Model pile calibration tank testing device and testing method thereof

Abstract

The application belongs to the technical field of model test equipment, and discloses a model pile calibration tank testing device and a testing method thereof, which comprises a loading frame, a calibration tank, a model pile and an actuator. The calibration tank is placed on the lower side of the loading frame, the inner side of the calibration tank is provided with a top plate, the middle of the upper end of the top plate is connected with the loading frame through a load transmission rod, the actuator is arranged on one end of the upper side of the calibration tank, the model pile is arranged on the side of the actuator, the model pile is driven to move up and down by the actuator, and the bottom of the calibration tank and the top plate are respectively provided with openings corresponding to the model pile. The indoor calibration tank model pile test has good parallel test effect and good consistency with the field test result.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of model test equipment, and particularly relates to a model pile calibration tank testing device and a testing method thereof. BACKGROUND

[0002] At present, with the construction of high-rise buildings, bridges and towers, the combined load borne by the single pile from the upper part is increasingly valued in engineering practice. The combined load is the load combined by the vertical load and the horizontal load acting on the pile body part above the ground. Under the combined load, additional bending moment is generated due to the appearance of pile body deflection deformation, thereby affecting the horizontal bearing capacity of the pile. The horizontal load causes the horizontal deformation of the pile body, thereby changing the resistance of the soil around the pile, and further affecting the vertical bearing characteristics of the pile.

[0003] Through the above analysis, the problems and defects of the prior art are:

[0004] The pile test effect of the prior art is poor, and does not have good consistency with the field test results. SUMMARY

[0005] In view of the problems existing in the prior art, the present application provides a model pile calibration tank testing device and a testing method thereof.

[0006] The present application is implemented as follows: a model pile calibration tank testing device and a testing method thereof comprise:

[0007] a loading frame, a calibration tank, a model pile and an actuator;

[0008] The calibration tank is placed on the lower side of the loading frame, the inner side of the calibration tank is provided with a top plate, and the upper end of the top plate is connected with the loading frame through a load transmission rod;

[0009] The actuator is arranged on one end of the upper side of the calibration tank, the model pile is arranged on the side of the actuator, the model pile is driven to move up and down by the actuator, and the bottom of the calibration tank and the top plate are respectively provided with openings corresponding to the model pile.

[0010] Further, the loading frame is provided with a longitudinal rod and a cross rod fixed on the upper end of the longitudinal rod through a pin shaft, and the outer end of the cross rod is connected with a weight through a pin shaft.

[0011] Further, a load sensor is arranged between the top plate and the load transmission rod.

[0012] Further, the bottom of the calibration tank is provided with an overhead base frame, and the upper end of the overhead base frame is provided with an opening corresponding to the bottom of the calibration tank.

[0013] Further, geotextile is laid on the inner side of the bottom of the calibration tank.

[0014] Another object of the present application is to provide a model pile calibration tank test method comprising:

[0015] The prepared slurry is placed in the calibration tank, a drainage hole is reserved at the bottom of the calibration tank, and two layers of drainage geotextiles are laid, and two layers of geotextiles are also placed on the top of the slurry;

[0016] Then, the top plate with a drainage hole is covered, a load is applied to the top plate for consolidation, and after the soil sample is consolidated, the model pile is pressed into the soil by the actuator;

[0017] Since the initial strength of the silt is 0, the consolidation pressure should be slowly applied at the beginning, and the settlement is monitored, and after each load level is basically stable, the next load level is applied;

[0018] After the model pile is driven into the clay for 1 day, the model pile uplift test is performed.

[0019] Further, when the load is applied, the load below 80 kg is directly applied by the weight, and the load above 80 kg is applied by a 1:5 lever.

[0020] Further, the penetration rate of the model pile is 0.5 mm / s, and the consolidation pressure of the soil sample is kept constant during the piling and subsequent uplift test process.

[0021] Further, the model pile uplift test adopts displacement control, and the displacement rate is 5 μm / s, and under this rate, the soft clay around the pile is in fully drained condition.

[0022] Further, in order to prevent the verticality problem of the pile during uplift, a steel strand is used to apply the uplift load.

[0023] In combination with all the above technical solutions, the present application has the following advantages and positive effects:

[0024] The indoor calibration tank model pile test has good parallel test effect, and has good consistency with the field test results.

[0025] The test shows that the pile-soil relative displacement obtained by the field test is significantly smaller than the standard slow-speed maintenance load method, that is, the pile vertical displacement test value corresponding to the peak value of the side resistance should be significantly greater than 7.3 mm, and the recommended value of 1.65 mm of the API specification is obviously too low. The measured pile-soil relative displacement value at the peak value of the side resistance is shown in Table 1. It can be seen that blindly applying the API specification will bring engineering risks and reliability problems that cannot be ignored, and a more scientific method should be explored to obtain the pile-soil interaction t-z curve.

[0026] Table 1 Comparison of pile-soil relative displacement values at peak value of side resistance

[0027] Item Pile outside diameter D (mm) Measured value (mm) API recommended value (mm) API range Measured value Laboratory model test 12.5 1.5 0.13 0.0025～0.02D 0.12D Field test 165 >7.3 1.65 0.0025～0.02D >0.044D

[0028] The vertical displacement rate of the pile in the indoor model test is 5 μm / s, which is completely drained condition for soft clay with consolidation coefficient of 0.1-0.5 mm / s, that is, the indoor model test carried out by using the test method is equivalent to the standard slow-speed maintaining load method test. The t-z normalized curve obtained by the indoor model test is also basically consistent with the API specification, but there are some differences in the relative displacement of the pile-soil corresponding to the peak side resistance and the residual side resistance. 2 / s of 0.1-0.5 mm BRIEF DESCRIPTION OF DRAWINGS

[0029] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings needed to be used in the embodiments of the present application will be briefly introduced as follows. Obviously, the drawings described below are only some embodiments of the present application, and other drawings can also be obtained by those skilled in the art without any creative effort on the basis of these drawings.

[0030] Figure 1 is a structural schematic diagram of a model pile calibration tank test device provided by the embodiments of the present application.

[0031] Figure 2 is a schematic diagram of indoor model pile uplift test results provided by the embodiments of the present application.

[0032] Figure 3 is a schematic diagram of comparison of indoor model test results and field test results provided by the embodiments of the present application.

[0033] In the figure: 1, loading frame; 2, overhead base frame; 3, calibration tank; 4, geotextile; 5, opening; 6, model pile; 7, top plate; 8, load sensor; 9, load transmission rod; 10, actuator; 11, weight. DETAILED DESCRIPTION

[0034] In order to make the purpose, technical solutions and advantages of the present application more clear, the present application will be further described in detail below by combining with embodiments. It should be understood that the specific embodiments described here are only used to explain the present application, and are not used to limit the present application.

[0035] In view of the problems in the prior art, the present application provides a model pile calibration tank test device and a test method thereof, which will be described in detail below by combining with the drawings.

[0036] As shown in Figure 1 , the loading frame 1, the calibration tank 3, the model pile 6 and the actuator 10 of the model pile calibration tank test device provided by the embodiments of the present application;

[0037] The calibration tank 3 is placed on the lower side of the loading frame 1, the inside of the calibration tank 3 is provided with a top plate 7, the middle of the upper end of the top plate 7 is connected with the loading frame 1 through a load transmission rod 9; an actuator 10 is arranged on one end of the upper side of the calibration tank 3, a model pile 6 is arranged on the side of the actuator 10, the model pile 6 is driven to move up and down through the actuator 10, and the bottom of the calibration tank 3 and the top plate 7 are respectively provided with an opening 5 corresponding to the model pile 6.

[0038] The loading frame 1 is provided with a longitudinal rod and a cross rod fixed on the upper end of the longitudinal rod through a pin shaft, and the outer end of the cross rod is connected with a weight through a pin shaft; a load sensor 8 is arranged between the top plate 7 and the load transmission rod 9.

[0039] The bottom of the calibration tank 3 is provided with an overhead chassis 2, and the upper end of the overhead chassis 2 is provided with an opening corresponding to the bottom of the calibration tank 3. The inside of the calibration tank is paved with geotextile.

[0040] The application will be further described below in combination with specific embodiments.

[0041] The disturbed clay sample is taken by excavator on site. The sampling depth is 4 m, the disturbed sample is taken by grab and then manually loaded into a large storage box. The sampling process prevents the mixture of sandy loam soil and vegetation, etc., to ensure that the test sample is uniform and representative.

[0042] The taken disturbed clay sample is placed in a large soil sample mixer, and the impurities such as gravel and shells larger than 0.5 mm in the clay sample are removed. The mud is prepared according to 2 times the liquid limit water content. The mud stirring process is carried out simultaneously with vacuumizing to remove air in the mud, avoiding air holes in the sample prepared later, causing uneven sample, and increasing the saturation of the soil sample.

[0043] The prepared mud is placed in the calibration tank, the bottom of the calibration tank is reserved with a drainage hole, and two layers of drainage geotextile are laid; two layers of geotextile are also placed on the top of the mud, and then a top plate with a drainage hole is covered, a load is applied to the top plate for consolidation, a load lower than 80 kg is directly applied through a weight, and a load higher than 80 kg is applied through a 1:5 lever. The clay consolidation in the calibration tank is as shown in Figure 1 The initial strength of the silt is 0, so the consolidation pressure should be slowly applied at the beginning, and the settlement is monitored. After each load is basically stable, the next level of load is applied. The design pressure of the top of the soil sample is 40 kPa.

[0044] During the process of driving the model pile. A steel pipe with an outer diameter of 12.5 mm is used to simulate the pile, the wall thickness is 1 mm, and the surface center line roughness is 3.6-4.4 μm. The total length of the pile is 360 mm, and the effective length of the pile into the soil is 300 mm. After the soil sample is consolidated, the model pile is pressed into the soil through the actuator, the penetration rate of the pile is 0.5 mm / s, and the consolidation pressure of the soil sample is kept constant during the process of pile driving and subsequent uplift test.

[0045] Indoor model pile uplift test:

[0046] According to the research of Doan and Lehane (2018), the pore pressure induced by pile driving can be fully dissipated after 1 day in clay. The indoor model pile uplift test is displacement-controlled with a rate of 5 pm / s, at which the surrounding soft clay is fully drained. To prevent the problem of pile plumbness during uplift, the uplift load is applied by steel strands.

[0047] The results of indoor model pile uplift test are shown in Figure 2 It can be seen that the parallel test results of indoor calibration tank model pile test are good, and the peak side resistance and residual side resistance are almost coincident, except for the 0.13 mm error of the relative displacement of pile-soil at the peak point of side resistance. The side resistance-displacement relationship is a typical softening type with post-peak decay and then stabilization, which is consistent with the API specification, and the residual value of side resistance is 70% of the peak value, which is consistent with the empirical range of 70% to 90% in the API specification.

[0048] Comparison of indoor calibration tank model test and field model test:

[0049] According to Meyerhof (1975) in Terzaghi's lecture, the side resistance of a driven pile in clay is closely related to the drained shear strength of remolded clay. Karlsrud (1985) of Norwegian Geotechnical Institute also proposed that the side resistance of the pile depends on the strength of remolded clay, and has no direct correlation with the strength of undisturbed clay. This is because the steel pipe pile driving method or static pressure method construction will cause strong disturbance to the soil within the range of 10-40 mm around the pile, and the shear failure of the pile in soft clay often occurs in this range, so the indoor model test can use remolded samples prepared by soft clay slurry, and has good consistency with the results of field test.

[0050] The side friction-displacement relationship curve is normalized according to the method of API specification, as shown in Figure 3The residual side resistance of indoor model test is 70% of the peak value, while the residual side resistance of field model pile test is about 65% to 70% of the peak value, both of which are in the experience range of 70% to 90% of API specification; since the field model pile test is stress-controlled by a jack, it is difficult to maintain the load after the bearing capacity reaches the peak value, thus the residual strength of field test is lower than that of indoor model test, and compared with the indoor test results, the side friction resistance of field model pile does not show an ideal stable trend with the increase of pile-soil relative displacement. The API specification has certain blindness in estimating the residual side resistance, and the experience range given by the specification is relatively large, especially for soft clay with pile side resistance, when the flexible pile reaches the ultimate pile bearing capacity, the side resistance often decays from the peak value to the residual value, and overestimation of the residual side resistance is unsafe for the project. On the other hand, blindly applying the lower limit value of the residual side resistance can lead to a waste of up to 22% of the bearing capacity.

[0051] The pile top displacement when the side resistance of indoor model test reaches the peak value is 1.5mm, and the pile top displacement when the side resistance of field test reaches the peak value is 7.3mm, API specification considers that the pile-soil relative displacement corresponding to the peak point is 0.0025~0.02D, and recommends 0.01D as the pile-soil displacement corresponding to the peak point of side resistance. Using API specification to estimate the pile-soil relative displacement value of the peak point of side resistance is significantly smaller than the measured value, thus overestimating the stiffness of pile-soil vertical interaction, which is dangerous for the displacement analysis of pile vertical loading.

[0052] Therefore, the pile-soil relative displacement obtained by field test is significantly smaller than the standard slow maintenance load method, that is, the measured value of pile vertical displacement corresponding to the peak point of side resistance should be significantly greater than 7.3mm, and the recommended value of 1.65mm by API specification is obviously too low. The measured pile-soil relative displacement value of the peak point of side resistance and the recommended value of the specification are shown in Table 1. It can be seen that blindly applying API specification will bring engineering risks and reliability problems that cannot be ignored, and a more scientific method should be explored to obtain the t-z curve of pile-soil interaction.

[0053] Table 1 Comparison of pile-soil relative displacement values of the peak point of side resistance

[0054] Item Pile outside diameter D (mm) Measured value (mm) API recommended value (mm) API range Measured value Laboratory model test 12.5 1.5 0.13 0.0025～0.02D 0.12D Field test 165 >7.3 1.65 0.0025～0.02D >0.044D

[0055] The pile vertical displacement rate of indoor model test is 5μm / s, which is completely drained condition for soft clay with consolidation coefficient of 0.1~0.5mm 2 / s, that is, the indoor model test carried out by this test method is equivalent to the standard slow maintenance load method. The t-z normalized curve obtained by indoor model test is also basically consistent with API specification, but there are some differences in the pile-soil relative displacement corresponding to the peak point of side resistance and the residual side resistance.

[0056] In the description of the application, unless otherwise specified, the meaning of "a plurality of" is two or more; the terms "upper", "lower", "left", "right", "inner", "outer", "front end", "rear end", "head", "tail" and the like indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, and are only for the purpose of facilitating the description of the application and simplifying the description, and do not indicate or imply that the device or element referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as a limitation on the application. In addition, the terms "first", "second", "third" and the like are only for the purpose of description and cannot be understood as indicating or implying relative importance.

[0057] The above is only a specific embodiment of the present application, but the protection scope of the present application is not limited thereto, and any modification, equivalent replacement and improvement made by any person skilled in the art within the technical scope disclosed by the present application, as long as it is within the spirit and principle of the present application, should be covered within the protection scope of the present application.

Claims

1. A model pile calibration tank test apparatus, characterised in that, The model pile calibration tank test device comprises a loading frame, a calibration tank, a model pile and an actuator. The calibration tank is placed on the lower side of the loading frame, and a top plate is arranged on the inner side of the calibration tank. The actuator is arranged on one end of the upper side of the calibration tank, and the model pile is arranged on the side of the actuator. The model pile calibration tank test method comprises the following steps: The prepared mud is placed in the calibration tank, and the bottom of the calibration tank is reserved with a drainage hole and is padded with two layers of drainage geotextiles. Then, the top plate with a drainage hole is covered, a load is applied to the top plate for consolidation, and after the soil sample is consolidated, the model pile is pressed into the soil through the actuator. Since the initial strength of the silt is 0, the consolidation pressure should be slowly applied at the beginning, and the settlement is monitored. After the model pile is driven into the clay for 1 day, the model pile uplift test is carried out. When the load is applied, the load below 80 kg is directly applied through the weight, and the load above 80 kg is applied through a 1:5 lever. The loading frame is provided with a longitudinal rod and a cross rod fixed on the upper end of the longitudinal rod through a pin shaft, and the outer end of the cross rod is connected with a weight through a pin shaft. A load sensor is arranged between the top plate and the load transmission rod.

2. The model pile calibration can test device of claim 1, wherein, The bottom of the calibration tank is provided with an elevated base frame, and the upper end of the elevated base frame is provided with an opening corresponding to the bottom of the calibration tank.

3. The model pile calibration can test apparatus of claim 1, wherein, The inner side of the calibration tank is paved with geotextile at the bottom.

4. The model pile calibration can test apparatus of claim 1, wherein, The penetration rate of the model pile is 0.5 mm / s, and the consolidation pressure of the soil sample is kept constant during the piling and subsequent uplift test.

5. The model pile calibration can test apparatus of claim 1, wherein, The model pile uplift test adopts displacement control, and the displacement rate is 5 μm / s.

6. The model pile calibration can test apparatus of claim 1, wherein, The uplift load is applied by using steel wire.

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