【摘要】：通過吹沙填海方式形成的場地,往往下伏著厚層海相淤泥質(zhì)軟土,加之沿海地區(qū)是地震頻發(fā)的地帶,軟土的不良工程特性對地震作用有很大的影響。因此本文從工程防震減災(zāi)的角度,通過動三軸試驗研究北海海積軟土在三種不同因素影響下的動力特性,并對循環(huán)荷載作用前后的土樣進(jìn)行壓汞試驗和掃描電鏡試驗,定性、定量研究土體微細(xì)觀結(jié)構(gòu)的變化規(guī)律及三種要素對細(xì)觀結(jié)構(gòu)的影響,建立細(xì)觀結(jié)構(gòu)特征參數(shù)與宏觀性質(zhì)參數(shù)之間的聯(lián)系,從細(xì)觀角度解釋土體位移、變形的力學(xué)行為。本文的主要研究成果如下:(1)北海海積軟土為含砂粒的黏性土,主要礦物成分為石英和高嶺土。人工制備樣在上覆荷載的作用下,豎向沉降變形量最大達(dá)44%,沉降變形速率隨著荷載等級的增加而逐漸放緩,最終趨于穩(wěn)定。制備土樣具有孔隙比大,飽和度高的特點。(2)動三軸試驗中,隨著動荷載幅值的增大,北海海積軟土的動變形從彈性穩(wěn)定狀態(tài)到塑性擴(kuò)張狀態(tài)演化,且不同要素對土體動力特性有不同程度的影響。固結(jié)比的增大會降低海積軟土的強(qiáng)度和剛度,固結(jié)比對土樣變形的影響相對較小。固結(jié)圍壓的增大能明顯提高土體的抗變形能力,會減小能量在土體中的傳遞消耗。振動頻率為0.05Hz～1Hz時,土樣變形曲線呈雙曲線穩(wěn)定型;當(dāng)頻率為2Hz時,變形曲線則呈硬化型;振動頻率越低越有利于動變形的開展,頻率的降低會減弱土體的強(qiáng)度和剛度。(3)壓汞試驗中根據(jù)海積軟土的孔隙特征,將孔徑劃分為4個等級,原狀樣孔徑呈單峰分布,中孔隙占有絕對優(yōu)勢。對比發(fā)現(xiàn),循環(huán)荷載作用后土樣總孔隙體積有所增加,但仍以中孔隙含量為主,各組孔徑間的分布均有調(diào)整,中、小孔隙含量減小,大孔隙含量有所增加。固結(jié)比越大,大孔隙含量越小,總孔隙體積越小;固結(jié)圍壓的增加能減少土體孔隙體積;在各因素耦合作用下,振動頻率對孔隙分布的影響較復(fù)雜。(4)對SEM圖像進(jìn)行定性分析發(fā)現(xiàn),北海海積軟土是面-面接觸為主、少量面-邊接觸的黏膠基質(zhì)片架-鑲嵌結(jié)構(gòu),基本單元體主要是由扁平顆粒片片相疊而成的“黏土疇”疊聚體,疊聚體在空間上犬牙交錯,形成架空-鑲嵌式孔隙,孔隙發(fā)育明顯,多呈條帶狀裂隙,形狀極不規(guī)則且連通性差。在較大上覆荷載的作用下,土體結(jié)構(gòu)單元呈現(xiàn)出一定的定向排列。(5)利用IPP軟件從SEM二值化圖像提取各單元體細(xì)觀結(jié)構(gòu)參數(shù),分析了三種要素影響下的細(xì)觀結(jié)構(gòu)變化規(guī)律。結(jié)合北海海積軟土動力特性,研究循環(huán)荷載作用前后土體細(xì)觀結(jié)構(gòu)演化規(guī)律,總結(jié)出細(xì)觀結(jié)構(gòu)演變的過程:結(jié)構(gòu)微調(diào)-結(jié)構(gòu)應(yīng)力集中-結(jié)構(gòu)整體失穩(wěn),對應(yīng)著宏觀變形的三個階段:彈性變形-應(yīng)力屈服-塑性變形。建立了孔隙結(jié)構(gòu)特征參數(shù)與宏觀特性參數(shù)之間的聯(lián)系,從細(xì)觀角度分析各因素對土體動力特性的影響機(jī)理。
[Abstract]:The site formed by sand blowing and reclamation is usually covered with thick layer of marine silt soft soil, and the coastal area is an area where earthquakes occur frequently. The poor engineering characteristics of soft soil have a great influence on earthquake action. Therefore, from the point of view of engineering earthquake prevention and disaster reduction, the dynamic characteristics of Beihai marine soft soil under the influence of three different factors are studied by dynamic triaxial test, and the mercury injection test and scanning electron microscope test are carried out on the soil samples before and after cyclic loading. The variation law of soil micro-structure and the influence of three elements on the micro-structure are studied quantitatively. The relationship between the characteristic parameters of meso-structure and the macro-property parameters is established, and the mechanical behavior of soil displacement and deformation is explained from the point of view of meso. The main results of this paper are as follows: (1) Beihai marine soft soil is sandy clay, and the main mineral components are quartz and kaolin. Under the action of overburden load, the maximum vertical settlement deformation is 44%, and the settlement deformation rate slows down gradually with the increase of load grade, and finally tends to be stable. The prepared soil samples have the characteristics of high porosity and high saturation. (2) in dynamic triaxial tests, with the increase of dynamic load amplitude, the dynamic deformation of Beihai marine soft soil evolves from elastic stable state to plastic expansion state. And different factors have different influence on soil dynamic characteristics. The strength and stiffness of marine soft soil will be decreased with the increase of consolidation ratio, and the effect of consolidation ratio on soil deformation is relatively small. The increase of consolidation confining pressure can obviously improve the deformation resistance of soil and reduce the energy transfer consumption in soil. When the vibration frequency is 0.05Hz~1Hz, the deformation curve of soil sample is hyperbolic and stable, and when the frequency is 2Hz, the deformation curve is hardening. The lower the vibration frequency is, the more favorable the dynamic deformation is, and the decrease of frequency will weaken the strength and stiffness of soil. (3) according to the pore characteristics of marine soft soil, the pore size is divided into four grades, and the pore size of the undisturbed sample is distributed as a single peak. The mesopore has the absolute advantage. It is found that the total pore volume of soil samples increases after cyclic loading, but the main pore content is medium pore content, the distribution of pore size in each group is adjusted, the small pore content decreases and the macropore content increases. The larger the consolidation ratio is, the smaller the macroporous content is, the smaller the total pore volume is, and the higher the consolidation confining pressure is, the less the pore volume of soil is. The effect of vibration frequency on pore distribution is complex under the coupling of various factors. (4) qualitative analysis of SEM images shows that the soft clay in Beihai Sea is a scaffold mosaic structure with surface to surface contact and a small amount of surface to edge contact. The basic unit is mainly composed of "clay domain" stacked by flat granular fragments, which interlaced in space to form aerial and inlaid pores, with obvious pore development, and most of them show banded fractures. Extremely irregular shape and poor connectivity. Under the action of large overburden load, the soil structure element presents a certain directional arrangement. (5) the meso-structure parameters of each unit are extracted from the SEM binary image by using IPP software, and the variation law of the meso-structure under the influence of three elements is analyzed. Combined with the dynamic characteristics of Beihai marine soft soil, the evolution law of soil meso-structure before and after cyclic loading is studied, and the evolution process of meso-structure is summarized: structure fine-tuning, structural stress concentration and overall structural instability. It corresponds to three stages of macroscopic deformation: elastic deformation, stress yield and plastic deformation. The relationship between pore structure characteristic parameter and macroscopic characteristic parameter is established, and the influence mechanism of each factor on soil dynamic characteristic is analyzed from the viewpoint of meso.
【學(xué)位授予單位】：廣西大學(xué)
【學(xué)位級別】：碩士
【學(xué)位授予年份】：2017
【分類號】：TU447

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