Method for calculating strength growth of soft soil by surcharge preloading

Abstract

The application discloses a kind of calculation methods for soft soil strength growth of preloading precompression foundation treatment, belong to the technical field of foundation treatment, the method of the present application innovatively uses the change of moisture content before and after preloading precompression drainage to obtain the growth of soil strength, without considering the consolidation state of soil, solve the limitation problem that current industry standard is only applicable to normal consolidation saturated soft clay for the strength growth calculation method of foundation soil under preloading, and is not applicable to overconsolidated soil and underconsolidated soil strength growth calculation;Solve the defect that current specification method only considers the strength growth caused by soil drainage consolidation under the action of compressive stress, but cannot consider the strength growth caused by shrinkage, can more accurately evaluate the treatment effect of preloading precompression method;Avoid the defect that current specification method can only consider the underestimated strength growth generated by one-dimensional consolidation of foundation and the cumbersome work of consolidation degree calculation, and is worth popularization and application.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of foundation treatment, in particular to a calculation method for strength growth of soft soil in preloading and surcharge foundation treatment. BACKGROUND

[0002] For deep soft clay foundation, preloading and surcharge method is an effective foundation treatment technology, which is often used in embankment or road embankment filling engineering. For embankment or road embankment filling, the filling rate should be determined according to the strength of the foundation soil. When the strength of the natural foundation soil meets the stability requirements of the foundation under the embankment or road embankment load, it can be loaded at one time. If it does not meet the requirements, it should be loaded gradually in stages, and when the strength growth of the foundation soil under the previous load meets the stability requirements of the foundation under the next load, it can be loaded. This involves the problem of evaluating the strength growth of the foundation soil under each preloading load.

[0003] The current industry standard "Technical Code for Building Foundation Treatment (JGJ79-2012)" gives a calculation method for the strength growth of the foundation soil under preloading load. At present, the strength growth evaluation in preloading and surcharge engineering design and construction practice is also carried out by using this method. However, the most obvious disadvantage of this method is that it is only applicable to normally consolidated saturated soft clay, and is not applicable to the strength growth calculation of overconsolidated soil and underconsolidated soil, which limits its scope of application. In addition, this method only considers the strength growth caused by the drainage consolidation of soil under pressure stress, and cannot consider the strength growth caused by shrinkage, which to some extent underestimates the treatment effect of preloading and surcharge method, resulting in increased investment and prolonged construction period of foundation treatment.

[0004] In order to solve the above problems, the present application provides a calculation method for strength growth of soft soil in preloading and surcharge foundation treatment, which can provide a reference for evaluating the strength growth of the foundation soil under preloading load. SUMMARY

[0005] The present application aims to overcome the shortcomings of the prior art and provides a calculation method for strength growth of soft soil in preloading and surcharge foundation treatment. The strength growth of the soil is calculated by using the change of water content before and after preloading and surcharge drainage, without considering the consolidation state of the soil, which solves the problem that the existing calculation method for strength growth of the foundation soil under preloading load is only applicable to normally consolidated saturated soft clay, but not applicable to overconsolidated soil and underconsolidated soil. It solves the defect that the existing standard method only considers the strength growth caused by the drainage consolidation of soil under pressure stress, and cannot consider the strength growth caused by shrinkage, which can more accurately evaluate the treatment effect of preloading and surcharge method. It avoids the defect that the existing standard method can only consider the underestimated strength growth caused by one-dimensional consolidation of the foundation, and the tedious work of calculating the degree of consolidation.

[0006] To solve the above technical problems, the present application realizes the following technical scheme:

[0007] A method for calculating the strength gain of soft soil in surcharge preloading foundation treatment includes:

[0008] Step 1: Let the water content of the soft soil before and after reinforcement be w0 and w1, respectively. Then w1 ≈ w L w L If the liquid limit of the soil is given, then the change in water content Δw of the soft soil before and after reinforcement is:

[0009] Δw=w0-w L (1)

[0010] Step 2: According to the generalized Hooke's law, the rate of change of soil volume and the effective normal stress are:

[0011]

[0012] In the formula: θ' is the rate of change of soil volume; λ and μ are Lamé elastic constants; σ' is the effective normal stress acting on the failure surface of the soil, σ'=σ-u, where σ is the total normal stress and u is the pore water pressure;

[0013] Step 3: In the above formula (2), λ and μ can be expressed using the soil elastic constants—Young's modulus E and Poisson's ratio ν:

[0014]

[0015]

[0016] Step 4: Substitute formulas (3) and (4) into formula (2), and we have:

[0017]

[0018] Step 5: According to the Mohr-Coulomb criterion, the effective strength τ of the soil is:

[0019]

[0020] In the formula: c', These are the effective cohesion and effective internal friction angle of the soil, respectively.

[0021] Step 6: Let the change in pore water pressure in the soil before and after reinforcement be Δu. Then the increase in soil strength Δτ is:

[0022]

[0023] Step 7: For saturated soft clay, since the pores within the soil are filled with water and contain no air, it is a two-phase system composed of soil particles and pore water. According to the definition of soil water content, we have:

[0024]

[0025] where m w , p w , V w are the mass, density and volume of the pore water, respectively; m s , p s , V s are the mass, density and volume of the soil particles, respectively;

[0026] Step 8, according to equation (8), the change of water content of the soil can be expressed as:

[0027]

[0028] Step 9, according to step 7, the soil volume V = V w + V s , let AV be the change of soil volume, AV v be the change of pore volume, AV w be the change of pore water volume, since the pores in the saturated soft clay are completely filled with water, the pore volume V v is equal to the volume of pore water V w ; assuming that the soil particles and the pore water are both incompressible, the change of soil volume before and after the soil is reinforced is equal to the change of soil pore volume or the change of pore water volume, i.e. AV = AV v = AV w , and the change rate of soil volume AV is:

[0029]

[0030] where V0 is the total volume of the soil before the ground treatment;

[0031] Step 10, according to step 7, for the saturated soft clay, the pores are completely filled with water, and the soil pore ratio e is:

[0032]

[0033] Step 11, let the initial pore water volume and the initial pore ratio of the soil before the ground treatment be V and e0, respectively, according to step 10, we have:

[0034]

[0035] Step 12, according to steps 8, 9 and 11, we have:

[0036]

[0037] Step 13, according to steps 4 and 12, we have:

[0038]

[0039] Step 14, according to step 6 and step 13, the strength growth amount △τ of the foundation soil body after preloading is:

[0040]

[0041] Preferably, according to step 1 and step 14, the maximum strength growth amount △τ of the foundation soil body after preloading is max :

[0042]

[0043] Preferably, considering the disturbance factor of soft soil caused by foundation treatment construction, according to engineering experience, formula (16) is moderately reduced, that is:

[0044]

[0045] In the formula: η is a field experience coefficient.

[0046] Compared with the prior art, the present application has the following advantages and beneficial effects:

[0047] The present application provides an evaluation method for the strength growth of soft soil treated by preloading, by using the unique relationship between the shear strength of soil, effective stress and water content, and innovatively uses the change of water content before and after preloading drainage to obtain the strength growth of soil, without considering the consolidation state of soil, solves the limitation problem that the calculation method of strength growth of foundation soil under preloading load in the current industry standard 'Technical Code for Building Foundation Treatment (JGJ79-2012)' is only applicable to normally consolidated saturated soft clay, and is not applicable to the strength growth calculation of overconsolidated soil and underconsolidated soil; solves the defect that the current standard method only considers the strength growth caused by soil drainage consolidation under the action of pressure stress, and cannot consider the strength growth caused by shear shrinkage, and can more accurately evaluate the treatment effect of preloading method; avoids the defect that the current standard method can only consider the underestimated strength growth caused by one-dimensional consolidation of foundation, and the tedious work of consolidation degree calculation, and is worth popularization and application. BRIEF DESCRIPTION OF DRAWINGS

[0048] Figure 1 It is a schematic diagram of two-phase composition of saturated soil.

[0049] In the figure: m w , V w are the mass and volume of pore water respectively; m s , V s are the mass and volume of soil particles respectively. DETAILED DESCRIPTION

[0050] In order to make the technical solution of the present application better understood by the skilled in the art, the preferred embodiments of the present application are described below in conjunction with specific examples, but it should be understood that the drawings are only used for illustrative description and cannot be understood as limiting the present patent; in order to better illustrate the present embodiment, some components in the drawings are omitted, enlarged or reduced, and do not represent the actual product size; it is understandable for those skilled in the art that some known calculation methods for the strength growth of soft soil in a preloading embankment foundation treatment and their descriptions in the drawings can be omitted. The positional relationship described in the drawings is only used for illustrative description and cannot be understood as limiting the present patent.

[0051] The preloading embankment method refers to that after a load is applied on a saturated soft foundation, the pore water in the foundation soil is slowly discharged, the pore volume is reduced, the foundation is deformed, at the same time, with the gradual dissipation of the excess pore water pressure, the effective stress of the soil is gradually increased, the strength is gradually increased, and the purpose of effectively reducing the post-construction settlement and improving the bearing capacity of the foundation is achieved. In order to achieve better treatment effect, a vertical drainage body such as a plastic drainage belt or a sand well is generally arranged in the foundation, which plays a role of vertical drainage channel and can accelerate the foundation treatment process.

[0052] The present application provides a calculation method for the strength growth of soft soil in a preloading embankment foundation treatment, comprising the following steps:

[0053] Step 1, practice shows that the water content of soft clay after preloading embankment is generally close to the liquid limit; let the water content of soft soil before and after reinforcement be w0 and w1 respectively, and w1 ≈ w L , w L is the liquid limit of the soil, then the water content change △w of the soft soil before and after reinforcement is:

[0054] Δw=w0-w L (1)

[0055] Step 2, according to the generalized Hooke's law, the volume change rate of the soil and the effective normal stress have:

[0056]

[0057] In the formula, θ' is the volume change rate of the soil; λ and μ are Lame elastic constants; σ' is the effective normal stress acting on the failure surface of the soil, σ' = σ - u, σ is the total normal stress, and u is the pore water pressure;

[0058] Step 3, λ and μ in the above formula (2) can be represented by the elastic constants of the soil, i.e. Young's modulus E and Poisson's ratio v:

[0059]

[0060]

[0061] Step 4, formula (3) and formula (4) are substituted into formula (2), and we have:

[0062]

[0063] Step 5, according to Mohr-Coulomb criterion, the effective strength τ of soil is:

[0064]

[0065] In the formula: c'、 are the effective cohesion and effective internal friction angle of soil, respectively;

[0066] Step 6, let the change of pore water pressure of soil before and after reinforcement be Δu, then the strength increment Δτ of soil is:

[0067]

[0068] Step 7, for saturated soft clay, since the pores in the soil are filled with water and do not contain air, it is a two-phase body composed of soil particles and pore water. According to the definition of soil moisture content, we have:

[0069]

[0070] In the formula: m w , ρ w , V w are the mass, density and volume of pore water, respectively; m s , ρ s , V s are the mass, density and volume of soil particles, respectively;

[0071] Step 8, according to formula (8), the change of soil moisture content can be expressed as:

[0072]

[0073] Step 9, according to step 7, the volume of soil V = V w + V s , let ΔV be the change of soil volume, ΔV v be the change of pore volume, ΔV w be the change of pore water volume. Since the pores in saturated soft clay are completely filled with water, the pore volume V v is equal to the volume of pore water V w . Assuming that both soil particles and pore water are incompressible, the change of soil volume before and after reinforcement is equal to the change of pore volume or pore water volume, i.e. ΔV = ΔV v = ΔV w , and the change rate of soil volume Δθ' is:

[0074]

[0075] In the formula, V0 is the total volume of the soil before ground treatment;

[0076] Step 10, according to step 7, for saturated soft clay, the pores are full of water, and the soil void ratio e is:

[0077]

[0078] Step 11, let the initial pore water volume of the soil before ground treatment (since the soil before ground treatment is saturated, the initial pore water volume is equal in value to the initial pore volume) and the initial void ratio be V0and e0respectively, according to step 10, we have:

[0079]

[0080] Step 12, according to steps 8, 9 and 11, we have:

[0081]

[0082] Step 13, according to steps 4 and 12, we have:

[0083]

[0084] Formula (14) shows that the increase in soil strength is independent of the consolidation state of the soil, solving the defect that the standard method is only applicable to normally consolidated saturated soft clay, and is no longer applicable to the calculation of strength increase for overconsolidated and underconsolidated soils.

[0085] Step 14, according to steps 6 and 13, the increase in strength of the ground soil after preloading is:

[0086]

[0087] According to steps 1 and 14, the maximum increase in strength of the ground soil after preloading is: max

[0088]

[0089] Considering the disturbance factor of ground treatment construction on soft soil, according to engineering experience, formula (16) is moderately reduced, that is:

[0090]

[0091] In the formula, η is an on-site experience coefficient, which can be taken as 0.8-0.9.

[0092] ​​Formula (17) shows that the strength growth of the soil body is linearly related to the change of the water content of the soil body, and is irrelevant to the test method (drained or undrained) and the load path, thereby solving the defect that the standard method only considers the strength growth caused by the drained consolidation of the soil body under the pressure stress, and cannot consider the strength growth caused by the shrinkage.

[0093] The application will be further described in detail below with reference to the accompanying drawings Figure 1 and specific embodiments, so as to make the application clear, but they do not constitute limitations on the application.

[0094] A certain silt foundation in the Pearl River Delta is treated by preloading + plastic drainage board, and the preloading load is 100 kPa. The natural water content of the silt is 57.5%, the natural density is 1.62×10 3 kg / m 3 , the initial void ratio is 1.579, the liquid limit is 47.2%, the Young's modulus is 1 MPa, the Poisson's ratio is 0.3, and the effective internal friction angle of the soil body is 14°. According to formula (17), the maximum strength growth value of the site is 17.2 kPa.

[0095]

[0096] Considering the different additional stresses and different degrees of consolidation at different depths of the foundation, the average strength growth of the foundation is taken as 17.2 kPa.

[0097] According to the current industry standard “Technical Code for Building Foundation Treatment (JGJ79-2012)”, when the consolidation degree of the foundation is assumed to be 100%, the maximum strength growth in the foundation is approximately Considering the different additional stresses at different depths of the foundation, the average strength growth of the foundation is taken as 12.5 kPa.

[0098] From the above preliminary calculation, it can be seen that the average strength growth value of the foundation obtained by the soft soil strength growth evaluation method of the preloading foundation treatment proposed in the application is 37.6% larger than the average strength growth value of the foundation obtained by the current standard, which further illustrates the defect that the current standard method only considers the strength growth caused by the drained consolidation of the soil body under the pressure stress, and cannot consider the strength growth caused by the shrinkage, and also illustrates the defect that the current standard method can only consider the underestimated strength growth caused by the one-dimensional consolidation of the foundation. In addition, for the case of graded loading, the application does not need to perform tedious calculation on the consolidation degree under each level of load, but only needs to determine the water content of the foundation soil through a simple test, and then the strength growth value under each level of load can be easily obtained by using the calculation method.

[0099] The above description is only the preferred embodiments of the present application, but the present application is not limited to the above specific embodiments. Those skilled in the art can make several modifications, supplements or use similar methods instead without departing from the principles of the present application, which should also be considered as the protection scope of the present application.

Claims

1. A method for calculating the strength increase of soft soil in surcharge preloading foundation treatment, characterized in that, include: Step 1: Let the water content of the soft soil before and after reinforcement be w0 and w1, respectively. Then w1 ≈ w L w L If the liquid limit of the soil is given, then the change in water content Δw of the soft soil before and after reinforcement is: Δw=w0-w L (1) Step 2: According to the generalized Hooke's law, the rate of change of soil volume and the effective normal stress are: In the formula: θ' is the rate of change of soil volume; λ and μ are Lamé elastic constants; σ' is the effective normal stress acting on the failure surface of the soil, σ'=σ-u, where σ is the total normal stress and u is the pore water pressure; Step 3: In the above formula (2), λ and μ are expressed using the soil elastic constants—Young's modulus E and Poisson's ratio ν: Step 4: Substitute formulas (3) and (4) into formula (2), and we have: Step 5: According to the Mohr-Coulomb criterion, the effective strength τ of the soil is: In the formula: c', These are the effective cohesion and effective internal friction angle of the soil, respectively. Step 6: Let the change in pore water pressure in the soil before and after reinforcement be Δu. Then the increase in soil strength Δτ is: Step 7: For saturated soft clay, since the pores within the soil are filled with water and contain no air, it is a two-phase system composed of soil particles and pore water. According to the definition of soil water content, we have: Where: m w ρ w V w These represent the mass, density, and volume of pore water, respectively; m s ρ s V s These represent the mass, density, and volume of the soil particles, respectively. Step 8: According to formula (8), the change in soil moisture content can be expressed as follows: Step 9: According to Step 7, the soil volume V = V w +V s Let ΔV be the change in soil volume. v The change in soil pore volume, ΔV w V represents the change in pore water volume. Since the pores in saturated soft clay are completely filled with water, the pore volume V is... v Equal to the volume V of pore water w Assuming that both soil particles and pore water are incompressible, the change in soil volume before and after reinforcement is equal to the change in soil pore volume or pore water volume, i.e., ΔV = ΔV v =ΔV w The rate of change of soil volume Δθ' is: Where: V0 is the total volume of soil before foundation treatment; Step 10: According to Step 7, for saturated soft clay, where the pores are completely filled with water, the soil void ratio e is: Step 11: Let the initial pore water volume and initial void ratio of the soil before foundation treatment be V, respectively. w0 And e0, according to step 10, we have: Step 12: Based on steps 8, 9, and 11, we have: Step 13: Based on steps 4 and 12, we have: Step 14: Based on Steps 6 and 13, the increase in soil strength Δτ after surcharge preloading is:

2. The method for calculating the strength increase of soft soil in surcharge preloading foundation treatment according to claim 1, characterized in that: Based on steps 1 and 14, the maximum increase in the strength of the foundation soil after surcharge preloading. △ τ max for:

3. The method for calculating the strength increase of soft soil in surcharge preloading foundation treatment according to claim 2, characterized in that: Considering the disturbance to soft soil caused by foundation treatment construction, and based on engineering experience, formula (16) is appropriately reduced, i.e.: In the formula: η is the field experience coefficient.

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