High-rise pile cap equipment under horizontal load and centrifuge test method for foundation bearing characteristics thereof

Abstract

Disclosed in the present invention are high-rise pile cap equipment under a horizontal load and a centrifuge test method for foundation bearing characteristics thereof. The equipment comprises a model box, a sand layer, a model pile cap, at least eight model piles, a horizontal loading device, at least two laser displacement sensors, an axial force sensor, and a plurality of strain gauges. The method comprises: S1, performing one-way horizontal loading on high-rise pile cap equipment under a horizontal load to obtain an ultimate foundation bearing capacity situation, a model pile body axial force distribution situation, a pile foundation axial force comparison situation, a model pile body bending moment distribution situation, and a model pile body bending moment comparison situation of the equipment; and S2, performing one-way variable amplitude cyclic loading on the high-rise pile cap equipment under the horizontal load to obtain a foundation displacement situation, an effect of the number of cycles on foundation stiffness, an effect of a cyclic load amplitude on the foundation stiffness, and a foundation inclination situation. In this way, the foundation design of the high-rise pile cap can be optimized, and a basis and a reference for evaluating safety of foundation long-term service are provided.

Description

High-pile cap equipment under horizontal load and centrifuge test method for bearing characteristics of foundation thereof TECHNICAL FIELD

[0001] The present application relates to the technical field of centrifuge test, in particular to high-pile cap equipment under horizontal load and centrifuge test method for bearing characteristics of foundation thereof.

[0002] BACKGROUND

[0003] The high-pile cap foundation of offshore wind turbine is derived from the high-pile wharf structure of port engineering, and has the advantages of high bearing capacity and small settlement, and is widely used in offshore wind farms in southeast China. However, unlike the high-pile wharf structure which needs to bear a large amount of vertical load such as cranes and vehicles for a long time, the high-pile bearing foundation of offshore wind power is frequently subjected to horizontal cyclic loads such as wind, wave and current, and its bearing mechanism is more complex. On the one hand, under the combined action of low-frequency and multi-directional load, the load transfer mechanism of high-pile cap foundation is not clear; on the other hand, under the action of long-term load, the pile-soil interaction weakens the stiffness of the foundation, and the evolution law of its bearing capacity and dynamic response characteristics is still unclear. Therefore, it is of great significance to master the nonlinear response law of pile-soil under different load combinations for the long-term safe service of high-pile cap.

[0004] Compared with single pile foundation, the research on high-pile cap foundation of offshore wind turbine is relatively less. The existing research mainly focuses on the bearing characteristics of pile group, the evolution law of foundation bearing capacity under horizontal cyclic load, the stress and deformation law of high-pile cap structure, etc. In the prior art, it is proposed by using finite element software ABAQUS that the vertical bearing capacity, horizontal bearing capacity and bending resistance of pile-platform composite foundation increase exponentially with the increase of the diameter of the additional platform, and it is proved that the bearing performance of pile-platform composite structure is better than that of single pile foundation. Or, based on Navier-Stokes equation and fluid volume method, the wave load at the cap is calculated, and the results show that the cap structure has a significant influence on the wave force of pile foundation, and the traditional morrison equation will underestimate the horizontal action of high-pile cap. Or, by using SESAM software, the stress and deformation characteristics of high-pile are studied, and it is proposed that a 2m pile diameter has safety and economy. Or, based on ABAQUS and Openfast, a numerical analysis method of wind turbine foundation considering pile-soil interaction is developed, and the pile-soil interaction significantly reduces the lateral and windward natural frequency of the wind turbine. Due to the dissipation effect of soil on pile foundation, the internal force of pile foundation at the mud surface elevation is reduced. It is convenient to consider the influence of different working conditions by using numerical simulation method to study the dynamic and bearing characteristics of offshore wind power high-pile cap foundation, but the numerical calculation precision and efficiency are difficult to meet at the same time.

[0005] There are also prior art based on field measured data on high pile cap foundation to carry out horizontal and vertical bearing capacity analysis, through dynamic test to obtain displacement time history curve, comparative analysis of single pile and multi-pile cap foundation dynamics law, it is proved that the stiffness of multi-pile cap foundation is greater than that of single pile foundation. Or have analyzed the wind turbine detection data of high pile cap in the past year, found that the yaw, typhoon and ship collision lead to strong vibration of the structure, long-term strong vibration will lead to fatigue damage of the foundation and tower. Or have systematically studied the influence law of cyclic load on the lateral bearing capacity of large diameter single pile foundation under 1g condition, analyzed the nonlinear characteristics of single pile foundation rate effect, hysteresis behavior and cumulative displacement under cyclic load, found that the loading amplitude and frequency have significant influence on the evolution of foundation stiffness, which provides research ideas and reference for high pile cap foundation. Or have compared the horizontal, eccentric and torsional bearing capacity of inclined pile and straight pile group pile foundation, and obtained the conclusion that the stiffness of inclined pile group pile foundation is greater and the bearing capacity is higher. Or have carried out bearing capacity characteristics test of open and closed pile under monotonic and cyclic load, obtained the rules that the bearing capacity of closed pile is higher than that of open pile under cyclic load, and the bearing capacity of dense sand body foundation decreases with the decrease of cycle number. Or have studied the lateral loading mechanism of multi-pile cap foundation by numerical simulation and centrifuge test, and proposed that the inclined pile provides additional axial force to resist the bending moment generated by horizontal load, and the size of additional axial force is related to the stiffness of upper structure, pile distribution and wind direction. The above research shows that high pile cap foundation has the advantages of strong resistance to horizontal load and large overall stiffness. However, the research on the weakening law of soil under long-term cyclic load is insufficient, and the research on the bearing capacity evolution law of foundation under horizontal cyclic load is insufficient. The cycle number used in the research on the bearing characteristics of long-term cyclic foundation is less, mostly hundreds of times, and the research on the stiffness and bearing capacity evolution of high pile cap foundation under high amplitude and large cycle number is lacking.

[0006] SUMMARY

[0007] To achieve the above purpose, one of the technical solutions adopted by the present application is: a high pile cap equipment under horizontal load, which is loaded on the bearing of a soil centrifuge to complete the centrifuge test of the foundation bearing characteristics of the model pile under horizontal load, the equipment comprises:

[0008] a model box 1;

[0009] a sand layer 2 located in the model box 1;

[0010] a model cap 3;

[0011] at least eight model piles 4 arranged symmetrically along the circumference of the model cap 3;

[0012] a horizontal loading device 5 for loading the model pile 4 with horizontal load;

[0013] At least two laser displacement sensors 6 are used to measure the lateral and vertical movement displacement of the model pile 4;

[0014] An axial force sensor 7 is used to measure the axial force of the model pile 4.

[0015] A plurality of strain gauges 8 are arranged along the length direction of the model pile 4.

[0016] Each model pile 4 is inserted into the sand soil layer 2.

[0017] Further, each model pile 4 is first fixed by strong glue after being embedded into the model pile cap 3, and then welded and reinforced at the connection between the model pile 4 and the model pile cap 3, so as to have sufficient rigidity at the connection between the model pile and the model pile cap 3.

[0018] The material of the model pile 4 is aluminum alloy, with a diameter of 20 mm, a wall thickness of 7 mm, a density of 2.75 g / cm3, a tensile strength of 280 MPa, and an elastic modulus of 72 GPa.

[0019] The distance between the model pile 4 and the outer side of the model pile cap 3 is 5.4 mm, the inclination angle is 11.3°, and the distance between the bottom surface of the model pile cap 3 and the bottom surface of the model pile 4 is 848 mm.

[0020] Further, the overall height of the sand soil layer 2 is 700 mm, and the sand soil layer 2 is prepared by the sand rain method.

[0021] Further, the horizontal loading device 5 is located at one half of the height of the model pile cap 3, and the distance from the bottom surface of the model pile cap 3 is 26 mm.

[0022] One of the technical solutions adopted by the present application is a centrifuge test method for bearing characteristics of a high-pile pile cap foundation under horizontal load, which is applicable to the high-pile pile cap equipment under horizontal load described above, and the method comprises:

[0023] S1, unidirectional horizontal loading is performed on the high-pile pile cap equipment under horizontal load to obtain the ultimate bearing capacity of the foundation, the axial force distribution of the model pile, the axial force comparison of the pile foundation, the bending moment distribution of the model pile, and the bending moment comparison of the model pile.

[0024] S2, unidirectional variable-amplitude cyclic loading is performed on the high-pile pile cap equipment under horizontal load to obtain the displacement of the foundation, the influence of the number of cycles on the stiffness of the foundation, the influence of the amplitude of the cyclic load on the stiffness of the foundation, and the inclination angle of the foundation.

[0025] Further, in S1, the unidirectional horizontal loading is performed by controlling the end displacement of the horizontal loading device 5, and the loading rate is 0.06 mm / s, and the sampling frequency is 10 Hz.

[0026] Further, in S1, the stress law of each model pile 4 of the high-pile pile cap equipment under the horizontal load is analyzed by the LPile software to determine the strain gauge 8 layout position of the test;

[0027] The axial force test position is 1#~9 measuring points, and the bending moment test position is 4#~10 measuring points.

[0028] Because the model pile 4 foundation is completely symmetrical, the test selects 1#~3 piles for load response analysis, and the axial force and bending moment loading time history data of a total of 30 measuring points are included.

[0029] Further, in S2, the unidirectional variable amplitude cyclic loading is controlled by the axial force sensor 7, a sinusoidal wave is used to apply the horizontal load, and the cyclic loading frequency is 1 Hz.

[0030] Compared with the prior art, the present application has the following beneficial effects:

[0031] The present application can reveal the foundation stiffness evolution law under cyclic loading by deeply studying the load transmission and distribution characteristics of the high-pile pile cap foundation. Two groups of soil centrifuge model tests are carried out, the foundation stress, displacement, pile foundation strain and other parameters are tested, the stress characteristics of the dense sand offshore wind turbine high-pile foundation are studied, the load transmission mechanism of the pile foundation at different positions is analyzed, and the foundation deformation accumulation, inclination change and other laws under long-term cyclic loading are revealed. The research results can provide basis and reference for the design optimization of the high-pile pile cap foundation and the long-term service safety evaluation of the foundation.

[0032] BRIEF DESCRIPTION OF DRAWINGS

[0033] Fig. 1 is a structural schematic diagram of a TK-500 soil centrifuge;

[0034] Fig. 2 is a front view schematic diagram of a pile cap model;

[0035] Fig. 3 is a top view schematic diagram of a pile cap model;

[0036] Fig. 4 is a particle size distribution curve diagram;

[0037] Fig. 5 is a sand rain method for preparing a seabed model diagram;

[0038] Fig. 6 is a front view of a high-pile pile cap model layout;

[0039] Fig. 7 is a top view of a high-pile pile cap model layout;

[0040] Fig. 8 is a pile cap horizontal load-displacement curve diagram;

[0041] Fig. 9 is a 1# pile body axial force distribution diagram under different loading amplitudes;

[0042] Fig. 10 is a 2# pile body axial force distribution diagram under different loading amplitudes;

[0043] Figure 11 is a distribution diagram of the axial force of the pile body under different loading amplitudes;

[0044] Figure 12 is a comparison diagram of the axial force of the pile body;

[0045] Figure 13 is a distribution diagram of the bending moment of the pile body under different loading amplitudes;

[0046] Figure 14 is a distribution diagram of the bending moment of the pile body under different loading amplitudes;

[0047] Figure 15 is a distribution diagram of the bending moment of the pile body under different loading amplitudes;

[0048] Figure 16 is a comparison diagram of the bending moment of the pile body;

[0049] Figure 17 is a cyclic loading history diagram;

[0050] Figure 18 is a diagram of the shape of the soil around the pile after horizontal cyclic loading;

[0051] Figure 19 is a diagram of the displacement of the pile cap under a 0.2Fu amplitude cyclic load;

[0052] Figure 20 is a diagram of the displacement of the pile cap under a 0.4Fu amplitude cyclic load;

[0053] Figure 21 is a diagram of the displacement of the pile cap under a 0.6Fu amplitude cyclic load;

[0054] Figure 22 is a diagram of the displacement of the pile cap under a 0.2Fu ， amplitude cyclic load;

[0055] Figure 23 is a diagram of the evolution of the stiffness of the foundation under cyclic loading;

[0056] Figure 24 is a diagram of the evolution of the stiffness of the foundation under cyclic loading;

[0057] Figure 25 is a diagram of the inclination angle of the pile cap under a 0.2Fu cyclic load;

[0058] Figure 26 is a diagram of the inclination angle of the pile cap under a 0.4Fu cyclic load;

[0059] Figure 27 is a diagram of the inclination angle of the pile cap under a 0.6Fu cyclic load;

[0060] Figure 28 is a diagram of the inclination angle of the pile cap under a 0.2Fu ， cyclic load.

[0061] In the drawings: 1, model box; 2, sand soil layer; 3, model pile cap; 4, model pile; 5, horizontal loading device; 6, laser displacement sensor; 7, axial force sensor; 8, strain gauge.

[0062] DETAILED DESCRIPTION

[0063] In order to make the technical personnel in the art better understand the technical solutions of the present application, the present application will be further described in detail below in conjunction with the drawings.

[0064] Embodiment 1

[0065] The device loads on the bearing of the geotechnical centrifuge, completes the centrifuge test on the foundation bearing characteristics under the horizontal load of the model pile, and the device comprises:

[0066] A model box 1;

[0067] Specifically, as shown in FIG. 1, the present test is carried out in the TK-500 geotechnical centrifuge of the State Key Laboratory of Port and Waterway Engineering Technology. The specific parameters of the centrifuge are shown in Table 1. The model design considers the size limitation of the model box, in order to avoid the boundary effect, the geometric scale is selected as 1:100, that is, the acceleration of the centrifuge test is 100 g.

[0068] Table 1 Physical and mechanical properties of foundation soil parameters

[0069] Effective capacity (g·t) Effective radius (m) Acceleration range (g) Model box size (m) 500 5.0 10 ~ 250 1.0 × 1.2 × 1.2

[0070] A sand layer 2 is located in the model box 1;

[0071] Specifically, the soil in the model test is sand, which is prepared from quartz sand with different particle sizes according to the particle size distribution in the geological survey report of a sea area in Bohai, and is prepared after being washed with deionized water and dried, and the particle size distribution curve is shown in FIG. 4. The overall height of the sand layer 2 is 700 mm, and the sand rain method is used to prepare the sand layer 2, as shown in FIG. 5, the sand sample falling distance is adjusted to control the dry density of 1.64 g / cm3, and other soil parameters are shown in Table 2. After the preparation of the seabed model reaches the designed elevation, the high-pile cap foundation is positioned, and the embedding method is used for fixation.

[0072] Table 2 Physical and mechanical properties of foundation soil parameters

[0073] Mass median particle size D 50 Coefficient of uniformity U e Curvature coefficient C e Minimum void ratio e min Maximum void ratio e max Specific gravity G s (kN / m 3 ) 0.18 3.2 0.49 0.94 19.4

[0074] A model cap 3;

[0075] Specifically, the model pile cap is made of aluminum blocks, and the geometric parameters are shown in FIGS. 2-3.

[0076] At least eight model piles 4 are arranged symmetrically along the circumference of the model pile cap 3;

[0077] Specifically, each model pile 4 is first fixed by strong glue after being embedded in the model pile cap 3, and then welded and reinforced at the connection between the model pile 4 and the model pile cap 3, so as to have sufficient rigidity at the connection between the model pile and the model pile cap 3.

[0078] The material of the model pile 4 is aluminum alloy, with a diameter of 20 mm, a wall thickness of 7 mm, a density of 2.75 g / cm3, a tensile strength of 280 MPa, and an elastic modulus of 72 GPa.

[0079] The distance between the model pile 4 and the outer side of the model pile cap 3 is 5.4 mm, the inclination angle is 11.3°, and the distance between the bottom surface of the model pile cap 3 and the bottom surface of the model pile 4 is 848 mm.

[0080] A horizontal loading device 5 is used to load the model pile 4 with horizontal load.

[0081] Specifically, the horizontal loading device 5 is located at the height of one-half of the model pile cap 3, with a distance of 26 mm from the bottom surface of the model pile cap 3.

[0082] The high-pile cap model and the pile foundation sensor arrangement are shown in FIGS. 6-7,

[0083] At least two laser displacement sensors 6 are used to measure the lateral and vertical displacement of the model pile 4.

[0084] An axial force sensor 7 is used to measure the axial force of the model pile 4.

[0085] A plurality of strain gauges 8 are arranged along the length direction of the model pile 4.

[0086] Each model pile 4 is inserted into the sand soil layer 2. Embodiment

[0087] The centrifuge test method for the bearing characteristics of the high-pile cap foundation under horizontal load is applicable to the high-pile cap device under horizontal load described above, and the method comprises:

[0088] S1, unidirectional horizontal loading is performed on the high-pile cap device under horizontal load, and the ultimate bearing capacity of the foundation, the axial force distribution of the model pile, the axial force comparison of the pile foundation, the bending moment distribution of the model pile, and the bending moment comparison of the model pile are obtained.

[0089] Further, in S1, the unidirectional horizontal loading is loaded by controlling the end displacement of the horizontal loading device 5, the loading rate is 0.06 mm / s, and the sampling frequency is 10 Hz.

[0090] Further, in S1, the stress law of each model pile 4 of the high-pile pile cap device under horizontal load is analyzed by LPile software to determine the layout position of the strain gauge 8 in the test;

[0091] The axial force test position is 1#~#9 measuring point, and the bending moment test position is 4#~10# measuring point.

[0092] Because the model pile 4 is completely symmetrical, the test selects 1#~3# piles for load response analysis, and a total of 30 groups of axial force and bending moment loading time history data of measuring points are included. The specific loading scheme is shown in Table 3. Table 3 Unidirectional variable amplitude loading scheme

[0093] Serial number loading amplitude cycle times 10.2Fu400020.4Fu400030.6Fu400040.2Fu4000

[0094] Specifically, (1) the ultimate bearing capacity of the foundation

[0095] The load-displacement curve of the high-pile pile cap foundation obtained by horizontal monotonic loading is shown in FIG. 8. With the increase of the horizontal displacement of the foundation, the displacement of the pile cap and the load in the initial stage of loading show an elastic relationship; then the horizontal displacement of the pile cap continues to increase, and the cumulative displacement of the foundation shows a continuous growth trend; when the load reaches 17.88 kN, the load-displacement curve shows a significant downward trend, at this time it is considered that the multi-pile pile cap foundation reaches the ultimate load Fu, and the ultimate bearing capacity of the prototype high-pile pile cap foundation is 17880 kN.

[0096] In order to further explore the cumulative deformation law of the foundation under different load amplitudes, the load change rate curve under different horizontal displacements is drawn. The displacement change rate k represents the horizontal displacement of the pile cap under unit load, which is calculated according to formula (1). When the load is lower than 0.46Fu, the displacement change rate k changes weakly, at this time the high-pile pile cap foundation occurs recoverable deformation. Then the load-displacement curve tends to be flat, the horizontal displacement of the pile foundation caused by the same load increment is larger, and the change rate k is unstable, and the plastic deformation of the soil around the multi-pile pile cap foundation occurs. Liu Jinchao et al.

[0027] The horizontal bearing characteristics of large-diameter single piles in sandy seabed are systematically studied, and the test and numerical simulation results show that the load-displacement of single pile loading is always approximately linear, which shows that the pile group foundation can gradually develop different pile bearing capacity during the increase of horizontal load, and the high-pile pile cap foundation has certain flexibility characteristics, and has stronger resistance to horizontal load.

[0097] (1)

[0098] (2) Pile shaft force distribution

[0099] Under the action of each level of load, the shaft force distribution of 1#~3# piles is shown in Figures 9-11. The 1# and 2# piles are in axial tension state under the action of horizontal load, and the 3# pile is in compression state, and the shaft force gradually decreases with the increase of buried depth. Among them, the shaft force of the 2# pile at the measuring point at the depth of -250 mm suddenly increases. Since the shaft friction of the pile gradually increases with the increase of the depth of the soil, the shaft force of the pile should gradually decrease, so the data of the measuring point at this position is determined as an abnormal point. Overall, the shaft force of 1#~3# piles gradually decreases at the depth of 0~-200 mm, and as the depth of the soil continues to increase, the shaft force of the pile tends to 0. Since the test uses a single dense sand layer, the observed shaft force attenuation rate along the depth of the soil is relatively stable. On the other hand, the shaft force of the 3# pile at -150 mm under the load of 0.2~0.8Fu tends to 0, indicating that a larger horizontal load will cause the side friction at the deep part of the pile to be exerted, and since the test high pile cap foundation pile is relatively long, the horizontal load is mainly provided by the friction between the pile and the soil, and the resistance of the inclined pile end has not been exerted.

[0100] (3) Pile foundation shaft force comparison

[0101] The shaft force comparison of 1#~3# piles under the horizontal load of 0.2Fu, 0.6Fu and 1.0Fu is shown in Figure 12. The horizontal load at the pile cap is mainly borne by the 1# and 3# piles, and the shaft force of the 2# pile is the smallest. During the increase of the horizontal load, the 1# and 2# piles are subjected to tensile stress, and the 3# pile is subjected to compressive stress. The maximum position of the shaft force of the pile foundation is selected, i.e. the mud surface, to calculate the axial load proportionality coefficient of the pile foundation, so as to represent the difference in load distribution of the shaft force of the pile foundation at different positions. As shown in Table 4, the shaft force of the 1# pile is 1.11 times that of the 3# pile, the shaft force of the 2# pile is 0.83 times that of the 3# pile, and the shaft force of the 1# pile is 1.50 times that of the 2# pile. The above rules show that the shaft force of the high pile cap foundation is closely related to the size and direction of the load, and the main load direction can be considered for reinforcement to improve the economy of the construction of the high pile cap foundation.

[0102] The existing technology studies the lateral load law of pile groups in sand. Three rows and three columns of pile groups are subjected to horizontal loading. The load distribution of the front row, middle row and rear row of piles is 20.2 kPa, 15.6 kPa and 11.6 kPa, respectively, which proves that the pile foundation close to the loading position bears larger horizontal load. For the inclined pile group foundation studied in this test, the pile foundation close to the loading position bears a relatively smaller proportion of external horizontal load, and the axial force distribution of pile foundations at different positions is relatively small. Therefore, the high-pile cap foundation type studied in this test has stronger integrity and is more likely to exhibit the overall performance of pile groups under horizontal load. Table 4 Comparison of axial forces of 1# pile and 3# pile

[0103] Proportion coefficient C Horizontal load C1 C2 C3 0.2 Fu-1.08-0.57 2.29 0.6 Fu-1.24-1.04 1.19 1.0 Fu-1.01-0.99 1.02 Average value C-1.11-0.83 1.50

[0104] (Note: C1, C2, C3 represent the axial force ratio of 1# pile to 3# pile, 2# pile to 3# pile, and 1# pile to 2# pile, respectively.)

[0105] (4) Pile bending moment distribution

[0106] The bending moment distribution of 1#-3# piles is shown in Figures 13-15. Under the action of horizontal load, the bending moment of 1#-3# piles increases in the upper part of the soil layer and then gradually decreases. With the increase of horizontal load, the bending moment of 1#-3# piles increases significantly, which is consistent with existing research, verifying the correctness of the test method in this paper. The maximum bending moment of 1#-3# piles occurs at a depth of-30 mm. The bending moment of the pile foundation obtained by testing can be considered as the superposition of the bending moment generated by horizontal force and the resistance provided by the soil, which causes the bending moment to increase first and then decrease along the depth of the soil. On the other hand, the bending moment of 1#-3# piles is in the same direction during the horizontal ultimate loading process, and the direction of the bending moment does not change with the size of the initial load.

[0107] (5) Comparison of pile bending moment

[0108] 1#~3# pile under 0.2Fu, 0.6Fu and 1.0Fu horizontal load, the bending moment of the pile body is compared as shown in Figure 16. Similar to the axial force distribution, the bending moment generated by the horizontal load of the pile cap is more borne by the 1# pile. The prior art studies the stress law of each pile foundation of the pile group pile cap foundation by theoretical calculation and numerical simulation method, and the results show that the bending moment of the pile foundation close to the loading direction is greater than that of the lateral pile foundation. In order to further study the bending moment transmission and distribution law of the high pile pile cap foundation, the maximum value of the bending moment curve of the 1#~3# pile under different horizontal loads is selected, and the bending moment proportion coefficient of the pile foundation is calculated, as shown in Table 5. The bending moment of the 1# pile is about 1.39 times that of the 3# pile, the bending moment of the 2# pile is about 1.28 times that of the 3# pile, and the bending moment of the 1# pile is about 1.08 times that of the 2# pile. Overall, the pile foundation close to the loading position bears larger bending moment load, but the difference in bending moment of the pile foundation caused by different positions is small (as shown in Figures 13-15). Combined with the axial force distribution law of 1#~3# piles, it can be seen that the axial force and bending moment load distribution of the pile foundation at different positions under horizontal load is small, therefore, the high pile pile cap foundation studied in this test has good integrity and strong collaborative stress capacity. Table 5 Comparison of bending moments of 1# pile~3# pile

[0109] Proportion coefficient D horizontal load D1 D2 D3 0.2Fu 1.46 1.49 1.18 0.6Fu 1.25 1.29 0.97 1.0Fu 1.17 1.06 1.10 Average value D 1.29 1.28 1.08

[0110] (Note: D1, D2, D3 represent the bending moment ratio of 1# pile to 3# pile, 2# pile to 3# pile, and 1# pile to 2# pile, respectively.)

[0111] S2, under horizontal load, the high pile pile cap equipment is subjected to unidirectional variable amplitude cyclic loading, and the foundation displacement of the equipment, the influence of cycle number on foundation stiffness, the influence of cyclic load amplitude on foundation stiffness, and the foundation inclination are obtained.

[0112] Further, in S2, the unidirectional variable amplitude cyclic loading is controlled by the axial force sensor 7, and the horizontal load is applied by using a sine wave, and the cyclic loading frequency is 1 Hz.

[0113] Specifically, (1) loading mode

[0114] The high-pile cap foundation cycle loading history is 0.2Fu→0.4Fu→0.6Fu→0.2Fu', the horizontal load amplitude and cycle number curve and its detailed characteristics are shown in Figure 17, the actual cycle loading period and amplitude are the same as the design scheme, it can be judged that the loading system works well. After long-term cyclic loading, the high-pile foundation is shown in Figure 18, at this time the high-pile cap foundation has tilted obviously. Under long-period, high-amplitude cyclic loading, the pile body has a large displacement, the soil around the pile rises outward, the soil particles move to the periphery and the lower part, and a conical subsidence area appears around the pile, which causes the reduction of the horizontal bearing capacity of the pile foundation. Existing researches on cyclic loading tests of different types of offshore wind turbine foundations have a cycle number of several hundred times [32-34] To further explore the evolution law of the bearing characteristics of high-pile foundations under long-term cyclic loading and extreme load environment, a larger cycle number and loading amplitude are used in the test.

[0115] (2) Foundation displacement

[0116] Figures 19-22 are the load-displacement response curves of the high-pile cap foundation under cyclic loading. Under the action of 0.4Fu and 0.6Fu cyclic load amplitudes, the foundation shows obvious hysteresis. For 0.2Fu and 0.2Fu', the foundation hysteresis is relatively small. With the increase of cycle number, the foundation cumulative deformation increases continuously, and the cumulative deformation of the foundation under 0.2Fu, 0.4Fu, 0.6Fu and 0.2Fu' cyclic load is 0.87 mm, 12.67 mm, 8.17 mm and 0.38 mm respectively, which shows that the cumulative deformation of the foundation gradually increases with the increase of the cyclic load amplitude. For 0.2Fu, 0.4Fu and 0.6Fu cyclic load, the cumulative deformation increases obviously at the beginning of loading, and then tends to be stable. When the cyclic load decreases from 0.6Fu to 0.2Fu, the displacement change of the foundation under this amplitude is 0.38mm, which is smaller than the 0.87mm displacement generated by the initial 0.2Fu cyclic load. On the other hand, the cumulative displacement of the foundation under 0.2Fu, 0.4Fu and 0.6Fu cyclic load gradually increases, while the cumulative displacement of the foundation under 0.2Fu' load decreases, because the surrounding soil of the dense sand is compacted under the action of a large number of 0.6Fu loads, the compaction degree of the surrounding soil decreases, and the cumulative displacement of the foundation increases. In addition, the load-displacement curve of the foundation under cyclic loading shows obvious hysteresis and ratchet effect. Under the same cyclic loading amplitude, the plastic deformation capacity of the soil around the pile decreases with the decrease of the cycle number, and the cumulative displacement of the foundation increases with the increase of the cycle number and the cyclic load amplitude.

[0117] With the increase of cycle times, the foundation stiffness under horizontal load and unloading is shown in Figure 23. In order to better reflect the trend of foundation stiffness, the moving average data processing is carried out, as shown by the black curve in Figure 23. For 0.2Fu load amplitude, the foundation stiffness increases with the increase of cycle load times, but the increase is not obvious. Studies have shown that for the same amplitude of cyclic load, the stiffness of dense sand almost remains unchanged with the increase of cycle times, which is consistent with the conclusion of about 0~1500 cycles in this test. After 1500 cycles of load, the foundation stiffness increases slowly due to the decrease of sand particle void caused by shear shrinkage effect. Under 0.4Fu cyclic load amplitude, the foundation stiffness decreases with the decrease of cycle load times. At this time, the soil particles around the pile are disturbed greatly, the soil particles around the pile are compacted and uplifted, and the density of the adjacent soil decreases, resulting in the weakening of the foundation stiffness. For 0.6Fu cyclic loading, the original position and arrangement of the soil particles around the pile are changed due to the severe action of the pile foundation, and the soil particles around the pile are plastically damaged under about 0~200 cycles. The density of the soil around the pile decreases, and the foundation stiffness decreases significantly in the initial stage. With the increase of cycle times, the arrangement of sand particles gradually stabilizes, so the subsequent soil stiffness almost remains unchanged. When the cyclic load amplitude decreases to 0.2Fu', the sand occurs shear shrinkage under low amplitude and large cycle load, the density of the soil around the pile increases, and the foundation stiffness quickly recovers and increases with the increase of cycle load times.

[0118] (4) Effect of cyclic load amplitude on foundation stiffness

[0119] Average value of cyclic load and unloading foundation stiffness under different load amplitudes The average values of cyclic load and unloading foundation stiffness under different load amplitudes are 2.10, 2.47, 1.80 and 2.72, as shown in Figures 24-28. When the cyclic load amplitude increases from 0.2Fu to 0.4Fu, the foundation stiffness increases significantly, and the larger horizontal displacement of the pile foundation leads to the increase of horizontal bearing capacity. When the load increases to 0.6Fu, the soil embedment effect on the pile decreases. When the cyclic load amplitude increases from 0.4Fu to 0.6Fu, the bearing capacity of the foundation decreases significantly (as shown in Figures 19-22), and the original soil structure may be damaged under larger cyclic load, resulting in a significant decrease in foundation bearing capacity. Subsequently, the cyclic load is restored to 0.2Fu, and the soil density increases under lower amplitude load, the interfacial friction resistance and horizontal resistance of sand particles around the pile increase, and the foundation stiffness is higher than that under 0.2Fu cyclic load in the first stage.

[0120] The results show that the number of cycles, the amplitude of cyclic loading and the density of sand all affect the stiffness and the horizontal bearing capacity of pile foundation. For the dense sand used in this test, the change of soil stiffness caused by the amplitude of cyclic loading is more significant than that caused by the number of cycles. When the amplitude of cyclic loading is the same, the change of soil stiffness with the number of cycles is related to the stress history. Therefore, the influence of extreme environmental load on soil stiffness should be considered for the long-term service of offshore wind turbine foundation with pile cap. The stiffness of foundation decreases significantly under large cyclic loading, and then increases under small cyclic loading due to the shear contraction of sand under horizontal loading.

[0121] (5) Foundation inclination

[0122] The displacement of pile cap is obtained by horizontal and vertical laser displacement meters, and then the inclination of pile cap is calculated. The black curve in Fig. 16 is the measured value, and the moving average data processing is performed to better reflect the change law of the inclination of pile cap, as shown by the red curve in the figure. When the amplitude of cyclic loading is 0.2Fu, the inclination of pile cap increases with the number of cycles and gradually stabilizes. The maximum change of inclination is 0.22°.

[0123] When the amplitude of cyclic loading is 0.4Fu, the inclination of pile cap increases significantly at the initial stage, and then stabilizes. When the number of cycles reaches 840, the inclination increases significantly, and then stabilizes again. Above 2741 cycles, the growth rate of foundation inclination becomes larger. Under the amplitude of 0.4Fu cyclic loading, the inclination of foundation increases in a step-like manner. For large amplitude cyclic loading, the cyclic shear of pile foundation on sand causes the redistribution of soil structure, resulting in a significant increase in the inclination of foundation, and then stabilizing. Due to the large number of cycles of the high-pile cap in this test, the soil has undergone repeated plastic failure and reached a stable state, which is consistent with the law that the soil stiffness gradually decreases with the increase of the number of cycles shown in Fig. 13. On the other hand, the total allowable inclination of the wind turbine foundation installation error and the cumulative deformation during service is 0.5°, so the inclination of the foundation after the 86th loading under the amplitude of 0.4Fu cyclic loading does not meet the specification requirements.

[0124] For 0.6Fu, the soil around the pile is significantly damaged at the initial stage of cyclic loading, the inclination of foundation increases significantly, and the density of soil further decreases. Then the inclination of foundation remains relatively stable, but a slow growth trend can still be observed. When the amplitude of cyclic loading decreases from 0.6Fu to 0.2Fu, the displacement of pile in soil decreases, and the inclination of pile cap shows a trend of “first decreasing and then stabilizing”. The reason is that under the amplitude of 0.6Fu cyclic loading, the sand far from the pile is more dense, while under the lower amplitude of cyclic loading, the sand particles that are more dense are disturbed and move to the position closer to the pile, which improves the density and stiffness of the sand in this area and shows a certain “self-healing” ability.

[0125] Through the centrifuge model test of offshore wind turbine high-pile cap foundation under horizontal monotonic and long-term cyclic loading, the load transfer and distribution law of pile foundation of high-pile cap structure is explored based on the distribution of pile shaft force and bending moment. The influence law of different cycle numbers and load amplitudes on foundation horizontal displacement, soil stiffness, deformation accumulation and inclination is studied. The main conclusions are as follows:

[0126] (1) With the monotonic increase of cap horizontal load, the foundation displacement and load are initially in linear relationship. The displacement generated by the foundation can be basically restored after unloading. When the load increases to 0.46Fu, the foundation displacement-load curve shows obvious nonlinear characteristics, the soil around the pile occurs plastic deformation, and the horizontal load required for the same displacement increment of the foundation decreases, showing a certain softening characteristics of high-pile foundation stiffness.

[0127] (2) The pile shaft force of high-pile cap gradually decreases with the depth of pile body into the soil. Due to the relatively high relative density of the sand used in the test, the shaft force along the pile body decays obviously and tends to be 0 at a depth of 200 mm (prototype depth of 20 m). The bending moment of the pile body increases with the depth of the soil, reaching a maximum at a depth of 30 mm (prototype depth of 3 m) and then decreasing. By comparing the pile shaft force and bending moment at different positions of the high-pile cap foundation, it is found that the horizontal load causes different positions of the pile to produce tension and compression differences, while the bending moment direction is the same, and the shaft force and bending moment of the pile near the loading position are larger. Therefore, the optimization of high-pile cap foundation should consider the long-term action direction of the load, analyze the bearing capacity required by the pile foundation at different positions combined with the mechanical properties of the soil at the construction site, and improve the economy of wind turbine high-pile cap foundation construction.

[0128] (3) Under long-term horizontal cyclic loading, the cap foundation produces obvious cumulative displacement. Under 0.2Fu, 0.4Fu and 0.6Fu cyclic load amplitudes, the foundation cumulative displacement increases with the cycle number. When the cyclic load amplitude decreases to 0.2Fu', the foundation cumulative displacement slightly decreases. The foundation stiffness significantly improves under 0.2Fu to 0.4Fu cyclic load amplitudes, and decreases under 0.6Fu cyclic load. With the cyclic load amplitude decreasing to 0.2Fu', the foundation stiffness shows a certain "self-healing" characteristics. Under different cyclic load amplitudes, a large number of cycles cause shear dilation and shear shrinkage of sand, resulting in changes in soil stiffness. However, the change in foundation stiffness caused by cycle number is smaller than that caused by cyclic load amplitude.

[0129] (4) Similar to the variation of the cumulative displacement, the inclination of the pile cap increases gradually under the cyclic load of 0.2Fu, 0.4Fu and 0.6Fu. The significant increase of the inclination of the pile cap occurs in the initial stage of the cyclic load, and the inclination of the pile cap increases further with the increase of the cycle number for a specific cyclic load amplitude. With the decrease of the cyclic load to 0.2Fu, the soil particles around the pile are densified under the continuous disturbance of the pile, which leads to the increase of the stiffness of the soil and the decrease of the inclination of the pile cap by 0.28°. The above shows and describes the basic principles, main features and advantages of the present application. It should be understood by those skilled in the art that the present application is not limited by the above examples, and the above examples and descriptions in the specification are only preferred examples of the present application and are not intended to limit the present application. Without departing from the spirit and scope of the present application, various changes and improvements can be made to the present application, and these changes and improvements all fall within the scope of the present application. The scope of protection of the present application is defined by the appended claims and their equivalents.

Claims

1. High-pile cap equipment under horizontal load, characterized by: The device is loaded at a bearing of a geotechnical centrifuge to complete a centrifuge test on foundation bearing characteristics of a model pile under horizontal load, and the device comprises: a model box (1); a sand soil layer (2) located in the model box (1); a model pile cap (3); at least eight model piles (4) arranged in a symmetrical manner along the circumference of the model pile cap (3); a horizontal loading device (5) for loading the model piles (4) with horizontal load; at least two laser displacement sensors (6) for measuring the lateral and vertical displacement of the model piles (4); an axial force sensor (7) for measuring the axial force of the model piles (4); a plurality of strain gauges (8) arranged along the length direction of the model piles (4); wherein each model pile (4) is inserted into the sand soil layer (2).

2. The high-pile cap equipment under horizontal load according to claim 1, characterized in that: Each model pile (4) is first fixed by strong glue after being embedded in the model pile cap (3), and then welded and reinforced at the connection between the model pile (4) and the model pile cap (3) to ensure sufficient rigidity at the connection between the model pile and the model pile cap (3). The material of the model pile (4) is aluminum alloy, with a diameter of 20 mm, a wall thickness of 7 mm, a density of 2.75 g / cm3, a tensile strength of 280 MPa, and an elastic modulus of 72 GPa. The distance between the model pile (4) and the outer side of the model pile cap (3) is 5.4 mm, the inclination angle is 11.3°, and the distance between the bottom surface of the model pile cap (3) and the bottom surface of the model pile (4) is 848 mm.

3. The high-pile cap equipment under horizontal load according to claim 2, characterized in that: The overall height of the sand soil layer (2) is 700 mm, and the sand soil layer (2) is prepared by the sand rain method.

4. The high-pile cap equipment under horizontal load according to claim 3, characterized in that: The horizontal loading device (5) is located at one-half of the height of the model pile cap (3), and the distance from the bottom surface of the model pile cap (3) is 26 mm.

5. The centrifuge test method for bearing characteristics of high-pile cap foundation under horizontal load, characterized in that: The method is applicable to the horizontal load high-pile pile cap device of any one of claims 1-4, and the method comprises: S1, unidirectional horizontal loading is performed on the horizontal load high-pile pile cap device to obtain the foundation ultimate bearing capacity, the axial force distribution of the model pile, the axial force comparison of the pile foundation, the bending moment distribution of the model pile, and the bending moment comparison of the model pile; S2, unidirectional variable amplitude cyclic loading is performed on the horizontal load high-pile pile cap device to obtain the foundation displacement, the influence of the number of cycles on the foundation stiffness, the influence of the cyclic load amplitude on the foundation stiffness, and the foundation inclination.

6. The centrifuge model test method for load bearing behavior of pile cap foundation under horizontal load according to claim 5, wherein, In S1, the unidirectional horizontal loading is performed by controlling the end displacement of the horizontal loading device (5) at a loading rate of 0.06 mm / s and a sampling frequency of 10 Hz.

7. The centrifuge model test method for load bearing behavior of pile cap foundation under horizontal load according to claim 5, wherein, In S1, the stress law of each model pile (4) of the horizontal load high-pile pile cap device is analyzed by LPile software to determine the strain gauge (8) layout position; The axial force test position is 1#~#9 measuring points, and the bending moment test position is 4#~10# measuring points; Since the model pile (4) is completely symmetrical, load response analysis is performed on 1#~3# piles, and a total of 30 groups of axial force and bending moment loading time data of measuring points are included.

8. The centrifuge model test method for load bearing behavior of pile cap foundation under horizontal load according to claim 5, wherein, In S2, the unidirectional variable amplitude cyclic loading is controlled by the axial force sensor (7), a sinusoidal wave is used to apply the horizontal load, and the cyclic loading frequency is 1 Hz.

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