基于单根纤维拉拔试验的波形纤维加筋土界面强度研究

纤维/土界面间的力学作用特性是决定纤维加筋土工程性质的关键因素。为了改善纤维/土界面作用力,开发了一种新型的波形纤维作为加筋材料,并自主设计了一套拉拔试验装置,对单根纤维加筋土开展了多组拉拔试验,定量获得了波形纤维加筋土的拉拔特性及界面剪切强度,通过与传统直线形纤维对比,分析了波形纤维/土界面的力学作用机理,并从理论上探讨了波形纤维的最大临界加筋长度。结果表明：提出的单根纤维拉拔试验方法及设计的试验装置为研究纤维/土界面力学作用提供了有效的途径,试验结果具有较好的可重复性;直线形纤维的拉拔曲线呈典型的单峰特征,拉力达到峰值后迅速减小到残余值并逐渐趋于稳定,而波形纤维的拉拔曲线呈显著的多峰特征,曲线波长与纤维的波长基本一致;通过对比,波形纤维/土界面剪切强度明显高于直线形纤维,强度值提高了178%,极大改善了纤维的加筋效果,此外,波形纤维拉拔曲线各峰值对应的界面剪切强度及残余剪切强度随拉拔位移呈指数递减趋势;利用测得的纤维/土界面剪切强度,结合纤维自身的抗拉强度和一些假设条件,能确定纤维的最大临界加筋长度,对实际工程设计有一定参考意义。     

Verification of unified effective stress theory based on the effect of moisture on mechanical properties of fiber reinforced unsaturated soil

The unified effective stress theory based on suction stress (SSCC theory) enables the characterization of soils under both saturated and unsaturated conditions with one closed-form relationship. This study provides experimental verification of this theory through the unconfined compressive strength test (UCS) and indirect tensile test strength (ITS) on silty clay soil stabilized with fiber. A series of matric suction, ITS, and UCS tests were conducted to validate the SSCC theory through the representation of the results of ITS and UCS tests in terms of mean total stress (p) versus deviatoric stress (q) and mean effective stress (p`) versus deviatoric stress (q). The results of the validation procedures showed that the SSCC theory is applicable and valid at a range of 6%–16% of water content on the silty clay and the silty clay fiber-reinforced soils. There is a small fluctuation in the increase of ITS and UCS values with increasing fiber content due to randomly oriented distribution of the fiber. The addition of glass fiber does not significantly affect the capacity of water retention of the soil. It improves the condition of the mechanical soil properties at the end of construction more than of the effective stress condition.     

Effect of fiber-reinforcement on the strength of cemented soils

This study aims to verify the differences in the strength of an artificially cemented sandy soil with and without fiber reinforcement. The controlling parameters evaluated were the amount of cement, porosity, moisture content, and voids/cement ratio. A series of unconfined compression tests and suction measures were carried out. The results show that fiber insertion in the cemented soil, for the whole range of cement studied, causes an increase in unconfined compression strength. The UCS increased linearly with the amount of cement and reduced with the increase in porosity (η) for both the fiber-reinforced and unreinforced specimens. A power function fits well as the relation between unconfined compressive strength (UCS) and porosity (η). Finally, it was shown that the voids/cement ratio is a good parameter in the evaluation of the unconfined compressive strength of the fiber-reinforced and unreinforced cemented soil studied.     

冻融循环对固化盐渍土的抗压强度与变形的影响

 北方地区冬季结冰与春季融化引起土的冻胀和融沉问题,弱化了土的抗压性能。以研究冻融循环对固化盐渍土抗压性能的影响为目的,完成冻融前后盐渍土、石灰固化盐渍土、石灰+SH固化盐渍土的抗压试验。结果表明：盐渍土、石灰固化土和石灰+SH固化土的抗压强度随冻融次数的增加而减小,石灰+SH固化土的抗压强度均高于另2种土;冻融前后石灰固化土和石灰+SH固化土均为应变软化型,盐渍土则由应变软化型转变为应变硬化型。冻融循环次数相同时,石灰+SH固化土的抗压强度随含水率的增加而减小,其应力–应变曲线逐渐趋于平缓,土的脆性减弱。石灰+SH固化土具有相对较好的抗冻融性能,含水率是影响冻融后土的抗压性能的首要因素。     

纤维加筋非饱和黏性土的剪切强度特性

纤维加筋是近些年发展起来的一种土质改良技术,系统掌握纤维加筋土的力学性质对评价纤维加筋土工程的稳定性和进一步推广该技术在工程中的应用具有重要意义。为了了解纤维加筋的剪切强度特性,以非饱和黏性土为研究对象,以聚丙烯纤维为加筋材料,在控制含水率和干密度条件下开展了一系列直剪试验。借助扫描电镜,从微观的角度探讨了纤维的增强机理,并对相关宏观力学性质进行了分析。结果表明：纤维加筋能有效提高土体的抗剪强度,且抗剪强度随纤维掺量的增加而增加;相对于内摩擦角,纤维对黏聚力的增强效果要明显得多;纤维加筋土的抗剪强度随含水率的增加而减小,随干密度的增加而增加;总体上,低含水率和高密实度条件有利于发挥纤维的拉筋效果,提高纤维对强度的贡献。此外,纤维加筋在提高土体峰值剪切强度的同时,还能增加土体破坏时对应的应变及破坏后的残余强度,改善土体的破坏韧性。由扫描电镜分析可知,单根纤维一维拉筋作用和纤维网三维拉筋作用是纤维加筋土的主要增强机理,增强效果则取决于纤维-土界面力大小;剪切面上的纤维在剪切过程中呈现拔出和拉断两种失效模式。     

Strength properties of soft clay treated with mixture of nano-SiO_2 and recycled polyester fiber

This paper investigates the effect of recycled polyester fiber, produced from polyethylene(PET) bottles, in combination with nano-Si O2 as a new stabilizer to improve the mechanical properties of soils. We intend to study the effect of adding nano-Si O2 and recycled polyester fiber on soil engineering properties,especially the shear strength and unconfined compressive strength(UCS), using clayey soil with low liquid limit. Three different combinations of fiber-soil ratios ranging between 0.1% and 0.5% as well as three different combinations of nano-soil ratios ranging between 0.5% and 1% are used. The shear strength and UCS of treated specimens are obtained from direct shear test and unconfined compression test, respectively. Results of this study show that the addition of recycled polyester fiber and nano-Si O2 increases the strength of soil specimens. Both the shear strength and UCS are improved by increasing the contents of recycled polyester fiber and nano-Si O2 in the soil mixture. The increase in the nano-Si O2 content leads to a reduction in failure strain, but the increase in the content of recycled polyester fiber leads to an increase in failure strain. The increase in the contents of recycled polyester fiber and nanoSi O2 leads to an increase in elastic modulus of soils. Based on the test results, the addition of recycled polyester fiber improves the mechanical properties of soils stabilized with nano-Si O2 as well as the recycled polyester fiber has a positive effect on soil behaviors.     

冻融循环对冻土-混凝土界面冻结强度影响的试验研究

为研究冻融循环作用对冻土-混凝土界面冻结强度的影响,对不同冻融循环次数、法向应力、试验温度及土体初始含水率条件下的冻结界面进行了系列直剪试验,研究经历冻融循环后界面峰值剪切强度、残余剪切强度及强度参数的变化规律。试验结果表明:冻融循环对界面剪切应力与水平位移曲线形态影响很小,经历20次循环后曲线仍是应变软化型。冻融循环对峰值剪切应力的影响强于对残余剪切应力的影响,表明其对界面胶结冰含量产生影响。当土体初始含水率较低且温度较高时,冻融循环使界面峰值剪切强度增加,但变化量较小。然而在含水率较高(20.8%)及试验温度较低时(-5℃),峰值剪切强度随着冻融循环增加而降低。因此在土体含水率较高且冻结温度较低时,对于发生小变形的冻结界面需要重视冻融循环对峰值剪切应力的影响。不同初始含水率、试验温度下冻融循环对残余剪切强度的影响较小且变化规律不明显。在试验温度为-1℃,-3℃,-5℃时,峰值黏聚力随冻融循环增加分别表现为增加、波动和下降,推测是由于界面胶结冰含量不同而引起。峰值摩擦角和残余摩擦角随冻融循环次数增加略有变化。     

Unconfined compressive strength and ductility of fiber-reinforced cemented sand

A series of unconfined compression tests were performed on specimens of fiber-reinforced cemented sand (FRCS) to evaluate how fiber inclusion affects the measured strength and ductility characteristics of cemented sand. Lightly cemented sand with three different cement ratios (2, 4, and 6% by weight of soil) was mixed with four different fiber ratios (0, 0.3, 0.6, and 1% by weight of soil) and then compacted into a cylindrical specimen. Polyvinyl alcohol (PVA) fiber, which adheres well to cement, was randomly distributed throughout the cemented sand. The test results indicate that the inclusion of PVA fiber has a significant effect on both the unconfined compressive strength (UCS) and the axial strain at peak strength. The increase in the UCS was most apparent in the 2% cemented specimen wherein the UCS increased more than three times as the fiber ratio increased up to 1%. The ductile behavior of the FRCS is quantified by the deformability index, D, which is a ratio of the axial strain at peak strength of fiber-reinforced specimen to that of non-fiber-reinforced specimen. In the cases of 1% fiber ratio, the values of D were greater than four, regardless of cement ratios.     

Evaluation of fiber reinforcement on the strength behaviors of dredging slurry cemented at high water content

This study evaluates the strength behaviors of dredging slurry at high water content reinforced by polypropene fibers. Variables, including cement content, curing time, fiber content, fiber length, and water content, were applied to study the effects on the strength of the treated slurry. The unconfined compressive strength was lower with slurry at higher initial water content at 174% than that at 145% at given cement content regardless of fiber contents; and that increases with increasing fiber content at all cement contents at the water content of 174%; whereas, the effect was less significant at water content of 145%. The fiber length was not observed to affect the strength of the treated slurry in this study. The fiber-soil network would form and be enhanced by the hydration products of cement, limiting the development of cracks. The cement content and curing time dominate the stress–strain behaviors of dredging slurry. In addition, three forms of failure were observed with fiber-reinforced dredging slurry. The admixture of polypropylene fiber would restrict the development of cracks upon failure, improving the brittleness and increasing strain at failure of treated slurry.     

Effect of fiber reinforcement and distribution on unconfined compressive strength of fiber-reinforced cemented sand

A series of unconfined compression tests were carried out to examine the effect of fiber reinforcement and distribution on the strength of fiber-reinforced cemented sand (FRCS). Nakdong River sand, polyvinyl alcohol (PVA) fiber, cement and water were mixed and compacted into a cylindrical sample with five equal layers. PVA fibers were randomly distributed at a predetermined layer among the five compacted layers. The strength of the FRCS increases as the number of fiber inclusion layers increases. A fiber-reinforced specimen, where fibers were evenly distributed throughout the five layers, was twice as strong as a non-fiber-reinforced specimen. Using the same amount of fibers to reinforce two different specimens, a specimen with five fiber inclusion layers was 1.5 times stronger than a specimen with one fiber inclusion layer at the middle of the specimen. The fiber reinforcement and distribution throughout the entire specimen resulted in a significant increase in the strength of the FRCS.     

Effect of polypropylene fiber and nano-zeolite on stabilized soft soil under wet-dry cycles

Nano-zeolite was used in the present study as a substitute for a part of lime and then inclusion polypropylene fiber in stabilized soil matrix to develop the soil stabilization method with lime and to improve the efficiency of this technique. In so doing, specimens of soft soil with 5, 10 and 15% of modifier L (lime), LZ (lime-nano-zeolite) and LZF (lime-nano-zeolite-fiber) were prepared, and were subjected to 1–7 wet-dry cycles. Then, microstructure and macrostructure tests were performed on the specimens. The results of the analyses, indicated that the optimal replacement of lime with nano-zeolite would be 40%, and the optimal amount of polypropylene fibers inclusion would be 1% in the stabilized soil matrix. Major reduction in lime consumption would yield a 40% increase in compressive strength and a 21% improvement in durability. The results also showed that the specimen containing 15%LZF would have excellent durability against environmental conditions and very good performance in terms of unconfined compressive strength (UCS), tensile strength and weight loss. Before and after applying 7 wet-dry cycles, the UCS increased by 39% and 16%, respectively. The results of this study indicate that LZF modifier is a suitable option for lime-based stabilization in areas under wet-dry cycles.     

纤维加筋土坯的蒸发过程及抗拉强度特性

土坯作为一种生态、低碳和环保的建筑材料,其力学性能是学界和工程技术人员关注的重点。为了提高土坯的综合抗拉特性,提出采用纤维加筋技术对土坯进行改性处理。通过模拟土坯的形成过程,制备了一系列不同纤维掺量(0~0.2%)、初始含水率(16.5%~20.5%)和干密度(1.50~1.70 g/cm³)的压实土坯试样,进行自然干燥处理,并对干燥后的土坯试样开展了一系列劈裂试验,重点分析了纤维掺量和初始压实状态对土坯干燥失水过程及抗拉强度的影响。结果表明:(1)纤维加筋对土坯的干燥失水过程没有明显影响,但加筋土坯的残余含水率随纤维掺量呈"先降后升"趋势;(2)纤维加筋能显著提高土坯的抗拉强度,但其对抗拉强度的贡献随掺量的参加呈"先升后缓"趋势,对南京地区的下蜀土而言,其最优纤维掺量为0.1%,且纤维加筋能有效抑制土坯的脆性破坏模式,改善土坯的残余抗拉强度和韧性;(3)提高土坯制作时的初始含水率和初始干密度对改善土坯的抗拉强度和纤维加筋效果有较好的正面作用;(4)纤-土界面的微观力学作用及纤维的"桥梁"作用是控制纤维加筋土坯综合抗拉特性的关键因素。     

冻融循环作用下橡胶颗粒混合土强度特性试验研究

对冻融循环作用后的橡胶颗粒混合土（RST混合土）进行无侧限抗压强度试验,分析强度影响规律。结果表明,在28d养护龄期,虽然掺入橡胶颗粒的RST混合土抗压强度相对水泥土会降低,但是其抗冻融性能比水泥土强;冻融循环5次后,随着水泥掺量的增大,掺有橡胶颗粒的试件强度的提高率明显强于未掺入橡胶颗粒的试件;冻融循环5次后,橡胶颗粒的掺入从一定程度上减缓了由于含水量的不同而导致的RST混合土强度的降低速率。     

纤维加筋微生物固化砂土的力学特性

微生物固化能有效提高砂土的强度,但同样会导致土体破坏时呈现明显的脆性。为了平衡微生物固化砂土脆性破坏的不利影响,提出纤维加筋与微生物固化相结合的改性方法,即将质量分数为0%,0.05%,0.15%,0.25%和0.30%的聚丙烯纤维与石英砂均匀混合,然后基于微生物诱导碳酸钙沉积(MICP)技术对土样进行固化,并开展了一系列无侧限抗压试验,同时采用酸洗法测定了各组试样中的碳酸钙含量,进一步分析了试样的微观结构及纤维–土颗粒之间的界面作用特征。结果表明:①在微生物固化砂土中掺入纤维,能极大提高土样的无侧限抗压强度和残余强度,并能显著改善土样破坏时的韧性;②纤维掺量对微生物固化砂土的力学特性有重要影响,无侧限抗压强度随纤维掺量总体上呈先增加后减小的趋势,最优纤维掺量为0.15%,峰后残余强度与纤维掺量呈单调正相关关系;③纤维加筋使微生物固化砂土的峰后应力–应变曲线呈阶梯式下降模式,局部存在波浪式起伏特征;④纤维加筋能够提高微生物诱导碳酸钙的沉积效率和产量,与此同时,碳酸钙的胶结作用对纤维加筋效果具有促进作用。纤维加筋技术与MICP技术相结合能够实现优势互补,对提高工程结构的安全性与稳定性具有积极意义。     

冻融对AS型固化剂改良土工程特性的影响

通过室内动静三轴试验,研究了Aught Set(AS)型土壤固化剂改良土的抗冻融耐久性,分析了AS型固化剂改良土的应力应变关系、静强度、动模量和临界动应力与龄期、冻融次数以及冷却温度等影响因素的相互关系.结果表明,试样的7d无侧限抗压强度达到28 d的75％;AS型固化剂改良土的应力应变关系为应变软化型曲线,呈脆性破坏形式;其静强度、临界动应力以及动回弹模量均随冻融次数增加呈指数形式衰减,且经历6次冻融后均趋于稳定;同一动应力水平下,冻融作用使得试样的累积塑性变形由变形稳定状态向变形破坏过渡;试样在经历多次冻融循环以后,负温越低对试样的力学特性影响越小.     

Enhancing mechanical behavior of micaceous soil with jute fibers and lime additives

Micaceous soils are common in many tropical countries and regions, and in some locations with moderate climate. The soils are spongy and unstable when loaded and are not considered suitable as construction material in earth structures. To resolve the issue, this work examined performance of micaceous soil reinforced with a combination of jute fibers, hydrated lime or slag-lime. A total of 28 sample sets were prepared at various dosages. Unconfined compression tests were conducted on the samples cured for 7 d and 28 d, respectively. The test results suggested that the unconfined compressive strength(UCS) and material stiffness were increased with the inclusion of up to 1% fiber and decreased if additional fibers were used. The ductility was improved consistently with up to 1.5% fiber content. The inclusions of fibers combined with hydrated lime or slag-lime further enhanced strength and stiffness of micaceous soil, and the improvement depended on the dosages used. For the dosages examined, jute fibers outweighed lime and slag in gaining ductility, and the optimal fiber content was 1% where strength and ductility were considered.     

冻融循环与持载对CFRP加固高强混凝土梁变形性能的影响

通过四点弯曲试验,研究了冻融循环与持载对碳纤维增强复合材料（CFRP）加固高强钢筋混凝土梁变形性能的影响。分析了不同环境作用下试验梁的承载力、刚度及破坏形态变化规律。结果表明:在冻融循环单独作用下,试验梁的性能变化很小;冻融与持载耦合作用时,两者均对梁的性能造成不利影响,且随着冻融循环次数的增加,承受持载梁的CFRP-混凝土界面黏结性能有所下降;在冻融循环作用下,CFRP-混凝土界面存在应力时会增大界面的劣化程度,从而引起加固梁性能的下降。     

聚丙烯纤维加筋膨胀土强度试验研究

为了研究聚丙烯纤维对膨胀土强度的影响,进行了大量的室内试验。试验结果表明,纤维膨胀土的强度比素土有了明显的提高,且随着纤维含量的增加,无侧限抗压强度增加,当纤维含量为0.3%时,无侧限抗压强度和纤维土的粘聚力达到最大值,随着纤维含量的继续增加,无侧限抗压强度和粘聚力降低,说明0.3%的纤维含量为最优含筋量。相同纤维含量的情况下,纤维土的强度随着纤维长度的增加而明显增加。同时聚丙烯纤维还可以增加纤维膨胀土的峰值强度,降低纤维膨胀土残余强度的损失,增加土样破坏的韧性,延缓破坏。     

微生物固化纤维加筋砂质黏性紫色土试验研究

微生物诱导方解石沉积(MICP)技术是一种新型土体加固措施,大量的研究表明,土体加固强化的同时也使得土体破坏呈现明显脆性。为了改善微生物固化紫色土的脆性破坏模式,采用纤维加筋与微生物固化相结合的加固方法,将质量分数为0.4%、0.6%、0.8%的纤维与紫色土混合,然后采用巨大芽孢杆菌和钙盐溶液对土样进行不同灌浆次数的固化试验(3次、5次、7次、9次)。通过无侧限压缩试验测定试样抗压强度,洗酸法试验测定试样碳酸钙含量,烘干法测定试样干密度,结果表明:(1)在微生物固化紫色土中掺入纤维,能显著提高试样固化后的无侧限抗压强度和峰值强度对应的轴向应变,改善了土体破坏时的韧性;(2)纤维掺量影响微生物固化紫色土的力学性质,其强度随纤维掺量总体上呈先增大后减小的趋势,最优纤维掺量为0.6%;(3)随着固化时间增加,试样的碳酸钙生成量和干密度逐级增加,强度与碳酸钙生成量呈正相关且有效碳酸钙沉积越来越少,强度趋于稳定;(4)纤维加筋可以提高碳酸钙沉积的效率和产量,土样内生成的碳酸钙对纤维加筋效果具有强化作用。研究成果可以为纤维加筋与MICP固化相结合的土体加固技术应用提供指导和参考。     

Geo-mechanical behavior of clay soils stabilized at ambient temperature with fly-ash geopolymer-incorporated granulated slag

Geopolymer is a cementitious material that can replace ordinary Portland cement in several geotechnical engineering applications, such as soil stabilization, with the advantages of much lower harmful emissions and energy consumption. This paper presents a rigorous evaluation of the geo-mechanical behavior of different types of clay soils treated with geopolymer, including the influence of soil characteristics and mineralogy. Two natural clay soils in addition to a commercially available kaolin clay were used for this investigation. Laboratory experiments were performed including unconfined compressive strength (UCS) and consolidated undrained (CU) triaxial compression tests under different confining pressures. The UCS and triaxial tests indicated that the addition of geopolymer considerably increased the yield stress and initial stiffness of all examined clays. With the increase of geopolymer content, the stress–strain behavior of treated clays was found to develop progressively from ductile response into a post-peak brittle fashion. The CU tests also demonstrated that the addition of geopolymer changed the initial characteristics of remolded clays from quasi-over-consolidated to heavily over-consolidated, rendering high yield surface and more effective shear strength parameters (i.e., cohesion and friction angle). Moreover, although the overall qualitative stress–strain and stress path responses of the clays were similar, significant quantitative differences were observed, particularly in terms of the attainable yield strength, stiffness, and shear strength. These differences can be attributed mainly to the heterogeneity associated with the soil mineralogy and the corresponding differences in the interaction between the clay/non-clay minerals and geopolymer.