Improved expansive soil and construction process

Abstract

According to the improved expansive soil and the construction technology, physical mixing is adopted, no chemical reaction is generated, and environmental pollution is low; on one hand, the sand cobbles are doped into the expansive soil, so that the compaction performance of the filler can be improved, the construction difficulty can be reduced, and the working efficiency can be improved, on the other hand, the sand cobbles are doped into the expansive soil, the content of coarse grains is increased, coarse-fine grain mixed soil is formed, the compression modulus is higher compared with fine-grained soil, the settlement amount of a high-fill foundation is favorably controlled, and meanwhile, compared with the fine-grained soil, the settlement amount of the high-fill foundation is reduced; the shear strength index is improved, and the stability of the foundation is enhanced.

Description

Technical Field

[0001] This application relates to the field of foundation engineering technology, and in particular to a modified expansive soil and its construction process. Background Technology

[0002] Expansive soil, rich in highly hydrophilic clay minerals such as montmorillonite, exhibits typical characteristics such as fissures and swelling / shrinkage, often leading to a series of engineering problems. Expansive soil cannot be directly used as fill material in engineering projects, but replacing it is often costly. To utilize expansive soil rationally and effectively, various methods are used to improve it. Research on improved expansive soil has significant practical engineering implications for ensuring project quality, reducing land occupation from borrowed or abandoned soil, and lowering project costs.

[0003] There are two main methods for improving expansive soil: chemical improvement and physical improvement. Chemical improvement essentially involves adding a certain amount of chemical modifier to the expansive soil, causing a complex physicochemical reaction that alters its physical and mechanical properties, thereby improving its poor engineering characteristics. Chemical improvement of expansive soil is the most common method and has been studied systematically. However, the addition of inorganic materials such as lime, cement, or fly ash can cause environmental damage, and the chemical reactions are time-sensitive. Summary of the Invention

[0004] This application provides an improved expansive soil and construction process, which can solve the technical problems in the chemical improvement methods of expansive soil, where the addition of inorganic materials such as lime, cement or fly ash can cause certain environmental damage, and the chemical reaction has a time-limited effect.

[0005] In one aspect, embodiments of this application provide an improved expansive soil, comprising sand and gravel and expansive soil, wherein the volume fraction of the sand and gravel is 25%-50%.

[0006] The improved expansive soil based on the embodiments of this application adopts physical mixing, which will not produce chemical reactions and causes low environmental pollution. The addition of sand and gravel can improve the compaction of the fill material, reduce construction difficulty, and improve work efficiency.

[0007] In some embodiments, the gravel particle size is no greater than 15 cm.

[0008] Based on the above embodiments, the particle size of sand and gravel is limited to avoid sand and gravel with excessively large particle size from increasing the porosity and defects of the improved expansive soil, reducing the compactness of the expansive soil, and thus reducing its strength.

[0009] In some embodiments, the mud content of the gravel does not exceed 5%.

[0010] Based on the above embodiments, the mud content of sand and gravel is limited to avoid sand and gravel with excessive mud content increasing the moisture content of the improved expansive soil, thereby causing problems such as deformation, cracking, and wear of the formed improved expansive soil and reducing the strength of the fill.

[0011] Secondly, embodiments of this application provide a construction process for improved expansive soil, including the steps of: determining a filling area; filling the filling area in layers with improved expansive soil obtained by mixing sand, gravel and expansive soil, wherein the improved expansive soil is as described in any one of the above; and compacting the improved expansive soil located in different layers.

[0012] The construction process of the improved expansive soil based on the embodiments of this application can effectively improve the compaction of the fill material by layering and compacting the mixed improved expansive soil. At the same time, it can also reduce the foundation settlement and deformation caused by the fill material, and ensure the stability and safety of the fill material.

[0013] In some embodiments, the modified expansive soil obtained by mixing sand and gravel with expansive soil is filled in layers in the filling area, including the following steps: first, expansive soil is laid in each layer, followed by sand and gravel; the sand and gravel and expansive soil are mixed evenly until the modified expansive soil reaches the optimal moisture content.

[0014] Based on the above embodiments, by first laying expansive soil and then laying sand and gravel, and finally mixing them evenly, the expansive soil and sand and gravel can be fully mixed and contacted, which can improve the compaction of the expansive soil.

[0015] In some embodiments, the paving height of each layer is between 30cm and 50cm.

[0016] Based on the above embodiments, the loose paving height is within this range, which facilitates the mixing of expansive soil and gravel, and ensures compaction during rolling.

[0017] In some embodiments, the modified expansive soil of adjacent layers is constructed by overlapping, with an overlap length of 5m-10m.

[0018] Based on the above embodiments, the stability of the connection is ensured by lap splicing and limiting the lap length.

[0019] In some embodiments, after determining the filling area, before filling the filling area with the modified expansive soil obtained by mixing sand, gravel and expansive soil in layers, the step includes: excavating steps at the cut-fill interface.

[0020] Based on the above embodiments, by adding steps, the friction between the fill material and the original soil can be increased to prevent soil sliding, and on the other hand, it is beneficial to the compaction at the junction.

[0021] In some embodiments, the height-to-width ratio of the step is 1:2; the top surface of the step is inclined to the excavation side; the step height is 45cm-55cm; a layer of bidirectional geogrid is provided within a preset range on both sides of the top of the cut-fill interface line, and the tensile strength of the bidirectional geogrid is not less than 50kN / m.

[0022] Based on the above embodiments, the stability of the filling area during layered filling is ensured by setting up steps, while the setting of bidirectional geogrid can rapidly improve the bearing capacity of the foundation and control the development of settlement, thereby effectively preventing ground cracking or collapse.

[0023] In some of these embodiments, the compaction degree of the improved expansive soil after rolling is not less than 0.97.

[0024] Based on the above embodiments, the stability of the foundation is ensured by limiting the degree of compaction.

[0025] Based on the improved expansive soil and construction technology of this application, this application adopts physical mixing, which does not produce chemical reactions, resulting in low environmental pollution, and is ready to use immediately without waiting for the chemical reaction to complete. Since the state of expansive soil is greatly affected by the moisture content and is difficult to compact, the addition of sand and gravel can improve the compaction of the fill material, reduce the construction difficulty, and improve the efficiency. On the other hand, the addition of sand and gravel to expansive soil is equivalent to increasing the coarse particle content, forming a coarse-fine particle mixed soil. Compared with fine-grained soil, it has a higher compression modulus, which is beneficial for controlling the settlement of high embankment foundations. At the same time, the shear strength index of the coarse-fine particle mixed soil is improved compared with fine-grained soil, and the stability of the foundation is enhanced. Attached Figure Description

[0026] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0027] Figure 1 A flowchart for improving the construction process of expansive soil; Figure 2 A flowchart for improving the construction process of expansive soil. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0029] Because the chemical modification methods for expansive soil in related technologies require the addition of inorganic materials such as lime, cement, or fly ash, which can cause some environmental damage and also presents time-sensitive technical issues.

[0030] To address the aforementioned technical problems, this application proposes an improved expansive soil, comprising sand and gravel and expansive soil, wherein the volume fraction of sand and gravel is 25%-50%.

[0031] The gravel used in this application is natural gravel commonly found in the strata of the Chengdu area. Its soil mechanics classification is gravel; it is variegated in color, and its parent rocks are mostly granite, quartzite, sandstone, and limestone. Gravel gravel with granite parent rock is strongly weathered and easily broken by hand into gravelly sand. The remaining gravel is mainly moderately weathered, with some areas being strongly weathered. It is sub-rounded, generally with a particle size of 2cm to 10cm, and locally contains boulders. The maximum particle size is about 30cm, and the gravel content is about 60% to 80%, of which the strongly weathered gravel content is about 15% to 30%. It is mainly filled with sand, with some areas filled with sand and clay. When used as a soil conditioner, its maximum particle size must be controlled to be no more than 15cm, and its mud content no more than 5%.

[0032] Specifically, the particle size of sand and gravel can be 8cm, 9cm, 10cm and 15cm, etc. The particle size of sand and gravel can be screened through a sieve to limit the particle size of sand and gravel, so as to avoid sand and gravel with excessively large particle size from increasing the porosity and defects of the improved expansive soil, thereby reducing the compactness of the expansive soil and thus reducing its strength.

[0033] The mud content should not exceed 5%, but can be 0%, 1%, 2%, 3%, 4%, or 5%, etc. The mud content can be tested using a water washing method, for example, adding the sample to a certain amount of water, stirring thoroughly, and then filtering out the suspended mud particles through a sieve. Finally, the mud particles on the sieve are dried and weighed, and the mud content of the sand and pebbles is calculated. Alternatively, it can be tested using acid-base methods, sieving methods, etc.

[0034] By limiting the mud content of sand and gravel, the excessive mud content of sand and gravel is prevented from increasing the moisture content of the improved expansive soil, which could lead to problems such as deformation, cracking, and wear of the formed improved expansive soil, thus reducing the strength of the fill.

[0035] The proportion λ of natural sand and gravel in expansive soil can be determined based on the free swelling rate of the expansive soil. The proportion λ of natural sand and gravel is determined by the following formula based on the principle of volume replacement. It is between 25% and 50%, that is, the volume fraction of natural sand and gravel is between 25% and 50%, which can be 25%, 30%, 40%, 45% and 50%, etc. The specific amount of natural sand and gravel can be further verified by small-scale tests.

[0036]

[0037] The improved expansive soil based on the embodiments of this application has the following beneficial effects: 1. It can replace the traditional method of adding lime, making it more low-carbon and environmentally friendly. The traditional treatment method for expansive soil is to add lime for improvement. However, lime production is a high-carbon emission process, and increasing the amount of lime used will increase carbon emissions. Furthermore, adding lime can pollute the soil and damage its natural environment. Additionally, the mixing and construction of lime-soil mixtures can easily generate dust pollution. Using natural sand and gravel instead of lime can avoid these environmental problems.

[0038] 2. Improve the compaction properties of the fill material and reduce construction difficulty. The state of expansive soil is greatly affected by its moisture content, making it difficult to compact. Adding natural sand and gravel can improve the compaction of the fill material, reduce construction difficulty, and increase work efficiency.

[0039] 3. Increase the compression modulus and control deformation. When natural sand and gravel are mixed into expansive soil, since natural sand and gravel are coarse-grained soil and expansive soil is fine-grained soil, the improved mixed soil is equivalent to increasing the content of coarse-grained soil, forming a coarse-fine-grained mixed soil. Compared with fine-grained soil, the improved expansive soil has a higher compression modulus, which is beneficial for controlling the settlement of foundations in high embankment foundations.

[0040] 4. Improve shear strength and enhance foundation stability 5. It can be used immediately after preparation, without waiting for the chemical reaction to complete, which greatly shortens the construction period.

[0041] This application also provides a construction process for improved expansive soil, which is applied in excavation and filling projects, specifically filling projects, including the following steps: S100, determining the filling area; S200, filling the filling area in layers with improved expansive soil obtained by mixing sand, gravel and expansive soil, the improved expansive soil being as described above; S300, compacting the improved expansive soil located in different layers.

[0042] The filling area can be determined by the construction drawings, and a layered filling method can be adopted. That is, the modified expansive soil in the bottom layer is first compacted by a roller, and then the modified expansive soil in the upper layer is compacted from bottom to top. This layered filling method can effectively improve the compaction of the filling material, and at the same time reduce the foundation settlement and deformation caused by the filling material, ensuring the stability and safety of the filling material.

[0043] In the construction process, since improved expansive soil is used as the main filler, no chemical reaction occurs, which can avoid dust pollution and reduce environmental problems. The addition of natural sand and gravel can improve the compaction of the filler, reduce construction difficulty, and improve work efficiency. The addition of natural sand and gravel to expansive soil is equivalent to increasing the coarse particle content, forming a coarse-fine particle mixed soil. Compared with fine-grained soil, it has a higher compression modulus, which is beneficial for controlling the settlement of high-fill foundations. The addition of natural sand and gravel to expansive soil is equivalent to increasing the coarse particle content, forming a coarse-fine particle mixed soil. Compared with fine-grained soil, the shear strength index is improved, and the stability of the foundation is enhanced.

[0044] In step S200, the improved expansive soil obtained by mixing sand, gravel and expansive soil is filled in layers in the filling area, including the following steps: S201, each layer is first laid with expansive soil, then with sand and gravel; S202, the sand, gravel and expansive soil are mixed evenly until the mixed improved expansive soil reaches the optimal moisture content.

[0045] By first laying expansive soil and then laying sand and gravel on top of it, the sand and gravel can fall into the gaps in the expansive soil during laying, which plays a certain role in pre-mixing. Then, a road mixer or a five-furrow plow is used to mix it again in situ until the mixed improved expansive soil reaches the optimal moisture content.

[0046] Unlike traditional fill materials, this application does not require thorough mixing in a mixer before layering and compaction. In this construction process, after the expansive soil and gravel are laid in sequence, they can be mixed evenly in situ using a road mixer or a five-furrow plow. This not only achieves a good mixing effect but also avoids the settlement of some raw materials in the fill material caused by the traditional method of mixing before filling, which can lead to uneven filling and affect the overall compaction of the foundation to some extent.

[0047] The loose paving height of each layer is between 30cm and 50cm. In some embodiments, the loose paving height can be 30cm, 40cm, or 50cm. Each layer is within this loose paving height, which facilitates the mixing of expansive soil and gravel and ensures compaction during rolling.

[0048] Understandably, the loose layer height refers to the thickness of the expansive soil and gravel before compaction.

[0049] When laying expansive soil and gravel in each layer, the thickness must follow the formula above. That is, the volume fraction of gravel must be determined first. For example, when the volume fraction of gravel is 25% and the loose laying height is 40cm, the height of expansive soil is 30cm and the height of gravel is 10cm.

[0050] The optimum moisture content can be determined by a compaction test, such as the compaction test method in the "Specifications for Geotechnical Testing of Highways" JTG3430-2020, to test the moisture content of modified expansive soil. This test method is simple and more accurate.

[0051] When the improved expansive soil reaches its optimum moisture content, compaction at this point results in a more stable and durable compacted layer with a more reliable compaction.

[0052] In the actual filling process, the filling of each layer is not completed in one go, but is divided into several work sections. In order to ensure good compaction of the filling and avoid the splicing surfaces of each layer being in the same direction, adjacent layers are constructed by overlapping, that is, alternating overlapping, and the overlap length is 5m-10m. Preferably, the overlap length can be 5m, 6m, 7m, 8m and 10m, etc. By overlapping construction and limiting the overlap length, the stability of the connection is ensured.

[0053] It is important to understand that when each layer is divided into multiple work sections for filling, the height difference between adjacent work sections should generally not exceed one loose paving thickness during construction to avoid excessive height difference, which could easily lead to collapse during continued filling and compaction.

[0054] When filling in multiple work sections, each work section follows the same procedure: first lay expansive soil, then lay sand and gravel. Mix the sand, gravel, and expansive soil evenly in situ using a road mixer or a five-furrow plow, then compact it. Repeat this process for multiple work sections. When compacting adjacent work sections, the compaction boundary line should be extended by more than 2 meters to ensure the stability of the connection.

[0055] Specifically, in step S100, after determining the filling area, the improved expansive soil obtained by mixing sand, gravel and expansive soil is filled in layers before the filling area, including the step: S101, excavating steps at the cut-fill interface.

[0056] Adding steps can increase the friction between the fill material and the original soil, preventing soil slippage. On the other hand, it facilitates compaction at the junction, thereby enhancing the overall anti-sliding stability of the fill material and reducing post-construction settlement.

[0057] Based on the previous embodiment, the height-to-width ratio of the steps is 1:2, the height of the steps is 45cm-55cm, and when the height of the steps is 50cm, the width of the steps is 100cm. The top surface of the steps is inclined to the excavation side, that is, the steps have a certain inward slope, with a slope of 2% to 3%, in order to enhance the connection between the new and old roadbeds.

[0058] A layer of bidirectional geogrid is installed within a range of 5±0.5m on both sides of the top of the cut-fill interface line. The tensile strength of the bidirectional geogrid is not less than 50kN / m, specifically 50kN / m, 53kN / m, 56kN / m, and 60kN / m. The installation of bidirectional geogrid can rapidly improve the bearing capacity of the foundation, control the development of settlement, and thus effectively prevent ground cracking or collapse.

[0059] In this embodiment, layered filling is adopted, that is, after each layer is filled and compacted, it is necessary to ensure that it has a certain degree of compaction before the next layer is filled. Specifically, the compaction degree of the improved expansive soil after compaction is not less than 0.97. In some implementations, vibratory compaction can be carried out by heavy rollers. The compaction degree can be measured by compaction test.

[0060] Because the higher the compaction degree, the smaller the gaps between soil particles, the more compacted the soil particles are arranged and the smaller the pores are, thus having a higher bearing capacity. Therefore, it is understandable that the compaction degree can also be 0.98 or even 0.99.

[0061] Throughout the construction process, earthwork filling must be carried out in layers, and the compaction coefficient must be tested for each layer. The compaction degree must meet the design requirements of each zone. Temporary drainage measures must be implemented during the earthwork filling process.

[0062] The present invention will be further described below with reference to embodiments, but this does not limit the scope of the invention.

[0063] Project Overview: In a project at an airport in Southwest China, a large area of ​​the site will be filled with weakly expansive soil excavated on-site. The maximum filling height is 8m. The free expansion rate of the expansive soil is 40%~53%, which means it has weak expansibility. The expansive soil has a high water content, is difficult to dry naturally, and is difficult to compact. It is a poor fill material and needs to be improved before it can be used.

[0064] The plan has been finalized: Considering the characteristics of the project site, the traditional method of adding lime for soil improvement is prone to dust pollution during construction, which may affect the normal operation of flights in the nearby airport area. The design selects locally available natural sand and gravel to physically improve the expansive soil. Based on the principle of volume replacement, the amount of natural sand and gravel to be added is calculated to be 25% (volume fraction). The plot test verifies that this amount of addition can eliminate the expansibility of the foundation, improve compaction, and increase the compression modulus of the foundation.

[0065] Construction steps: Determine the filling area, excavate steps at the cut-fill interface, with a height-to-width ratio of 1:2, a step height of 50cm, and an inward slope of 2%. Set a layer of bidirectional geogrid 5m on each side of the top of the cut-fill interface line, with a tensile strength of 60kN / m. Layered filling and compaction are carried out, with each layer having a loose thickness of 40cm. Each layer is divided into several work sections for filling. For different work sections, 30cm of expansive soil is laid first, followed by 10cm of sand and gravel. The moisture content of the fill material is controlled within ±2% of the optimum moisture content. Then, a road mixing machine or a five-furrow plow is used for on-site mixing. After uniform mixing, a heavy road roller is used for vibratory compaction, with a compaction degree of 0.97. Adjacent layers are constructed using an alternating overlapping method, with an overlap length of 5m. When compacting adjacent work sections, the compaction boundary line should be extended by more than 2m on the side that was constructed later, in order to ensure the stability of the connection.

[0066] The above-mentioned construction technology for improved expansive soil can eliminate expansibility, reduce the optimum moisture content of the fill material, make it easier to compact, and improve the shear strength and compression modulus of the foundation, which is beneficial to the stability and deformation control of the foundation.

[0067] Adding natural sand and gravel to expansive soil is equivalent to increasing the coarse particle content, forming a coarse-fine particle mixed soil. Compared with fine-grained soil, the shear strength index is improved, and the stability of the foundation is enhanced.

[0068] In this embodiment of the application, corresponding tests were conducted on unmodified expansive soil and modified expansive soil, and the following table was obtained:

[0069] Note: The free expansion rate remained unchanged because the free expansion rate test used fine particles <0.5mm, which removed coarse particles. Physical modification does not change the properties of fine particles.

[0070] The test method for dry density is: JTG3430-2020; The optimal moisture content test method is: JTG3430-2020; The test method for free expansion rate is GB / T 50123-2019; The test method for the 50 kPa load expansion rate is GBT50123-2019; The test method for expansion force is GB / T 50123-2019; As can be seen from the table above, the improved expansive soil has better performance in all aspects than the unimproved expansive soil.

[0071] In the accompanying drawings of this embodiment, the same or similar reference numerals correspond to the same or similar components. In the description of this application, it should be understood that if terms such as "upper," "lower," "left," and "right" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, they are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, the terms used to describe positional relationships in the accompanying drawings are only for illustrative purposes and should not be construed as limiting this patent. For those skilled in the art, the specific meaning of the above terms can be understood according to the specific circumstances.

[0072] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A modified expansive soil, characterized in that, include: The mixture consists of sand, gravel, and expansive soil, wherein the volume fraction of the sand and gravel is 25%-50%.

2. The improved expansive soil according to claim 1, characterized in that, The size of the sand and gravel is no greater than 15cm.

3. The improved expansive soil according to claim 1, characterized in that, The mud content of the sand and gravel shall not exceed 5%.

4. A construction process for improving expansive soil, characterized in that, Including the following steps: Determine the filling area; The improved expansive soil obtained by mixing sand, gravel and expansive soil is filled in layers in the filling area, wherein the improved expansive soil is the improved expansive soil as described in any one of claims 1-3; The improved expansive soil located in different layers is compacted.

5. The construction process for improved expansive soil according to claim 4, characterized in that, The modified expansive soil, obtained by mixing sand, gravel, and expansive soil, is layered and filled into the filling area, including the following steps: Each layer is first laid with expansive soil, then with sand and gravel; Mix the sand, gravel, and expansive soil evenly until the improved expansive soil reaches its optimal moisture content.

6. The construction process for improved expansive soil according to claim 4, characterized in that, The unlaid height of each layer is between 30cm and 50cm.

7. The construction process for improved expansive soil according to claim 4, characterized in that, The improved expansive soil in adjacent layers is constructed by overlapping, with an overlap length of 5m-10m.

8. The construction process for improved expansive soil according to claim 4, characterized in that, After determining the filling area, the modified expansive soil, obtained by mixing sand, gravel, and expansive soil, is layered and filled into the filling area before the filling area, including the following steps: Excavate steps at the cut-fill interface.

9. The construction process for improved expansive soil according to claim 8, characterized in that, The height-to-width ratio of the steps is 1:2; And / or, The top of the step slopes towards the excavation side; And / or, The step height is 45-55cm; And / or, A layer of bidirectional geogrid is installed within a preset range on both sides of the top of the cut-fill interface line, and the tensile strength of the bidirectional geogrid is not less than 50kN / m.

10. The construction process for improved expansive soil according to claim 4, characterized in that, The compaction degree of the improved expansive soil after rolling is not less than 0.97.

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