A method for calculating bearing capacity of composite foundation with bond pile considering axial force change of pile

Abstract

The present application relates to the field of composite foundation bearing capacity calculation, and discloses a method for calculating the bearing capacity of a bonded pile composite foundation by considering the axial force change of the pile, which comprises the following steps: firstly, judging whether the influence of the negative friction of the soil around the pile needs to be considered; if not, calculating the first single-pile bearing capacity characteristic value and the second single-pile bearing capacity characteristic value respectively, and taking the minimum value as the single-pile bearing capacity of the bonded pile composite foundation; if yes, calculating the third single-pile bearing capacity characteristic value and the fourth single-pile bearing capacity characteristic value respectively, and taking the minimum value as the single-pile bearing capacity of the bonded pile composite foundation; and finally, calculating the bearing capacity characteristic value of the bonded pile composite foundation. The present application considers the axial force change of the pile, the different strengths of the pile formed by different soil layers and the influence of the negative friction, is more accurate than the existing calculation methods, and can save resources.

Description

Technical Field

[0001] This invention relates to the field of foundation bearing capacity calculation, and in particular to a method for calculating the bearing capacity of bonded pile composite foundations that takes into account the variation of pile axial force. Background Technology

[0002] Bonded pile composite foundations are suitable for reinforcing foundations in silt, silty soil, cohesive soil (soft plastic to plastic), silty soil (slightly dense to medium dense), sandy soil (loose to medium dense), loess, and fill foundations. They are widely used in foundation treatment due to their low cost and good reinforcement effect. However, the current "Technical Code for Building Foundation Treatment" does not consider the change in axial force with depth when calculating the vertical bearing capacity of a single pile. Therefore, it is conservative in calculating the bearing capacity of general bonded pile composite foundations. Furthermore, when calculating the characteristic value of the bearing capacity of bonded pile composite foundations with negative skin friction between piles, it fails to consider the increase in axial force caused by negative skin friction, resulting in calculated results that are larger than the actual values. Summary of the Invention

[0003] To address the aforementioned technical problems, this invention provides a method for calculating the bearing capacity of bonded pile composite foundations that considers the variation of axial force in the pile body. This method can solve the problem of insufficient consideration of the variation of axial force in the bonded pile body with depth in existing specifications.

[0004] To achieve the above objectives, the embodiments of the present invention adopt the following technical solutions:

[0005] This invention provides a method for calculating the bearing capacity of bonded pile composite foundations considering changes in pile axial force, the method comprising the following steps:

[0006] Determine whether negative frictional resistance needs to be considered;

[0007] If negative skin friction is not required, calculate the characteristic values ​​of the first and second single pile bearing capacities, and take the minimum value as the single pile bearing capacity of the bonded pile composite foundation. The first single pile bearing capacity characteristic value is determined by the skin friction between the surrounding soil and the pile side and the end resistance of the soil at the pile tip. The second single pile bearing capacity characteristic value is determined by the axial force at the top surface of the i-th soil layer counting downwards from the pile top and the pile strength formed by the i-th soil layer counting downwards from the pile top.

[0008] If negative skin friction needs to be considered, the characteristic values ​​of the third and fourth single pile bearing capacities are calculated, and the minimum value is taken as the single pile bearing capacity of the bonded pile composite foundation. The characteristic value of the third single pile bearing capacity is determined by the skin friction between the surrounding soil and the pile side and the end resistance of the soil at the pile tip; the characteristic value of the fourth single pile bearing capacity is determined by the axial force at the bottom of the soil layer above the neutral point, the axial force at the top of the soil layer below the neutral point, and the pile strength formed by the soil layers.

[0009] The characteristic value of the bearing capacity of the bonded pile composite foundation is calculated based on the determined single pile bearing capacity of the bonded pile composite foundation.

[0010] The method for determining whether negative skin friction needs to be considered includes: determining whether the bonded pile composite foundation meets any of the following conditions; if it does, then negative skin friction needs to be considered; otherwise, negative skin friction does not need to be considered.

[0011] The pile penetrates from the first soil layer into the second soil layer, where the second soil layer has higher stiffness than the first soil layer, and the stiffness of the second soil layer reaches a predetermined value.

[0012] There is a soil layer around the pile with a softness level that reaches a set value, and the pile side ground surface bears a load exceeding a set value for a set duration within a predetermined range.

[0013] The reduction in groundwater level increases the effective stress change around the pile to a preset value, resulting in compressive settlement within a predetermined range.

[0014] The characteristic value of the bearing capacity of the first single pile is calculated by the following formula:

[0015]

[0016] Where u is the perimeter of the bonded pile; q si Let l be the characteristic value of the pile side skin friction of the i-th soil layer counting downwards from the pile top; i α is the thickness of the i-th soil layer counting downwards from the top of the pile; p A is the pile end resistance utilization factor; p q represents the cross-sectional area of ​​the pile. p This represents the characteristic value of the pile end resistance.

[0017] The characteristic value of the bearing capacity of the second single pile is calculated by the following formula:

[0018]

[0019] Among them, f cui To measure the average cubic strength of pile specimens formed from the i-th soil layer counting downwards from the pile top, after a predetermined number of days of standard curing, η is the pile strength reduction factor; A p q is the cross-sectional area of ​​the pile; u is the perimeter of the bonded pile; sm Let l be the characteristic value of the side skin resistance of the m-th soil layer counting downwards from the pile top; m The thickness of the m-th soil layer is measured downwards from the top of the pile.

[0020] The method further includes: (The second single pile bearing capacity characteristic value is multiple; the method also includes:)

[0021] Select the minimum second single pile bearing capacity characteristic value from multiple second single pile bearing capacity characteristic values;

[0022] Then, the minimum value between the minimum second single pile bearing capacity characteristic value and the first single pile bearing capacity characteristic value is selected as the single pile bearing capacity of the bonded pile composite foundation.

[0023] The location of the neutral point in the bonded pile composite foundation is calculated using the following formula:

[0024] l=kl0

[0025] Where l0 is the distance from the top of the pile to the lower limit of the weak layer around the pile, k is the neutral point calculation coefficient, k = 0.5 for cohesive soil and silt at the pile tip, and k = 0.6 for sandy soil at the pile tip, and l is the distance from the top of the pile to the neutral point of the pile.

[0026] The characteristic value of the bearing capacity of the third single pile is calculated by the following formula:

[0027]

[0028] Where, α p A is the pile end resistance utilization factor; p q represents the cross-sectional area of ​​the pile. p The characteristic value of the pile end resistance; u is the perimeter of the bonded pile; l i q represents the thickness of the i-th soil layer counting downwards from the top of the pile; si Q represents the characteristic value of the side skin resistance of the i-th soil layer counting downwards from the pile top; ni The standard value of the pull-down load on the bonded pile caused by the negative skin friction generated by the i-th soil layer;

[0029] Wherein, Q ni Calculated by the following formula:

[0030]

[0031] Where, q n (x) represents the negative skin friction of the pile at a depth x below the pile top, h i-1 <x≤h i h i h is the distance from the top of the pile to the bottom surface of the i-th soil layer from the pile top; i-1 It is the distance from the top of the pile to the bottom surface of the (i-1)th layer of soil from the top of the pile;

[0032] Wherein, the q n (x) is calculated by the following formula:

[0033]

[0034] When q n (x)>q s Take q n (x)=q s h i-1 <x≤hi ;

[0035] in, β is the effective internal friction angle of the i-th soil layer from the pile top; β is the bearing capacity utilization coefficient of the soil between piles; f sk To process the characteristic value of the bearing capacity of the soil between piles; γ m ' is the effective unit weight of the m-th soil layer from the pile top; l m γ is the thickness of the m-th soil layer counting downwards from the top of the pile; i ' is the effective unit weight of the i-th layer of soil from the top of the pile.

[0036] The characteristic value of the bearing capacity of the fourth single pile is calculated by the following formula:

[0037]

[0038] Among them, f cui To measure the average cubic strength of pile specimens formed from the i-th soil layer counting downwards from the pile top, after a predetermined number of days of standard curing, η is the pile strength reduction factor; A p Q is the cross-sectional area of ​​the pile; u is the perimeter of the bonded pile; nj Let q be the standard value of the pull-down load on the bonded pile caused by the negative skin friction generated by the j-th soil layer. sj Let l be the characteristic value of the side skin resistance of the j-th soil layer counting downwards from the pile top; j The thickness of the j-th soil layer is counted downwards from the top of the pile.

[0039] The method further includes: (The fourth single pile bearing capacity characteristic value is multiple; the method also includes:)

[0040] Select the minimum fourth single pile bearing capacity characteristic value from among the multiple fourth single pile bearing capacity characteristic values;

[0041] Then, the minimum value between the minimum fourth single pile bearing capacity characteristic value and the third single pile bearing capacity characteristic value is selected as the single pile bearing capacity of the bonded pile composite foundation.

[0042] The characteristic value of the bearing capacity of the bonded pile composite foundation, calculated based on the determined single pile bearing capacity, is determined by the following formula:

[0043]

[0044] Where β is the bearing capacity utilization coefficient of the soil between piles, f sk The characteristic value of the bearing capacity of the soil between piles after processing, m is the area replacement ratio of the bonded piles in the bonded pile composite foundation, and f is the characteristic value of the bearing capacity of the soil between piles after processing. spk A represents the characteristic value of the bearing capacity of the bonded pile composite foundation. p R is the cross-sectional area of ​​the pile end. a λ represents the bearing capacity of a single pile in a bonded pile composite foundation, and λ is the single pile bearing capacity utilization coefficient.

[0045] The embodiments of the present invention have the following beneficial effects:

[0046] This invention provides a method for calculating the bearing capacity of bonded pile composite foundations considering variations in pile axial force. First, it determines whether the influence of negative skin friction on the pile side needs to be considered. If the influence of negative skin friction is not considered, the method calculates a first characteristic value of the single pile bearing capacity determined by the skin friction between the surrounding soil and the pile side, and the end resistance of the soil at the pile tip. It also calculates a second characteristic value of the single pile bearing capacity determined by the axial force at the top surface of the i-th soil layer counting downwards from the pile top, and the pile strength formed by the i-th soil layer counting downwards from the pile top. The minimum value is taken as the single pile bearing capacity of the bonded pile composite foundation. If the influence of negative skin friction needs to be considered, the method calculates a third characteristic value of the single pile bearing capacity determined by the skin friction between the surrounding soil and the pile side, and the end resistance of the soil at the pile tip. Finally, it calculates a fourth characteristic value of the single pile bearing capacity determined by the axial force at the bottom surface of the soil layer above the neutral point, the axial force at the top surface of the soil layer below the neutral point, and the pile strength formed by the soil layer. The minimum value is taken as the single pile bearing capacity of the bonded pile composite foundation. Finally, the method calculates the characteristic value of the bearing capacity of the bonded pile composite foundation. This invention takes into account the changes in pile axial force, the differences in pile strength formed by different soil layers, and the influence of negative skin friction. Compared with existing calculation methods, it is more accurate and can save resources.

[0047] Of course, implementing any product or method of the present invention does not necessarily require achieving all of the advantages described above at the same time. Attached Figure Description

[0048] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0049] Figure 1 This is a flowchart illustrating a method for calculating the bearing capacity of bonded pile composite foundations considering changes in pile axial force, as described in an embodiment of the present invention.

[0050] Figure 2 This is a diagram of pile side friction resistance without considering negative skin friction resistance in this embodiment of the invention;

[0051] Figure 3 This is a diagram of the axial force of the pile body without considering negative skin friction in this embodiment of the invention;

[0052] Figure 4 This is a diagram of the side friction resistance of a pile considering negative skin friction in an embodiment of the present invention;

[0053] Figure 5 This is a diagram of the axial force of the pile body considering negative skin friction in an embodiment of the present invention. Detailed Implementation

[0054] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations. It should be noted that, without conflict, the embodiments and features in the embodiments of the present invention can also be combined with each other.

[0055] It should be noted that similar reference numerals and letters in the following figures indicate similar items; therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures. In the description of the invention, the terms "first," "second," "third," "fourth," etc., are used only to distinguish descriptions and should not be construed as merely or implying relative importance.

[0056] This invention provides a method for calculating the bearing capacity of bonded pile composite foundations considering variations in pile axial force. First, it determines whether the influence of negative skin friction on the pile side needs to be considered. If the influence of negative skin friction is not considered, the characteristic value of the single pile bearing capacity is calculated based on the skin friction between the surrounding soil and the pile side, the end resistance of the soil at the pile tip, and the characteristic value of the single pile bearing capacity is calculated based on the axial force at the top surface of the i-th soil layer counting downwards from the pile top and the pile strength formed by the i-th soil layer counting downwards from the pile top. The minimum value is taken as the single pile bearing capacity of the bonded pile composite foundation. If the influence of negative skin friction on the pile side needs to be considered, the characteristic value of the single pile bearing capacity is calculated based on the skin friction between the surrounding soil and the pile side, the end resistance of the soil at the pile tip, and the characteristic value of the single pile bearing capacity is calculated based on the axial force at the bottom surface of the soil layer above the neutral point, the axial force at the top surface of the soil layer below the neutral point, and the pile strength formed by the soil layer. The minimum value is taken as the single pile bearing capacity of the bonded pile composite foundation. The characteristic value of the bearing capacity of the bonded pile composite foundation is then calculated. This invention takes into account the changes in pile axial force, the differences in pile strength formed by different soil layers, and the influence of negative skin friction. Compared with existing calculation methods, it is more accurate and can save resources.

[0057] like Figure 1 The diagram shown is a flowchart of a method for calculating the bearing capacity of bonded pile composite foundations considering changes in pile axial force, provided by an embodiment of the present invention. The method includes the following steps:

[0058] Step S1: Determine whether negative frictional resistance needs to be considered;

[0059] The judgment method provided in the embodiments of the present invention can be as follows: when the bonded pile composite foundation meets one of the following conditions, negative skin friction needs to be considered; otherwise, negative skin friction does not need to be considered.

[0060] 1) The pile (i.e., bonded pile) penetrates from the first soil layer into the second soil layer, where the second soil layer has higher stiffness than the first soil layer, and the stiffness of the second soil layer reaches a predetermined value. The first soil layer includes: loose fill with a predetermined thickness, self-weight collapsible loess, under-consolidated soil, and liquefiable soil layer; the predetermined value can generally be taken as the compressibility coefficient of the second soil layer being less than 0.1 MPa. -1 The thickness of the first type of soil layer is greater than 1m, but this invention is not limited to this.

[0061] 2) The soil around the pile is weak to a certain degree (the set value may be, for example, a soil compressibility coefficient greater than 0.5 MPa). -1 The soil layer is subjected to a load where the soil layer is subjected to pressure exceeding a predetermined value (e.g., 100 kPa) for a predetermined duration (e.g., 2 days) within a predetermined range (the predetermined range can be 2 m) around the pile. For example, the soil layer is subjected to a load where the soil layer is subjected to local pressure exceeding a predetermined value for a predetermined duration or a large-area load on the soil layer around the pile. The soil layer is subjected to a load where the soil layer is subjected to pressure exceeding a predetermined value for a predetermined duration or ...

[0062] 3) Due to the lowering of the groundwater level, the effective stress change around the pile increases to the preset value (50 kPa), and compressive settlement within a predetermined range (greater than 10 mm) occurs.

[0063] If negative friction resistance does not need to be considered, proceed to step S2; if negative friction resistance needs to be considered, proceed to step S3.

[0064] Step S2: Calculate the characteristic value of the first single pile bearing capacity and the characteristic value of the second single pile bearing capacity, and take the minimum value as the single pile bearing capacity of the bonded pile composite foundation.

[0065] The first characteristic value of the single pile bearing capacity is determined by the frictional resistance of the soil around the pile and the end resistance of the soil at the pile tip; the second characteristic value of the single pile bearing capacity is determined by the axial force at the top surface of the i-th soil layer counting downwards from the pile top and the pile body strength formed by the i-th soil layer counting downwards from the pile top.

[0066] Without considering negative skin friction, the side resistance of a bonded pile with side skin friction is as follows: Figure 2 As shown, the axial force variation of the bonded pile is as follows: Figure 3 As shown, the average cube strength (kPa) of bonded pile test blocks (150mm side length cubes) formed from the same soil layer after 28 days of standard curing is f cui Since they are the same, the compressive bearing capacity of the pile body remains unchanged in this soil layer. Therefore, the bearing capacity of the bonded pile body in the i-th soil layer can be expressed by the following formula:

[0067] R i =ηf cui A p (1)

[0068] Among them, f cui To measure the average cubic strength (kPa) of a pile specimen (150mm cube) formed from the i-th soil layer from the top of the pile after 28 days of standard curing, η is the pile strength reduction factor; A p This represents the cross-sectional area of ​​the pile.

[0069] When a load is applied to the top of the pile, the axial force in the pile decreases continuously with increasing pile depth due to the resistance of the surrounding soil. The specific changes are as follows: Figure 3 As shown, the axial force of the pile at the top of the same soil layer is the largest. Therefore, it is sufficient to verify that the axial force of the pile at the top of the soil layer meets the bearing capacity requirements of the bonded pile. Based on the bonded pile strength at the pile section at the top surface of the i-th soil layer, the following formula can be obtained:

[0070]

[0071] Among them, R ai Let q be the characteristic value of the single pile bearing capacity determined by the pile body strength of the bonded pile at the top surface of the i-th soil layer. u is the perimeter of the bonded pile; q sm Let l be the characteristic value of the side skin resistance of the m-th soil layer counting downwards from the pile top; m The thickness of the m-th soil layer is measured downwards from the top of the pile.

[0072] Transforming the above formula, we can obtain the characteristic value R of the second single pile bearing capacity, which is determined by the pile strength of the i-th soil layer counting downwards from the pile top. ai As shown in the following formula:

[0073]

[0074] The characteristic value R of the first monopile bearing capacity, determined by the skin friction of the surrounding soil and the soil resistance at the pile tip, can be calculated using the following formula. as :

[0075]

[0076] Where u is the perimeter of the bonded pile; q si Let l be the characteristic value of the pile side skin friction of the i-th soil layer counting downwards from the pile top; i α is the thickness of the i-th soil layer counting downwards from the top of the pile; p A is the pile end resistance utilization factor; p q represents the cross-sectional area of ​​the pile. p This represents the characteristic value of the pile end resistance.

[0077] Take the characteristic value R of the bearing capacity of the first single pile as and the characteristic value R of the second single pile bearing capacity ai The minimum value in the formula is taken as the bearing capacity of a single pile in a bonded pile composite foundation, and can be expressed by the following formula:

[0078] R a =min{R as ,R a1 ,R a2 ,R a3 ,……,R an} (5)

[0079] Among them, the characteristic value R of the second single pile bearing capacity ai If there are multiple second single pile bearing capacity characteristic values, the smallest second single pile bearing capacity characteristic value among the multiple second single pile bearing capacity characteristic values ​​can be selected first; or the minimum value of the multiple second single pile bearing capacity characteristic values ​​and the first bearing capacity characteristic value can be directly calculated using the above formula (5) as the single pile bearing capacity of the bonded pile composite foundation.

[0080] Step S3: Calculate the characteristic values ​​of the bearing capacity of the third and fourth single piles, and take the minimum value as the bearing capacity of the single pile of the bonded pile composite foundation.

[0081] The third characteristic value of the single pile bearing capacity is determined by the frictional resistance of the soil around the pile to the pile side and the end resistance of the soil at the pile tip; the fourth characteristic value of the single pile bearing capacity is determined by the axial force of the bottom surface of the soil layer above the neutral point, the axial force of the top surface of the soil layer below the neutral point, and the pile body strength formed by the soil layers.

[0082] When considering negative skin friction, the location where the pile skin friction changes from negative to positive, i.e., the neutral point, must first be calculated. The location of the neutral point in the bonded pile composite foundation is calculated using the following formula:

[0083] l=kl0 (6)

[0084] Where: l0 is the distance from the pile top to the lower limit depth of the weak layer around the pile, k is the neutral point calculation coefficient, k = 0.5 for cohesive soil and silt at the pile tip, and k = 0.6 for sandy soil at the pile tip. l is the distance from the pile top to the pile neutral point. For bonded pile composite foundations that need to consider negative skin friction in clause 1), the weak layer is a relatively thick layer of loose fill, collapsible loess, underconsolidated soil, or liquefiable soil; for bonded pile composite foundations that meet clause 2), the weak layer is a layer of soft soil; for bonded pile composite foundations that meet clause 3), the weak layer is the lower limit depth of a significantly compressible soil layer.

[0085] For ease of calculation, the soil layer at the neutral point is divided into two layers along the horizontal plane at the neutral point. The number of soil layers above the neutral point is s, and the number of soil layers below the neutral point is t.

[0086] Figure 4 To consider the negative skin friction of the pile side skin friction diagram, Figure 5 To consider the axial force diagram of the pile body under negative skin friction, such as Figure 4 and Figure 5As shown, the characteristic value R of the third single pile bearing capacity, determined by the skin friction of the surrounding soil and the bearing capacity of the soil at the pile tip, can be calculated using the following formula. as :

[0087]

[0088] Among them, l i q represents the thickness of the i-th soil layer counting downwards from the top of the pile; si Q represents the characteristic value of the side skin resistance of the i-th soil layer counting downwards from the pile top; ni The standard value of the pull-down load on the bonded pile caused by the negative skin friction generated by the i-th soil layer;

[0089] The standard value of the pull-down load on the bonded pile caused by the negative skin friction generated by the i-th soil layer can be calculated by the following formula:

[0090]

[0091] Where, q n (x) represents the negative skin friction of the pile at a depth x below the pile top, h i-1 <x≤h i h i h is the distance from the top of the pile to the bottom surface of the i-th soil layer from the pile top; i-1 It is the distance from the top of the pile to the bottom surface of the (i-1)th layer of soil from the top of the pile;

[0092] The vertical stress in the soil layer at the location of negative skin friction can be calculated using the following formula:

[0093]

[0094] Where β is the bearing capacity utilization coefficient of the soil between piles; f sk To process the characteristic value of the bearing capacity of the soil between piles; γ m ' is the effective unit weight of the m-th soil layer from the pile top; l m γ is the thickness of the m-th soil layer counting downwards from the top of the pile; i 'The effective unit weight of the i-th soil layer counting downwards from the top of the pile;

[0095] Calculate the horizontal stress in the soil layer at the point of negative skin friction based on the static earth pressure of the soil layer:

[0096]

[0097] Where K0 is the at-rest earth pressure coefficient of the i-th soil layer. The effective internal friction angle of the i-th soil layer from the pile top;

[0098] The negative skin friction q of the pile side at a depth x below the pile top in the i-th soil layer counting downwards from the pile top can be calculated using the following formula. n (x),

[0099]

[0100] Substituting equations (11), (10), and (9) into equation (8), we can obtain the negative skin friction q at a depth x below the pile top. n (x), as shown in equation (12):

[0101]

[0102] Since the negative skin friction of the pile side should be less than or equal to the skin friction of the pile side, when q is calculated by equation (12) n (x)>q si When, take q n (x)=q si ;

[0103] in, β is the effective internal friction angle of the i-th soil layer from the pile top; β is the bearing capacity utilization coefficient of the soil between piles; f sk To process the characteristic value of the bearing capacity of the soil between piles; γ m ' is the effective unit weight of the m-th soil layer from the pile top; l m γ is the thickness of the m-th soil layer counting downwards from the top of the pile; i ' is the effective unit weight of the i-th layer of soil from the top of the pile.

[0104] Depend on Figure 4 , Figure 5 It can be seen that, considering negative skin friction, the axial force of the pile body first increases continuously with the increase of depth, reaches its peak at the neutral point, and then decreases continuously with the increase of depth.

[0105] The average cube strength (kPa) of the bonded pile test blocks (150mm side length cubes) formed from the same soil layer after 28 days of standard curing was f. cui Since they are the same, the compressive bearing capacity of the pile body remains unchanged in this soil layer. Therefore, the bearing capacity of the bonded pile body in the i-th soil layer can be expressed by the following formula:

[0106] R i =ηf cui A p (13)

[0107] Among them, f cui To measure the average cubic strength (kPa) of a pile specimen (150mm cube) formed from the i-th soil layer from the top of the pile after 28 days of standard curing, η is the pile strength reduction factor; A p This represents the cross-sectional area of ​​the pile.

[0108] Considering negative skin friction, the axial force of the pile initially increases with depth, reaching a peak at the neutral point, and then decreases with further depth. However, the compressive bearing capacity of bonded piles formed in the same soil layer is the same. Therefore, in soil layers above the neutral point, only the pile strength at the bottom of the layer needs to be checked; in soil layers below the neutral point, only the pile strength at the top of the layer needs to be checked. This yields the characteristic value R of the fourth single pile bearing capacity, determined by the pile strength formed in the i-th soil layer. ai That is, the characteristic value of the bearing capacity of the fourth single pile is determined above the neutral point by the axial force at the bottom surface of the i-th soil layer counting downwards from the pile top and the pile strength formed by the i-th soil layer counting downwards from the pile top; and below the neutral point by the axial force at the top surface of the i-th soil layer counting downwards from the pile top and the pile strength formed by the i-th soil layer counting downwards from the pile top.

[0109]

[0110] Among them, f cui To measure the average cubic strength of pile specimens formed from the i-th soil layer counting downwards from the pile top, after a predetermined number of days of standard curing, η is the pile strength reduction factor; A p Q is the cross-sectional area of ​​the pile; u is the perimeter of the bonded pile; nj Let Q be the standard value of the pull-down load on the bonded pile caused by the negative skin friction generated by the j-th soil layer, and its calculation method is the same as that for the standard value of the pull-down load on the bonded pile caused by the negative skin friction generated by the i-th soil layer. ni The calculation method is the same, but i needs to be replaced by j during the calculation; q sj Let l be the characteristic value of the side skin resistance of the j-th soil layer counting downwards from the pile top; j The thickness of the j-th soil layer is counted downwards from the top of the pile.

[0111] The bearing capacity of a single pile in a bonded pile composite foundation is calculated using the following formula:

[0112] R a =min{R as ,R a1 ,R a2 ,R a3 ,……R an} (15)

[0113] Among them, the characteristic value R of the bearing capacity of the fourth single pile ai If there are multiple fourth single pile bearing capacity characteristic values, the minimum fourth single pile bearing capacity characteristic value among the multiple fourth single pile bearing capacity characteristic values ​​can be selected first; alternatively, the minimum value of the multiple second single pile bearing capacity characteristic values ​​and third bearing capacity characteristic values ​​can be directly calculated using the above formula (5) as the single pile bearing capacity of the bonded pile composite foundation.

[0114] Step S4: Calculate the characteristic value of the bearing capacity of the bonded pile composite foundation based on the determined single pile bearing capacity of the bonded pile composite foundation.

[0115] The bearing capacity of the bonded pile composite foundation can be calculated using the following formula:

[0116]

[0117] Where β is the bearing capacity utilization coefficient of the soil between piles, f sk The characteristic value of the bearing capacity of the soil between piles after processing, m is the area replacement ratio of the bonded piles in the bonded pile composite foundation, and f is the characteristic value of the bearing capacity of the soil between piles after processing. spk A represents the characteristic value of the bearing capacity of the bonded pile composite foundation. p R is the cross-sectional area of ​​the pile end. a λ represents the bearing capacity of a single pile in a bonded pile composite foundation, and λ is the single pile bearing capacity utilization coefficient.

[0118] As can be seen from the above technical solutions, the bearing capacity calculation method for bonded pile composite foundations considering the change of pile axial force provided by the present invention first determines whether the influence of the negative skin friction of the soil around the pile needs to be considered; if the influence of the negative skin friction of the soil around the pile does not need to be considered, then the characteristic value of the single pile bearing capacity determined by the skin friction of the soil around the pile to the pile side and the end resistance of the soil at the pile tip are calculated respectively, and the single pile bearing capacity determined by the axial force of the top surface of the i-th soil layer counting downwards from the pile top and the pile strength formed by the i-th soil layer counting downwards from the pile top is calculated. The minimum value of the bearing capacity characteristic value is taken as the single pile bearing capacity of the bonded pile composite foundation. If the influence of the negative skin friction of the soil around the pile needs to be considered, the characteristic value of the single pile bearing capacity is determined by the skin friction of the soil around the pile to the pile side and the end resistance of the soil at the pile tip, and the characteristic value of the single pile bearing capacity is determined by the axial force of the soil layer at the bottom surface above the neutral point, the axial force of the soil layer at the top surface below the neutral point, and the pile strength formed by the soil layers. The minimum value is taken as the single pile bearing capacity of the bonded pile composite foundation. The characteristic value of the bearing capacity of the bonded pile composite foundation is also calculated. This invention considers the changes in pile axial force, the differences in pile strength formed by different soil layers, and the influence of negative skin friction. Compared with existing calculation methods, the calculation is more accurate and can save resources.

[0119] The above description is merely a preferred embodiment of the present invention and an explanation of the technical principles employed, and is not intended to limit the scope of the claimed invention, but merely to illustrate preferred embodiments of the invention. Those skilled in the art should understand that the scope of the invention is not limited to the specific combinations of the above-described technical features, but should also cover other technical solutions formed by arbitrary combinations of the above-described technical features or their equivalents without departing from the inventive concept. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

Claims

1. A method for calculating the bearing capacity of bonded pile composite foundations considering variations in pile axial force, characterized in that, The method includes the following steps: Determine whether negative frictional resistance needs to be considered; If negative skin friction is not required, calculate the characteristic values ​​of the first and second single pile bearing capacities, and take the minimum value as the single pile bearing capacity of the bonded pile composite foundation. The first single pile bearing capacity characteristic value is determined by the skin friction between the surrounding soil and the pile side, and the end resistance of the soil at the pile tip. The second single pile bearing capacity characteristic value is calculated by counting downwards from the pile top... i Axial force at the top surface of the soil layer and the number of layers from the top of the pile downwards i Determining the strength of the pile body formed by the soil layers; The characteristic value of the bearing capacity of the first single pile is calculated by the following formula: ， in, u The perimeter of the bonded pile; q si The number of downwards from the top of the pile i Characteristic value of pile side friction resistance of soil layer; l i The number of downwards from the top of the pile i Soil layer thickness; α p This is the coefficient for the utilization of pile end resistance; A p The cross-sectional area of ​​the pile body; q p This represents the characteristic value of the pile end resistance. The characteristic value of the bearing capacity of the second single pile is calculated by the following formula: ， in, f cui The test measurement is performed from the top of the pile downwards. i The average cube strength of pile test blocks formed by layered soil after a predetermined number of days of standard curing. η This is the reduction factor for pile strength; A p The cross-sectional area of ​​the pile body; u The perimeter of the bonded pile; q sm The number of downwards from the top of the pile m Characteristic values ​​of side friction resistance of soil layers; l m The number of downwards from the top of the pile m Soil layer thickness; If negative skin friction needs to be considered, the characteristic values ​​of the third and fourth single pile bearing capacities are calculated, and the minimum value is taken as the single pile bearing capacity of the bonded pile composite foundation. The characteristic value of the third single pile bearing capacity is determined by the skin friction of the surrounding soil to the pile side and the end resistance of the soil at the pile tip; the characteristic value of the fourth single pile bearing capacity is determined by the axial force at the bottom of the soil layer above the neutral point, the axial force at the top of the soil layer below the neutral point, and the pile strength formed by the soil layers. The characteristic value of the bearing capacity of the third single pile is calculated by the following formula: ， in, α p This is the coefficient for the utilization of pile end resistance; A p The cross-sectional area of ​​the pile body; q p This represents the characteristic value of the pile end resistance. u The perimeter of the bonded pile; l i The number of downwards from the top of the pile i Soil layer thickness; q si The number of downwards from the top of the pile i Characteristic values ​​of side friction resistance of soil layers; Q ni For the first i Standard value of the pull-down load of the bonded pile caused by the negative skin friction generated by the soil layer; Among them, the Q ni Calculated by the following formula: ， in, q n ( x The depth below the pile top is... x Negative skin friction on the pile side at the location, h i-1 < x ≤ h i ; h i The number of the distance from the top of the pile to the top of the pile. i The distance between the bottom surfaces of the soil layers; h i-1 The number of the distance from the top of the pile to the top of the pile. i -1 layer of soil bottom surface; Among them, the q n ( x ) is calculated using the following formula: ， when q n ( x )> q s ,Pick q n ( x )= q s ; h i-1 < x ≤ h i ; in, φ i For the number of pile tops i Effective internal friction angle of the soil layer; β This is the bearing capacity utilization coefficient of the soil between piles; f sk To process the characteristic value of the bearing capacity of the soil between piles; γ m 'The number of the first pile from the top of the pile' m Effective unit weight of the soil layer; l m The number of downwards from the top of the pile m Soil layer thickness; γ i 'The number of the first pile from the top of the pile' i Effective unit weight of the soil layer; The characteristic value of the bearing capacity of the fourth single pile is calculated by the following formula: ， in, f cui The test measurement is performed from the top of the pile downwards. i The average cube strength of pile test blocks formed by layered soil after a predetermined number of days of standard curing. η This is the reduction factor for pile strength; A p The cross-sectional area of ​​the pile body; u The perimeter of the bonded pile; Q nj For the first j The standard value of the pull-down load of the bonded pile caused by the negative skin friction generated by the soil layer. q sj The number of downwards from the top of the pile j Characteristic values ​​of side friction resistance of soil layers; l j The number of downwards from the top of the pile j Soil layer thickness; The characteristic value of the bearing capacity of the bonded pile composite foundation is calculated based on the determined single pile bearing capacity of the bonded pile composite foundation.

2. The method for calculating the bearing capacity of bonded pile composite foundations considering changes in pile axial force according to claim 1, characterized in that, The method for determining whether negative skin friction needs to be considered includes: determining whether the bonded pile composite foundation meets any of the following conditions. If it does, then negative skin friction needs to be considered; otherwise, negative skin friction does not need to be considered. The pile penetrates from the first soil layer into the second soil layer, where the second soil layer has higher stiffness than the first soil layer, and the stiffness of the second soil layer reaches a predetermined value. There is a soil layer around the pile with a softness level that reaches a set value, and the pile side ground surface bears a load exceeding a set value for a set duration within a predetermined range. The reduction in groundwater level increases the effective stress change around the pile to a preset value, resulting in compressive settlement within a predetermined range.

3. The method for calculating the bearing capacity of bonded pile composite foundations considering changes in pile axial force according to claim 1, characterized in that, The second single pile bearing capacity characteristic value is multiple, and the method further includes: Select the minimum second single pile bearing capacity characteristic value from multiple second single pile bearing capacity characteristic values; Then, the minimum value between the minimum second single pile bearing capacity characteristic value and the first single pile bearing capacity characteristic value is selected as the single pile bearing capacity of the bonded pile composite foundation.

4. The method for calculating the bearing capacity of bonded pile composite foundations considering changes in pile axial force according to claim 1, characterized in that, The location of the neutral point in a bonded pile composite foundation is calculated using the following formula: ， in, l 0 represents the distance from the top of the pile to the lower limit of the weak layer around the pile. k The coefficient is calculated for the neutral point; for pile tip soils that are cohesive or silty, the coefficient is taken as follows: k =0.5, for pile tip soil is sandy soil. k =0.6, l This is the distance from the top of the pile to the center point of the pile.

5. The method for calculating the bearing capacity of bonded pile composite foundations considering changes in pile axial force according to claim 1, characterized in that, The fourth single pile bearing capacity characteristic value is multiple, and the method further includes: Select the minimum fourth single pile bearing capacity characteristic value from among the multiple fourth single pile bearing capacity characteristic values; Then, the minimum value between the minimum fourth single pile bearing capacity characteristic value and the third single pile bearing capacity characteristic value is selected as the single pile bearing capacity of the bonded pile composite foundation.

6. The method for calculating the bearing capacity of bonded pile composite foundations considering changes in pile axial force according to claim 1, characterized in that, The characteristic value of the bearing capacity of the bonded pile composite foundation, based on the determined single pile bearing capacity, is calculated by the following formula: ， in, β This is the bearing capacity utilization coefficient of the soil between piles. f sk To process the characteristic value of the bearing capacity of the soil between piles, m For the area replacement ratio of bonded piles in bonded pile composite foundations, f spk The characteristic value of the bearing capacity of the bonded pile composite foundation is... A p The cross-sectional area of ​​the pile tip. R a For the bearing capacity of a single pile in a bonded pile composite foundation, λ This is the single pile bearing capacity utilization coefficient.

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