Soil Plug Research Articles

Displacement analysis of open pipe piles sinking based on strain path method

The displacement field in the process of opening pipe piles sink has always been a hot issue in the study, especially in the area of pile, the soil body has produced a large strain, regardless of the large deformation effect of the soil, there will be a large calculation error. The strain path method can obtain the flow velocity of the soil because it does not rely on the constitutive relationship of the soil, which is a favorable theory for solving large strain problems. In this paper, the modified strain path method is used as the theoretical basis. The open pipe pile is equivalent to a closed pipe pile through the soil plug effect, and the sunk pile displacement of the open pipe pile is calculated to further reveal the soil plug effect and the soil squeeze effect of the open pipe pile mutual restraint and mutual influence, thus providing a theoretical basis for the pile displacement of open pipe piles.

Model Tests on the Penetration Resistance of Bucket Foundations for Offshore Wind Turbines in Sand

Driven by the rapid development of offshore wind farms, bucket foundations have come to constitute a very promising form of foundation for offshore wind turbines, mainly owing to their efficient construction. However, the penetration resistance of the suction penetration of a bucket foundation, when calculated inaccurately, may lead to installation failure of the foundation. In this study, model tests were performed on the suction penetration of a mono-bucket mono-compartment foundation and a mono-bucket multi-compartment foundation in saturated fine marine sand, aiming to compare their penetration resistances and critical suctions, and the development of a soil plug in the two models was analyzed. The results will provide a design reference for the penetration construction of bucket foundations for offshore wind turbines.

Study of plugging of steel H piles, jacked in sand, in a calibration chamber

When predicting the bearing capacity of an H pile, the design engineer is faced with the questions of which perimeter of the pile to consider for lateral resistance, and which base area for tip resistance. These questions derive from the doubt regarding the occurrence of ‘plugging’. The phenomenon, in which soil adheres strongly to the pile, causes failure to occur not at the steel–soil interface but at an enveloping area. There is little experimental data specifically on the plugging of H piles, in which the contribution of shaft and tip loads were properly separated. Different test interpretations by researchers and designers have led to different recommendations for the design of such piles, some considering and others dismissing soil plugging. The research herein described was conducted on instrumented model H piles jacked in fine sand in a large flexible calibration chamber. The results indicate that – even in loose sand – plugging does occur in H piles close to the pile tip and that the tip resistance is an important component of the bearing capacity of such piles.

Movement of an Open-Ended Pipe with a Soil Plug Under a Longitudinal Impact

The process of horizontal immersion into soil of an open-ended steel pipe of circular cross section containing a soil plug is under study. The effect of both single and multiple longitudinal strikes is considered. A one-dimensional mathematical model of the movement of the pipe and soil plug is proposed that takes into account changes in radial stress caused by the lateral interaction of the pipe and soil plug. In this model, the interaction of the pipe with internal and external soil is described by the law of external dry friction. The resistance of the soil in front of the pipe is taken into account by specifying viscous conditions at the pipe end. The influence of the parameters of the pipe and soil on the process of immersing an open-ended pipe into the soil is studied numerically by the finite-difference method. In addition, the results of a field experiment are given which was carried out by the authors in dense urban environment. As a result of comparing the data obtained in numerical and field experiments, it is concluded that the proposed model can be used to describe the results of field experiments.

Effect of diameter on the load carrying capacity of Closed-Open Ended Pipe piles

This study investigates the behavior of the components on the pile loading capacity and the effect of removing the components. The sand is used as a fine, sandy soil. It was prepared with different densities using rain technology. The soil plug was removed by a device that was manufactured to remove the soil column trapped inside the piles of the pipes during installation by driving and pressing devices. The present study focuses on determining the effect of soil plug on the maximum load capacity of the open-end steel pipe pile and comparing it with the closed pipe pile. A new type of piles of pipes is proposed; piles of open-ended organic pipes driven and compressed in sandy soils of different densities (medium and dense) where axial pressure tests were carried out on the piles of the model. The end of the pile will be open at a predetermined depth in order to facilitate the penetration of the pile base and close it at a distance to increase the resistance of the pile base. The open pile models were modified by closing the ends of the pile by a welded plate at a distance of two, three, and four D (where D is the diameter of the pile) by the pile.It was concluded that increasing the diameter of the pile from 20 mm to 40 mm caused increase in the pile bearing capacity by about 275 – 615%, while in closed-ended piles, the same increase in pile diameter caused increase in the bearing capacity of about 320 – 860%. On the other hand, in closed-open-ended piles, this increase improves the bearing capacity by about 490 – 1020%. The results prove the controlling importance of the pile diameter in the proposed type of piles; closed-open-ended type.

The loading behavior of innovative monopile foundations for offshore wind turbine based on centrifuge experiments

Wind energy is a promising source of renewable energy and is projected to shift to offshore areas increasingly. Monopile foundation is one of the most commonly used foundations for offshore wind applications with the priority in load bearing capability and initial cost. This study describes an innovative monopile foundation, which institutes a creative strategy over the traditional large diameter monopile foundation to achieve higher axially load bearing capacity. This is achieved by adding a restriction plate inside the pile to intensify the soil plug effect. This design is based on the soil plug mechanism, and the arching effects and plug resistance mobilizations are considered. In this study, an extensive amount of geotechnical centrifuge experiments was conducted to analyze the bearing behaviors of the innovative monopile with restriction plates. The pile with 1-hole restriction plate and the pile with 4-hole restriction plate are considered to discuss effects of the plate shape. Twelve models with different diameters and restriction plate types are investigated. The traditional open-ended and close-ended piles are included for comparisons. The static tests are conducted in saturated silica sand first to determine the ultimate bearing capacity of the innovative pile, after which the cyclic tests are performed. The innovative pile is proved to provide a larger bearing capacity than the pipe pile. An analytical method is proposed to estimate the capacity of the innovative pile. The study aims to develop the design code for innovative piles and provide design reference to large-scale offshore wind turbine projects.

The Seepage and Soil Plug Formation in Suction Caissons in Sand Using Visual Tests

The rapid development of offshore wind energy in China is becoming increasingly relevant for movement toward green development. This paper presents the results of visual tests of a suction caisson used as foundation for offshore wind turbines. The distribution of hydraulic gradients of sand at the mudline in the caisson was obtained to find out the relationship with the heights of soil plugs. The relationship equation was proposed and obtained by using quadratic regression, guiding project designs, and construction. It was found that there was no soil plug in the caisson when small suction was applied during the suction penetration. The relationship between the heights of the soil plugs and the hydraulic gradient of the soil was proposed and obtained by using quadratic regression to predict (roughly) the height of soil plugs in suction caissons in sand during suction penetration. The influence of settlement outside caissons on the soil plug was found to decrease as the buried depth rose.

Torsional complex impedance of pipe pile considering pile installation and soil plug effect

Torsional vibrations of pipe pile are more complicated compared to solid pile. In actual engineering, pile surrounding soil may be strengthened or weakened due to different pile installation methods, which is regarded as pile installation effect. At the same time, soil plug effect which describes the interaction between the soil plug and pile wall is also non-negligible in bearing analysis. Along the model of inhomogeneous medium, this paper subdivides the inhomogeneous soil region into certain sub-zones whose shear complex stiffness are derived by virtue of continuity conditions and plain strain model. To study the influence of soil plug effect, the additional mass model is raised and utilized in this paper. In this model, the pile-soil plug system is vertically split into infinite sections, and each pipe pile section and soil plug section is connected by distributed Voigt model. The analytical solution of torsional complex impedance is acquired and compared with results obtained from former studies to verify its rationality. Finally, a parametric research is presented to exhibit the influence of the pile installation and soil plug effect on the torsional complex impedance of pipe pile head.

Model tests on the bearing capacity of precast open-ended micro pipe piles in soft soil

Laboratory 1g model tests were carried out to study the bearing capacity and load transfer mechanisms of precast open-ended micro pipe piles in soft soil. The single pile, 2 × 2 and 3 × 3 pile groups were designed and tested under vertical compression, uplift and horizontal loads, respectively. A steep fall pattern occurred in the load–displacement curves of piles under vertical compression and uplift loads. The measured soil plug length and axial force indicated that the piles could be considered as fully plugged friction piles under vertical compression load. The pile groups behaved in a non-integral failure mode under vertical uplift load and a conical wedge failure mode under horizontal load. At the early loading stages under vertical compression and uplift loads, the pile shaft resistance of the upper part tended to be fully mobilised earlier than that of the lower part. At the later loading stages, the pile shaft resistance of the upper part decreased, whereas the shaft resistance of the lower part still showed a little increase. The maximum bending moment of the pile occurred at a depth of about 10Dp under horizontal load.

Numerical and Experimental Study on Soil Plug Resistance of Open‐Ended Pipe Pile with a Restriction Plate

The end bearing capacity of open‐ended pipe piles is highly dependent upon the soil plugging behavior inside the pipe wall. For large‐diameter open‐ended pipe piles, the arching effect due to inner soil‐pile friction may be insufficient to generate a fully plugged condition, which compromises the end bearing capacity after the pile installation. Here, we propose two innovatively designed restriction plates installed inside the pipe to facilitate the soil plugging process, that is, a restriction plate with one circular hole and a restriction plate with four semisized circular holes. By use of the discrete element method, the mechanical behaviors of soil plugs in cohesionless soils with different restriction plates are analyzed. The numerical model has been validated by comparing the simulations to the results of a series of laboratory‐scale experiments over different pile diameters, plug length‐diameter ratios, and different types of restriction plates. It is shown that the numerical model can accurately predict the soil plug resistance and the particle‐scale force transmission. Both numerical simulations and experiments validate that the arching effect is significantly enhanced by the restriction plate, especially with the four‐hole restriction plate. Furthermore, the geotechnical centrifuge principle is adopted into the DEM model to study the mechanical behavior of large‐diameter soil plugs. It is found that the effectiveness of restriction plate decreases as the pile diameter increases. However, even for the 2 m in diameter soil plugs, the plug resistance is increased by 252% with one‐hole restriction plate and 281% with four‐hole restriction plate compared to that of traditional pipe piles.

Интерпретация результатов испытаний забивных трубчатых свай с грунтовой пробкой

The paper describes the results of dynamic pile tests performed accounting to wave impact theory and analyzed utilizing specific devices and program codes as PDI and CAPWAP. It is noted, that the program codes do not take into account an occurrence of soil plugging during pile driving process. The results of pile tests utilizing wave impact theory, strain gauges and accelerometers are processed and compared to the theoretical solution of the wave equation, which describes pile driving process allowing for soil plugging.

Mist chamber extraction for improved diagnosis of Meloidogyne spp. from golf course bermudagrass

Meloidogyne spp. are among the most damaging plant-parasitic nematodes to golf course bermudagrass in the southern United States. Diagnostic samples processed by centrifugal flotation often recovered only low numbers of vermiform Meloidogyne spp. life stages (J2 and males) from soil, while roots were found to be heavily infested by sedentary life stages. Therefore, the University of Florida Nematode Assay Lab (NAL) evaluated mist extraction from turf plugs as a method for diagnosis of Meloidogyne spp. from golf course bermudagrass. Soil and turf plugs were obtained from 596 golf course bermudagrass small plots from multiple locations and cultivars over several years, and vermiform Meloidogyne spp. extracted from 100 cm3 of soil by centrifugal flotation and by mist chamber extraction from four 3.8-cm-diam. turf plugs were compared. Additionally, both extraction methods were performed on 431 golf course bermudagrass diagnostic samples received by the NAL from Florida, 36 golf course bermudagrass diagnostic samples from Texas, and 34 golf course bentgrass/bluegrass samples from California. In the small plots, and the bermudagrass samples from Florida and Texas, mist extraction had higher detection and recovery rates of vermiform Meloidogyne spp. than did centrifugal flotation. However, centrifugal flotation had higher detection and recovery rates than mist extraction from bentgrass/bluegrass samples from California. Mist extraction from turf plugs is superior to centrifugal flotation from soil for diagnosis of Meloidogyne spp. on golf course bermudagrass, but not on golf course bentgrass and bluegrass.

Centrifuge modelling of uplift response of suction caisson groups in soft clay

This paper describes a program of centrifuge model tests on the uplift behaviour of suction caisson foundations. The parameters considered were the loading rate, caisson diameter (D), soil strength profile, and type of footing (i.e., mono-caisson and tetra-caissons group). The loading responses were examined in terms of total uplift resistance, suction beneath the caisson lid, and the vertical displacements of the caisson and at the soil surface. There exists a critical uplift displacement, approximately 0.02D and 0.01D for the mono-caisson and the tetra-caissons groups, respectively, at which a turning point can be identified in the load–displacement curve. This was found to be attributed to the adhesion on the caisson–soil interface reaching a peak response and then dropping. Of interest is that the tetra-caissons group exhibits much greater normalized uplift resistance than the mono-caisson group before reaching an uplift displacement of about 0.02D, suggesting superiority of the former in term of serviceability. However, a reversed trend was observed at greater displacement, and accordingly an empirical model was derived to quantify the shadowing effect of caisson groups. Regarding the cyclic response, several cycles of large-amplitude loading are sufficient to reduce the ultimate bearing capacity of caisson(s) to below the self-weight of the inner soil plug(s), indicating a transition of failure mechanism.

Effect of soil plug removal on the load-carrying capacity of symmetric and non-symmetric pile groups

ABSTRACT This study comprises experimental tests that consist of 138 tests that are divided as follows: 36 tests for single pile, 36 tests for pile group (2 × 1), 36 tests for pile group (2 × 2) and 30 tests for pile group (2 × 3). All tubular piles are tested in poorly graded sand. Different parameters are considered such as the method of installation, relative density, removal of soil plug with respect to length of plug and pile length-to-diameter ratio. Strain gauges are used to separate skin friction with two components of shear resistance and end bearing. It was concluded that the removal of soil plug from piles (single or group) will cause a reduction in single and group capacities due to the elimination of two components, internal friction and the confining in end-bearing zone. The maximum reduction occurs in the dense state of soil with 61.5% and 83% for single and group of piles, respectively.

Detection sensitivity analysis of pipe pile defects during low-strain integrity testing

The detection sensitivity of pipe pile defects during low-strain integrity testing is investigated with the consideration of soil plug, the soil sandwiched in neck defect and high-frequency interference at pile head. The additional mass model and fictitious ring-soil pile model (FRSPM) are introduced to simulate the soil plug effect and the dynamic interaction between the sandwiched soil and neck defect, respectively. The high-frequency interference is realized by considering the stress wave propagating both in the vertical and circumferential directions. An analytical solution for the dynamic velocity response of pipe pile is derived in the frequency domain through the transfer function method, and the corresponding semi-analytical solution in the time domain is then obtained by using the inverse Fourier transform. The reasonability and accuracy of the developed solution are verified by a series of field experiments. The dynamic interaction between the neck defect and the FRSPM is clarified via the parameter sensitivity analysis. The non-detection zone of pipe pile during low-strain integrity testing is analyzed. It is shown that the main factors to affect detection accuracy are the reduction degree and the location of defects, and there exists a location non-detection zone near the pipe pile toe.

Mechanisms of soil plug formation of open-ended jacked pipe pile in clay

The mechanisms of soil plug formation during the jacking of an open-ended pipe pile into clay is investigated by using Coupled Eulerian-Lagrangian (CEL) method. According to the evolution of soil velocity field, the formation of soil plug is divided into: the upwelling stage, the partially plugged stage and the fully plugged stage. A concave arch is formed below the pile tip when the stress bulb expands from the pile wall to the pile axis. The initial soil plug and the fully soil plug are formed when the vertical stress increment of the soil below the pile tip reaches 6 and 9–10 times undrained shear strength (Su), respectively. The results show that soil plug is easier to be formed in the pile with smaller diameter. The pipe pile shows a fully plugged mode in the stiff soil overlaying soft soil, while pipe pile shows a partially plugged mode in the soft soil overlaying stiff soil. The soil plug is hard to be formed in inhomogeneous clay due to the higher soil strength of deeper soil. An effective method is proposed to estimate the formation of soil plug with the equation for vertical stress increment of the soil below the pile tip.

Effects of strain softening on the penetration resistance of offshore bucket foundation in nonhomogeneous clay

The concrete-made bucket foundations were employed as emerging solution to support the wind turbine or breakwater, where the relatively large thicknesses of skirts and the strain softening effect related to the soil sensitivity and ductility were necessary to be explicitly accounted for. However, few of current study concerned this topic. This paper therefore reported an extensive parametric investigation of penetrating behaviour for bucket foundation in nonhomogeneous clay to allow a better estimation of penetration resistance of bucket foundation. The three-dimensional (3D) large deformation finite element (LDFE) analysis model incorporating the effect of strain softening on soil strength was set up and validated by comparing to the published centrifuge data. Then the evolution of soil flow mechanism and the distribution of soil strength softening were examined with the soil plug generated inside skirt assessed. Based on the numerical results, a new analytical model was proposed to predict the penetration resistance of bucket foundation in clay accounting for a partially-remoulded condition.

Soil plug heave induced by suction bucket installation on sand via centrifuge model tests

Suction pressure created during suction bucket installation may disturb the surrounding soil and affect its properties. Soil heaving can also occur owing to upward seepage inside the bucket embedded in sand, which hinders soil penetration; however, this mechanism is not well understood. Centrifuge model tests were performed to investigate this soil behaviour and its effect on the installation. Suction pressure, seepage flow, and soil plug heave were monitored under various installation conditions. Bender element and cone penetration tests were also conducted to evaluate the effect of soil plug loosening indirectly. The results illustrate the effects of upward seepage on soil plug heave during suction bucket installation. This study highlights the changes in the soil properties inside the bucket owing to the suction installation. This is an important design consideration for bucket installation, which also affects the bearing capacity of the suction bucket foundation.

Observations during suction bucket installation in sand

Suction buckets represent a viable solution as foundations for offshore wind turbines. Installation in sand is relatively straightforward, albeit with limited understanding of the resulting changes in soil state. This paper describes an experimental methodology that allows for visualisation and quantification of changes in soil state during suction bucket installation, validated in sand. Insights obtained from particle image velocimetry analyses, performed on images of a half-bucket installing against a Perspex window taken in a geotechnical centrifuge are discussed. Compared with the initial self-weight penetration, the deformation mechanism governing the suction-assisted phase shows a preference for the soil below the skirt tips to move inwards and upwards inside the bucket. The installation process is responsible for changes in relative density and permeability within the bucket. In these experiments, the majority of the soil plug heave can be attributed to the soil displaced inwards by the advancing skirts, with a minor contribution caused by dilation. The confidence in the experimental methodology provided through the results of suction bucket installation in sand discussed herein now enables suction bucket installation in more complex seabeds to be investigated.

Load Transfer of Offshore Open-Ended Pipe Piles Considering the Effect of Soil Plugging

Open-ended pipe piles have been increasingly used as the foundations for offshore structures. Considering the soil plugging effect, a novel analytical model is proposed in this paper to study the load transfer mechanism of open-ended pipe piles. A trilinear model for the external shaft friction was introduced, while a rigid plastic model was adopted to describe the load transfer at the pile-plug interface. Furthermore, an equilibrium equation of the soil plug was proposed, based on the hypothesis of a trilinear distribution of lateral earth pressure. The pile end resistance was analyzed by dividing it into two parts, i.e., the soil plug and pile annulus, the behaviors of which were described by the double broken line model. A calculation example was carried out to analyze the load transfer properties of the open-ended pipe piles. As a validation, similar load transfer processes of the open-ended pile were also captured in a newly built discrete element method model, mimicking the 100g centrifuge testing conditions.