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1.
Shuwang Yan Jingjing Zhang Jian Chu Zhiliang Huo 《Marine Georesources & Geotechnology》2017,35(3):435-440
Concrete suction caissons have been successfully used as breakwaters or seawalls in recent years. The relative large wall thickness-to-diameter ratio of a concrete caisson can lead to the formation of a full soil heave plug that may cause difficulties in the installation of concrete caisson in clay. One way to overcome this limitation is to use a tampered tip for the caisson wall. An analytical method is proposed in this article to calculate the minimum suction pressure required to penetrate a caisson and the maximum allowable suction pressure that can be applied to avoid too much soil heave plug during the installation of the suction caisson. Four model tests were conducted in normally consolidated clay to study the installation process of a concrete suction caisson with tampered tip and to verify the proposed analytical method. The height of the soil heave plug in the caisson with a tampered tip is observed to be about half of that in the caisson with a flat tip. 相似文献
2.
Hai-lei Kou Dan-liang Yang Wang-chun Zhang Yi-fan Wu Qiang Fu 《Marine Georesources & Geotechnology》2020,38(8):980-988
AbstractThe performance of steel caisson during and after installation with different penetration velocities in medium dense sand is presented. The applied jacking forces, the amount of formed soil heave and bearing capacity were measured in the model tests. The influence of penetration velocities on jacking forces, soil heave and bearing capacity were also discussed in detail. The results indicated that the jacking forces for caisson in medium dense sands were significantly affected by the penetration velocity. The larger the penetration velocity, the more soil flowed into the caisson cavity during installation. This will lead to larger inner shaft resistance and in turn more jacking forces required for the same penetration depth. The height of soil heave during installation increases with penetration velocity. The m value calculated by the ratio of the volumes of the soil heave to that of the penetrated caisson wall can be used to evaluate the soil heave. The larger the applied velocity, the larger the m value and larger bearing capacity of caisson after installation. The relationship between the m value and penetration velocity can be used to control the soil heave for a steel caisson with a wall thickness to external diameter ratio of 4.2% in medium dense sand by jacking method. 相似文献
3.
针对吸力式沉箱在黏性底床中沉贯安装的减阻问题,基于黏性泥沙的流变特性,开展了一系列不同振动荷载作用下的室内沉贯模型试验,分析了振动荷载对吸力式沉箱沉贯过程的影响和沉贯减阻效果。试验结果表明：在压力沉贯阶段或吸力沉贯阶段施加高频振动荷载,均能促使吸力式沉箱侧壁周围的土体发生流化,有效降低沉箱的沉贯阻力;减阻效果与振动频率和沉箱的长径比有关,与振动频率成正比,与沉箱的长径比成反比;存在一个临界频率,当振动频率大于该频率后,沉贯阻力随频率的减小不明显;施加高频振动荷载有助于减小吸力沉贯阶段沉箱内的土塞高度,促进沉箱沉贯到位。研究成果可为黏性泥沙流变减阻技术在吸力式沉箱中的应用提供理论依据和技术参考。 相似文献
4.
吸力基础与海洋工程大直径钢桩相比,具有成本低、安装周期短、对环境影响小、不受海况影响及可回收再利用等优点,近年来在海上风电工程中得到推广应用。吸力基础沉贯至海床预定位置,是其发挥承载力和确保服役稳定性的前提。海床地基土体常以分层土形式分布,且各层土体强度、压缩性和渗透性等存在显著差别,导致吸力基础吸力沉贯机理非常复杂。明确吸力基础在分层土中沉贯特性,有助于指导吸力基础在海上风电工程中的推广应用。对目前吸力基础在分层土中沉贯特性研究进行综述和总结,归纳了其沉贯机理研究进展,并对影响吸力基础在分层土中沉贯因素进行了分析;提出了分层土中吸力基础沉贯的研究方向和改进的沉贯方法。 相似文献
5.
The bearing behavior of suction caissons supporting offshore wind turbines under two-way cyclic lateral loading and dead load in clay was investigated with consideration of soil strength degradation and adhesive interface friction between caisson walls and heterogeneous clay using the finite-element package ABAQUS.An ABAQUS built-in user subroutine was programmed to calculate the adhesive interface friction between clay and caisson walls.The results of parametric studies showed that the degradation of bearing capacity could be aggravated by the decrease of the aspect ratio.The offset between the rotation point of the soil inside the caisson and the central axis of the caisson increased with the increasing vertical load and number of cycles.The linearly increasing strength profile and adhesive interface led to the formation of an inverted spoon failure zone inside the caisson.The settlement-rotation curves in each cycle moved downwards with increasing number of cycles due to the soil strength degradation. 相似文献
6.
筒型基础安装过程中,筒内的负压作用会导致土塞现象的出现。以试验模型为基础建立可变形离散元计算模型,通过VC＋＋编制的程序SPSAⅡ对筒型基础负压沉贯中土塞的生成过程进行可视化模拟计算。考虑渗流力、筒壁内外摩擦阻力和筒内负压吸力作为土塞产生的主要外荷载,分别模拟渤海地区3种典型土（黏土、粉土和砂土）的情况。其中,粉土情况下。程序的收敛速度最快,黏土情况下,土塞的计算高度最接近试验测量值,数值计算结果表明该计算方法作为筒型基础施工前筒内土塞高度的预测方法是可行的。 相似文献
7.
Offshore wind power is a rapidly growing area of electricity in China. In the present paper, interaction mechanisms between the caisson for wind turbines and saturated silt sand are investigated with laboratory tests based on two different installation methods, jacking installation and suction installation. For the jacking installation process, the results indicate that the soil pressures inner and outer the skirt of the caisson vary with a similar feature and the magnitudes of the two are nearly balanced. The tip resistance plays a key role in the total jacking installation resistance. This paper examines the predictive performance of qc method and API approach for jacking installation resistance. It is demonstrated that the qc method provides better predictions. The resistance coefficients are recommended. For the case of suction installation, however, the changes of soil pressures inner and outer the skirt are contrasting. Specifically, the inner pressure and tip resistance fall dramatically, but the outer pressure increases when suction is applied. Seepage effect is found to be an important mechanism for the installation of suction caisson. The reduction ratios of the inner friction and tip resistance follow a power-function with the normalized suction. Based on the test results, a prediction method for the required suction has been developed and evaluated. 相似文献
8.
9.
Knut H. Andersen Philippe Jeanjean Dirk Luger Hans Petter Jostad 《Ocean Engineering》2005,32(7):845-863
Three centrifuge model tests were performed in normally consolidated Speswhite Kaolin to study the penetration of suction anchors in soft clay. The suction anchors could be penetrated by means of underpressure to a depth of about 12.4 to a little more than 14.5 times the diameter. When the anchors were penetrated by underpressure, all clay displaced by the skirt moved into the anchor. At a penetration depth of about half the maximum penetration depth, the volume of the soil heave inside the anchor actually increased more than the volume of the displaced clay. When a material coefficient of 1.5 against plug failure was mobilized, more than the clay displaced by the skirts had accumulated inside the anchor. The penetration resistance increased by 42 and 26% during rest periods of 4.5 and 0.8 days prototype time, respectively. Some uncertainty in the shear strength of the clay beds gave some uncertainty with respect to the interpretation, but the observed behaviour generally confirmed the theoretical analyses. 相似文献
10.
某工程拟在深水软土地基上修筑防波堤,为了尽量减少地基处理充分利用天然地基,创新性设计出一种轻型薄壁的预制防波堤结构,其挡浪部分为直立薄壁圆筒,基础部分则为倒扣的薄壁椭圆形桶,并且椭圆形下桶为外壁和内隔板分成9个格室,防波堤结构浮运至指定位置后,拟采用负压工法施工安装就位。这种新型防波堤结构为国内外首次提出,其下沉施工设计尚无规范可循,为此开展了土工离心模型试验,在模型加速过程中模拟了椭圆形下桶基础在淤泥层中的自重下沉,之后利用新研发了一种大行程作动加载装置给椭圆形下桶施加下推力,让其继续向下贯入直至穿越整个淤泥层,以模拟负压工法的贯入下沉。试验测量了下桶贯入下沉过程中的推力与贯入位移,还尝试测量了桶壁和内隔板断面的压应变,由此分析了下桶基础的下沉总阻力、桶壁摩擦力以及截面压应变随贯入位移的变化。结果发现,这些曲线均出现了转折点,根据转折点对应的下沉总阻力确定了椭圆形下桶基础贯入过程所遭遇的临界下沉总阻力值,据此估算了负压工法中所需施加的压力差。 相似文献
11.
AbstractAn experimental study of the performance of concrete pipe piles during installation under different penetration speeds and static load tests on the piles in sand is presented. The applied jacking force, the amount of pile penetration, length of soil plug formed and ultimate bearing capacity were measured during the model tests. The results showed that the concrete pipe piles were partially plugged and the behavior of the soil plug was significantly affected by the penetration speed. The lower the penetration speed, the larger the soil plug formed which in turn leads to a greater ultimate bearing capacity. The size of soil plug can be evaluated by the m value defined as the ratio of the volume of the soil plug to that of the penetrated pile wall. The relationship between the m value and the penetration speeds can be used to estimate the amount of soil plug and the depth of penetration for an open-ended concrete pipe pile jacked into sand. 相似文献
12.
A series of model tests were conducted on Perspex-made suction caissons in saturated dense marine sand to study the sand plug formation during extraction. Suction caissons were extracted by pullout loading or by pumping air into the suction caisson. Effects of the pullout rates, aspect ratios and loading ways (monotonic or sustained) on the pullout capacity, and plug formation were investigated. It was found that the ultimate pullout capacity of the suction caisson increases with increasing the pullout rate. The sand plug formation under the pullout loading is significantly influenced by the pullout rate and the loading way. When the suction caisson is extracted at a relatively slow rate, the general sand boiling through the sand plug along the inner caisson wall occurs. On the contrary, the local sand boiling will occur at the bottom of the suction caisson subjected to a rapid monotonic loading or a sustained loading. Test results of the suction caisson extracted by pumping air into the caisson show that the pressure in the suction caisson almost follows a linear relationship with the upward displacement. The maximum pressures for suction caissons with aspect ratios of 1.0 and 2.0 during extraction by pumping air into the caisson are 1.70 and 2.27 times the maximum suction required to penetrate the suction caisson into sand. It was found that the sand plug moves downward during extraction by pumping air into the caisson and the variation in the sand plug height is mainly caused by the outflow of the sand particles from the inside of the suction caisson to the outside. When the suction caisson model is extracted under the pullout rate of 2?mm/s (0.28?mm/s for the prototype), the hydraulic gradient along the suction caisson wall increases to the maximum value with increasing the penetration depth and then reduces to zero. On the contrary, when extracted under the pullout rate of 10?mm/s (1.4?mm/s for the prototype), the hydraulic gradient along the suction caisson wall increases with increasing the pullout displacement. When extracted by pumping air into the caisson, the hydraulic gradient reaches the critical value, and at the same time, the seepage failure occurs around the suction caisson tip. 相似文献
13.
1 .IntroductionSuctioncaissons have been widely usedfor offshore oil exploration duetothe advantages of econo-my and simple installation over traditional piles (Huanget al .,2003) .For tensionleg platforms andspar platforms in deep ocean,suction caissons … 相似文献
14.
讨论了浅海桶形基础平台负压沉贯渗流场有限元分析方法.通过模型实验结果与有限元分析结果的对比,验证有限元法的可靠性.将有限元法用于桶形基础平台负压沉贯渗流场的计算,得到沉贯过程中许用负压随插深的变化,用于指导平台就位施工时的负压控制.不同插深时,相应许用负压下的桶内、外壁渗流水头和水头梯度分布的计算结果,用于沉贯阻力的计算. 相似文献
15.
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. 相似文献
16.
AbstractThis study established a Couple Eulerian–Lagrange model to simulate monopile vibratory penetration for the investigation of soil plugging effect during high-frequency penetration of monopiles for wind turbine. Simulation analysis is focused particularly on soil plugging effect of a large diameter monopile during vibratory penetration into sand, clay, or layered soil. The results of the numerical simulation show that soil plugging effect is unlikely to occur during monopile penetration into the clay soil, while partial soil plugging may occur during the sand penetration. Penetration resistance at the pile toe is transferred to the radial stress around the pile wall. At a critical point penetration process, internal shaft friction becomes larger than external shaft friction. Moreover, radial pressure factors increase during partial soil plugging effect. For layered soil, the topsoil not only has great influence on the soil plugging effect, but also affects shaft friction in the subsoil during monopile penetration. 相似文献
17.
吸力式导管架基础具备高承载力、高施工效率、高环境友好度、低造价等特点,使用其作为海上风电底部支撑结构有利于产业的规模化发展,实现上述愿景的前提在于使筒型基础顺利安装以满足设计要求。基于此,在不同初始施加泵压下,对吸力式导管架的沉贯效率特征值、基础内外围渗流变化、筒裙端部土压力特点进行探究。通过抱桩器使吸力式导管架在吸力安装过程中仅具备竖向自由度,从而进行纯沉贯试验。结果表明：初始泵压2 kPa时沉贯效率特征值最高;沉贯过程中基础外围渗流水压小于基础内围渗流水压;筒裙外侧所受土压要大于筒壁内侧所受土压力。 相似文献
18.
筒型基础负压沉贯就位后,筒内表面土的沉降会使土层上部产生一层水膜。严重的水膜现象会影响到筒基平台的正常使用,为此专门设计本项试验来模拟筒内水膜形成的机理,并且提出了向筒内灌浆的方法解决水膜问题的方案。该试验通过配土、设计模型筒基、负压沉贯和筒内灌浆等操作过程,设置土槽、安装孔压传感器等实验手段,从土体破坏的角度,寻求负压沉贯过程中沉贯负压、沉贯阻力及孔隙水压力等因素对水膜形成的影响,试图探询水膜形成的规律,并且通过不同泥浆的灌浆试验来寻找有效解决水膜现象的方法。 相似文献
19.
Suction-induced seepage is pivotal to the installation of caisson foundations in sand. Indeed, the upward pore water flow on the inner side of the caisson wall causes a release of a fraction of soil resistance due to the reduction of the lateral effective stress. A safe caisson installation requires a reliable prediction of soil conditions, especially soil resistance and critical suction for piping. These soil conditions must be predicted for the whole installation process.In this paper, we examine the effect on such prediction of the assumed permeability profile, which is described as a function of depth below the mudline. This study is motivated by the fact that marine sediments generally exhibit a permeability that decreases with depth because of consolidation under gravity. Hence, the question is whether conventional theories based on a constant permeability lead to a conservative prediction of soil conditions or not. Our conclusion is affirmative only regarding piping condition. As for soil resistance, a prediction based on the assumption of a constant permeability is non-conservative. This is due to an overestimated reduction in effective stresses under suction-induced seepage. 相似文献
20.
The seepage field generated by suction penetration of a bucket foundation into sea floor is numerically simulated with a finite element method. The water head and its gradient on both sides of the bucket wall in the seepage field are computed and the results are applied to the calculation of penetration resistance versus penetration depth. Also, allowable suction pressures with various penetration depths are provided for platform installation. The engineering parameters given in this paper were adopted in the design of the first platform (CB20B) with suction-penetrated buckets as foundations in China. This platform has successfully been launched into operation for the offshore oilfield development at the Yellow River Submersed Delta. 相似文献