富水软岩隧道突泥塌方及地层沉降的模型试验

在富水软岩地层中修建隧道容易出现突泥塌方,对施工及周边环境产生严重影响。北岗隧道围岩强度低且地下水丰富,在施工中多次遇到突泥涌水现象,出现了围岩大面积坍塌、地表开裂、地下水位下降等严重灾害问题。以北岗隧道为原型,开展了室内模型试验,再现了该隧道的破坏现象,研究了隧道突泥塌方的影响因素、破坏规律及影响范围。结果表明:（1）地表未出现裂缝时,隧道开挖导致的地表和地层内部变形可以采用Peck的正态分布曲线描述;（2）涌水是造成隧道围岩发生严重坍塌的根本原因;（3）隧道破坏从拱顶开始,呈拱形破坏型式;（4）地表的两条主裂缝在隧道塌方初期形成,塌方对地表沉降和裂缝的主要影响区域位于两条主裂缝之间。     

深隧道围岩分区破裂的数学模拟

该研究为先前深隧道围岩分区破裂现象内变量梯度塑性模型的进一步发展。利用应变梯度模型研究了深隧道围岩的分区破裂现象。作为额外的状态变量,在此引入应变梯度这一新变量。利用虚功原理得到了深隧道围岩的平衡方程、边界条件和流动准则,利用Clausius-Duhem不等式获得了岩体的本构方程。对于圆形深隧道,由上述模型的一般方程得到了弹性变形情况下、具有下降段的弹塑性变形情况下和不考虑弹性变形的塑性变形情况下圆形深隧道围岩的支配方程,得到了解析解,并讨论了解析解的性质。这一模型不仅扩展了隧道围岩的经典弹塑性模型,也为下一步数值研究深隧道围岩的分区破裂现象奠定了理论基础。     

含初始缺陷地铁隧道的抗震动力学行为研究

针对含初始缺陷的既有地铁隧道受邻近穿越隧道施工影响的首都地铁隧道工程背景,基于有限差分方法,通过对既有地铁车站结构受邻近隧道穿越后的抗震性能研究,揭示其抗震动力响应机理。结果表明,在邻近隧道穿越影响下,含初始缺陷的既有隧道各薄弱控制点在地震作用下位移和应力动力时程的波动趋势具有相似性,说明衬砌结构是整体运动的,其位移、应力较穿越前均有所增加,边墙和仰拱在穿越后抗震性能影响最大,剪切破坏主要以车站结构周边土体为主,新线穿越后其剪切破坏分布范围有所增大,应进行注浆加固处理,与自身结构刚度相匹配,提高抗震能力。     

盾尾注浆硬性浆液固结变形数值计算模型构建

为了更为合理地评价盾构壁后注浆控制地层变形机制,采用自制的硬性浆液固结变形试验装置,对水泥胶结作用影响硬性浆液的固结过程进行了试验研究。在此基础上,分析并建立了能够反映水泥胶结作用的硬性浆液固结变形模型,编制了相应的数值计算程序,并通过试验验证了该模型的合理性。硬性浆液固结变形数值计算模型的构建对于提高盾构施工模拟精细度将起到一定的推动作用。     

隧道水平冻结施工引起地表冻胀的历时预测模型

隧道水平冻结施工过程中,土体冻结引起体积膨胀,进而会在地表产生冻胀现象。实际工程一般采用多根冻结管形成冻结壁。冻结壁交圈前,地表冻胀由多个冻土柱的叠加膨胀变形引起;冻结壁交圈后,地表冻胀则由整个冻结壁的膨胀变形引起。鉴于此,考虑冻结壁的形成过程,基于随机介质理论,建立了隧道水平冻结施工引起地表冻胀位移的历时预测模型。同时对冻结外锋面半径和冻胀区域外半径这2个关键参数的取值方法进行了相关探讨。最后针对两个工程案例,采用该计算模型对地表冻胀位移进行分析,得到地表冻胀位移随时间的变化规律,并与现场实测结果相比较,验证了模型的可靠性。该模型应用于隧道水平冻结施工前、冻结期内任意时刻的地表冻胀位移预测,可为工程冻结实施方案的合理确定提供有效依据。     

隧道掘进面接近地质界面时围岩的应力特征研究

隧道开挖地层中往往呈现出明显的地质变异性,如软硬岩层的不均匀分布等,使隧道开挖后的力学行为十分复杂.本文采用FLAC3D初步分析了隧道掘进面接近地质界面时围岩的应力状态,并探讨侧向应力、纵向应力、断面形式对围岩应力的影响规律.分析结果表明:当隧道由硬岩往软岩或由软岩往硬岩向地质界面掘进时,掘进面前方围岩均有应力集中现象.隧道无论由硬岩往软岩还是由软岩往硬岩掘进至地质界面时,掘进面前方边墙位置围岩径向应力均随侧向应力的增大而增大,掘进面前方拱顶和边墙位置围岩纵向应力均随纵向应力的增大而增大.马蹄彤隧道开挖至硬岩与软岩的地质界面时,掘进面后方边墙位置围岩应力均小于圆形隧道相应位置的围岩应力.     

黄河源区玛多县沙漠化成因与发展过程

通过分析黄河源区玛多县沙漠化的成因和过程,结果表明,玛多县沙漠化成因类型有三种,分别为固定沙丘/古沙丘活化、滑塌陡坎及风蚀斑块,其形成都与多年冻土退化相关.在固定沙丘或古沙丘分布区,冻土退化导致热融沉陷,形成沉陷坑,沉陷坑边缘形成拉裂缝或陡坎,使下伏松散沙露出.在斜坡上,冻土退化形成滑塌陡坎,使底层土壤从侧面暴露.在平坦的冲积平原,差异性冻胀和融沉导致草皮拉裂,形成积水坑洼,冻土退化导致土壤变干,土壤的底层暴露.下伏风成沉积物暴露后,遭受风蚀,形成侧向凹槽,致使上部土体坍塌,使更多的风成沉积物暴露,这些过程的不断重复,使风蚀坑、陡坎、风蚀斑块扩大相连,最终形成流动沙丘、风蚀劣地、戈壁等地貌.     

高速铁路隧道围岩抗力系数现场试验与理论研究

以沪昆高速铁路隧道为依托,通过现场径向液压枕试验,测量各级压力下半径向变形,获取了隧道研究点处岩石抗力系数实测值。在实测数据的基础上,结合国内外相关的弹性抗力本构模型,建立了沪昆高速铁路隧道围岩抗力系数的5种理论计算模型：理想弹塑性模型、爆破裂缝弹塑性模型、塑性软化岩体模型、Lade-Duncan准则模型、统一强度理论模型,并有针对性的将所研究的20座隧道进行分类,分别计算其围岩抗力系数理论计算值。为了弥补现场试验点数目的限制,对所研究的隧道展开了数值模拟分析。通过对不同的埋深、围岩级别、地质特征下岩石抗力系数现场试验值、理论计算值、数值模拟值和规范值进行对比和综合,提出了高速铁路大断面隧道的围岩抗力系数推荐值,该推荐值突破了普通铁路隧道规范中关于抗力系数建议值仅限于弹性模型的限制,为优化隧道结构设计,充分利用围岩的自承能力提供了理论依据。     

Centrifuge modeling of offshore wind foundations under earthquake loading

The construction of large offshore wind turbines in seismic active regions has great demand on the design of foundations. The occurrence of soil liquefaction under seismic motion will affect the stability of the foundations and consequently the operation of the turbines. In this study, a group of earthquake centrifuge tests was performed on wind turbine models with gravity and monopile foundations, respectively, to exam their seismic response. It was found that the seismic behavior of models was quite different in the dry or saturated conditions. Each type of foundation exhibited distinct response to the earthquake loading, especially in the offshore environment. In the supplementary tests, several remediation methods were evaluated in order to mitigate the relatively large lateral displacement of pile foundation (by fixed-end pile and multi-pile foundation) and excessive settlement of gravity foundation (by densification, stone column, and cementation techniques).     

Importance of hydrogeological conditions during formation of the karstic bauxite deposits,Central Guizhou Province,Southwest China: A case study at Lindai deposit

Karstic bauxite deposits are widespread in Central Guizhou Province, SW China, and high-grade ores are frequently sandwiched with overlying coal and underlying iron-rich layers and form a special “coal–bauxite–iron” structure. The Lindai deposit, which is one of the most representative karstic bauxite deposits in Central Guizhou Province, was selected as a case study. Based on textural features and iron abundances, bauxite ores in the Lindai deposit are divided into three types of ores, i.e., clastic, compact, and high-iron. The bauxite ores primarily comprise diaspore, boehmite, kaolinite, illite, and hematite with minor quartz, smectite, pyrite, zircon, rutile, anatase, and feldspar. The Al₂O₃ (53–76.8 wt.%) is the main chemical contents of the bauxite ore samples in the Lindai district, followed by SiO₂, Fe₂O₃, TiO₂, CaO, MgO, S, and P etc. Our geological data on the Lindai deposit indicated that the ore-bearing rock series and its underlying stratum have similar rare earth elements distribution pattern and similar Y/Ho, Zr/Hf, and Eu/Eu¹ values; additionally, all ore-bearing rock samples are rich in MgO (range from 0.16 wt.% to 0.68 wt.%), and the plots of the dolomites and laterites lie almost on or close to the weathering line fit by the Al-bearing rocks in Zr vs. Hf and Nb vs. Ta diagrams; suggesting that the underlying Middle Cambrian Shilengshui Formation dolomite is the parent rock of bauxite resources in the Lindai district.Simulated weathering experiments on the modern laterite from the Shilengshui Formation dolomite in the Lindai bauxite deposit show that hydrogeological conditions are important for karstic bauxite formation: Si is most likely to migrate, its migration rate is several magnitudes higher than those of Al and Fe under natural conditions; the reducing inorganic acid condition is the most conducive to Al enrichment and Si removal; Fe does not migrate easily in groundwater, Al enrichment and Fe removal can occur only in acidic and reducing conditions with the presence of organic matter.The geological and experimental studies show that “coal–bauxite–iron” structure in Lindai deposit is formed under certain hydrogeological conditions, i.e., since lateritic bauxite or Al-rich laterite deposited upon the semi-closed karst depressions, Si can be continuously removed out under neutral/acidic groundwater conditions; the coal/carbonaceous rock overlying the bauxitic materials were easily oxidized to produce acidic (H₂S, H₂SO₄, etc.) and reductant groundwater with organic materials that percolated downward, resulting in enrichment of Al in underlying bauxite; it also reduced Fe³⁺ to its easily migrating form Fe²⁺, moving downward to near the basal carbonate culminated in precipitating of ferruginous (FeS₂, FeCO₃, etc.) strata of the “coal–bauxite–iron” structure. Thus, the bauxitic materials experienced Al enrichment and Si and Fe removal under above certain hydrogeological conditions forming the high-quality bauxite.