High filling abutment back roadbed structure and construction method

Abstract

The high filling abutment back roadbed structure comprises reinforced concrete driven piles, a broken stone hardcore, permeable geotextile, a permeable concrete raft-shaped structure, a gravel permeable layer, light soil and a retaining wall, drainage blind ditches are formed in the two sides of the gravel permeable layer, and latticed reinforced concrete ribbed beams are composed of side ribbed beams and middle ribbed beams; pervious concrete is poured in the inter-rib cavities. The construction method comprises the following steps: a) pile foundation construction; b) paving a cushion layer; c) paving water-permeable geotextile; d) pouring the ribbed beams; (e) pervious concrete is poured; f) laying blind ditches; (g) pre-pressing construction is carried out; h) pouring the retaining wall; i) laying a metal net; j) laying a top layer of geotechnical cloth; and k) carrying out layered filling to improve a soil layer. According to the high filled soil abutment back roadbed structure and the construction method, pre-pressing drainage and timely drainage of seepage water in the later service process are effectively achieved, the seepage water retention problem of the high filled soil roadbed is solved, and the stability of the abutment back roadbed is ensured.

Description

Technical Field

[0001] The present invention relates to a subgrade structure and a construction method, belonging to the technical field of bridge subgrades. More specifically, it particularly relates to a high-fill abutment back subgrade structure and a construction method. Background Art

[0002] With the rapid development of highway traffic construction, high-fill subgrade projects across gullies or soft soil areas are increasing day by day. As the transition section between structures such as bridges and culverts and embankments, the settlement control of the abutment back subgrade is directly related to driving comfort and structural safety. However, traditional high-fill abutment back subgrades generally face the problem of large settlement after construction. Especially when the fill height exceeds 10m, significant post-construction settlement is likely to occur under the action of foundation compression, self-weight of the filler, and traffic loads, resulting in frequent "bump at bridgehead" phenomena, which severely restricts the service life of the road.

[0003] In the prior art, reinforced concrete pile foundations are often used to reinforce the foundation, and the load is transmitted to the subgrade foundation through the pile body. However, practice shows that when solely relying on pile foundation treatment, the pile-soil interaction is insufficient, stress concentration still exists in the pile top area, and the consolidation settlement of the soil between piles is difficult to completely eliminate. Especially for cohesive soil foundations with poor permeability, even with pile foundation treatment, without preloading or improper drainage measures, the post-construction settlement may still exceed 30cm, which cannot meet the post-construction settlement standard of ≤10cm for high-grade highways. In addition, the drainage system in the traditional structure is imperfect, and water accumulation in the abutment back is likely to cause problems such as softening of the filler and an increase in the lateral pressure of the retaining structure, further exacerbating the risk of settlement and structural instability.

[0004] Therefore, there is an urgent need for a comprehensive treatment technology that integrates deep reinforcement, drainage diversion, and staged preloading. Based on the improvement of the bearing capacity of the pile foundation, by optimizing the structural stiffness distribution, accelerating soil consolidation, and coordinating deformation, the effective control of the settlement of the high-fill abutment back subgrade can be achieved. Summary of the Invention

[0005] The present invention aims to overcome the above technical problems and provides a high-fill abutment back subgrade structure and a construction method.

[0006] The high fill subgrade structure of the present invention includes a reinforced concrete driven pile, a gravel cushion layer, a permeable geotextile, a permeable concrete raft structure, a gravel permeable layer, a bottom waterproof geotextile, a lightweight soil filling layer, and two retaining walls, which are arranged in sequence from bottom to top. The reinforced concrete driven pile is sunk into the roadbed foundation. The gravel cushion layer is laid around the top of the reinforced concrete driven pile. The permeable geotextile is laid on the gravel cushion layer. The permeable concrete raft structure is fixedly connected to the upper end of the reinforced concrete driven pile. The two retaining walls are cast on both sides above the permeable concrete raft structure. The gravel permeable layer, the bottom waterproof geotextile, the lightweight soil filling layer, and the top waterproof geotextile are arranged in sequence from bottom to top between the two retaining walls. It is characterized in that drainage blind ditches are arranged on both sides of the gravel permeable layer, and the drainage blind ditches are located inside the bottom of the retaining wall. The permeable concrete raft structure is composed of grid-shaped reinforced concrete rib beams and permeable concrete. The grid-shaped reinforced concrete rib beams are composed of two side rib beams and several middle rib beams. The two side rib beams are located on both sides in the road width direction. All the middle rib beams are located between the two side rib beams. Intercostal cavities are formed between the middle rib beams and between the middle rib beams and the side rib beams, and permeable concrete is poured in the intercostal cavities. The thickness and width of the side rib beams are both greater than those of the middle rib beams, and the retaining walls are cast on the side rib beams.

[0007] In the high fill subgrade structure of the present invention, the reinforced concrete driven pile is composed of a first pile foundation and a second pile foundation. The first pile foundation is located below the side rib beam, and the second pile foundation is located below the middle rib beam. The diameter and length of the first pile foundation are both greater than those of the second pile foundation. Pile top steel bars are pre-buried at the tops of the first pile foundation and the second pile foundation and are respectively inserted into the side rib beam and the middle rib beam, and the length of the pile top steel bars inserted into the side rib beam and the middle rib beam is not less than 30 cm.

[0008] In the high fill subgrade structure of the present invention, drain pipes communicating with the drainage blind ditches are uniformly buried in the gravel permeable layer; the drainage blind ditches extend longitudinally along the roadbed and have a longitudinal slope of 1% - 3%.

[0009] In the high fill subgrade structure of the present invention, a metal mesh is laid on the upper part of the lightweight soil filling layer, and the metal mesh is located below the top waterproof geotextile; an improved soil layer is laid above the top waterproof geotextile.

[0010] In the high fill subgrade structure of the present invention, the reinforced concrete driven piles are arranged in a square grid, the pile spacing of the reinforced concrete driven piles is 3.0 - 5.0 times the pile diameter, and the diameter of the first pile foundation is 10 cm - 20 cm larger than that of the second pile foundation.

[0011] For the high-fill platform-back subgrade structure of the present invention, the grid size of the grid-shaped reinforced concrete rib beams is 1.5 m × 1.5 m to 2.5 m × 2.5 m. The thickness of the cross-section of the side rib beams is (1 ± 10%) * 50 cm, and the width is (1 ± 10%) * 80 cm. The thickness of the cross-section of the middle rib beams is (1 ± 10%) * 30 cm, and the width is (1 ± 10%) * 50 cm. Reserved bars connected to the bottom of the retaining wall are pre-buried at the upper part of the side rib beams. A settlement joint is reserved every 10 m to 15 m for the grid-shaped reinforced concrete rib beams.

[0012] For the high-fill platform-back subgrade structure of the present invention, the side rib beams are composed of concrete and side rib upper-layer steel bars, side rib lower-layer steel bars, side rib structural steel bars, and side rib stirrups cast in the concrete. The middle rib beams are composed of concrete and middle rib upper-layer steel bars, middle rib lower-layer steel bars, middle rib structural steel bars, and middle rib stirrups cast in the concrete. The side rib upper-layer steel bars, side rib lower-layer steel bars, middle rib upper-layer steel bars, and middle rib lower-layer steel bars all adopt HRB400 steel bars with a diameter of 12 mm to 18 mm, and the spacing between the steel bars is 10 cm to 15 cm. The side rib structural steel bars and middle rib structural steel bars all adopt HRB400 steel bars with a diameter of 12 mm to 14 mm, and the spacing between the steel bars is 20 cm to 30 cm. The side rib stirrups and middle rib stirrups all adopt HRB300 steel bars with a diameter of 6 mm to 8 mm, and the spacing between the steel bars is 15 cm to 20 cm. The concrete strength grade adopted for the middle rib beams and side rib beams is C30 to C40.

[0013] For the high-fill platform-back subgrade structure of the present invention, the permeable concrete meets the following indicators: The porosity is between 15% and 25% and presents a connected pore structure. The compressive strength ≥ C25. The permeability coefficient ≥ 1 × 10⁻² cm / s. The grading of the gravel permeable layer meets the following requirements: the coefficient of uniformity Cu ≥ 5, the coefficient of curvature Cc = 1 to 3, and the mud content < 3%.

[0014] The construction method of the high-fill platform-back subgrade structure of the present invention is characterized in that it is realized through the following steps: a). Pile foundation construction: Locate the positions of the first pile foundation and the second pile foundation to be driven according to the design of the grid-shaped reinforced concrete rib beams, and use the static pressure method to sink the first pile foundation and the second pile foundation into the subgrade foundation, and control the pile top elevation error ≤ 2 cm and the pile verticality deviation ≤ 1%. b). Laying the cushion layer: Lay gravel with a thickness of 30 cm ± 5 cm on the top of the reinforced concrete driven piles, and use a small roller to roll 6 to 8 times with a compaction degree ≥ 90% to form a gravel cushion layer. c). Laying permeable geotextile: Lay permeable geotextile on top of the gravel cushion layer, with the overlap width of the permeable geotextile ≥30cm; d). Casting grid-shaped reinforced concrete rib beams; tying the steel bars in the grid-shaped reinforced concrete rib beams, i.e. tying the upper steel bars, lower steel bars, structural bars and stirrups of the side ribs to form the side rib beams, tying the upper steel bars, lower steel bars, structural bars and stirrups of the middle ribs to form the middle rib beams, and making the top steel bars of the first pile foundation and the second pile foundation extend into the side rib beams and the middle rib beams respectively; then, integrally cast the side rib beams and the middle rib beams, and curing for 5 days after casting; e). Pouring permeable concrete: Pouring permeable concrete in the inter-rib cavity, and using an inserted vibrator to compact it, and curing for 7 days to form a permeable concrete raft structure; f). Lay permeable layer, blind ditch and waterproof geotextile; lay a layer of gravel on the permeable concrete raft structure to form a gravel permeable layer, lay drainage blind ditch on both sides of the gravel permeable layer, and ensure that the drainage blind ditch has a longitudinal slope of 1%~3%, and then evenly lay drainage pipes connected to the drainage blind ditch in the gravel permeable layer; then, lay a layer of waterproof geotextile on the gravel permeable layer to form the bottom waterproof geotextile; g). Preloading construction: Use graded pile loading preloading, and the load distribution is uniform loading of the entire cross section of the roadbed. Preloading is carried out according to steps g-1) to g-3): g-1). In the first stage, the load is 40% of the design load and the load is maintained for 7 days; g-2). The second stage is loading to 80% of the design load and holding the load for 7 days; g-3). In the second stage, load to 100% of the design load and hold the load until the settlement stability standard is met; the settlement stability standard is: the monitoring data for 3 consecutive days shows that the daily settlement is ≤0.5mm / d; h). Cast the retaining wall and lightweight soil; unload the preload, then cast the retaining wall in layers with a height of 1m each time. After each retaining wall casting, cure for 3 days and cast lightweight soil in two layers. The thickness of the lightweight soil cast each time is 50cm; until the height of the cast retaining wall and lightweight soil filling layer meets the design requirements; i). Laying metal mesh; Lay a layer of metal mesh on the top of the lightweight soil filling layer, and then pour another layer of lightweight soil on top of the metal mesh; j). Lay the top layer of waterproof geotextile with overlap width ≥50cm; k). Improve the soil layer by filling in layers, with the compaction thickness of each layer ≤30cm and the compaction degree ≥95%.

[0015] For the construction method of the high fill abutment back roadbed structure of the present invention, in step e), the permeable concrete in the intercostal cavity is poured by the alternate bay casting method, in step g), woven bags filled with stones are used as the preloading load; in step h), the lightweight soil is poured by the sectional propulsion method, and a 20-cm-thick foam plastic board is arranged between adjacent sections as a deformation joint.

[0016] The beneficial effects of the present invention are as follows: The high fill abutment back roadbed structure of the present invention comprises, from bottom to top, reinforced concrete driven piles, a gravel cushion layer, a permeable geotextile, a permeable concrete raft structure, a gravel permeable layer, a bottom waterproof geotextile, and a lightweight soil filling layer. Retaining walls are arranged on both sides of the upper part of the permeable concrete raft structure, and drainage blind ditches are arranged between the two sides of the gravel permeable layer and the retaining walls. The permeable concrete raft structure is composed of two side rib beams and a middle rib beam arranged therebetween. Permeable concrete is poured into the intercostal cavity between the side rib beam and the middle rib beam. In this way, during the preloading of the abutment back roadbed and the subsequent service process, the upward seepage water of the roadbed foundation successively penetrates through the gravel cushion layer, the permeable geotextile, the permeable concrete, and the gravel permeable layer and enters the drainage blind ditch for discharge, effectively realizing preloading drainage and timely discharge of seepage water during the subsequent service process, solving the problem of seepage water retention in the high fill roadbed, and ensuring the stability of the abutment back roadbed; at the same time, by filling concrete in the intercostal cavity, the overall stiffness of the permeable concrete raft structure is increased, and the permeable concrete raft structure evenly distributes the upper load received to each pile foundation, and then each pile foundation acts the load on the roadbed foundation (i.e., the deep stable soil layer), solving the deficiencies of the existing pile-soil cooperative force and the stress concentration problem in the pile top area; at the same time, for the construction method of the high fill abutment back roadbed structure of the present invention, the abutment back roadbed structure is preloaded in three stages by the method of staged surcharge preloading, which is beneficial to reducing the post-construction settlement; it can be seen that the high fill abutment back roadbed structure and the construction method of the present invention integrate the design scheme of "pile foundation system + permeable concrete raft structure + staged preloading construction", realizing the synergistic effect of piles, rafts, lightweight soil, retaining walls, and composite drainage systems, and meeting the requirement that the settlement of the high fill abutment back roadbed is ≤ 10 cm.

[0017] Furthermore, in the permeable concrete raft structure, the thickness and width of the side rib beam supporting the retaining wall are greater than those of the middle rib beam, and the side rib beam is supported by the first pile foundation with a larger diameter and length, while the middle rib beam is supported by the second pile foundation with a smaller diameter and length. Therefore, the bearing capacity of the side rib beam is enhanced specifically, reducing the risk of uneven settlement of the abutment back roadbed and further reducing the post-construction settlement of the abutment back roadbed.

[0018] Furthermore, during the construction of the high-fill platform-back subgrade structure, a loading method of applying 40% of the design load in the first stage, 80% in the second stage, and 100% in the third stage is adopted. And in the third stage, a settlement stability criterion for ending the loading is adopted that "the daily settlement amount shown by the continuous 3-day monitoring data ≤ 0.5 mm / d", so that the usually over 6-month long preloading can significantly reduce the post-construction settlement. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a schematic structural diagram of the high-fill platform-back subgrade structure of the present invention; Figure 2 It is a top-view structural diagram of the permeable concrete raft structure in the present invention; Figure 3 It is a connection diagram of the permeable concrete raft structure with the first pile foundation and the second pile foundation in the present invention.

[0020] In the figure: 1 first pile foundation, 2 second pile foundation, 3 gravel cushion layer, 4 permeable geotextile, 5 permeable concrete raft structure, 6 side rib beam, 7 middle rib beam, 8 permeable concrete, 9 drainage blind ditch, 10 gravel permeable layer, 11 bottom waterproof geotextile, 12 retaining wall, 13 lightweight soil filling layer, 14 metal mesh, 15 top waterproof geotextile, 16 improved soil layer, 17 concrete, 18 upper side rib steel bars, 19 lower side rib steel bars, 20 side rib structural steel bars, 21 side rib stirrups, 22 upper middle rib steel bars, 23 lower middle rib steel bars, 24 middle rib structural steel bars, 25 middle rib stirrups, 26 subgrade foundation, 27 cavity between ribs, 28 pile top steel bars. DETAILED DESCRIPTION OF THE EMBODIMENTS

[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 high-fill platform-back subgrade structure of the present invention is given, Figure 2 and a top-view structural diagram of the permeable concrete raft structure in the invention is given, Figure 3The connection schematic diagram of the permeable concrete raft structure with the first pile foundation and the second pile foundation in the present invention is given; the shown high fill abutment back roadbed structure is composed of reinforced concrete driven piles, gravel cushion layer 3, permeable geotextile 4, permeable concrete raft structure 5, sand and gravel permeable layer 10, bottom waterproof geotextile 11, lightweight soil filling layer 13, which are arranged successively from bottom to top, and retaining walls 12 arranged on both sides above the permeable concrete raft structure 5. The reinforced concrete driven piles penetrate into the roadbed foundation 26. The load of the entire high fill abutment back roadbed is transmitted to the roadbed foundation 26 (i.e., the deep stable soil layer) through the reinforced concrete driven piles (including the first pile foundation 1 and the second pile foundation 2). The gravel cushion layer 3 is laid on the roadbed foundation 16, outside the bottom of the reinforced concrete driven piles. The permeable geotextile 4 is laid on the gravel cushion layer 3. The permeable concrete raft structure 5 is located above the permeable geotextile 4, and the bottom of the permeable concrete raft structure 5 is fixedly connected to the tops of the reinforced concrete driven piles (including the first pile foundation 1 and the second pile foundation 2).

[0023] The shown permeable concrete raft structure 5 is composed of grid-shaped reinforced concrete rib beams and permeable concrete 8. The grid-shaped reinforced concrete rib beams are composed of two side rib beams 6 and several middle rib beams 7 arranged between the two side rib beams 6. The length direction of the side rib beams 6 is consistent with the road driving direction. The several middle rib beams 7 between the two side rib beams 6 are arranged perpendicular to each other. In this way, the side rib beams 6 and the middle rib beams 7 form a grid shape. Intercostal cavities 27 are formed between adjacent side rib beams 6 and between the side rib beams 6 and the middle rib beams 7, and the permeable concrete 8 is poured in the intercostal cavities 27.

[0024] The shown retaining walls 12 on both sides are poured above the two side rib beams 6, and reserved bars extending into the bottom of the retaining walls 12 are pre-buried in the upper parts of the side rib beams 6. The sand and gravel permeable layer 10, the bottom waterproof geotextile 11, the lightweight soil filling layer 13 and the top waterproof geotextile 15 are arranged successively from bottom to top between the two retaining walls 12. Drainage blind ditches 9 are laid between the two sides of the sand and gravel permeable layer 10 and the retaining walls 12. In order to ensure the smooth drainage of seepage water, drain pipes connected to the drainage blind ditches 9 can be evenly laid in the sand and gravel permeable layer 10. A metal mesh 14 is laid on the upper part of the shown lightweight soil filling layer 13. The metal mesh 14 is located below the top waterproof geotextile 15, and an improved soil layer 16 is laid above the top waterproof geotextile 15.

[0025] It can be seen that since the permeable concrete raft structure is composed of the side rib beam 6, the middle rib beam 7 and the permeable concrete 8 poured in the inter-rib space 27, the seepage generated by the roadbed foundation 26 during the preloading and later service of the entire high-fill platform back roadbed structure passes through the crushed stone cushion layer 3, the permeable geotextile 4, the permeable concrete 8, and the sand and gravel permeable layer 10 in turn into the drainage blind ditch 9 for discharge, avoiding the retention of the seepage of the platform back roadbed. At the same time, since the permeable concrete is poured in the inter-rib space 27 between the side rib beam 6 and the middle rib beam 7, the overall stiffness of the permeable concrete raft structure 5 formed by it is effectively enhanced, so that the upper load received by the permeable concrete raft structure 5 can be evenly transmitted to each reinforced concrete driven pile (including the first pile foundation 1 and the second pile foundation 2), and evenly transmitted to the roadbed foundation 26 below, which can be said to achieve the coordinated acceptance of piles and soils and avoid the problem of stress concentration at the top of the pile foundation.

[0026] Since the heavy retaining wall 12 is arranged on the side rib beam 6, in order to avoid uneven settlement of the permeable concrete raft structure 5, the thickness and width of the side rib beam 6 are greater than the thickness and width of the middle rib beam 7; at the same time, the side rib beam 6 is supported by the evenly distributed first pile foundation 1, and the middle rib beam 7 is supported by the evenly distributed second pile foundation 2, and the diameter and length of the first pile foundation 1 are greater than the diameter of the second pile foundation 2.

[0027] For example, the mesh size of the grid-shaped reinforced concrete rib beam is 1.5m×1.5m~2.5m×2.5m, the thickness of the cross section of the side rib beam 6 is (1±10%)*50cm, the width is (1±10%)*80cm, the thickness of the cross section of the middle rib beam 7 is (1±10%)*30cm, the width is (1±10%)*50cm. The diameter of the first pile foundation 1 is 10cm~20cm larger than the diameter of the second pile foundation 2.

[0028] In order to achieve fixed connection between the top of the first pile foundation 1 and the second pile foundation 2 and the side rib beam 6 and the middle rib beam 7, the top of the first pile foundation 1 and the second pile foundation 2 are pre-embedded with pile top steel bars 28, and the length of the pile top steel bars 28 at the top of the first pile foundation 1 and the second pile foundation 2 extending into the side rib beam 6 and the middle rib beam 7 is not less than 30 cm.

[0029] In order to ensure that the seepage water is discharged smoothly through the drainage blind ditch 9, the drainage blind ditch 9 is arranged in an inclined form with a high side close to the pedestal and a low side away from the pedestal. The drainage blind ditch 9 has a longitudinal slope of 1% to 3% along the longitudinal direction of the roadbed. In this way, the seepage water can be discharged smoothly through the drainage blind ditch 9, avoiding the problem of seepage water retention.

[0030] Among them, the reinforced concrete driven piles are arranged in a square grid, and the pile spacing of the reinforced concrete driven piles is 3.0 to 5.0 times the pile diameter. In the case where the length of the permeable concrete raft structure 5 is relatively large, it should be set in sections, that is, a settlement joint is reserved every 10m to 15m for the grid-shaped reinforced concrete rib beams to avoid affecting the overall performance due to different settlement performances of different sections.

[0031] The shown side rib beam 6 is composed of concrete 17 and side rib upper reinforcement 18, side rib lower reinforcement 19, side rib structural reinforcement 20 and side rib stirrups 21 cast in the concrete. The middle rib beam 7 is composed of concrete and middle rib upper reinforcement 22, middle rib lower reinforcement 23, middle rib structural reinforcement 24 and middle rib stirrups 25 cast in the concrete.

[0032] The side rib upper reinforcement 18, side rib lower reinforcement 19, middle rib upper reinforcement 22 and middle rib lower reinforcement 23 all adopt HRB400 steel bars with a diameter of 12mm to 18mm, and the spacing between the steel bars is 10cm to 15cm. The side rib structural reinforcement 20 and middle rib structural reinforcement 24 all adopt HRB400 steel bars with a diameter of 12mm to 14mm, and the spacing between the steel bars is 20cm to 30cm. The side rib stirrups 21 and middle rib stirrups 25 all adopt HRB300 steel bars with a diameter of 6mm to 8mm, and the spacing between the steel bars is 15cm to 20cm. The concrete 17 used for the middle rib beam and side rib beam has a strength grade of C30 to C40.

[0033] In order to ensure that the water seeping below can pass through the permeable concrete 8 smoothly, the permeable concrete 8 meets the following indicators: the porosity is between 15% and 25% and presents a connected pore structure; the compressive strength ≥ C25; the permeability coefficient ≥ 1×10⁻² cm / s. The gradation of the gravel permeable layer 10 meets: the coefficient of uniformity Cu ≥ 5, the coefficient of curvature Cc = 1 to 3, and the mud content < 3%.

[0034] The construction method of the high embankment abutment subgrade structure of the present invention is realized through the following steps: a). Pile foundation construction; Locate the positions of the driven piles required according to the designed grid-shaped reinforced concrete rib beams, and use the static pressure method to sink the first pile foundation 1 and the second pile foundation 2 into the roadbed foundation 26, and control the pile top elevation error ≤ 2cm and the pile verticality deviation ≤ 1%; b). Laying the cushion; Lay gravel with a thickness of 30cm ± 5cm on the top of the reinforced concrete driven piles, and use a small roller to roll 6 to 8 times, with a compaction degree ≥ 90% to form a gravel cushion 3; c). Laying the permeable geotextile; Lay the permeable geotextile 4 above the gravel cushion 3, and the lap width of the permeable geotextile ≥ 30cm; d). Pour grid-shaped reinforced concrete rib beams; tie the steel bars inside the grid-shaped reinforced concrete rib beams, that is, tie the upper-layer steel bars, lower-layer steel bars, structural steel bars, and stirrups (18, 19, 20, 21) of the side rib beams 6, and tie the upper-layer steel bars, lower-layer steel bars, structural steel bars, and stirrups (22, 23, 24, 25) of the middle rib beams 7, and make the pile top steel bars 28 at the tops of the first pile foundation and the second pile foundation extend into the side rib beams and the middle rib beams respectively; then, pour the side rib beams and the middle rib beams integrally, and cure for 5 days after pouring and forming; e). Pour permeable concrete; pour permeable concrete into the intercostal cavities 27, and vibrate it densely with an inserted vibrator, and cure for 7 days to form a permeable concrete raft structure 5; f). Lay a permeable layer, blind drains, and waterproof geotextiles; lay a layer of gravel above the permeable concrete raft structure to form a gravel permeable layer 10, lay drainage blind drains 9 on both sides of the gravel permeable layer, and ensure that the drainage blind drains 9 have a longitudinal slope of 1% - 3%, and then evenly lay drain pipes communicating with the drainage blind drains in the gravel permeable layer 10; then, lay a layer of waterproof geotextile above the gravel permeable layer 10 to form a bottom waterproof geotextile 11; g). Preloading construction; adopt staged surcharge preloading, and the load distribution is uniformly loaded across the full width of the roadbed cross-section, and preload according to steps g-1) to g-3): g-1). Load to 40% of the design load in the first stage and hold the load for 7 days; g-2). Load to 80% of the design load in the second stage and hold the load for 7 days; g-3). Load to 100% of the design load in the second stage and hold the load until the settlement stability standard is met; the settlement stability standard is: the daily settlement amount ≤ 0.5 mm / d shown by the monitoring data for 3 consecutive days; h). Pour retaining walls and lightweight soil; unload the preloading load, and then pour the retaining walls 12 in layers in the way of pouring 1 m height each time. Cure for 3 days after each pouring of the retaining wall and pour the lightweight soil in two layers. The thickness of each poured lightweight soil is 50 cm; until the heights of the poured retaining walls 12 and the lightweight soil filling layers 13 meet the design requirements; i). Lay a metal mesh; lay a layer of metal mesh 14 on the top of the lightweight soil filling layer 13, and then pour another layer of lightweight soil above the metal mesh 14; j). Lay the top waterproof geotextile 15 with a lap width ≥ 50 cm; k). Fill the improved soil layer 16 in layers, with the compaction thickness of each layer ≤ 30 cm and the compaction degree ≥ 95%.

[0035] Among them, in step e), the pervious concrete 8 in the intercostal cavity 27 is poured by the skip-joint method. In step g), the woven bag filled with stones is used as the preloading load. In step h), the lightweight soil is poured by the sectional advancing method, and a 20-cm-thick foam plastic board is set between adjacent sections as the deformation joint.

[0036] In step h), since the retaining wall 12 is poured in layers, the lightweight soil filling layer 13 is also poured in layers corresponding to the retaining wall 12. In this way, it can ensure that the lightweight soil filling layer 13 filled (using the pouring process) has good uniformity and compactness, and ensures the stability of the lightweight soil filling layer 13 after construction.

[0037] In summary, the high-fill abutment back roadbed structure and construction method of the present invention integrate a multi-layer collaborative design scheme of a pile foundation system + a pervious concrete raft structure + staged preloading construction for the problems of high-fill roadbed settlement control, drainage requirements, and lateral stability, realizing the synergistic effect of piles, rafts, lightweight soil, retaining walls, and composite drainage systems; the pile foundations are arranged with different diameters (the first pile foundation > the second pile foundation), specifically enhancing the bearing capacity of the side ribs and reducing the risk of uneven settlement; the pervious concrete raft structure enhances the overall stiffness, solves the deficiency of the cooperative force between piles and soil, and there is still a stress concentration problem in the pile top area; the three-dimensional drainage design of the pervious concrete raft structure and the blind ditch system solves the problem of water seepage retention in the high-fill roadbed; staged preloading can significantly reduce the post-construction settlement; the lightweight soil material reduces the filling load and relieves the lateral pressure; these designs and construction methods meet the requirement that the settlement of the high-fill abutment back roadbed is ≤ 10 cm.

Claims

1. A high-fill subgrade structure at the back of a platform, comprising a reinforced concrete driven pile, a crushed stone cushion layer (3), a permeable geotextile (4), a permeable concrete raft structure (5), a sand and gravel permeable layer (10), a bottom waterproof geotextile (11), a lightweight soil filling layer (13) and two retaining walls (12) arranged in sequence from bottom to top. The reinforced concrete driven pile is sunk into the roadbed foundation (26). The crushed stone cushion layer is laid around the top of the reinforced concrete driven pile. The permeable geotextile is laid on the crushed stone cushion layer. The permeable concrete raft structure is fixedly connected to the upper end of the reinforced concrete driven pile. The two retaining walls are cast on both sides above the permeable concrete raft structure. The sand and gravel permeable layer, the bottom waterproof geotextile, the lightweight soil filling layer and the top waterproof geotextile are arranged in sequence from bottom to top between the two retaining walls. It is characterized in that: Drainage blind ditches (9) are arranged on both sides of the sandstone permeable layer, and the drainage blind ditches are located on the inner side of the bottom of the retaining wall; The permeable concrete raft structure (5) is composed of grid-shaped reinforced concrete rib beams and permeable concrete (8), wherein the grid-shaped reinforced concrete rib beams are composed of two side rib beams (6) and a plurality of middle rib beams (7), wherein the two side rib beams are located on both sides of the road width direction, and all the middle rib beams are located between the two side rib beams, and inter-rib cavities (27) are formed between the middle rib beams and between the middle rib beams and the side rib beams, and the permeable concrete (8) is poured in the inter-rib cavities; the thickness and width of the side rib beams are greater than the thickness and width of the middle rib beams, and the retaining wall is poured on the side rib beams.

2. The high-fill embankment subgrade structure according to claim 1, characterized in that: The reinforced concrete driven piles are composed of a first pile foundation (1) and a second pile foundation (2), the first pile foundation being located below the side rib beam (6), the second pile foundation being located below the middle rib beam (7), the diameter and length of the first pile foundation being greater than the diameter and length of the second pile foundation; the tops of the first pile foundation and the second pile foundation are pre-embedded with pile top steel bars (28) respectively inserted into the side rib beam (6) and the middle rib beam (7), the length of the pile top steel bars inserted into the side rib beam and the middle rib beam being not less than 30 cm.

3. The high-fill embankment subgrade structure according to claim 1 or 2, characterized in that: Drainage pipes communicating with the drainage blind ditch (9) are evenly buried in the sandstone permeable layer (10); the drainage blind ditch (9) extends longitudinally along the roadbed and has a longitudinal slope of 1% to 3%.

4. The high-fill embankment subgrade structure according to claim 1 or 2, characterized in that: A metal mesh (14) is laid on the upper part of the lightweight soil filling layer (13), and the metal mesh is located below the top waterproof geotextile (15); and an improved soil layer (16) is laid above the top waterproof geotextile.

5. The high fill subgrade structure at the back of the platform according to claim 2, characterized in that: The reinforced concrete driven piles are arranged in a square grid, the distance between the reinforced concrete driven piles is 3.0 to 5.0 times the pile diameter, and the diameter of the first pile foundation (1) is 10 cm to 20 cm larger than the diameter of the second pile foundation (2).

6. The high-fill embankment subgrade structure according to claim 1 or 2, characterized in that: The grid size of the grid-like reinforced concrete rib beam is 1.5m×1.5m~2.5m×2.5m, the thickness of the cross section of the side rib beam (6) is (1±10%)*50cm, the width is (1±10%)*80cm, the thickness of the cross section of the middle rib beam (7) is (1±10%)*30cm, the width is (1±10%)*50cm; the upper part of the side rib beam is pre-embedded with reserved reinforcement connected to the bottom of the retaining wall (12), and the grid-like reinforced concrete rib beam is pre-reserved with a settlement joint every 10m~15m.

7. The high-fill embankment subgrade structure according to claim 1 or 2, characterized in that: The side rib beam (6) is composed of concrete (17) and side rib upper layer reinforcement (18), side rib lower layer reinforcement (19), side rib structural reinforcement (20) and side rib stirrups (21) cast in the concrete, and the middle rib beam (7) is composed of concrete and middle rib upper layer reinforcement (22), middle rib lower layer reinforcement (23), middle rib structural reinforcement (24) and middle rib stirrups (25) cast in the concrete; The upper and lower steel bars of the side ribs, the upper and lower steel bars of the middle ribs, the upper steel bars of the middle ribs and the lower steel bars of the middle ribs are all made of HRB400 steel bars with a diameter of 12 mm to 18 mm, and the spacing between the steel bars is 10 cm to 15 cm; the structural steel bars of the side ribs and the middle ribs are all made of HRB400 steel bars with a diameter of 12 mm to 14 mm, and the spacing between the steel bars is 20 cm to 30 cm; the stirrups of the side ribs and the middle ribs are all made of HRB300 steel bars with a diameter of 6 mm to 8 mm, and the spacing between the steel bars is 15 cm to 20 cm; the concrete strength grade used for the middle rib beam and the side rib beam is C30 to C40.

8. The high-fill embankment subgrade structure according to claim 1 or 2, characterized in that, The permeable concrete (8) meets the following indicators: The porosity is between 15% and 25% and presents a connected pore structure; The compressive strength ≥ C25; The permeability coefficient ≥ 1×10⁻² cm / s; The grading of the gravel permeable layer (10) meets the requirements: the coefficient of uniformity Cu ≥ 5, the coefficient of curvature Cc = 1 to 3, and the mud content < 3%.

9. A construction method for the high-fill embankment subgrade structure according to claim 1, characterized in that, It is realized through the following steps: a). Pile foundation construction; Locate the positions of the piles to be driven according to the designed grid-shaped reinforced concrete rib beams, and use the static pressure method to sink the first pile foundation (1) and the second pile foundation (2) into the roadbed foundation (26), and control the pile top elevation error ≤ 2 cm and the pile verticality deviation ≤ 1%; b). Laying the cushion layer; Lay gravel with a thickness of 30 cm ± 5 cm on the top of the reinforced concrete driven piles, and use a small roller to roll 6 to 8 times, with a compaction degree ≥ 90% to form a gravel cushion layer (3); c). Laying the permeable geotextile; Lay the permeable geotextile (4) above the gravel cushion layer (3), and the lap width of the permeable geotextile ≥ 30 cm; d). Pouring the grid-shaped reinforced concrete rib beams; Bind the steel bars inside the grid-shaped reinforced concrete rib beams, that is, bind the upper and lower steel bars, structural steel bars, and stirrups (18, 19, 20, 21) of the side rib beams (6) that form the side rib beams, and bind the upper and lower steel bars, structural steel bars, and stirrups (22, 23, 24, 25) of the middle rib beams (7) that form the middle rib beams, and make the pile top steel bars (28) at the tops of the first pile foundation and the second pile foundation extend into the side rib beams and the middle rib beams respectively; Then, pour the side rib beams and the middle rib beams integrally, and cure for 5 days after pouring and forming; e). Pouring the permeable concrete; Pour the permeable concrete into the intercostal cavity (27), and use an inserted vibrator to vibrate it densely, and cure for 7 days to form a permeable concrete raft structure (5); f). Laying the permeable layer, blind ditch and waterproof geotextile; Lay a layer of gravel above the permeable concrete raft structure to form a gravel permeable layer (10), lay drainage blind ditches (9) on both sides of the gravel permeable layer, and ensure that the drainage blind ditch (9) has a longitudinal slope of 1% to 3%, and then evenly lay drain pipes connected to the drainage blind ditch in the gravel permeable layer (10); Then, lay a layer of waterproof geotextile above the gravel permeable layer (10) to form a bottom waterproof geotextile (11); g). Preloading construction; Adopt staged surcharge preloading, and the load distribution is evenly loaded across the entire width of the roadbed cross-section, and preload according to steps g-1) to step g-3): g-1). The first stage is to load to 40% of the design load and maintain the load for 7 days; g-2). The second stage is to load to 80% of the design load and maintain the load for 7 days; g-3). The second stage is to load to 100% of the design load and maintain the load until the settlement stability standard is met; the settlement stability standard is: the daily settlement amount shown by the monitoring data for 3 consecutive days ≤ 0.5 mm / d; h). Pour the retaining wall and lightweight soil; unload the preloading load, and then pour the retaining wall (12) in layers in the way of pouring 1 m height each time. After each pouring of the retaining wall, cure for 3 days and pour the lightweight soil in two layers. The thickness of the lightweight soil poured each time is 50 cm; until the heights of the poured retaining wall (12) and the lightweight soil filling layer (13) meet the design requirements; i). Lay the metal mesh; lay a layer of metal mesh (14) on the top of the lightweight soil filling layer (13), and then pour another layer of lightweight soil above the metal mesh (14); j). Lay the top waterproof geotextile (15) with an overlap width ≥ 50 cm; k). Fill the improved soil layer (16) in layers, with the compaction thickness of each layer ≤ 30 cm and the compaction degree ≥ 95%.

10. The construction method of the high fill platform back subgrade structure according to claim 9, characterized in that: In step e), the permeable concrete (8) in the intercostal cavity (27) is poured by the alternate bay casting method. In step g), the preloading load is the gravel filled in woven bags. In step h), the lightweight soil is poured by the sectional advancing method, and a 20-cm-thick foam plastic board is set as the deformation joint between adjacent sections.

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