Abutment back soft roadbed structure and construction method

Abstract

The abutment back soft roadbed structure comprises a rubble structure layer, a broken stone hardcore, a first waterproof geotechnical cloth, a second waterproof geotechnical cloth, a light soil layer, an improved soil layer, a water stabilizing layer, a sleeper beam and a butt strap, a steel corrugated pipe is arranged in the light soil layer under the sleeper beam, a bottom cushion layer is laid under the steel corrugated pipe, and the bottom cushion layer is laid under the bottom cushion layer. The angle corresponding to the arc length of the portion, embedded in the middle of the bottom cushion layer, of the steel corrugated pipe is not smaller than 120 degrees. Annular sealing rings are arranged at the two ends of the steel corrugated pipe. The construction method comprises the following steps: a) riprapping; b) pouring the retaining wall; c) paving broken stones; d) laying a cushion layer; e) mounting a corrugated pipe; (f) sealing; g) laying geotechnical cloth; h) filling light soil; i) filling and improving soil; j) pouring the sleeper beam; and k) laying a butt strap. According to the roadbed structure and the construction method, the volume weight value of the steel corrugated pipe is close to 0, the requirement for the bearing capacity of a foundation is remarkably reduced, the upper load is dispersed to the surrounding soil body, and vertical sedimentation of the sleeper beam is reduced.

Description

Technical Field

[0001] The present invention relates to a subgrade structure and a construction method, and more specifically, to a soft subgrade structure at the back of a abutment and a construction method. Background Art

[0002] In road and bridge engineering, affected by the small settlement of bridge pile foundations and relatively large settlement of subgrades, uneven settlement will occur at the connection position between the bridge and the road subgrade. If not treated, the phenomenon of "bump at bridge head" will occur. Bump at bridge head will cause the wheels to lose part of the traction force and may even lead to traffic accidents. Currently, a transition slab is usually set at the connection between the bridge head and the subgrade. One end of the transition slab is placed on the corbel of the bridge pile foundation cap, and the rest is laid on the subgrade at the back of the abutment to solve the bump at bridge head caused by the uneven settlement between the bridge pile foundation and the subgrade.

[0003] However, for soft soil subgrades, their bearing capacity is usually less than 120 kPa. After setting the transition slab at the bridge head, due to the larger settlement of the soft soil subgrade, the uneven settlement amount at both ends of the transition slab is increased. The large differential settlement on both sides of the transition slab is the core problem causing the "bump at bridge head" phenomenon. Most traditional structures at the back of the abutment use pile foundation treatment, but there are the following defects: long construction period of pile foundation (the preloading period of pile foundation at the back of the abutment is usually 4 - 6 months), high cost, and strict requirements for the bearing capacity of the foundation, which is not suitable for soft soil subgrades. Although lightweight materials have been tried in the prior art, there is a lack of systematic optimization design for load transfer, and it is difficult to achieve a coordinated improvement in structural settlement control.

[0004] This article presents a soft subgrade structure at the back of the abutment and a construction method, which is additionally provided with a steel corrugated pipe for load dispersion, and cooperates with a bottom cushion layer, a lightweight soil layer, an improved soil layer, a cement stabilized layer and a sleeper beam to support the transition slab, and can effectively reduce the uneven settlement of the transition slab on the soft subgrade at the back of the abutment. Summary of the Invention

[0005] The present invention aims to overcome the above technical problems and provides a soft subgrade structure at the back of the abutment and a construction method.

[0006] The structure of the soft subgrade at the back of the abutment of the present invention includes a rubble structure layer, a crushed stone cushion layer, a first waterproof geotextile, a lightweight soil layer, a second waterproof geotextile, an improved soil layer, a water-stable layer, a sleeper beam and a slab. Retaining walls are cast on both sides of the rubble structure layer; the rubble structure layer is laid on the soft soil foundation, the crushed stone cushion layer is laid on the rubble structure layer, the first waterproof geotextile is laid on the crushed stone cushion layer, the lightweight soil layer is filled on the first waterproof geotextile, the second waterproof geotextile is laid on the lightweight soil, the improved soil layer is filled above the second waterproof geotextile, the water-stable layer is laid above the improved soil layer, the sleeper beam is cast along the transverse direction of the road on the improved soil layer, and the sleeper beam is located in the water-stable layer; the slab is located above the water-stable layer, the front end of the slab is placed on the corbel of the front wall of the bridge, and the rear end of the slab is placed on the sleeper beam; its characteristics are: a steel corrugated pipe is arranged in the lightweight soil layer directly below the sleeper beam, the length direction of the steel corrugated pipe is consistent with the width direction of the road, and the axis of the steel corrugated pipe and the midline in the length direction of the sleeper beam are in the same vertical plane; a steel corrugated pipe bottom cushion for supporting it is laid below the steel corrugated pipe, the steel corrugated pipe bottom cushion is laid on the rubble structure layer, and a first waterproof geotextile for separating it from the lightweight soil layer is laid above the steel corrugated pipe bottom cushion; The bottom of the steel corrugated pipe is buried in the steel corrugated pipe bottom cushion, and the angle corresponding to the arc length of the part where the steel corrugated pipe is buried in the steel corrugated pipe bottom cushion is not less than 120°; an annular sealing ring composed of hot asphalt and felt is arranged between the outer periphery of both ends of the steel corrugated pipe and the inner wall of the retaining wall.

[0007] In the structure of the soft subgrade at the back of the abutment of the present invention, a fine sand layer with a particle size ≤ 5mm and a thickness of 10 cm to 20 cm is filled between the bottom of the steel corrugated pipe and the steel corrugated pipe bottom cushion.

[0008] In the structure of the soft subgrade at the back of the abutment of the present invention, the annular sealing ring is composed of an inner hot asphalt layer, a geotextile layer, an outer hot asphalt layer and a felt layer which are distributed in sequence from the inside to the outside. The inner hot asphalt layer is coated on the outer surface of the end of the steel corrugated pipe and the inner wall of the retaining wall around the end of the steel corrugated pipe. The geotextile layer is located between the inner hot asphalt layer and the outer hot asphalt layer, and the felt layer is on the periphery of the outer hot asphalt layer.

[0009] In the structure of the soft subgrade at the back of the abutment of the present invention, the contact surface between the lightweight soil layer and the road subgrade is a multi-level step-shaped contact surface. The width of each step along the driving direction of the road is not less than 3m, and the height of each step is not less than 2m.

[0010] In the structure of the soft subgrade at the back of the abutment of the present invention, the diameter of the steel corrugated pipe is 2.0m to 5.0m, and the wall thickness of the steel corrugated pipe is 3.0mm to 6.0mm; the width of the steel corrugated pipe bottom cushion is not less than 2 times the diameter of the steel corrugated pipe, and the thickness of the steel corrugated pipe bottom cushion is not less than 0.25 times the diameter of the steel corrugated pipe.

[0011] The structure of the soft subgrade at the back of the abutment of the present invention, the density of the lightweight soil used in the lightweight soil layer is 0.6 g / cm 3 ~1.2 g / cm 3 , and the compressive strength is not less than 0.5 MPa; the permeability coefficients of the first waterproof geotextile and the second waterproof geotextile ≤ 1×10 -11 m / s, and the lapping width of the first waterproof geotextile and the second waterproof geotextile ≥ 30 cm.

[0012] For the structure of the soft subgrade at the back of the abutment of the present invention, a retaining wall foundation is provided at the bottom of the retaining wall, and the distance from the outer edge of the retaining wall foundation to the outer edge of the rubble structure layer is not less than 2 m.

[0013] For the structure of the soft subgrade at the back of the abutment of the present invention, the sleeper beam is of a reinforced concrete structure. The length of the sleeper beam along the road driving direction is 2 m to 3 m. The width of the sleeper beam is the same as the width of the slab. The concrete grade forming the sleeper beam is C25 to C30, and the thickness of the sleeper beam is 25 cm to 35 cm; the distance from the bottom of the sleeper beam to the top of the steel corrugated pipe is 0.5 m to 1.0 m, and the larger the diameter of the steel corrugated pipe, the larger the value of the distance from it to the sleeper beam.

[0014] The construction method of the structure of the soft subgrade at the back of the abutment of the present invention is characterized in that it is realized through the following steps: a). Rock throwing; Rock is thrown in layers on the soft soil foundation at the back of the abutment. After each layer of rubble is thrown, it is compacted to form a rubble structure layer; b). Pouring the retaining wall; Pour a retaining wall of reinforced concrete structure on both sides of the rubble structure layer; c). Laying gravel; Lay gravel at one end close to the front wall between the two retaining walls to form a gravel cushion layer; d). Laying the bottom cushion layer; Lay gravel between the two retaining walls at one end close to the road subgrade to form a bottom cushion layer for the steel corrugated pipe. Then, an arc-shaped groove for accommodating the steel corrugated pipe is dug on the upper part of the bottom cushion layer of the steel corrugated pipe, and a layer of fine sand is laid in the arc-shaped groove to form a fine sand layer; e). Installing the steel corrugated pipe; The steel corrugated pipe is hoisted into the arc-shaped groove dug in step d). Gravel is continuously filled and tamped within the range of 120° at the bottom of the steel corrugated pipe to form the entire bottom cushion layer of the steel corrugated pipe; f). Sealing the end of the steel corrugated pipe; Coat a ring-shaped sealing ring composed of hot asphalt and tarpaulin on the end of the steel corrugated pipe and the inner wall of the retaining wall to achieve the sealing of the end of the steel corrugated pipe; g). Laying the lower geotextile; First, the contact surface between the road subgrade and the lightweight soil layer is processed into a contact surface in the form of multi-level steps. Then, lay a layer of waterproof geotextile on the laid gravel cushion layer, the bottom cushion layer of the steel corrugated pipe, and the multi-level steps of the road subgrade to form the first waterproof geotextile; h). Fill with lightweight soil; fill the front, back and top sides of the steel corrugated pipe with lightweight soil to form a lightweight soil layer, and then cover a layer of waterproof geotextile on the lightweight soil layer to form the second waterproof geotextile; i). Fill with improved soil; layer by layer fill with improved soil above the second waterproof geotextile to form an improved soil layer; j). Cast a reinforced concrete crosstie beam directly above the steel corrugated pipe, and pour cement stabilized mortar on both sides and the ends of the crosstie beam to form a water stable layer; k). Finally, place the front end of the approach slab on the bracket and the rear end on the crosstie beam.

[0015] For the construction method of the abutment back soft subgrade structure of the present invention, anti-corrosion treatment is carried out before the installation of the steel corrugated pipe in step d); in step e), the specific steps for sealing the end of the steel corrugated pipe are as follows: First, apply an internal heat asphalt layer on the outer surface of the end of the steel corrugated pipe and the inner wall of the retaining wall, then bond a geotextile layer on the internal heat asphalt layer, then apply another external heat asphalt layer on the geotextile layer, and finally bond a layer of tar paper on the external heat asphalt layer.

[0016] The beneficial effects of the present invention are as follows: For the abutment back soft subgrade structure of the present invention, a rubble structure layer, a gravel cushion layer, a first waterproof geotextile, a lightweight soil layer, a second waterproof geotextile, an improved soil layer, a water stable layer, a crosstie beam and an approach slab are arranged from bottom to top above the soft soil foundation. The approach slab is cast on the improved soil layer, and both ends of the crosstie beam are respectively placed on the bracket and the approach slab; a steel corrugated pipe is arranged in the lightweight soil directly below the approach slab, and a bottom cushion layer of the steel corrugated pipe is laid on the rubble structure layer below the steel corrugated pipe, so that it has the following beneficial effects: (1) The vehicle load borne by the approach slab is transmitted to the steel corrugated pipe through the crosstie beam and the improved soil layer. Through the interaction between the steel corrugated pipe and the surrounding soil, the load is dispersed to the surrounding soil, forming an "arch effect" formed by the horizontal arrangement of the steel corrugated pipe, diffusing the upper load to both sides and below the crosstie beam, reducing the vertical settlement of the crosstie beam, reducing the risk of differential settlement before and after the approach slab, effectively avoiding cracks or fractures in the approach slab, and further avoiding the occurrence of vehicle bumping at the bridge head.

[0017] (2) Under the action of the upper load on the steel corrugated pipe, the steel corrugated pipe can absorb the vibration energy generated by the load through micro-deformation to form a flexible transition zone, effectively alleviating the sudden change in stiffness, and further avoiding the settlement of the crosstie beam and the improved soil layer, lightweight soil layer and rubble structure layer below, so that the abutment back soft soil subgrade structure of the present invention has good structural stability during the service life.

[0018] (3) Since a steel corrugated pipe with a cavity inside is adopted, its unit weight value is close to 0. When combined with lightweight soil, the self-weight of the entire soft subgrade structure at the back of the abutment can be reduced by 40%-60%, significantly reducing the demand for the bearing capacity of the foundation, enabling it to meet the use on soft soil foundations with a bearing capacity ≤ 120 kPa (usually a bearing capacity ≥ 80 kPa is required). Description of the Drawings

[0019] Figure 1 It is a schematic structural diagram of the soft subgrade structure at the back of the abutment of the present invention; Figure 2 is Figure 1 the sectional view of the A-A section in Figure 3 is Figure 2 the partial enlarged view of area B in

[0020] In the figure: 1 soft soil foundation, 2 rubble structure layer, 3 gravel cushion layer, 4 first waterproof geotextile, 5 steel corrugated pipe, 6 steel corrugated pipe bottom cushion layer, 7 lightweight soil layer, 8 second waterproof geotextile, 9 improved soil layer, 10 slab, 11 sleeper beam, 12 cement stabilized layer, 13 fine sand layer, 14 front wall, 15 corbel, 16 road subgrade, 17 retaining wall, 18 retaining wall foundation, 19 annular sealing ring, 20 internal heat asphalt layer, 21 geotextile layer, 22 external heat asphalt layer, 23 tar paper layer, 24 capping beam, 25 pile foundation. Detailed Embodiment

[0021] The present invention will be further described below in conjunction with the drawings and embodiments.

[0022] As Figure 1 shown, a schematic structural diagram of the soft subgrade structure at the back of the abutment of the present invention is given. Figure 2 gives Figure 1 the sectional view of the A-A section in, and the shown soft subgrade structure at the back of the abutment is composed of a rubble structure layer 2, a gravel cushion layer 3, a first waterproof geotextile 4, a lightweight soil layer 7, a second waterproof geotextile 8, an improved soil layer 9, a cement stabilized layer 12, a sleeper beam 11, a slab 10, a retaining wall 17, a steel corrugated pipe 5 and a steel corrugated pipe bottom cushion layer 6 arranged on the soft soil foundation 1. The rubble structure layer 2 is laid on the soft soil foundation 1 to increase the bearing capacity of the soft soil foundation 1. Retaining walls 17 are poured on both sides of the rubble structure layer 2. A gravel cushion layer 3 is laid at one end of the rubble structure layer 2 between the two retaining walls 17 close to the front wall 14, and a steel corrugated pipe bottom cushion layer 6 is laid at one end of the rubble structure layer 2 between the two retaining walls 17 close to the road subgrade 16. The steel corrugated pipe bottom cushion layer 6 realizes the support for the steel corrugated pipe 5.

[0023] The contact surface between the shown lightweight soil layer 7 and the road subgrade 16 is a multi-level stepped contact surface. A first waterproof geotextile 4 is laid on the stepped contact surfaces of the crushed stone cushion layer 3, the bottom cushion layer 6 of the steel corrugated pipe, and the road subgrade 16 with the lightweight soil layer 7. The lightweight soil layer 7 is filled above the first waterproof geotextile 4. The lightweight soil layer 7 has a certain bearing capacity and a relatively small density, which can reduce the self-weight of the entire soft subgrade structure at the back of the abutment. A second waterproof geotextile 8 is laid above the lightweight soil layer 7. The first waterproof geotextile 4 and the second waterproof geotextile 8 wrap the lightweight soil layer 7 to prevent external moisture from entering the lightweight soil layer 7 and affecting its structural stability.

[0024] A water-stable layer 12 is laid above the shown second waterproof geotextile 8. The water-stable layer 12 is composed of cement-stabilized mortar. The sleeper beam 11 is cast on the water-stable layer 12. The length direction of the sleeper beam 11 is consistent with the road width direction; the slab 10 is located above the water-stable layer 12. The front end of the slab 10 is placed on the corbel 15 of the front wall 14. Below the front wall 14 is the capping beam 24, and below the capping beam 24 is the pile foundation 25. The rear end of the slab 10 is placed on the sleeper beam 11.

[0025] When a traveling vehicle passes through the slab 10, the load applied to the front end of the slab 10 acts on the corbel 15, and the load applied to the rear part of the slab 10 mainly acts on the sleeper beam 11. Therefore, in order to achieve stable support for the sleeper beam 11 and prevent excessive settlement of the sleeper beam 11, a steel corrugated pipe 5 is arranged in the lightweight soil layer 7 directly below the shown sleeper beam 11. The lower part of the steel corrugated pipe 5 is buried in the bottom cushion layer 6 of the steel corrugated pipe. In order to ensure the stable support of the bottom cushion layer 6 of the steel corrugated pipe for the steel corrugated pipe 5, it is required that the angle corresponding to the arc length of the part where the steel corrugated pipe 5 is buried in the bottom cushion layer 6 of the steel corrugated pipe is not less than 120°.

[0026] At the same time, in order to ensure that the load of the sleeper beam 11 can be effectively transmitted to the steel corrugated pipe 5 and dispersed to the surrounding soil through the steel corrugated pipe 5, the center line of the sleeper beam 11 in the length direction and the axis of the steel corrugated pipe 5 are located in the same vertical plane. The length of the steel corrugated pipe 5 is equal to the distance between the two retaining walls 17, that is, the two ends of the steel corrugated pipe 5 are adjacent to the inner walls of the two retaining walls 17. In order to seal the two ends of the steel corrugated pipe 5, annular sealing rings 19 are arranged on the outer walls of the two ends of the steel corrugated pipe 5 and the inner walls of the retaining walls 17 around the ends of the steel corrugated pipe 5 to prevent moisture from entering the steel corrugated pipe 5.

[0027] It can be seen that since the steel corrugated pipe 5 is arranged in the lightweight soil layer 7 directly below the bolster 11, the vehicle load borne by the approach slab 10 is transmitted to the steel corrugated pipe 5 through the bolster 11 and the improved soil layer 9, and then the steel corrugated pipe 5 disperses the load to the surrounding lightweight soil layer 7 and the steel corrugated pipe bottom cushion layer 6 at the bottom, realizing the diffusion of the upper load to both sides and downward, and effectively reducing the vertical settlement of the bolster. At the same time, the inside of the steel corrugated pipe 5 is a cavity, making its unit weight close to 0, significantly reducing the self-weight of the entire soft subgrade structure at the back of the abutment, and thus reducing the demand for the bearing capacity of the underlying soft soil foundation 1, making it suitable for use on soft soil foundations with a bearing capacity lower than ≤120 kPa (usually a bearing capacity of ≥80 kPa).

[0028] At the same time, while the steel corrugated pipe 5 transmits and disperses the upper load, it can also undergo a small amount of deformation by itself to absorb energy, forming a flexible transition zone, effectively alleviating the stiffness mutation, and thus avoiding the settlement of the bolster and the underlying improved soil layer, lightweight soil layer and riprap structural layer.

[0029] In order to ensure the stable support of the steel corrugated pipe bottom cushion layer 6 for the steel corrugated pipe 5, a fine sand layer 13 is arranged between the steel corrugated pipe bottom cushion layer 6 and the steel corrugated pipe 5. The particle size of the fine sand used in the fine sand layer 13 is ≤5 mm, and the thickness of the fine sand layer 13 is between 10 cm and 20 cm.

[0030] As Figure 3 shown, a partial enlarged view of area B in Figure 2 is given. The shown annular sealing ring 19 is composed of an inner heat asphalt layer 20, a geotextile layer 21, an outer heat asphalt layer 22 and a tarpaulin layer 23 which are distributed in sequence from the inside to the outside. The inner heat asphalt layer 20 is coated on the outer surface of the end of the steel corrugated pipe 5 and the inner wall of the retaining wall 17 around the end of the steel corrugated pipe. The geotextile layer 21 is located between the inner heat asphalt layer 20 and the outer heat asphalt layer 22, and the tarpaulin layer 23 is outside the outer heat asphalt layer. In this way, the annular sealing ring 19 that effectively seals the end of the steel corrugated pipe 5 is formed to prevent external moisture from entering the cavity inside the steel corrugated pipe 5.

[0031] In the multi-step contact surface between the shown lightweight soil layer 7 and the road subgrade 16, the width of each step along the road driving direction is not less than 3 m, and the height of each step is not less than 2 m. The diameter of the steel corrugated pipe 5 is 2.0 m to 5.0 m, and the wall thickness of the steel corrugated pipe 5 is 3.0 mm to 6.0 mm; the width of the steel corrugated pipe bottom cushion layer 6 is not less than 2 times the diameter of the steel corrugated pipe 5, and the thickness of the steel corrugated pipe bottom cushion layer 6 is not less than 0.25 times the diameter of the steel corrugated pipe 5.

[0032] The density of the lightweight soil used in the lightweight soil layer 7 is 0.6 g / cm 3 ~1.2 g / cm 3, the compressive strength is not less than 0.5 MPa; the permeability coefficients of the first waterproof geotextile 4 and the second waterproof geotextile 8 are ≤ 1×10 -11 m / s, and the lapping width of the first waterproof geotextile 4 and the second waterproof geotextile 8 is ≥ 30 cm.

[0033] A retaining wall foundation 18 is provided at the bottom of the shown retaining wall 17, and the distance from the outer edge of the retaining wall foundation to the outer edge of the rubble structural layer 20 is not less than 2 m. The sleeper beam 11 is of reinforced concrete structure. The length of the sleeper beam 11 along the road driving direction is 2 m to 3 m. The width of the sleeper beam 11 is the same as the width of the approach slab 10. The concrete grade forming the sleeper beam 11 is C25 - C30, and the thickness of the sleeper beam 11 is 25 cm to 35 cm; the distance from the bottom of the sleeper beam 11 to the top of the steel corrugated pipe 5 is 0.5 m to 1.0 m, and the larger the diameter of the steel corrugated pipe, the larger the value of its distance from the sleeper beam; that is, when the diameter of the steel corrugated pipe is 2 m, the distance from the bottom of the sleeper beam 11 to the top of the steel corrugated pipe 5 is 0.5 m, and when the diameter of the steel corrugated pipe is 5 m, the distance from the bottom of the sleeper beam 11 to the top of the steel corrugated pipe 5 is 1.0 m.

[0034] The construction method of the back - of - abutment soft subgrade structure of the present invention is specifically realized through the following steps: a). Rock filling; Rock is filled in layers on the soft soil foundation 1 at the back of the abutment. After each layer of rubble is filled, it is compacted to form a rubble structural layer 2; b). Pouring the retaining wall; Reinforced concrete retaining walls 17 are poured on both sides of the rubble structural layer; c). Laying gravel; Gravel is laid at one end close to the front wall 14 between the two retaining walls to form a gravel cushion layer 3; d). Laying the bottom cushion layer; Gravel is laid at one end close to the road subgrade 16 between the two retaining walls to form a bottom cushion layer 6 for the steel corrugated pipe. Then, an arc - shaped groove for accommodating the steel corrugated pipe 5 is dug in the upper part of the bottom cushion layer 6 for the steel corrugated pipe, and a layer of fine sand is laid in the arc - shaped groove to form a fine sand layer 13; e). Installing the steel corrugated pipe; The steel corrugated pipe 5 is hoisted into the arc - shaped groove dug in step d). Gravel is continuously filled and tamped within the range of 120° at the bottom of the steel corrugated pipe to form the entire bottom cushion layer 6 for the steel corrugated pipe; f). Sealing the end of the steel corrugated pipe; A ring - shaped sealing ring composed of hot asphalt and tarpaulin is coated on the end of the steel corrugated pipe and the inner wall of the retaining wall to realize the sealing of the end of the steel corrugated pipe 5; g). Laying the lower geotextile; First, the contact surface between the road subgrade 16 and the lightweight soil layer 7 is processed into a contact surface in the form of multi - level steps. Then, a layer of waterproof geotextile is laid on the laid gravel cushion layer 3, the bottom cushion layer 6 for the steel corrugated pipe, and the multi - level steps of the road subgrade to form a first waterproof geotextile 4; h). Fill with lightweight soil; fill the front, back and top of the steel corrugated pipe 5 with lightweight soil to form a lightweight soil layer 7, and then cover a layer of waterproof geotextile on the lightweight soil layer to form a second waterproof geotextile 8; i). Fill with improved soil; layer by layer fill with improved soil above the second waterproof geotextile to form an improved soil layer 9; j). Cast a reinforced concrete crosstie 11 in situ directly above the steel corrugated pipe 5, and pour cement stabilized mortar on both sides and the ends of the crosstie 11 to form a water stable layer 12; k). Finally, place the front end of the approach slab on the bracket 15 and the rear end on the crosstie 11.

[0035] Among them, anti-corrosion treatment is carried out on the steel corrugated pipe 5 before installation in step d); in step e), the specific steps for sealing the end of the steel corrugated pipe are as follows: First, apply an internal heat asphalt layer 20 on the outer surface of the end of the steel corrugated pipe 5 and the inner wall of the retaining wall 17, then bond a layer of geotextile layer 21 on the internal heat asphalt layer 20, then apply another layer of external heat asphalt layer 22 on the geotextile layer 21, and finally bond a layer of felt layer 23 on the external heat asphalt layer 22.

[0036] It can be seen that for the abutment back soft subgrade structure and construction method of the present invention, since the steel corrugated pipe 5 is arranged in the lightweight soil layer 7 directly below the crosstie 11, the self-weight of the entire subgrade structure is effectively reduced, making it suitable for being laid on the soft soil foundation 1; at the same time, the load of the approach slab 10 is transmitted to the steel corrugated pipe 5 through the crosstie 11, and the steel corrugated pipe 5 disperses the load to the surrounding lightweight soil layer 7 and the steel corrugated pipe bottom cushion layer 6 below. At the same time, the steel corrugated pipe 5 can also absorb and buffer the vibration rod load through micro deformation, finally reducing the vertical settlement of the crosstie 11, achieving effective protection of the approach slab 10, avoiding the occurrence of cracks or fractures in the approach slab 10, and further avoiding the occurrence of vehicle bumping at the bridgehead.

Claims

1. A subgrade structure for the back of a platform, comprising a rubble structure layer (2), a crushed stone cushion layer (3), a first waterproof geotextile (8), a lightweight soil layer (7), a second waterproof geotextile (8), an improved soil layer (9), a water-stable layer (12), a sleeper beam (11) and a slab (10) arranged above a soft soil foundation (1). Retaining walls (17) are cast on both sides of the rubble structure layer; the rubble structure layer is laid on the soft soil foundation, the crushed stone cushion layer is laid on the rubble structure layer, the first waterproof geotextile is laid on the crushed stone cushion layer, the lightweight soil layer is filled on the first waterproof geotextile, the second waterproof geotextile is laid on the lightweight soil, the improved soil layer is filled above the second waterproof geotextile, the water-stable layer is laid above the improved soil layer, the sleeper beam is cast in the transverse direction of the road on the improved soil layer and the sleeper beam is located in the water-stable layer; the slab is located above the water-stable layer, the front end of the slab is placed on the corbel of the front wall (14) of the bridge, and the rear end of the slab is placed on the sleeper beam; and it is characterized in that: A corrugated steel pipe (5) is provided in the lightweight soil layer directly below the bolster. The length direction of the corrugated steel pipe is consistent with the road width direction, and the axis of the corrugated steel pipe and the midline in the length direction of the bolster are in the same vertical plane. A corrugated steel pipe bottom cushion layer (6) for supporting it is laid below the corrugated steel pipe. The corrugated steel pipe bottom cushion layer is laid on the rubble structural layer, and a first waterproof geotextile for separating it from the lightweight soil layer is laid above the corrugated steel pipe bottom cushion layer. The bottom of the corrugated steel pipe is buried in the corrugated steel pipe bottom cushion layer, and the angle corresponding to the arc length of the part where the corrugated steel pipe is buried in the corrugated steel pipe bottom cushion layer is not less than 120°. An annular sealing ring (19) composed of hot asphalt and tarpaulin is provided between the outer peripheries of both ends of the corrugated steel pipe and the inner wall of the retaining wall.

2. The abutment back soft subgrade structure according to claim 1, characterized in that: A fine sand layer (13) with a particle size ≤ 5 mm and a thickness of 10 cm to 20 cm is filled between the bottom of the corrugated steel pipe (5) and the corrugated steel pipe bottom cushion layer (6).

3. The backfill soft subgrade structure according to claim 1 or 2, characterized in that: The annular sealing ring (19) is composed of an inner hot asphalt layer (20), a geotextile layer (21), an outer hot asphalt layer (22), and a tarpaulin layer (23) which are distributed in sequence from the inside to the outside. The inner hot asphalt layer is coated on the outer surface of the end of the corrugated steel pipe (5) and the inner wall of the retaining wall (17) on the outer periphery of the end of the corrugated steel pipe. The geotextile layer is located between the inner hot asphalt layer and the outer hot asphalt layer, and the tarpaulin layer is on the outer periphery of the outer hot asphalt layer.

4. The abutment back soft subgrade structure according to claim 1 or 2, characterized in that: The contact surface between the lightweight soil layer (7) and the road subgrade (16) is a contact surface in the form of multi-level steps. The width of each step along the road driving direction is not less than 3 m, and the height of each step is not less than 2 m.

5. The structure of the soft subgrade at the back of the abutment according to claim 1 or 2, characterized in that: The diameter of the corrugated steel pipe (5) is 2.0 m to 5.0 m, and the wall thickness of the corrugated steel pipe is 3.0 mm to 6.0 mm. The width of the corrugated steel pipe bottom cushion layer (6) is not less than 2 times the diameter of the corrugated steel pipe, and the thickness of the corrugated steel pipe bottom cushion layer is not less than 0.25 times the diameter of the corrugated steel pipe.

6. The abutment back soft subgrade structure according to claim 1 or 2, characterized in that: The density of the lightweight soil used in the lightweight soil layer (7) is 0.6 g / cm 3 ~1.2 g / cm 3 , and the compressive strength is not less than 0.5 MPa; the permeability coefficients of the first waterproof geotextile (4) and the second waterproof geotextile (8) are ≤ 1×10 -11 m / s, and the lapping width of the first waterproof geotextile and the second waterproof geotextile is ≥ 30 cm.

7. The structure of the soft subgrade at the back of the abutment according to claim 1 or 2, characterized in that: The bottom of the retaining wall (17) is provided with a retaining wall foundation (18), and the distance from the outer edge of the retaining wall foundation to the outer edge of the rubble structural layer (20) is not less than 2 m.

8. The structure of the soft subgrade at the back of the abutment according to claim 1 or 2, characterized in that: The bolster (11) is of reinforced concrete structure. The length of the bolster along the road driving direction is 2 m to 3 m. The width of the bolster is the same as the width of the approach slab (10). The concrete grade of the bolster is C25 - C30, and the thickness of the bolster is 25 cm to 35 cm. The distance from the bottom of the bolster to the top of the corrugated steel pipe (5) is 0.5 m to 1.0 m, and the larger the diameter of the corrugated steel pipe, the larger the value of its distance from the bolster.

9. A construction method for the abutment back soft subgrade structure according to claim 1, characterized in that, It is realized through the following steps: a). Rock throwing; Rock is thrown in layers on the soft foundation (1) at the back of the abutment, and each layer of rock is compacted after throwing to form a rubble structural layer (2). b). Pouring the retaining wall; Reinforced concrete retaining walls (17) are poured on both sides of the rubble structural layer. c). Laying gravel; Gravel is laid at one end close to the front wall (14) between the two retaining walls to form a gravel cushion layer (3). d). Lay the bottom cushion layer; lay gravel near one end of the roadbed (16) between the two retaining walls to form the bottom cushion layer (6) of the corrugated steel pipe, then dig an arc-shaped groove for accommodating the corrugated steel pipe (5) above the bottom cushion layer (6) of the corrugated steel pipe, and lay a layer of fine sand in the arc-shaped groove to form the fine sand layer (13); e). Install the corrugated steel pipe; hoist the corrugated steel pipe (5) into the arc-shaped groove dug in step d), and continue to fill and tamp gravel within a 120° range at the bottom of the corrugated steel pipe to form the entire bottom cushion layer (6) of the corrugated steel pipe; f). Seal the ends of the corrugated steel pipe; apply a ring-shaped sealing gasket composed of hot asphalt and felt on the inner wall of the end of the corrugated steel pipe and the retaining wall to seal the ends of the corrugated steel pipe (5); g). Lay the lower geotextile; first, treat the contact surface between the roadbed (16) and the lightweight soil layer (7) as a contact surface in the form of multiple steps, and then lay a layer of waterproof geotextile on the laid gravel cushion layer (3), the bottom cushion layer (6) of the corrugated steel pipe, and the multiple step surfaces of the roadbed to form the first waterproof geotextile (4); h). Fill with lightweight soil; Fill lightweight soil on the front, back, and top of the corrugated steel pipe (5) to form the lightweight soil layer (7), and then cover a layer of waterproof geotextile on the lightweight soil layer to form the second waterproof geotextile (8); i). Fill with improved soil; Fill the improved soil in layers above the second waterproof geotextile to form the improved soil layer (9); j). Cast a reinforced concrete crosstie beam (11) in situ directly above the corrugated steel pipe (5), and pour cement stabilized mortar on both sides and the ends of the crosstie beam (11) to form the water-stabilized layer (12); k). Finally, place the front end of the approach slab on the bracket (15) and the rear end on the crosstie beam (11).

10. The construction method of the abutment back soft subgrade structure according to claim 1, characterized in that: In step d), anti-corrosion treatment is carried out before installing the corrugated steel pipe (5); in step e), the specific steps for sealing the ends of the corrugated steel pipe are as follows: first, apply an inner hot asphalt layer (20) on the outer surface of the end of the corrugated steel pipe (5) and the inner wall of the retaining wall (17), then bond a layer of geotextile layer (21) on the inner hot asphalt layer (20), then apply an outer hot asphalt layer (22) on the geotextile layer (21), and finally bond a layer of felt layer (23) on the outer hot asphalt layer (22).

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