酸性环境下黄土湿陷性试验与湿陷敏感性研究

 黄土的胶结连接是黄土湿陷的重要内在因素,对其湿陷敏感性有着直接影响。为了对胶结连接在黄土湿陷敏感性中的作用有更清楚的认识并提出一个能科学的判定黄土湿陷敏感性的评价方法,在黄土湿陷性试验的浸入水溶液里添加不同浓度的硝酸,测试在酸性环境中,不同压力下黄土的湿陷系数、稳定时间、湿陷速率等湿陷敏感性指标。同时用扫描电镜观察试验前后土样的孔隙、结构变化情况。试验结果表明：在酸性溶液中,湿陷性黄土在低压状态下的湿陷系数增大较多,在各级压力下的湿陷稳定时间均有所延长;在水溶液和酸性溶液中,黄土有着不同的湿陷敏感压力区间;湿陷性黄土的结构强度对湿陷性黄土的深度有着重要影响等。在此试验基础上,吸取前人的经验,提出用完成90%湿陷量的用时和湿陷系数这2个指标对黄土及其场地进行湿陷敏感性评价,并以发生过湿陷的场地--抽黄渠为实例对此方法进行验证。验证结果表明,该方法能准确地反映湿陷场地的湿陷敏感性,对湿陷场地上的工程建设具有一定的指导意义。此外,该方法简单、科学,又能与现行规范相结合,因此,易于被业界接受并推广,是对现有少数关于黄土湿陷敏感性研究的有益补充。     

灰土桩处理湿陷性黄土地基试验研究

通过工程实例,设计了试验方案,测量了石灰桩膨胀法加固湿陷性黄土时黄土的各项力学参数的变化以及进行了评价.结果表明土桩挤密地基可以用来处理全部湿陷性土层,也可用于处理部分湿陷性土层,对用水量很大,地基大量浸水难于避免的建(构)筑物,地基亦处理全部湿陷性土层,至少要消除极大部分的湿陷性,对湿陷性黄土地基用挤密地基解决湿陷问题是主要的,只要湿陷问题解决了,一般压缩性、承载力和防水抗渗也可以满足.若将挤密地基做成防水抗渗性能较好的垫层时,则应适当提高桩间土的密实度要求.场地土的含水量偏低,适当的增湿能提高地基处理的效果,由于本次试验场地存在砂石夹层,增湿深度受到限制,增湿灌水量不宜控制,使增湿后含水量增大不大明显.但挤密后的效果均能满足设计要求.     

Effect of water salinity on Atterberg limits of El-Hodna sabkha soil

Sabkha soils are recent saline sediments widely distributed in Algeria and other countries. As they are characterised by low strength and high compressibility, they are considered to be difficult foundation soils that pose special problems for design and construction engineering. This study deals with the effect of salts on the determination of the water content and Atterberg limits of sabkha soils. The tests were performed using distilled water, natural sabkha brine and saline solutions with different salt concentrations. The results indicate that the liquid and plastic limits decrease with pore fluid salinity when the conventional water content procedure is used, but increase when the fluid content method is used. Also, the results revealed that the oven-drying time of saline soils is highly affected by the size of soil clods and surface salt precipitation.     

Environmental Effects Of The Dead Sea-Red Sea Canal

A project to link the Dead Sea to the Red Sea via a canal is undergoing extensive study. This project will transform the physical and chemical characteristics of the Sea and therefore has significant environmental impacts. The predicted behaviour of the Dead Sea with a canal project was examined in this study, and the environmental impacts were then identified and classified into positive and negative. The main positive impact of the canal is the restoration of the Dead Sea into its 1930s level, and the improvement in life quality in the area. Negative impacts include the risk of groundwater contamination, the re-establishment of stratification condition, seasonal precipitation of chemicals, and the growth of micro-organisms in the Sea.     

Enhancing mechanical behaviors of collapsible soil using two biopolymers

This study aims to investigate the possibility of using biopolymer (environmental friendly material) to enhance the mechanical behaviors of collapsible soil. Two types of biopolymers were (xanthan gum and guar gum) used in this study due to their stable behaviors under severe conditions and their availability with reasonable prices. The experimental program focused on three major soil properties, i.e. compaction characterizations, collapsible potential and shear parameters. These three properties are essential in process of soil improvement. Different biopolymer concentrations were used in this study and the experimental program was performed at two curing periods (soon after mixing the soil with the biopolymer and after one week curing time). Shear parameters were measured for the treated specimens under both soaked and unsoaked conditions, while a collapsible potential test was performed under different mixing conditions (wet mix and dry mix). A numerical model was built to predict the behavior of the treated collapsible soil after and before water immersing. The results indicated that the ability of both xanthan gum and guar gum can be used as improvement materials for collapsible soil treatment. The collapsible potential has been reduced from 9% to 1% after mixing the soil with 2% biopolymer concentration in the wet case. After one week curing, the cohesion has been increased from 8.5 kPa to 105 kPa by increasing the xanthan gum concentration from zero to 2%, leading to an overall improvement in soil shear strength. It also proves that the guar gum is superior to the xanthan gum. The shear strength of soil can be increased by about 30% when using the guar gum in comparison with the xanthan gum at the same conditions; however, the collapsible potential of soil material will be reduced by about 20%.     

Effect of added polymer on the desiccation and healing of a geosynthetic clay liner subject to thermal gradients

The desiccation and subsequent hydraulic conductivity of both a standard (GCL_A) and polymer-enhanced (GCL_B) Na-bentonite GCL hydrated from a well-graded sandy subsoil under 20 kPa, then subjected to a thermal gradient, and finally rehydrated and permeated with distilled water or 0.325 mol/L Na⁺ synthetic brine are reported.With moderate temperature of 40 °C applied to the top of the liner, GCL_B experienced less cracking than GCL_A, but this advantage disappeared when temperatures increased. Both desiccated specimens of GCL_A and B showed significant self-healing when permeated with distilled water and their hydraulic conductivities quickly reduced to around 10⁻¹¹ m/s at 20 kPa upon rehydration. However, when GCL_B desiccated specimens were permeated with the synthetic brine, their hydraulic conductivities were found to be one to two orders of magnitude higher than corresponding values obtained with distilled water. On the other hand, GCL_A (with no polymer treatment) maintained its hydraulic conductivities at the same level obtained with distilled water. It is concluded that caution should be exercised in using polymer-bentonite in applications in which GCLs are subjected to significant thermal gradients unless there is data to show they are resistant to thermal effects.     

大厚度湿陷性黄土隧道现场浸水试验研究

对于穿越大厚度湿陷性黄土地层的隧道,其围岩湿陷变形会威胁隧道结构的稳定性。为了分析黄土围岩湿陷变形对隧道衬砌结构的影响机制,选取典型大厚度湿陷性黄土隧道场地,通过开展隧道场地地面浸水试坑试验及隧道仰拱浸水试验,测试了地面入渗和隧道基底入渗过程中不同埋深地层的湿陷沉降变形及地基的沉降变形、入渗过程中围岩的体积含水率变化分布、试坑周边地层的侧向位移、衬砌结构接触压力和轴力,研究了既有隧道黄土地层的湿陷变形特性及水分运移规律、隧道结构力学响应。结果表明,隧道开挖、衬砌作用扰动黄土结构,增大了围岩及深层黄土的渗透性;与天然黄土场地试坑浸水入渗比较,增大了竖向浸水范围,减小了水平向浸水范围。隧道围岩湿陷变形改变了围岩与衬砌结构的相互作用性状。围岩湿陷和地基软化作用增大了二次衬砌结构侧墙竖向荷载和侧墙围岩的挤压作用,引起拱脚地基承载力减小和沉降变形发展,拱顶、拱肩接触面呈受拉状态;仰拱中部地基土的抗力作用抑制其沉降变形,从而使得拱脚和仰拱中部出现显著的沉降差,导致仰拱混凝土开裂,形成纵向裂缝。此外,浸水范围内黄土的湿陷变形不仅引起竖向沉降变形,还会引起周围土体产生侧向水平位移;洞口边坡场地黄土的湿陷性和地层湿陷变形差异较大,反映了黄土山岭黄土场地地层条件复杂多变的特征。     

人工制备湿陷性黄土地基地下连续墙浸水试验研究

为研究湿陷性黄土地基地下连续墙基础竖向极限承载特性及浸水后负摩阻力分布特征,选用石英粉、砂、膨润土、石膏和工业盐制备了人工湿陷性黄土,对人工制备湿陷性黄土的物理力学特性进行分析;采用人工制备湿陷性黄土填筑模型试验,进行地下连续墙基础承载特性试验研究。研究结果表明:人工制备湿陷性黄土的物理力学参数与天然黄土基本一致,可用于湿陷性黄土与构筑物相互作用模型试验相似材料。地下连续墙竖向承载力达到其极限时,外墙和内墙总侧摩阻力荷载分担比为67%,确定地下连续墙为端承摩擦型基础。地基浸水湿陷后,中性点深度比为0.64~0.73,试验结果与桩基浸水试验测试结果较为一致。由于地下连续墙基础具有良好的整体性和防渗性,芯土不受水的影响,内墙侧摩阻力与承台土反力能够得以发挥,有效减小地下连续墙基础的沉降。     

兰州地铁大厚度湿陷性黄土地层的现场浸水试验研究

为了研究大厚度湿陷性黄土地层的湿陷性对城市轨道交通地下结构的影响,针对传统室内试验评价结果不准确的缺点,依托兰州地铁3号线一期工程—陡道沟站,选取典型大厚度湿陷性黄土施工场地,通过开展场地地面浸水试验,测试了地面入渗过程中不同深度地层的湿陷沉降变形及地表的沉降变形,研究了既有黄土地层的湿陷变形特性,并结合室内试验的结果进行验证。结果表明：①场地内黄土的湿陷性具有突发性的特点,地表土层及深部土层的湿陷变形大体呈现陡增、骤降和平稳三个阶段;②场地内黄土的湿陷系数随着黄土深度的增加而降低,自重湿陷系数与深度的关系曲线符合幂函数关系,相关性为0.983;③兰州地区湿陷性黄土地层自重湿陷变形计算值的修正系数建议取值为1.675。研究结果可为兰州地区大厚度湿陷性黄土地层地铁设计及施工提供借鉴。     

湿陷性黄土地基的处理与边坡加固措施

介绍了黄土的特性,分析了水、水压力、土压力对黄土湿陷性的影响,对灰土或素土垫层、重锤夯实及强夯法、石灰土或二灰挤密桩等湿陷性黄土的地基处理方法进行了阐述,同时给出了湿陷性黄土的边坡加固措施。     

湿陷性黄土高贴坡变形模式和稳定性分析

采用原状黄土与重塑黄土联合制作模型,针对湿陷性黄土高贴坡在天然含水量及饱和状态时的稳定性和变形模式开展离心试验研究。结果表明：湿陷性黄土高贴坡在天然含水量状态下稳定性较好,贴坡体固结变形是高贴坡变形的主导因素,高贴坡工后变形量及变形速率前期较大,后期较小,且贴坡体厚度越大,工后沉降也越大,变形稳定所需的时间也越长,贴坡体厚度与工后沉降量呈现线性关系;当土体饱和时,贴坡体固结及黄土湿陷共同导致高贴坡沉降变形,若沉降变形过大,坡体可能沿水分浸入时形成的软弱带开裂破坏;湿陷性黄土层的强度决定湿陷性黄土高贴坡的稳定性,坡体破坏时滑裂面将通过湿陷性土层,其位置取决于湿陷性黄土层与其相邻土层的强度差异。当强度差异较大以致湿陷性黄土层与相邻土层的接触面形成软弱夹层时,则接触面必为滑裂面的一部分,且强度相对较小的接触面首先破坏,滑裂面上部土层表现出比较典型的平移滑动模式;反之,滑裂面近似圆弧,且与接触面之间存在一定厚度的过渡层。     

大厚度湿陷性黄土地层的现场砂井浸水试验研究

针对大厚度黄土湿陷变形室内试验评价不准确,现场原位浸水试坑试验评价方法周期长、费用高且难以适应线性工程技术要求等不足,提出了一种新的现场试验评价方法:砂井浸水试验方法。其核心是利用湿陷变形土体与未湿陷变形土体之间产生相对沉降差及地裂缝,通过湿陷性黄土场地设置砂井,将水直接导入某一深层湿陷性黄土地层及砂井圆周边土体,以此来测定砂井井底下地层和砂井孔深范围内黄土的湿陷变形量。该方法具有操作简便、花费小、周期短和灵活性高等特点。依托宝兰客专建设项目,在具有代表性的大埋深自重湿陷性黄土场地开展了4个不同深度的砂井浸水试验,测试了砂井场地的沉降变形及井底湿陷性土层的沉降变形,同时配合井底土层含水量的量测,分析了井底黄土的湿陷性变形特征。参考现有规范中建议的该地区自重湿陷量修正系数,对比砂井浸水试验结果与室内试验结果,初步论证了砂井浸水试验的合理性,及其在大厚度湿陷性黄土线性工程上运用的优势。     

湿陷性黄土区复杂地基上高层建筑岩土勘察设计要点浅析

高层建筑基底压力大,对沉降敏感,破坏后果很严重,相较于其他一般建筑,对地基和基础的稳定性要求更高,而湿陷性黄土在未受水浸湿时强度较高,浸水后,土结构迅速破坏,进而产生不均匀沉降,造成基础破裂。为了解决在湿陷性黄土峁、梁地区高层建筑建设过程中碰到的岩土工程技术问题,在查阅文献资料的基础上,结合工程实际经验,对湿陷性黄土峁、梁地区高层建筑工程勘察、设计及施工中主要考虑的问题进行分析,采用严格分阶段进行勘察、地基处理消除湿陷、按设计填筑土方并对坡面防护、桩基无水分段施工然后快速浇筑并扩大检测范围等手段,使得湿陷性黄土区上的高层建筑基础得以安全可靠投入使用。由此可知,详尽的勘察、针对性设计、良好的施工加之可靠的检测是湿陷性黄土区复杂地基上进行高层建筑所必不可少的。     

碎石桩加固深厚湿陷性黄土地基的试验研究

为明确碎石桩加固深厚湿陷性黄土地基的效果,依托某深厚湿陷性黄土地基处理工程,开展了碎石桩加固地基的系列试验研究。通过多种方式检测了加固后地基土体的物理力学参数、承载能力及沉降变形,并分析其变化规律。试验结果表明:采用碎石桩加固深厚湿陷性黄土地基,桩身的密实度在松软土层较低,在坚硬土层较密实,且相同深度的桩身密实度随桩间距的减小而提升;碎石桩对深度1～3 m内的桩间土有较好的挤密效果,对深度3 m以下土层则无显著影响;碎石桩能有效加固湿陷性黄土地基,可消除地基土湿陷性,经加固后的复合地基承载力特征值可提升56.25 %～152.78 %,压缩模量可提升62.90 %～152.78 %,而地基土的干密度和孔隙比与处理前的平均参考值相比则变化较小;桩间距对地基土体的处理效果有较明显的影响,减小桩间距可降低地基土层的后期沉降量;试验区合理的碎石桩置换率为7 %。研究结果可为碎石桩处理深厚湿陷性黄土地基的工程方案及预期目标的设计提供参考。     

Feasibility of using electrokinetics and nanomaterials to stabilize and improve collapsible soils

Loess as a subcategory of collapsible soils is a well-known aeolian deposit generally characterized as a highly-porous medium with relatively low natural density and water content and a high percentage of fine-grained particles.Such collapsible soil sustains large stresses under a dry condition with natural water content.However,it can experience high and relatively sudden decreases in its volume once it reaches a certain water content under a certain load and therefore,the natural condition of the soil might not be suitable for construction if the possibility of the exposure of the soil to excessive water exists during the lifetime of the project.This research presents the utilization of an innovative method for stabilization and improvement of Gorgan loessial soil.This method uses electrokinetics and nanomaterials to instigate additives to move through soil pores,as an in situ remedial measure.To assess the acceptability of this measure,the deformability and strength characteristics of the improved collapsible soil are measured and compared with those of the unimproved soil,implementing several unsaturated oedometer tests under constant vertical stress and varying matric suction.The result emphasizes the importance of the matric suction on the behavior of both improved and unimproved soils.The test results indicate that the resistance of the soil was highly dependent on the water content and matric suction of the soil.The oedometer tests on samples improved by 3% lime and 5% nanomaterials show considerable improvement of the collapse potential.Results also reveal that stabilized samples experience notably lower volume decrease under the same applied stresses.     

研究黄土湿陷性的新方法

为了研究吸力对黄土湿陷变形的影响,研制了一套可以控制吸力的非饱和土湿陷三轴仪。该三轴仪底座为二元结构,既能控制吸力又能浸水。将该三轴仪和GDS压力/体积控制器相结合,实现了浸水过程中量测浸水体积的功能,并可施加浸水压力。该设备能精确量测试样体变;控制偏应力;精确量测浸水量。利用该仪器探讨了原状湿陷性黄土控制吸力三轴湿陷试验的方法,并进行了净围压相同吸力不同和吸力相同净围压不同的双线法三轴湿陷试验。结果表明:吸力越大湿陷变形越大,净围压对湿陷变形有显著影响。非饱和土湿陷三轴仪为研究黄土的湿陷特性提供了有力的工具。     

大厚度自重湿陷性黄土地基处理深度和湿陷性评价试验研究

 为解决大厚度自重湿陷性黄土地区地基处理深度和湿陷性评价等难题,在湿陷性黄土厚度大于36.5 m的场地进行以下浸水试验：不同深度的挤密桩处理地基深层浸水载荷试验,不同深度的孔内深层强夯处理地基载荷浸水试验,不打注水孔、埋设TDR水分计的原位浸水试验。研究结果表明：(1) 大厚度自重湿陷性黄土地基处理6～12 m、深层浸水时,发生显著地基下沉;15～20 m时,地基沉降较小;处理深度大于20 m时,地基沉降基本可忽略。(2) 浸水试坑22.5～25.0 m以上土体含水率增加较快,甚至达到饱和,以下土体含水率增加缓慢,基本没有发生湿陷。建议22.5～25.0 m作为大厚度自重湿陷性黄土地基处理和湿陷性评价的临界深度。(3) 大厚度自重湿陷性黄土地基在采取有效的综合处理措施之后,甲类建筑可以不全部消除湿陷量,乙、丙类建筑可以根据控制建议适当放宽对剩余湿陷量的要求。(4) 不同地区、不同微结构类型土的湿陷性应当采用不同的湿陷系数 来判定,即“湿陷系数 = 0.015”在自基础底面至基底下15 m的范围内可继续使用;15 m以下适当放宽,按不同深度对 进行修正,可使大厚度自重湿陷性黄土湿陷性评价趋于合理,有效节约大量地基处理费用。     

湿陷性黄土场地PHC桩竖向承载性状试验研究

研发了一种PHC管桩内力测试方法,应用该方法对湿陷性黄土场地中PHC管桩分天然和浸水工况进行了载荷试验过程中的内力测试,研究了湿陷性黄土地基中PHC管桩的承载力性状和侧阻力分布规律。结果表明:干黄土中的多节PHC桩,浅部桩体的侧阻力存在发挥不佳的可能,该情况下土体浸水也不能有效提高侧摩阻力,同时桩周土自重湿陷沉降在桩侧产生的负摩阻力也较小;下部桩体侧阻力随深度的分布曲线呈单峰状,峰值深度随荷载增大逐渐向下移动;黄土与古土壤互层地层结构中的PHC管桩,在极限荷载作用下桩身压缩量是构成总沉降量的主要部分。     

自重湿陷性黄土与单桩负摩阻力离心模型试验

 为研究和分析黄土的湿陷变形性质与桩基的负摩阻力,采用离心模型试验的方法分别对原状自重湿陷性黄土与重塑湿陷性黄土进行模拟浸水试验。试验研究结果表明：原状和重塑湿陷性黄土浸水湿陷过程主要分为3个阶段,即显著湿陷变形阶段、湿陷稳定变形阶段以及水位下降后土体的固结变形阶段。根据试验结果,对于以沉降观测为目的的试验研究中,用重塑黄土代替原状黄土进行离心模型试验模拟其湿陷变形的方法是可行的。进而分析基桩负摩阻力分布规律及中性点位置的变化规律。单桩的负摩阻力分布及中性点位置是一动态变化过程,中性点位置与桩长的比例为0.68～0.82。     

黄土隧道地基湿陷变形评价方法探讨

 隧道穿越自重湿陷性黄土地层时,可能遭受浸水作用下地基湿陷变形的附加作用而产生结构破坏。针对隧道衬砌结构的湿陷性黄土地基,结合隧道围岩及地基的自重湿陷变形特征,首先,提出浅埋隧道围岩压力、衬砌结构自重荷载构成基底压力和隧道两侧基底面分布土层自重共同作用下地基土的附加应力计算方法,以及考虑地基土自重应力的湿陷压缩应力计算方法。其次,在基本物性与构度、构度与结构压缩屈服应力、孔隙比和初始孔隙比比值与压缩应力和结构压缩屈服应力比值关系的基础上,建立自重湿陷系数和湿陷系数的计算方法。依据大厚度自重湿陷黄土场地不同埋深范围黄土具有不同自重湿陷系数门槛值的特征,得到了场地的自重湿陷变形和隧道地基的湿陷变形的计算方法。最后,通过数值计算分析,模拟隧道地基湿陷变形不同沉降差作用下衬砌结构应力场和塑性域发展,随着不均匀湿陷变形的增加,隧道衬砌结构塑性区范围不断增大,并结合铁路路基沉降控制标准,建议隧道地基湿陷变形0～5 cm为一级、5～10 cm为二级、大于10 cm为三级。