A displacement observation device and analysis method for self-balanced static load test of foundation piles

Abstract

The present invention relates to a displacement observation device and an analysis method for self-balanced static load test of foundation piles, belonging to the technical field of self-balanced static load test. It includes a load cell, a pressurizing mechanism, a displacement detector and a data acquisition instrument. The load cell includes an upper connecting plate, a lower connecting plate and a hydraulic cylinder. The two ends of the hydraulic cylinder are respectively fixedly connected to the upper connecting plate and the lower connecting plate. The displacement detector is installed in the load cell and detects the distance between the upper connecting plate and the lower connecting plate. The pressurizing mechanism is located on the ground and communicates with the hydraulic cylinder. The data acquisition instrument is electrically connected to the displacement detector and the pressurizing mechanism. The data acquisition instrument draws a load-displacement curve by corresponding the detection results of the displacement detector with the loading force of the pressurizing mechanism one by one. The present invention solves many deficiencies of the existing data analysis methods for self-balanced tests; improves the displacement observation system, does not require the setting of reference piles and foundation pile beams, and improves the adaptability of self-balanced tests to the site and environment.

Description

Technical Field

[0001] The present invention belongs to the technical field of self-balanced static load tests, and particularly relates to a displacement observation device and an analysis method for self-balanced static load tests of foundation piles. Background Art

[0002] The existing methods for observing the displacement of the load cell in self-balanced static load tests are usually the methods of embedding displacement pipes or displacement wires. Then, by measuring the upward displacement of the upper pile section and the downward displacement of the lower pile section, the load-displacement curves of the upper and lower pile sections are obtained, and the ultimate compressive bearing capacity of the pile is calculated. Since the self-balanced pile testing method can greatly reduce the reaction force or does not require a reaction platform, it has great advantages in the static load tests of foundation piles in areas with high bearing capacity or limited site conditions.

[0003] The existing methods for observing the displacement of the load cell in self-balanced static load tests are usually the methods of embedding displacement pipes or displacement wires. Then, by measuring the upward displacement of the upper pile section and the downward displacement of the lower pile section, the load-displacement curves of the upper and lower pile sections are obtained, and the ultimate compressive bearing capacity of the pile is calculated. Since the self-balanced pile testing method can greatly reduce the reaction force or does not require a reaction platform, it has great advantages in the static load tests of foundation piles in areas with high bearing capacity or limited site conditions.

[0004] However, there are some deficiencies in the current data analysis and processing of self-balanced static load tests. The theoretical basis of the equal displacement method and the equal load method is insufficient; both the simplified method and the precise method are based on assumed conditions that do not conform to the actual situation, and some of the assumed conditions are not reasonable; when the downward displacement is less than the upward displacement, the equivalent displacement method cannot obtain a complete load-displacement curve and cannot judge the ultimate bearing capacity; the precise method needs to carry out calculations based on a large number of test and monitoring data arranged in the pile body, and the cost of data acquisition and the calculation requirements are high; the numerical analysis method or the analytical algorithm is also based on a large number of assumed conditions, test data and geotechnical parameters, and the precise results need to be inverted by the conventional static load test piles under the same test conditions, and the implementation cost and difficulty are very high; while the conventional method (the method of adding the bearing capacities of the upper and lower pile sections) completely separates the upper and lower pile sections, independently analyzes the bearing capacity and then adds them, and cannot obtain the load-displacement curve of the whole pile, and cannot consider the upper and lower pile sections as a whole for overall consideration, which is sometimes obviously unreasonable and the obtained results are on the small side.

[0005] In addition to the unreasonableness of some basic assumed conditions, the properties of the soil and rock mass around the pile are often uneven (mainly due to different types of soil), and the properties of the same type of soil and rock mass often also vary greatly (the discreteness of the soil). The distribution of the side resistance of the pile is related to factors such as the composition of the soil layer around the pile, the magnitude of the load, and the pile length. Even if there is only one layer of soil around the pile, it will not be evenly distributed. These assumed conditions often deviate from the actual situation in different ways, and the analysis results generally have large limitations and cannot be widely applied.

[0006] In addition, the displacements of the upper and lower segments of the self-balanced test pile, i.e., the displacements of the upper and lower connecting plates of the load cell, are easily affected by general wind forces, temperature, slight vibrations, and even when the reference pile and the pile foundation beam are slightly touched. As a result, the displacement values of the upper and lower connecting plates of the load cell are further affected, which in turn affects the data obtained from the test, leading to relatively large errors in the data.

[0007] Therefore, there is an urgent need to establish a method for determining the load-displacement curve of the entire pile based on self-balanced test pile data that is theoretically more perfect. Summary of the Invention

[0008] To solve the above problems existing in the prior art, the present invention provides a displacement observation device and its analysis method for self-balanced static load test of pile foundations, which solves the problem of a relatively high probability of errors in the data of the ultimate compressive bearing capacity of pile foundations obtained from existing self-balanced tests. At the same time, many deficiencies in the analysis of existing self-balanced test pile data are improved, and the reference pile and pile foundation beam for ground displacement observation are cancelled. On the one hand, it improves the adaptability of the self-balanced static load test to the site and environment, and on the other hand, it may significantly reduce the construction period and cost.

[0009] The object of the present invention can be achieved through the following technical solutions: A displacement observation device for self-balanced static load test of pile foundations includes a load cell, a pressurizing mechanism, a displacement detector, and a data acquisition instrument. The load cell includes an upper connecting plate, a lower connecting plate, and a hydraulic cylinder. The two ends of the hydraulic cylinder are respectively fixedly connected to the upper connecting plate and the lower connecting plate. The upper connecting plate and the lower connecting plate are respectively fixedly connected to the upper segment pile and the lower segment pile. The displacement detector is installed inside the load cell and detects the distance between the upper connecting plate and the lower connecting plate. The pressurizing mechanism is located on the ground and is connected to the hydraulic cylinder. The pressurizing mechanism pressurizes the hydraulic cylinder to make the hydraulic cylinder push the upper segment pile and the lower segment pile to move. The data acquisition instrument is electrically connected to the displacement detector and the pressurizing mechanism. The data acquisition instrument plots the load-displacement curve by corresponding the detection results of the displacement detector with the loading force of the pressurizing mechanism one by one.

[0010] As a preferred technical solution of the present invention, the displacement detector includes a laser rangefinder, a guiding tube, a sliding tube, and a detection plate. The guiding tube is installed on the top surface of the lower connecting plate. The sliding tube is installed on the bottom surface of the upper connecting plate. The sliding tube is located inside the guiding tube and is in sliding fit with the guiding tube. Both the guiding tube and the sliding tube are parallel to the central axis of the upper connecting plate and the lower connecting plate. The detection plate is embedded in the top surface of the sliding tube. The laser rangefinder is installed on the inner bottom surface of the guiding tube and measures the position change of the detection plate.

[0011] As a preferred technical solution of the present invention, the pressurizing mechanism includes an oil pump and a high-pressure oil pipe. The oil pump is connected to the hydraulic cylinder through the high-pressure oil pipe. The length of the end of the high-pressure oil pipe connected to the hydraulic cylinder is telescopic.

[0012] As a preferred technical solution of the present invention, at least two hydraulic cylinders are provided, and the hydraulic cylinders are distributed in an annular array along the center of the upper connecting plate.

[0013] As a preferred technical solution of the present invention, both the upper connecting plate and the lower connecting plate are annular plates, and a plurality of through grooves and a plurality of reinforcing ribs are provided on both the upper connecting plate and the lower connecting plate.

[0014] As a preferred technical solution of the present invention, two through holes are further provided on the top surface of the upper connecting plate, and two grouting pipes are respectively arranged in the two through holes.

[0015] Based on the above displacement observation device for self-balanced static load test of foundation piles, the present invention also provides a displacement analysis method for self-balanced static load test of foundation piles, including the following steps:

[0016] S1: Fix and weld the upper connecting plate and the steel reinforcement cage of the upper pile section, and weld reinforcing bars between the upper connecting plate and the upper steel reinforcement cage; fix and weld the lower connecting plate and the steel reinforcement cage of the lower pile section, and weld reinforcing bars between the lower connecting plate and the lower steel reinforcement cage;

[0017] S2: Connect the pressurizing mechanism with the hydraulic cylinder;

[0018] S3: Lower the entire steel reinforcement cage to the designated position;

[0019] S4: Lower the conduit, and the conduit passes through the central hole between the upper connecting plate and the lower connecting plate and reaches the lower part of the steel reinforcement cage, and pour concrete to form a pile;

[0020] S5: Pressurize through the pressurizing mechanism, inject oil into the hydraulic cylinder to make it elongate, so that the upper connecting plate moves upward, driving the upper pile to move upward, and the lower connecting plate moves downward, driving the lower pile section to move downward;

[0021] S6: The displacement detector will detect the displacement amount during the movement of the upper connecting plate and the lower connecting plate and transmit the detection result to the data acquisition instrument;

[0022] S7: The data acquisition instrument will draw a load-displacement curve by corresponding the detection result of the displacement detector and the loading force of the pressurizing mechanism one by one;

[0023] S8: Inject cement slurry or grouting material upward between the upper connecting plate and the lower connecting plate through the grouting pipe to play a role in firm connection.

[0024] As a preferred technical solution of the present invention, in step S5, the pressurizing mechanism pressurizes step by step, and each stage loading value is 1 / 10 of the maximum loading value.

[0025] As a preferred technical solution of the present invention, in step S8, pressure relief is carried out in stages, and the pressure relief value for each stage is 2 times the graded load during loading, and equal unloading is carried out step by step.

[0026] As a preferred technical solution of the present invention, in step S6, after each stage of load is applied, the detection value of the displacement detector measures the displacement change at the 5th minute, 15th minute, 30th minute, 45th minute and 60th minute, and then detects the displacement change every 30 minutes.

[0027] The beneficial effects of the present invention are as follows: By correspondingly plotting the load-displacement curve of the displacement change between the upper and lower connecting plates in the load cell and the loading value of the pressurizing mechanism, the ultimate loading compressive bearing capacity data of the foundation pile can be obtained, avoiding the influence of the environment on the foundation pile during the loading process, resulting in changes in the displacement values of the upper and lower piles, so that the displacement value of a certain section of the pile reaches the limit value while the displacement value of the other section of the pile does not reach the displacement value, resulting in the situation that the ultimate loading compressive bearing capacity data of the foundation pile obtained at this time does not conform to the actual situation. It solves the problem that the error probability of the ultimate compressive bearing capacity data of the foundation pile obtained by the existing self-balanced test is relatively large. In addition, it also solves many problems such as insufficient theoretical basis, unreasonable assumption conditions, incomplete curve, and lack of unified analysis of the upper and lower piles in the existing analysis method. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] For the convenience of those skilled in the art to understand, the present invention will be further described below with reference to the accompanying drawings.

[0029] Figure 1 It is a schematic structural diagram of the displacement observation device of the present invention;

[0030] Figure 2 It is a schematic structural diagram of the load cell of the present invention;

[0031] Figure 3 It is a schematic structural diagram of the displacement detector of the present invention;

[0032] Figure 4 It is a flow chart of the observation method of the present invention;

[0033] Figure 5 It is a schematic diagram of a typical self-balanced static load test curve and the principle of the total displacement method;

[0034] Figure 6 It is a schematic diagram of the relationship principle between a typical self-balanced static load test curve and an equivalent load-total displacement method curve;

[0035] DESCRIPTION OF MAIN ELEMENT SYMBOLS

[0036] In the figure: 1. Load cell; 11. Upper connecting plate; 12. Lower connecting plate; 13. Hydraulic cylinder; 2. Pressurizing mechanism; 21. Oil pump; 22. High-pressure oil pipe; 3. Displacement detector; 31. Laser rangefinder; 32. Guide pipe; 33. Sliding pipe; 34. Detection plate; 4. Data acquisition instrument. Detailed implementation manners

[0037] To further elaborate on the technical means and effects adopted by the present invention to achieve the predetermined invention purpose, the following, in conjunction with the accompanying drawings and preferred embodiments, details the specific implementation manners, structures, features and their effects according to the present invention as follows.

[0038] When detecting the ultimate bearing capacity of a foundation pile by the self-balanced method, usually the displacements of the upper and lower sections of the pile are detected, and then after reaching the ultimate point, the ultimate bearing capacity of the foundation pile is calculated according to the loading value at this point, or when the load cell reaches the loading limit and the upper and lower sections of the pile have not reached the ultimate point at this time, the maximum loading value is taken. In theory, when the upper and lower sections of the pile reach the ultimate point simultaneously, the data obtained at this time is the ultimate bearing capacity of the foundation pile. However, the properties of the rock and soil around the foundation pile are often uneven (mainly due to different types of soil), and the properties of the same type of rock and soil often also vary greatly (the discreteness of the soil). The distribution of the pile side resistance is related to factors such as the composition of the soil layer around the pile, the magnitude of the load, and the pile length. Even if there is only one layer of soil around the pile, it will not be evenly distributed. Therefore, it is impossible to accurately judge the placement position of the load cell, resulting in a large deviation value of the ultimate bearing capacity of the foundation obtained during the actual test process.

[0039] Please refer to Figure 5 - Figure 6 , points A and B are respectively the ultimate load points of the lower and upper sections of the pile, and the corresponding loads are 、 , and the corresponding displacements are ( ), ( ). At the load , the lower section of the pile reaches the limit, the upper section of the pile does not reach the limit, and the whole pile does not reach the limit; at the load , the upper section of the pile reaches the limit, the lower section of the pile exceeds the limit, and the whole pile exceeds the limit; obviously, there is a load between 、 corresponding to the limit of the whole pile. When the lower end plate of the load cell displaces downward by , the lower section of the pile reaches the limit, and when the upper end plate of the load cell displaces upward by , the upper section of the pile reaches the limit. Therefore, when the total displacement of the upper and lower end plates (i.e., the relative displacement of the upper and lower end plates) = , the bearing capacities of the upper and lower sections of the pile both reach the limit, that is, the load The total displacement of the corresponding upper and lower end plates (the length of CD in the figure) is . Considering the continuity of the curve and the monotonicity of the displacement varying with the load, the point exists and is unique. When the upper pile reaches the limit earlier than the lower pile or they reach the limit simultaneously, the situation is similar.

[0040] In the above description, no specific requirements are made for the displacements of points A and B. Points A and B can be arbitrarily selected. That is, for any points A and B on the Q-s curves of the lower pile and the upper pile, the corresponding loads are 、 , the corresponding displacements are 、 . At the load , , the total displacement of the upper pile and the lower pile is less than or equal to ; at the load , the total displacement of the upper pile and the lower pile is greater than or equal to . Obviously, there is a load 、 between (or = ) corresponding to which the total displacement of the upper pile and the lower pile is equal to . Thus, the total displacement of the upper pile and the lower pile (i.e., the relative displacement between the upper pile and the lower pile) = corresponds to the load 、 which is the equivalent load.

[0041] From the above analysis, the relationship between the equivalent load and the corresponding total displacement for the load value is as follows:

[0042] (1)

[0043] (2)

[0044] (3)

[0045] (4)

[0046] where λ is the uplift coefficient of the soil on the side of the upper pile and is the weight of the upper pile. For any load and its corresponding displacements of the upper pile and the lower pile, formulas (1) to (4) are all valid.

[0047] After one side of the upper and lower (side) piles of the load cell reaches the ultimate load , the other side has not reached the limit yet. As the load and displacement increase, until the whole pile reaches the ultimate load At this time, the displacement on the side that reaches the limit first increases by ; as the load and displacement continue to increase, the other pile also reaches the limit , and at this time, the displacement of this side is also increased compared to the displacement when the entire pile reaches the ultimate load . It can also be understood from the perspective of displacement balance or displacement compensation. When the entire pile reaches the ultimate load, the loading value of the load cell is between the ultimate loads of the upper and lower sections of the pile. At this time, the displacements of the upper and lower sections of the pile are both different by Δs compared to the displacements when they reach their respective ultimate loads, except that one side is larger and the other side is smaller. The curves determined by formulas (1) and (4) are the equivalent load - total displacement curves. Taking the average value of the displacements in formula (4), it is the equivalent load - average displacement curve, which is equivalent to the load - displacement curve of the traditional static load test. When determining the ultimate bearing capacity according to the equivalent load - total displacement curve, the displacement limit value is the sum of the displacement limit values of the upper section of the pile and the lower section of the pile; when determining the ultimate bearing capacity according to the equivalent load - average displacement curve, the displacement limit value is half of the former.

[0048] Please refer to Figure 6 , where Curve 1 is: load - upper displacement curve; Curve 2 is: load - lower displacement curve; Curve 3 is: load - total displacement curve (the corresponding displacements of Curves 1 and 2 are added); Curve 4 is: equivalent load - total displacement curve (the displacements are the same as 3, and the loads are added after conversion).

[0049] In order to better obtain the ultimate bearing capacity of the foundation pile, the following is a case analysis of the actual construction process:

[0050] For a cast - in - place pile in a certain project with a pile diameter of 1000 mm, the pile length above the load cell is 42 m, the self - weight is 808 kN, and the uplift coefficient of the upper section of the pile is λ. The test loading and related displacements are shown in Table 2.1 - 1.

[0051] Table 2.1 - 1 Test Loading and Related Displacements

[0052]

[0053] For the value of the ultimate bearing capacity in each specification, generally, there are two cases according to the displacement: 40 mm and 0.05d. In the following Tables 2.1 - 2 and 2.1 - 3, Q´us and Qux are respectively taken according to these two cases, and the total displacement rule is taken as 80 mm and 0.10d accordingly. In order to obtain more data for comparison, the uplift coefficient is extended to the entire possible range (0.6 - 1.0).

[0054] Table 2.1 - 2 Results of the Summation Method of the Ultimate Loads of the Upper and Lower Sections of the Pile and the Total Displacement Method (s = 40 mm, 80 mm)

[0055] Note: Qus is the uplift force of the upper pile segment, Q´us is the converted compressive bearing capacity of Qus, and Qz is the bearing capacity determined by the total displacement method. The same applies hereinafter.

[0056] Table 2.1-3 Results of the summation method of the ultimate loads of the upper and lower pile segments and the total displacement method (s = 50mm, 100mm)

[0057]

[0058] As can be seen from Tables 2.1-1 to 2.1-3, when the total displacement reaches the sum of the control displacements of the upper and lower pile segments (when the ultimate control displacement is reached on one side of the pile while not reached on the other side, the total displacement reaches the sum of the control displacements on both sides, or the control displacements are reached on both sides of the pile), the bearing capacity obtained by the total displacement method is very consistent with the summation of the ultimate loads of the upper and lower pile segments.

[0059] A steel pipe pile is used for the foundation of a certain offshore wind turbine. The pile diameter is 1.70m, the wall thickness of the steel pipe pile is 25mm - 30mm, the length of the pile above the load cell is 72m, the self-weight is 880kN, and the load cell is 2m away from the pile bottom. The uplift coefficient of the upper pile segment is λ, and the test loading and related displacements are shown in Table 2.2-1.

[0060] Table 2.2-1 Test Loading and Displacements

[0061]

[0062] Table 2.2-2 Results of the summation method of the ultimate loads of the upper and lower pile segments and the total displacement method (s = 40mm, 80mm)

[0063]

[0064] Table 2.2-3 Results of the summation method of the ultimate loads of the upper and lower pile segments and the total displacement method (s = 85mm, 170mm)

[0065]

[0066] As can be seen from Tables 2.2-1 to 2.2-3, when the total displacement does not reach the sum of the control displacements of the upper and lower pile segments (when the ultimate control displacement is reached on one side of the pile while not reached on the other side, the total displacement does not reach the sum of the control displacements on both sides, or the control displacements are not reached on both sides of the pile), the bearing capacity obtained by the total displacement method is very consistent with the summation of the ultimate loads of the upper and lower pile segments.

[0067] Therefore, by detecting the sum of the control displacements of the upper and lower pile segments to obtain the ultimate bearing capacity of the foundation pile is consistent with the data obtained by separately detecting the displacements of the upper and lower pile segments. Therefore, the total displacement of the upper and lower pile segments can be used to complete the data calculation, and when using the total displacement for calculation, the situation of deviation in the ultimate bearing capacity of the foundation pile can be reduced, as shown in the following case:

[0068] For a certain engineering pile foundation, the pile diameter is 1.4 m, the pile length above the load cell is 8.0 m, and the self-weight is 307.7 kN. The uplift coefficient of the upper pile segment is λ = 0.9. The test loading and related displacements are shown in Table 3.1-1.

[0069] Table 3.1-1 Test Loading and Displacements

[0070]

[0071] When the downward displacement drops steeply, the upward displacement is relatively small, and the skin friction on the pile side is far from being fully exerted, which does not conform to the conventional static load test. This is caused by the characteristics of the self-balanced method, and it is unreasonable to simply take values according to the existing specifications directly. From the perspective of the force of a conventional pile and the static load test, when a load is applied to the pile top, the skin friction on the upper pile segment is exerted first, and then the skin friction and end resistance on the lower pile segment are gradually exerted. When the bearing capacity of the lower pile segment is insufficient and the displacement increases, the skin friction on the upper pile segment will be exerted, which will support the lower pile segment and slow down the increase in the displacement of the lower pile segment. This situation of mutual support and joint bearing is basically consistent with the principle of displacement compensation between the upper and lower pile segments of the total displacement method, and is more reasonable than considering the upper and lower pile segments separately. The steep drop in this case can be considered a "pseudo-steep drop".

[0072] Therefore, under the action of 2240 kN, the increment of the downward displacement is 5.2 times that under the previous level of load, and it is judged as a steep drop in displacement. Qux = 1960 kN, Qus = 2800 kN, Q´us = 2769.2 kN. The ultimate load obtained by adding the ultimate loads of the upper and lower pile segments is 4729.2 kN, and this ultimate load is obviously not the true ultimate load of the foundation pile. According to the total displacement method, under the action of a load of 2240 kN, the displacement increment generated is 3.9 times that under the previous level of load, and it cannot be judged as a steep drop. The converted ultimate load is 5569.2 kN, which is 18% higher than the former. The total displacement is 75.98 mm, slightly less than the limit displacement of 80 mm. If the loading continues, there may still be a little room for the ultimate load to increase; if the total displacement is controlled at 140 mm (2×0.05d), there may be a large room for the bearing capacity to increase.

[0073] For a certain cast-in-place pile foundation, the pile diameter is 0.8 m, the pile length above the load cell is 6.5 m, and the self-weight is 81.5 kN. The uplift coefficient of the upper pile segment is λ = 0.9. The test loading and related displacements are shown in Table 3.2-1.

[0074] Table 3.2-1 Test Loading and Displacements

[0075]

[0076] As can be seen from the above table, the displacement of one side of the pile is very large, while that of the other side is very small. The bearing capacity of the lower section of the pile is taken as 2420.0 kN, and the compressive bearing capacity of the upper section of the pile is taken as 2088.0 kN. The ultimate bearing capacity of the upper and lower sections of the pile added together is 4508.0 kN. The ultimate bearing capacity obtained by the total displacement method (s = 80 mm) is 4885.4 kN, which can reasonably increase by 8% compared with the former.

[0077] Therefore, through the study of the typical displacement curves of the self-balanced static load test and in combination with the test principle of the self-balanced method, the converted load-displacement curves, namely the load-total displacement method curve and the load-average displacement curve, are deduced.

[0078] The results obtained by the total displacement method are very consistent with those obtained by the method of adding the ultimate loads of the upper and lower sections of the pile. In special cases such as the appearance of "pseudo-steep drop" or extremely large displacement differences between the upper and lower sections of the pile, the total displacement method can reasonably correct or compensate for the results of the method of adding the ultimate loads of the upper and lower sections of the pile.

[0079] The total displacement method is based on the measured data of the self-balanced test pile, without making assumptions about models and parameters, and without the need to use numerical methods such as data fitting for analysis and processing. It is simple, applicable, and the results are more reasonable and accurate.

[0080] The self-displacement method is a type of total displacement method, characterized by not requiring an external displacement observation reference system. They all have the characteristics of being less affected by external environmental factors such as wind, temperature changes, and vibrations. When the existing displacement observation reference system observes the foundation pile on the sea surface, due to the large wind and waves on the sea surface, the support columns of the displacement observation reference system will continuously sway under the action of external forces, resulting in the reference plane of the action system not being able to maintain balance, resulting in relatively large errors in the displacement data of the observed foundation pile, thereby affecting the judgment of the bearing capacity of the foundation pile. By measuring the relative displacement change inside the foundation pile to calculate the bearing capacity of the foundation pile, the influence of external factors can be greatly reduced.

[0081] Therefore, please refer to Figure 1 - Figure 4, this embodiment provides a displacement observation device for self-balanced static load test of foundation piles, including a load cell 1, a pressurizing mechanism 2, a displacement detector 3 and a data acquisition instrument 4. The load cell 1 includes an upper connecting plate 11, a lower connecting plate 12 and a hydraulic cylinder 13. Both ends of the hydraulic cylinder 13 are fixedly connected to the upper connecting plate 11 and the lower connecting plate 12 respectively. The upper connecting plate 11 and the lower connecting plate 12 are respectively fixedly connected to the upper section pile and the lower section pile. The displacement detector 3 is installed in the load cell 1 to detect the distance between the upper connecting plate 11 and the lower connecting plate 12. The pressurizing mechanism 2 is located on the ground and communicated with the hydraulic cylinder 13. The pressurizing mechanism 2 pressurizes the hydraulic cylinder 13 to make the hydraulic cylinder 13 push the upper section pile and the lower section pile to move. The data acquisition instrument 4 is electrically connected to the displacement detector 3 and the pressurizing mechanism 2. The data acquisition instrument 4 draws a load-displacement curve by corresponding the detection results of the displacement detector 3 with the loading force of the pressurizing mechanism 2 one by one.

[0082] By arranging the displacement detector 3 in the load cell 1 to detect the displacement of the upper connecting plate 11 and the lower connecting plate 12, the displacement of the upper section pile and the lower section pile during the self-balanced static load test can be obtained to calculate the ultimate bearing capacity of the foundation pile. At the same time, since the displacement detector 3 is located in the load cell 1, the displacement detection of the upper and lower section piles will not be affected by the environment and cause detection errors.

[0083] In order to enable the displacement detector 3 to be free from external interference and affect the detection effect during the detection process, in this embodiment, the displacement detector 3 includes a laser rangefinder 31, a guiding tube 32, a sliding tube 33 and a detection plate 34. The guiding tube 32 is installed on the top surface of the lower connecting plate 12. The sliding tube 33 is installed on the bottom surface of the upper connecting plate 11. The sliding tube 33 is located in the guiding tube 32 and is in sliding fit with the guiding tube 32. Both the guiding tube 32 and the sliding tube 33 are parallel to the central axes of the upper connecting plate 11 and the lower connecting plate 12. The detection plate 34 is embedded in the top surface of the sliding tube 33. The laser rangefinder 31 is installed on the inner bottom surface of the guiding tube 32 to measure the position change of the detection plate 34. When the hydraulic cylinder 13 pushes the upper connecting plate 11 and the lower connecting plate 12 to move, the sliding tube 33 will slide in the guiding tube 32, and the laser rangefinder 31 located in the guiding tube 32 detects the displacement of the sliding tube 33, so as to obtain the total displacement variable of the load cell 1 during the pressurization process, and then calculate the ultimate bearing capacity of the foundation pile according to the pressurization value of the pressurizing mechanism 2. At the same time, during the pouring of the foundation pile, the entire load cell 1 will be covered by concrete. Therefore, in order to protect the detection result of the displacement detector 3 from interference, the laser rangefinder 31 is protected by arranging the guiding tube 32 and the sliding tube 33, so as to avoid detection errors caused by the occlusion of concrete during the distance detection.

[0084] In order to successfully complete the loading process of the pressurizing mechanism 2, in one embodiment, the pressurizing mechanism 2 includes an oil pump 21 and a high-pressure oil pipe 22. The oil pump 21 is connected to the hydraulic cylinder 13 through the high-pressure oil pipe 22. One end of the high-pressure oil pipe 22 connected to the hydraulic cylinder 13 has a telescopic length. Since the upper and lower piles will separate during the loading process, in order to avoid the situation where the high-pressure oil pipe 22 is stretched due to the change in the position of the load cell 1 relative to the ground during continuous pressurization, resulting in damage to the oil pipe and oil leakage, which may lead to insufficient loading force of the load cell 1 and affect the calculation of the ultimate bearing capacity of the foundation pile.

[0085] In order to better apply forces to the upper and lower piles during the loading process of the load cell 1 and avoid the situation where the direction of the acting force is not in the same direction as the central axis of the foundation pile, resulting in the loading force not being fully applied to the foundation pile and causing calculation errors in the ultimate bearing capacity of the foundation pile. In one embodiment, at least two hydraulic cylinders 13 are provided. The multiple hydraulic cylinders 13 are distributed in a circular array along the center of the upper connecting plate 11, so that the hydraulic cylinders 13 are evenly distributed on the annular surface of the upper connecting plate 11. When the oil pump 21 pressurizes, the multiple hydraulic cylinders 13 can be lifted synchronously, and then the upper and lower piles can be in force balance when pushed by the upper connecting plate 11 and the lower connecting plate 12, thereby avoiding the situation where the displacement trajectory deviates due to the offset of the force application point when the upper and lower piles are displaced.

[0086] In order to reduce the additional influence of the load cell 1 on the upper and lower piles during the displacement process during the test, in one embodiment, both the upper connecting plate 11 and the lower connecting plate 12 are annular plates. A number of through grooves and a number of reinforcing ribs are provided on both the upper connecting plate 11 and the lower connecting plate 12. By providing through grooves on both the upper connecting plate 11 and the lower connecting plate 12, the weight of the load cell 1 is reduced. At the same time, in order to avoid the strength problem of the upper connecting plate 11 and the lower connecting plate 12 when reducing the weight of the load cell 1, a number of reinforcing ribs are provided to improve the structural strength of the upper connecting plate 11 and the lower connecting plate 12 to avoid the situation where the load cell 1 breaks during the loading process.

[0087] During the loading process of the load cell 1, the foundation pile will separate at the position of the load cell 1. Therefore, in order to reduce the use cost, in one embodiment, two through holes are further provided on the top surface of the upper connecting plate 11. Two grouting pipes are respectively provided in the two through holes. After the test is completed, cement slurry or grouting material that meets the design strength requirements and has good fluidity is injected into the load cell 1 through the grouting pipes, which plays a role in filling and firmly connecting, so that the test pile can also be used as an engineering pile after the test is completed, reducing the use cost.

[0088] Based on the above-mentioned displacement observation device for self-balanced static load test of foundation piles, a method for analyzing the displacement of self-balanced static load test of foundation piles is also proposed, including the following steps:

[0089] S1: Fix and weld the upper connecting plate 11 and the steel reinforcement cage of the upper pile section, and weld the reinforcing bars between the upper connecting plate 11 and the upper steel reinforcement cage; fix and weld the lower connecting plate 12 and the steel reinforcement cage of the lower upper pile section, and weld the reinforcing bars between the lower connecting plate 12 and the lower steel reinforcement cage, so that when the load cell 1 is loaded, the entire upper pile section or lower pile section will not fail to move due to insufficient strength of the steel reinforcement cage connected to the upper connecting plate 11 and the lower connecting plate 12, resulting in the failure of the test;

[0090] S2: Connect the high-pressure oil pipe 22 in the pressurizing mechanism 2 to the hydraulic cylinder 13 to facilitate subsequent pressurization of the hydraulic cylinder 13 during the test to push the upper and lower pile sections to move;

[0091] S3: Lower the entire steel reinforcement cage to the designated position;

[0092] S4: Lower the conduit, which passes through the central hole between the upper connecting plate 11 and the lower connecting plate 12 and reaches the lower part of the steel reinforcement cage, and pour concrete to form a pile;

[0093] S5: Pressurize through the pressurizing mechanism 2, inject oil into the hydraulic cylinder 13 to make it extend, so that the upper connecting plate 11 moves upward, driving the upper pile to move upward, and the lower connecting plate 12 moves downward, driving the lower pile section to move downward;

[0094] S6: The displacement detector 3 will detect the displacement during the movement of the upper connecting plate 11 and the lower connecting plate 12 and transmit the detection results to the data acquisition instrument 4;

[0095] S7: The data acquisition instrument 4 will draw the load-displacement curve by corresponding the detection results of the displacement detector 3 and the loading force of the pressurizing mechanism 2 one by one. When the data acquisition instrument 4 finishes collecting the detection results of the displacement detector 3 and the pressurizing value of the pressurizing mechanism 2, it will correspond according to the pressurizing value during each pressurization and the relative displacement between the upper connecting plate 11 and the lower connecting plate 12 detected by the displacement detector 3, and draw the load-displacement curve to better judge the loading value corresponding to the ultimate bearing capacity node of the cast-in-place pile and calculate the ultimate bearing capacity of the cast-in-place pile;

[0096] S8: Inject cement slurry or grouting material between the upper connecting plate 11 and the lower connecting plate 12 from the grouting pipe to play a role in fastening connection, so that the test pile can also be used as an engineering pile after the test is completed, reducing the use cost.

[0097] During the test, both the upper pile segment and the lower pile segment will undergo displacement. During the displacement process, in order to prevent the loading force of the pressurizing mechanism 2 from being too large, which may cause the displacement of the upper pile segment or the lower pile segment to be too large, resulting in misjudgment when determining whether the ultimate point has been reached. Therefore, in one embodiment, in step S5, the pressurizing mechanism 2 is pressurized step by step, and each level of loading value is 1 / 10 of the maximum loading value. Let the pressurizing mechanism 2 be pressurized step by step, so that the upper pile segment or the lower pile segment can adapt to the applied pressure provided by the pressurizing mechanism 2 during the test, avoiding unrealistic displacement during the movement process, and thus avoiding the situation where the tester mistakenly believes that the ultimate loading node of the foundation pile has been reached due to large displacement changes, resulting in errors in calculating the bearing value of the ultimate bearing capacity of the foundation pile.

[0098] After the test is completed, since the upper pile segment will gradually exceed its original casting position during the test and is also subjected to a large load, the pile body will undergo certain deformation. Therefore, in order to reduce the stress and deformation of the test pile and thus reduce the risk of its failure, in one embodiment, in step S8, the pressure relief is carried out in stages, and each level of pressure relief value is 2 times the graded load during loading, and the load is unloaded equally in each stage. By gradually reducing the load size, relieving pressure in stages, and unloading equally, the stress and deformation of the test pile can be reduced, thereby reducing the risk of its failure.

[0099] During the test, the loading force of the pressurizing mechanism 2 will gradually increase, and the increasing loading force will also cause the resistance of the soil layer around the foundation pile to continuously increase. However, due to the delay of the self-weight friction of the soil layer itself, this will cause a delay in the displacement of the upper and lower pile segments after the loading is completed. Therefore, in order to avoid errors in the detection results of the displacement detector 3, in one embodiment, in step S6, the detection value of the displacement detector 3 measures the displacement change amount at the 5th minute, 15th minute, 30th minute, 45th minute, and 60th minute after each level of load is applied, and then detects the displacement change amount every 30 minutes. During the loading process, since the subsequent loading force will increase successively, and due to the different resistances that the soil layer where the foundation pile is located can provide, when the loading force becomes larger and exceeds the resistance provided by the soil layer, a large displacement amount will be generated at this time. At the same time, due to the delay of the resistance provided by the soil layer, the displacement detector 3 can be allowed to extend a period of time after each loading before detecting, so as to ensure the detection accuracy.

[0100] Therefore, it can be seen from the above test analysis that:

[0101] (1)The total displacement curve of the equivalent load deduced in this paper proposes the total displacement method based on the self-balanced method of pile testing; this method can overcome the influences of wind force, temperature changes (including sunlight irradiation), wave impact, ground vibration, and human touch, and also has good adaptability to the site, environment, etc.

[0102] (2)This paper proposes a self-displacement method that does not require external reference piles and reference beams, which greatly simplifies the displacement observation of the self-balanced static load test and will significantly save the project duration and cost (such as for pile testing at sea, the construction of reference piles can be avoided);

[0103] (3)This paper uses a large number of on-site cases for verification and analysis, confirming the credibility and superiority of this method. The test results of the upper and lower sections of the pile can be comprehensively analyzed, and obvious unreasonable results that occur when analyzing the upper and lower sections of the pile during the "pseudo-steep drop" can be avoided.

[0104] (4)This method can avoid the deficiencies of many existing analysis methods, can obtain a complete load-displacement curve, does not require a large number of conditional assumptions, nor a large number of parameter assumptions and conditional inversions. The principle is simple, the applicability is strong, and the application is convenient.

[0105] The above are only the preferred embodiments of the present invention and do not impose any form of limitation on the present invention. Although the present invention has been disclosed as above with preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or refinements into equivalent embodiments with equivalent changes within the scope of the technical solution of the present invention. However, as long as it does not depart from the content of the technical solution of the present invention, any simple modification, equivalent change, and refinement made to the above embodiments based on the technical essence of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A displacement observation method for self-balanced static load test of foundation piles, characterized in that: It includes a load cell, a pressurizing mechanism, a displacement detector and a data acquisition instrument. The load cell includes an upper connecting plate, a lower connecting plate and a hydraulic cylinder. The two ends of the hydraulic cylinder are respectively fixedly connected to the upper connecting plate and the lower connecting plate. The upper connecting plate and the lower connecting plate are respectively fixedly connected to the upper pile section and the lower pile section. The displacement detector is installed in the load cell and detects the distance between the upper connecting plate and the lower connecting plate. The pressurizing mechanism is located on the ground and is connected to the hydraulic cylinder. The pressurizing mechanism pressurizes the hydraulic cylinder to make the hydraulic cylinder push the upper pile section and the lower pile section to move. The data acquisition instrument is electrically connected to the displacement detector and the pressurizing mechanism. The data acquisition instrument draws a load-displacement curve by corresponding the detection result of the displacement detector with the loading force of the pressurizing mechanism one by one; The displacement detector includes a laser rangefinder, a guiding tube, a sliding tube and a detection plate. The guiding tube is installed on the top surface of the lower connecting plate. The sliding tube is installed on the bottom surface of the upper connecting plate. The sliding tube is located inside the guiding tube and is in sliding fit with the guiding tube. Both the guiding tube and the sliding tube are parallel to the central axes of the upper connecting plate and the lower connecting plate. The detection plate is embedded in the top surface of the sliding tube. The laser rangefinder is installed on the inner bottom surface of the guiding tube and measures the position change of the detection plate; The load-displacement curve is obtained by processing and converting the load-total displacement curve and the load-average displacement curve by the total displacement method; S1: Fix and weld the steel cage of the upper connecting plate and the upper pile section, and weld reinforcing bars between the upper connecting plate and the upper steel cage; fix and weld the steel cage of the lower connecting plate and the lower pile section, and weld reinforcing bars between the lower connecting plate and the lower steel cage; S2: Connect the pressurizing mechanism to the hydraulic cylinder; S3: Lower the entire steel cage to the designated position; S4: Lower the conduit. The conduit passes through the central hole between the upper connecting plate and the lower connecting plate and reaches the lower part of the steel cage, and pour concrete to form a pile; S5: Pressurize through the pressurizing mechanism, inject oil into the hydraulic cylinder to make it extend, so that the upper connecting plate moves upward, driving the upper pile to move upward, and the lower connecting plate moves downward, driving the lower pile section to move downward; S6: The displacement detector will detect the displacement amount during the movement of the upper connecting plate and the lower connecting plate and transmit the detection result to the data acquisition instrument; S7: The data acquisition instrument draws a load-displacement curve by corresponding the detection result of the displacement detector with the loading force of the pressurizing mechanism one by one; S8: Inject cement slurry or grouting material between the upper connecting plate and the lower connecting plate from the grouting pipe to play a role in firm connection.

2. The displacement observation method for self-balanced static load test of foundation piles according to claim 1, characterized in that: The pressurizing mechanism includes an oil pump and a high-pressure oil pipe. The oil pump is connected to the hydraulic cylinder through the high-pressure oil pipe. The length of the end of the high-pressure oil pipe connected to the hydraulic cylinder is telescopic.

3. A displacement observation method for self-balanced static load test of foundation piles according to claim 1, characterized in that: At least two hydraulic cylinders are provided, and the hydraulic cylinders are distributed in a circular array along the center of the upper connecting plate.

4. A displacement observation method for self-balanced static load test of foundation piles according to claim 1, characterized in that: Both the upper connecting plate and the lower connecting plate are annular plates, and a number of through grooves and a number of reinforcing ribs are provided on both the upper connecting plate and the lower connecting plate.

5. A displacement observation method for self-balanced static load test of foundation piles according to claim 4, characterized in that: Two through holes are further provided on the top surface of the upper connecting plate, and two grouting pipes are respectively arranged in the two through holes.

6. A displacement observation method for self-balanced static load test of foundation piles according to claim 1, characterized in that: In step S5, the pressurizing mechanism pressurizes step by step, and each stage loading value is 1 / 10 of the maximum loading value.

7. A displacement observation method for self-balanced static load test of foundation piles according to claim 1, characterized in that: In step S8, pressure relief is carried out in stages, with the pressure relief value for each stage being twice the graded load during loading, and unloading is carried out in equal amounts step by step.

8. A displacement observation method for self-balanced static load test of foundation piles according to claim 1, characterized in that: In step S6, after each stage of load is applied, the detection value of the displacement detector measures the displacement change at the 5th minute, 15th minute, 30th minute, 45th minute, and 60th minute, and then detects the displacement change every 30 minutes.

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