Method for determining reinforcement parameters of flexible supporting structure of expansive soil cutting slope

Abstract

This invention provides a method for determining reinforcement parameters of flexible support structures for expansive soil slopes. The method first establishes a compatibility equation between soil expansion and geogrid deformation. Then, a CNC universal testing machine is used to test the tensile modulus and tensile strength of the geogrid. Next, a load-bearing expansion rate test is conducted using a consolidation apparatus to establish a calculation model for the expansion deformation of the expansive soil. Furthermore, based on the fact that the tensile stress experienced by the geogrid during normal operation is less than or equal to its tensile strength, a calculation model for the reinforcement thickness is obtained. Finally, a stress model for the free zone and anchorage zone of the reinforcement is established, thereby obtaining a calculation model for the reinforcement length. This invention calculates various forces on reinforced expansive soil based on changes in soil moisture content after rainfall and proposes calculation models and methods for reinforcement thickness and length, overcoming the shortcomings of previous flexible support structure designs that relied on experience.

Description

Technical Field

[0001] This invention belongs to the field of geotechnical engineering technology, and particularly relates to a method for determining reinforcement parameters of flexible support structures for expansive soil trench slopes. Background Technology

[0002] Expansive soil is a special type of soil mainly composed of the highly hydrophilic clay mineral montmorillonite and its mixed-layer clay minerals. Its typical engineering characteristics include overconsolidation, fissility, swelling and softening upon water absorption, cracking upon drying and shrinkage after water loss, and repeated deformation. Expansive soil is widely distributed; to date, it has been found in more than forty countries and regions worldwide.

[0003] For a long time, the treatment of landslides on expansive soil cut slopes has been a challenging problem and an important research topic in geotechnical engineering. To address this, researchers have proposed a variety of treatment techniques. Summarizing domestic and international treatment techniques for expansive soil cut slopes, existing treatment schemes can be divided into two categories: rigid support schemes and flexible support schemes. Rigid support refers to a treatment scheme that primarily uses masonry structures, supplemented by other necessary comprehensive treatment measures. However, rigid support cannot allow excessive expansion and deformation of the soil; when the soil expansion and deformation are significant, the support will fail. Flexible support refers to a treatment scheme that primarily uses chemical amendments and geotextiles, supplemented by other necessary comprehensive treatment measures. Typical flexible support schemes include... Figure 2 As shown. The advantages of flexible support are that it can coordinate the expansion or contraction of the soil and prevent rainwater from directly eroding the slope. At the same time, it can effectively drain the fissure water in the slope and maintain the stability of the slope's water content, thereby reducing the expansion potential energy of the expansive soil on the slope and reducing the possibility of support measures failing or structural damage caused by expansion deformation. This makes it a major measure for the treatment of expansive soil road cut slopes.

[0004] Chinese invention patent application number CN200510031717.3 discloses a flexible reinforced support structure and its construction method, specifically disclosing the following technical features: a permeable foundation layer is formed at the bottom of the support structure; a permeable layer exists between the support structure and the excavation face, and this permeable layer is integral with the permeable foundation layer; a seepage trench is excavated beneath the permeable foundation layer; geogrid reinforcement is laid throughout the entire support structure, with each layer of geogrid reinforcement exceeding the width of the support structure, wrapping around the previous layer, and connecting with the reinforcement of the previous layer; each layer of reinforcement is backfilled and compacted with excavated expansive soil. This structure has been widely applied in provinces with abundant expansive soil, such as Guangxi, Henan, Hainan, Hubei, and Yunnan, effectively preventing landslides on expansive soil road cut slopes. Although this structure is widely used, its design has mainly relied on empirical methods due to insufficient understanding of the stress characteristics of flexible support structures in the past. That is, the length of the geogrid reinforcement is determined by dividing the local atmospheric influence depth by the slope and then multiplying by a safety factor. The commonly used value is 3.5 to 4.5 m. The reinforcement thickness is determined based on experience and is generally taken as 0.5 m. It can be seen that the empirical method does not take into account the stress of the geogrid or the expansion characteristics of expansive soil. That is, regardless of whether the backfill is strongly expansive soil, moderately expansive soil, or weakly expansive soil, the length and thickness of the reinforcement may be taken as the same value.

[0005] For highly expansive fill materials, current reinforcement length and thickness values ​​may be too risky, potentially leading to structural damage. Conversely, for weakly expansive soils, these values ​​may be too conservative, resulting in material waste. Furthermore, due to the friction between the geogrid interface and the soil, and the effect of the stirrups limiting soil expansion and contraction, soil cracking and rainwater infiltration depth are significantly reduced. The significant impact zone of the wet-dry cycle for reinforced soil will be much smaller than the atmospheric impact depth for unreinforced soil. Following traditional design methods would result in substantial waste. Summary of the Invention

[0006] The purpose of this invention is to provide a method for determining the reinforcement parameters of flexible support structures for expansive soil trenches, so as to solve the technical problem that improper values ​​of reinforcement length and thickness can have adverse effects on engineering projects.

[0007] To solve the above-mentioned technical problems, the specific technical solution of the present invention is as follows:

[0008] A method for determining reinforcement parameters of a flexible support structure for expansive soil trench slopes, comprising the following steps:

[0009] Step 1: Establish the compatibility equation between soil expansion and grid deformation:

[0010]

[0011] In the formula: E Tα is the tensile modulus of the grid to resist expansion deformation, P is the expansion pressure generated by the soil after water absorption, h is the reinforcement thickness, w0 is the initial moisture content, w is the moisture content during the water absorption process, and α(P,w,w0) is the lateral expansion amount.

[0012] Step 2: Establish a calculation model for the expansion deformation of expansive soil:

[0013]

[0014] In the formula: α(P,w,w0) is the lateral expansion amount, A, B, C, and D are all fitting parameters, w0 is the initial moisture content, w is the moisture content during the water absorption process, and P is the expansion pressure generated by the soil after water absorption, which is numerically equal to the overlying stress.

[0015] Step 3: Establish the stress compatibility equation for the geogrid:

[0016]

[0017] In the formula: P is the expansion pressure generated by the soil after absorbing water, h is the reinforcement thickness; T s The tensile strength of the geogrid;

[0018] Step 4: Substitute formula (2) from Step 2 into formula (1) from Step 1, and combine it with formula (3) from Step 3 to obtain the equation containing the stiffening thickness h:

[0019]

[0020] In the formula: P is the expansion pressure generated by the soil after absorbing water, h is the reinforcement thickness, A, B, C, and D are all fitting parameters, and T s Let w0 be the initial moisture content, w be the moisture content during the water absorption process, and E be the tensile strength of the geogrid. T The tensile modulus of the geogrid;

[0021] Furthermore, following step four, the following is also included:

[0022] Step 5: Determine the free zone and anchorage zone of the slope based on the maximum rainfall infiltration depth, and obtain the length l of the free zone. 自 Based on the cohesion c at the soil-reinforcement interface and the internal friction angle φ, the length l of the anchorage zone is constructed. 锚 Computational model:

[0023]

[0024] In the formula: P is the expansion pressure generated by the soil after absorbing water, which is numerically equal to the overburden stress; h is the reinforcement thickness; A, B, C, and D are all fitting parameters; T s For tensile strength, w0 is the initial moisture content, w is the moisture content during the water absorption process, and E is the tensile strength. Tσ is the tensile modulus, c is the vertical pressure at the reinforcement-soil interface, φ is the cohesion at the reinforcement-soil interface, and φ is the internal friction angle at the reinforcement-soil interface.

[0025] Step 6: Measure the length of the free region l 自 and anchorage length l 锚 Construct a calculation model for the stiffening length l:

[0026] l = l 自 +l 锚 (6).

[0027] Furthermore, the tensile modulus E of the grid resists expansion deformation T and geogrid tensile strength T s The tensile properties of geogrid were obtained by using a CNC universal testing machine and the single-rib test method.

[0028] Furthermore, in step two, a wet heavy compaction test is required on the soil samples taken from the site to obtain the maximum dry density. Based on the requirement of controlling the compaction degree to be greater than 93%, the dry density is determined, specimens with different initial moisture contents are made, and a load expansion rate test is conducted using a consolidation apparatus to obtain a calculation model for the expansion deformation of expansive soil.

[0029] Furthermore, in step three, assuming the geogrid in the flexible support structure is in normal working condition, the tensile stress Ph on the geogrid should be less than or equal to the tensile strength T of the geogrid. s From this, we can obtain the stress compatibility equation for the geogrid.

[0030] The method for determining the reinforcement parameters of flexible support structures for expansive soil cut slopes proposed in this invention has the following advantages: It overcomes the drawbacks of previous flexible support structure designs that relied on experience. Based on the change in soil moisture content after rainfall, it calculates the various forces acting on the reinforced expansive soil, proposes calculation models and methods for reinforcement thickness and length, and obtains the reinforcement length and thickness of the flexible support structure based on relevant experiments and calculation formulas. This method for determining the reinforcement parameters of flexible support structures for expansive soil cut slopes can be extended to the calculation of flexible support structures for expansive red clay road cut slopes. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the stress analysis of the reinforced soil unit after expansion and stabilization according to the present invention;

[0032] Figure 2 A schematic diagram of a geogrid-reinforced expansive soil slope structure.

[0033] The markings in the diagram are as follows: 1. Backfill expansive soil; 2. Geogrid reverse wrapping part; 3. Reinforcement spacing; 4. Free zone; 5. Anchorage zone; 6. Geogrid. Detailed Implementation

[0034] To better understand the purpose, structure, and function of this invention, the invention will be described in further detail below with reference to the accompanying drawings.

[0035] The method for determining the reinforcement parameters of the flexible support structure for expansive soil trench slopes of the present invention includes the following steps:

[0036] Step 1: Based on the principle that the swelling deformation of expansive soil due to water absorption should be equal to the tensile deformation of the geogrid, establish a compatibility equation between soil swelling and geogrid deformation:

[0037]

[0038] In the formula: E T α is the tensile modulus of the grid to resist expansion deformation, P is the expansion pressure generated by the soil after water absorption, h is the reinforcement thickness, w0 is the initial moisture content, w is the moisture content during the water absorption process, and α(P,w,w0) is the lateral expansion amount.

[0039] Step 2: According to the "Specifications for Geotechnical Testing of Highways" (JTG3430-2020), wet heavy compaction tests were conducted on soil samples taken from the site to obtain the maximum dry density of the fill in the flexible support structure. Based on the requirement of controlling the compaction degree to be greater than 93%, the dry density was determined, and specimens with different initial moisture contents were prepared. Loaded swelling rate tests were conducted using a consolidation apparatus to establish a calculation model for the swelling deformation of expansive soil.

[0040]

[0041] In the formula: α(P,w,w0) is the lateral expansion amount, A, B, C, and D are all fitting parameters, w0 is the initial moisture content, w is the moisture content during the water absorption process, and P is the expansion pressure generated by the soil after water absorption, which is numerically equal to the overlying stress.

[0042] Step 3: Considering that the geogrid in the flexible support structure is in normal working condition, the tensile stress Ph on the geogrid should be less than or equal to the tensile strength T of the geogrid. s The stress compatibility equation of the geogrid can be obtained as follows:

[0043]

[0044] In the formula: P is the expansion pressure generated by the soil after absorbing water, h is the reinforcement thickness; T s The tensile strength of the geogrid;

[0045] Step 4: Substitute formula (2) from Step 2 into formula (1) from Step 1, and combine it with formula (3) from Step 3 to obtain the equation containing the stiffening thickness h:

[0046]

[0047] In the formula: P is the expansion pressure generated by the soil after absorbing water, h is the reinforcement thickness, A, B, C, and D are all fitting parameters, and T s Let w0 be the initial moisture content, w be the moisture content during the water absorption process, and E be the tensile strength of the geogrid. T The tensile modulus of the geogrid; the tensile modulus E of the geogrid against expansion deformation. T and geogrid tensile strength T s The tensile properties of geogrids were obtained by conducting tests using a CNC universal testing machine and the single-rib test method in accordance with the "Test Procedure for Geosynthetics" (SL / T235-1999).

[0048] Step 5, as follows Figure 1 As shown, the free zone 4 and anchorage zone 5 of the slope are determined based on the maximum infiltration depth of rainfall, and the length l of the free zone is obtained. 自 Since the pull-out force generated by the expansion of the expansive soil in the free zone is borne by the friction between the geogrid and the soil in the anchorage zone, that is, by the friction of the four faces of the two geogrids on the upper and lower parts of the soil unit, according to the "Test Procedure for Geosynthetics" (SL237-1999), the strength parameters of the geogrid-reinforced soil-reinforcement interface are obtained: the cohesion c of the reinforcement-soil interface, the internal friction angle φ, and the anchorage zone length l is constructed. 锚 Computational model:

[0049]

[0050] In the formula: P is the expansion pressure generated by the soil after absorbing water, which is numerically equal to the overburden stress; h is the reinforcement thickness; A, B, C, and D are all fitting parameters; T s Let E be the tensile strength, w0 be the initial moisture content, and w be the moisture content during the water absorption process. T σ is the tensile modulus, c is the vertical pressure at the reinforcement-soil interface, φ is the cohesion at the reinforcement-soil interface, and φ is the internal friction angle at the reinforcement-soil interface.

[0051] Step 6: Measure the length of the free region l 自 and anchorage length l 锚 Construct a calculation model for the stiffening length l:

[0052] l = l 自 +l 锚 (6).

[0053] Specifically, taking the grayish-white expansive soil of the Nanning-Youyiguan Expressway as an example, the relationship between the loaded swelling rate and the overlying pressure, initial moisture content, and process moisture content was obtained (related data reference: Wei Bingxu, Zhou Yufeng. Experimental study on laterally confined loaded swelling deformation of expansive soil in Ningming[J]. Mechanics and Practice, 2006(06):64-68).

[0054]

[0055] The geogrid used was model TGDG35, and its tensile strength T was obtained according to the "Test Procedure for Geosynthetics". s 35kN, elongation ≤10%, E T It is 300 kN / m.

[0056] From formulas (3) and (4), we can obtain:

[0057]

[0058] Grayish-white expansive soil was used as the fill material. The initial moisture content w0 of the specimens was 15%, and the dry density was 1.66 × 10⁻⁶. 3 kg / m 3 The slope ratio of the flexible support is 1:1.5. Based on large-scale indoor model tests and numerical simulations, the rainwater infiltration depth of the flexible support structure after extreme rainfall is 70 cm, and the average moisture content w in the affected area is 20%. Therefore, h ≤ 0.70 m.

[0059] Since the slope of the flexible support is 1:1.5, and the infiltration depth of rainwater is 0.7m, the influence range of the water content in the horizontal direction is 1.05m, i.e., l 自 =1.05m.

[0060] The expansive soil in the anchorage zone is unaffected by moisture content, which is 15% of the initial moisture content. According to the "Test Procedures for Geosynthetics" (SL237-1999), the soil-grid interface parameters in the anchorage zone are tested, and the strength parameter c at the soil-grid interface is 7.4 kPa. At 4.50°, Figure 1 The triangular load is converted to a rectangular load according to the slope ratio, that is, the σ of the reinforced soil element is 0.668 × 10. 4 N

[0061] Substituting the above values ​​into formula (5), we can obtain l 锚 ≤2.29. Therefore, the reinforcement length l≤3.34 calculated according to formula (6) is 3.34m.

[0062] Therefore, this invention first proposes a compatibility equation for soil expansion and geogrid deformation based on the stress characteristics of reinforced expansive soil flexible support structures and relevant theories of elasticity. The tensile modulus and tensile strength of the geogrid are tested using the single-rib test method as specified in the "Test Procedure for Geosynthetics". Assuming that the remolded (compacted) expansive soil is an isotropic material, its water absorption and expansion deformation calculation model can be obtained using a consolidation apparatus and a loaded expansion rate test. Considering that the tensile force on the geogrid under normal working conditions should be less than or equal to its tensile strength, a calculation model for the reinforcement thickness is established. A stress model for the expansion and stabilization of the reinforced soil unit is then established, leading to a calculation model for the reinforcement length. The soil-reinforcement interface parameters are obtained through geogrid pull-out tests.

[0063] It is understood that the present invention has been described through some embodiments, and those skilled in the art will recognize that various changes or equivalent substitutions can be made to these features and embodiments without departing from the spirit and scope of the invention. Furthermore, under the teachings of the present invention, these features and embodiments can be modified to adapt to specific situations and materials without departing from the spirit and scope of the invention. Therefore, the present invention is not limited to the specific embodiments disclosed herein, and all embodiments falling within the scope of the claims of this application are within the protection scope of the present invention.

Claims

1. A method for determining the reinforcement parameters of a flexible support structure for an expansive soil cutting slope, characterized in that, Includes the following steps: Step 1: Establish the compatibility equation between soil expansion and grid deformation: or (1) In the formula: is the tensile modulus of the lattice against swelling deformation, is the swelling pressure generated after the soil absorbs water, is the reinforcement thickness, is the initial water content, is the water content during the water absorption process, is the lateral swelling amount; Step 2: Establish a calculation model for the expansion deformation of expansive soil: （2） In the formula: , , , All are fitted parameters. Numerically equal to the overlying stress; Step 3: Establish the stress compatibility equation for the geogrid: ,Right now (3) In the formula: The tensile strength of the geogrid; Step 4: Substitute formula (2) from Step 2 into formula (1) from Step 1, and combine it with formula (3) from Step 3 to obtain the thickness containing reinforcement. The equation: （4）； Step 5: Determine the free zone and anchorage zone of the slope based on the maximum rainfall infiltration depth, and obtain the length of the free zone. Based on the cohesion at the reinforcement-soil interface Internal friction angle of reinforced soil interface Construct the anchorage zone length Computational model: （5） In the formula: The vertical pressure at the reinforced soil interface; Step Six: Pass and , construction reinforcement length Computational model: （6）。 2. The method for determining the reinforcement parameters of the flexible support structure for expansive soil trench slopes according to claim 1, characterized in that, The tensile modulus of the grid against expansion deformation and geogrid tensile strength The tensile properties of geogrid were obtained by using a CNC universal testing machine and the single-rib test method.

3. The method for determining the reinforcement parameters of the flexible support structure for expansive soil trench slopes according to claim 1, characterized in that, In step two, a wet heavy compaction test is required on the soil samples taken from the site to obtain the maximum dry density. Based on the requirement of controlling the compaction degree to be greater than 93%, the dry density is determined, specimens with different initial moisture contents are made, and a load expansion rate test is conducted using a consolidation apparatus to obtain a calculation model for the expansion deformation of expansive soil.

4. The method for determining the reinforcement parameters of the flexible support structure for expansive soil trench slopes according to claim 1, characterized in that, In step three, assuming the geogrid in the flexible support structure is in normal working condition, the tensile stress on the geogrid should be... Less than or equal to the tensile strength of the geogrid From this, we can obtain the stress compatibility equation for the geogrid.

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