土工格室规格对加筋土剪切性能的影响

土工格室加筋结构由于抗震性好、施工简便、造价低廉,而广泛应用于公路、铁路等交通基础设施中。目前土工格室加筋结构中仅考虑了土工格室的抗拉强度,而未考虑土工格室规格的影响,使土工格室的选用主要依靠工程经验。通过对5种不同规格土工格室开展室内直剪试验,研究了条带高度、结点间距及法向应力对土工格室–砾砂剪切力学特性的影响,通过引入加筋强度系数评价了不同法向应力、土工格室规格的加筋效果,最后分析了土工格室规格对剪切强度参数的影响。试验结果表明：不同规格土工格室均可有效提高加筋结构的抗剪强度,其中抗剪强度随条带高度的增大、结点间距的减小而增大,同时条带高度对剪切强度的贡献约是结点间距的1.8倍。土工格室加筋砾砂的抗剪强度随法向应力增大而增大,但其加筋强度系数随法向应力的增大而减小。50kPa作用下,条带高度对加筋强度系数的增幅在12.57%以上,而结点间距对加筋强度系数的增幅却不足3.80%。土工格室加筋可显著提高填料的黏聚力,其中条带高度对黏聚力的提高尤为显著,增幅约为25%,而对内摩擦角提高相对较少,增量最大为5.11°。试验结果可为土工格室在实际工程中的应用和理论研究提供实验基础。     

条形荷载下土工格室加筋砂土路堤模型试验研究

为了探讨不同土工格室加筋方式对路堤应力变形特性和不均匀沉降的控制效果,分别对纯砂路堤边坡和土工格室加筋路堤进行多组模型试验,研究了土工格室焊距、埋深、加筋层数以及压实度对路堤承载力特性和变形特性的影响,同时结合土工格室材料应变的变化规律和加筋路堤的坡面变形状态分析了土工格室加筋路堤的破坏模式。试验结果表明:格室加筋效果随着焊距、加筋深度的减小而增加,随着压实度、加筋层数的增加而增加;土工格室加筋路堤承载力是纯砂的2.5倍左右,提高了路堤的极限承载力,减小了路堤沉降,并且土工格室加筋路堤坡面侧向位移比纯砂路堤减小了75%。     

土工格室不同结点连接方式失效机制试验研究

结点连接方式对土工格室的性能至关重要。结点在加筋结构中往往会受到不同方向的作用力,然而对结点在不同受力状态下的失效机制缺乏系统的研究。通过对焊接、插接、铆接3种结点连接方式的土工格室进行单轴拉伸试验,研究了结点连接方式对土工格室条带性能的影响,比较了在不同受力状态下结点的失效模式及抗拉强度。此外,通过引入"条带强度保持率"、"条带变形保持率"、"结点强度发挥率"评价了不同结点连接方式的性能。结果表明:焊接结点对HDPE土工格室条带拉伸性能的协调发挥影响较低,为4.82%;而铆接结点对PET土工格室条带影响较高,为22.2%。焊接、铆接、插接结点主要受剥离强度的控制,但焊接结点的剥离结点强度发挥率可达28.3%,分别是插接、铆接结点强度发挥率的11.32倍、6.58倍,体现了焊接结点的性能优势。插接结点在3种受力状态下结点强度相差很大,需采取注塑等措施来改善结点在剪切、剥离作用下的强度,从而均衡发挥插接结点的性能。试验结果可为土工格室的合理选用以及加筋机理的研究提供参考。     

基于响应面法的聚丙烯高强土工格室拉伸性能

为研究土工格室整体抗拉承载力与条带抗拉承载力、节点抗拉承载力之间的力学响应关系,采用双轴拉伸试验测定了不同高度、焊距的高强土工格室在不同加载速度下的破坏承载力;利用节点抗拉承载力与条带抗拉承载力之间的匹配关系,给出了土工格室抗拉承载力的设计参考值;采用响应面多因素试验分析法,研究了土工格室双轴拉伸承载力和变形对各影响因素的响应规律.结果表明:双轴拉伸试验中高强土工格室的破坏均发生在节点处,工程设计时应重点关注;对土工格室整体抗拉承载力的影响程度由大到小为土工格室高度>土工格室焊距>加载速度,研究结果可以为工程设计中土工格室的技术指标选择提供理论依据.     

填方地基组合加筋模型试验及稳定性探析

为研究不同加筋处治技术在填方地基边坡中的稳定性效应,开展土工格栅加筋以及土工格栅、土工格室组合加筋地基模型试验,对比测试两种技术加固条件下,边坡和地基的应力及变形,并通过理论,分析边坡整体稳定性和坡角局部稳定性。结果表明：组合加筋技术较土工格栅加筋对软基边坡的固结效果提升更佳;加筋效果随着坡体位移的增大而逐渐显现;土工格栅加筋使边坡整体稳定性提升32.4%,组合加筋则可以提高37.8%;土工格室碎石垫层可显著提高坡角局部稳定性。     

基于渐进均匀化方法的土工格室加筋地基沉降计算与分析

土工格室作为一种三维加筋材料已广泛应用于土木、港口等多领域,土工格室加筋地基也随之得到了广泛的应用。由于地基结构在受荷载作用后会产生应力、应变、位移,为保证地基质量,对地基沉降值进行计算与分析是十分必要的。采用渐进均匀化方法计算土工格室加筋层各组分等效弹性模量等力学参数,表明土工格室加筋层是一种横观各向同性的负泊松比材料。利用所得的力学参数建立格室不同埋深时的参考试验地基模型,进行数值模拟,结果表明格室埋深影响地基沉降值,埋深越大,地基沉降值越大。     

高填方加筋新旧路堤现场试验与数值模拟分析

 结合山区高速公路拓宽工程,对土工格室处治高填方新旧路堤进行现场试验,分析加宽高填方路堤侧向位移、沉降及土压力变化规律,研究格室处治效果。在现场试验的基础上,采用三维薄膜单元模拟土工格室的立体加筋性能,建立三维弹塑性模型,分析土工格室受力特点,通过对相关参数的敏感性分析,揭示高填方加宽路堤的变形规律。结果表明,采用三维薄膜单元,能较好地反映土工格室处治现场高填方新旧路堤的规律。与现场试验相比,利用数值试验不仅能得到现场的加筋效果,而且还能通过分析筋材与填料参数的变化和筋材铺设间距来研究格室处治高填方路堤的规律,从而可进一步探讨格室加筋的机制。高填方路堤在加宽路基自重荷载作用下沉降主要集中在加宽路堤的中上部,侧向位移从路基顶面到底部依次逐渐减少。土工格室所在层位起到扩散荷载、减少侧向变形和不均匀沉降的作用。填料与筋材模量愈高,加筋间距愈小,加筋效果愈好,较为合理的铺设间距为2～3 m。该研究成果对高填方路堤加筋处理和新旧路基结合部处理均有借鉴意义。     

土工格室加筋砂土地基沉降试验研究

土工合成材料可以有效提高地基的承载力与减小地基的表面沉降差异。在静荷载作用下,采用室内模型试验方法对纯砂地基和土工格室加筋地基的地基承载力和沉降情况进行了对比分析,研究了格室埋深、格室高度及筋材层数对距离基础不同远近处地基沉降的影响。研究结果表明,在荷载较小时,土工格室加筋地基作用效果相近;在荷载较大时,土工格室加筋效果提高显著;土工格室加筋地基不仅有效控制了基础沉降,而且减小了基础附近地基的沉降差异;筋材调节地基不均匀沉降的加筋效果随筋材埋深减小、筋材层数增加、格室高度增加而有不同程度的提高。     

炉渣加筋的大型直剪试验研究

炉渣为城市生活垃圾焚烧后的产物,具有砂性材料类似的性质,可作为路基填料。通过大型土工直剪仪分别对纯炉渣、双向土工格栅与炉渣加筋、土工格室与炉渣加筋在不同垂压下进行直剪试验,对比几种筋材的加筋效果,并研究分析其加筋机理。试验结果表明:炉渣加筋的抗剪强度明显大于纯炉渣的抗剪强度,筋材的加筋效果显著;土工格室加筋效果优于双向土工格栅,加筋后的内摩擦角与纯炉渣基本接近,而黏聚力提高显著,炉渣加筋主要是通过提高黏聚力提高抗剪强度。     

桩承式变刚度加筋垫层复合地基数值模拟

提出采用高强土工格栅和土工格室对天然砂石与建筑固体废弃物再生混凝土骨料的混合料进行加筋,通过添加适量水泥、石灰或粉煤灰等黏结材料,构造变刚度加筋垫层,替代桩筏复合地基中的混凝土板,形成采用柔性筏板的桩筏复合地基。通过建立有限元数值模型进行了对比分析,研究结果表明:土工格室加筋垫层可以通过提兜效应和柔性筏板效应,充分调动桩墙的承载能力,有效减小桩间土荷载,提高桩土应力比;增大垫层弹性模量对土工格室加筋垫层工况工作性状的改善更为显著;实际工程中加筋垫层厚度不宜过大。     

Development and mechanical properties of HDPE/PA6 blends: Polymer-blend geocells

Geocells are three-dimensional expandable panels composed of polymers such as polyolefin polymers. Currently, geocells are being extensively used in various geotechnical engineering applications; however, its applications are limited because of the sizeable long-term deformation under constant loading and poor tensile strength. Owing to the rapid growth rate of geocells, it has become necessary to develop a polymer material with excellent creep resistance and tensile strength. To this end, a polymer-blend geocell (PBG) is developed in this study using a twin-screw extruder with high-density polyethylene (HDPE), polyamide 6, and compatibilizer. The polymer formula is determined by the tear fracture surface and scanning electron microscopy. The tensile properties of the blends with different formulas are studied in terms of yield strength, tensile strength, and elongation at break. Finally, three types of PBG and HDPE geocells are selected to study the long-term creep behavior using accelerated creep tests. The analysis results of raw creep data, master creep curve, and isochronous creep curves indicated that the PBG had a better creep resistance than the HDPE geocells.     

Investigation of factors influencing behavior of single geocell-reinforced bases under static loading

Geocell, one type of geosynthetics manufactured in the form of three-dimensional interconnected cells, can be used as a reinforcement to improve the behavior of base courses by providing lateral confinement to increase their stiffness and strength and reduce surface permanent-deformation. However, the use of geocells for base reinforcement is hindered by the existing gap between applications and theories. This study experimentally investigated the factors influencing the behavior (stiffness and bearing capacity) of single geocell-reinforced bases including shape, type, embedment, height of geocells, and quality of infill materials. Three of the four types of geocells investigated in this study were made of novel polymeric alloys using a new manufacturing technology. Repeatability and potential scale effects on test results were examined. The test results showed that the geocell placed in a circular shape had a higher stiffness and bearing capacity than that placed in an elliptical shape. The performance of the geocell-reinforced base depended on the elastic modulus of the geocell sheet. The unconfined geocell had a lower stiffness but a higher ultimate load capacity than the confined geocell. The benefit of the geocell was minimized when the infill material, quarry waste with apparent cohesion, was used as compared with the Kansas River sand without apparent cohesion. The single geocell-reinforced base had a lower stiffness and bearing capacity than the multiple geocell-reinforced base.     

Failure mechanisms of geocell walls and junctions

Geocell panels are honeycomb-like systems used to provide earth reinforcement. Strips of perforated high-density polyethylene sheets, also known as cell-walls, are welded together at locations known as junctions. The cell-wall and junctions are designed to support and transfer tensile and shear loads and the integrity of these is essential for the appropriate performance of geocells in practice. Nevertheless, there is no standardized test procedure to assess the strength of the cell-wall or junction, and limited research has been undertaken regarding the failure mechanisms of geocell panels when subjected to various loading scenarios. This paper aims to examine the responses of geocell junctions and cell-walls under various loading conditions. An extensive testing program was undertaken to assess the geocell junctions, which included uniaxial tensile, shear, peeling and splitting strength tests. The uniaxial tensile strength, trapezoidal tearing strength, and creep tests were carried out on the geocell walls. A ductility ratio was developed to measure the rapidness of failure under different short-term loading scenarios for both the cell-wall and junction. This paper presents the observed failure patterns and an evaluation of the implications of the practical uses of geocells.     

Pullout tests on diagonally enhanced geocells embedded in sand to improve load-deformation response subjected to significant planar tensile loads

This paper presents pullout test results on conventional (ordinary) and diagonally enhanced geocells under surcharge pressures of 3, 13, 23 and 33 kPa. Extensive pullout tests on scaled geocells embedded in silica sand are performed to investigate the effects of improvements on load-deformation response, strength and stiffness. Conventional web-shaped geocells are having a small stiffness when subjected to planar tension attributed to deformability of webs. Therefore, conventional geocells may not function properly when subjected to tensile forces along the main plane in service. A special geocell is fabricated in this study, similar to tendoned geocells, through adding diagonal members along the induced tensile load to overcome the shortcomings of conventional geocells. The test results have shown that both the stiffness and ultimate resistance of the diagonally enhanced geocells have significantly improved with respect to the conventional ones. Afterwards, three experiments were carried out on a small-scale shallow footing resting on sand reinforced with geocells, indicating improvement in bearing capacity as well as load-settlement response of footings supported by the diagonally enhanced geocells as compared to conventional geocells.     

Internal stability analysis of geocell-reinforced slopes subjected to seismic loading based on pseudo-static approach

Seismic stability analysis of geocell-reinforced slopes (GRSs), considering shear and moment strength in addition to tensile resistance for geocells, is a novel topic for which scarce studies are found in the literature. Despite few available studies, an analytical approach is presented in this study to investigate the seismic internal stability of GRSs, employing the pseudo-static method based on a limit state approach. Results are given in terms of conventional design charts representing the required total strength and critical length of geocells. The results show that with increasing the horizontal seismic acceleration (k_h), the internal stability degenerates since the required strength and critical length of geocells increase. For GRSs subjected to greater k_h, the effect of increasing the vertical seismic component (k_v) on increasing the required strength and length of geocells is more considerable than those subjected to lower k_h values. Parametric analyses are conducted, under various seismic conditions, to investigate the effect of increasing the geocell height and raising the number of geocell layers, leading to the reduction in the required strength and length of geocells. Such effects are found to be dependent on the parameters such as the intensity of seismic excitation, material properties and geometry of slope.     

Static stability analysis of geocell-reinforced slopes

This paper presents an analytical approach to investigate the stability of geocell-reinforced slopes using the limit equilibrium method (LEM). The so-called Horizontal Slice Method (HSM) is employed to simulate horizontal geocell layers. Each geocell layer acts as a beam providing bending and shear resistance in addition to axial strength. A formula is devised by picking relevant governing equilibrium equations, fitted to the new concept employed exclusively for analysis of geocell-reinforced slopes. Parametric studies are conducted to evaluate the effects of increasing the geocell height and replacing geogrids by geocells with various heights for slopes with different characteristics. The results showed that such actions would reasonably reduce the required tension and length of the reinforcement layers, meaning that the stability condition is improved and the less lengthy reinforcement system is formed. Output values also showed dependency on the slope angle and its material properties.     

Evaluation of creep behavior of high density polyethylene and polyethylene-terephthalate geogrids

The tensile creep behavior of polyethylene-terephthalate (PET) and high density polyethylene (HDPE) geogrids was evaluated using five test methods: the short- and long-term stepped isothermal method (SIM), the short- and long-term time-temperature superposition (TTS), and the conventional method. SIM and TTS are acceleration tests using elevated temperatures. SIM uses a single specimen throughout all temperature steps in contrast to TTS in which a new specimen is employed for each temperature step. The test results indicate that at the same percentage of ultimate tensile strength, PET geogrid exhibited less creep deformation than the HDPE geogrid. The HDPE geogrid exhibited primary, secondary, and tertiary creep stages before rupture, whereas only primary creep and tertiary creep were detected in the PET geogrid. Furthermore, the strain rate of the primary creep stage was found to be independent of the applied loads for the PET geogrid, while it increased exponentially for the HDPE geogrid. The activation energies deduced from different accelerated creep tests were very similar for the PET geogrid. In contrast, the activation energies were higher from the short-term acceleration tests than from the long-term tests for the HDPE geogrid. The four-parameter Weibull model was able to predict the linear and non-linear creep behavior up to 100 years based on 10-h creep testing data. The creep reduction factor of 100 years design life was evaluated and higher values were resulted from the HDPE geogrid than from the PET geogrid.     

Response of pavement foundations incorporating both geocells and expanded polystyrene (EPS) geofoam

The suitability of geocell reinforcement in reducing rut depth, surface settlements and/or pavement cracks during service life of the pavements supported on expanded polystyrene (EPS) geofoam blocks is studied using a series of large-scale cyclic plate load tests plus a number of simplified numerical simulations. It was found that the improvement due to provision of geocell constantly increases as the load cycles increase. The rut depths at the pavement surface significantly decrease due to the increased lateral resistance provided by the geocell in the overlying soil layer, and this compensates the lower competency of the underlying EPS geofoam blocks. The efficiency of geocell reinforcement depends on the amplitude of applied pressure: increasing the amplitude of cyclic pressure increasingly exploits the benefits of the geocell reinforcement. During cyclic loading application, geocells can reduce settlement of the pavement surface by up to 41% compared to an unreinforced case – with even greater reduction as the load cycles increase. Employment of geocell reinforcement substantially decreases the rate of increase in the surface settlement during load repetitions. When very low density EPS geofoam (EPS 10) is used, even though accompanied with overlying reinforced soil of 600 mm thickness, the pavement is incapable of tolerating large cyclic pressures (e.g. 550 kPa). In comparison with the unreinforced case, the resilient modulus is increased by geocell reinforcement by 25%, 34% and 53% for overlying soil thicknesses of 600, 500 and 400 mm, respectively. The improvement due to geocell reinforcement was most pronounced when thinner soil layer was used. The verified three-dimensional numerical modelings assisted in further insight regarding the mechanisms involved. The improvement factors obtained in this study allow a designer to choose appropriate values for a geocell reinforced pavement foundation on EPS geofoam.     

Uniaxial compressive behavior of scrapped tire and sand-filled wire netted geocell with a geotextile envelope

Cellular structures are widely used in civil engineering. Their design is based on the understanding of the mechanical behavior of geocells. This paper investigates the response of a single geocell to a uniaxial compression test. The geocells were cubic, either 500 mm or 300 mm on a side. The fill materials were sand and scrapped tire and sand mixtures in different mass ratios. The envelope of the geocell was made up of a hexagonal wire netting cage and a containment geotextile. The response of the geocell is discussed based on the axial load and displacement measurements as well as the change in geocell volume.The axial load was found to be globally governed by the interaction between the fill material and the envelope, which depends on the shape of the wire mesh and the volumetric behavior of the fill material.     

Combining EPS geofoam with geocell to reduce buried pipe loads and trench surface rutting

This paper reports full scale experiments, under simulated heavy traffic, of geocell and EPS (expanded polystyrene) geofoam block inclusions to mitigate the pressure on, and deformation of, shallow buried, high density polyethylene (HDPE) flexible pipes while limiting surface settlement of the backfilled trench. Geocell of two pocket sizes and EPS of different widths and thickness are used. Soil surface settlement, pipe deformation and transferred pressure onto the pipe are evaluated under repeated loading. The results show that using EPS may sometimes lead to larger surface settlements but can alleviate pressure onto the pipe and, consequentially, result in lower pipe deformations. This benefit is enhanced by the use of geocell reinforcement, which not only significantly opposes any EPS-induced increase in soil surface settlement, but further reduces the pressure on the pipe and its deformation to within allowable limits. For example, by using EPS geofoam with width 0.3 times, and thickness 1.5 times, pipe diameter simultaneously with geocell reinforcement with a pocket size 110 × 110 mm² soil surface settlement, pipe deformation and transferred pressure around a shallow pipe were respectively, 0.60, 0.52 and 0.46 times those obtained in the fully unreinforced buried pipe system. This would represent a desirable and allowable arrangement.