Evaluation of Undrained Shear Strength and Consolidation Characteristics of Disturbed Marine Sedimentary Clay due to SCP Installation

The behaviors of the marine sedimentary ground improved by sand compaction pile (SCP) method are analyzed. To do this, the results of upheaval characteristics of the sea floor, undrained shear strength, and horizontal consolidation coefficient (consolidation) are investigated. Due to SCP installation on ground, as thickness of a soft clay layer increases, upheaval height increases and upheaval angle decreases. Undrained shear strength of disturbed ground due to SCP construction decreases in early stage after completion of construction, but it shows a trend of recovering as months elapse. As the result of piezocone penetration dissipation tests, consolidation delay phenomenon by the disturbance due to SCP installation clearly is identified and its degree is dependent on the replacement area ratio of SCP and the location of ground.     

Stress Sharing Behavior and Its Mechanism during Consolidation Process in Composite Ground Improved by Sand Compaction Piles with Low Replacement Area Ratio

In order to accurately design a sand compaction pile (SCP) with low replacement area ratio, it is important to understand the mechanical interaction between the sand pile and clay ground and its mechanism during consolidation process in composite ground. In this article, therefore, a series of numerical analyses on composite ground improved by SCP with low replacement area ratio were carried out. The applicability of numerical analyses, in which an elasto-viscoplastic consolidation finite element method was applied, were confirmed by comparing the results obtained from a series of laboratory model tests with the composite ground improved by SCP. Through the results of the numerical analyses, mechanical behavior of the sand pile and clay in composite ground during consolidation is elucidated, together with a stress sharing mechanism between sand pile and clay.     

Stress Sharing Behavior and Its Mechanism during Consolidation Process in Composite Ground Improved by Sand Compaction Piles with Low Replacement Area Ratio

In order to accurately design a sand compaction pile (SCP) with low replacement area ratio, it is important to understand the mechanical interaction between the sand pile and clay ground and its mechanism during consolidation process in composite ground. In this article, therefore, a series of numerical analyses on composite ground improved by SCP with low replacement area ratio were carried out. The applicability of numerical analyses, in which an elasto-viscoplastic consolidation finite element method was applied, were confirmed by comparing the results obtained from a series of laboratory model tests with the composite ground improved by SCP. Through the results of the numerical analyses, mechanical behavior of the sand pile and clay in composite ground during consolidation is elucidated, together with a stress sharing mechanism between sand pile and clay.     

Experimental Evaluation of Recycled Aggregate Porous Concrete Piles for Soft Ground Improvement

The use of sand compaction pile or gravel compaction pile is nowadays a common approach for soft ground improvement. In this article, a recycled aggregate porous concrete pile has been developed by replacing natural aggregates with recycled aggregates to overcome issues related to bulging failure or reduced section geometries. Such issues may arise during installation and during the early stages of operation. In addition, the proposed approach utilizes recycled aggregates instead of natural materials. To investigate the applicability of the recycled aggregate porous concrete pile method as a ground improvement technique, a series of laboratory model consolidation tests was performed on soft clay soil reinforced with sand compaction pile, gravel compaction pile, and recycled aggregate porous concrete pile, respectively. The results indicated that the settlement reduction effect of recycled aggregate porous concrete pile was significantly higher than the sand compaction pile and gravel compaction pile methods. The stress sharing ratio from the experimental program showed good agreement with those calculated by elasticity theory. Comparative analyses of the recycled aggregate porous concrete pile versus sand compaction pile and gravel compaction pile approaches, under the same replacement area ratio and surcharge pressure, showed significantly improved consolidation time, settlement reduction, and stress sharing effect.     

Dynamic Compaction of Ultra-High Energy in Combination with Ground Replacement in Coastal Reclamation Areas

This paper presents a case study of dynamic compaction (DC) on backfill ground for planned oil tanks located at Nanjing Bay. The ultra-high energy DC level of 18,000 kN · m was applied in the area tamping phase of the dynamic compaction. In combination with ground replacement technique by forcing crushed stones into the underlain cohesive layer, the high-compressibility of this layer was properly tackled. The construction techniques and ground improvement mechanisms are introduced and discussed. Field evaluations before and after the dynamic compaction, including surface wave test, plate load test, and soil sampling, showed that the ultra-high energy dynamic compaction in combination with ground replacement achieved much greater improvement depth and ground bearing capacity than conventional DC. Results show that dynamic compaction of 18,000 kN · m combined with ground replacement is of great merit in dealing with backfill ground with weak cohesive content in harbor development.     

强夯法处理粉煤灰地基效果评价

在青岛娄山河污水处理厂使用强夯法对粉煤灰地基进行处理,通过定量计算分析强夯前后土工试验、原位测试结果,评价粉煤灰物理力学性质的改善状况;分析原位测试试验成果之间的内在联系,对强夯法加固粉煤灰地基的效果评价进行了初步研究。结果显示,强夯有效加固深度内的粉煤灰层物理力学性质发生了明显改善,但其孔隙比仍大于1.0;点夯过程宜及时排水,以提高夯实效率。本文研究结论对粉煤灰地区的加固具有一定的指导和参考意义。     

Immersed tunnel foundation on marine clay improved by sand compaction piles

The sand compaction pile (SCP) method can be applied to soft marine clay ground that is a reinforcement of composite ground consisting of compacted sand piles and surrounding clay. The application of SCP method in the immersed tunnel of Hong Kong–Zhuhai–Macao Bridge verify SCP method is a robust solution to limit the total settlement and differential longitudinal settlement and to promote smooth transition from immersed tunnel to artificial island. The SCP method has significant settlement reduction effect on marine clay. The SCPs can also function as a drainage path to accelerate the consolidation process in marine clay. It is also found that the consolidation rate of SCP-improved ground is delayed compared with that predicted program which is most probably because of the soil disturbance effect during the installation of SCPs.     

Seabed Ground Improvement in Coastal Reclamation Area by Heavy Tamping After Rockfilling Displacement

Considering the characteristics of seabed ground in coastal reclamation area, ultra soft clay improvement method by heavy tamping after rockfilling displacement was proposed combined with a case study. The improvement mechanisms of the method can primarily be attributed to rockfilling displacement (RD), dynamic compaction (DC), dynamic replacement (DR) and dynamic replacement and mixing (DRM). For the case study given in this article, heavy tamping construction program was proposed based on field pilot tests. Furthermore, the effectiveness of the proposed ground improvement method was verified through in situ plate load test, sand fill test and the analyses of observed settlement data. Thus, the method of heavy tamping after rockfilling displacement is applicable for the improvement of seabed ground in coastal reclamation area. It is proposed for similar projects that heavy tamping of fills may be performed by layers and correspondingly tamping energy increased to further enhance the improvement effect of DC, DR and DRM. In addition, delayed improvement effect or time effect of soil mass after heavy tamping is still an issue to be further studied.     

考虑桩土作用独桩海洋平台横向振动特性研究

采用动Winkler弹性地基梁模型模拟桩土问动力相互作用，并考虑了流体与桩问相互作用，通过组合成层土中、水中桩单元的刚度阵，推得了独桩海洋平台连续系统横向振动的动刚度阵及在波浪力作用下平台甲板处的频率响应函数，进而求得了在确定性波浪力及随机波浪力作用下桩身任意点的位移响应。最后，通过算例研究和分析了在随机波浪力作用下成层土参数、甲板上重量及冲刷淘深等因素对平台振动响应的影响。     

浅谈挤密碎石桩的施工方法

挤密碎石桩施工法是一种振动成桩法,即先用桩管振动成孔,然后填入足够数量的碎石,最后振动密实成桩体。通过振动、挤密的成桩过程,将原地基土振动夯实,桩体与桩间土形成复合地基,达到既处理可液化地基又增强地基的效果。介绍了挤密碎石柱的施工方法和质量检测方法。     

Evaluation of the Effect of Penetration Ratios on Composite Grounds Improved with Sand Compaction Piles

In this paper, centrifuge model tests were conducted in order to understand the deformation characteristics and behavior of sand compaction piles (SCPs) reinforced grounds in relation to area replacement ratios and penetration ratios. To simulate ground stress conditions, preliminary compaction was conducted to form grounds that maintained a certain level of strength. SCPs were installed in the grounds using compaction methods, and the relationship between loads and settlement as well as stress under rigid loading conditions were compared and analyzed. In addition, finite element analyses were conducted in order to verify the results of the centrifuge model tests and assess the effects of penetration ratios and depths on variations in stress. According to the results of the analyses, stress concentration ratios gradually decreased as depths increased, and the decreasing rate increased as penetration ratios decreased. However, in regions close to the surface layer in depth in which SCPs were installed, stress concentration ratios showed almost the same range regardless of penetration ratios. Stress concentration ratios showed proportional relations with penetration ratios. However, they showed similar values in regions close in depth to the surface of the ground. In particular, they showed very close ranges at penetration ratios of 100% and 80%.     

Application of High Energy Dynamic Compaction in Coastal Reclamation Areas

High energy dynamic compaction (HEDC) is adopted in a coastal reclamation area because the grain size of backfilled soil mostly ranges between 20 cm and 100 cm. The in situ tests for evaluating the effectiveness of HEDC were performed on the backfilled soil ground. The crater depth per drop and the whole test zone elevations before and after HEDC were measured and analyzed. Dynamic penetration tests and spectral analysis of surface wave (SASW) tests were used for investigating the improvement depth. Furthermore, the allowable bearing capacity of HEDC treated ground was determined based on the results of plate-load tests. It was found that HEDC did not cause the ground surface heave during construction, and was more effective than low energy dynamic compaction (LEDC) in terms of applied energy utilization. Based on the test results, the improvement depth of HEDC at this site was not less than 14 m, and there was no obvious weak layer within the range of improvement depth. The allowable bearing capacities were larger than 160 kPa. The investigation results indicate that the HEDC technique is an effective way for improving backfilled coarse-grained soil in coastal reclamation areas. This technique helps to achieve both greater improvement depths and higher ground bearing capacities as compared with LEDC.     

Performance of small group of geosynthetic-reinforced granular piles

Granular piles are frequently used as a method of improving soft grounds as they provide increased bearing capacity and reduce foundation settlements. However, in very soft clayey soils, they may not derive their load-carrying capacity by low confining pressure provided by the surrounding soil. In such circumstances, granular piles may be reinforced with suitable geosynthetic to increase its load-carrying capacity and to reduce excessive bulging. In this study, the performance of small group of geosynthetic-reinforced granular piles (GRGPs) is examined in terms of load-carrying capacity, settlement, and modulus by laboratory model tests. The parameters investigated include modulus of reinforcement material, area replacement ratio (ARR) based on the column diameter and reinforcement length. The results indicated that increasing the modulus of the reinforcement and the ARR based on the column diameter enhances the overall performance of the GRGP group. It was also observed that reinforcement on top portion of the granular pile is sufficient to substantially increase the load-carrying capacity of granular pile group.     

有限元法在桩靴承载力计算中的应用

自升式钻井平台插桩是地基土在桩靴荷载作用下发生连续的塑性破坏的动态过程,当地基极限承载力等于桩靴荷载时插桩完成。经典土力学极限承载力理论对土体潜在滑动面做了假设,无法有效分析土体内部的破坏过程。本研究应用有限元法（FEM ）对插桩过程进行了模拟,得到地基土的破坏机制以及中间荷载下土体的应力、应变情况,通过和各理论公式计算的极限承载力进行对比分析,分析影响地基极限承载力的因素。研究表明,基础宽度与硬土层厚度的比值 B/H越大,下卧软土层越容易发生塑性破坏,极限承载力明显下降,当B/H＜0．286时,可以忽略下卧软土层对地基承载力的影响。     

Vertical vibration capacity of a single pile in dry sand

In this study, the dynamic response of pile foundation in dry sandy soil excited by two opposite rotary machines was considered experimentally. A small scale physical model was manufactured to accomplish the experimental work in the laboratory. The physical model consists of two small motors supplied with eccentric mass (0.012?kg) and eccentric distance (20?mm) representing the two opposite rotary machines, an aluminum shaft as the pile, and a steel plate a pile cap. The experimental work was achieved taking the following parameters into considerations: pile embedment depth ratio (L/d, where L is the pile length and d is its diameter) and operating frequency of the rotary machines. All tests were conducted in medium dense fine sandy soil with 60% relative density. Twelve tests were performed to measure the change in load transferred through the pile’s tip to the underlying soil. To predict precisely the dynamic load that will be induced from the rotary machines, a mini load cell with a capacity of 100?kg was mounted between the aluminum plate (the machine base) and the steel plate (pile cap). The results revealed that, before machine operation, the pile tip load was approximately equal to the static load (machine and pile cap), whereas during machines’ operation, the pile tip load decreased for all embedment depth ratios and operating frequencies. This reduction was due to the action of skin friction that was mobilized along the pile during operation, and as a result the factor of safety against pile bearing failure increases. For all operating frequencies and pile lengths, the factor of safety against bearing failure increased during machines’ operation, where the pile tip load became less than its value before starting operation. During operation, the skin friction resistance mobilized along pile length led to decrease the bearing load.     

Longitudinal dynamic impedance of a static drill rooted nodular pile embedded in layered soil

The static drill rooted nodular (SDRN) pile is a new type of precast pipe pile with equally spaced nodes distributed along the shaft and wrapped by the surrounding cemented soil. In this paper, the longitudinal dynamic response of the SDRN pile embedded in layered soil is investigated with respect to the complexity of the pile body structure and the pile–soil contact condition. First, the shear complex stiffness transfer model is used to simulate the radial inhomogeneity of the surrounding soil. Then, the governing Equations of the pile–soil system subjected to longitudinal dynamic loading are established. The analytical solution for the dynamic response at the pile head is obtained by the shear complex stiffness transfer method and the impedance function transfer method. The degenerate case of the present solution is compared with the published solution to verify its reliability, and the complex impedance of the SDRN pile is compared with that of the precast pipe pile and the bored pile. Finally, a parametric study is conducted to investigate the influence of pile–soil parameters on the complex impedance at the pile head within the low frequency range concerned in the design of the dynamic foundation.     

海洋含黏粒砂土共振柱试验研究

本文利用英国GDS公司生产的RCA共振柱系统测试海洋含黏粒砂土动剪切模量,同时对比纯砂样的动剪切模量,系统研究固结应力、初始密实度、黏粒含量等因素对砂土最大动剪切模量的影响。试验结果表明:最大动剪切模量随有效固结应力增大而增大;随初始密实度增大而增大;随黏粒含量的增加而降低。     

Vertical dynamic impedance of large-diameter pile considering its transverse inertia effect and construction disturbance effect

The vertical dynamic impedance of the large-diameter pile is theoretically investigated considering the construction disturbance effect. First, the Rayleigh–Love rode model is introduced to simulate the large-diameter pile with the consideration of its transverse inertia effect. The shear complex stiffness transfer model is proposed to simulate the radial inhomogeneity of the pile surrounding soil caused by the construction disturbance effect. Then, the pile–soil system is divided into finite segments, and the governing equation of the pile–soil system subjected to vertical dynamic loading is established. Following this, the analytical solution of vertical dynamic impedance at the pile head is obtained by means of the shear complex stiffness transfer method and the impedance function transfer method. Based on the present solution, a parametric analysis is conducted to investigate the influence of the transverse inertia effect on the vertical dynamic impedance at the pile head and its relationship with the pile–soil parameters. Finally, comparisons with published solutions are carried out to verify the reliability of the present solution.     

循环荷载作用下粉土的动力学特征

选用近海分布广泛的粉土为研究对象,利用动三轴压缩试验结果得到了动荷载作用下粉土的应力-应变关系、孔压发展模式及动强度与临界循环次数之间的关系;探讨固结围压和固结比对粉土动力学性质的影响。动力学试验结果表明,动剪切模量随着固结围压的增大而增大,动阻尼比随动剪应变幅的变化关系受围压影响不大;不同围压对动剪应力的比值影响很小,同一围压下随着固结比的增大,动剪应力比也会随之增大;不同围压及不同固结比对以Nf表示的峰值孔隙水压力发展模式影响很小。     

Torsional Dynamic Impedance of a Tapered Pile Considering its Construction Disturbance Effect

The dynamic response of a tapered pile (considering its construction disturbance effect) is investigated when the tapered pile is subjected to a time-harmonic torsional loading. For most engineering conditions, the surrounding soil may be weakened or strengthened owing to the construction disturbance effect of the tapered pile, resulting in the soil becoming radially inhomogeneous. In order to consider this problem, the circumferential shear complex stiffness transfer model is proposed to simulate the radial inhomogeneity of soil. Then, the governing equations of a tapered pile-soil system subjected to torsional dynamic loading are established. By virtue of the circumferential shear complex stiffness transfer method and the impedance function transfer method, the analytical solution of torsional dynamic impedance at the head of the tapered pile is derived. Based on the presented solution, the influence of the construction disturbance effect of the surrounding soil on the torsional dynamic impedance at the pile head is investigated within the low-frequency range concerned in the design of a dynamic foundation. The results show that, even if the hardening range and softening range of the surrounding soil vary within a smaller scale, the hardening effect and softening effect also have a notable influence on the torsional dynamic impedance at the pile head.