CN103572785B - All-weather anti-seepage and air-tight landfill soil covering system, preparation method and application - Google Patents

Abstract

The invention provides an all-weather anti-seepage airtight refuse landfill soil coverage system, a preparation method and application. The soil coverage system is laid on the garbage to be buried or a gas collection layer covered on the garbage; the soil covering system is characterized by comprising a clay layer and a capillary blocking covering layer, wherein the clay layer is positioned below the capillary blocking covering layer, and the capillary blocking covering layer sequentially comprises a gravel layer and a silt layer from bottom to top. The soil coverage system can prevent liquid from infiltrating through the capillary retardation of the silt layer and the gravel layer under the condition that the soil body is dry. Under the condition that the soil body is wet and saturated, the liquid can be prevented from infiltrating by the additionally arranged clay layer. The clay layer can effectively prevent the landfill gas from escaping under all weather (any dry and wet weather) conditions. The materials used by the soil coverage system are all natural soil materials, and the soil coverage system has the characteristics of convenient material acquisition, simple construction, good durability, low cost and the like.

Description

All-weather anti-seepage and air-tight landfill soil covering system, preparation method and application

technical field

The invention relates to the technical field of landfill sealing and covering systems, in particular to an all-weather anti-seepage and air-tight soil covering system for landfill treatment, a preparation method and an application.

Background technique

Landfill is the main way to dispose of municipal waste at present and for a long time to come. A large amount of leachate and landfill gas (mainly composed of methane and carbon dioxide) are produced during the accumulation and degradation of garbage. The spread of landfill gas pollutes urban air, contributes to the greenhouse effect, and can cause fires and explosions. The infiltration of liquids such as rainwater and snowwater will intensify the leachate pollution of groundwater. Therefore, the main function of the landfill closure cover system is to prevent liquid infiltration and landfill gas escape, that is, seepage-proof and air-tight.

Waste landfills are generally filled with cols and plains, mainly in the form of plains and valleys. Therefore, there are mainly two forms of landfill closure and covering systems: horizontal and slope. Traditional covering systems include a single compacted clay covering layer and a geocomposite covering system composed of a compacted clay layer and a geomembrane, all of which use the low permeability of the material to achieve the purpose of preventing seepage and sealing air. However, in these traditional cover systems, the clay layer is the surface soil layer. Clay layers exposed to air are prone to cracking in dry weather, making them more permeable and unable to block liquid infiltration and landfill gas escape (see "Albrecht, B. & Benson, C.( 2001). Effect of Desiccation on Compacted Natural Clays. Journal of Geotechnical and Geoenvironmental Engineering, ASCE, 127(1), 67–75.”; “Albright, W., Benson, C., Gee, G., Abichou, T., McDonald, E., Tyler, S. & Rock, S. (2006). Field Performance of a Compacted Clay Landfill Final Coverata Humid Site. Journal of Geotechnical and Geoenvironmental Engineering, ASCE, 132(11), 1393–1403.”). However, the geomembrane has problems such as easy sliding and instability along the interface and poor durability (see literature: "Stark, T.D., Arellano, D., Evans, W.D., Wilson, V.L., and Gonda, J.M. (1998). Unreinforced geosynthetic clay liner case history. Geosynthetics International , 5(5), 521-544."). Therefore, the long-term performance of these traditional overlay systems is not satisfactory.

In recent years, the Tengfa soil cover system based on the principle of water storage and release has gradually received attention and development. In the early days, the evaporative soil cover system was mainly a single fine-grained soil layer planted with vegetation, which stored water under rainfall conditions and released water through ground evaporation and plant transpiration under dry conditions. However, this single-soil type evacuation covering system is only suitable for arid areas with little rainfall, and the required soil layer thickness is thicker, up to 2m (see literature: "Benson, C.H. & Khire, M.V. (1995) . Earthen covers for semi-aridandarid climates. Geotechnical Special Publication, No. 53, 201-217."). Therefore, the capillary barrier-type soil cover system has been developed (see literature: "Khire, M.V., Benson, C.H. & Bosscher, P.J. (2000). "; "Aubertin, M., Cifuentes, E., Apithy, S.A., Bussiere, B., Molson, J. & Chapuis, R.P. (2009). , H., Santoso, V.A., Leong, E.C., Ng, Y.S. & Hua, C.J. (2012). Performance of an instrumented slope covered by a capillary barrier system. Journal of Geotechnical and Geoenvironmental Engineering, ASCE, 138(4), 481-490.”). The capillary retardation covering system is composed of a layer of fine-grained soil under an unsaturated state and a layer of coarse-grained soil lying below it. The capillary retardation at the interface of the coarse-grained soil layer is used to increase the water storage capacity of the upper fine-grained soil layer. Improve lateral drainage capacity and reduce rainwater infiltration. The above-mentioned unsaturated state refers to a state in which both water and air exist in the pores of the soil body, and the water and air in the soil body are connected to each other respectively. The research results show that the capillary barrier covering system can achieve effective anti-seepage function in arid and semi-arid climate areas, but the anti-seepage effect is not good in humid climate areas (see literature: "Benson, C.H. & Khire, M.V. (1995).Earthencoversforsemi -aridandaridclimates.GeotechnicalSpecialPublication, No.53,201-217."; "Benson, C.H., Albright, W.H., Roesler, A.C. & Abichou, T. (2002).Evaluationoffinalcoverperformance:fielddatafromtheAlternativeCoverAssessmentProgram(ACAP).Proc.Waste2agement,1-Vol. 15.").

Later, some scholars proposed the method of adding an unsaturated drainage layer in the middle of the coarse and fine-grained soil layer of the capillary retardation cover layer to further improve its lateral drainage capacity (see literature: "Stormonont, J.C. & Morris, C.E. (1997). Unsaturated drainage layers for diversion of filtering water .Journal of Irrigation and Drainage Engineering, ASCE,123(5),364-367."; "Deng Linheng, Zhan Liangtong, Chen Yunmin, Jia Guanwei (2012). Model test research on the performance of capillary barrier overburden with unsaturated drainage layer. Geotechnical Engineering Acta, Vol. 34, No. 1, 75-80.”). Chinese patent application publication CN102493496A (application number is No. 201110441895.9, the title of the invention is "a kind of capillary retardation overburden structure with lateral drainage capacity") is to use unsaturated sand in the middle of the silt layer and gravel layer Layer method to improve its lateral drainage capacity. However, this covering system is still only applicable when the soil is in an unsaturated state. When the water content in the soil is close to or reaches saturation under long-term heavy rainfall conditions, the covering system will lose its anti-seepage effect. In addition, the covering system is a slope type, and if it is a horizontal type, its anti-seepage effect is questionable. The covering system has a vegetation layer on the surface for aesthetics, and a geotextile layer between the sand layer and the gravel layer to prevent sand from being embedded in the gravel, but it has no practical effect on seepage control of the landfill.

In addition, a very important point is that none of the above-mentioned traditional and improved capillary arrest covering systems can effectively prevent landfill gas from escaping, that is, they have no gas-holding function.

So far, there is no cover system that can achieve long-term anti-seepage and air-tight function under any climatic conditions, especially cannot effectively prevent landfill gas from escaping and cannot effectively prevent liquid infiltration when the soil is close to or reaches saturation.

Contents of the invention

The purpose of the present invention is to overcome the above-mentioned defects and provide a novel all-weather and multifunctional landfill closure soil covering system, which is suitable for any climatic conditions and can effectively prevent liquid infiltration and landfill gas escape , and has good durability.

In order to solve the above-mentioned technical problems, the present invention provides an all-weather anti-seepage and air-tight soil covering system, preparation method and application for landfill treatment.

Specifically, the present invention provides:

(1) A soil cover system for landfills, which is laid over the waste to be landfilled or, in the case of such waste covered with a gas-collecting layer, over the gas-collecting layer; It is characterized in that the soil covering system includes a clay layer and a capillary blocking covering layer, and the clay layer is located below the capillary blocking covering layer and above the garbage to be landfilled; wherein, the capillary blocking covering layer The lagging cover layer includes a gravel sand layer and a silt layer, and the silt layer is located above the gravel sand layer.

(2) The soil covering system according to (1), characterized in that the clay layer is compacted, wherein the compaction process includes compaction at an optimal water content.

(3) The soil covering system according to (1), characterized in that the thickness of the clay layer is 0.3-0.8m, the soil particle size of the clay layer is less than 0.075mm, and the saturation of the clay layer The water permeability coefficient is not higher than 1×10 ^-9 m/s.

(4) The soil covering system according to (1), characterized in that the thickness of the gravel sand layer is 0.2-0.3m, the soil particle size of the gravel sand layer is 0.5-5mm, and the gravel sand layer The saturated water permeability coefficient of the sand layer should not be lower than 1×10 ^-4 m/s.

(5) The soil covering system according to (1), characterized in that the thickness of the silt layer is 0.3-0.6m, the soil particle size of the silt layer is not greater than 2mm, and the silt layer The saturated water permeability coefficient of the layer is 1×10 ^-8 -1×10 ^-5 m/s.

(6) The soil covering system according to (1), characterized in that the soil covering system is a horizontal or slope type, and the slope of the slope type is: the ratio of the vertical direction to the horizontal direction is not greater than 1:3.

(7) The soil covering system according to (1), characterized in that the soil covering system is provided with a drainage ditch every 20-40m along the horizontal direction.

(8) A method for preparing the soil covering system according to any one of (1)-(7), the method comprising:

1) Provide a horizontal or sloped landfill with a flat surface;

2) Optionally, laying a gas collection layer on the surface of the landfill body (for example, a gas collection layer composed of gravel);

3) On the surface of the gas collection layer (if there is one) or the landfill body (if there is no gas collection layer) a layer of clay is laid, compacted and leveled at an optimal water content ;

4) laying a gravel sand layer on the surface of the clay layer, compacting and leveling; and

5) A silt layer is laid on the surface of the gravel layer, compacted and leveled to obtain the soil covering system.

(9) Use of the soil cover system according to any one of (1)-(7) for preventing liquid from penetrating into a landfill under all-weather (any wet or dry climate) conditions.

(10) Use of the soil covering system according to any one of (1)-(7) for preventing gas escape from landfills under all weather conditions.

Compared with the prior art, the method of the present invention has the following advantages and positive effects:

(1) Under any dry and wet climate conditions, especially when the soil is close to or reaches saturation, it can effectively prevent liquid (especially rainwater) from seeping into the landfill;

(2) Under any climatic conditions, it can effectively prevent landfill gas from escaping;

(3) The constituent materials of the soil covering system of the present invention are all natural soil materials, which are rich in sources, convenient to obtain materials, and low in cost;

(4) The soil covering system of the present invention is simple in construction, good in durability, and low in later maintenance costs;

(5) The soil covering system of the present invention can be used for horizontal and slope landfills, especially, even if the landfill is horizontal, the soil covering system of the present invention can effectively achieve the effect of anti-seepage and air-tightness all-weather .

Description of drawings

Fig. 1 is the structural composition of the soil covering system of the present invention and the schematic diagram of the anti-seepage and air-tight principle;

Fig. 2 is the relation function schematic diagram of each soil layer water permeability coefficient and suction in the soil covering system of the present invention;

Fig. 3 is the relational function schematic diagram of each soil layer air flow and air pressure in the soil covering system of the present invention;

Fig. 4 is the numerical simulation result figure of a specific example of capillary retardation covering system as comparison;

Fig. 5 is the numerical simulation result figure of a concrete example of soil covering system of the present invention;

Fig. 6 is the soil column model test result figure of a concrete example of the anti-seepage performance of soil covering system of the present invention;

Fig. 7 is the soil column model test result figure of a specific example of the air-trap performance of the soil covering system of the present invention;

The reference signs in each figure indicate respectively: 1 is a silt layer, 2 is a gravel sand layer, 3 is a clay layer, 4 is an optional gas collection layer, 5 is a landfill body, 6 is a landfill gas, 7 is the water flow, 8 is the soil covering system, 9 is the geotextile, 10 is the drainage ditch, S ₁ is the suction value corresponding to the intersection point of the water permeability coefficient function of the silt layer and the gravel sand layer, S ₂ is the gravel sand layer and the clay layer The suction value corresponding to the intersection point of the water permeability coefficient function, A is the air intake value of the clay layer.

detailed description

The present invention will be further described below by the description of specific embodiment and with reference to accompanying drawing, but this is not limitation of the present invention, those skilled in the art can make various modifications or improvements according to the basic idea of the present invention, but as long as not departing from The basic idea of the present invention is within the scope of the present invention.

None of the existing capillary blocking covering systems can achieve long-term anti-seepage and air-tight functions under any climatic conditions, especially cannot effectively prevent landfill gas from escaping and cannot effectively prevent liquid infiltration when the soil is close to or reaches saturation. . And the inventor surprisingly found that as long as a layer of clay layer, especially a compacted clay layer is added below the capillary retardation cover layer composed of silt layer and gravel sand layer, it can be used under any climatic conditions, especially in soil. When the body is close to saturation or reaches saturation, it can effectively prevent liquid (especially rainwater) from penetrating into the landfill body, and effectively prevent landfill gas from escaping.

And if the clay layer is set on the surface of the covering system, it is impossible to achieve the effect of long-term anti-seepage and air-tightness under any climatic conditions. This is because, in dry weather, the clay layers exposed to the air are extremely prone to cracking, making them more permeable and no longer able to stop liquids from seeping in or gases from escaping. In the present invention, the clay layer is arranged at the bottom layer of the covering system, that is, below the silt layer and gravel sand layer, and has a high degree of saturation, so there is no problem of drying and cracking.

In this context, "all weather" refers to any climatic conditions, including any wet and dry climates, and more specifically, includes: humid, semi-humid, semi-arid, and arid climatic conditions.

Herein, "soil body" refers to the clay layer, gravel sand layer, silt layer or their general term in the covering system of the present invention.

The definitions and divisions of "silt", "gravel" and "clay" described in this article are known in the art. For details, please refer to the National Standard of the People's Republic of China "Engineering Classification Standard for Soil GB/T50145-2007" And "Code for Design of Building Foundation GB50007-2002", which is mainly determined according to the particle size distribution range.

In this article, "liquid" refers to various forms of liquid including rainwater, snowwater, etc.

Specifically, the present invention provides an all-weather anti-seepage and air-tight soil covering system for landfill treatment, which includes a clay layer, a gravel sand layer and a silt layer; it is characterized in that the clay layer, gravel sand Layer and silt layer are laid sequentially from bottom to top on the garbage to be landfilled, or in the case of a gas collection layer covering the garbage, on the gas collection layer, that is, on the gravel A layer of said clay layer is added below the capillary retardation covering layer composed of the layer and said silt layer.

Preferably, said clay layer is compacted, wherein said compaction includes compaction at an optimum water content.

The definition of "optimum water content" described herein is known in the art, and it refers to the corresponding water content when the soil reaches the maximum compaction under certain compaction conditions according to the compaction test (for details, please refer to The National Standard of the People's Republic of China "Standards for Soil Test Methods" GB/T50123-1999").

Fig. 1 schematically shows a kind of soil covering system 8 of the present invention, and it is mainly made up of clay layer 3, gravel sand layer 2 and silt layer 1 laid in sequence from bottom to top, namely between silt layer 1 and gravel layer A clay layer 3 is added below the capillary retardation covering layer composed of the sand layer 2 .

Also preferably, the thickness of the clay layer 3 is 0.3-0.8m, and the soil particle size of the clay layer is less than 0.075mm, and the saturated water permeability coefficient of the clay layer is not higher than 1×10 ^-9 m /s; the thickness of the gravel sand layer is 0.2-0.3m, the soil particle size of the gravel sand layer is 0.5-5mm, and the saturated water permeability coefficient of the gravel sand layer is not less than 1×10 ^-4 m/s; the thickness of the silt layer is 0.3-0.6m, the soil particle size of the silt layer is not greater than 2mm, and the saturated water permeability coefficient of the silt layer is 1×10 ^-8 -1 ×10 ^-5 m/s. The thickness range of the soil layer can be obtained through parametric analysis, for example, by using numerical simulation software known in the art (eg, GeoStudio2007 software, available from GEO-SLOPE, Inc. located in Canada). Different soil layer thicknesses and different water permeability coefficients can be used for parameter analysis, so that the soil layer thickness range that can achieve the best anti-seepage and air-tight effect can be obtained under any climatic conditions from saturated to dry.

Also preferably, the soil covering system can be horizontal or slope-type, and the slope of the slope-type is: the ratio of the vertical direction to the horizontal direction is not greater than 1:3.

Also preferably, the soil covering system is laid on a horizontal or slope-shaped landfill body, and the slope of the slope body is: the ratio of the vertical direction to the horizontal direction is not greater than 1:3.

In the present invention, there is no special limitation on the area size of the soil covering system. Those skilled in the art can choose according to actual needs (such as the actual size of the landfill body), and the soil covering system can be horizontally spaced every 20 Set up a drainage ditch at -40m. When the climate is very dry for a long time, the method of recharge can be used to pour water into the gravel layer on the upper part of the slope to moisten the clay layer. The setting and refilling method of the drainage ditch can be carried out by conventional means or methods in the art.

In addition, for the soil covering system of the present invention, when the particle diameters of adjacent two layers of soil particles differ greatly, an air-permeable and water-permeable geotextile layer 9 can be optionally laid to prevent the particles from interfering between different soil layers. mix. For example, when the average particle size difference between the silt layer and the gravel sand layer is greater than 5 times, a geotextile layer can be laid between the silt layer and the gravel sand layer to prevent the silt particles from being embedded in the gravel sand layer. Similarly, when the average particle size difference between the gravel sand layer and the clay layer is greater than 5 times, a geotextile layer can be laid between the gravel sand layer and the clay layer.

In addition, when the soil covering system of the present invention is applied to a landfill body, a gas collection layer may optionally be laid on the landfill body. The gas collection layer is known in the prior art, and it is used to migrate and diffuse the gas generated by the landfill body, so as to improve the gas collection efficiency. Therefore, the gas collection layer can also be called a gas diffusion layer. The gas collection layer may be composed of crushed rock, but is not limited thereto.

The present invention also provides a preparation method of the soil covering system, the method comprising:

1) Provide a horizontal or sloped landfill with a flat surface;

2) Optionally, laying a gas collection layer on the surface of the landfill body (for example, a gas collection layer composed of gravel);

3) On the surface of the gas collection layer (if there is one) or the landfill body (if there is no gas collection layer) a layer of clay is laid, compacted and leveled at an optimal water content ;

4) laying a gravel sand layer on the surface of the clay layer, compacting and leveling; and

5) A silt layer is laid on the surface of the gravel layer, compacted and leveled to obtain the soil covering system.

Preferably, the soil covering system of the present invention can be used according to the following steps (see Figure 1):

1) leveling the surface of the landfill body 5;

2) Optionally, laying a gas collection layer 4 on the surface of the landfill body 5 (for example, a gas collection layer composed of gravel);

3) lay clay layer 3 on the surface of gas collection layer 4 (if there is no gas collection layer 4, lay clay layer 3 on the surface of landfill body 5), and perform leveling treatment after compaction with optimal water content;

4) laying gravel sand layer 2 on the surface of clay layer 3, compacting and leveling;

5) The silt layer 1 is laid on the surface of the gravel layer 2, compacted and leveled.

The present invention also provides the use of the soil covering system for preventing liquid (especially rainwater) from seeping into landfills under all-weather conditions. The present invention also provides the use of the soil covering system for preventing gas from escaping from landfills under all-weather conditions.

The anti-seepage function of the soil covering system of the present invention can be realized by the following principles:

Those skilled in the art can understand that when the soil is saturated to dry, the saturation decreases, the water content decreases, and the corresponding suction increases, and the water permeability coefficient of the soil decreases with the increase of suction (see literature: "Ng, C.W.W. & Menzies, B. (2007). Advanced Unsaturated Soil Mechanics and Engineering. Taylor & Francis, London and NY. ISBN: 978-0-415-43679-3.687p.").

The definition of "suction" described herein is known in the art, and it refers to the difference between gas pressure and water pressure in a soil mass.

The definition of "saturation" described herein is known in the art, which refers to the ratio of the volume occupied by water in the pores of the soil to the volume of the pores of the soil.

The definition of "water permeability coefficient" described herein is known in the art, which refers to the unit water flow rate in the soil body under the unit hydraulic gradient, and its numerical unit may be "m/s".

The definition of "saturated water permeability coefficient" described herein is known in the art, and it refers to the corresponding water permeability coefficient when the soil reaches saturation (ie, the saturation is 100%).

Figure 2 is a schematic diagram of the relationship function between water permeability coefficient and suction of each soil layer in the soil cover system. The following description will be made with reference to FIG. 2 .

(1) When the soil suction in the soil cover system is less than S ₁ , that is, under humid climate conditions, the water content of the soil is high, saturated or close to saturation, the water permeability coefficient of the gravel layer 2 is the highest, and the clay layer 3 The water permeability coefficient is the lowest. Because the water permeability coefficient of the gravel layer 2 is higher than that of the silt layer 1, capillary retardation fails, so the ultra-low water permeability of the clay layer 3 is relied on to prevent rainwater from seeping into the landfill. On the other hand, because the water permeability coefficient of the gravel sand layer 2 is the largest, the lateral drainage mainly occurs in the gravel sand layer 2, as shown in Fig. 1 .

(2) When the soil suction in the soil cover system is between S ₁ and S ₂ (see Figure 2), that is, in semi-humid climate conditions, the soil is relatively moist, and the water permeability coefficient of silt layer 1 is the highest. Clay layer 3 has the lowest water permeability coefficient. Because the water permeability coefficient of the silt layer 1 is the largest, the lateral drainage mainly occurs in the silt layer 1. However, some water may still seep into the gravel sand layer 2, so it is still necessary to rely on the ultra-low water permeability of the clay layer 3 to prevent rainwater from seeping into the landfill.

(3) When the soil suction in the soil cover system is greater than S ₂ , that is, in semi-arid or arid climate conditions, the soil is relatively dry, and the water permeability coefficient of the silt layer 1 is much higher than that of the gravel layer 2 , the capillary arrest effect takes effect. The rainwater is mainly stored in the silt layer 1 or drained away along the side of the slope in the silt layer 1, and does not infiltrate into the gravel sand layer 2, so that it does not infiltrate into the landfill body.

The gas-holding function of the soil covering system of the present invention is realized by the following principles:

Figure 3 is a schematic diagram of the relationship function between the air flow rate of each soil layer and the landfill air pressure in the soil cover system. The following description will be made with reference to FIG. 3 . When the landfill gas pressure is low, the air flow in the soil is almost zero, and then the air flow increases rapidly with the increase of the landfill gas pressure. Taking the clay layer 3 as an example, point A in the figure is the breakdown pressure of the clay layer 3 or the air intake value, that is, the corresponding air pressure value when the gas starts to enter the soil. When the landfill gas pressure is lower than A, the landfill gas cannot enter the soil, and the air flow in the clay layer 3 is almost zero. When the landfill gas pressure is greater than A, the gas begins to enter the clay layer 3, and the gas flow increases rapidly. The clay layer 3 has the highest breakdown pressure (i.e. air intake value) in the soil cover system, and the clay layer 3 is at the bottom layer, directly in contact with the landfill gas 6 (see Figure 1). The pressure of landfill gas 6 is usually below 20kPa (refer to literature: "Wei Haiyun (2007). Research on gas migration law in municipal solid waste landfill. Doctoral dissertation of Zhejiang University."). Therefore, compacting the clay layer 3 so that the breakdown air pressure (that is, the air intake value) A is higher than the air pressure generated by the landfill gas 6 can effectively prevent the landfill gas from escaping.

The content of the present invention is further explained or illustrated by way of examples below, but these examples should not be construed as limiting the protection scope of the present invention.

example one

Numerical simulations were performed in the following examples using GeoStudio 2007 software (available from GEO-SLOPE, Inc., Canada). GeoStudio software is a simulation software for geotechnical engineering and geotechnical environment simulation calculation. According to reports, this software was used to simulate the landfill and soil cover system, and it was found that the simulation results were in good agreement with the real model test results (see literature: "Jia Guanwei (2010). Moisture in the soil cover layer at the end of the solid waste yard Research on migration laws and regulation methods. Doctoral dissertation of Zhejiang University.").

Comparative example 1

As a comparison, the traditional capillary retardation covering system is simulated first, that is, only the silt layer and the gravel sand layer are used as the covering system, and the gravel sand layer is under the silt layer. Among them, the thickness of the gravel layer is 0.3m, the particle size of the soil is 0.5-5mm, and the saturated water permeability coefficient is 2×10 ^-4 m/s; the thickness of the silt layer is 0.3m, and the particle size of the soil is not greater than 2mm. The saturated water permeability coefficient is 3×10 ^-6 m/s. The vertical to horizontal ratio of the slope is 1:3.

GeoStudio2007 software was used for numerical simulation, and the climate conditions were set as follows: heavy rainfall with an intensity of 100mm/hour for 2 hours. The results are shown in Figure 4. The direction of the arrow in the figure represents the direction of the water flow, and the size of the arrow represents the speed of the water flow. This result demonstrates that the traditional capillary barrier cover system cannot prevent rainwater from seeping into the landfill under heavy rainfall conditions, that is, when the soil is close to or reaches saturation.

Example 1

In a soil covering system of the present invention, a clay layer is added below the traditional capillary retardation covering system composed of a silt layer and a gravel sand layer. The thickness of the clay layer is 0.3m, the soil particle size is less than 0.075mm, and the saturated water permeability coefficient is 1×10 ^-9 m/s; the thickness of the gravel sand layer is 0.3m, the soil particle size is 0.5-5mm, and the saturated water The permeability coefficient is 2×10 ^-4 m/s; the thickness of the silt layer is 0.3m, the soil particle size is not more than 2mm, and the saturated water permeability coefficient is 3×10 ^-6 m/s. The vertical to horizontal ratio of the slope is 1:3.

GeoStudio2007 software was used for numerical simulation, and the climate conditions were set as follows: heavy rainfall with an intensity of 100mm/hour for 2 hours. The results are shown in Figure 5. The direction of the arrow in the figure represents the direction of the water flow, and the size of the arrow represents the speed of the water flow. This result proves that the soil covering system of the present invention can effectively prevent rainwater from seeping into landfills by utilizing the ultra-low water permeability of the clay layer under heavy rainfall conditions, that is, when the soil is close to or reaches saturation.

Example 2

In a soil covering system of the present invention, the thickness of the clay layer is 0.5m, the particle size of the soil is less than 0.075mm, and the saturated water permeability coefficient is 1× ^10-9 m/s; the thickness of the gravel layer is 0.2m, The soil particle size is 0.5-5mm, the saturated water permeability coefficient is 1×10 ^-3 m/s; the thickness of the silt layer is 0.3m, the soil particle size is less than 2mm, and the saturated water permeability coefficient is 1×10 ^-8 m /s. The vertical to horizontal ratio of the slope is 1:4.

GeoStudio2007 software was used for numerical simulation, and the climate conditions were set as follows: heavy rainfall with an intensity of 70mm/hour for 4 hours. The result proves that the soil covering system of the present invention can effectively prevent rainwater from infiltrating into the landfill under the condition of heavy rainfall, that is, when the soil is close to or reaches saturation.

Example two

In the following example, a one-dimensional soil column is used to conduct indoor tests to verify the anti-seepage and air-tight performance of the horizontal soil cover system. When preparing the soil column, the bottom clay layer and the upper gravel sand layer were compacted according to the optimal water content and 95% compaction degree, and the upper silt layer was compacted according to the optimal water content and 80% compaction degree, and then The upper and lower passages of the soil column are sealed to make the moisture distribution of each layer of soil as uniform as possible. The anti-seepage or air-tight test is carried out by controlling the upper and lower boundaries of the soil column, and the pore water pressure, pore air pressure and water content are measured along different depths of the overburden layer during the test.

Example 1

A soil covering system of the present invention is subjected to a one-dimensional soil column rainfall simulation test. The thickness of the clay layer is 0.4m, the particle size of the soil is less than 0.075mm, and the saturated water permeability coefficient is lower than 1× ^10-9 m/s; The thickness of the layer is 0.2m, the particle size of the soil is 0.5-5mm, and the saturated water permeability coefficient is higher than 1×10 ^-4 m/s; the thickness of the silt layer is 0.4m, the particle size of the soil is less than 2mm, and the saturated water permeability The coefficient is 5×10 ^-7 m/s. Rainfall is applied above the cover system, and the distribution of volumetric water content in the cover system after experiencing different intensities of rainfall is shown in Figure 6, where the rainfall intensity "once in x years" is determined with reference to the Hong Kong climate data literature (refer to the literature: "Lam , CC and Leung, YK (1995). Extreme rain fall statistics and design rain storm profile sat selected locations in Hong Kong. Technical Note No. 86, Royal Observatory, Hong Kong."). Even under rainfall intensities of 1 in 200 years or higher, the water content of the soil at the bottom of the clay layer in the cover system remains basically unchanged. This means that for a cover system with a typical design life of 120 years, rainwater will not penetrate the clay layer during its service life.

Example 2

A gas breakdown test is carried out on the clay layer in a soil covering system of the present invention. The thickness of the clay layer is 0.4m, the particle size of the soil is less than 0.075mm, and the saturated water permeability coefficient is lower than 1×10 ^-9 m/s. Apply gas pressure below the clay layer (shown by abscissa "bottom gas pressure" in Figure 7), and observe changes in air flow above the clay layer (shown by ordinate "top gas flow" in Figure 7). The test results are shown in Figure 7. In the saturated state of the clay layer (that is, the saturation is 100%, that is, in humid climate conditions), even if the air pressure reaches 50kPa, the clay layer still cannot be broken down; the clay layer with a saturation of 80% ( That is, in humid and semi-humid climate conditions), the breakdown pressure is about 35kPa; for a clay layer with a saturation of 60%, the breakdown pressure is about 22kPa. Since the pressure of the landfill gas in the landfill is usually below 20kPa, even under dry soil conditions, the saturation of the clay layer is 60%, which can still effectively prevent the escape of landfill gas.

Claims (9)

1. A soil covering system for landfills, which is laid over the waste to be landfilled or, in the case of said waste being covered with a gas-collecting layer, over the gas-collecting layer; its It is characterized in that the earthy covering system includes a clay layer and a capillary-blocking covering layer, and the clay layer is located below the capillary-blocking covering layer and above the garbage to be landfilled; wherein, the capillary-blocking The covering layer includes a gravel layer and a silt layer, and the silt layer is located above the gravel layer; the thickness of the clay layer is 0.3-0.8m, and the soil particle size of the clay layer is less than 0.075mm , and the saturated water permeability coefficient of the clay layer is not higher than 1×10 ^-9 m/s. 2. The soil cover system of claim 1, wherein the clay layer is compacted, wherein the compaction includes compacting at an optimum moisture content. 3. The soil covering system according to claim 1, characterized in that, the thickness of the gravel sand layer is 0.2-0.3m, and the soil particle diameter of the gravel sand layer is 0.5-5mm, and the gravel sand layer The saturated water permeability coefficient of the layer is not lower than 1×10 ^-4 m/s. 4. soil covering system according to claim 1, is characterized in that, the thickness of described silt layer is 0.3-0.6m, and the soil particle diameter of described silt layer is not more than 2mm, and the thickness of described silt layer The saturated water permeability coefficient is 1×10 ^-8 -1×10 ^-5 m/s. 5. The soil covering system according to claim 1, characterized in that, the soil covering system is horizontal or slope type, and the slope of the slope type is: the ratio of vertical direction to horizontal direction is not greater than 1:3. 6. The soil covering system according to claim 1, characterized in that, the soil covering system is provided with a drainage ditch every 20-40m along the horizontal direction. 7. A method for preparing the soil covering system according to any one of claims 1-6, the method comprising: 1) Provide a horizontal or sloped landfill with a flat surface; 2) Optionally, laying a gas collection layer on the surface of the landfill body; 3) laying a clay layer on the surface of the landfill, or laying a clay layer on the surface of the gas collection layer if the gas collection layer is provided, compacting and carrying out smoothing; 4) laying a gravel sand layer on the surface of the clay layer, compacting and leveling; and 5) laying a silt layer on the surface of the gravel sand layer, compacting and leveling, so as to obtain the soil covering system. 8. Use of the soil cover system according to any one of claims 1-6 for preventing the infiltration of liquids into landfills under all weather conditions. 9. Use of a soil cover system according to any one of claims 1-6 for preventing the escape of gases from landfills under all weather conditions.