Skirt Wall Research Articles

Bearing capacity of large-diameter pile foundation under combined VHM loadings in soft clay

ABSTRACT The large-diameter pile foundation is with an intermediate depth, widely used in engineering fields such as ports, breakwaters and rapid construction of artificial islands as a supporting structure to resist lateral loads. However, there is limited research on the bearing performance of such large-diameter, intermediate-depth pile foundations. In this paper, the finite element method is used to investigate the bearing capacity of the large-diameter pile foundation with different embedment ratios in homogeneous soil and normally consolidated soil and the prediction formulas for uniaxial and combined bearing capacity are developed. This study also sets up three different skirt wall–soil interface properties to discuss their influence on the bearing performance of the large-diameter pile foundation. The research is systematic, and relevant analysis can provide practical suggestions for engineering design.

Numerical model for suction anchor under vertical impact loading

In marine engineering, suction anchors are widely employed as the supporting foundation for many types of fixed or floating structures. One of the important control conditions for the service of marine foundations is the impact loading in extreme environments. This study proposes a numerical model to predict the dynamic behavior of suction anchors under impact loading. A mass module is added to the numerical model to quantify the effect of anchor inertia. The t-z springs are positioned along the depth to reflect the soil layer stress and accurately calculate the skirt wall friction. Considering the seepage effect, the differential pressure development, internal soil stress loss and external soil stress enhancement are also reflected. Subsequently, the influence of anchor aspect ratio and loading history are investigated. As the foundation uplift movement is further pronounced, the friction resistance gradually works, resulting in the weakening of inertial force. The existence of pretension loading makes friction resistance take effect earlier, and accelerates its attenuation during the impact loading stage.

Twelve-Month Clinical and Histopathological Performance of a Novel Synthetic Cornea Device in Rabbit Model.

To report the biological stability and postoperative outcomes of a second-generation, single-piece, flexible synthetic cornea in a rabbit model. Device materials and design were amended to enhance biointegration. Optic skirt design devices were made from compact perfluoroalkoxy alkane with porous expanded polytetrafluoroethylene ingrowth surface overlying the skirt and optic wall. Sixteen devices were implanted into intrastromal pocket in rabbit eyes. Rabbits were randomly assigned to 6- and 12-month follow-up cohorts (n = 8 in each) postoperatively. Monthly examinations and optical coherence tomography assessed cornea-device integration, iridocorneal angle, optic nerve, and retina. There were no intraoperative complications. All devices were in situ at exit, with clear optics. No retroprosthetic membrane, glaucoma, cataract formation, or retinal detachment was observed. Two rabbits in the 6-month group had mild, focal anterior lamella thinning without retraction adjacent to the optic near tight sutures. Three postoperative complications occurred in the 12-month group. One rabbit diagnosed with endophthalmitis was euthanized on day 228. Mild sterile focal retraction of anterior lamella occurred in two rabbits, which were terminated on days 225 and 315. Light microscopic examination of enucleated globes demonstrated fibroplasia with new collagen deposition into the porous scaffold without significant inflammation, encapsulation, or granuloma formation. Clinical evaluations, imaging, and histopathological findings indicate favorable outcomes of this synthetic corneal device in a rabbit model. Early feasibility studies in humans are being planned. Favorable 12-month results of the device in rabbits demonstrate vision-restoring potential in corneally blind individuals at high risk of failure with donor keratoplasty.

Experimental study on an isolated oscillating water column wave energy converting device in oblique waves

The oscillating water column is the most widely used wave energy conversion technology for long-term operations, which has the potential to play a critical role in massive applications of wave energy. In this study, an isolated OWC model employing a generic cubic air chamber and an impulse turbine model was tested in a wave tank using regular and irregular wave scenarios. The wave distribution characteristics surrounding the isolated OWC model in oblique waves with various incident angles and other testing conditions were reported. As the incident waves were forward, the symmetrical wave-height distributions could be observed with enhancement and reduction zones in front of the skirt wall and in the sheltered area of the model. Furthermore, the wave distributions were asymmetrical and more complicated in oblique waves. In addition, the effects of the incident wave angle, height, period, and rotation speed of the turbine model on the staged and overall energy-harvesting performances were analyzed. The OWC model obtained the peak energy-harvesting performance, which reduced as the incident angle decreased. As the incident angle decreased to 60° and zero, the overall capture width ratio reduced to approximately two-thirds and a quarter of the value for the incident angle of 90°. Based on experimental results, the annual energy production of a prototype isolated OWC plant in a typical sea state in North China was predicted. The annual energy production of the plant in the actual sea condition was approximately two-thirds of that only in the forward wave incidence.

Investigation of composite bucket foundation bearing characteristics in various soil properties

A composite bucket foundation (CBF) is a new foundation type of offshore wind turbine. The investigation of the bearing characteristic of CBF is a crucial part of the structural design and safety assessment of offshore wind turbines. This study investigates the bearing characteristic of CBF in sand and clay foundations using centrifuge model test and numerical simulation. Analysis revealed that as load increases, the rotation center of the CBF moves toward the central line and shallow area. The buried depth of the CBF rotation center in a clay foundation is shallower than in a sand foundation when the CBF is under ultimate loading. Due to the rotation and translation of CBF, the soil pressure–depth curves on the skirt wall in different directions will resemble parabolic, line, and triangle shapes, respectively. Under ultimate loading, the relationship between the depth and horizontal coordinate of the rotation center and embedment ratio is linear. The CBF's bearing capacity trend presents quadratic function characteristics as the embedment ratio increases. However, the bearing capacity ratio of the CBF in the clay foundation is faster than that in the sand foundation. The findings provide guidelines for offshore wind turbines.

Analytical Study on the Pullout Resistance of Suction Caisson in Clay

Abstract The pullout resistance is a design consideration for suction caisson-supported offshore structures, such as offshore platforms and wind turbines. An analytical method is proposed in this paper to calculate the pullout resistance of a single caisson foundation embedded in clay. Comparisons with laboratory model tests were also made to verify the accuracy of the proposed analytical method. Parametric studies were conducted to investigate the effects of key factors on the pullout resistance of caisson. This study found the pullout force nonlinearly increases with the increase of caisson diameter squared but linearly increases with the increase of caisson–soil frictions. For caisson wall thickness over height ratio ranging from 0.00125 to 0.00325 and caisson diameter over height ratio ranging from 0.25 to 0.75, the normal stress decreases nonlinearly along with the depth of the caisson skirt wall and is heavily influenced by the skirt wall thickness but slightly influenced by caisson diameter.

Resistance to skirt-tip with external bevels of suction caissons penetrating clay

Suction caisson is an alternative to the foundation for offshore wind turbines due to easy installation and retrieval for reuse. It is partially installed by self-weight followed by insertion to the final depth by applying suction. During suction penetration, the soil accommodated by skirt wall will move into the caisson inside, leading to soil plug heave that prevents the caisson from penetrating to the desired depth. To mitigate the effect of the soil plug heave, suction caissons are fabricated with external bevels, so as to force the more expelled soil to the outside of the caisson. This paper presents failure mechanisms for calculating the penetration resistance to skirt-tip with various external bevels under two limit conditions. For the smooth base, the failure mechanism consists of two active shear zones below the smooth base and two radial and passive shear zones that are identical to those of a deep foundation. For the rough base, two active shear zones formed below the smooth base are replaced by the radial shear zones. According to the slip-line theory, the penetration resistance of skirt-tip with various external bevels is deduced in terms of su and weighted average of σi and σo (where su is undrained shear strength, σi and σo are equivalent overburdens inside and outside the caisson at the skirt-tip level). Theoretical result shows that the resistance factor Nc of skirt-tip increases with increasing αi and αo and decreasing δ (where αi and αo are adhesion factors, and δ is the angle of external bevel from the horizontal). The difference of resistance factor Nc between rough and smooth bases identically equals 0.57. Finally, numerical simulations borrowed from the literature are used to testify the rationality of the analytical solution presented in this study.

Penetration Resistance of Skirt-Tip with Rough Base for Suction Caissons in Clay

Suction caissons can readily penetrate into the seabed under the combination of the self-weight and suction resulted from the encased water being increasingly pumped out. During suction-assisted penetration, the equivalent overburden at the skirt-tip level outside the caisson is generally higher than that inside because the vertical stress within the soil plug is reduced by the exerted suction. This may result in a uniform shear stress developing over the base of the skirt-tip as the soil below the skirt-tip tends to move into the caisson, which leads to an asymmetric failure wedge existing below the base of the skirt-tip. Besides, different adhesion factors along the inside (α1) and outside (αo) of the skirt wall will cause asymmetric plastic zones inside and outside the caisson. Accordingly, an asymmetric failure mechanism is therefore proposed to calculate the penetration resistance of the skirt-tip. The proposed failure mechanism is the first to consider the effect of different adhesion factors (αi) and (αo) on the failure mechanism at the skirt-tip, and involves the contribution from the weighted average of equivalent overburdens inside and outside caisson at the skirt-tip level. The required suction pressure can be obtained in terms of force equilibrium of the caisson in a vertical direction. Finally, the asymmetric failure mechanism at the skirt-tip is validated with the FE calculations. By comparing with the measured data, the predictions of the required suction pressure are found to be in good agreement with the experimental results.

Scattering of gravity waves by a pontoon type breakwater with a series of pervious and impervious skirt walls

ABSTRACT The scattering of surface gravity waves by a skirt wall breakwater is analysed in two dimensions under the assumption of small-amplitude wave theory. The oscillatory wave past the porous skirt walls is assumed to follow Darcy’s law. The boundary value problem is solved using multi-domain boundary element method to obtain the numerical results for the different number of porous skirt walls, relative pontoon width, relative submergence and different porosity of the skirt walls. The study reveals that an increase in the number of skirt walls from one to four can reduce the wave transmission by 61% but at the expense of an increase in horizontal wave force by 90% and decrease in the vertical force by 31%. A minimum of two-skirt walls is needed for reducing wave transmission. Skirt wall porosity of 0.40 is recommended for field applications considering the trade-off between the wave transmission and wave forces.

TWO-DIMENSIONAL PHYSICAL MODELING OF SINGLE CHAMBER SKIRT BREAKWATER (SCSB)

Gravity-type breakwaters such as the rubble mound type are ineffective in intermediate-depth and deep waters due to high construction costs. In intermediate-depth and deep waters, the skirt-type breakwater is one of the best alternatives due to the low construction cost because the breakwater structure isconsists of a pile at the bottom part and a skirt wall in the upper part of the structure. To assess the effectiveness of the skirt type breakwaters, two-dimensional physical modeling conducted on the wave flume in Ocean Engineering Laboratory at Bandung Institute of Technology, Indonesia. In this study, a Single Chamber Skirt Breakwater (SCSB) model structure was investigated. The relationship between the transmission coefficient, (CT) and the reflection coefficient (CR) with environmental and structure variables are examined. The physical modeling concluded that SCSB is effective in intermediate depth in the region of 1.5<kh<2.0. In this region,the value of CT and CR are optimum where the CT value is 0.28-0.49, and the CR value is 0.28-0.52.

Solution to critical suction pressure of penetrating suction caissons into clay using limit analysis

The critical suction pressure will lead to the difference in equivalent overburdens at the skirt-tip level between outside and inside the caisson exceeds Ncrsu (Ncr is the local reverse bearing capacity factor, and su is the undrained shear strength of clay at the skirt-tip), which may induce the local reverse bearing capacity failure of soil near the skirt-tip. As a result, soil flows into the caisson cavity, making soil plug heave higher that prevents the caisson from penetrating to its desired depth. This paper is the first to apply the limit analysis to obtain a theoretical study on determining the critical suction pressure based on the local reverse failure mechanism near the skirt-tip. This study is also the first to consider the effects of adhesion factors along the inside (αi) and outside (αo) of the skirt wall on the local reverse bearing capacity failure mechanism reflecting that Ncr includes αi and αo. It is shown that Ncr ranges from 5.14 (αi=αo=0) to 8.28 (αi=αo=1), and the values of αo and αi are all between 0 and 1. In addition, the critical penetration depth can be determined under the condition that the critical suction pressure equals the required suction pressure. And, the proposed method of calculating the critical penetration depth is testified to be in good agreement with the experimental results.

Observation of sand movement during bucket installation

The occurrence of soil-plug heave during bucket installation is one of the major problems in successful installation of bucket foundation at a desired depth. It is usually caused by the upward seepage flow created by continuous pumping out of water from inside the bucket as well as due to shear failure of the soil along the skirt wall during skirt penetration. This soil-plug heave eventually hinders the bucket to be installed at a targeted depth and hence affects its performance. This paper, therefore, presents the result of a series of bucket installation tests using half-bucket foundation performed in a box equipped with a transparent window for directly observing the soil movement around the bucket during installation. Continuous images were captured simultaneously during the entire suction installation process and subsequently analysed by particle image velocimetry to quantify the soil movement. The results provide: (a) the trend of suction pressure with wall embedment; (b) soil-displacement pattern and (c) the effects of penetration rate (i.e. water-pumping rate) and wall thickness on the soil displacement. The results also provide comprehensive insights into the mechanism of soil disturbance, including soil-heave formation, during suction installation.

Theoretical studies on penetration resistance of suction caissons in clay

ABSTRACTThe suction caisson is commonly a top-closed cylindrical steel structure with large diameter, short length and much thinner skirt wall thickness. The resistance to penetrating is calculated as the sum of the tip bearing capacity and the adhesion on the both sides of the skirt wall. Since the thickness of the skirt wall is very small, the downward adhesion produced by the skirt wall will cause the additional vertical stress and shear stress in the soil at the skirt tip level, increasing the skirt tip resistance. However, the increase in skirt tip resistance caused by the additional vertical stress rather than shear stress in soil at the skirt tip level was only considered, this may lead to an inaccurate estimation for the tip bearing capacity and the suction required. Thus, a modified slip-line field is put forward in this study to estimate the tip resistance. The expression of obtaining the minimum suction to install the suction caisson in clay is derived in terms of the force equilibrium. Results from calculations of the minimum suction have been proved to be in a good agreement with the measured data.

Bearing capacity of composite bucket foundations for offshore wind turbines in silty sand

A composite bucket foundation (CBF) with seven compartments arranged in a similar honeycomb structure is a relatively new type of structure for offshore wind turbines. Field tests were conducted at a natural water pool with a water depth of 0.4 m in saturated silty sand off the coast of Jiangsu to estimate the horizontal bearing capacity of the CBF. The distribution of soil pressures along the skirt of the CBF with different loading steps helps in explaining the soil-bucket interaction due to the deformation of the CBF. The results show that an increased load will result in a significant increase in the soil pressure in passive zones outside the bucket skirt whereas the soil pressure on the skirt wall in the corresponding active zone remains largely the same. Moreover, the suction pressure inside the separation between the bucket lid and the soil results in a potential bearing capacity with larger loading application, though it will dissipate with further displacement because seepage is observed in the silty sand. The load-displacement relationships obtained using a FEM simulation are almost in same trend with the test results when the elasticity modulus of the soils in the FEM model is set approximately four or five times the compression modulus obtained from the laboratory test. The rotation center of the CBF obtained from the test and FEM results is approximately 0.8 skirt height below the soil surface around the bulkhead along the loading direction under the ultimate horizontal loading.

Determination of Maximum Penetration Depth of Suction Caissons in Sand

The suction caisson is a large top-closed cylindrical steel structure in diameter, short in length and much thinner in skirt wall thickness. The total resistance of the suction caisson during installation consists of the tip resistance and the skirt wall friction. However, since the thickness of the skirt wall is very small, the skirt wall friction may produce additional vertical stress and shear stress in soil at the skirt tip level, and this additional vertical stress and shear stress will contribute to the increase in the skirt tip resistance. At the same time, seepage induced by suction also causes the tip resistance to reduce significantly. A modified slip-line field is proposed in this study estimating the tip resistance in terms of the slip-line theory. The expression obtaining the minimum suction to install the suction caisson is also proposed in terms of the force equilibrium. In addition, the critical suction is determined based on the mechanism of sand piping. Thus, the maximum penetration depth of the suction caisson can be reached when the critical suction equals the minimum suction. Results from calculations of the minimum suction and the maximum penetration depth have been proved to be in a good agreement with the measured data.

Efficiency checking of use of stiffening ribs for piston skirt of a low-sized diesel engine

The study actuality is connected with a problem of high mechanical losses due to friction in naturally aspirated high-speed low-cylinder four-stroke diesel engines. The research aims to check the efficiency of application of an experimental piston with rigid skirt that according to preliminary data provides the decrease of mechanical losses in the cylinder-piston group. The check method consists in comparison of benchmarks of a serial piston and an experimental one. Following indices are accepted as benchmarks: the piston friction force and mechanical losses formed by its work; the temperatures in characteristic zones of the piston; the temperatures of cylinder wall and motor oil caused by the piston friction; the moment of resistance to turning of crankshaft of installation with piston in the cylinder. The comparison is made by means of both modeling and experiments on a model installation designed on the base of 1Ch 85/80 (TMZ-450D) low-sized diesel engine. The comparison objects are a serial piston of diesel engine and an experimental piston with improved rigidity of skirt provided by special stiffening ribs joining the skirt wall with piston bosses. The results of modeling show the advantage of the experimental piston over serial one by signs of decrease of mechanical losses by 4%, of skirt wear by 33%, of temperatures of combustion chamber center and piston top edge accordingly by 5 and 10%. The experimental check allows to establish that the experimental piston with other equal conditions of turning without compression, combustion and cooling provides the decrease in the moment of resistance to turning by 3%, in temperatures of cylinder wall by 9% and motor oil by 6%. The results of comparative modeling and experiment highlight the certain prospect of substitution of serial piston for the experimental one, which is caused by possibility of significant decrease of temperature of the piston top and mechanical losses.

Earth pressures on modified suction caisson in saturated sand under monotonic lateral loading

A modified suction caisson (MSC) can be used to support the offshore wind turbine. This paper presents an experimental study and a numerical simulation on the earth pressure distribution along the MSC embedded in saturated marine fine sand. Results show that the yield envelops for the MSC and the corresponding regular suction caisson in the H-M space can be represented by a linear relationship. It was also found that the Prasad and Chari method, which is popular in analyzing the earth pressure distribution along lateral loaded rigid plies, can well predict the maximum net earth pressure on the regular suction caisson. However, for the MSC, since the external skirt can resist a considerable portion of the lateral load, Prasad and Chari method overestimates the maximum net earth pressures acting on the internal compartmental shaft of the MSC. Numerical simulation results indicate that in the ultimate limit state the maximum net earth pressure acting on the external skirt wall in the loading direction is larger than that on the internal compartment wall. However, the maximum net earth pressure along the caisson embedded depth is obtained at the MSC base opposite the loading direction. In addition, the separation between the top lids of the internal compartment and external skirt occurs opposite the loading direction. The lateral load and the resulting overturning moment acting on the MSC are mainly carried by the passive earth pressure zones along the inner and outer shafts of the internal compartment and the external skirt both in the loading and opposite the loading directions and under the external skirt lid in the loading direction. The main reason of the suction caisson failure is that the sand in the lower passive earth pressure zone opposite the loading direction yields completely.

Model tests of soil heave plug formation in suction caisson

An experimental study of the formation of the soil heave plug inside a concrete caisson during installation in clay under suction is presented. The soil heave plug, the amount of caisson penetration and the vacuum pressure applied were measured during the model tests. The results show that the soil heave plug can completely fill in the cavity and prevent the caisson from penetrating further. The soil heave plug is affected by the geometry of the caisson. The higher the consolidation stress or the stiffer the soils, the higher is the suction pressure required, and a higher suction will in turn induce a greater soil heave plug. The m value, defined as the ratio of the volume of the soil heave plug to that of the penetrated caisson skirt wall, is adopted to evaluate the soil heave plug. The relationship between the m value and the undrained shear strength of the soil can be used to estimate the amount of soil heave plug and the caisson penetration for a concrete caisson with a wall thickness to external diameter ratio of 5% in normally consolidated clay conditions.

실트질모래 지반에서 버켓기초의 압입저항력에 대한 원심모형실험 연구

Penetration resistance of bucket foundations with skirt wall in the silty sand of the western coast of Korea was analyzed by centrifuge modelling. The penetration resistance is induced when the bucket foundations are jacked into the soil without suction, and is directly related to the self-weight penetration depth. The procedure by Houlsby and Byrne (2005), which takes into account the effect of stress increase by frictional resistance of skirt wall, was utilized to generate the penetration resistance similar to the experimental results. This paper describes the methods by which major parameters such as lateral earth pressure coefficient and friction angle between the skirt wall and the soil are evaluated. The effect of changes in these parameters on the predictions is analyzed. Also, observed soil behaviour during jacking penetration is investigated.

The Effects of Piston Skirt Profiles on Secondary Motion and Friction

Piston skirt form deviating from a perfect cylinder is investigated numerically for an improved frictional performance. Three features defining the barrel and oval form of the skirt are compared in the search for lower friction power loss. Radius of curvature around the bulge of the barrel is changed to obtain a flatter or more-rounded lubricated area with respect to a hot-piston profile as well as the axial location of this bulge. On the other hand, the circumferential variation in the separation between the skirt and cylinder wall is represented by an elliptical piston and the aspect ratio is varied for comparison. These different skirt profiles are used in a developed piston secondary dynamics model solving for the lateral movement of the piston by calculating the hydrodynamic and boundary normal forces acting on the piston together with friction. Finally, an improved skirt profile is suggested to obtain better frictional efficiency.