Wu Taihang
Zhenning Highway Management Section of Guizhou Province
Abstract: The mechanical properties of soft soil subgrade are poor, and it is easy to produce excessive post-construction settlement under the action of self-weight stress and additional stress. This paper analyzes the stress and settlement calculation methods of highway soft soil subgrade, discusses the application of replacement method, drainage consolidation method, strong compaction method and pile-soil composite foundation method in the treatment of soft soil subgrade, and takes a highway project as the research object, uses the finite element software Pluxis to establish a calculation model, and analyzes the settlement change law of soft soil subgrade before and after treatment with cement mixing piles.
Keywords: soft soil subgrade; settlement calculations; treatment measures; finite element software; cement mixing pile;
About author:WU Taihang (1986—), male, undergraduate, engineer, engaged in highway and bridge construction management.
0 Introduction
Highway engineering is a linear structure with a long construction mileage, and it is inevitable that it will pass through special geotechnical areas, such as soft soil. Compared with the general roadbed, the mechanical properties of soft soil are poor, such as low bearing capacity, low strength, high compressibility, and obvious thixotropy. If effective treatment measures are taken for the soft soil subgrade during the design and construction process, it may lead to a large post-construction settlement of the highway, resulting in cracks in the road surface or slope collapse, affecting driving comfort and safety, and in serious cases, it will lead to large casualties and economic losses [1]. In view of this, scholars at home and abroad have carried out a lot of research on the settlement and treatment of soft soil subgrade, and have achieved many advanced results. However, when engineering and technical personnel choose soft soil treatment measures, most of them rely on engineering experience or apply other project drawings, which makes the treatment plan unreasonable or uneconomical, resulting in a certain degree of waste of resources. Therefore, it is necessary to further study the settlement calculation method and treatment countermeasures of soft soil subgrade.
1. Stress analysis and settlement calculation of highway soft soil subgrade
1.1 Calculation of stress in soft soil subgrade
The subgrade settlement of soft soil consists of two parts: the compression settlement of the subgrade itself and the settlement of the foundation soil, and the settlement depends on the self-weight stress of the subgrade and the additional stress of the foundation [2].
1.1.1 Subgrade self-weight stress
The vehicle load is complex and in dynamic change, as can be seen from the Code for Design of Highway Asphalt Pavement (JTG D50-2017), the pavement design load is a single-axle-double-wheel combined load (standard axle load 100 k N, wheel pressure 0.7 MPa). When calculating the self-weight stress of the subgrade, the vehicle load transferred to the top surface of the subgrade can be statically equivalent. The formula for calculating the self-weight stress of the subgrade is as follows:
Subgrade self-weight stress = vehicle load + pavement load + subgrade fill load
Thereinto:
Pavement load = asphalt mixture gravity × pavement thickness
Subgrade fill load = subgrade fill weight × subgrade thickness
The subgrade fill load increases with the increase of the fill height, but the weight of each layer of fill is also different due to the different compaction control standards of the upper roadbed, the lower roadbed, the upper embankment and the lower embankment. When calculating the self-weight stress of the subgrade caused by the filling, it should be calculated in layers.
1.1.2 Additional stresses on foundations
The additional stress caused by the subgrade fill in the foundation is attenuated with the depth of the foundation, and the relevant specifications for subgrade design or construction do not give a clear calculation method. In the project, you can refer to the "Code for Design of Highway Bridge and Culvert Foundation and Foundation" (JTG 3363-2019) to calculate the additional stress of the foundation, that is, the subgrade is regarded as a strip foundation, and the additional stress coefficient is checked according to the ratio of the calculated point depth to the foundation width, and the product of the foundation pressure and the additional stress coefficient is the additional stress of the foundation at the point.
1.2 Calculation method of soft soil subgrade settlement
1.2.1 Tiered summation method
Total settlement of soft soil foundation = instantaneous settlement Sd + main consolidation settlement Sc + secondary consolidation settlement Ss. According to the Code for the Design of Highway Subgrade (JTG D30-2015) (hereinafter referred to as the "Code"), the main consolidation settlement of soft soil foundation can be mainly settled by the "layered sum method", which is calculated in Equation (1) [3]:
In the formula, ,nn———— the number of layers of foundation soil divided into layers;;hhii———— the thickness of the second layer of soil (mm);; ee0i———— the ratio of the porosity ratio of the self-weight stress force at the midpoint of the second layer of soil to the corresponding porosity-to-void ratio;;ΔΔee1i———— the porosity ratio corresponding to the self-weight stress at the midpoint of the ith layer of soil + the additional stress.
The mechanism of instantaneous settlement and sub-consolidation settlement is complex, the calculation theory is immature, and the calculation is difficult, so it is not considered in practical engineering, and the product of the main consolidation settlement Sc and the settlement coefficient ms is generally used to represent the total settlement of soft soil foundation. The calculation of the settlement coefficient ms can be referred to Equation (2):
where, γ - packing weight (k N/m3); H - the height of the center of the embankment (m); θ - the coefficient of foundation treatment type, V - the correction coefficient of loading rate, and Y - the correction coefficient of geological factors.
1.2.2 Finite element method
Compared with the "layered sum method", the finite element method is to calculate the soft soil foundation and embankment as a whole, which has the advantages of low cost, high calculation efficiency and good repeatability, and can calculate the subgrade settlement under multiple working conditions in a limited time, and obtain the deformation and stress at any point in the foundation and embankment. At the same time, the finite element method can also consider the influence of factors such as stress history, complex boundary conditions, step-by-step loading of construction, rainfall seepage and other factors on the subgrade settlement of soft soil subgrade.
2 Analysis of treatment measures for highway soft soil subgrade
2.1 Soft soil subgrade treatment objectives
The goal of highway soft soil subgrade treatment is to control the settlement of subgrade after construction, so as to avoid excessive settlement causing subgrade slope instability, slippage and road surface cracking, which will affect driving safety and comfort. The allowable post-construction settlement control of highway soft soil subgrade is related to the highway grade and subgrade location, as shown in Table 1 [4]:
Table 1 Allowable post-construction settlement of soft soil subgrade Download the original drawing
/cm
2.2 Soft soil subgrade treatment
The selection of the highway soft soil foundation reinforcement scheme should follow the principle of "simple first and then complex, and local materials", comprehensively consider the thickness and nature of the soft soil, the height of the embankment, the allowable settlement after construction, the construction environment, the construction period and other factors, and carry out technical and economic necessary selection. For soft soil foundations with poor properties, complex foundation conditions, tight construction periods, and lack of fillers, comprehensive treatment measures should be adopted.
2.2.1 Replacement method
The replacement method is to remove part of the soft soil that needs to be replaced, backfill the materials with high strength, large bearing capacity and hard materials (such as medium sand, coarse sand, ash soil, gravel, etc.), and use large machinery to tamp or compaction to improve the bearing capacity of the pressed foundation. The replacement method is convenient and economical, but it is mainly aimed at shallow soft soil foundation treatment, and the treatment depth is <3 m.
2.2.2 Drainage consolidation method
The drainage consolidation system of soft soil subgrade is composed of two parts: drainage system and pressurization system, in which the drainage system has vertical drainage bodies such as bagged sand wells and plastic drainage boards, and the pressurization system has stacking preload, vacuum preloading, etc. The vertical drainage body should be arranged according to the equilateral triangle, and its length is determined by the calculation results of the settlement of the soft soil subgrade. If the soft soil layer is thin, the vertical drainage body should pass through the soft soil layer and penetrate into the lower harder soil layer for at least 50 cm. At the same time, the settlement of the foundation during the preloading period should be greater than the difference between the settlement at the end of the design service life of the pavement and the allowable settlement after construction, and the preloading period should not be less than 6 months.
2.2.3 High compaction method
The strong compaction method is to lift the rammer to a set height and fall freely, convert its gravitational potential energy into power ramming energy, and tamp the soft soil foundation. In order to ensure the tamping effect, a layer of 30~50 cm gravel or gravel cushion should be laid on the top of the soft soil foundation before strong compaction.
The effective reinforcement depth d of the strong compaction method can be estimated according to the test tamping or equation (3):
In the formula, α - correction coefficient, the value range is 0.34~0.80; m - rammer mass (t); h - rammer drop distance (m).
The key to the design of the strong compaction method is to determine the number of ramming and the tamping energy, so as to avoid the surrounding soil uplift or the ramming pit too deep. When the ramming energy of the click is < 2 000 k N·m, the average tamping and sinking requirements of the last two strokes are ≤50 mm, when the ramming energy of the click is 2 000~4 000 k N·m, the average tamping and sinking requirements of the last two strokes are ≤100 mm, respectively, and when the ramming energy of clicking is > in the range of 4 000 k N·m, the average tamping and sinking requirements of the last two hits are ≤200 mm, respectively.
2.2.4 Pile-soil composite foundation method
In deep soft soil areas, cement mixing piles, CFG piles, prefabricated pipe piles, etc. are often used to form a composite foundation with soft soil to bear the external load. Among them, the cement mixing pile is to add the cement material, external admixture, etc. to the surrounding foundation soil in the way of powder spraying or slurry spraying, the reinforcement depth of the powder spraying pile is ≤ 12 m, and the reinforcement depth of the slurry spraying pile is ≤ 20 m, which has the advantages of fast construction speed, short construction period and low price; CFG pile is mixed by crushed stone, fly ash, cement and other materials in a certain proportion, and a layer of gravel and gravel should be laid on the top of the pile as a mattress cushion (thickness 30~50 cm); prefabricated pipe pile includes PC, PHC and PTC, and has high bearing capacity of single pile, Deep reinforcement depth, high construction efficiency, no pollution at the construction site and other advantages.
The reinforcement mechanism of composite foundation is mainly reflected in the following three aspects [5]: first, the compaction effect. During the construction of the pile, the soil around the pile will be partially or completely extruded to the pile side, so that the soil between the piles is more compact; The pile body in the composite foundation has high strength, and the load borne by the pile is much greater than that of the soil between the piles, and the load can be transferred to the depth of the foundation. The pile body is coordinated with the soil around the pile, that is, the pile body restricts the soil body, so that the soil body does not appear shear failure under high stress, and the soil body restricts the pile body and maintains the shape of the pile body.
3 Case study of subgrade settlement calculation and treatment of highway soft soil
3.1 Project Overview
Taking the soft soil subgrade of a highway in Guizhou Province as the research object, the settlement calculation method of a highway in Guizhou Province was discussed by using the finite element software Plaxis. The design speed of the highway is 100 km/h, the design load is highway-I, and the main line is 4 lanes in both directions. The cross-sectional station number of the subgrade settlement calculation is K10+800 (adjacent to the abutment and the subgrade), the width of the subgrade is 25 m (0.75 m for the dirt shoulder + 2.5 m for the hard shoulder + 2×3.75 m + 3.5 m for the median + 2×3.75 m for the roadway + 2.5 m for the hard shoulder + 0.75 m for the dirt shoulder), the filling height is 6 m, and the slope slope ratio is 1:1.5. The area where the project is located belongs to the warm temperate monsoon continental climate, with high temperature and rain in summer, low temperature and little rain in winter, large annual temperature difference, large annual precipitation, and an average annual rainfall of about 986 mm, which is mainly concentrated in June to September.
According to the geological survey data, the subgrade filler is a well-graded gravel soil. From top to bottom, the foundation soil is silt, silty silty clay and silty clay. The physical and mechanical calculation parameters of different rock and soil masses are shown in Table 2:
Table 2 Calculation parameters of each layer of soil Download the original drawing
3.2 Soft soil treatment scheme
The burial depth of silty silty clay at the K10+800 section is larger, about 6.5~8.8 m, and it is not suitable to use the replacement method or strong compaction method, and the construction period is tight, and the sufficient preloading time cannot be guaranteed, so the drainage consolidation method is not applicable. After the technical scheme and economic necessity, it is proposed to use cement mixing pile composite foundation to treat soft soil. The cement mixing pile has a diameter of 0.5 m, a pile length of 10 m, and a pile spacing of 1.5 m.
3.3 Finite element model establishment
3.3.1 Yield Criterion
Soil is a three-phase substance composed of soil particles, gases and liquids, which is compressive but not tensile resistant, so the Drucker-Prager criterion built into the Pluxis software was selected to calculate the settlement of the foundation and subgrade. The expression of the Drucker-Prager yield criterion is shown in (4) [7]:
where α, K, and J are all material constants, F is the compressive strength of the material, and I is the sum of the principal stresses.
3.3.2 Boundary conditions and meshing
The boundaries of the left and right sides of the soft soil subgrade are undrained boundaries and are horizontally constrained. The lower boundary is an undrained boundary, fully constrained, and has no deformation in any direction. The top surface of the roadbed and the slope surface of the slope are the drainage boundaries, which can be freely deformed in the X, Y and Z directions. Under the condition of comprehensive consideration of calculation accuracy and efficiency, the soft soil subgrade model was meshed by tetrahedral elements, and the mesh size was 2.5 m, and a total of 958 elements and 1 022 nodes were divided, as shown in Figure 1:
Fig.1. Calculation model of soft soil subgrade Download the original drawing
3.3.3 Load simulation
In order to accurately simulate the construction process of soft soil subgrade (fill rate of 60 mm/d), the time step must be selected reasonably. When the time step set in the software < the minimum critical value, stress oscillations are prone to occur.
3.4 Settlement and deformation law of soft soil subgrade before and after treatment
According to the "settlement symmetry theory" of subgrade, only half of the subgrade is taken when analyzing the settlement and deformation law of soft soil subgrade in K10+800 section, that is, one settlement monitoring point is set every 2 m with the road centerline as the benchmark, and the calculation results of the post-construction settlement of each monitoring point are shown in Figure 2.
It can be seen from Figure 2 that at the center point of the subgrade, the settlement of the soft soil subgrade after construction is the largest, and the farther away from the center point, the smaller the subgrade settlement. When the soft soil subgrade is not treated, the maximum post-construction settlement is 16.1 cm, and the settlement value is too large, which does not meet the "Specification". After the soft soil subgrade was treated with cement mixing piles, the post-construction settlement of each monitoring point decreased significantly, and the maximum settlement was only 7.5 cm, a decrease of 53.4%. This shows that the design scheme of cement mixing pile composite foundation is reasonable and feasible, and the treatment effect of soft soil subgrade is good.
Fig.2 Subgrade settlement of soft soil at different monitoring points Download the original map
4 Conclusion
In this paper, the calculation methods and common treatment measures of soft soil subgrade are studied, and the settlement variation law of soft soil subgrade before and after treatment is calculated by using finite element software Pluxis software based on a highway project, and the following conclusions are drawn:
(1) The settlement of soft soil subgrade depends on the self-weight stress of subgrade and the additional stress of foundation, which can be calculated by layered sum method or finite element method.
(2) The common reinforcement measures for soft soil foundation include replacement and filling method, drainage consolidation method, strong compaction method, composite foundation method, etc., which should be selected in combination with factors such as soft soil thickness, embankment height, construction environment, and construction period.
(3) The settlement of soft soil subgrade is the largest after construction at the center line position, and the farther away from the center point, the smaller the settlement.
(4) Cement mixing pile can significantly reduce the settlement of soft soil subgrade engineering, and the reduction range can reach 53.4%.
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