Phase change heat storage material combined with small heat pipe permafrost composite roadbed and construction method thereof

Abstract

The present application relates to a kind of phase change heat storage material combined small heat pipe frozen soil composite roadbed and its construction method.The existing phase change heat storage material mixed frozen soil roadbed cannot pass the cooling effect of phase change heat storage material to underlying frozen soil.The present roadbed includes gravel clay layer, silty clay layer, roadbed fill layer and asphalt pavement layer from bottom to top;Silty clay layer and roadbed fill layer are provided with first phase change material heat storage layer, and second phase change material heat storage layer is arranged between roadbed fill layer and asphalt pavement layer;Roadbed further includes small heat pipe, and small heat pipe is vertically arranged in roadbed fill layer, first phase change material heat storage layer and silty clay layer.The present application sets two layers of phase change heat storage material at different positions of roadbed, improves phase change heat storage capacity using new material compatibility, adds small heat rod on natural frozen soil upper limit, combines phase change heat storage material and further expands the cooling effect of phase change heat storage material to underlying frozen soil.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of roadbed construction, in particular to a phase change heat storage material combined small heat pipe frozen soil composite roadbed and a construction method thereof. BACKGROUND

[0002] Traffic roads passing through large areas of permafrost need to withstand extreme environmental tests. For example, the degradation of underlying permafrost of roadbed caused by heat absorption of asphalt pavement accelerates the uneven settlement of roadbed, affecting the smoothness of the road; the long cold season of permafrost seriously affects the road surface due to snow accumulation and frost, and the traffic capacity of the road decreases. In order to prevent and control the diseases of traffic roads in permafrost regions, the optimization of roadbed structure and construction has been carried out based on the influencing factors of heat exchange between frozen soil roadbed and external environment, the heat transfer process of frozen soil under the action of multi-field coupling, the quantitative calculation model of frozen soil roadbed heat budget, and the response mechanism of underlying frozen soil to thermal disturbance of highway engineering.

[0003] However, in order to ensure the thermal stability of frozen soil roadbed, the overall heat design of the ground-air coupling heat exchange system of roadbed-ground-air should be carried out, especially for special roadbeds, the asphalt underpad, the interface between the special roadbed structure layer and the filling soil, and other key interfaces such as the top plate of the active layer, all of which need to accurately control the heat flux. At present, in order to achieve the purpose of accurately controlling the heat flux, phase change heat storage materials can be mixed in the frozen soil roadbed, which can have a certain effect on the lifting of the frozen soil upper limit of the underlying frozen soil of the frozen soil roadbed. However, due to the limitation of mixing ratio and mixing layer position, combined with the high cost of phase change heat storage materials, the cooling effect of the phase change heat storage materials mixed in the frozen soil roadbed cannot be transferred to the high-temperature unstable frozen soil disc of the underlying frozen soil, thereby reducing the area of the high-temperature unstable frozen soil disc.

[0004] Therefore, it is necessary to propose new measures to overcome the above-mentioned defects. SUMMARY

[0005] The purpose of the present application is to provide a phase change heat storage material combined small heat pipe frozen soil composite roadbed and a construction method thereof, so as to solve the problem that the phase change heat storage material mixed in the frozen soil roadbed cannot transfer the cooling effect to the underlying frozen soil.

[0006] In order to achieve the above-mentioned purpose, the technical scheme adopted by the present application is as follows:

[0007] The phase change heat storage material combined small heat pipe frozen soil composite roadbed comprises a gravel-containing clay layer, a silty clay layer, a roadbed filling soil layer and an asphalt pavement layer from bottom to top.

[0008] A first phase change material heat storage layer is arranged between the silty clay layer and the roadbed filling soil layer, and a second phase change material heat storage layer is arranged between the roadbed filling soil layer and the asphalt pavement layer.

[0009] The subgrade further comprises small heat pipes vertically arranged in the subgrade filling layer, the first phase change material heat storage layer and the silty clay layer.

[0010] Further, the top of the silty clay layer is an active layer, the active layer being a part of the silty clay layer, and the upper limit of frozen soil being between the active layer and the silty clay layer.

[0011] The first phase change material heat storage layer is above the active layer, and the small heat pipes pass through the active layer.

[0012] Further, the small heat pipes are two-phase closed thermosyphon heat pipes, comprising a condensation section, a transition section and an evaporation section from top to bottom.

[0013] The condensation section is in the subgrade filling layer, the transition section is in the first phase change material heat storage layer and the active layer, and the evaporation section is in the active layer and the silty clay layer below the active layer.

[0014] Further, the first phase change material heat storage layer and the second phase change material heat storage layer are paved with phase change heat storage materials.

[0015] The phase change heat storage materials comprise RT2-HC, RT3-HC and RT-4.

[0016] Further, in the phase change heat storage materials, the volume mixing ratio of RT2-HC, RT3-HC and RT-4 is 6:2:2.

[0017] Further, the small heat pipes are arranged in the road shoulder range and the road surface range between the road shoulders on both sides.

[0018] Further, the top of the subgrade filling layer is paved with a semi-rigid base layer, and an asphalt pavement layer is paved above the semi-rigid base layer.

[0019] The second phase change material heat storage layer is below the semi-rigid base layer.

[0020] In another aspect, a construction method of the phase change heat storage material combined with the small heat pipe frozen soil composite subgrade is provided, and the method comprises:

[0021] Cleaning the original ground surface, i.e. the silty clay layer, and tamping the foundation;

[0022] Drilling holes in the silty clay layer, and vertically placing the evaporation sections of the small heat pipes in the holes;

[0023] Paving the first phase change material heat storage layer and surrounding the transition sections of the small heat pipes;

[0024] The subsoil layer is paved on the first phase change material heat storage layer, and the condensation section of the small heat pipe is buried;

[0025] After the subsoil layer is compacted and tamped layer by layer, the second phase change material heat storage layer is paved by rolling;

[0026] The semi-rigid base layer is paved on the second phase change material heat storage layer, and the asphalt pavement layer is paved on the upper layer;

[0027] The left and right road shoulders and the slope protection of the asphalt pavement layer.

[0028] Further, RT2-HC, RT3-HC and RT-4 are uniformly mixed at a volume mixing ratio of 6:2:2 to obtain the first phase change material heat storage layer and the second phase change material heat storage layer.

[0029] Further, in the small heat pipe, the working medium vapor rises after the liquid working medium is heated in the evaporation section, reaches the condensation section, and then condenses after the rising working medium vapor drops in temperature and returns to the evaporation section.

[0030] Compared with the prior art, the application has the following beneficial effects:

[0031] The application provides a frozen soil composite subgrade combined with a phase change heat storage material and a small heat pipe and a construction method thereof. On the basis of adding the phase change heat storage material to the frozen soil subgrade, two layers of phase change heat storage materials are optimized for different positions, the phase change heat storage capacity is improved by using new material matching, a small heat rod is added between the subsoil area and the upper limit of natural frozen soil, the cooling effect of the frozen soil subgrade mixed with the phase change heat storage material on the underlying frozen soil is further expanded, the phase change heat storage material and the small heat pipe are used for heat conduction and temperature control of the subgrade, and through active and passive modes, the frozen soil degradation can be cooled and prevented, the integrity of the subgrade can be maintained, the traffic capacity can be ensured, engineering diseases can be reduced, the thermal stability and structural stability of the highway engineering in the frozen soil area can be improved, and the application can be widely applied to the technical field of frozen soil subgrade freeze-thaw disaster prevention. BRIEF DESCRIPTION OF DRAWINGS

[0032] In order to more clearly illustrate the technical solutions in the embodiments of the application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or the prior art description. Obviously, the drawings in the following description are only some embodiments of the application, and for those skilled in the art, other drawings can be obtained from these drawings without creative labor.

[0033] Figure 1 is a subgrade temperature control schematic diagram of the application.

[0034] Figure 2 is a subgrade structure diagram of the application.

[0035] Figure 3is a local structure of the application.

[0036] Figure 4 is a schematic diagram of a heat pipe point position plane of the application.

[0037] Figure 5 is a structure diagram of a small heat pipe of the application.

[0038] Figure 6 is a statistical comparison diagram of different roadbed annual maximum thaw depth.

[0039] Figure 7 is a comparison diagram of different roadbed frozen soil upper limit. Wherein, A is a frozen soil upper limit schematic diagram of the roadbed without measures, B is a local enlarged view of the roadbed in A, C is a frozen soil upper limit schematic diagram of the roadbed of the application, and D is a local enlarged view of the roadbed in C.

[0040] Identified in the figure is:

[0041] 1-gravel-containing clay layer, 2-silty clay layer, 3-active layer, 4-first phase change material heat storage layer, 5-roadbed filling layer, 6-second phase change material heat storage layer, 7-semi-rigid base layer, 8-asphalt pavement layer, 9-small heat pipe, 10-fully weathered mudstone layer, 11-condensing section, 12-transition section, 13-evaporating section, 14-frozen soil upper limit, 15-slope protection, 16-road shoulder, and 17-natural ground surface. DETAILED DESCRIPTION

[0042] In order to facilitate the understanding of the present application, the present application will be described in more detail below with reference to the relevant drawings. The preferred embodiments of the present application are shown in the drawings. However, the present application can be realized in many different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the disclosure of the present application more thorough and comprehensive.

[0043] In the description of the present application, it should be understood that the terms "upper", "lower", "vertical", "horizontal", "top", "bottom", "inner", "outer" and the like indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, and are only for the purpose of facilitating the description of the present application and simplifying the description, and therefore cannot be understood as indicating or implying that the devices or elements referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as limiting the present application.

[0044] In the description of the present application, it should be noted that unless otherwise explicitly specified and limited, the terms "provided", etc. should be understood broadly, for example, it can be fixedly connected, provided, or detachably connected, provided, or integrally connected, provided. For those skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

[0045] Meanwhile, in the description of the present application, the terms "first", "second", and the like are used only to distinguish different descriptions, and cannot be understood as indicating or implying relative importance. Of course, such objects can be interchanged under appropriate circumstances, so that the embodiments of the present application described herein can be implemented in an order other than that illustrated or described herein.

[0046] It should also be noted that, although the step sequence is involved in the method description, in some cases, it can be performed in a different order than here, and should not be understood as a limitation on the step sequence.

[0047] Frozen soil is a special rock and soil with a temperature of 0℃ or below 0℃ and containing ice. It is very sensitive to temperature change, and climate warming and engineering construction will cause frozen soil to warm and degrade, affecting the stability of the upper road. Understanding the heat exchange mechanism of frozen soil subgrade is the basis for ensuring the thermal stability of frozen soil subgrade. The heat exchange process of frozen soil subgrade in the atmosphere-soil coupling system is a complex unsteady heat exchange process, which is affected by many factors such as climate environment, physical properties and engineering design scheme.

[0048] The phase change heat storage material can store heat in the form of latent heat, construct temperature regulating pavement, control the temperature distribution of asphalt / concrete pavement, realize ice and snow melting of pavement, reduce road diseases, and prolong the service life of road.

[0049] Based on this, the present application provides a kind of phase change heat storage material combined small heat pipe frozen soil composite subgrade (PCM-SHP), as shown in Figures 1-4 , the subgrade includes gravel clay layer 1, silty clay layer 2, subgrade filling layer 5 and semi-rigid base 7 from bottom to top, and asphalt pavement layer 8 is laid on the semi-rigid base 7. The top of the silty clay layer 2 is the active layer 3, and the frozen soil upper limit 14 is between the active layer 3 and the silty clay layer 2. The active layer 3 changes with seasonal temperature, and the thickness changes with it, which is part of the top of the silty clay layer 2, and the frozen soil upper limit 14 also changes with seasonal temperature. The semi-rigid base 7 is a structure with certain rigidity but relatively low strength, which can choose cement stabilized type, cement fly ash stabilized type or lime-fly ash stabilized type materials, which has both bearing capacity and good flexibility and toughness, and can adapt to the road section with rough terrain.

[0050] Among them, the first phase change material heat storage layer 4 is arranged between the silty clay layer 2 and the subgrade filling layer 5, and the second phase change material heat storage layer 6 is arranged between the subgrade filling layer 5 and the semi-rigid base 7. The first phase change material heat storage layer 4 and the second phase change material heat storage layer 6 are laid by phase change heat storage material, and the phase change heat storage material contains RT2-HC, RT3-HC and RT-4 (manufactured by Rubitherm Technologies GmbH company in the United Kingdom). In the phase change heat storage material, the volume mixing ratio of RT2-HC, RT3-HC and RT-4 is 6:2:2.

[0051] In addition, the roadbed further comprises small heat pipes 9 vertically arranged in the roadbed filling layer 5, the first phase change material heat storage layer 4 and the silty clay layer 2. The small heat pipes 9 are two-phase closed heat siphon type heat pipes, comprising, from top to bottom, a condensation section 11, a transition section 12 and an evaporation section 13, wherein the transition section 12 is also an adiabatic section. The condensation section 11 is located in the roadbed filling layer 5, the transition section 12 is located in the first phase change material heat storage layer 4 and the active layer 3, and the evaporation section 13 is located in the active layer 3 and penetrates into the silty clay layer 2 below the active layer 3, and the evaporation section 13 penetrates through the frost table 14. The small heat pipes 9 are arranged in the road surface range between the road shoulders 16 and the two sides of the road shoulders 16.

[0052] In the roadbed of the present application, the small heat pipes 9 are active defense structures, and the first phase change material heat storage layer 4 and the second phase change material heat storage layer 6 are passive heat preservation structures. The structure of the present application comprehensively adopts active defense and passive heat preservation measures, which can effectively prevent disasters of the roadbed in the cold season and significantly improve the anti-freeze-thaw stability of the roadbed structure in the permafrost region.

[0053] The construction method of the permafrost composite roadbed of the phase change heat storage material combined with the small heat pipes comprises the following steps:

[0054] S1: clean the original ground surface, i.e. the silty clay layer 2, and tamp the foundation;

[0055] S2: drill a hole in the silty clay layer 2, and vertically place the evaporation section 13 of the small heat pipe 9 in the hole;

[0056] S3: spread the first phase change material heat storage layer 4 and surround the transition section 12 of the small heat pipe 9;

[0057] S4: spread the roadbed filling layer 5 on the first phase change material heat storage layer 4, and bury the condensation section 11 of the small heat pipe 9;

[0058] S5: after the roadbed filling is tamped layer by layer, lay and compact the second phase change material heat storage layer 6;

[0059] S6: lay the semi-rigid base layer 7 on the second phase change material heat storage layer 6, and then lay the asphalt pavement layer 8 on the upper layer;

[0060] S7: the asphalt pavement layer 8, the left and right road shoulders 16 and the revetment 15.

[0061] RT2-HC, RT3-HC and RT-4 are mixed uniformly at a volume mixing ratio of 6:2:2 to obtain the first phase change material heat storage layer 4 and the second phase change material heat storage layer 6.

[0062] In addition, the small heat pipe 9 is a two-phase closed heat siphon type heat pipe, and after the liquid working medium in the evaporation section 13 is heated, the working medium vapor rises, passes through the transition section 12, reaches the condensation section 11, and then condenses with the temperature dropping, and returns to the evaporation section 13.

[0063] Embodiment:

[0064] The structure of the application and the construction method thereof will be further described in detail through specific construction implementation:

[0065] The composite roadbed in the frozen earth area is constructed from bottom to top as follows:

[0066] 1. Gravel clay layer 1;

[0067] 2. Silty clay layer 2;

[0068] 3. Active layer 3, which changes with seasonal temperature changes and thickness, is part of the top of the silty clay layer 2;

[0069] 4. First phase change material heat storage layer 4, which is uniformly laid on the bottom of the roadbed with a thickness of about 0.5m, and the phase change material extends to the ends during laying;

[0070] 5. Roadbed filling layer 5, which has a filling height of about 1.85m, and the road shoulder 16 is also covered with filling;

[0071] 6. Second phase change material heat storage layer 6, which has a laying thickness of about 0.2m and a laying width same as the width of the asphalt pavement;

[0072] 7. Semi-rigid base layer 7, which has a laying thickness of about 0.33m and a laying width same as the width of the asphalt pavement;

[0073] 8. Asphalt pavement layer 8, which is the roadbed surface layer, has a laying thickness of about 0.12m, and a laying width of the entire width except 1.5m on both sides of the road shoulder 16;

[0074] 9. Small heat pipes 9 are vertically and spacedly arranged between the silty clay layer 2, the active layer 3, the first phase change material heat storage layer 4, and the roadbed filling layer 5;

[0075] 10. The slope 15 on both sides of the roadbed structure has a slope of 1:1.5.

[0076] The small heat pipe 9 is a two-phase closed heat siphon type heat pipe, which is divided into a condensation section 11, a transition section 12, and an evaporation section 13 from top to bottom; the lengths of the condensation section 11 and the evaporation section 13 are each 1.5m, the length of the transition section 12 is 1m, the inner diameter of the heat pipe is 0.08m, the outer diameter is 0.1m, and the heat exchange coefficient of the inner wall surface of the evaporation section 13 is 5000[W·(m 2 ·℃).-1] The inner wall surface heat exchange coefficient of the transition section 12 is 6000 [W·(m2·℃) 2 ]·℃) -1 The inner wall surface heat exchange coefficient of the condensing section 11 is 6000 [W·(m2·℃) -1 ]·℃) 2 The thermal conductivity of the heat pipe wall is 10 [W·(m2·℃) -1 ]·℃)

[0077] The heat pipe heat transfer mainly consists of six links: (1) the heat conduction process of the heat pipe evaporation section solid wall; (2) the evaporation (boiling) heat exchange process of the heat pipe evaporation section; (3) the condensation heat exchange process of the heat pipe condensing section; (4) the heat conduction process of the heat pipe condensing section solid wall; (5) the axial heat conduction process of the heat pipe evaporation section to the condensing section solid wall; (6) the heat exchange process between the heat pipe condensing section outer wall and the cold source.

[0078] The heat pipes are arranged at intervals in the direction perpendicular to the extension direction of the heat pipes. The evaporation section 13 of the heat pipe is arranged in the silty clay layer 2, 0.5m of the transition section 12 is arranged in the active layer 3, the remaining 0.5m is arranged in the first phase change material heat storage layer 4, and the condensing section 11 is arranged in the roadbed fill layer 5. The interval between two adjacent heat pipes is 12.85m. Of course, according to actual needs and the size of the frozen soil roadbed, the number of heat pipes and the interval between the heat pipes can be appropriately adjusted to ensure that the evaporation section of the heat pipe is located in the lower layer of the upper limit of the natural frozen soil under the roadbed, and at the same time, the terminal of the condensing section 11 is located in the middle layer of the roadbed. The heat pipe roadbed system of the frozen soil roadbed in the present application includes three heat pipes. In specific embodiments, the heat pipe roadbed system of the frozen soil roadbed can also include one, three and more heat pipes. The arrangement of multiple heat pipes can increase the strength of the frozen soil roadbed adjusted by the heat pipes, improve the adjustment efficiency, and be more conducive to the internal temperature regulation of the frozen soil roadbed.

[0079] The present application adopts two layers of phase change heat storage layer to isolate the heat exchange between a large amount of roadbed and pavement and the heat exchange between the roadbed and natural frozen soil. The second phase change material heat storage layer 6 arranged in the composite roadbed is mainly used for isolating the heat exchange between the asphalt pavement layer 8 and the roadbed filling layer 5, and mainly isolating solar radiation. The first phase change material heat storage layer 4 arranged in the composite roadbed is mainly used for isolating the heat exchange between the roadbed filling layer 5 and the silty clay layer 3, and mainly preventing the leakage of underground cold. Meanwhile, small heat pipes 9 are vertically and equidistantly arranged in the silty clay layer 2, the active layer 3, the first phase change material heat storage layer 4 and the roadbed filling layer 5. On the one hand, the heat pipes arranged in the frozen soil roadbed return the heat generated by the frozen soil, maintain the temperature balance inside the frozen soil, and on the other hand, temporarily lead out the excess heat in the frozen soil, prevent the accumulation of heat, and accelerate the melting process of the frozen soil.

[0080] The construction method is specifically as follows: firstly, the original ground, i.e., the silty clay layer and the active layer, is cleaned, and the foundation is rammed; in the silty clay layer and the active layer, a drilling machine is used to drill three cylindrical holes with a depth of more than 2 m and a diameter of more than 0.1 m every 2 m along the road direction, with the center as the base point and extending 12.85 m to the left and right, and the evaporation section of the two-phase closed thermosyphon type heat pipe is placed in the lower side of the hole and vertically placed in the hole, so as to ensure that the adiabatic section of the heat pipe is 0.5 m in the silty clay layer; then the soil in the hole is filled and rammed, and attention is paid to protecting the heat pipe wall from being damaged and keeping it vertical during filling, and a first layer of phase change heat storage material is spread on the ground, with a thickness of about 0.5 m, and rolled until the compaction degree reaches the standard, and the phase change heat storage material is extended to the two ends during laying; attention is paid to avoiding the heat pipe placement point during ramming around the heat pipe with a small compaction device to prevent damage to the heat pipe; it is ensured that the condensation section of the heat pipe is completely placed in the air at this time, and the adiabatic section is not exposed to the air area; then the first layer of phase change heat storage material is spread on the roadbed fill, the sand grain size is not more than 10 mm, and the sand soil clay content is not more than 5%, and the filling height of the fill after compaction is about 1.85 m, and the condensation section of the heat pipe is directly covered with soil, and the treatment method is the same as that of the phase change heat storage material layer; after the heat pipe is completely covered; the roadbed fill is rolled and rammed layer by layer using a ramming device, and the initial heat pipe position and the adjacent heat pipe spacing are accurately positioned during the rolling process, so as to ensure seamless contact between the roadbed and the heat pipe and ensure that the adjacent heat pipe spacing meets the design requirements. After the roadbed fill layer is treated, the second layer of phase change heat storage material is laid and rolled, the laying thickness of the second layer of phase change heat storage material is about 0.2 m, the laying width is the same as the width of the asphalt pavement, and the second layer of phase change heat storage material is compacted by a mechanical device; when the compaction degree is qualified, the semi-rigid base layer is laid on the second layer of phase change heat storage material and compacted by a mechanical device, and the thickness of the semi-rigid base layer after compaction is about 0.33 m; after the laying is completed, asphalt is laid on the upper layer, and the thickness of the asphalt after compaction is about 0.12 m, and the width is about 24 m; 1.5 m is extended to the left and right of the asphalt layer according to the slope foot coordinate point, and the fill is covered and rammed at a slope ratio of 1:1.5.

[0081] As Figure 6 ,

[0082] As Figure 7 ,

[0083] The above application of specific examples is used to illustrate the present application and is not used to limit the present application. According to the idea of the present application, those skilled in the art can make several simple deductions, deformations or substitutions.

Claims

1. A permafrost composite roadbed combined with phase change material and small heat pipes, characterized in that: the roadbed comprises, from bottom to top, a gravel-containing clay layer (1), a silty clay layer (2), a roadbed fill layer (5), and an asphalt pavement layer (8); a first phase change material heat storage layer (4) is arranged between the silty clay layer (2) and the roadbed fill layer (5), and a second phase change material heat storage layer (6) is arranged between the roadbed fill layer (5) and the asphalt pavement layer (8); the roadbed further comprises small heat pipes (9) vertically arranged in the roadbed fill layer (5), the first phase change material heat storage layer (4), and the silty clay layer (2); the top of the silty clay layer (2) is an active layer (3), which is a part of the silty clay layer (2), and the active layer (3) and the silty clay layer (2) are separated by a permafrost upper limit (14); the first phase change material heat storage layer (4) is located above the active layer (3), and the small heat pipes (9) pass through the active layer (3); the small heat pipes (9) are two-phase closed heat siphon type heat pipes, comprising, from top to bottom, a condensation section (11), a transition section (12), and an evaporation section (13); the condensation section (11) is located in the roadbed fill layer (5), the transition section (12) is located in the first phase change material heat storage layer (4) and the active layer (3), and the evaporation section (13) is located in the active layer (3) and extends into the silty clay layer (2) below the active layer (3); the top of the roadbed fill layer (5) is paved with a semi-rigid base layer (7), and the semi-rigid base layer (7) is paved with an asphalt pavement layer (8) above; and the second phase change material heat storage layer (6) is located below the semi-rigid base layer (7).

2. The permafrost composite roadbed combined with phase change material and small heat pipes according to claim 1, characterized in that: the small heat pipes (9) are arranged in the road shoulder (16) range and the road surface range between the two road shoulders (16).

3. A construction method of the permafrost composite roadbed combined with phase change material and small heat pipes according to claim 2, characterized in that: the method comprises: cleaning the original ground surface, i.e., the silty clay layer (2), and tamping the foundation; drilling a hole in the silty clay layer (2) and vertically placing the evaporation section (13) of the small heat pipe (9) in the hole; paving the first phase change material heat storage layer (4) and surrounding the transition section (12) of the small heat pipe (9); paving the roadbed fill layer (5) on the first phase change material heat storage layer (4) to cover and bury the condensation section (11) of the small heat pipe (9); after the roadbed fill is tamped layer by layer, paving the second phase change material heat storage layer (6); paving the semi-rigid base layer (7) on the second phase change material heat storage layer (6) and then paving the asphalt pavement layer (8) on the upper layer; and paving the road shoulders (16) and the slope protection (15) on both sides of the asphalt pavement layer (8).

4. The construction method of the permafrost composite roadbed combined with phase change material and small heat pipes according to claim 3, characterized in that: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ In the small heat pipe (9), the working medium steam rises after the liquid working medium is heated in the evaporation section (13), passes through the transition section (12) to reach the condensation section (11), and the rising working medium steam is condensed with the temperature dropping to return to the evaporation section (13).

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