Effect of tempering on corrosion properties of mold steel

There are many strongly developed karsts in South China, which bring great uncertainty to the construction stability of underground projects. This paper takes the deep foundation pit project of a station in a sandy soil area as the background, simulates the deep foundation pit excavation process based on finite element software, and comparatively analyzes the influence of the existence of karst holes on the foundation pit support structure. The results show that the effective treatment of solution holes in the process of foundation pit excavation can reduce the risk of surface settlement around the foundation pit but has less influence on the lateral displacement of the foundation pit support structure. The cavity grouting reinforcement technology is also introduced in detail in the paper, which can provide a reference for the reinforcement of deep foundation pit projects in sandy soil stratum containing strong development of large cavities.


Introduction
With the rapid development of China's modernization, many large-scale underground projects have emerged, of which tunnels account for the most significant proportion.In station construction, there are usually three kinds of geological conditions: soft soil (clay, silt, and sand), sand and gravel, and mixed strata [1].There are many studies on deep foundation excavation in sandy soil areas, but there are few discussions on deep foundation excavation in sandy soil formations containing cavities [2].Many buried caverns in South China contain high fissure water, are highly susceptible to collapse, are highly permeable, and are characterized by irregular distribution [3].The karst environment can significantly influence the excavation of deep foundation pits [4].The uncertainty and high permeability of the distribution of the caverns may lead to the generation of gushing water accidents in foundation pit excavation.The disturbance of pit excavation and settlement caused by construction disturbance will again aggravate the cavern's collapse and other unfavorable situations, causing long-term instability in the cavern area.In this paper, with the help of three-dimensional nonlinear finite element software, a station's deep foundation pit excavation case is modeled and analyzed [5], and the impact of the cavern on the deep foundation pit construction and construction recommendations are derived.It provides a reference for subsequent similar projects.

Overview of pit works
A station pit is located in the Pearl River Delta plain landscape, and soft soil distribution is vast; there is sand liquefaction; the local section of the cave development is substantial and even has traces of fracture; the site for the building of seismic unfavorable area, the surrounding buildings, construction excavation and precipitation process is straightforward to cause the existing building settlement and side shift, endangering the safety of the building, the pit plan location in figure .1. the comprehensive judgment of the environmental risk level of level one.The station is an underground three-story single-column double-span structure with a total length of 240.0m, standard section pit width of 23.5m, pit depth of 29.0-31.6m,and pit safety level of one.The main structure is constructed by open excavation method.
The pit support design adopts a 1000mm thick underground diaphragm wall and four internal support supports, 1-17 axes enclosure structure adopts 1000mm@750mm biting piles, 17-29 axes adopts 1000mm thick underground diaphragm wall, in-pit descending is carried out before excavation, underground diaphragm wall is also used as a water-stopping curtain, and water-stopping joints are adopted for welded I-beam steel sheet joints.The bottom of the wall enters into the medium-weathering rock layer with low water permeability.

Engineering geological conditions
The station foundation pit is located in the Pearl River Delta Plain.The geotechnical profile of the site mainly consists of miscellaneous fill (from 1.0 to 2.05 m depth), silty clay (from 2.05 to 7.46 m depth), medium-coarse sand (from 7.46 to 19.14 m depth), fully weathered siltstone (from 19.14 to 25.60 m depth), moderately weathered siltstone (from 25.60 to 35.82 m depth), and slightly weathered siltstone (from 35.82 to 50.80 m depth),and slightly weathered siltstone (from 35.82 to 50.80 m depth).Thirteen of the 19 construction boreholes at the site have developed caves, and the karst cave rate is 68.4%.Three representative caves are selected for modeling and analysis in this paper.The degree of karst development is substantial; the caves are mainly half-filled or filled, with a small amount of unfilled, bead-like caves developed, and their fillings are mainly flow, plastic clay, gravel, and a small amount of rock clods, with some of the drilling tools falling from the drilling tools, some can be pressed directly into the drilling, and some of the drilling is faster.There are generally leaks of water, slurry leakage phenomenon.In limestone deposition, primitive pores were formed, and tectonic movements caused faults, creating conditions for rainwater to corrode the rocks.The formation of the caverns is presumed to be influenced by the concealed fracture, as the bedrock fissure water in the site washed over the underlying bedrock, and many calcareous components were lost, resulting in the formation of the caverns.
Groundwater at the site mainly consists of loose rock pore water, bedrock fissure water, and karst water.Open rock pore water primarily exists in the fill and sand soil layer; bedrock fissure water mainly exists in the weathering fissures of the whole ～ middle weathered siltstone; karst water primarily exists in the grey rock of the Meishui Group of the Lower Carboniferous System and the marble of the Shidengzi Group, with the development of dissolving fissures and caves, and abundant water quantity.During the survey, the measured depth of groundwater level was 2.00-4.90m, the elevation of groundwater level was 38.11-40.81m,and the annual change of groundwater level is about 3.0-5.0mwith different precipitation amounts in this area.

Finite element modeling
The finite element software is used to establish the three-dimensional finite element model of the cavern pit, as shown in figure 2. Literature research shows that the length X and width Y in the horizontal direction outside the pit of the 3D finite element model should be taken as 3~5 times the excavation depth of the pit, and Z in the vertical direction should be taken as 2~4 times the excavation depth of the pit.In order to reduce the influence of boundary conditions on the main areas of stress and strain of the bottomless foundation pit, the size of the 3D finite element model should be larger than the influence area of foundation pit excavation, i.e., the horizontal direction outside the foundation pit is more significant than four times the excavation depth of the foundation pit.The vertical direction outside the foundation pit is more significant than three times the excavation depth of the foundation pit.Therefore, the size of the model is chosen to be 360m×250m×105m.The model constrains the average direction of all the surrounding areas; and the bottom surface is fully constrained, and the upper surface is unconstrained.The bottom surface is fully constrained, and the upper surface is unconstrained.Global gravity load was applied to the model, and 20kPa design overload was uniformly set on the construction surface.The contact between the ground-connected wall and the soil on both sides is set as face-to-face contact, and the constraints between the bottom surface of the ground-connected wall, the bottom surface of the base plate, and the soil are set as binding constraints.There are 74,505 units in the model, among which 65,664 C3D8RP units simulate the soil layer, 4,638 C3D8R units simulate the diaphragm wall, 3,164 B31 units simulate the internal support and column, 153 S4R units simulate the diaphragm wall, and 526 S4R units simulate the precipitation well.The cavern was full-fill, replacing the original soil with cavern-fill units.1.The friction coefficients of the soil layer from top to bottom in face-to-face contact between the soil and the ground-connecting wall are 0.31, 0.36, 0.41, 0.43, and 0.51, respectively.

Simulation of construction process
The pit excavation process is simulated by the "unit life and death" technique; after the initial ground stress, the ground connecting wall, columns, and the waterfalling wells, followed by in-pit waterfalling Effective friction angles and applying the first internal support, and the waterfalling simulation is to set the boundary of the pore pressure of the waterfalling wells above the working surface to zero, and then excavating the first layer of soil, and then cycling the above operation until the fourth layer of soil excavation is completed, and then finally laying the bottom slab.

Simulation results analysis
This paper establishes two working conditions to calculate and analyze the cavern model.Some caverns are not treated, and there are caverns and effective treatment of the two working conditions, see table 2. In this paper, the cavern is simplified to be treated as a regular rectangular body, and the detailed characteristics of the cavern, such as morphology, are not considered.figure 3 and 4 show the surface settlement outside the pit and the deflection of the diaphragm wall for the two conditions, respectively.It is worth noting that there is a representative cavern directly below the north diaphragm wall, with a buried depth of 30.15 meters at the top of the cavern, a solubility height of 18 meters, and a thickness of only 1.5 meters at the top of the cavern roof, which belongs to a large cavern.As can be seen in figure 3, the ultimate bearing capacity of the cavern is related to the thickness of the roof plate of the cavern, and the thickness of the roof plate of the cavern, in this case, is small, close to the minimum thickness of the roof plate of the cavern proposed in the relevant literature.Hence, the pit excavation has a more significant impact on it, and the surface settlement outside the wall becomes more significant with the increase of the excavation depth.This is because the untreated cavern can be destabilized by the disturbance of the pit excavation, resulting in the upper part of the cavern soil falling.This part of the falling soil is mainly manifested in the surface settlement outside the wall of the ground-connected wall, and with the increase of the distance outside the wall, the smaller the impact on the surface settlement.As seen in figure .4, the deformation of the ground-connected wall is spindle-shaped, with two small ends and a large middle, similar to the deformation mentioned in many research papers on embedded rock-ground-connected walls.The horizontal displacement of the ground-connected wall becomes more significant with excavating the foundation pit, but the presence of the cavern has little effect on it.In conclusion, the effect of untreated cavities on the surface settlement outside the wall as the pit is excavated is greater than the effect of horizontal displacement of the ground-connected wall.The cavern is generally stable, but if it receives interference, it is easy to continue to expand.When the soil hole develops upward to a certain extent, it will cause the station structure floor to be dehollowed, causing damage to the station and jeopardizing the safety of the project.At the same time, the development of caverns also brings certain security risks to the safety of the surrounding buildings.Under certain conditions, geological disasters may occur, resulting in ground collapse seriously threatening the safety of the surrounding buildings.In order to ensure the construction safety and the stability of the station main body and to provide sufficient bearing capacity for the neighboring buildings, it is recommended to carry out the necessary cavern treatment, taking into account the characteristics of the soil and rock and whether the cavern has already been filled with crushed stone and other materials with high stability.
Usually, mud grouting is used for small caves, and reinforced concrete support is used for large caves.The first step is to conduct preliminary exploration of the caverns to understand as much as possible the number, location, and development of the caverns.Commonly used detection techniques include borehole detection, Rayleigh wave exploration, elastic wave CT, and geo-radar.Before drilling and grouting the caverns, it is necessary to carry out supplementary karst exploration (e.g., over-drilling.) in the range of 3 meters around the pit and 5 meters under the bottom plate in order to determine the boundaries of the caverns and find out as much as possible about the undulation of the rock layer at the bottom of the enclosure structure as well as the development of caverns around the periphery, and to take corresponding cavern treatment measures based on the results of the supplementary exploration.Finally, core sampling can be taken for the uniaxial compressive strength test (UCS) and standard penetration test (SPT), but also through the touch probing method to check the effect of the cavern treatment; only after passing the test can the pit enclosure construction be carried out, in order to ensure the quality and safety of the pit excavation.
For this project's representative large-volume beaded cavern, the cavern can be treated with grouting filling.Grouting holes in the bottom of the karst hole at a depth of 0.2 to 0.3 meters, 2-meter spacing matrix distribution.After drilling the grouting holes, introduce the grouting pipe to the bottom of the hole and start grouting.Because of the uncertainty of the thickness of the top plate of the cave, pressure grouting is used, and the pressure is controlled within 1MPa in the actual construction.The initial grouting pressure can be 0.3~0.4MPa,and the grouting speed is 30~70L/min.When the grouting pressure reaches 0.5MPa, the pipe is lifted by 0.5m to continue grouting.Repeat this process until the grouting pipe reaches the top of the cavern.When the grouting pressure reaches 1.2MPa and no longer rises, keep the steady state for 10 minutes, then stop grouting and smooth the hole to restore the road surface.Repeat this process until all the grouting holes are filled, and the hole is grouted.In grouting, if the grouting pressure has not risen, there are large cracks in the cavern or connected to other caverns.At this time, you should increase the proportion of water glass in the cement slurry, increase the initial solidification time, pause the grouting until the cement slurry reaches the initial solidification state, and continue to grout many times.In the grouting process, if you find that the grouting pressure has been rising, then you need to use two-liquid slurry; the specific gravity of the water glass needs to be determined according to the actual situation, according to experience can be generally taken as 4% ~8%.

Figure 5. Schematic diagram of grouting points
For this project in figure 5, the effectiveness of the cavity grouting treatment testing is mainly through sampling geotechnical tests, according to 10% of the number of grouting holes to take samples for testing, and uniaxial compressive strength as a test standard, supplemented by a standard

Conclusion
The water-rich sandy soil layer of the cavern will be affected by the pit excavation disturbance; combined with the finite element model, it can be seen that the existence of the cavern in the pit excavation process will affect the surface settlement around the pit, the pit excavation disturbance will aggravate the surrounding soil fall, thus increasing the surface settlement, the excavation disturbance on the horizontal displacement of the ground wall itself has less impact, through effective treatment of the cavern can be a greater degree of enhancement of the overall stability of the pit and its surroundings.In summary, for the strong development of the cavern rich in sand and soil and other permeable solid layer of the station pit, there is a risk of karst collapse, etc., should be based on the actual distribution of the station pit and the development of the cavern and other essential characteristics, combined with the risk of collapse in the process of excavation of the pit, comprehensively determine the cavern treatment program.Cave cement slurry grouting reinforcement is a more economical and effective reinforcement program, which mainly includes cave exploration, drilling, grouting, and testing the effect of grouting treatment.The cavern treatment program of this project can provide a reference for similar projects.

Figure 3 .
Figure 3.Comparison of surface settlement under different working conditions

Figure 4 .
Figure 4. Comparison of horizontal displacement of wall under different working conditions

Table 1 .
Parameters of finite element model

Table 2 .
Numerical study of the effect of the penetration test: testing requirements core sample integrity, and 28d unconfined compressive strength of not less than 0.2MPa.