Effects of solar radiation and fine roots on suction of Amorpha fruticose-vegetated soil

The thickness of shallow landslides is generally less than 2 m,which is of the same order of magnitude as the growth range of vegetation roots.Vegetation roots can improve the stability of shallow soil through mechanical and hydraulic effects.Therefore,the landslide process is closely related to the plant roots growing on the slope surface.Plant roots play a dominant role in the regulation of soil suction through solar radiation induced transpiration.However,little is known about the correlation between cumulative solar radiation and soil suction.Moreover,the specific effects of fine roots on the suction distribution are not clear in most previous studies.In this study,a vegetated soil of a drought-tolerant and water-tolerant shrub,namely Amorpha fruticose,was adopted.The suction and volumetric water content of bare and vegetated soils were monitored under natural conditions for 4 months.The results demonstrate that there is a nearly linear relationship between cumulative solar radiation and suction ranging from zero to 100 kPa.Regarding the modeling of the soil-plant-atmosphere interactions,this relationship could serve a significant role in calculating the root water uptake under given solar radiation conditions.In addition,higher suctions were observed at the lower layer of the vegetated soil than those at the middle layer,which is different from the results of vegetated soil from previous investigations.This is due to the fact that the root area index(RAI)of fine roots at the lower layer is twice that of the middle layer.Importantly,the higher concentration of fine roots at the lower layer of vegetated soil sample resembles the root distribution of shrub near the soil-bedrock interface on shallow bedrock landslides.The fine roots would increase soil suction through transpiration,and hence reduce the permeability and increase shear strength of landslides.Eventually,these new findings serve as a preliminary step on the evaluation of the stability of this common type of landslides.

原状和压实黄土持水特性及湿陷性对比试验研究

通过对比研究具有相同初始干密度和含水率的原状和压实黄土,揭示黄土结构对其持水特性和湿陷性影响。利用扫描电镜观察原状和压实黄土微观结构,对试验结果辅助分析。研究显示:压实黄土持水特征曲线进气值较原状黄土大75%。这可能是由于原状黄土中存在超大团粒间孔隙,造成显著的瓶颈效应。另外,原状黄土持水特征曲线的滞回度在低吸力范围(小于7 kPa)较压实黄土大,而在中间吸力范围(7~80 kPa)较压实黄土小。这是由于原状和压实黄土经历不同脱-吸湿历史。对于湿陷性,高含水率时(大于18%)原状和压实黄土湿陷系数差别不大,而在低含水率(16%)时,原状黄土湿陷系数大于压实黄土。这是由于原状黄土中存在黏土颗粒胶结,使其具有较大抵抗加载变形能力(竖向应力200 kPa)。随后的注水,导致黏土颗粒胶结作用失效,引起较大湿陷变形。此外,原状黄土屈服应力较压实黄土屈服应力增大的程度随含水率减小而显著增加,这说明了原状黄土的结构性随含水率的降低而显著增强。

考虑土体孔隙比和比表面积影响的未冻结体积含水率曲线模型

土体未冻结含水率曲线模型描述温度和未冻结含水率之间的关系,该模型对计算冻土强度、变形以及水热迁移具有重要意义。考虑孔隙水毛细和吸附作用,提出了一个新的土体未冻结体积含水率曲线模型。该模型假设在较高温度下(0~−2℃),毛细作用支配土体中孔隙水冻结,该过程中毛细水冻结受孔隙比影响。另一方面,在较低温度下(<−2℃),吸附作用在孔隙水冻结中起主导作用,该过程中吸附水冻结受比表面积影响。结合文献中已有土体未冻结体积含水率曲线的测试数据(包括不同土体孔隙比、比表面积以及广温度范围),对模型进行验证。同时将所提模型与文献中3种常见模型计算结果进行对比,讨论新模型的优越性。结果发现,只有新提出的模型可只用一套参数计算不同土体孔隙比的未冻结体积含水率曲线。此外,针对土体在广温度范围条件下的未冻结体积含水率曲线,新提出模型比已有模型有更好的计算结果。

基于时域反射技术(TDR)的黄土湿陷原位评价研究

现有黄土湿陷评价方法具有取样扰动、耗时和费用高等缺点,急需探索原位评价黄土湿陷的新技术。基于时域反射技术(TDR)原位测试黄土介电常数和电导率,计算干密度和质量含水率。依据原位所测黄土干密度、质量含水率以及物性指标(相对质量密度、液塑限),采用经验模型对黄土进行湿陷评价。结果表明,TDR原位测试黄土干密度和室内烘干法结果相对误差在±6%以内,测试黄土质量含水率和室内烘干法结果绝对误差在±0.02以内。另一方面,基于经验的湿陷评价模型,利用TDR原位所测干密度、质量含水率以及物性指标不仅可判定黄土有无湿陷,且具有区分强烈和中等湿陷程度的潜力。本研究丰富了黄土地区勘察技术手段,为黄土湿陷原位勘察提供有益探索。

Compression characteristics of saturated re-compacted glacial tills in Tianmo Gully of Tibet, China

Glacial tills are highly compressible soils composed of loosely cemented sandy silt particles.Their role,for instance,as initial filler materials in geo-technical projects along temperate glacier regions warrant a systematic evaluation of their compressive characteristics.As such,we carry out a series of onedimensional compression tests on re-compacted glacial till samples obtained from Tianmo Gully,Tibet,with the aims of evaluating their compressive behavior under controlled initial stress conditions.The yield stress was determined from the void ratio versus consolidation pressure(e-log)planes.Its compression and swelling behaviors were observed during the primary and secondary compressions of the consolidation tests.From the compression curves,a correlation can be found between the consolidation stress and the void index.In addition,we find that the compression curves of glacial tills collapse into a single curve when normalized by the compression index.The experimental results in this study provide a basic understanding of the compression characteristics of the glacial tills in Tibet,China.

Effects of water content on the shear behavior and critical state of glacial till in Tianmo Gully of Tibet, China

Glacial tills are widely distributed in Tibet, China, and are highly susceptible to landslides under intense rainfalls. Failures of the slope during rainfall are closely related to the shear behavior of glacial tills at different moisture conditions. This study investigates the shear behavior and critical state of saturated and unsaturated glacial tills through a series of drained direct shear tests. The tests were conducted on two types of compacted glacial tills with different water contents and total normal stresses. A strain softening mode of failure is observed for all water content conditions accompanied by noticeable dilation. Dilatancy is found to decrease with increasing water content. Unsaturated samples showed increased rates of dilation as water content is decreased for all applied normal stresses a behavior which cannot be predicted well by classical stressdilatancy models. Furthermore, it was found that the Critical State Line(CSL), plotted on the(e-ln) plane, can be used to define the shear behavior of unsaturated glacial tills at different water contents.The CSL of saturated glacial tills run parallel to this line. The experimental results in this study are aimed to provide a basic understanding to the underlying failure mechanisms of glacial tills.