Method for calculating end bearing force of coral reef geological driven pile foundation

Abstract

The invention discloses a method for calculating the end bearing force of a coral reef geology driven pile foundation, which comprises the following steps of: S1, acquiring original coral rock soil on site, implementing an indoor crushing test, and quantifying the particle crushing degree of the coral rock soil into a relative crushing rate; s2, a pore pressure dissipation test is carried out on site, and the pressure dissipation time of 50% dissipation of the ultra-static pore pressure at the pile tip is measured; s4, calculating a crushing reduction coefficient and a pore pressure lag reduction coefficient; and S5, establishing an end bearing force calculation formula. The calculation method combines test results of field sampling and field implementation, quantifies a particle crushing effect and a pore pressure lag effect of coral rock soil, explores a reduction coefficient relational expression according to a historical database, calculates a crushing reduction coefficient and a pore pressure lag reduction coefficient of the field coral rock soil, couples the coefficients to an original standard formula of pile foundation end bearing force, and calculates the pile foundation end bearing force according to the original standard formula of the pile foundation end bearing force. And the end bearing force calculation result is corrected, so that the end bearing force calculation formula better conforms to the actual construction condition, and the field pile foundation construction design is effectively, reliably and accurately guided.

Description

Technical Field

[0001] This invention relates to the fields of marine geological engineering, geotechnical engineering, and port engineering, and is particularly applicable to the design and construction of pile foundations under coral reef geological conditions. Specifically, it refers to a method for calculating the end bearing capacity of driven pile foundations in coral reef geological conditions. Background Technology

[0002] Coral reef strata are a common type of strata in island and reef construction projects. They have three main characteristics: high porosity, high internal friction, and easily broken particles. For medium and large-sized structures with large loads, the coral reef strata are prone to uneven settlement and other problems due to insufficient bearing capacity. They cannot meet the requirements as a foundation bearing layer and require pile foundations for pile end bearing.

[0003] When driving piles into coral reef strata, the bearing capacity of the coral reef strata at the pile bottom differs from that of sand and clay strata. Under stress, the coral reef rocks undergo particle breakage, altering the pile tip density, stress transfer effectiveness, and shear strength, thus reducing the end bearing capacity. Simultaneously, the pore pressure hysteresis effect reduces the effective stress in the reef strata, delaying the development of end bearing capacity, increasing the compressibility of the coral rock and soil, and increasing pile foundation settlement, further affecting the development of end bearing capacity. Therefore, the end bearing capacity values ​​calculated according to the sand end bearing capacity formula in the "Port Engineering Pile Foundation Code" (JTS 167-2018) are generally more than 30% higher than the measured values. Adding the errors from pile side friction and other factors, the calculated single pile bearing capacity will deviate significantly from the actual construction conditions.

[0004] In addition, according to engineering geological classification, coral soil includes coral reef rock and coral sand. The bearing capacity of the two is very different. Coral rock has high strength, and its strength must be considered during construction to enhance the bearing capacity. Coral sand has low pile end resistance, especially loose coral sand layers with low strength, whose bearing capacity can be ignored.

[0005] Existing technologies for calculating end bearing capacity do not consider the effects of coral reef soil particle breakage, pore pressure hysteresis, and coral reef rock strength. Therefore, there is an urgent need for a method to calculate the end bearing capacity of driven pile foundations in coral reef geology to solve the above problems. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method for calculating the end bearing capacity of driven pile foundations in coral reef geology, so as to solve the problems mentioned in the background art.

[0007] To achieve the above objectives, the technical solution of the present invention is as follows:

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

[0009] 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. ;

[0010] 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 ;

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

[0012] ;

[0013] Dissipation time t 50 Substitute the hysteresis reduction formula into the orifice pressure to calculate the orifice pressure hysteresis reduction coefficient. :

[0014] ;

[0015] Where k is the reduction effect index, a is the upper limit of the reduction, and b is the pore pressure dissipation reduction rate;

[0016] S5, Coupling Breakage Reduction Coefficient and the reduction coefficient after hysteresis Establish a system for calculating the end bearing capacity Q of each stratum. bi General formula for calculating end bearing capacity:

[0017] ;

[0018] 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 coral rock soil, and D as diameter of pile tip section.

[0019] Furthermore, when B r For values ​​≤0.35, perform a first-order Taylor expansion of the breakage reduction formula to obtain the simplified breakage reduction formula: , where c is the slope and d is the intercept.

[0020] Furthermore, it also includes step S3: collecting coral reef soil from the site, conducting a saturated unconfined compressive strength test, and determining the saturated unconfined compressive strength q of the coral reef soil. u Establish the formula for the bearing resistance of coral rock soil:

[0021] ;

[0022] 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;

[0023] 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:

[0024] .

[0025] Furthermore, the coral soil is loose coral sand, or the saturated unconfined compressive strength q of the coral soil. u Strength reduction factor when <1MPa If the value is 0, the end bearing capacity is calculated using the general end bearing capacity calculation formula.

[0026] Furthermore, the coral reef geology is coral reef rock, and 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 the strength is ≥1MPa, its strength reduction factor It ranges from 0.4 to 0.6.

[0027] Furthermore, 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.

[0028] Furthermore, 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 were verified by pile testing.

[0029] Compared with the prior art, the method for calculating the end bearing capacity of a driven pile foundation in coral reef geology according to the present invention has the following beneficial effects:

[0030] 1. Combining the results of field sampling and field experiments, the particle fragmentation effect and pore pressure hysteresis effect of coral rock soil are quantified to generate a relative fragmentation rate. and pressure dissipation time t 50 Based on historical database research - and -t 50 The relationship is used to calculate the fragmentation reduction factor of the coral reef soil in the field. and the reduction coefficient after hysteresis The original standard formula for pile foundation end bearing capacity is coupled to modify the calculation results of end bearing capacity, 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. This calculation method can complete the core test within 24 hours, with a small workload. Compared with the traditional static load test, it is more efficient and economical. On-site verification data shows that the error of the calculation results is controlled within 8%. In addition, this calculation method can be seamlessly integrated with the current specifications, which can significantly reduce the cost of offshore exploration. Attached Figure Description

[0032] Figure 1 This is a flowchart of the method for calculating the end bearing capacity of pile foundations disclosed in this invention. Detailed Implementation

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

[0034] The term "embodiment" as used herein means that a particular method, step, or content described in connection with an embodiment may be included in at least one embodiment of the invention. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a mutually exclusive, independent, or alternative embodiment. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0035] Example 1: Coral reef geology includes coral rock and coral sand. This example uses loose coral sand as an example. Figure 1 As shown, the calculation method for the end bearing capacity of driven pile foundations in coral reef geology includes the following contents and steps:

[0036] S1. Collect undisturbed coral sand from the construction site. The collection points should at least cover the test pile points and their surrounding area, maintaining the natural moisture content and natural gradation of the undisturbed coral sand. Determine its initial gradation and conduct consolidated drained triaxial shear tests and model pile driving tests using a high-pressure triaxial apparatus. The model pile diameter is φ75 mm, and the drop hammer weight is 1.5 t.

[0037] The triaxial shear test process involves dividing undisturbed coral sand into at least 12 equal-volume fragmentation test samples of the same gradation, simulating multi-stage burial pressure, and testing the confining pressure ∈ The breakage rate of coral sand under confining pressures of at least 50 MPa, 100 MPa, 200 MPa, and 400 MPa was tested. Each confining pressure was repeated at least three times. The average particle breakage degree of the coral sand under the same confining pressure was calculated, and the relative breakage rate was used as the basis for the determination. Quantification representation:

[0038] (1)

[0039] In equation (1), This refers to the content of unprocessed coral sand particles smaller than 0.075mm in the original gradation. The content of undisturbed coral sand particles smaller than 0.075 mm after triaxial shear test;

[0040] Obtain the relative fracture rate under different confining pressures , , and ......;

[0041] Simultaneous end-side resistance and side resistance tests were conducted using model piles to measure the changes in peak friction angles at various depth levels.

[0042] S2. Conduct at least 10 sets of pore pressure dissipation tests in the designed pile foundation area. Based on the depth of each test pile, pre-embed micro-pore pressure gauges 1D~2D directly below the pile tip. Combined with high-strain pile testing, determine the pressure dissipation time t required for 50% dissipation of excess pore pressure at the pile tip. 50 ;

[0043] S3. Collect coral sand and conduct a saturated unconfined compressive strength test. For obviously loose coral sand, step S3 can be omitted, as the bearing resistance of the coral sand to the pile end is considered to be very small and negligible, and no test is needed to determine its saturated unconfined compressive strength. For coral rock or coral sand with a certain cementation strength, it is necessary to determine its saturated unconfined compressive strength q through a saturated unconfined compressive strength test. u The saturated unconfined compressive strength test was performed according to GB / T 50266-2013 Standard for Test Methods of Engineering Rock Mass, and the test specimens were cemented undisturbed coral sand; when the saturated unconfined compressive strength q of the coral sand... u When the pressure is <1MPa, the bearing resistance of the coral sand to the pile tip is ignored, that is, let The end bearing capacity is 0, and the end bearing capacity is calculated according to formula (5). The bearing resistance term is defaulted to 0.

[0044] S4. Using the model pile test and triaxial shear test results data from step S1, according to... Calculate the crushing reduction factor, and fit the data to the crushing reduction factor minus the relative crushing rate to find the peak friction angle. The relationship between the relative breakage rate and the power-law decreasing relationship is obtained. - The formula for reducing breakage:

[0045] = (2)

[0046] The breakage reduction formula is quite complex. To reduce the computational difficulty, it can be simplified using a Taylor expansion. The approximation error of the simplified expansion is within B. r ≤0.35 is acceptable, therefore, in B r Performing a first-order Taylor expansion at any point ≤0.35 yields the following result: - The simplified Taylor expansion is:

[0047] (3)

[0048] In equation (2), The original peak friction angle is the original coral sand or original coral reef rock, and k is the reduction effect index. The slope c and intercept d in equation (3) are obtained by expanding equation (2).

[0049] Based on the experimental results of step S2, it was found that the peak terminal resistance increases with... By linearly decreasing the pressure and performing statistical regression on the experimental data, the hysteresis reduction formula for pore pressure is obtained:

[0050] (4)

[0051] In equation (4), a is the upper limit of the reduction, and b is the pore pressure dissipation reduction rate. When the time for pore pressure dissipation of 50% or more is equal to or less than 1 minute, The value is set to 0, and the maximum duration of the pore pressure dissipation test is 60 minutes. When the time for 50% pore pressure dissipation exceeds 60 minutes, The value is assigned to 2;

[0052] S5. Calculate the breakage reduction coefficient based on the test data. and the reduction coefficient after hysteresis Establish a general formula for calculating end bearing capacity, and calculate the end bearing capacity Q of each stratum. bi :

[0053] (5)

[0054] In equation (5), 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, where... , , For the effective vertical stress at the pile tip, denoted as saturated unit weight of coral rock soil, and D as diameter of pile tip section.

[0055] Example 2: This example uses coral reef rock. Due to its high strength, coral reef rock has a certain bearing capacity for the foundation piles, significantly increasing its end resistance. Figure 1 As shown, the strengthening term for increasing strength and bearing capacity extends the end-bearing capacity calculation formula of Embodiment 1 to make it suitable for calculating the end-bearing capacity of coral reef rock. The reduction effect of coral reef rock fracturing and pore pressure hysteresis on the end-bearing capacity is consistent with that of coral sand. The basic terms in the calculation formula for the end-bearing capacity of coral reef rock are consistent with the experimental and calculation methods of Embodiment 1, and will not be repeated in this embodiment. Only the experimental and calculation methods for the strengthening term are described:

[0056] Unconfined coral reef core samples were collected, with sample dimensions of φ75mm×150mm. The saturated unconfined compressive strength q was determined according to the saturated unconfined compressive strength test described in step S3 of Example 1. u Establish the bearing resistance R of coral rock soil u Calculation formula:

[0057] (6)

[0058] in, The correction factor for pile type is 0.6 to 0.8 for open piles and 1 for closed piles. q is the strength reduction factor for coral rock soil at the pile tip. u <1MPa, Assign a value of 0 to q u When ≥1MPa, Take a value of 0.4 to 0.6;

[0059] The formula for calculating end bearing capacity enhancement is:

[0060] (7)

[0061] Application example: Perform end bearing capacity calculations for a certain island / reef according to the methods and steps described above:

[0062] Data from an experiment on artificial islands in an archipelago was obtained through regression analysis. - The formula for reducing the breakage rate is: When B r When ∈[0,0.35], a first-order Taylor expansion yields the simplified form as follows: The regression yields the hysteresis reduction formula for pore pressure: When the pore pressure dissipation time is equal to or less than 1 minute, the pore pressure hysteresis reduction coefficient It is believed that the bearing capacity at the end of the hole is basically not reduced;

[0063] The island reef can be established using the database of the aforementioned archipelago. - The simplified formula for fracture reduction and the formula for pore pressure hysteresis reduction are used. PHC pipe pile foundations are employed, with D=1000mm. Based on steps S1 and S2 of Example 1, the relative fracture rate B of a certain stratum of the coral reef rock on this island reef is determined. r =0.2 and pressure dissipation time t 50 =5min, substitute into equations (2) and (3) respectively, and calculate the fragmentation reduction factor of the coral rock of the island reef. and the reduction coefficient after hysteresis Determine the saturated unconfined compressive strength q according to step S3. u =8MPa, substituting into equation (6) to calculate the bearing resistance of the coral reef, the original peak friction angle of the coral reef is =42°, substitute into formula (7) to calculate the end bearing capacity of the stratum. The static load test result of this stratum is 6.1MN with an error of 3.3%, which meets the engineering requirements.

[0064] Example 3: Refer again Figure 1 If the relevant experiments or measurements in steps S1 and S2 cannot be performed, then step S6 is performed, and default values ​​are assigned to the breakage reduction coefficient and the pore pressure hysteresis reduction coefficient in step S5. , , =0.5, the calculation results of the end bearing capacity are checked by pile testing, the end bearing capacity calculation formula is corrected, and the error of the end bearing capacity calculation results is guaranteed to be within 8%.

[0065] Application Example: Following the methods and steps described above, a pile foundation construction design was carried out on an island in the Maldives. Since a three-axis measuring instrument was unavailable on site, default values ​​were used. , During the construction phase, two additional test piles were used for verification. The calculated value of the corrected pile side friction was analyzed by subsequent static load tests, and the error was controlled within 5.8%, which met the project requirements.

[0066] Through the above description of the embodiments, those skilled in the art can clearly understand that the various embodiments of this application can be implemented by means of software or software combined with necessary general-purpose hardware platforms, and of course, they can also be implemented by hardware functions. Based on this understanding, the technical solution of this application, in essence or the part that contributes to the prior art, can be embodied in the form of a software product. The software product is stored in a storage medium and includes several instructions to cause a computer device, such as including but not limited to a personal computer, server, or network device, to execute all or part of the steps of the method described in any embodiment of this application.

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

Claims

1. A method of calculating the end bearing force of a coral reef geologic driven pile foundation, characterized by, Comprise the following steps and content: S1, collecting the coral rock soil at the construction site, simulating a multi-level buried depth pressure to implement an indoor pressure crushing test on the coral rock soil, and quantifying the particle crushing degree as a relative crushing rate ; S2, field implementation of pore pressure dissipation test, to determine the level of buried depth under the pile end where the excess pore pressure dissipation of 50% of the dissipation pressure time t 50 ; S4, the relative breakage rate Substitute the breakage reduction formula to calculate the breakage reduction coefficient : ； The pressure relief time t 50 Substitute the hole pressure lag reduction formula to calculate the hole pressure lag reduction coefficient : ； Wherein, k is the reduction influence index, a is the reduction upper limit value, b is the pore pressure dissipation reduction rate; S5, coupling the crushing reduction factor and the pore pressure lag reduction factor , to establish the general end bearing force calculation formula of each level of stratum end bearing force Q bi : ； wherein A p is the pile tip cross-sectional area, N q and N r is the bearing capacity coefficient related to the internal friction angle of the soil in the Meyerhof theory, is the effective vertical stress at the pile tip, is the saturated unit weight of the coral soil, and D is the pile tip cross-sectional diameter.

2. The method of calculating the end-bearing capacity of a coral reef geologic driven pile foundation according to claim 1, wherein: 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 of calculating the end-bearing capacity of a coral reef geologic driven pile foundation according to claim 1, wherein: Further comprising a step S3 of collecting the coral soil on site, performing a saturated unconfined compressive strength test, and determining the saturated unconfined compressive strength q of the coral soil u , and establishing a formula for the bearing resistance of the coral soil ； wherein, is the bearing resistance, is the pile type correction factor, is the strength reduction factor of the pile end coral rock soil; The influence of increasing the bearing resistance of coral soil in step S5 is established to calculate the end bearing force enhancement formula: 。 4. The method of calculating the end-bearing capacity of a coral reef geologic driven pile foundation according to claim 3, wherein: said coral soil is loose coral sandy soil, or said saturated unconfined compressive strength q of said coral soil u said strength reduction factor is 0 when the strength is less than 1 MPa said end bearing force is calculated by using said general end bearing force calculation formula.

5. The method of calculating the end-bearing capacity of a coral reef geologic driven pile foundation according to claim 3, wherein: The coral reef geology is coral reef rock, the sample of the saturated unconfined compressive strength test adopts a φ75mmx150mm undisturbed coral reef core, the saturated unconfined compressive strength q of the undisturbed coral reef core u ≥1MPa, the strength reduction coefficient is 0.4~0.

6.

6. The method of calculating the end-bearing capacity of a reef geologic driven pile foundation according to any one of claims 1 to 5, characterized in that: The pore pressure dissipation test adopts a buried micro pore pressure gauge, and the embedding depth of the micro pore pressure gauge is 1D~2D below the pile end.

7. The method of calculating the end-bearing capacity of a coral reef geologic driven pile foundation according to claim 6, wherein: When the relevant experiments or determinations of steps S1 and S2 cannot be implemented, the broken reduction coefficient and the pore pressure lag reduction coefficient in step S5 are assigned default values, that is , , and the calculation result of the end bearing force is checked by trial pile.

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