Calculation method of end bearing force of coral reef geological driven pile foundation

Abstract

The application discloses a method for calculating the end bearing force of a coral reef geological driven pile foundation, comprising the following steps: S1, collecting undisturbed coral rock and soil in the field, performing indoor crushing test, and quantifying the particle crushing degree of the coral rock and soil into a relative crushing rate; S2, performing a pore pressure dissipation test in the field, and measuring the dissipation time of 50% of the excess pore pressure at the pile end; S4, calculating the crushing reduction coefficient and the pore pressure lag reduction coefficient; and S5, establishing an end bearing force calculation formula. The method combines the field sampling and the test results performed in the field, quantifies the particle crushing effect and the pore pressure lag effect of the coral rock and soil, explores the reduction coefficient relational expression according to a historical database, calculates the crushing reduction coefficient and the pore pressure lag reduction coefficient of the coral rock and soil in the field, and is coupled to the original specification formula of the end bearing force of the pile foundation, so that the end bearing force calculation result is corrected, the end bearing force calculation formula is more in line with the actual construction situation, and the method can effectively, reliably and accurately guide the design of the pile foundation construction in the field.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of marine geology engineering, geotechnical engineering and port engineering, and is especially suitable for pile foundation design and construction under the condition of coral reef geology, and particularly relates to a method for calculating the end bearing force of a driven pile foundation in coral reef geology. BACKGROUND

[0002] As a common stratum in island reef construction engineering, the coral reef stratum has three characteristics of high porosity, high internal friction and easy particle breakage. For medium and large structures with large loads, uneven settlement and other problems are prone to occur due to insufficient bearing strength of the coral reef stratum, which cannot meet the requirements of the foundation bearing stratum, and it is necessary to set a pile foundation for pile end bearing.

[0003] When driving a pile on a coral reef stratum, the bearing capacity of the pile bottom coral stratum is different from that of sand and clay strata in terms of the degree of exertion. The coral reef stratum is subjected to stress and particle breakage, which changes the compactness of the pile end, the effectiveness of stress transmission and the shear strength, thereby reducing the end bearing force. At the same time, the pore pressure hysteresis effect reduces the effective stress of the reef stratum, delays the exertion of the end bearing force, increases the compressibility of the coral rock and soil, increases the pile foundation settlement and affects the exertion of the end bearing force. Therefore, the end bearing force value calculated according to the sand end bearing force specification formula in the Port Engineering Pile Foundation Specification (JTS 167-2018) is generally 30% higher than the measured value. In addition, the cumulative error of the pile side friction resistance will make the single pile bearing capacity calculation result deviate from the actual construction situation;

[0004] In addition, according to the engineering geological classification, the coral rock and soil includes coral reef rock and coral sand, and the bearing capacity of the two is quite different. The coral rock has high strength, and its own strength needs to be considered when driving a pile. The end resistance of the coral sand is small, especially the loose coral sand layer with small strength, and its bearing capacity can be ignored.

[0005] The existing end bearing force calculation process does not consider the influence of coral reef soil particle breakage, pore pressure hysteresis effect and coral reef stone strength. Therefore, a method for calculating the end bearing force of a driven pile foundation in coral reef geology is needed to solve the above problems. SUMMARY

[0006] The purpose of the present application is to overcome the defects of the prior art and provide a method for calculating the end bearing force of a driven pile foundation in coral reef geology to solve the problems raised in the background.

[0007] In order to achieve the above purpose, the technical scheme of the present application is as follows:

[0008] A method for calculating the end bearing force of a driven pile foundation in coral reef geology, comprising the following contents and steps:

[0009] S1, collecting coral rock and soil at the construction site, simulating multi-level buried pressure to implement indoor pressure crushing test of coral rock and soil, and quantifying the particle crushing degree as relative crushing rate ;

[0010] S2, implementing hole pressure dissipation test on site to measure the dissipation time t of excess static hole pressure at the pile end under each buried depth 50 ;

[0011] S4, substituting the relative crushing rate into the crushing reduction formula to calculate the crushing reduction coefficient :

[0012] ;

[0013] substituting the dissipation time t into the hole pressure hysteresis reduction formula to calculate the hole pressure hysteresis reduction coefficient 50 :

[0014] :

[0015] wherein k is the reduction influence index, a is the upper limit value of reduction, and b is the hole pressure dissipation reduction rate

[0016] S5, coupling the crushing reduction coefficient and the hole pressure hysteresis reduction coefficient to establish a general end bearing force calculation formula for calculating the end bearing force Q of each stratum bi :

[0017] :

[0018] wherein A is the pile end cross-sectional area, N and N are bearing capacity coefficients related to the internal friction angle of soil in Meyerhof theory, p q r is the effective vertical stress at the pile end, is the saturated density of coral rock and soil, and D is the pile end cross-sectional diameter.

[0019] Further, when B r ≤0.35, the crushing reduction formula is first-order Taylor expanded to obtain a simplified crushing reduction formula: wherein c is the slope and d is the intercept.

[0020] Further, it further includes step S3: collecting on-site coral rock and soil, implementing saturated unconfined compressive strength test, determining the saturated unconfined compressive strength q of coral rock and soil u , and establishing a coral rock and soil bearing resistance formula: ​​​​

[0021] ;

[0022] wherein, is the bearing resistance, is the pile type correction coefficient, is the strength reduction coefficient of the pile end coral rock soil;

[0023] The influence of the increase of the bearing resistance of the coral rock soil in step S5 is established to calculate the end bearing force enhancement formula:

[0024] .

[0025] Further, when the coral rock soil is loose coral sand, or the saturated unconfined compressive strength q u of the coral rock soil is less than 1MPa, the strength reduction coefficient is 0, and the general end bearing force calculation formula is used to calculate the end bearing force.

[0026] Further, when the coral reef geology is coral reef rock, the sample for the saturated unconfined compressive strength test is a φ75mm*150mm undisturbed coral reef core, and when the saturated unconfined compressive strength q u of the undisturbed coral reef core is greater than or equal to 1MPa, the strength reduction coefficient is 0.4-0.6.

[0027] Further, the pore pressure dissipation test uses a buried micro pore pressure gauge, and the buried depth of the micro pore pressure gauge is 1D-2D below the pile end.

[0028] Further, when the relevant experiments or determinations of steps S1 and S2 cannot be implemented, the broken reduction coefficient and the pore pressure hysteresis reduction coefficient in step S5 are assigned default values, that is, , , and the calculation result of the end bearing force is checked by the test pile.

[0029] Compared with the prior art, the end bearing force calculation method of the coral reef geology driven pile foundation has the following beneficial effects:

[0030] 1. The particle crushing effect and the pore pressure hysteresis effect of the coral rock soil are quantified by combining the field sampling and the test results, to generate the relative broken rate and the pore pressure dissipation time t 50 , the relationship between - and -t 50 is explored according to the historical database, and the broken reduction coefficient and the pore pressure hysteresis reduction coefficient The original specification formula coupled to the end bearing force of the pile foundation is modified to correct the calculation result of the end bearing force, so that the calculation formula is more in line with the actual situation of the construction site, so as to effectively, reliably and accurately guide the on-site pile foundation construction design.

[0031] 2、The calculation method can complete core testing within 24 hours, the test workload is small, the efficiency is high, and the economy is good compared with the traditional static load test, and the error of the calculation result is controlled within 8% according to the on-site calibration data; in addition, the calculation method can be seamlessly connected with the current specification, and the cost of offshore exploration can be significantly reduced. BRIEF DESCRIPTION OF DRAWINGS

[0032] Figure 1 A flowchart of the pile foundation end bearing force calculation method disclosed in the present application is shown. DETAILED DESCRIPTION

[0033] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only the best embodiments of the present application, not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0034] In this document, referring to "embodiments" means that the specific methods, steps or contents described in combination with the embodiments can be included in at least one embodiment of the present application. The appearance of this phrase in various places in the specification does not necessarily mean the same embodiment, nor is it an independent or alternative embodiment to other embodiments. Those skilled in the art explicitly and implicitly understand that the embodiments described herein can be combined with other embodiments.

[0035] Embodiment one: the coral reef geology includes coral rock and coral sandy soil, and the present embodiment takes loose coral sandy soil as an example, as shown in Figure 1 The end bearing force calculation method of the driven pile foundation in the coral reef geology includes the following contents and steps:

[0036] S1, collect the undisturbed coral sandy soil at the construction site, the collection points should cover at least the test pile point and its pile circumference, keep the natural water content and natural gradation of the undisturbed coral sandy soil, determine the initial gradation, and use a high-pressure triaxial apparatus to implement a consolidated drained triaxial shear test and a model pile sinking test, the model pile diameter is φ75 mm, and the drop hammer weight is 1.5 t;

[0037] The process of the triaxial shear test is as follows: the undisturbed coral sandy soil is divided into at least 12 equal broken test samples with the same gradation, the multi-level buried pressure is simulated, the confining pressure ∈ , at least test the breakage rate of coral sand under confining pressures of 50 MPa, 100 MPa, 200 MPa and 400 MPa respectively, each group of confining pressure at least repeat test three times, to obtain the average degree of particle breakage of coral sand under the same confining pressure, with the relative breakage rate Quantitative expression is carried out:

[0038] (1)

[0039] In formula (1), is the particle content of the original coral sand below 0.075 mm in the original gradation, is the particle content of the original coral sand below 0.075 mm after triaxial shear test;

[0040] Obtain the relative breakage rate under different confining pressures 、 、 And ......;

[0041] Synchronous test of tip resistance and side resistance is carried out by using model pile, and the change of peak friction angle at each depth is measured;

[0042] S2, at least 10 groups of pore pressure dissipation tests are carried out at the design pile foundation point area, according to the depth of each test pile, the miniature pore pressure gauge is pre-buried at 1D~2D below the pile tip, combined with high strain test pile detection, the dissipation time t required for the excess static pore pressure at the tip to dissipate by 50% 50 ;

[0043] S3, saturated unconfined compressive strength test is carried out on the collected coral sand, for obvious loose coral sand, this step S3 can be omitted, it is considered that the bearing resistance of coral sand to pile tip is very small and can be ignored, and the saturated unconfined compressive strength of the coral sand does not need to be tested; for coral rock or coral sand with certain cementation strength, the saturated unconfined compressive strength q u of the coral sand needs to be determined by saturated unconfined compressive strength test; the saturated unconfined compressive strength test is carried out according to the standard of “GB / T 50266-2013 Engineering Rock Mass Test Method”; the sample for test is cemented original coral sand; when the saturated unconfined compressive strength q u of the coral sand is less than 1 MPa, the bearing resistance of the coral sand to the pile tip is ignored, i.e. q = 0, the end bearing force calculation formula is calculated according to formula (5), and the bearing resistance term is defaulted as 0;

[0044] S4, the broken reduction coefficient is calculated according to using the model pile test and triaxial shear test results data of step S1, the data fitting of broken reduction coefficient-relative breakage rate is carried out, and it is found that the peak friction angle With the power-law descending relationship of relative breakage rate, the breakage reduction formula of - is obtained:

[0045] = (2)

[0046] The breakage reduction formula is relatively complex. In order to reduce the calculation difficulty, the breakage reduction formula can be simplified by Taylor expansion. The approximate error of the expansion simplified formula is acceptable within the range of B r ≤0.35, so the first-order Taylor expansion is carried out within any point of B r ≤0.35, and the Taylor expansion simplified formula of - is obtained:

[0047] (3)

[0048] In formula (2), is the original peak friction angle of the original coral sand or the original coral reef rock, k is the reduction influence index, and the slope c and the intercept d in formula (3) are obtained according to the expansion calculation of formula (2);

[0049] According to the test results of step S2, the end resistance peak value linearly decreases with , and the pore pressure hysteresis reduction formula is obtained by statistical regression of the test data:

[0050] (4)

[0051] In formula (4), a is the upper limit value of reduction, b is the pore pressure dissipation reduction rate, when the time of 50% and above of pore pressure dissipation is equal to or less than 1 min, is assigned as 0, the maximum duration of the pore pressure dissipation test is 60 min, when the duration of 50% of pore pressure dissipation exceeds 60 min, is assigned as 2;

[0052] S5, the breakage reduction coefficient and the pore pressure hysteresis reduction coefficient are calculated in combination with the test data, the general end bearing force calculation formula is established, and the end bearing force Q bi of each stratum is calculated:

[0053] (5)

[0054] In formula (5), A p is the pile end section area, N q and N r are the bearing capacity coefficients related to the internal friction angle of soil in the Meyerhof theory, wherein, , , is the effective vertical stress at the pile tip, is the saturated unit weight of the coral rock soil, and D is the cross-sectional diameter of the pile tip.

[0055] Example Two: This example takes coral reef rock as an example. Since the strength of the coral reef rock is relatively large, it has a certain bearing capacity for the foundation pile, significantly increasing the end resistance. As shown in formula (2), the enhanced term for increasing the strength bearing is used to expand the end bearing force calculation formula of example one, so that it is suitable for the end bearing force calculation of the coral reef rock; wherein the reduction effect of the coral reef rock crushing and pore pressure lag on the end bearing force is consistent with the coral sand, and the calculation formula of the end bearing force of the coral reef rock is consistent with the test and calculation method of example one. This example will not be described again, and only the test and calculation of the enhanced term will be described. Figure 1

[0056] The undisturbed coral reef rock core sample is collected, the sample structure size is φ75mmx150mm, and the saturated unconfined compressive strength q u is determined according to the saturated unconfined compressive strength test described in step S3 of example one. u The calculation formula of the bearing resistance R

[0057] (6)

[0058] wherein, is the pile type correction coefficient, and the open pile takes 0.6-0.8, and the closed pile takes 1; is the strength reduction coefficient of the pile tip coral rock soil, q u <1MPa, is assigned as 0, and q u ≥1MPa, takes 0.4-0.6;

[0059] The end bearing force enhanced calculation formula is:

[0060] (7)

[0061] Application Example: According to the above method and steps, the end bearing force of a certain island is calculated:

[0062] According to the test data of a certain artificial island of a group of islands, the crushing reduction formula of - is obtained by regression as: When B r ∈[0,0.35], the simplified formula is obtained by first-order Taylor expansion as ; the pore pressure lag reduction formula is obtained by regression as: When the pore pressure dissipation time is equal to or less than 1min, the pore pressure lag reduction coefficient ​It is considered that the pore pressure has no reduction on the end bearing force;

[0063] The island can use the above-mentioned island database established by a group of islands - The broken reduction simplified formula and the pore pressure hysteresis reduction formula of the broken rock, the PHC pipe pile foundation, D = 1000mm, according to the steps S1 and S2 of example one, the relative broken rate B of a stratum of the island coral reef rock is determined r =0.2 and the pore pressure time t 50 =5min, respectively, into formula (2) and formula (3), the broken reduction coefficient of the island coral rock And the pore pressure hysteresis reduction coefficient According to step S3, the saturated unconfined compressive strength q u =8MPa, into formula (6) to calculate the bearing resistance of the coral rock, the original peak friction angle of the coral reef rock is =42°, into formula (7) to calculate the end bearing force of the stratum The static load test result of the stratum is 6.1MN, the error is 3.3%, which meets the engineering requirements.

[0064] Example three: refer to Figure 1 When the relevant experiments or determinations of steps S1 and S2 cannot be implemented, then implement step S6, the broken reduction coefficient and the pore pressure hysteresis reduction coefficient in step S5 are assigned by default, , , =0.5, the calculation result of the end bearing force is checked by the test pile, the end bearing force calculation formula is revised to ensure that the error of the end bearing force calculation result is within 8%;

[0065] Application example: according to the above method and step, the pile foundation construction design of a certain island in Maldives is carried out: because there is no triaxial equipment on site, the default value , ; Two test piles are supplemented in the construction stage to check the revised pile side friction resistance calculation value, and the error is controlled within 5.8% through subsequent static load test analysis, which meets the engineering requirements.

[0066] Those skilled in the art can clearly understand the above-mentioned embodiments of the present application can be realized by means of software or software combined with necessary general hardware platform, of course, can also be realized by hardware function, based on such understanding, the technical scheme of the present application essentially or the part of contribution to the prior art can be embodied in the form of software product, the software product is stored in a storage medium, includes several instructions to make a computer device, for example, includes but is not limited to personal computer, server, or network device etc., to execute all or part of the steps of the method described in any embodiment of the present application.

[0067] Although embodiments of the present application have been shown and described, it would be appreciated by those skilled in the art that changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the application, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for calculating the end bearing capacity of driven pile foundations in coral reef geology, characterized in that, Includes the following steps and content: S1. Collect coral rock soil from the construction site and conduct indoor pressure crushing tests on the coral rock soil to simulate multi-level burial pressure, quantifying the degree of particle crushing as a relative crushing rate. ; S2. Conduct on-site pore pressure dissipation tests to determine the time t required for 50% dissipation of excess pore pressure at the pile tip at each depth. 50 ; S4, the relative breakage rate Substitute into the crushing reduction formula to calculate the crushing reduction coefficient. : ； The pressure dissipation time t 50 Substitute the hysteresis reduction formula into the orifice pressure to calculate the orifice pressure hysteresis reduction coefficient. : ； Where k is the reduction effect index, a is the upper limit of reduction, and b is the pore pressure dissipation reduction rate; S5, Coupled with the aforementioned breakage reduction coefficient and the reduction coefficient after hysteresis of the orifice Establish a system for calculating the end bearing capacity Q of each stratum. bi General formula for calculating end bearing capacity: ； Among them, A p N is the cross-sectional area of ​​the pile tip. q and N r This is the bearing capacity coefficient related to the internal friction angle of soil in Meyerhof theory. For the effective vertical stress at the pile tip, denoted as saturated unit weight of the coral rock soil, and D as the diameter of the pile end section.

2. The method for calculating the end bearing capacity of a coral reef geological driven pile foundation according to claim 1, characterized in that: When B r For values ​​≤0.35, perform a first-order Taylor expansion on the aforementioned breakage reduction formula to obtain the simplified breakage reduction formula: , where c is the slope and d is the intercept.

3. The method for calculating the end bearing capacity of a coral reef geological driven pile foundation according to claim 1, characterized in that: The method also includes step S3: collecting the coral rock soil from the site, conducting a saturated unconfined compressive strength test, and determining the saturated unconfined compressive strength q of the coral rock soil. u Establish the formula for the bearing resistance of coral rock soil: ； in, To withstand resistance, This is the pile type correction factor. This is the strength reduction factor for the coral rock soil at the pile tip; In step S5, the influence of the bearing resistance of coral rock soil is added, and a calculation formula for end bearing capacity enhancement is established: 。 4. The method for calculating the end bearing capacity of a coral reef geological driven pile foundation according to claim 3, characterized in that: The coral rock soil is loose coral sand, or the saturated unconfined compressive strength q of the coral rock soil is... u When <1MPa, the strength reduction factor If the value is 0, the end bearing force is calculated using the general end bearing force calculation formula.

5. The method for calculating the end bearing capacity of a coral reef geological driven pile foundation according to claim 3, characterized in that: The coral reef geology is coral reef rock. The specimens for the saturated unconfined compressive strength test are undisturbed coral reef cores with a diameter of φ75mm × 150mm. The saturated unconfined compressive strength q of the undisturbed coral reef core is... u When ≥1MPa, the strength reduction factor It ranges from 0.4 to 0.

6.

6. The method for calculating the end bearing capacity of a driven pile foundation for coral reef geological conditions according to any one of claims 1 to 5, characterized in that: The pore pressure dissipation test uses an embedded micro pore pressure gauge, which is buried at a depth of 1D to 2D directly below the pile end.

7. The method for calculating the end bearing capacity of a coral reef geological driven pile foundation according to claim 6, characterized in that: When the relevant experiments or measurements in steps S1 and S2 cannot be performed, the breakage reduction coefficient and the pore pressure hysteresis reduction coefficient in step S5 are assigned default values, i.e. , The calculation results of the end bearing capacity are then verified by pile testing.

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