is an important part to play the effective role of soil nails. The constraint provided by the panel depends on the friction between the surface of the soil nail and the soil. When the composite soil cracks and expands into blocks, only the stable composite soil behind the cracking area produces friction.

Step 4 share roles

In composite soil, soil nails have high tensile strength, shear strength and bending strength. When the soil enters the plastic state, the stress gradually shifts to the soil nail. When the soil is cracked, the sharing effect of soil nails is more obvious. Corresponding composite stress, such as bending shear, tensile shear, etc. , produced in the soil nail, will lead to the slurry outside the soil nail fracture, steel yield.

Application of soil nailing wall construction technology in deep foundation pit support of building engineering

With the development of China's construction industry, in order to ensure the quality of deep foundation pit construction, construction enterprises must attach importance to its construction technology and standardize the construction process. Only in this way can we improve the overall quality of the project and realize its economic benefits.

1. Drill the nail hole. The foundation pit is supported by soil nailing, and the hole-forming tool is Luoyang drilling rig. The aperture is set to 80mm, and the depth shall be ensured to exceed the length of soil nail 100mm and the hole-forming inclination 15 degrees. Drill once every 1m and measure the inclination angle to avoid deviation.

2, soil nailing installation. Combined with the design requirements of soil nailing wall support of foundation pit in this project, soil nailing support is carried out to ensure its length is above the design length. Set a set of soil nails every 1.5m, connect the soil nails with lap welding, and control the height of the welding seam at 6mm, and put the soil nails in the hole after the hole is formed.

3. Grouting. The grouting method at the bottom of the hole is selected for grouting the foundation pit support of soil nailing wall. The operation process is to insert grouting pipe into the bottom of the hole to ensure that the nozzle is 200mm away from the bottom of the hole. Grouting pipe should carry out grouting and pulling-out operations at the same time, so as to ensure that the bottom of grouting pipe can be lower than the slurry level and can flow out from the orifice smoothly during grouting. Grouting check valve is set at the orifice, and pressure grouting is adopted for construction, so as to ensure that the strength of cement slurry is M20 and the grouting pressure is controlled between 1 ~ 2 MPa.

4. Hang the steel mesh and weld it firmly with the tail of the soil nail. Steel mesh is selected for soil nailing wall construction, and the spacing is set to 200 mm, and steel bars are bound on the slope manually; The length of reinforcement on the overlapping slope should be about 300mm, and then short reinforcement should be welded at both ends of the soil nail along the length direction of the soil nail. At the same time, the reinforcement at the end of similar soil nails should be connected on the surface and welded firmly.

5. Install the drain pipe. The drainage pipe of soil nailing wall foundation pit support should be made of PVC pipe. The length of the drainage pipe must be above 450 mm, and drilling operations should be carried out near the pipe, with the number of holes controlled at 5 to 8, and then nylon mesh cloth should be wrapped outside the pipe. The vertical and horizontal spacing of drainage holes is set to 2m, and the layout shape is plum blossom, so as to ensure the firmness of installation.

6. Re-spray the surface concrete to the design thickness. When shotcrete is used in soil nailing wall construction, its design strength must be about C20 and its thickness should be controlled at 80 mm.. Firstly, dry mixing is selected, and the mixture must be mixed according to the corresponding mixture ratio. According to the actual situation, shotcrete accelerator with 5% of cement weight can be added to concrete during shotcrete construction. After earthwork excavation and slope repair, soil nailing should be completed in time, and shotcrete should be carried out in time after welding steel mesh construction. Choose the method of layered spraying and spray concrete from bottom to top. The spraying thickness of the first layer should be controlled between 4 cm, and then the second layer of concrete should be sprayed until its thickness meets the design requirements.

3 quality control of soil nailing wall construction technology

1, the deviation between the center of the casing and the center of the pile shall not exceed 5cm, the buried depth shall not be less than 1m, the specific gravity of mud shall be controlled at1.1.2, and the thickness of sediment at the bottom of the hole shall not exceed15 cm; The reinforcing cage is placed accurately, and the reinforcement connection meets the specification requirements; The underwater concrete pouring construction needs continuous operation, so as to ensure that the buried depth of the conduit is not less than 2m, and the speed is appropriate, so as to avoid blocking the conduit or floating the reinforcing cage, and at the same time, the pile head is over-poured1m.. After the concrete curing of cast-in-place piles is completed, quality inspection shall be carried out according to relevant specifications and design requirements to ensure the quality is qualified.

2, soil anchor in the excavation of the deep foundation pit wall or excavation of the vertical wall soil drilling, reach the required depth and then expand the bottom of the hole, generally form a column. In the construction of bolt support technology, steel bars, wire ropes or other types of tensile materials are mainly put into holes, and then grouting materials are poured to combine them with soil layers to form powerful tensile bolts. This support technology can make the support system bear great tension, which is beneficial to protect its structural stability and prevent deformation. At the same time, it can save materials and manpower and speed up the construction progress.

3. During the construction period after the completion of deep foundation pit support, the pit wall did not collapse, and no obvious deformation was found in the surrounding buildings through instrument monitoring. The concrete cast-in-place pile and anchor support can ensure the smooth progress of the project and the safety of surrounding buildings, so it is feasible to implement the construction scheme of deep foundation pit support.

4 conclusion

To sum up, construction projects are important projects related to national economic growth. With the rapid development of China's real estate industry, its construction requirements are also constantly improving. As one of the important technologies in construction engineering, the scientific and reasonable choice of construction technology will directly affect the quality of the whole project and the safety of people's lives. Only by ensuring the standardization of its construction technology and fully grasping its technical points can its overall quality be effectively improved.

The above are collected and sorted by Zhong Da Consulting Company.

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