Mechanical Characteristics of Assembled Overhanging Composite Roadbed Structure

Abstract

In order to solve the problems of narrow roads, difficult construction, long construction period and high impact on the surrounding environment encountered during the construction of mountainous highways, railroads and other infrastructures, this paper proposes an assembled overhang composite roadbed structure. The mechanical analysis of the assembled overhang is carried out, and the three-dimensional finite element numerical model of the assembled roadbed structure is established, and the force characteristics of the overhang structure are analyzed. The results show that there is stress concentration between the two spans of the road panel, and the deformation of the road panel is mainly concentrated in the overhang side, and the deformation of the side span is larger than that of the middle span. The stress concentration phenomenon appeared on the outer side of the connection end of the middle overhanging and column, and the horizontal displacement of the middle column was larger, and the displacement was about twice that of the edge column. The research results can provide useful guidance for the design and optimization of assembled overhanging roadbeds.

1 Introduction

With the implementation of major national strategies such as Sichuan-Tibet Railway and Transportation Power, the construction technology of mountainous highways, railroads and other infrastructure has become a hot concern in the field of engineering construction [1, 2]. In the changing environment of mountainous terrain, the cast-in-place concrete construction scheme is not convenient. And the levelness and stability of the ground in mountainous areas are not easily guaranteed, which has an impact on the construction parts that require support formwork or other enclosure measures.

The emergence of overhanging structures has led to a richer and more efficient construction solution for mountainous roadbeds. Wang [3] first proposed the design concept of widening mountain roads with monolithic overhanging structures in 2007. This concept forms the embankment with monolithic poured wall-post retaining wall without destroying the original ecological environment, and completes the road under-width with overhanging structure. For the problem of narrow roads in mountainous areas, Liu et al. [4, 5] considers the joint action of the overhanging structure and the geotechnical body, and proposes a widening scheme for mountainous roads. Zhuo [6] takes the re-expansion project of the tertiary road from Ju Shui Town to Gao Chuan Township, An County, Sichuan Province as an example, and introduces in detail the application of overhanging slabs in the widening of the roadbed of a mountainous cliff section with high and steep slopes. Other scholars proposed a road widening scheme of buried overhang structure for a mountainous road modification and expansion project [7, 8]. In terms of theoretical research, Tian [9] proposed the mechanical model and calculation principle of the overhanging roadbed based on the simplified single-bay overhang structure. Guo et al. [10] carried out a systematic study on the structural form, external load type and load combination method of overhanging U-shaped roadbed, and proposed the design and analysis methods of structural bearing capacity, fatigue resistance, crack control and deflection deformation of overhanging U-shaped roadbed. In terms of numerical simulation, You [11] used ABAQUS to analyze the working performance of the overhanging structure of slope lattice foundation with various foundation conditions, various side slope angles and various overhanging beam lengths. Wei et al. [12] established a two-dimensional finite element model of overhanging structure and proposed the appropriate structural dimensions of overhanging slab. Yao et al. [13] established a three-dimensional finite element model of overhanging-support roadbed structure and proposed the optimal size selection of counterweight blocks, support columns and other components. In terms of engineering application, Yuan [14, 15] proposed the general design idea of assembled overhang structure and supporting construction plan for the problems faced by overhang composite roadbed. Wang et al. [3] conducted a study on the application of monolithic overhang structure in Tibetan class Chang highway, and summarized the processing techniques of structural support columns and anchorage points of overhanging beams under different geological conditions and different widening requirements. Li [16] presents detailed construction schemes and technical points for the engineering application of composite roads with integral overhanging structures. Previous studies on overhanging structures stay on the structural calculation principles and the influence of site conditions on overhanging structures, lacking a more systematic study on the mechanical characteristics of assembled overhanging structures, and not revealing the mechanical behavior characteristics of assembled overhanging roadbeds under different overhanging lengths and anchor placement methods.

The assembled structure has many advantages such as improving production efficiency, shortening construction period, reducing resource consumption, and having little impact on the environment [17, 18], so it is widely used in the process of engineering construction. The application of assembled overhang structure in road foundation engineering can achieve less ecological damage, reduce excavation and filling, low cost, simple construction, and apply to different terrain and geological conditions, which can effectively reduce the safety hazards of road foundation construction in mountainous areas. Therefore, new road construction solutions suitable for mountainous areas are gradually mentioned and increasingly in demand. This paper adopts finite element analysis method to study the main influencing factors of overhanging roadbed, combine with engineering Conditions, and propose an optimized construction technology scheme of assembled overhanging roadbed.

2 Working Principle and Mechanical Analysis of Assembled Overhang Structure

The assembled overhang structure is a structural form consisting of columns, outer longitudinal beams, overhanging beam, precast slabs, inner longitudinal beams, retaining slabs and anchors in one piece. The column and the soil are often in contact with each other by filling foam concrete, so the column is subjected to the pressure from the foam concrete, which is recorded as No. 1 soil pressure. The lower end of the column is embedded in the soil body, and the soil pressure generated by the interaction between the soil body and the column is recorded as No. 2 soil pressure. Therefore, the column is mainly subjected to the two types of earth pressure as shown in Fig. 1. The part above the embedded section of the column is calculated according to the geotechnical lateral pressure, and the internal force below the embedded section can be calculated according to the bending moment and shear force at the ground level of the embedded end combined with the foundation coefficient method.

Fig. 1.

Simplified schematic diagram of assembled overhang structure subjected to soil pressure

As shown in Fig. 2, the connection between the column and the soil can be regarded as rigid connection, and the overhanging beam is bolted to the column, which can be regarded as rigid connection. The left side of the overhanging beam is reinforced by anchor rods, which provide anchoring tension, and the force is influenced by the structural load, and the connection between the anchor rods and the overhanging beam is regarded as hinged. The loads on the overhanging structure are mainly the self-weight of the structure, the lane load consisting of inner and outer longitudinal beams and precast slabs, and the vehicle load on the road.

Fig. 2.

Simplified calculation diagram of overhanging structure

3 3D Numerical Model of Assembled Road Base Structure

3.1 Introduction of the Overhanging Structure Scheme

The relying project is located in the section of G4012 Liyang-Ningde Expressway from Huangshan to Qiandao Lake, starting and ending at pile number k9 + 258–k15 + 482, with a total length of 6.244 km, using assembled overhanging structure roadbed and high and low mountain bridge to widen the road surface. Figure 3 is the schematic diagram of the assembled overhanging roadbed.

Fig. 3.

Schematic diagram of assembled overhanging roadbed

3.2 Model Building

ABAQUS finite element software is used to simulate the overhanging structure, and the most steeply inclined slope in the geographic location of the overhanging structure is selected as the model cross section, and the soil is restrained by the normal displacement around and vertical displacement at the bottom, as shown in Fig. 4 below. The anchor rod adopts truss unit, and the built-in area function is used to simulate the contact between the anchor rod and soil. The Mohr-Coulomb principal structure is used for the geotechnical body, and the geotechnical material parameters of the slope are shown in Table 1 below.

Table 1. Rock parameters of the slope

Fig. 4.

Schematic of the slope model

The dimensions of the model are: the spacing between the overhanging beam and columns is 8 m; the overhanging length is 12.75 m, the beam height is 0.75 m–1.5 m, and the overhanging length is 4.25 m; the column cross-section size is 1.5 m × 1.5 m, and the columns are connected by reinforced concrete internal and external longitudinal beams to form a whole frame structure; the width of the external longitudinal beam and retaining plate is 50 cm, and the width of the internal longitudinal beam is 100 cm. The overhanging structure and the effect are shown in Fig. 5 The overhang structure and the effect are shown in Fig. 5. The precast overhanging beam are made of C50 concrete, the precast hollow slab, outer longitudinal beam and retaining slab are made of C30 concrete, and the inner longitudinal beam, inner longitudinal beam foundation, column and retaining slab foundation are made of C30 cast-in-place concrete. Foam concrete is filled between the permanent protection surface of the slope and the retaining slab, and each concrete parameter is shown in Table 2.

Table 2. Concrete parameters

Fig. 5.

Finite element model of the overhang structure

3.3 Load Application

According to the General Design Specification for Highway Bridges and Culverts (JTG D60-2018), the relevant loads are applied in combination with the most unfavorable position of the overhanging structure. The narrow strip area of the overhanging pavement is selected to convert the line load and concentrated load imposed in the code into a uniform load to achieve the effect of simulating the lane load and vehicle load.

Analyzing the characteristics of the overhang structure and the assembly type, there are two most unfavorable load arrangements for the overhang structure, which are set as Condition 1 and Condition 2. Condition 1 is to distribute the lane load on the whole overhang road according to the specification spacing, under this arrangement, the overhang structure has adverse effects on the pressure of the embedded section at the bottom of the column, the pressure of the assembly node and the side slope soil. In this arrangement, the overturning moment inside the overhanging structure is the largest, and the connection part with the soil such as the anchorage end and the inner side of the column is the most unfavorable. The load application method of the overhang structure is shown in Fig. 6.

Fig. 6.

Schematic illustration of load application on overhanging structure

4 Analysis of Results

4.1 Slope Soil Displacement and Stress Analysis

Figure 7(a) and (b) shows the displacement contours in the X-direction for condition 1 (overhanging structure under full lane load) and condition 2 (overhanging structure under overhanging side lane load), respectively. It can be seen that the slope under the action of Condition 1 has increased displacement and there is a sliding surface. As the embedded foundation is driven by the overhanging beam, the soil in the embedded section shows an obvious displacement in X direction. Comparing condition 1 and condition 2, it can be found that because the soil under the action of full lane load in condition 1 is subjected to the largest force, the largest deformation and a larger range of sliding surface, is used to analyze the deformation and stability of the soil under the most unfavorable load in. In condition 2, the overhang is prone to overturn under the lane load on the overhang side, and the column deformation is the largest. Therefore, condition 2 can be used to analyze the stresses and displacements of the overhanging structure under the most unfavorable conditions.

Fig. 7.

Displacement contour on the X-direction of the slope

Figure 8 shows the stress contour in the X-direction under the action of condition 1. The soil at the embedded end portion of the side slope of the overhanging structure is subjected to a full lane load and the self-weight of the structure. As shown in the black dashed box in Fig. 8, the soil in this area presents a large tensile stress along the side slope, which leads to a slip surface on the side slope. Therefore, foam concrete is often filled between the column and the slope in practical engineering, thus containing the slip damage on the overhanging slope.

Fig. 8.

Stress contour on the X-direction of the slope

4.2 Soil Stability Analysis

The reduction factor F_s in the strength reduction method is defined as the ratio of the maximum shear strength of the soil in the slope to the actual shear stress produced by the external load in the slope, while the external load is kept constant. In the finite element calculation, as the reduction factor F_s increases, the shear strength of the soil decreases until it is reduced to a certain degree and the calculation cannot be converged, which means that the shear strength of the soil at this time is not enough to support the stability of the soil and the soil is in an unstable state. The reduced shear strength index is calculated according to Eq. (1) and Eq. (2)

$$ C_F = C/F_S $$

(1)

$$ \varphi_F = \tan^{( - 1)} (\tan \varphi /F_S ) $$

(2)

where: C is the cohesion of soil before reduction, ϕ is the angle of friction within the soil before reduction, C_F is the cohesion of soil after reduction, and ϕ_F is the angle of friction within the soil after reduction.

Based on the strength reduction method, the internal friction angle and cohesion of the slope soil are reduced according to a certain proportion, and the following Fig. 9 is calculated by the reduction analysis contour map of the plastic zone of the slope soil. It can be seen that after the sliding surface is reduced by strength, the sliding surface of the slope extends from the location of the excavation and slope release of the overhanging structure to the top of the slope. To extract the calculation results, take the horizontal displacement at the top of the slope as the vertical axis and the reduction coefficient as the horizontal axis, and draw the reduction process curve. Figure 10 below shows the curve of reduction coefficient and displacement of slope top. From the Figure, we can see that the inflection point of the curve is located at about 1.71, and the stability coefficient of the slope is 1.71 according to the principle of strength reduction method, which means that the slope is stable.

Fig. 9.

Contour of plastic zone of slope soil

Fig. 10.

Plot of reduction factor and slope top displacement

4.3 Stress and Displacement Analysis of the Overhanging Structure

The main stress-bearing elements in the whole overhanging structure are the road panel, overhanging beam and columns, so these parts are also the parts with large deformation. Figure 11 below is the road panel stress contour. As the concentrated load of the lane is applied to the middle part of each span of the overhanging structure, a stress concentration phenomenon occurs between the two spans. This is due to the middle three span road panel each span is supported by both sides of the overhanging beam, bearing a large negative bending moment to resist the road panel downward depression. The stress concentration also occurs in the road panel directly above the overhanging beam, which is caused by the bending moment generated by the interaction between the overhanging beam section and the road panel after the lane load is applied to the road panel. Figure 12 shows the deformation cloud diagram of the roadway panel. From this figure, we can see that the deformation is mainly concentrated in the overhang side, and the side span deformation is larger than the middle span. The reason is that the middle span overhanging plate is subjected to large negative bending moment at both pivot points, which plays a role in resisting plate deformation. While the side span only near the middle span side by the same size of the negative bending moment. In the most edge side of the restraint is smaller, so the deformation of the plate is relatively large. And the road panel in the overhanging beam between the obvious sag, overhanging beam upper road panel due to the overhanging beam support deformation is small, and the outermost side of the road panel lack of restraint, resulting in the road panel upward bending. The actual project should strengthen the outermost sides of the road panel restraint.

Fig. 11.

Stress contour of the road panel

Fig. 12.

Deformation contour of the road panel

Figures 13 and 14 below show the stress and displacement contours of the overhanging skeleton, which mainly consists of three parts: overhanging beam, columns and external longitudinal beams. According to the stress cloud diagram, it can be seen that the stress concentration appears on the outer side of the connection between the four middle overhanging beams and the column, indicating that the stress concentration occurs due to the extrusion of the four middle overhanging beams at the connection with the outer edge of the column due to the large bending moment. In Fig. 14, it can be seen that the displacement of the overhanging section of the middle four overhanging beam is larger, about 5 mm, while the displacement of the two spans at the edge is smaller, about 3 mm. Figure 15 shows the moment-shear diagram of the connection between the column and the overhanging beam, and it can be seen that the middle four overhanging beam are subjected to about twice the moment and shear force than the two spans at the edge. Under the action of external load, the four overhanging beams in the middle of the overhanging structure are in the most unfavorable force condition.

Fig. 13.

Overhanging structures skeleton stress contour

Fig. 14.

Deformation of overhanging structures skeleton.

Fig. 15.

Shear moment diagram of the connection point between the overhanging beam and the column

The upper end of the column is connected to the overhanging beam, and the lower part is embedded into the interior of the soil body of the slope, bearing the vertical load and bending moment transmitted from the upper part, the deflection and deformation of the column will directly affect the stability of the upper structure, especially the X-direction displacement of the column directly affects the stability of the overhanging structure. Figure 16 shows the X-direction displacement contour diagram of the column, and Fig. 17 shows the horizontal displacement curve from top to bottom of the unembedded soil section of the column. It can be seen that the X-direction displacement of the middle four columns is larger, especially at the top. According to the displacement curve, the displacement of the middle column is twice that of the edge column, which is mainly due to the outward bending of the column caused by the transfer of the bending moment of the upper beam and the deflection deformation caused by the action of the upper load, and the superposition of the two produces a deformation curve similar to the “S” shape of the column.

Fig. 16.

X-direction displacement contour of column

Fig. 17.

Horizontal displacement curve of the unembedded soil section of the column

5 Conclusion

In this paper, a three-dimensional finite element numerical model of the assembled overhang roadbed structure is established, and the mechanical characteristics of the overhang structure under the slope stress and displacement, slope soil stability and load are analyzed. The research findings are summarized as follows.

• (1) After strength reduction, the sliding surface of the slope extends from the location of the excavated and released slope of the overhanging structure to the top of the slope. The stability coefficient of the slope is 1.71, which indicates that the slope is stable.

• (2) Stress concentration phenomenon between the two spans of road panels, road panel deformation is mainly concentrated in the overhang side, and side span deformation is larger than the middle span, road panels in the part between the overhanging beam appeared obvious sagging deformation, the actual project should strengthen the outermost sides of the road panel restraint.

• (3) The stress concentration phenomenon appeared on the outer side of the middle overhanging and column connection end because the middle overhanging was subjected to about twice the bending moment and shear force than the two spans at the edge. Under the action of external load, the middle overhanging beam of the overhanging structure is in the most unfavorable state of force.

• (4) The horizontal displacement of the middle column in X direction is larger, especially the displacement at the top of the column is larger, and the displacement of the middle column is about twice of the edge column. This is mainly due to the bending of the column caused by the transfer of the bending moment of the upper beam and the deflection deformation caused by the action of the upper load, under the superposition of the two, the column produces a deformation curve similar to “S” type.