A method for estimating the ultimate uplift capacity of a single post-grouting bored pile

Abstract

The present application relates to the field of geotechnical engineering design, in particular to a method for estimating the ultimate uplift capacity of a single post-grouting bored pile. The main method is: according to the measured data, a quantitative post-grouting reinforcement coefficient calculation formula is given; the grouting reinforcement coefficient β of each soil layer in the grouting reinforcement section is calculated si ; the ultimate uplift capacity of the post-grouting bored pile is estimated by substituting the formula. The present application solves the problem of inaccurate assessment relying on experience in construction.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of geotechnical engineering, in particular to a method for estimating the ultimate uplift capacity of a single post-grouting bored pile. BACKGROUND

[0002] At present, the post-grouting process is widely used in bored piles to improve the bearing capacity of the pile. However, there is no clear provision on how to determine the ultimate uplift capacity of a single pile after grouting, which is greatly influenced by human experience.

[0003] Currently, the estimation of the ultimate uplift capacity of a single post-grouting bored pile is mainly based on empirical parameters, i.e., multiplying the ultimate side friction value of the pile by an uplift coefficient (λi) less than 1 to obtain the post-grouting uplift side friction value. However, it is not clear how to consider the post-grouting enhanced section, and there is a lack of quantitative calculation.

[0004] According to the "Technical Code for Building Pile Foundation" (JGJ 94-2008), the current estimation of the ultimate bearing capacity of a single post-grouting pile is based on static load test data, multiplying the side resistance by the uplift coefficient, as follows:

[0005] T uk =∑λ i β si q sik u i l i

[0006] In the formula:

[0007] T uk — Standard value of the ultimate uplift capacity of the pile when the whole is not broken;

[0008] u i — Perimeter of the pile when the whole is not broken, for equal diameter piles u = πd, for expanded base piles, the value is taken from Table 5.4.6-1 of the present specification; for the whole, u is the peripheral length of the pile group;

[0009] q sik — Standard value of the ultimate compressive side resistance of the i-th layer of soil on the pile surface before grouting, which can be taken from Table 5.3.5-1 of the present specification;

[0010] β si — Post-grouting side resistance enhancement coefficient, which can be taken from Table 5.3.10 of the present specification.

[0011] λ i — Uplift coefficient, which can be taken from Table 5.4.6-2 of the present specification.

[0012] Table 5.3.10 Post-grouting side resistance enhancement coefficient β si

[0013]

[0014] Table 5.4.6-2 uplift coefficient λ

[0015] Soil name λ value Sand 0.50-0.70 Clay, silt 0.70-0.80

[0016] Note: λ takes a small value when the ratio of pile length l to pile diameter d is less than 20.

[0017] The uplift coefficient in the specification is given according to the results of uplift static load tests in various parts of the country, but the value range is large, slightly rough, depends on experience, and does not consider the reinforcement effect of the post-grouting reinforcement section. A method for quantitatively estimating the single pile uplift ultimate bearing capacity of post-grouting bored piles is needed. SUMMARY

[0018] In order to solve the problem that the estimation of the single pile uplift ultimate bearing capacity of post-grouting bored piles relies on experience and is inaccurate, the present application summarizes and proposes a quantitative post-grouting reinforcement coefficient calculation formula according to measured data, and calculates the reinforcement coefficient β of the post-grouting reinforcement section si The post-grouting bored pile single pile uplift ultimate bearing capacity is estimated by substituting the descendant into the specification formula.

[0019] The present application specifically adopts the following schemes:

[0020] S1: pile side post-grouting: laying 2 or more uniform and symmetrical pile-long grouting pipes; the amount of grouting cement is not less than D / 10 tons per meter;

[0021] S2: after grouting, the standard value of the pile side friction resistance is divided into two parts: the side friction resistance without grouting and the post-grouting reinforcement;

[0022] S3: obtaining the experimental data of steps S1-S2, and analyzing to obtain the post-grouting reinforcement coefficient β si calculation formula;

[0023] S4: calculating the side friction resistance in the post-grouting reinforcement section range according to the post-grouting reinforcement coefficient, adding the side friction resistance without grouting, and substituting into the specification formula, so as to estimate the single pile uplift ultimate bearing capacity of the post-grouting bored pile.

[0024] Wherein D is the pile diameter, unit: m.

[0025] In some embodiments disclosed in the present application, the experimental data of S1-S2 includes the pile side soil internal friction angle The pile side soil internal friction angle is calculated in sections.

[0026] In some embodiments disclosed in the present application, the reinforcement coefficient calculation formula of the post-grouting reinforcement section is:

[0027] In some embodiments disclosed in the present application, the specification formula is:

[0028] wherein T uk is the standard value of the ultimate bearing capacity of the base pile when not being integrally destroyed;

[0029] u i is the perimeter of the pile body when not being integrally destroyed;

[0030] lambda i is the uplift coefficient;

[0031] q sik is the standard value of the pre-grouting compressive ultimate side resistance of the i-th layer of soil on the side surface of the pile.

[0032] The present application has the following beneficial effects:

[0033] The present application quantifies the post-grouting reinforcement coefficient by summarizing the calculation formula of the post-grouting reinforcement coefficient based on a large amount of measured data. The post-grouting reinforcement coefficient can be directly calculated by the measured data on the construction site, and compared with the prior art, the present application can avoid the error caused by empiricism, and provides a reference method for calculating the single-pile ultimate bearing capacity of the post-grouting bored pile. Specific embodiments

[0034] The embodiments of the present application are described below through specific examples, and those skilled in the art can easily understand other advantages and effects of the present application from the disclosure. The present application can also be implemented or applied through different specific embodiments, and the details in the specification can be modified or changed based on different views and applications without departing from the spirit of the present application. It should be noted that the drawings provided in the following embodiments only illustrate the basic concept of the present application in a schematic manner, and the following embodiments and features in the embodiments can be combined with each other without conflict.

[0035] It is found through research and statistics that the post-grouting of the bored pile meets the following conditions:

[0036] Pile side post-grouting: 2 or more uniform and symmetrical pile-long grouting pipes need to be arranged; the amount of grouting cement is not less than D / 10 (tons) per meter (D is the pile diameter, unit: meter).

[0037] After grouting, the standard value of the pile side friction resistance is divided into two parts: the ungrouted side friction resistance and the post-grouting reinforcement.

[0038] The reinforcement coefficient of the post-grouting reinforcement part is: wherein is the internal friction angle of the soil on the side surface of the pile, which should be calculated in sections.

[0039] According to the above formula, the uplift coefficient after grouting can be calculated, and the ultimate uplift bearing capacity of the pile after grouting can be calculated according to the compression side resistance after grouting.

[0040] First, calculate the side friction in the range of the post-grouting reinforcement section, and then add the side friction of the ungrouted section to the specification formula:

[0041] T uk = u∑λ i q sik l i +u∑β si q sik l i

[0042] That is:

[0043] The ultimate single pile uplift bearing capacity of the post-grouting pile can be estimated.

[0044] Example 1:

[0045] The following is an example of a pile foundation project in Shanghai to prove the reasonableness of this formula and compare it with the recommended value of the specification. The design pile diameter is 700mm, the pile length is 46.40m-46.68m, the pile end is buried with 2 grouting pipes, the pile end grouting volume is not less than 2.0t, the pile side is buried with 2 grouting pipes (12m above the pile end), and the pile side grouting volume is not less than 2.1t. A total of 5 single pile vertical uplift static load tests were conducted, with a maximum design load of 4700kN, and all 5 piles reached the maximum test load of 4700kN, ending the test. The maximum uplift of 5# pile is 40.98mm, and the uplift under the last level load exceeds 2 times the uplift under the previous level load, indicating that it is close to the limit state. The maximum uplift of the remaining 4 piles is 15.24mm-29.56mm.

[0046] Shanghai Pile Foundation Project Example

[0047]

[0048] As shown in the above table, the estimated single pile uplift ultimate bearing capacity according to the patent technical solution is basically between the upper and lower limits of the theoretical value calculated according to the post-grouting reinforcement coefficient recommended by the specification, especially the maximum uplift of 619# pile is 40.98mm, and the uplift under the last level load exceeds 2 times the uplift under the previous level load, indicating that it has not reached but is close to the failure state (with a small amount of residual capacity), which is also close to the measured value. It is proved that The estimated formula is reasonable.

[0049] The above description is only the description of the preferred embodiments of the present application, and is not any limitation on the scope of the present application. Any change and modification made by the person skilled in the art according to the above disclosure is within the protection scope of the claims.

Claims

1. A method for predicting the ultimate pull-out bearing capacity of a single post-grouting bored cast-in-place pile, characterized in that, Includes the following steps: S1: Post-grouting on the pile side: Install two or more uniformly symmetrical grouting pipes with a continuous pile length; the amount of grouting cement used shall not be less than D / 10 tons per meter; where D is the pile diameter in meters. S2: After grouting, the standard value of pile side friction is divided into two parts: the side friction without grouting and the post-grouting reinforcement; the experimental data of S1-S2 include the internal friction angle of the soil around the pile. The internal friction angle of the soil around the pile is calculated in segments; S3: Obtain the experimental data from steps S1-S2 and analyze them to obtain the post-grouting reinforcement coefficient. β si The calculation formula for the post-grouting reinforcement coefficient is as follows: ; S4: Calculate the side friction resistance within the post-grouting reinforcement section based on the post-grouting reinforcement coefficient, add the side friction resistance of the un-grouted section, and then substitute it into the standard formula to estimate the single pile pull-out ultimate bearing capacity of the post-grouting cast-in-place pile. The standard formula is: ,Right now ; in T uk This represents the standard value of the ultimate tensile bearing capacity of the foundation pile when it fails as a whole. u i The perimeter of the pile body when it fails as a whole; λ i It is the pull-out coefficient; q sik The standard value of the ultimate compressive lateral resistance of the i-th layer of soil on the pile side surface before grouting.

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