In order to study the deformation mechanism of sand around the spudcan, the model test for spudcan-sand interaction was carried out by employing transparent media, which is also called "transparent soil". By means of the technology of Particle Image Velocimetry (PIV), the displacement vector field and contour map of sand around the spudcan were obtained. According to shallow strain path method (SSPM), the theoretical value of soil displacement around the spudcan was calculated. Compared with the experimental results, the correlation between the variation law of horizontal displacement of each group and the theoretical value reached 60%~80% in a certain range, which further verifies the rationality of the experimental method and theoretical analysis.
Research Article
Model investigation on spudcan-sand interaction using transparent media
https://doi.org/10.21203/rs.3.rs-2067406/v1
This work is licensed under a CC BY 4.0 License
Journal Publication
published 07 Sep, 2023
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Spudcan
Transparent media
Model test
PIV
SSPM
Model test for transparent soil test for spudcan-sand interaction
The displacement vector field and contour map of sand around the spudcan
Theoretical value of soil displacement around the spudcan
As the supporting structure of self-elevating platform, the stability of spudcan is closely related to the safety of the platform. Due to the specificity of spudcan structure, the traditional theoretical analysis method of equal-diameter piles cannot be applied indiscriminately, so theoretical models or experimental studies need to be reconstructed. In the experimental research on penetration of spudcan into soil, Tani and Craig (1995) found through model test study that when the spudcan penetrates into soil, soil above the spudcan will flow back. When studying the mechanism of spudcan penetrating into soil by centrifuge model test, Hossain et al. (2005) found that there are two obvious stages of soil deformation. In the first stage, the soil is squeezed to both sides of the spudcan, and holes appear above the spudcan. In the second stage, the soil on both sides above the spudcan flows back into the hole. Le et al. (2021) establish a finite-element model which considered the materia land geometrical nonlinearity in the structure and the nonlinear interaction between soil and structure to investigate the collision response between the spudcan and seabed during lowering spudcan. Neither the model test nor the field test can directly obtain the displacement field of soil beside the pile when the spudcan penetrates, which results in the lack of comparison between deformation theory and internal deformation test results.
Many scholars have carried out indoor model tests on the compaction effect of pile driving. Iskander et al. (1994) proposed the technology of artificial synthesis of transparent soil. The principle of this technology is to use granular materials with the same refractive index and pore fluid to prepare transparent materials, so that the soil becomes transparent and visible, and the displacement field inside the soil is visualized. Yin et al. (2018) carried out the model tests on the penetration of six flat-ended piles in synthetic transparent soil. The movement of pile-soil interaction was determined by comparing the images before and after penetrations. Qi et al. (2017) used a small-scale 1-g transparent soil model to investigate soil deformations caused by casing-assisted pile jacking of plastic tube cast-in-place concrete piles. It was found that under unconfined conditions, soil response to jacking is significantly affected by the pile shoe shape, with displacements adjacent to a conical shoe markedly smaller compared to a flat shoe. In recent decades, particle image velocimetry (PIV) has been rapidly developed in geotechnical engineering. White et al. (2003) combined semi-model test with PIV technology to study the displacement field of pile penetration effect in soil.
At present, the theory of soil deformation mainly includes cavity expansion method, strain path method and shallow strain path method. The cavity expansion method (CEM) (1972) is the most widely used method to solve the impact of pile sinking on the surrounding soil, which is simple in form and easy to solve. However, CEM only takes into account radial deformation. For this purpose, Baligh (1985) proposed the strain path method (SPM), which can reflect two-dimensional deformation of soil and usually applies to deep foundation problems. When the pile-soil interaction is predicted with SPM, the surface soil deformation does not match the actual deformation. To this end, Sagaseta and Whittle (2001) proposed the shallow strain path method (SSPM) on the basis of SPM, which can predict the amount of surface subsidence caused by tunnel excavation, as well as the soil deformation caused by pile setting. Such deformation theory has been widely used in pile foundation, but its effectiveness in spudcan is still debatable.
In order to study the the deformation mechanism of sand around the spudcan, the transparent soil test of spudcan were carried out. Then, the theoretical value of horizontal displacement of sand around the spudcan were derived by using the SSPM. By comparing with the model test results, a discussion was conducted against the effectiveness of SSPM applied to the spudcan, to solve the incomplete issues about the research on spudcan penetrating into sand.
2.1 Transparent Soil
The transparent soil, which was composed of aggregate (fused quartz) mixed with pore solution (Puretol7 and Krystol40), developed by Ezzein et al. was employed in this test. The grain gradation of fused quartz is shown in Fig. 1. The pore solution was prepared with Puretol7 and Krystol40 in a volume ratio of 1:2.77. At 20℃, the refractive indexes of the pore solution prepared and fused quartz were 1.458 and 1.459, respectively. According to the experiments by Qi (2015), Ezzein (2011), Guzman et al. (2014), the physical indexes of the new transparent soil used in this experiment can be basically regarded as consistent with that of natural saturated sand under relevant gradation conditions. Through the test, the prepared transparent soil exhibited a maximum dry density of 1.222g/cm3, a minimum dry density of 0.963g/cm3, and relative density of 0.231, 0.665, 0.821 and 0.930, respectively, simulating the test conditions of loose sand, medium-dense sand and dense sand. The test conditions of transparent soil are shown in Table 1.
| Test Number | Relative Density of Soil Sample | Recording Point of Penetration Depth | |
|---|---|---|---|
| T1 | 0.930 | 1.3R | 2.1R |
| T2 | 0.821 | 1.3R | 2.1R |
| T3 | 0.665 | 1.3R | 2.1R |
| T4 | 0.231 | 1.3R | 2.1R |
2.2 Test Device
The transparent soil model of spudcan is shown in Fig. 2. The model test device includes model box, transparent soil, spudcan foundation, digital camera, laser and driver. The model box was made of 20mm thick organic glass (dimension: length×width×height = 236mm×164mm×164mm). A He-Ne laser was set on the left side of the model trough, and the laser beam emitted from the laser was projected as the laser surface through the line generator to enhance the visualization effect of transparent soil particles. The spudcan model penetrated into the transparent soil through the pressure of the driver on it. In the process of penetration, the whole process of spudcan penetration was recorded by a digital camera set up in front.
2.3 Model Pile Size
In this experiment, a spudcan model with a reduction ratio of 1:1000 to the actual spudcan was made by acrylic plastics, and the size is shown in Fig. 3. The maximum section diameter of the spudcan model was 24mm, the pile tip angle was 70°, the included angle of the cone on both sides of the middle was 20°, and the central partition was 4mm thick.
2.4 Test Image Processing
In the transparent soil test, the images of this experiment were processed by GEO-PIV software, which was developed by White et al. (2002) in the process of studying the soil deformation caused by pile sinking using PIV. Based on the PIV principle and MATLAB module, the digital image was analyzed to obtain the deformation displacement field. In addition, the refractive index of model glass trough and transparent soil, and lens distortion will affect the analysis results to varying degrees, so attention should be paid to the correction in advance.
3.1 Displacement Vector of Sand around Spudcan
The displacement vectors of spudcan penetrating into transparent soil with relative density of 0.821 and 0.930 (dense sand) are shown in Fig. 4 and Fig. 5. According to the analysis of Fig. 4 and Fig. 5, with the increase of penetration depth, the disturbance range of soil around the spudcan also expands. When penetrating into 1.3R, the affected range of surrounding soil is 2.5R ~ 3.5R in the horizontal direction, and is 1R ~ 1.5R in the vertical direction. When the penetration depth increases to 2.1R, the affected range of surrounding soil expands to 3.5R ~ 5R in the horizontal direction, and is 1.5R ~ 2.5R in the vertical direction.
With soil around the spudcan as the research object, its displacement trend was analyzed in detail. On the whole, under the combined action of horizontal force applied during the penetration of spudcan and the occlusal friction between soil particles, soil around the spudcan presented an upward overall displacement trend. Vertically, the soil on the upper pile tip had a large displacement near the surface. Horizontally, the displacement of soil around the spudcan decreased as the distance between soil and pile increased.
Locally, when the penetration depth of spudcan is relatively shallow, due to the specific structure of spudcan, the soil shifts to both sides and a cavity is formed at the top of spudcan. This is because soil particles fail to flow back in time due to the shallow penetration depth and the short penetration time. With the further penetration of spudcan, backflow appears at the upper part of the spudcan, and the displacement of soil particles that flow back is smaller than that of soil around the spudcan.
The displacement vectors of spudcan penetrating into transparent soil with relative density of 0.665 (medium-dense sand) and 0.231 (loose sand) are shown in Fig. 6 and Fig. 7.
Compared with Fig. 4 and Fig. 5, soil displacement vectors observed in Fig. 6 and Fig. 7 are more disordered. As the relative density of soil decreases, the occlusal friction between soil particles also decreases, which results in that the squeezing force caused by penetration of spudcan into soil decreases more slowly in the transfer process and expanding the influence range and displacement degree of soil.
3.2 Contour Map of Horizontal Displacement of Sand around the Spudcan
To quantitatively describe the displacement of soil around the spudcan, Fig. 8 shows the normalized horizontal displacement contour map of the surrounding soil when the spudcan penetrates into the transparent soil with a relative density of 0.930 to 2.1R. As shown in Fig. 8, the horizontal displacement of soil around the spudcan decreases with the increase of the distance from spudcan, and the closer the soil layer is to the surface, the slower the trend of displacement reduction. At the same time, due to the backflow of soil at the upper part of the spudcan, the displacement of soil particles that flow back is smaller than that of soil around the spudcan.
Figure 9 and Fig. 10 exhibit the normalized horizontal displacement contour map of soil around the spudcan when the spudcan penetrates into the transparent soil with relative density of 0.665 and 0.231 to 2.1R. The displacement trend of each group of soil is roughly similar, but due to the decrease in the relative density of soil, the occlusal friction between soil is greatly reduced, so that the range of soil affected by the horizontal squeezing force that is generated by the penetration of spudcan into soil is larger, and accordingly, the horizontal displacement of soil also increases greatly.
4.1 Theoretical Introduction
4.1.1 SSPM
At present, the theoretical methods for predicting soil deformation caused by soil-foundation interaction mainly include: CEM, SPM and SSPM. According to the SSPM model proposed by Sagaseta and Whittle (2001), when a pile with a radius of R and a length of h is driven, the radial displacement L at the ground surface r away from the central axis of the pile can be expressed by the following equation:
5Wherein, R is the pile diameter, the maximum section diameter of spudcan, which shown in Fig. 3.
4.2 Comparative Analysis
4.2.1 Horizontal Displacement of Soil around the Spudcan
The horizontal displacement of soil around the spudcan in the T1, T3 and T4 tests were compared with the normalized SSPM result. As shown in Fig. 11, the penetration depth H of the spudcan was selected to be 2.1R, and the horizontal displacement of soil at 1R, 2R, 3R, 4R and 5R along the spudcan height z was selected and compared with the theoretical results. In the Fig. 11, the abscissa is normalized radius r/R, and the ordinate is normalized horizontal displacement L/R.
By analyzing Fig. 11, the test results of each group are basically consistent with the SSPM theoretical solution regarding the change law of horizontal displacement. Considering the specificity of the spudcan structure and the boundary effect of the model test, when z/R = 2 and r/R < 4, the correlation between the horizontal displacement change law of T1, T2 and T3 and the theoretical solution is 60%~80%.
From the overall analysis, apparently, with the decrease of soil compactness, the influence degree and range of spudcan penetration on the surrounding soil displacement become increasingly larger. From the local analysis, when z/R = 1 and r/R < 4.5, the test data of T3 is greater than that of the SSPM, which is caused by the weak occlusal friction between soil particles with low relative density. When z/R = 1 and r/R > 4.5, the horizontal displacements of T1, T3 and T4 are smaller than that of the SSPM, which is mainly due to the boundary effect of the model test. When z/R = 5, the horizontal displacements of T1, T3 and T4 are basically same, which are close to 0, indicating that the influence range of spudcan penetration on the soil at the bottom of the pile pile is roughly 3R.
Based on digital photography/photogrammetry, transparent soil and PIV, the transparent soil test of spudcan penetrating into sand was designed to study the displacement field of sand around the spudcan. The conclusions are drawn as below:
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It is observed from the transparent soil test that soil around the spudcan moves upwards obliquely. At a relatively shallow penetration depth, the soil at the upper part of spudcan is squeezed to both sides to form a cavity in the upper part. As the penetration depth further increases, the soil on the pile side flows back to both sides. The relative density of soil contributes to the displacement change of soil around the spudcan. As the relative density of soil decreases, the influence range of soil around the spudcan expands, and the displacement of soil particles decreases slowly.
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The horizontal displacement field of soil around the spudcan was obtained through the transparent soil model test based on PIV technology. The test results were compared with the SSPM theoretical solution, and their variation laws are basically the same. When z/R = 2 and r/R < 4, the correlation between the horizontal displacement change law of each group and the theoretical value reaches 60%~80%, which verifies the reliability of the model test.
- Tani K., and Craig W. 1995. Bearing capacity of circular foundations on soft clay of strength increasing with depth. Soils and Foundations. 35(4): 21-35.
- Hossain, M.S., Hu, Y., Randolph, M.F., and White, D.J. 2005. Limiting cavity depth for spudcan foundations penetrating clay. Géotechnique. 55(9): 679-690.
- Le, C.H., Li, Y.E., H, L., Ren, J.Y., Ding, H.Y., and Zhang, P.Y. 2021. Collision analysis between spudcan and seabed during the process of jack-up platform lowering jack-up legs. China Ocean Eng. 35(5): 779-788.
- Iskander, M., Lai, J., Oswald, C., et al. 1994. Development of a transparent material to model the geotechnical properties of soils. Geotechnical Testing Journal. 17(4): 425-433.
- Yin, F., Xiao, Y., Liu, H.L., Zhou, H., and Chu, J. 2018, Experimental Investigation on the Movement of Soil and Piles in Transparent Granular Soils.Geotechnical and Geological Engineering. 36(8): 783-791.
- Qi, C.G., Iskander, M., and Omidvar, M. 2017. Soil Deformations During Casing Jacking and Extraction of Expanded-Shoe Piles, Using Model Tests. Geotechnical and Geological Engineering. 35(2): 809-826.
- White, D.J., Take, W.A., and Bolton, M.D. 2003. Soil deformation measurement using particle image velocimetry(PIV) and photogrammetry. Geotechnique. 53(7): 619-631.
- Vesic, A.S. 1972. Expansion of cavity in infinite soil mass. Journal of the Soil Mechanics and Foundations Division. 98(3): 265-289.
- Baligh, M.M. 1985. Strain path method. Journal of Geotechnical Engineering. 111(9): 1108-1136.
- Sagaseta, C., and Whittle, A.J. 2001. Prediction of ground movements due to pile driving in clay. Journal of Geotechnical and Geoenvironmental engineering, ASCE. 127(1): 55-66.
- Qi, C.G., Chen, Y.H., Wang, X.Q., and Zuo, D.J. 2015. Physical modeling study on buckling of slender pile using transparent Soil. Chinese Journal of Rock Mechanics and Engineering. 34(04):838-848. (in Chinese)
- Ezzein, F.M., and Bathurst, R.J. 2011. A transparent sand for geotechnical laboratory modeling. Geotechnical Testing Journal. 34(6): 590-601.
- Guzman, I.L., Iskander, M., Suescun-Florez, E., and Omidvar, M. 2014. A transparent aqueous-saturated sand surrogate for use in physical modeling. Acta Geotechnica. 9(2): 187-206.
- Li, H.D., Tang, C.S., Cheng, Q., Li, S.J., Gong, X.P., and Shi, B. 2019. Tensile strength of clayey soil and the strain analysis based on image processing techniques. Engineering Geology. 253: 137-148.
- White, D.J., Take, W.A. 2002. GeoPIV: Particle image velocimetry(PIV) software fou use in geotechnical testing[M].
- Zhou, H., Kong, G.Q., and Liu, H.L. 2014. Study on pile sinking compaction effect of hydrostatic wedge pile using cavity expansion theory. China Journal of Highway and Transport. 27(04): 24-30. (in Chinese)
- Liu, F.C., Shang, S.P., and Wang, H.D. 2011. Study of shear properties of silty clay-concrete interface by simple shear tests. Chinese Journal of Rock Mechanics and Engineering. 30(08): 1720-1728. (in Chinese)
- Zhang, Q.Q, Li, S.C., Li, L.P., and Chen, Y.J. 2013. Simplified method for settlement prediction of pile groups considering skin friction softing and end resistance hardening. Chinese Journal of Rock Mechanics and Engineering. 32(03): 615-624. (in Chinese)