【作者】 刘婷婷；

【导师】 吕岩；

【作者基本信息】 吉林大学 ， 土木工程， 2022， 硕士

【摘要】 草炭土是一种具有高含水率,高有机质含量、高压缩性和高孔隙比等特殊工程地质特性的腐殖质土,常存在于特殊的沼泽湿地环境中,对调节周围环境的生态平衡起到重要作用。草炭土主要分布于中国的东北区域、青藏高原、川西若尔盖高原和长江中下游等植被发育、常年积水的地势低平地区。当草炭土在季冻区分布时,会出现冻胀融沉现象,常出现道路破坏,路基软化,公路承载能力降低等工程问题。而近年来,由于我们国家经济建设快速发展,以及公路铁路工程的日益发达,很多新建的线路工程穿越季冻区沼泽草炭土分布区是无法避免的,为了保护沼泽草炭土湿地以及减少工程问题的出现,有必要对草炭土的冻胀融沉特性展开探究,为工程建筑设计提供理论基础。本文基于国家自然科学基金项目“季冻区沼泽草炭土冻胀机理研究”,以敦化地区典型的季冻区沼泽草炭土为主要研究对象,首先通过常规的土工试验,测定了草炭土的基本物理化学指标,然后选取天然含水率、干密度、有机质、分解度四个影响因素,在不同冻结温度下对草炭土展开封闭条件下的冻胀融沉试验,建立了基于极限学习机(ELM)的草炭土冻胀和融沉预测模型,得出以下结论:(1)由Pearson相关性分析可知,草炭土的含水率,干密度,有机质含量以及分解度四个因素之间有显著的线性关系,含水率与干密度成负相关,与有机质含量和分解度均成正相关;干密度与有机质含量和分解度均成负相关;而有机质含量和分解度二者成正相关。(2)不同深度的草炭土含水率与干密度差距较大,冻胀力与含水率和干密度均成正比关系,但干密度过小便不会产生明显的冻胀力。深度为0-0.4m的土样,干密度不超过0.41g/cm~3时,几乎不产生冻胀力,随着深度增加,含水率逐渐增加,干密度逐渐降低,不产生明显冻胀力的界限值在逐渐降低,深度为0.8-1.6m的草炭土,当干密度低于0.25 g/cm~3后不会产生明显的冻胀力。(3)草炭土属于强冻胀土,其冻胀率与融沉系数成显著的正比关系;含水率和分解度对与冻胀率和融沉系数的影响规律相同,均成正比关系,并且冻胀率和融沉系数均随着有机质含量的增加先增加后减小;另外含水率越大,未冻水含量越高,冻胀完成的温度也越低;而干密度越大,孔隙率越小,冻胀率越大,冻结时所需的温度降低;对于融沉系数,干密度增加,其先逐渐减小后增加,即存在一个临界干密度值使得草炭土的融沉系数最小。(4)分别建立了基于ELM的草炭土冻胀和融沉预测模型并检验,通过此模型得到的预测值和实测值偏差相对较小,整体拟合度较好,可应用于季冻区草炭土路基工程中冻胀融沉特性的初步预测,为实际工程提供一定参考。更多还原

【Abstract】 Turfy soil is a type of humus soil with exceptive engineering geological characteristics,such as high moisture content and organic matter content,high compressibility and high porosity ratio.It often exists in special marsh environment,which plays an significant role in adjusting the ecological balance of the surrounding environment.It is widely distributed in northeast China,Qinghai-Tibet Plateau,Western Sichuan Ruoergai Plateau and the lower-middle Yangtze River and other low-lying areas with vegetation development and perennial water accumulation.When turfy soil is distributed in seasonal freezing area,frost heaving and thawing subsidence will occur,which will lead to road damage,subgrade softening,reducing the highway carrying capacity and other engineering problems.In the last few years,national economic construction is developing rapidly,highway and railway construction is also developing,so it is inevitable that many new line projects pass through the seasonal frozen swamp turfy soil distribution area.In order to protect swamp turfy soil wetland and reduce the occurrence of engineering problems,it is necessary to explore the characteristics of frost heaving and thawing settlement of turfy soil,and provide a theoretical basis for engineering architectural design.Based on the National Natural Science Foundation of China project"Study on frost heaving mechanism of swamp turfy soil in the seasonal frozen region",this paper takes typical swamp turfy soil in Dunhua region of Jilin Province as the subject investigated.Firstly,the basic physical and chemical indexes of turfy soil are determined by conventional geotechnical tests.Four influencing factors including natural water content,dry density,organic matter content and decomposition degree were selected to conduct frost heaving and thawing settlement experiments on turfy soil under closed conditions at different freezing temperatures.A prediction model of frost heaving and thawing settlement of turfy soil based on ELM was established,and the conclusions were as follows:(1)According to Pearson correlation analysis,there was a significant linear correlation among moisture content,dry density,organic matter content and decomposition degree of turfy soil.Percentage of moisture was negatively correlated with dry density,but is directly proportional to organic matter content and decomposition degree.The dry density is inversely proportional to both the organic matter content and the decomposition degree.The organic matter content and decomposition degree were positively correlated.(2)There is a great difference between the moisture content and dry density of turfy soil at different depths,and the frost heave force is proportional to both moisture content and dry density,but the frost heave force will not be obvious when the dry density is too small.When the dry density of 0-0.4m soil sample is less than0.41g/cm~3,there is almost no frost heave force.With the increase of depth,moisture content increases gradually,dry density decreases gradually,and the limit value of no obvious frost heave force decreases gradually.When the dry density of turfy soil is lower than 0.25 g/cm~3at the depth of 0.8-1.6m,no obvious frost heave force occurs.(3)Turfy soil belongs to strong frost heaving soil,and the frost heaving rate has a significant proportional relationship with thawing settling coefficient.The effect of water content and decomposition degree on frost heaving rate and thawing settlement coefficient is the same,and is proportional to each other.They are both increase first and then decrease with the increase of organic matter content.In addition,the higher moisture content,the higher the unfrozen water content,the lower the temperature of frost heaving.The higher dry density is,the smaller porosity is,the higher the frost heaving rate is,and the lower the temperature required for freezing is.As for the thawing settlement coefficient,the dry density increases,which is gradually decreased first and then increased,that is,there is a critical dry density value that minimizes the thawing settlement coefficient of turfy soil.(4)The prediction models of frost heaving and thawing settlement of turfy soil based on ELM were constructed and tested respectively.The deviation between the predicted values and the measured values obtained by the model was small,and the overall fitting degree was good,which could be applied to the preliminary prediction of frost heaving and thawing settlement characteristics of turfy soil subgrade engineering in seasonal freezing region,providing some reference for practical engineering.更多还原