渗透侵蚀下砂性土细颗粒流失率预测方法

渗流过程中内部不稳定砂性土细颗粒易于流失而形成渗透侵蚀,将对土工建筑物或地基造成不良影响。细颗粒流失率是土体力学性能劣化及稳定性分析的关键参数,为了对渗透侵蚀下砂性土细颗粒的流失率进行预测,笔者讨论了判断土体是否会发生渗透侵蚀的内部稳定性评价准则;介绍了细颗粒起动临界坡降,并对应力折减系数进行修正,以考虑土颗粒受力的差异性;提出了“细颗粒起动概率”,对细颗粒起动与否进行量化;分析细颗粒在土体孔隙网络中的迁移过程,给出了细颗粒通过收缩的概率和渗透迁移距离;根据细颗粒起动和迁移规律给出了细颗粒侵蚀和沉积定律,基于质量守恒定律形成细颗粒流失率预测方法。采用预测方法进行流失率计算,计算结果和试验结果吻合较好,误差基本在15%以内。     

酸性溶液对花岗岩残积土物理力学特性的影响

花岗岩残积土是华南地区进行工程建设时的常见地层之一,它的工程性质易受到周围环境的影响。采用人工方法制备了4种不同浓度的硫酸溶液浸泡后的试样,同时与原状试样进行对比,系统研究了不同浓度的硫酸溶液、不同的酸液溶蚀时间对花岗岩残积土试样物理、力学特性的影响。试验结果表明:(1)随着酸液浓度及酸液溶蚀时间的增大,试样的土粒比重减小、孔隙比增大、颗粒粒径级配变差;(2)试样的剪应力-剪切位移关系曲线类型会随着法向压力及酸液溶蚀作用而发生变化;(3)硫酸溶液浸泡后,试样的抗剪强度参数粘聚力和内摩擦角都降低了,但粘聚力的降低效果更明显。     

Experimental study on suffusion under multiple seepages and its impact on undrained mechanical responses of gap-graded soil

An experimental investigation of multiple seepage-induced suffusion and its impact on the mechanical responses of internally unstable gap-graded soil, with a fines content of 25%, is presented in this paper. Using a modified triaxial erosion apparatus, with a redesigned seepage control system, erosion tests under multiple seepage conditions, as well as undrained monotonic and cyclic compression tests, are performed. It is found that multiple seepages cause an eroded mass without a marked change in volume and with a change in hydraulic conductivity as the number of seepage cycles increases. The monotonic compression tests show that eroded soil presents a smaller peak strength, residual strength, and a greater contractive response than non-eroded soil. The peak strength and stiffness of eroded soil are seen to decrease considerably as the number of seepage cycles increases. Multiple seepage-induced suffusion may create a collapsible soil structure in eroded soil, as shown by sudden decreases in deviator stress and stiffness, coupled by sharp increases in pore water pressure at small strain levels. As eroded soil might already be unstable, cyclic loading causes it to collapse, revealing a sudden increase in the generation of cyclic pore water pressure and a decrease in liquefaction resistance. The results highlight the importance of conducting laboratory tests to determine the impact of internal erosion on the strength and liquefaction resistance.     

基于离散元法的接触冲刷影响因素研究

成层土间容易发生接触冲刷破坏,对水工建筑物的安全运行有不利影响。为此探讨了粗层孔隙率、粗层颗粒形状和剪切位移对接触冲刷的影响,采用离散元法对成层无粘性土进行了接触冲刷数值模拟,得到以下结论:粗土层孔隙率的增加对土层抗渗流冲刷不利,对于D₁₀ /d₁₀比值为10的土层,当粗土层的孔隙率相对较大时,在一定的渗流压力作用下,试样的渗透流速在初始阶段会有一定减小,随着细颗粒流失比的不断增大,渗透流速也随之增大,土层结构发生较大改变,也会发生接触冲刷破坏,在工程实践中应该引起一定的重视;当粗层颗粒形状为非球形时,细土层颗粒流失比减小,土层抗冲刷破坏能力有所提高;剪切变形后试样中粗土层孔隙率会有一定增大,孔隙结构也会发生改变,细土层颗粒流失比增大,加剧了土层发生接触冲刷破坏的风险。     

宽级配粗粒土的内部侵蚀试验及其稳定性判别

强降雨入渗条件下宽级配粗粒土中的细颗粒会由于孔隙流体的作用发生内部侵蚀。细颗粒的运移改变了土体的微观结构,土体的水力、力学性质随之变化。利用自行研发的刚性壁渗流侵蚀实验装置,对颗粒粒径介于0.002～10 mm的9种间断及连续颗粒级配粗粒土试样进行渗流侵蚀试验,提出一种新的基于试验的内部稳定性判别标准。实验结果表明:对于宽级配粗颗粒土,内部侵蚀改变了土体的渗透系数,最终导致渗透系数下降;粗颗粒的缺失导致内部稳定性降低,渗流作用下,连续颗粒级配土也可能是内部不稳定土;侵蚀土颗粒累计质量可以度量土体内部稳定性,在容许水力梯度范围内侵蚀量超过总质量5%的土体可划分为内部不稳定土;Kezdi几何标准更适用于评价宽级配粗粒土的内部稳定性。     

细粒土不均匀分布对粗粒土力学特性的影响

为了研究冻融循环条件下细粒土的不均匀分布特性对粗粒土力学特性的影响,对不同冻融循环次数、冻结温度、围压条件下的含有不同细粒土分布的粗粒土进行常规的静三轴剪切试验,研究冻融循环后具有不均匀细粒土分布的试样的最大剪胀位置、应力–应变关系曲线、起始屈服强度、起始屈服应变、静强度、破坏应变变化规律。研究结果表明,在相同的细颗粒与粗颗粒不均匀分布的条件下,试验后试样的最大剪胀位置随着试样上两层中细粒土含量的增加而沿着试样高度上升,随着冻融循环次数的增加,最大剪胀位置的上升速率是不断增加的。细颗粒与粗颗粒不均匀分布的程度越高,尤其是粗颗粒的富集程度越高,试样的脆性越高,试样的应变软化程度越明显。冻融循环次数的增加,降低了粗、细颗粒较均匀分布的3种试样的应力–应变软化程度的差异性,即冻融循环次数的增加可以使粗颗粒和细颗粒的组合结构趋于稳定。在相同的围压条件下,冻融循环次数的增加削弱了因细粒土不均匀分布而导致的起始屈服强度的差异性和静强度的差异性。试样中的粗颗粒和细颗粒分布越不均匀,试样的起始屈服强度和静强度越大。当围压为100 k Pa时,静强度与起始屈服强度的比值随冻融循环次数波动较大,当围压增加到300 k Pa时,比值随冻融循环次数的波动较为平缓。细粒土不均匀分布是起始屈服应变和破坏应变的最显著性影响因素,但是围压对破坏应变的影响程度明显大于围压对起始屈服应变的影响程度。细粒土不均匀分布对破坏应变影响的显著性明显小于它对起始屈服应变影响的显著性。     

考虑潜蚀影响的降雨入渗边坡稳定性分析

在降雨入渗作用下土坡内细颗粒会随渗流发生运移产生潜蚀作用。通过联立颗粒运移方程与非饱和土渗流方程,结合非饱和土水力特性方程和土体潜蚀本构关系,建立了渗流潜蚀耦合的边坡稳定性分析模型。采用有限元方法模拟了降雨入渗作用下潜蚀和入渗的相互作用,分析了初始饱和渗透系数、进气值参数对入渗和边坡稳定性的影响。结果表明：潜蚀主要发生在湿润锋范围内浅层土体,潜蚀加速了湿润锋下行速度,进一步降低边坡稳定性;饱和渗透系数、进气值是影响渗流潜蚀作用的主要因素;当降雨入渗强度大于饱和渗透系数时,饱和渗透系数越大,潜蚀对入渗和边坡稳定性的影响越显著;进气值越小,潜蚀作用对入渗和边坡稳定性的影响越显著。     

间断级配砂土中管线破损引起的渗流侵蚀模型试验

频发的路面塌陷事故对城市安全造成了巨大威胁,主要原因是地下管线破损引起周围土体的渗流侵蚀,进而产生了地下空洞所致。针对目前常见的地下无压管线破损问题,提出了一种研究间断级配砂土中破损管线周围渗流侵蚀特性的模型试验方法。基于试验中的水土流失变化和受侵蚀区域的发展规律,揭示了间断级配砂土中侵蚀过程可分为快速侵蚀阶段、稳定侵蚀阶段和收敛阶段,土体中由于侵蚀变形产生的松动区和空洞扩展规律与砂土流失变化一致。对比不同工况的侵蚀量和最终侵蚀状态,得到了土体饱和度和水位高度通过影响渗流力而改变侵蚀量和空洞形态,土体级配则决定了可流失细砂含量,从而影响侵蚀量和空洞大小,而当管线破损形状只导致细粒流失时,破损尺寸的增加将增多流失路径数量,进而增大了侵蚀量,但此因素影响较小。     

Experimental study on stiffness degradation and monotonic response of reconstituted volcanic ash induced by internal erosion

Volcanic ash soils are a common geological body in earthquake prone regions, and they are widely used as construction materials. The present study discusses the contribution of initial density, stress state and seepage time on seepage-induced internal instability, stiffness degradation, and monotonic response of volcanic ash collected from Satozuka, Japan. Reconstituted specimens are tested using an erosion triaxial apparatus. The results exhibit that the rate of erosion is influenced by initial density, stress state and hydraulic gradient. The mercury intrusion porosimeter tests are conducted to explore the distribution of constriction sizes after erosion. Post-erosion stress-strain responses are also affected by the stress state during erosion, initial density and seepage time. Particularly, internal erosion affects the dilatancy response of relatively loose specimens and has an impact on the critical state line, brittleness, and peak strength. Further, the maximum shear modulus of eroded soil is found to be greater than that of non-eroded soil, mainly due to the particle rearrangement and removal of fines. However, a sharp reduction in stiffness during monotonic shearing is maybe evidence that temporary reinforced soil packing collapses at a large strain. This indicates that the elastic yield surface has expanded for eroded soils.     

Variability in the shear strength characteristics of an amphibolite derived laterite soil

Variability in the shear strength characteristics of a fine grained amphibolite derived laterite soil within an area of one square metre in the same horizon is investigated by means of consolidated undrained triaxial compression tests. The shear strength parameters in terms of both effective and total stresses show remarkable variabilities. Cohesion values have coenfficients of variation of 65.1% and 54.7% respectively while the angle of internal friction values have coefficients of variation of 16.1% and 23.6% respectively. The chemistry of the soil has been found to be essentially constant while the spatial distribution of the goethite present was found to be non-uniform The variability in the shear strength characteristics of the soil is attributed to the inconsistent cementation of the soil particles by the goethite.     

含浅层强透水层堤基的上覆砂层管涌破坏试验研究

堤基中往往存在局部浅层强透水层并形成渗流优先通道,该通道不能大幅度削减流体的水头势能,易引起堤基管涌破坏,此类堤基管涌破坏机理的研究尚不明朗,仍需进一步研究。采用砂槽试验模拟堤基渗流,试验中通过抬升水箱水位,观察砂土中细颗粒流失现象,并分析渗流量、渗透坡降、测压管水头、砂土颗粒级配、锥头阻力、沉降量等关键参数。试验结果表明,水箱水位增大至48cm,浅层强透水层上覆砂层被"击穿"发生管涌破坏,管涌破坏分为稳定渗流阶段、细颗粒流失阶段(0.05d≤0.075粒级砂土流失)、较细颗粒流失阶段(0.075d≤0.1粒级砂土流失)、管涌破坏扩大阶段(0.1d≤0.25粒级砂土流失)。管涌破坏过程中,细颗粒砂土流失,锥头阻力降低,砂土层发生沉降,且较细颗粒流失阶段的沉降较为突出。细颗粒砂土流失导致砂土层孔隙率和渗透系数上升,渗流量和渗透坡降随之增大。     

改性钠羧甲基纤维素胶结固化土质边坡机制与抗冲蚀特性研究

 改性钠羧甲基纤维素(简称改性CMC)溶液在岩土体表面具有较强的渗析胶结与吸附胶结作用,按一定浓度就地拌合原状土,形成12～15 cm固化层可对土质斜坡起到加固、防渗与抗冲蚀作用。取寒区公路边坡粉砂土,利用不同浓度改性CMC进行拌合加固,对干燥后的试样进行水稳(遇水崩解),抗剪强度与渗透系数测试,并进行冻融循环条件下加固体强度与变形影响因素研究。结果显示,随改性CMC浓度增加,加固土水稳性增强,抗剪强度增大,渗透系数降低,至1.1%加量的改性CMC加固土,其崩解程度为0,黏聚力提高1.2倍,内摩擦角提高0.8倍,渗透系数从2.17×10－4 cm/s减小到1.0×10－5 cm/s,冻融强度损失小于25%。设计降雨冲刷试验研究改性CMC加固土抗冲蚀性能,结果表明：按120 mm/min雨强冲刷8.7 min,未加固土边坡即形成沟蚀,沟槽最大宽度13 cm,深9.0 cm,侵蚀面积占整个坡面30%,平均产沙速率达0.23 kg/min,而1.1%改性CMC加固土边坡冲刷60 min,无沟蚀产生,产砂量近于0,且坡面植被完好;改性材料已在多处现场应用,效果良好。     

不同管片张开量下隧道外水土流失规律试验研究

盾构隧道管片接缝漏水并导致管片外土体侵蚀、甚至发生漏水漏砂,是富水砂层中盾构隧道安全的主要风险。设计了一种模拟管片在不同张开量下土体流失状态的试验设备,对福建标准砂和天津典型细砂在不同管片张开量及水压下的土体流失状态进行了试验研究。试验揭示了管片外砂土逐步流失的机理,发现不考虑土颗粒作用的传统水密性试验会高估弹性密封垫的防水性能;提出了临界侵蚀张开量的理论公式,并通过试验结果初步验证了其合理性。研究表明,临界侵蚀张开量与土颗粒竖向所受应力和土层厚度成一次正比关系,与缝隙处水头、土体孔隙率成一次反比关系;当土体通过管片缝隙发生侵蚀后,侵蚀质量与管片张开量和水压成正比,与缝隙周围的有效应力大小成反比,侵蚀使福建标准砂级配曲线变得更加平缓;针对福建标准砂及天津细砂,提出了便于实际应用的考虑水压及管片张开量的土体流失状态评估方法。     

不同养护龄期下凹凸棒土改良黏性土三轴压缩试验研究

随着人们对环境保护的重视,凹凸棒土被越来越多的用于土体改良中。目前研究主要针对凹凸棒土对土壤污染修复、团粒结构和水力参数的影响,缺少对黏性土土体强度特性改善方面的研究。本文通过不固结不排水三轴压缩试验,探究凹凸棒土改良黏性土在不同养护龄期条件下的强度变化规律,结合扫描电镜图像(SEM)对凹凸棒土改良土体的作用机理进行讨论。研究发现,随着凹凸棒土置换量和养护时间的增加,试样的脆性增加,内摩擦角逐渐增大,黏聚力表现出先增加后减小的趋势。凹凸棒土吸附周围土颗粒和水形成的团聚体增加了改良试样内部的滑动摩擦力和咬合摩擦力,从而提升了改良试样的内摩擦角。在团聚体趋于稳定过程中形成的表观黏聚力变化是试样黏聚力先增后降的主要原因。研究结果为凹凸棒土在道路路基工程与边坡防护工程中的应用提供一定的参考依据。     

Erosional behavior of gravel-sand mixtures stabilized by microbially induced calcite precipitation (MICP)

The objective of this study is to evaluate the effectiveness of Microbially Induced Calcite Precipitation (MICP) for improving internal erosion resistance of gravel-sand mixtures. Two gravel-sand mixtures with 25% sand/75% gravel and 50% sand/50% gravel were used; the former was susceptible to suffusion whereas the latter was internally stable. The MICP treatment was conducted by either mixing a urea-calcium solution with the tested soils prior to bacteria injection (the pre-mixing method) or injecting the bacteria prior to the urea-calcium solution injection (the injection method). A series of pressure-controlled erosion tests was performed on specimens placed inside a column erosion test apparatus under different levels of axial stress. During the erosion test, the erosion rate, axial deformation, and hydraulic conductivity were measured. Without the MICP treatment, the specimens with 25% sand/75% gravel exhibited much faster backward erosion and suffusion. In contrast, the specimens with 50% sand/50% gravel showed slow backward erosion only. Within the tested conditions, MICP was very effective in mitigating internal erosion for the soil with 25% sand/75% gravel. However, for the soil with 50% sand/50% gravel, the MICP treatment was only successful when the injection method was applied and the erosion test was performed at a low axial stress.     

石灰偏高岭土改良粉砂土强度特性与微观机理

以河南地区典型粉砂土为研究对象,制备不同石灰及偏高岭土掺量下的改良粉砂土试样,对其进行了无侧限抗压强度试验、扫描电镜(SEM)与X射线衍射(XRD)测试,并与水硬性石灰改良粉砂土进行对比分析.结果 表明:随着偏高岭土掺量的增加,改良粉砂土破坏应变增大,无侧限抗压强度提高,但强度增长率呈现先增加后减小的规律,并在偏高岭土掺量为4％时达到峰值;当养护龄期从7d增至28d时,石灰偏高岭土改良粉砂土的强度增长率明显高于石灰改良粉砂土.采用6％石灰+4％偏高岭土、8％石灰+4％偏高岭土可分别有效替代8％、10％的水硬性石灰;偏高岭土掺入后形成的水化产物可联结土颗粒并填充于孔隙,使改良粉砂土微观结构更加密实,具有一定的水硬性.     

含水量及冻融循环对阳曲黄土压缩特性的影响分析

压缩特性是黄土的重要力学特性,是季节性冻土区黄土工程设计的重要依据。为研究冻融循环和含水量变化对黄土压缩特性的影响,以山西阳曲1号隧道为背景,对不同含水量的黄土进行冻融循环后的系列试验。结果表明:冻融循环通过强风化作用于土中自由水,使得土中自由水发生了原位的冻胀融沉和迁移,破坏了土颗粒的胶结物质,改变了土样固体颗粒的粒径分布,粗颗粒减小,细颗粒增加。使得土体的孔隙比、干密度、体积均增加,压缩系数的变化与含水量和冻融循环次数正相关,压缩模量的变化与含水量和冻融循环次数反相关,压缩性增强。本文的研究结果可对阳曲甚至山西的黄土工程提供帮助。     

Post-erosion mechanical responses of internally unstable gap-graded soil under drained torsional simple shear and triaxial compression

Internal erosion is a major threat to hydraulic earth structures, such as river levees and dams. This paper focuses on suffusion and suffosion phenomena which are caused by the movement of fine particles in the granular skeleton due to seepage flow. The present study investigates the impact of internal erosion on the dynamic response under cyclic torsional shear and monotonic responses under triaxial compression and torsional simple shear. A series of experiments, using a gap-graded silica mixture with a fines content of 20%, is performed under loose, medium, and dense conditions using a novel erosion hollow cylindrical torsional shear apparatus. The erosion test results indicate that the critical hydraulic gradient and the rate of erosion are density-dependent, where a transition from suffosion to suffusion is observed as the seepage continues. Regardless of the sample density, variations in the radial strain and particle size distribution, along the specimen height after erosion, are no longer uniform. Furthermore, the dynamic shearing results show that the small-strain shear modulus increases, but the initial damping ratio decreases after internal erosion, probably due to the removal of free fines. In addition, the elastic threshold strain and reference shear strain values are found to be higher for the eroded and non-eroded specimens, respectively. Finally, based on drained monotonic loading, the post-erosion peak stress ratio increases remarkably under triaxial compression, while that under torsional simple shear depends on the relative density where the direction of loading is normal to the direction of seepage. These observations indicate that the horizontal bedding plane becomes weaker, while the vertical one becomes stronger after downward erosion.     

Experimental study on the erosion resistance of rubberized cement-soil

This study examines the effects of granular rubber on the erosion resistance of Cement Soil (CS). Rubber powder was incorporated into CS where the Cement Content Ratios (CCR) of the samples were 10%, 15%, 20% and 25% at five different Rubber Particles Content Ratios (RCR) of 0%, 5%, 10%, 15% and 20%. The main tests included Erosion test, Permeability test and Crystallization test, and theses test results were used to analyze the influence and mechanism of rubber particles on the erosion resistance of CS. Under the action of NaCl and Na₂SO₄ solutions, the mass and unconfined compressive strength of rubberized cement soil (RCS) went up first, and then decreased with the increase of RCR. When the CCR is more than 20%, the increase of CCR has no obvious effect on the strength after the erosion process. The compressive strength was stronger with bigger size of the rubber particles. Under the same depth, the chloride ion content of RCS decreased with the increase of RCR. When the RCR is more than 15%, there is no significant effect on the erosion resistance with adding more rubber particles in CS. The reasons for increasing of erosion resistance of RCS can be summed up into three aspects: blocking capillary action, elastomer action and preventing crack propagation action. The effect of elastomer delays the time of cracks developing of CS. From the SEM test, it can be observed that the rubber powder hindered the development of micro-cracks since it offered adequate restraint to prevent the shorter cracks from propagating. All the above results show that rubber powder can effectively improve the erosion resistance of cement soil.     

三峡库区侏罗系顺层滑坡滑带土的剪切蠕变特性

滑带土的蠕变行为对滑坡的预测预报和长期稳定性分析具有重要作用。选取三峡库区侏罗系典型顺层滑坡—万州兴福寺滑坡为研究实例,针对滑带土重塑样,先进行三轴压缩试验测定其峰值及残余抗剪强度指标,然后开展不同法向应力和不同含水率下滑带土试样的剪切蠕变试验。试验结果表明:①兴福寺滑坡滑带土在初始破坏之前经历瞬时蠕变和衰减蠕变阶段,并最终稳定在低速率的等速蠕变阶段。②法向应力越大,滑带土剪切蠕变速率和变形量越小,各级法向应力下蠕变破坏时的剪应变均小于5%。③含水率对滑带土剪切蠕变速率的影响不甚明显。含水率低于塑限时滑带土剪切蠕变模量最大。随着剪切蠕变的进行,低含水率和饱和试样的剪切蠕变模量均在短时间内急剧降低并分别稳定在某一稳定值,而含水率略高于塑限的试样则呈缓慢降低趋势。与此相反的是含水率接近液限的试样呈缓慢增大趋势。④滑带土长期黏聚力为峰值和残余黏聚力的70%和15.0倍,长期内摩擦角为峰值和残余内摩擦角的69%,70%,各强度指标均受滑带土细颗粒含量的影响。