【作者】 孙立强；

【导师】 闫澍旺；

【作者基本信息】 天津大学 ， 岩土工程， 2010， 博士

【摘要】 随着天津滨海新区的开发建设,越来越多的土地是由围海造陆形成的吹填土地基。以往吹填土地基在吹填完毕后,经过2-3年的晾晒,然后才进行真空预压加固,而随着滨海新区开发建设脚步的加快,真空预压有了新的工艺,可使吹填土地基几乎不经过晾晒,直接进行真空预压加固,所以现在真空预压加固的对象变为不经过晾晒的新近吹填土地基。对于真空预压法的固结机理和加固效果,国内外众多学者和工程师已经做了大量的研究工作,得到了很多有意义的研究成果,真空预压法的工作机理和计算方法已经成熟。由于新近吹填土地基厚度大、含水量高、压缩性比较大,与以往经过晾晒的吹填土地基不同,导致在真空预压计算和实际工程中出现很多问题。由于新近吹填土地基的特点,使其在真空预压过程中会发生很大的沉降,使排水板的工作机理和固结计算方法都发生了很大的改变,利用以往计算方法计算新近吹填土地基真空预沉降与现场实测值存在很大的偏差,故以往的真空预压计算理论不适应于新近吹填土地基,本文针对这些问题,对新近吹填土地基真空预压从理论和试验上进行了以下研究:1.提出了新近吹填土地基真空预压沉降计算方法。本文首先分析了新近吹填土与以往吹填土地基的不同,通过分析新近吹填土地基的形成过程和特点,提出新近吹填土是欠固结土,加固前土体内存在由吹填土有效自重形成的超孔压,在插板期间会产生一定的沉降。本文认为新近吹填土地基真空预压沉降是由抽真空前的插板期间沉降和真空预压期间沉降两部分组成,通过研究吹填土地基的在插板期间的土层的附加荷载和抽真空期间土层的附加荷载情况,提出吹填土地基真空预压插板期间的沉降计算方法和抽真空期间的沉降计算方法。通过与现场监测数据相比,证明了计算方法的可行性,这种方法已经为相关的计算规程所采用。2.对新近吹填土地基真空预压进行了室内模型试验研究。新近吹填土地基是由含水量在80%以上的超软土形成。随着围海造陆的发展,吹填区底面标高越来越低,吹填土厚度越来越厚,使真空预压法进行地基加固时发生很大的压缩变形。在地基加固过程中出现很多理论上难以解释的现象,针对这些问题本文进行了超软土真空预压室内模型试验。通过试验可以发现:(1)土体加固后由于超软土发生很大的压缩变形,使排水板发生了很大的扭曲变形,甚至发生局部折断现象,改变了排水板的工作状态。(2)在加固土体变形基本稳定后,进行了二次插板再加固的试验,结果表明土体的沉降进一步发展,含水量进一步降低,十字板强度进一步提高,且十字板强度沿深度递减幅度大为减少。证明了第一次加固过程中由于土体发生大变形使排水板效率降低,地基不能达到预期的加固效果;采用二次插板可使吹填土产生较好的加固效果。同时通过实验对排水板中和土体中的真空度的分布规律和变化规律进行了研究。研究成果为以后的吹填土地基真空预压的理论分析和有限元计算提供了依据。3.通过对新近吹填土地基特点和加固过程的分析,提出适应于吹填土地基真空预压的有限元计算方法。新近吹填土地基真空预压有限元计算分为两个过程:一是插板期间的固结,另一个为真空预压过程。考虑真空预压不同过程的荷载形式,将非线性本构关系(Duncan-Chang模型)引入Biot固结理论,编制了通用的有限元程序,对新近吹填土地基真空预压法进行平面应变有限元分析;利用本文提出的有限元程序分别对新近吹填土地基整个加固过程和正常固结土地基的真空—堆载联合预压进行了有限元分析,计算结果与实测结果对比表明,该方法较好的反映了实际工程沉降及孔隙水压力的发展趋势。同时本文利用有限元软件ABAQUS模拟了吹填土地基真空预压的整个过程,计算结果与实际工程比较吻合。4.为了探索更适合新近吹填土地基真空预压的新工艺,针对新近吹填土的特点,本文设计了可控通气真空预压试验,这种方法可以加速水体的排出,提高真空预压效率。对可控通气真空预压和常规真空预压进行了对比研究,结果表明可控通气真空预压在相同的条件下加固效果好于常规真空预压。最后阐述了可控通气真空预压法加固机理。更多还原

【Abstract】 With the development and construction of Tianjin Binhai New Area, more and more land is made up with reclaimed soil. In the past the reclaimed soil foundation is dried in 2 to 3 years, and then it was reinforced in vacuum preloading method. But with the development and construction of Tianjin Binhai New Area, there is new technics of vacuum preloading, so the reclaimed soil can be reinforced without dried, the object was changed to recently reclaimed soil. Vacuum preloading method is an effective reinforcement to soft soil foundation; and this method is suitable for reclaimed soil foundation. Many foreign and domestic scholars and engineers have done a lot of research and made many interesting research results in the Vacuum preloading mechanism. However, the recently reclaimed soil characterized by low-intensity, high water content, large thickness, there is many problems due to these characters in vacuum preloading method. The theory and calculation method is not accurate to the recently reclaimed soil foundation. In order to solve these problems the paper did some studies on the calculation theory and test as followings:First, the settlement calculation method of recently reclaimed soil foundation was put forward. The formation process and character of the recently reclaimed soil are analyzed, and it was proposed that the reclaimed soil is unconsolidated soil, there is excessive pore water pressure which is caused by the self weight in it before the reinforcement. So there will be settlement during the PVDs installment. It is considered that the vacuum preloading settlement include the settlement during the PVDs installment and the settlement during the vacuum preloading. The settlement calculation method was put forward, and at the same time, the calculation result was compared to the field test data, it shows that the method is feasible.The indoor model test study was conducted to the recently reclaimed soil.The reclaimed soil have the water content of 80% and it is super-soft soil. With the development of land of Reclamation, the bottom elevation of the reclaimed area getting lower and lower, the thickness of the reclaimed soil reached 10m from the original thickness of 2m now, so that a great deformation occured in the course of the vacuum preloading. There are some phenomena that hard to explain. In order to solve these problems, this paper carried out an ultra-soft soil vacuum preloading indoor model test. Some research have been done through the experiments: (1)Though the indoor model experiment it was found that the PVDs has undergone a drastic distortion and even part of it was breaken due to large deformation of the ultra-soft soil during the vacuum preloading, this result the efficiency of the PVDs reduced, so that the soil reinforcement is ineffective and can not achieve the desired bearing capacity of foundation requirements. (2) in order to make secondary reinforcement, secondary PVDs installation was carried out, results show that the settlement was further developed,water content was further reduce,vane strength was further improved, and the Vane strength decline rate greatly developed. the distribution of vacuum degree along the PVDs and in the soil was also be studied. The test result provide reference to explore a more effective method of reinforcing the deep reclaimed soil foundation.Though the analysis of the the character of reclaimed soil and the consolidation process, the FE calculation method was put forward. The recently reclaimed soil foundation vacuum preloading is divided into two processes, the consolidation during PVDs installing and vacuum preloading. The paper incorporates the nonlinearity constitutive relation (Duncan-Chang’s model) into the Biot’s consolidation theory program. Considering different load conditions, a general-purpose program was compiled to analyze the behavior of the soft clay consolidation with plain strain FEM. Two actual projects were calculated using this program. Comparing to the field measurements data, it indicates that the method reflects very well the developing trend of settlement and pore water pressure for the projects. At the same time, the vacuum preloading of reclaimed soil foundation was analyzed using ABAQUS FE soft ware, the result goes well with the field data.At last, the paper presents a new method of vacuum preloading by improving the drainage system. An indoor experiment was made to compare the ventilating vacuum preloading with the ordinary vacuum preloading. On the basis of test data, described the theory of the ventilating vacuum preloading.更多还原