Application of the hottest soil nailing wall techn

Application of soil nailing wall technology in deep foundation pit engineering in recent years, in the process of foundation pit construction, the proportion of pit wall collapse accidents has increased due to the failure to set up a safe slope according to the soil conditions and do a good job of wall fixing support. Composite materials have stable mechanical properties and high flexibility. Therefore, the standard for safety inspection of building construction (JGJ) lists the construction of foundation pit as a safety inspection content, and requires special design and support for deep foundation pit. At present, there are many mature technologies for deep foundation pit support, among which soil nailing wall support is a relatively new technology

1. Characteristics and scope of application of soil nailing wall support structure soil nailing wall support structure is an in-situ soil reinforcement technology, which is to install or drive soil nails into the soil mass of foundation pit slope, and strengthen the soil mass into a self stable gravity retaining structure. This technology was formed in the 1970s. In 1980, this technology was first applied to the slope support project of Liuwan Coal Mine in Shanxi Province (excavation depth 10.2m, slope α= 80°）。 Since the 1990s, many engineering experts and scholars in China have conducted in-depth research and application of this technology, which has proved to be a technically feasible, safe and reliable technology with considerable economic benefits. It has been successfully applied to the foundation pit support in non soft soil sites, and the depth of the foundation pit has exceeded 20m

in terms of working mechanism, soil nailing wall is stressed by high-strength soil nailing, shotcrete surface and undisturbed soil, which enhances the ductility of soil destruction, changes the nature of sudden slope collapse, and is conducive to safe construction; In terms of technology, the method of excavation and support is adopted, so the working face is not limited and the construction period is shortened; In terms of investment, because soil nails make use of the self bearing capacity of soil, the soil around the foundation pit is transformed into a part of the supporting structure, and the economic benefit is considerable

soil nailing wall support is generally suitable for slopes with relatively uniform soil mass such as plain fill, ordinary cohesive soil, cohesive sand and silt above the groundwater level or after dewatering and drainage measures. In recent years, this technology has been rapidly developed in the foundation pit excavation in the southeast coastal area. It is widely used not only in the foundation pit excavation in sandy soil, but also in the filling and soft soil layer. Using the cement soil pile combined soil nailing wall support technology, this technology can be applied to the high water level stratum with precipitation. When there are soil layers and rock masses with different weathering degrees at the site, the application of soil nailing wall support is particularly advantageous. The application range of soil nailing wall support is very wide, mainly including:

1.1 temporary support during soil excavation. It is used for deep foundation pit excavation of high-rise buildings, underground structure construction excavation, soil slope excavation, etc

1.2 permanent retaining structure. Such works are generally combined with the temporary support during construction excavation, such as the retaining wall at the end of the tunnel portal and the retaining walls on both sides of the portal, the cutting soil slope retaining wall, the abutment retaining wall, etc

1.3 rescue and reinforcement under repair and reconstruction of existing retaining structure and support. Such as the maintenance and reinforcement of various retaining walls, as well as the rescue and reinforcement in case of instability or excessive deformation of various supports

1.4 slope stability. It is used to reinforce the embankment slope that may be unstable

2. Structure of soil nailing wall

the structure of soil nailing wall is composed of soil nails and surface layer. Soil nails mainly include two forms: bored grouting soil nails and driven soil nails

drilling grouting soil nail is the most commonly used soil nail, which is generally used φ 6~ φ 32mm HRB335, HRB400 rebar, placed in φ 70~ φ In the 120mm drill hole, cement or cement mortar with strength grade not lower than M10 shall be injected into the hole to form. The water cement ratio of cement slurry is generally about 0.5, the mix ratio of cement mortar is generally 1:1~1:2, and the water cement ratio is 0.38~0.45. The grouting soil shall be nailed with positioning support to center the reinforcement, and the orifice shall be provided with grout stop plug and exhaust pipe

driven soil nails are usually made of steel pipe materials. Driven soil nails are generally simpler and faster in length than in construction, but they are not easy to be used in dense cemented soil. When the driven steel pipe is a closed steel pipe with holes around, grouting can be carried out in the pipe after driving to enhance the adhesion between soil nails and soil and improve the pull-out resistance of soil nails. Grouting methods include low-pressure grouting and high-pressure jet grouting

the length of soil nails is generally 0.5~1.2 times of the excavation depth, the spacing is 1~2m, and the included angle with the horizontal is generally 5 ° ~20 °. The applicable wall slope of soil nail wall should not be greater than 1:0.1

the surface course is an important part of the soil nailing wall. Generally by φ 6~ φ 10mm reinforcement with spacing of 150~300mm, composed of sprayed concrete with strength grade not lower than C20, and the thickness of surface course is generally 80~150mm. In order to ensure the effective connection between the soil nail and the wall surface, the reinforced reinforcement can be connected with the soil nail and the distributed reinforcement, or the bearing pad method can be used

3. Measures to increase the pull-out resistance of soil nails

in the weak soil layer of the wall soil, or in the case of good soil quality, but the design length of soil nails is limited due to the surrounding environment constraints, in order to increase the pull-out resistance of soil nails, in addition to adjusting the horizontal and vertical distance of soil nails, the following measures can be considered:

3.1 increase the diameter of soil nails. Properly increasing the diameter of soil nails is a way to improve the pullout resistance of soil nails. According to engineering practice, when the diameter of soil nail is increased to 120mm, the pull-out force in sandy soil can be increased to 16n/m, and that in fill soil can be increased to 13kn/m

3.2 increase the grouting pressure. Increasing the grouting pressure can increase the volume of the anchor, thus improving the adhesive force between the anchor and the surrounding wall of the tunnel and the friction between them. The grouting pressure currently used in Hangzhou is generally not less than 0.4 MPa, but when the grouting pressure is increased to not less than 0.8 MPa, the pull-out resistance of soil nails is significantly improved

3.3 carry out secondary grouting. If there is only one grouting, since the upper end of the grouting hole is open, the grouting pressure will not be too high. The bonding force between the anchor and the soil mass of the tunnel wall to put the required cylinder into the groove and the friction between them are low. The secondary grouting can increase the friction between the anchor and the soil, and make the maximum compressive stress of the soil to the anchor reach the self weight stress of the soil at this depth. When the cement mortar injected at the first time is about to set, it is injected with cement slurry pressure. Since the first cement mortar is used to cover, the pressure of the second grouting can be controlled to a large value, so that the cement slurry can penetrate into the soil, and the pressure of the soil on the anchor body can reach the self weight of the soil at that place. The primary grouting pressure shall not be less than 0.4 MPa, and the secondary grouting pressure can be increased to not less than 0.8~1.2 MPa

4. Composite soil nailing wall technology in recent years, Shanghai has gradually summarized an improved composite soil nailing wall technology in the process of applying soil nailing wall technology. In addition to setting short and dense soil nails in the reinforced soil, the composite soil nailing wall is also provided with cement mixing piles with appropriate width and inserted into a certain depth at the bottom of the foundation pit. Due to the setting of cement mixing piles, the uplift and piping at the pit bottom are prevented, and a water stop curtain is established to form a closed waterproof system; Secondly, the self-supporting height is formed to improve the stability of the foundation pit slope at the excavation stage. Composite soil nailing wall is a new technology of foundation pit support design, which has an important reference for support design in similar soft soil layers

5. General principles and requirements for soil nailing wall construction

as a retaining structure, soil nailing wall support should meet the specified requirements for strength, stability, deformation and durability. When soil nailing wall support is used for deep foundation pit excavation in urban building intensive areas, it becomes more important to control and limit the deformation of the support. The construction of soil nailing wall support for deep foundation pit excavation has the following characteristics:

5.1 the construction process must be combined with on-site testing and monitoring from beginning to end. Through deformation and other measurement data and construction process, the on-site geological conditions can be continuously explored to guide the next construction in time

5.2 the impact of surface runoff and groundwater shall be fully considered. If the water seepage is serious during construction, the surface concrete cannot be sprayed, and it is easy to cause slope collapse and hole collapse. When the groundwater flow is large, special measures shall be taken to reduce the groundwater level during construction. Under the action of groundwater level, the surface pressure and internal force of soil nails will increase obviously, especially the shear capacity of clay and the pull-out bond capacity of soil nails in clay are closely related to the water content

6. Application example of soil nailing culture

6.1 engineering situation

a commercial and residential building project in Guangzhou, 22 floors underground, two floors underground structure, and 12m deep foundation pit. As shown in the figure. The project site has buildings on three sides and one side is adjacent to the street. The geological conditions of the slope from top to bottom are as follows:

Fig. profile of main nail wall

(1) artificial accumulation layer, with a thickness of about 2.4m, including silty fill and construction waste soil, with a medium to low compactness

(2) sandy clay, about 5m thick, moderately dense

(3) gravelly clay, about 8m thick, moderately dense

(4) completely weathered granite and strongly weathered granite

the groundwater level of the site is about 5m, and the soil permeability coefficient is small

according to the engineering, hydrogeological conditions and surrounding environment of the project site, and considering the factors such as cost, construction period and technical feasibility, it is decided to adopt soil nailing wall structure for slope protection. Compared with cast-in-place pile and underground continuous wall, soil nailing wall can save investment

6.2 soil nailing wall support design

6.2.1 design parameters: the slope of soil nailing wall is 75 °; The soil nails are arranged in quincunx shape, and the vertical and horizontal spacing of soil nails is 1.2m, ф 225hrb335 grade reinforcement, with the length as shown in the remittance note in Figure 2, the drilling diameter is 110mm, the grouting pressure is not less than 0.5MPa, and the syringe slurry is cement mortar, with the strength not less than 20MPa; The surface structure adopts ф 8○a200 × The surface course is sprayed twice, and a drain hole is set every 2.4m

6.2.2 design calculation

checking calculation of uplift bearing capacity of soil nails: the first row of soil nails tk=19.6kn, tu=28.5kn ≥ 1.25tk=24.5kn (the checking calculation of other rows of soil nails is omitted)

checking calculation of overall stability of soil nail wall: the most dangerous sliding surface is taken for calculation, and the simple slice method of circular arc sliding is adopted, with a safety factor of k=1.38 1.30

6.3 main construction technology of soil nailing wall

construction process of soil nailing wall: construction preparation → excavation of the first layer → spraying the first layer of concrete → hole forming → placing soil nails → grouting → making drainage hole reinforcement and welding anchor head → spraying the second layer of concrete → cleaning

slope top settlement observation points, displacement monitoring points, soil inclinometer holes and water level observation holes are designed around the foundation pit, and soil pressure monitoring points are set in the soil. It is required to closely monitor the deformation of the retaining structure during construction

prepare the construction scheme before construction, and make preparations for surveying and setting out, mechanical commissioning, material mobilization inspection, soil nail processing and monitoring measurement and distribution

well point dewatering is adopted for the foundation pit, and a drainage ditch is set 2m away from the pit edge. During the construction process, it is also very important for the stress stability of soil nailing wall to keep the underground water level below the excavation surface and create the condition of no water excavation

the foundation resistance shall be excavated in layers and sections in strict accordance with the design requirements, and the soil nails shall be constructed layer by layer. The layer height shall be within 1.5m, and the segment length shall be within 20m. After excavation, the soil mass and construction soil nails shall be closed in time. Backhoe excavators and trucks are used to excavate and transport soil. 300mm soil mass is reserved for manual slope construction. After the slope is repaired, the first layer of concrete about 50mm thick is sprayed in time to prevent rain wash and collapse. Reinforcement marks shall be made during construction to control the thickness of the second shotcrete