Design method for rigid pile composite foundation structure in deep and thick melting interlayer development area

Abstract

The invention discloses a design method for a rigid pile composite foundation structure in a deep and thick melting interlayer development area, and relates to the field of rigid pile composite foundations, which comprises the following steps: step 1, adding roadbed-foundation temperature field analysis before the existing rigid pile bearing capacity calculation process; 2, adding groundwater analysis; 3, determining compression modulus and bearing capacity characteristic values of soil layers of a seasonal freeze-thaw layer, a (water-rich) thaw interlayer and a permafrost layer, and checking single-pile bearing capacity characteristic values when materials with different pile lengths, pile diameters and pile body stiffness are adopted at different positions of the cross section of the roadbed; and fourthly, partition unequal-strength design is conducted, the pile length, the pile diameter and the pile body rigidity are adjusted, and the characteristic value range of the single-pile bearing capacity of all the piles on the cross section is made to be lower than a design allowable value. The pile foundation design is effectively optimized, material waste is reduced, the engineering cost is reduced, and meanwhile the safety and stability of the structure are ensured.

Description

Technical Field

[0001] The present invention relates to the technical field of rigid pile composite foundations, and specifically to a design method for the structure of a rigid pile composite foundation in areas with well-developed thick thawing interlayers. Background Art

[0002] Frozen soils are widely distributed in China, ranking third in the world in terms of area. The areas of permafrost and seasonal frozen soils account for two-thirds of the total national territory. Permafrost refers to the soil and rock layers that have been frozen continuously for many years. The soil and rock masses near the surface usually show seasonal freezing and thawing, with relatively drastic temperature changes, and are also called active layers; below the active layer is the soil and rock mass that has been frozen for many years, called the permafrost layer, with a stable temperature maintained below 0°C. The foundation of the permafrost area on the Qinghai-Tibet Plateau presents a typical stratigraphic sequence of seasonal freeze-thaw layer, (water-rich) thawing interlayer, and permafrost layer. Currently, there is no special design method for the composite foundation structure for this kind of stratigraphic combination. Therefore, we provide a design method for the structure of a rigid pile composite foundation in areas with well-developed thick thawing interlayers to solve the above problems. Summary of the Invention

[0003] The purpose of the present invention is to make up for the deficiencies of the existing technology and provide a design method for the structure of a rigid pile composite foundation in areas with well-developed thick thawing interlayers, which solves the problem that there is currently no special design method for the composite foundation structure for this kind of stratigraphic combination.

[0004] To solve the above technical problems, the present invention provides the following technical solutions:

[0005] A design method for the structure of a rigid pile composite foundation in areas with well-developed thick thawing interlayers, comprising the following steps:

[0006] Step 1, add a subgrade-foundation temperature field analysis before the existing rigid pile bearing capacity calculation process, measure or predict the freezing and thawing depths, cross-sectional freezing and thawing zone shapes, permafrost degradation rate and temperature rise rate of the subgrade and shallow foundation, and determine the compression modulus, bearing capacity characteristic value, and pile tip soil layer resistance characteristic value of each regional soil layer according to the soil type, temperature value, and cross-sectional freezing and thawing zone range shape;

[0007] Step 2, add a groundwater analysis, measure or predict the thickness, water content, and seasonal fluctuation law of the water-rich layer of the thawing interlayer, and determine the compression modulus and bearing capacity characteristic value of the thawing interlayer according to the characteristics of groundwater changes;

[0008] Step 3, according to the compression modulus and bearing capacity characteristic value of each regional soil layer determined in Step 1 and Step 2, check the bearing capacity characteristic value of a single pile when using different pile lengths, pile diameters, and pile body stiffness materials at different positions of the subgrade cross-section, and calculate according to the following formula:

[0009]

[0010] In the formula: R a is the characteristic value of the bearing capacity of a single pile, with the unit of kN; q pa is the characteristic value of the end resistance of the pile, with the unit of kPa; q sja is the characteristic value of the lateral resistance of the soil around the pile at the j-th layer, with the unit of kPa; l j is the length of each section of the pile divided by soil layer, with the unit of m; A p is the cross-sectional area of the pile bottom, with the unit of m 2 ; U p is the perimeter length of the pile body, with the unit of m; m is the number of layers of frozen and thawed soil layers;

[0011] Step 4, in the cross-sectional direction of the subgrade, due to the spatial non-uniformity of the physical and mechanical parameters of the foundation soil layer, when the range of the characteristic values of the bearing capacity of a single pile calculated with the same pile length, pile diameter, and pile body stiffness at different positions exceeds the design allowable value, unequal strength design for different zones should be carried out, and the pile length, pile diameter, and pile body stiffness should be adjusted so that the range of the characteristic values of the bearing capacity of a single pile for all piles in the cross-section is lower than the design allowable value.

[0012] As a further description of the present invention, in Step 1, the calculation of the subgrade-foundation temperature field is obtained from the upper surface hydrothermal boundary calculation formula; the upper surface hydrothermal boundary calculation formula is divided into the upper surface moisture boundary calculation formula and the upper surface temperature boundary calculation formula, and the hydrothermal coupling migration model is obtained according to the calculation formulas of the upper surface moisture boundary and the upper surface temperature boundary.

[0013] Furthermore, the upper surface moisture boundary calculation formula includes:

[0014] M = C m (T air - T mel )

[0015]

[0016] In the formula, M is the melting amount, C m is the melting coefficient, T air is the air temperature, T mel is the melting temperature, PE is the potential evaporation, Q n is the radiation amount, E0 is the evaporation latent heat, and both Γ and η are coefficients affecting the potential evaporation.

[0017] Furthermore, the upper surface temperature boundary calculation formula includes:

[0018] T = T a + T i

[0019] T i = λR s

[0020] R s = R a ηa0

[0021] Wherein, T is the surface temperature, T a is the ambient air temperature, T s is the internal temperature increment of the boundary layer, λ is the transfer coefficient of the passing heat flux, R s is the solar radiation on the ground surface, R a is the total solar radiation, η is the reflectivity, and a0 is the effective utilization rate of solar radiation.

[0022] Furthermore, the calculation formula of the hydrothermal coupling calculation model is:

[0023]

[0024] Wherein, T is the surface temperature, t is the time, α is the thermal diffusivity, k is the thermal conductivity, ρ is the density, c is the specific heat capacity, Q is the internal heat source term, θ is the soil water content, D is the water diffusion coefficient, S is the water source term, Q(T,θ) is the heat source term related to the temperature and water content, and D(T) is the water diffusion coefficient related to the temperature.

[0025] As a further illustration of the present invention, in step three, the calculation formula of the pile length is:

[0026] L = D f + H w + ΔL

[0027] Wherein, D f is the thawing or freezing depth, H w is the special thickness of the water-rich layer, and ΔL is the additional safety length; the calculation formula of the pile diameter is:

[0028]

[0029] Wherein, Q design is the design load, K is the safety factor, and σ max is the allowable bearing capacity of the soil layer;

[0030] The calculation formula of the pile spacing is:

[0031] S = 2d

[0032] Wherein, d is the pile diameter;

[0033] The calculation formula of the bending stiffness of the pile is:

[0034]

[0035] Wherein, E is the elastic modulus of the pile body;

[0036] The calculation formula for the size of the pile cap is as follows:

[0037]

[0038] In the formula, Q1 is the total load borne by the pile cap, q allow is the allowable bearing capacity of the soil layer at the bottom of the pile cap, h is the buried depth of the pile cap, and γ is the weight of the soil per unit volume.

[0039] Furthermore, the calculation formula for the design load is as follows:

[0040] Q design = Q p + Q s

[0041] Q p = A t ·σ′

[0042] Q s = A s ·τ

[0043] τ = σ′·tan(Φ)

[0044] σ′ = σ - u

[0045] In the formula, Q p is the bearing capacity at the end of the pile, Q s is the frictional force on the side of the pile, A t is the end area, A s is the side area, τ is the side frictional force, σ′ is the effective stress, σ is the total stress, and u is the pore water pressure.

[0046] Compared with the prior art, the design method of the rigid pile composite foundation structure in the area with developed deep thawing interlayers has the following beneficial effects:

[0047] 1. Through the pre - placed temperature field analysis, the present invention can more accurately predict the freezing and thawing states of the soil layer, thereby better understanding the physical and mechanical properties of the soil mass. According to the physical and mechanical parameters of different soil layers, different pile lengths, pile diameters, and pile spacings can be adopted in the design, which can effectively optimize the pile foundation design, reduce material waste, lower the engineering cost, and ensure the safety and stability of the structure. At the same time, by analyzing the seasonal fluctuation law of groundwater and the characteristics of the thawing interlayer, a foundation structure adaptable to different seasons and climatic conditions can be designed, enhancing the adaptability and durability of the project.

[0048] 2. By optimizing the pile foundation design and material selection, the present invention can improve the seismic resistance of the foundation, ensure the structural safety under extreme conditions such as earthquakes, and based on detailed temperature and hydrological analyses, can formulate a more accurate construction plan, reduce the uncertainties during the construction process, thereby improving the construction efficiency. At the same time, through the analysis of the characteristics of different soil layers, the soil layer resources can be utilized more reasonably, unnecessary earthwork can be avoided, and the environmental impact can be reduced.

[0049] Other advantages, objectives, and features of the present invention will be described to some extent in the subsequent specification, and to some extent, will be obvious to those skilled in the art based on the study of the following text, or can be learned from the practice of the present invention. Detailed implementation manners

[0050] Next, in combination with the embodiments of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without making creative efforts belong to the scope of protection of the present invention.

[0051] Embodiment 1: A design method for a rigid pile composite foundation structure in an area with well-developed deep thawing interlayers, comprising the following steps:

[0052] Step 1, add a subgrade-foundation temperature field analysis before the existing rigid pile bearing capacity calculation process, measure or predict the freezing and thawing depths of the subgrade and shallow foundation, the cross-sectional freezing and thawing zone morphology, the degradation rate and warming rate of permafrost in the lower soil layer, and determine the compression modulus, bearing capacity characteristic value, and pile tip soil layer resistance characteristic value of each area soil layer according to the soil type, temperature value, and cross-sectional freezing and thawing zone range morphology;

[0053] Step 2, add a groundwater analysis, measure or predict the thickness, water content, and seasonal fluctuation law of the water-rich layer of the thawing interlayer, and determine the compression modulus and bearing capacity characteristic value of the thawing interlayer according to the characteristics of groundwater changes;

[0054] Step 3, according to the compression modulus and bearing capacity characteristic value of each area soil layer determined in Step 1 and Step 2, check the single pile bearing capacity characteristic value when different pile lengths, pile diameters, and pile body stiffness materials are used at different positions of the subgrade cross-section, and calculate according to the following formula:

[0055]

[0056] In the formula: R a is the single pile bearing capacity characteristic value, with the unit of kN; q pa is the pile tip resistance characteristic value, with the unit of kPa; q sjais the characteristic value of the lateral resistance of the soil around the pile at the j-th layer, with the unit of kPa; l j is the length of each section of the pile divided by soil layer, with the unit of m; A p is the cross-sectional area of the pile bottom, with the unit of m 2 ; U p is the perimeter length of the pile body, with the unit of m; m is the number of layers of frozen and thawed soil layers;

[0057] Step 4, in the cross-sectional direction of the subgrade, due to the spatial non-uniformity of the physical and mechanical parameters of the foundation soil layer, when the range of variation of the characteristic values of the bearing capacity of a single pile calculated with the same pile length, pile diameter, and pile body stiffness at different positions exceeds the design allowable value, unequal strength design for different zones shall be carried out, adjusting the pile length, pile diameter, and pile body stiffness so that the range of variation of the characteristic values of the bearing capacity of all single piles in the cross-section is lower than the design allowable value.

[0058] Example 2: A design method for a rigid pile composite foundation structure in a region with well-developed thick thawing interlayers, comprising the following steps:

[0059] Step 1, add the analysis of the subgrade-foundation temperature field before the existing rigid pile bearing capacity calculation process, measure or predict the freezing and thawing depths of the subgrade and shallow foundation, the morphology of the freezing and thawing regions in the cross-section, the degradation rate and warming rate of the permafrost in the lower soil layer, and determine the compression modulus, bearing capacity characteristic value, and pile tip soil resistance characteristic value of each region's soil layer according to the soil type, temperature value, and the range and morphology of the freezing and thawing regions in the cross-section;

[0060] The calculation of the subgrade-foundation temperature field is obtained from the upper surface hydrothermal boundary calculation formula; the upper surface hydrothermal boundary calculation formula is divided into the upper surface moisture boundary calculation formula and the upper surface temperature boundary calculation formula, and the hydrothermal coupling migration model is obtained according to the calculation formulas of the upper surface moisture boundary and the upper surface temperature boundary.

[0061] Step 2, add the groundwater analysis, measure or predict the thickness, water content, and seasonal fluctuation law of the water-rich layer of the thawing interlayer, and determine the physical and mechanical parameters and bearing capacity characteristic value of the soil layer of the thawing interlayer according to the variation characteristics of the groundwater;

[0062] Step 3, according to the compression modulus, bearing capacity characteristic value, etc. determined in Step 1 and Step 2; check the characteristic values of the bearing capacity of single piles when using different pile lengths, pile diameters, and pile body stiffness materials at different positions in the subgrade cross-section, and calculate according to the following formula:

[0063]

[0064] In the formula: R a is the characteristic value of the bearing capacity of a single pile, with the unit of kN; q pa is the characteristic value of the pile tip resistance, with the unit of kPa; q sjais the characteristic value of the lateral resistance of the soil around the pile in the j-th layer, with the unit of kPa; l j is the length of each section of the pile divided by soil layer, with the unit of m; A p is the cross-sectional area of the pile bottom, with the unit of m 2 ; U p is the perimeter length of the pile body, with the unit of m; m is the number of layers of frozen and thawed soil layers;

[0065] Step 4, in the cross-sectional direction of the subgrade, due to the spatial non-uniformity of the physical and mechanical parameters of the foundation soil layer, when the range difference of the characteristic values of the bearing capacity of single piles calculated with the same pile length, pile diameter, and pile body stiffness at different positions exceeds the design allowable value, unequal strength design should be carried out in zones, adjusting the pile length, pile diameter, and pile body stiffness to make the range difference of the characteristic values of the bearing capacity of all single piles in the cross-section lower than the design allowable value.

[0066] As a further description of Embodiment 2 of the present invention, the upper surface moisture boundary calculation formula in Step 1 includes:

[0067] M = C m (T air - T mel )

[0068]

[0069] In the formula, M is the melting amount, C m is the melting coefficient, T air is the air temperature, T mel is the melting temperature, PE is the potential evaporation, Q n is the radiation amount, E0 is the evaporation latent heat, and both Γ and η are coefficients affecting the potential evaporation.

[0070] As a further description of Embodiment 2 of the present invention, the upper surface temperature boundary calculation formula in Step 1 includes:

[0071] T = T a + T i

[0072] T i = λR s

[0073] R s = R a ηa0

[0074] In the formula, T is the surface temperature, T a is the ambient air temperature, T s is the temperature increment inside the boundary layer, λ is the transfer coefficient of the heat flux passing through, R s is the solar radiation on the surface, R a is the total solar radiation, η is the reflectivity, and a0 is the effective utilization rate of solar radiation.

[0075] As a further illustration of Embodiment 2 of the present invention, the calculation formula of the hydrothermal coupling calculation model in Step 1 is:

[0076]

[0077] In the formula, T is the surface temperature, t is the time, α is the thermal diffusivity, k is the thermal conductivity, ρ is the density, c is the specific heat capacity, Q is the internal heat source term, θ is the soil water content, D is the water diffusion coefficient, S is the water source term, Q(T,θ) is the heat source term related to the temperature and water content, and D(T) is the water diffusion coefficient related to the temperature.

[0078] As a further illustration of Embodiment 2 of the present invention, the calculation formula of the pile length in Step 3 is:

[0079] L = D f + H w + ΔL

[0080] In the formula, D f is the thawing or freezing depth, H w is the special thickness of the water-rich layer, and ΔL is the additional safety length; the calculation formula of the pile diameter is:

[0081]

[0082] In the formula, Q design is the design load, K is the safety factor, and σ max is the allowable bearing capacity of the soil layer;

[0083] The calculation formula of the pile spacing is:

[0084] S = 2d

[0085] In the formula, d is the pile diameter;

[0086] The calculation formula of the bending stiffness of the pile is:

[0087]

[0088] In the formula, E is the elastic modulus of the pile body;

[0089] The calculation formula of the pile cap size is:

[0090]

[0091] In the formula, Q1 is the total load borne by the pile cap, q allow is the allowable bearing capacity of the soil layer at the bottom of the pile cap, h is the buried depth of the pile cap, and γ is the weight of the soil per unit volume.

[0092] Furthermore, the calculation formula of the design load is:

[0093] Q design = Q p + Q s

[0094] Q p = A t · σ′

[0095] Q s = A s · τ

[0096] τ = σ′ · tan(Φ)

[0097] σ′ = σ - u

[0098] In the formula, Q p is the bearing capacity at the pile tip, Q s is the skin friction of the pile, A t is the tip area, A s is the side area, τ is the skin friction, σ′ is the effective stress, σ is the total stress, and u is the pore water pressure.

[0099] For those skilled in the art, it is obvious that the present invention is not limited to the details of the above exemplary embodiments, and the present invention can be implemented in other specific forms without departing from the spirit or basic characteristics of the present invention. Therefore, in any regard, the embodiments should be regarded as exemplary and non-limiting. The scope of the present invention is defined by the appended claims rather than the above description. Therefore, all changes falling within the meaning and scope of the equivalent elements of the claims are intended to be embraced within the present invention.

Claims

1. A method for designing a rigid pile composite foundation structure in an area with a thick melting interlayer, characterized in that: The following steps are involved: Step 1: Add the roadbed-foundation temperature field analysis to the existing rigid pile bearing capacity calculation process, measure or predict the freezing and thawing depth of the roadbed and shallow foundation, the cross-sectional freezing and thawing area morphology, the permafrost degradation rate and heating rate of the lower soil layer, and determine the compression modulus, bearing capacity characteristic value, and pile end soil resistance characteristic value of each area according to the soil type, temperature value, and cross-sectional freezing and thawing area range morphology; Step 2: Add groundwater analysis, measure or predict the thickness, water content, and seasonal fluctuation of the water-rich layer in the melt interlayer, and determine the compression modulus and bearing capacity characteristic values ​​of the melt interlayer according to the characteristics of groundwater changes; Step 3: According to the compression modulus and bearing capacity characteristic values ​​of the soil layers in each region determined in Step 1 and Step 2, calculate the bearing capacity characteristic values ​​of single piles at different locations of the roadbed cross section using different pile lengths, pile diameters, and pile body stiffness materials, and calculate according to the following formula: Where: R a is the characteristic value of the single pile bearing capacity, in kN; q pa is the characteristic value of pile end resistance, in kPa; q sja is the characteristic value of the soil resistance around the pile in the jth layer, in kPa; l j It is the length of each pile section divided by soil layer, in meters; A p is the cross-sectional area at the bottom of the pile, in m 2 ; U p is the length of the pile perimeter in meters; m is the number of frozen and thawed soil layers; Step 4. In the cross-sectional direction of the roadbed, due to the spatial heterogeneity of the physical and mechanical parameters of the foundation soil layer, when the extreme difference of the characteristic value of the single pile bearing capacity calculated using the same pile length, pile diameter, and pile body stiffness at different locations exceeds the design allowable value, a zoned unequal strength design should be performed to adjust the pile length, pile diameter, and pile body stiffness so that the extreme difference of the characteristic value of the single pile bearing capacity of all piles in the cross section is lower than the design allowable value.

2. The method for designing a rigid pile composite foundation structure in a region with a deep melting interlayer according to claim 1 is characterized by: In step one, the calculation of the roadbed-foundation temperature field is obtained by the upper surface hydrothermal boundary calculation formula; the upper surface hydrothermal boundary calculation formula is divided into the upper surface moisture boundary calculation formula and the upper surface temperature boundary calculation formula, and the hydrothermal coupling migration model is obtained according to the upper surface moisture boundary and the upper surface temperature boundary calculation formula.

3. The method for designing a rigid pile composite foundation structure in a region with a deep melting interlayer according to claim 2 is characterized by: The upper surface moisture boundary calculation formula includes: M=C m (T air -T mel ) Where M is the melting amount, C m is the melting coefficient, T air is the air temperature, T mel is the melting temperature, PE is the potential evaporation, Q n is the radiation, E0 is the evaporation potential, and Γ and η are coefficients that affect potential evaporation.

4. The method for designing a rigid pile composite foundation structure in a region with a deep melting interlayer according to claim 2 is characterized by: The upper surface temperature boundary calculation formula includes: T=T a +T i T i =λR s R s =R a ηa0 Where T is the surface temperature, T a Ambient temperature, T s The temperature increment inside the boundary layer, λ is the transfer coefficient of the heat flux passing through, R s is the solar radiation at the surface, R a is the total solar radiation, η is the reflectivity, and a0 is the effective utilization rate of solar radiation.

5. The method for designing a rigid pile composite foundation structure in a region with a deep melting interlayer according to claim 2 is characterized by: The calculation formula of the water-heat coupling calculation model is: Where T is the surface temperature, t is time, α is the thermal diffusivity, k is the thermal conductivity, ρ is the density, c is the specific heat capacity, Q is the internal heat source term, θ is the soil moisture content, D is the moisture diffusion coefficient, S is the moisture source term, Q(T,θ) is the heat source term related to temperature and moisture content, and D(T) is the moisture diffusion coefficient related to temperature.

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