Elastic-viscoplastic behaviors of polymer-blend geocell sheets:Numerical and experimental investigations

Polymer-blend geocell sheets(PBGS)have been developed as substitute materials for manufacturing geocells.Various attempts have been made to test and predict the behaviors of commonly used geogrids,geotextiles,geomembranes,and geocells.However,the elastic-viscoplastic behaviors of novel-developed geocell sheets are still poorly understood.Therefore,this paper investigates the elastic-viscoplastic behaviors of PBGS to gain a comprehensive understanding of their mechanical properties.Furthermore,the tensile load-strain history under various loading conditions is simulated by numerical calculation for widespread utilization.To achieve this goal,monotonic loading tests,short-term creep and stress relaxation tests,and multi-load-path tests(also known as arbitrary loading history tests)are performed using a universal testing machine.The results are simulated using the nonlinear three-component(NLTC)model,which consists of three nonlinear components,i.e.a hypo-elastic component,a nonlinear inviscid component,and a nonlinear viscid component.The experimental and numerical results demonstrate that PBGS exhibit significant elastic-viscoplastic behavior that can be accurately predicted by the NLTC model.Moreover,the tensile strain rates significantly influence the tensile load,with higher strain rates resulting in increased tensile loads and more linear load-strain curves.Also,parametric analysis of the rheological characteristics reveals that the initial tensile strain rates have negligible impact on the results.The rate-sensitivity coefficient of PBGS is approximately 0.163,which falls within the typical range observed in most geosynthetics.

Effect of coir geotextile and geocell on ephemeral gully erosion in the Mollisol region of Northeast China

The unique geomorphological features and farming methods in the Mollisol region of Northeast China increase water catchment flow and aggravate the erosion of ephemeral gully(EG).Vegetation suffers from rain erosion and damage during the growth stage,which brings serious problems to the restoration of grass in the early stage.Therefore,effects of coir geotextile and geocell on EG erosion under four confluence intensities were researched in this study.Results of the simulated water discharge erosion test showed that when the confluence strength was less than 30 L/min,geocell and coir geotextile had a good effect on controlling EG erosion,and sediment yield of geocell and coir geotextile was reduced by 25.95%–37.82%and 73.73%–88.96%,respectively.However,when confluence intensity increased to 40 L/min,protective effect of coir geotextile decreased,and sediment yield rate increased sharply by 189.03%.When confluence intensity increased to 50 L/min,the protective effect of coir geotextile was lost.On the other hand,geocell showed that the greater the flow rate,the better the protective effect.In addition,with the increase in confluence intensity,erosion pattern of coir geotextile developed from sheet erosion to intermittent fall and then to completion of main rill,and the protective effect was gradually weakened.In contrast,the protective effect of EG under geocell was gradually enhanced from the continuous rill to the intermittent rill and finally to the intermittent fall.This study shows that coir geotextile and geocell can prevent EG erosion,and the effect of geocell is better than that of coir geotextile on the surface of EG.

Sediment yield and erosion–deposition distribution characteristics in ephemeral gullies in black soil areas under geocell protection

Investigating the effect of geocells on the erosion and deposition distribution of ephemeral gullies in the black soil area of Northeast China can provide a scientific basis for the allocation of soil and water conservation measures in ephemeral gullies.In this study,an artificial simulated confluence test and stereoscopic photogrammetry were used to analyze the distribution characteristics of erosion and deposition in ephemeral gullies protected by geocells and the effect of different confluence flows on the erosion process of ephemeral gullies.Results showed that when the confluence flow was larger,the effect of geocell was more evident,and the protection against ephemeral gully erosion was stronger.When the confluence flow rates were 0.6,1.8,2.4,and 3.0 m^(3)/h,ephemeral gully erosion decreased by 37.84%,26.09%,21.40%,and 35.45%.When the confluence flow rates were 2.4 and 3.0 m^(3)/h,the average sediment yield rate of the ephemeral gully was close to 2.14 kg/(m^(2)•min),and the protective effect of ephemeral gully erosion was enhanced.When the flow rate was higher,the surface fracture of the ephemeral gully was more serious.With an increase in confluence flow rate,the ratio of erosion to deposition increased gradually,the erosion area of ephemeral gullies was expanded,and erosion depth changed minimally.In conclusion,geocell measures changed erosion patterns by altering the rill erosion/deposition ratio,converting erosion from rill erosion to sheet erosion.

Bearing capacity of circular footings on multi-layered sand-waste tire shreds reinforced with geogrids

The presence of waste tires poses an environmental challenge as they occupy a significant amount of land and are expensive to dispose in landfills.However,reusing waste tires can address this issue when waste tires are used in geotechnical applications.To determine the viability of this approach,laboratoryscale tests were conducted to investigate load-bearing capacity of circular footings on sand-tire shred(STS)mixtures with shredded waste tire contents of 5%e15%by weight and three different widths of shreds.The investigation focused on analyzing the thickness of layers composed of STS mixtures,the soil cap,and the impact of geogrids on bearing capacity.The results indicate that a specific mixture of sand and tire shreds provides the highest footing-bearing capacity.In addition,the optimal shred content and size were found to be 10%by weight and 2 cm×10 cm,respectively.Furthermore,for a given tire shred width,a particular length provides the largest bearing capacity.The results agree well with that of previous research conducted by the first author and his colleagues in direct shear and California bearing ratio(CBR)tests.The primary finding of this research is that the use of two-layered STS mixtures reinforced by geogrids significantly enhances the bearing capacity.

Residual settlement calculation of geocell cushion over gravel piles

The calculation of residual settlement of bidirectional reinforced composite foundation, which is composed of geocell cushion over gravel piles, was studied. The geocell cushion was modeled as a thin flexible plate with large deflection. Based on the Kirchhoff hypothesis, the governing differential equations and boundary conditions of the deformation of geocell cushion under working load were founded using von Karman method and solved by Galerkin method. On theses bases, the gravel piles and inter-pile soils were assumed as Winkler ground with variable spring stiffness so as to execute the approximate calculation of the residual settlement of the bidirectional reinforced composite foundation. The calculation method was verified by two laboratory experiments concerning settlement of embankments. One experiment was with just geocell cushion installed to treat the soft clay under embankments; another one was with both geocell cushion and gravel piles installed. The results show that the calculated settlement curve and the maximum settlement are closed to the observed ones.

Large-scale direct shear testing of geocell reinforced soil

The tests on the shear property of geocell reinforced soils were carried out by using large-scale direct shear equipment with shear-box-dimensions of 500 mm×500 mm×400 mm (length×width×height). Three types of specimens, silty gravel soil, geocell reinforced silty gravel soil and geocell reinforced cement stabilizing silty gravel soil were used to investigate the shear stress-displacement behavior, the shear strength and the strengthening mechanism of geocell reinforced soils. The comparisons of large-scale shear test with triaxial compression test for the same type of soil were conducted to evaluate the influences of testing method on the shear strength as well. The test results show that the unreinforced soil and geocell reinforced soil give similar nonlinear features on the behavior of shear stress and displacement. The geocell reinforced cement stabilizing soil has a quasi-elastic characteristic in the case of normal stress coming up to 1.0 GPa. The tests with the reinforcement of geocell result in an increase of 244% in cohesion, and the tests with the geocell and the cement stabilization result in an increase of 10 times in cohesion compared with the unreinforced soil. The friction angle does not change markedly. The geocell reinforcement develops a large amount of cohesion on the shear strength of soils.

土工格栅加筋粉煤灰改良黄土力学特性研究

随着西部黄土地区城镇化进程的加快,填挖结合的新增用地方式要求地基处理技术需要重点解决填方区的强度与变形问题。论文将粉煤灰作为改性填料对黄土进行改良,通过直剪试验、三轴试验、湿陷试验与扫描电镜试验,分析不同粉煤灰掺入比条件下加筋与未加筋黄土试样的强度变化规律、宏观破坏特征和微结构特征。结果表明:采用粉煤灰改良和土工格栅加筋后,黄土的最优含水率增大,最大干密度降低。当粉煤灰掺入比λ=20%时,加筋改良效果最好,黄土的峰值强度、残余强度和等效内摩擦角均有大幅提高,侧向变形显著减小,且能有效降低其湿陷性。微结构分析表明:粉煤灰对黄土的改良主要体现在颗粒直接填充和化学结晶体交织成的网状填充两种类型的强化作用,改良后黄土的孔隙率与孔隙尺寸均有所下降。研究结果可为黄土地区填方边坡与填方路基的改良加固提供参考。

基于拉拔试验的加筋土界面摩擦特性

加筋土具有良好的工程性能,被广泛应用于加固路基和边坡支挡等领域,筋土界面摩擦强度对支挡结构的整体性和稳定性具有重要影响.为研究不同加筋土的界面摩擦特性,自制拉拔装置,对黏土-土工格栅、黏土-碳纤维布、黏土-土工格室和碎石土-土工格室等4种加筋土进行拉拔试验.研究结果表明,4种加筋土中,土工格室与碎石土的界面摩擦强度最高,构成的加筋体承受变形的能力最强;黏土-碳纤维布构成的加筋体界面摩擦强度最低,但具有筋土间相互作用发挥较快、允许变形小的特点;不同加筋体的允许变形差别较大,在实际工程中应充分考虑.提出了一种考虑土工格室加强区的无黏性土-土工格室界面最大摩阻力计算理论模型,研究结果可为加筋土工程筋材选取提供参考.

高速铁路土工格栅加筋膨胀土边坡作用机制

土工格栅加筋处治是解决膨胀土边坡问题的关键技术。为深入研究格栅与膨胀土相互作用机制,依托南宁铁路改柳南线工程,开展加筋设计及格栅抗拔稳定性计算。将考虑膨胀土侧向膨胀影响及格栅反包约束作用的设计方案应用于实际工程中,并通过预埋设的监测元件,对自然降雨−蒸发气候下加筋边坡的含水率、应力、位移及格栅应变的变化特征进行长期监测。研究结果表明:在考虑侧向膨胀及格栅约束的影响下,土工格栅的抗拔稳定安全系数仍满足规范要求。基于监测数据发现,受大气环境影响,体积含水率的波动幅度由浅至深逐渐减弱;格栅应变变化趋势与土体应力变化趋势基本一致,随季节性干湿气候呈“波浪”式变化;靠近坡面土体受大气干湿循环的影响更为显著,对降雨入渗敏感,其土工格栅应变变化迅速,峰值出现时间早;格栅应变峰值远小于格栅允许拉应变,且边坡累积水平位移量较小,表明加筋边坡滑移破坏风险较低;在降雨过程中,格栅对边坡土体施加弹性约束,允许坡面发生一定程度的膨胀,释放坡体因增湿产生的膨胀势,从而避免边坡因侧向应力增大形成渐进式滑坡,体现了土工格栅加筋膨胀土边坡“以柔治胀”的作用机制。研究成果可为高速铁路膨胀土边坡的土工格栅加筋设计与施工提供理论和技术支撑。

土工格室加筋路基振动压实行为及预应力效应

通过建立路基离散元模型和土工格室有限差分模型,开展了一系列有限差分法-离散元法(finite difference method-discrete element method,简称FDM-DEM)耦合数值计算,探明了土工格室加筋路基在振动荷载作用下的压实行为。进一步揭示了土工格室对路基压实后水平残余应力的贡献,并在此基础上,提出了土工格室加筋路基的预应力效应,以体现施工期格室填料在经历加载并卸载后引起的格室撑阔对加筋效果的提升。结合微观接触组构、配位数变化、压实过程的应力路径,探讨土工格室加筋路基预应力效应的形成机制。研究结果表明,相较于未加筋路基,土工格室可以提高加筋路基的回弹模量,增加压实路基的水平残余应力。在振动荷载作用下,土工格室呈现上开口的喇叭形,土工格室口袋被撑阔,格室壁的最大应变为0.17%~0.21%。接触力分布也表明,在振动压实作用后,力链由竖直向水平方向发展,这体现在水平残余应力的增加,而土工格室则进一步提升了颗粒的横向接触力。

高摩阻超静定土工格栅在粗粒土夹层中的剪胀作用研究

在加筋土工程中,采用异型格栅和设置粗粒土夹层可以有效增强筋土相互作用。然而粗粒土夹层厚度的确定方法仍有待进一步研究。基于一种带有凸起节点结构的高摩阻超静定土工格栅(high resistance hyperstatic geogrids,简称HRHG)与砾石的直剪试验结果,建立了剪切硬化筋土界面的剪胀本构模型,并进一步研究了筋土剪胀应力在土体内引起附加应力的分布规律。通过开展不同法向压力(30、50、80 kPa)下的直剪试验,研究了不同粗粒土夹层厚度(60、100、140、180 mm)对筋土相互作用的影响,并与筋土界面剪胀应力的分布规律进行了对比分析。结果表明,筋土界面剪胀本构模型可以有效计算剪缩位移和剪胀位移,且最终剪胀位移随法向压力增加而减小。由界面剪胀应力引起的粗粒土中附加应力随着与筋土界面距离的增加而降低,但是剪胀范围逐步增大。粗粒土夹层厚度的增加可以有效提高界面剪切强度,但存在最优夹层厚度使界面剪切强度的增幅迅速降低。最优夹层厚度随着法向压力的增加而减小。通过对比分析最优夹层厚度与剪胀应力比之间的关系,提出了基于筋土界面剪胀本构模型的确定最优夹层厚度的半经验公式,可为HRHG在工程中的设计或应用提供参考。

珊瑚砂与土工格栅界面的剪切特性及本构模型

采用南海原位级配珊瑚砂和聚丙烯双向土工格栅,开展了不同法向应力下珊瑚砂和筋土界面大型直剪试验。试验结果表明,珊瑚砂和筋土界面剪应力—剪切位移曲线呈现明显的应变软化特征,峰值抗剪强度线呈现双折线形态,残余抗剪强度线则呈现较好的线性关系;各级法向应力下珊瑚砂与土工格栅界面摩擦比均值达到1.37左右,远高于石英砂与土工格栅界面摩擦比,说明在珊瑚砂中加筋能充分发挥加筋效果;珊瑚砂和筋土界面在剪切过程中总体上经历了相对剪缩—相对剪胀—相对剪缩的过程,对珊瑚砂加筋可显著减小其剪缩变形;珊瑚砂和筋土界面直剪试验试样的相对破碎率接近,均随法向应力的增大而增大,但增幅逐渐趋缓。采用邓肯—张模型和剪切软化模型分别描述南海原位级配珊瑚砂与聚丙烯双向土工格栅界面峰前和峰后剪应力—剪切位移曲线,所建立的筋土界面本构模型能够很好地反映其剪切特性。

新型单向拉伸土工格栅在边坡加固中的性能初步研究

单向拉伸高密度聚乙烯(HDPE)土工格栅是一种具有高断裂强度和低蠕变率的拉伸格栅筋条,最大幅宽4m,最大抗拉强度可达260kN/m。本文以一典型加筋边坡工程为例,将其与常规土工格栅进行加固计算对比。通过结果分析:新型土工格栅抗拉强度和屈服强度比常规土工格栅提高约63%;在相同土工格栅布置的情况下,可有效提高边坡整体稳定系数约18%;在相同的边坡加固效果下,采用新型土工格栅可显著节省筋材用量约36%;在相同的边坡稳定状态下,采用新型土工格栅可显著增加边坡填筑高度约67%。因此,该新型土工格栅在边坡加固中具有显著的优势和广阔的应用前景。

高寒区隧道洞口段覆雪波纹钢棚洞减载方法研究

高寒区隧道洞口段易受上方覆雪及落石影响,采用现有大跨径波纹钢棚洞会产生较大变形和失稳,影响边疆公路运行安全。为此,依托某线路隧道洞口段波纹钢防雪棚洞项目,从拱效应及加筋桥减载原理出发,运用有限元软件构建了EPS板、EPS板+加筋雪、EPS板+加筋冰等10种减载方案的三维波纹钢棚洞模型,研究不同减载方案对棚洞应力、棚洞变形和覆雪压力的影响,对比分析棚洞受力变形特征及减载效果,筛选出最优减载方案。结果表明:施加减载材料后,棚洞上方覆雪压力均不同程度分散到棚洞两侧,减小了棚洞结构的受力;随着覆雪高度增加,减载效果更显著,其中30 cmEPS板+加筋冰组合式方案的减载效果最佳,棚顶应力减少32.72%,棚顶位移减小22.63%;针对波纹钢棚洞的减载问题,建议隧道洞口30 m范围内采用30 cmEPS板+加筋雪方案,距隧道口30 m以外范围可采用30 cmEPS板+加筋冰方案。

土工格室加筋正常固结粉质黏土应力应变响应

土工格室加筋正常固结粉质黏土对工程建设意义重大。基于土工格室与填土的相互作用机制和增量法,采用弹塑性理论、邓肯-张双曲线模型、修正剑桥屈服函数以及剪胀方程,推导了土工格室加筋正常固结黏土的应力应变响应计算模型,进行了土工格室加筋正常固结黏土的三轴固结排水剪切试验,实测不同围压下的应力应变关系,采用三轴试验结果验证了所提理论模型的正确性和可靠性。此外,采用所提理论模型进行了参数敏感分析,研究了填土的力学性质参数、格室的网格尺寸和力学性质参数对于加筋土应力应变响应的影响。研究结论表明,与砂砾料相似,土工格室约束作用对于正常固结黏土的内摩擦角影响较小,而黏聚力有所增加;随着轴向应变的增加或围压的减小,加筋效果逐渐明显;加筋正常固结黏土的强度和刚度随着非线性弹性常数k的增加、强度参数φ的增加和Rf的减小而增大,常数n对于加筋土应力应变响应影响较小。本研究成果可为工程建设提供理论借鉴。

双向增强复合地基桩土应力比计算方法研究

研究目的:桩土应力比是反映双向增强复合地基工作特性的重要指标。由于路堤荷载下双向增强复合地基承载变形机理复杂,本文以双桩范围内的路堤与双向复合地基为研究对象,考虑路堤土拱效应、加筋层网兜效应、应力扩散效应等多重效应的共同作用,建立路堤-水平加筋垫层-竖向桩体-桩间土协调变形的二维理论分析模型,探究各影响因素对桩土应力比的影响规律。研究结论:(1)桩土应力比随填土重度、桩间距的增加而减小,随填料内摩擦角的增大呈先增加后减小趋势,随路堤填筑高度、格栅层数、填料黏聚力的增加而增大;(2)桩土应力比对各参数的敏感性由高到低排序为:桩间距、路堤填筑高度、路堤填土重度、路堤填料黏聚力、格栅层数及路堤填料内摩擦角;(3)本研究成果可为类似工程的桩土应力比计算提供理论依据及工程借鉴。

不同垫层下煤矸石桩网复合路基承载特性

为综合利用固体废弃物煤矸石,将其取代部分传统细骨料制备煤矸石混凝土桩。开展了土工格栅和土工格室加筋垫层下煤矸石桩网复合路基室内模型试验,分析路堤顶面荷载-沉降关系、加筋垫层传荷能力和桩身应力的差异性。同时,建立有限元计算模型,研究不同加筋材料应力和变形规律,揭示土工格室应用于加筋垫层作用机制。结果表明:在本次试验中,同土工格栅相比,土工格室加筋垫层显著减小路堤顶面沉降约29.71%。土工格室加筋垫层下桩顶和桩间处土压力衰减量分别约61.51%、56.35%,充分发挥了筏板效应和应力扩散效应。采用土工格室加筋垫层,路堤荷载能够被有效传递至煤矸石桩体,在路基中心处和沿路基横断方向桩身应力极值分别采用土工格栅加筋垫层的1.78、1.80倍。土工格室和土工格栅应力分布规律分别呈“V”型和“U”型变化特征,且土工格室的应力最大值大于土工格栅。土工格室特有的蜂窝状结构对路堤填料产生侧向环箍和肋条摩阻力作用,通过自身张拉变形有效地扩散路堤荷载。

基于离散元的冲击荷载下加筋道砟细观研究

为了从细观上探究冲击荷载下加筋材料加固作用机制,通过离散元方法建立道砟颗粒模型,采用平行黏结模型建立土工格栅、土工格室柔性模型;通过对比室内道砟冲击试验和数值模拟结果,从宏观、细观上研究道砟竖向累积沉降和侧向变形,并对道砟应力链分布、颗粒接触和位移等进行细观分析。结果表明:在冲击高度为250 mm时,相较于未加筋道砟,土工格栅加筋道砟的竖向沉降和侧向变形分别减少24.4%和12.7%,土工格室加筋道砟分别减少33.5%和24.0%;由于道砟加固区存在垫层效应,土工格栅和土工格室加筋道砟的颗粒接触数分别增加9.3%和42.6%,平均接触力分别减少5.6%和16.7%;对比道砟颗粒平均位移,土工格栅和土工格室加筋工况相较于未加筋工况分别减少13.3%和21.1%,土工格室加固效果比土工格栅更为显著。

含水率和冻融循环对筋土界面剪切特性的影响

制作了一套可实现温度控制的筋土界面直剪试验设备。为了研究含水率、界面温度、冻融循环次数对筋土界面剪切特性的影响,开展11组土工格栅-砂土界面直剪试验。研究结果表明,筋土界面的黏聚力和摩擦角均随含水率的增加而减小,当含水率提高时,筋土间的抗剪强度减弱。加筋可显著提高冻土的抗剪强度,当界面温度为-10℃时,土工格栅-砂土界面剪应力峰值较冻结后砂土的剪应力增加了约20%。筋土界面剪应力随着界面温度的降低而增大,当界面温度在0℃以下时,剪应力较大且剪应力-剪切位移曲线会出现峰值强度和残余强度,而在无冻结情况下,筋土界面剪应力稳定值基本相同。冻融循环后筋土界面的抗剪强度减小,筋土界面的黏聚力和摩擦角均随着冻融循环次数的增加而减小,但在4次冻融循环后趋于稳定。研究成果可为冻土地区土工格栅加筋土结构的设计和应用提供理论依据。

土工格室与沙漠沙界面特性大型直剪试验

为探明土工格室与沙漠沙的筋土界面特性,开展了多项大型直剪试验研究,包括不同土工格室位置条件下、不同剪切速率条件下、不同温度条件下的土工格室加固沙漠沙界面直剪试验。结果表明:在50,100,150 kPa的法向应力条件下,以土工格室加筋层上界面或下界面作为剪切面,不同剪切速率(0.5,1.0,2.0 mm/min),不同温度(30,15,0,-15℃)影响下的筋土界面剪应力-剪切位移曲线均呈应变软化特征;纯沙漠沙对剪切速率的变化不敏感,剪切速率由0.5 mm/min升至3 mm/min时沙漠沙抗剪强度基本持平,而土工格室加固沙漠沙的界面对剪切速率的变化敏感,界面抗剪强度随剪切速率的增加而减小;在较低剪切速率0.5 mm/min下,土工格室提供的似黏聚力对抗剪强度的贡献更大;土工格室加固沙漠沙在30℃与15℃下峰值剪应力变化不大,区别在于峰值过后30℃抗剪强度存在更快的衰减至平稳阶段的情况;随着温度的降低,土工格室-土复合体呈现出更大的刚性,界面剪应力峰值提高并前移;土工格室刚性的增加使土工格室壁不易受拉变形,造成紧箍作用削弱而水平摩擦力作用增强,综合体现在加筋土剪切面的似黏聚力降低,而内摩擦角增大。