Strength properties of xanthan gum and guar gum treated kaolin at different water contents

Nowadays, using biopolymer as a ground improvement method has become very popular. However,since biopolymers are organic and degradable, their long-term effect is not fully known. In this study, the effects of biopolymers on the mechanical behavior of kaolin clay were investigated through a comprehensive program of experiments. Two types of biopolymer, i.e. xanthan gum and guar gum were chosen to investigate the effect of biopolymer type. For this purpose, specimens were prepared using standard Proctor energy at four different water contents(25%, 30% 35% and 40%) with 0.5%, 1%, 1.5% and 2%biopolymer inclusions. The specimens were cured for 1 d, 7 d, 28 d and 90 d. Moreover, some of the specimens were kept in the curing room for 3 years to observe the long-term effect of the biopolymers.At the end of the curing periods, the specimens were subjected to unconfined compression test, and scanning electron microscopy(SEM) analysis was performed to observe the mechanism of strength improvement. The results revealed that the unconfined compressive strength(UCS) of the specimens treated with biopolymers increased in all biopolymer inclusion levels and water contents up to a 90-d curing period. For specimens containing xanthan gum, the maximum strength increase was observed at 25% water content and 2% xanthan gum with 90-d curing. The strength increased 5.23 times induced by xanthan gum addition when compared to the pure clay. Moreover, the increase in strength reached 8.53 times in specimens treated with guar gum. Besides, increasing water content caused more ductile behavior, thus increasing the axial deformation.     

Geotechnical behaviour of guar gum-treated soil

Soil stabilization through biological methods is a sustainable, efficient and long-term alternative to conventional techniques. An elaborate study on the engineering behaviour of guar gum-treated soil, a gum-based biopolymer, is carried out through a concise experimental programme. The dry density of the treated soil shows a marginal increase and its optimum moisture content decreases with the increase in guar gum content. The marginal change in dry density over a range of water contents indicates that guar gum-treated soil can be compacted at low energies and that its sensitivity to changes in water content is not appreciable. The stress-strain behaviour of the treated soil indicates that guar gum stiffens the soil matrix, enabling it to resist loads at lower strains. The gain in strength of the soil-guar matrix is seen to depend on both the biopolymer content and the curing time. There is a substantial increase in the unconfined compressive strength of the treated soil – nearly 45% immediately and 131% after 90 days of curing at a 2% guar gum content. The compressibility of the soil is effectively reduced as the addition of guar gum results in the formation of hydrogels that not only clog the pore spaces, but also stiffen the soil matrix. Guar gum is susceptible to degradation; and hence, durability tests are conducted. The results show that there is no appreciable degradation of the treated soil samples within the investigated period of 90 days.     

Geotechnical characterization of a clay–cement mix

Soft clay deposits are highly plastic, normally consolidated fine grained soils characterized by their low inherent shear strength. The mixing of soft clays with cement as a chemical stabilizer has become a well-known stabilization technique. The resulting strength of the clay–cement mix is controlled by different factors, but mainly the water to cement ratio, the cement content, and the curing conditions. It is crucial to develop a clear understanding of the changes in engineering behavior of the clay–cement mix that result from changes in controlling factors. A phase diagram was established to define the initial conditions of the mass–volume relationships of air, cement, clay, and water of a typical clay–cement mix. This phase diagram was then used to determine the total dry density, void ratio, and specific gravity of the clay–cement mix as a function of the cement content and water to cement ratio. The main objective of this work was to develop generalized trends for the geotechnical properties of clay–cement mixes. These trends were evaluated based on unconfined compressive strength as well as consistency tests carried out on soft clay samples before and after mixing with cement and at different curing times. A reduction in the plasticity index (PI) of 16 % and an increase in the unconfined shear strength of more than 200 kPa were obtained from the addition of 15 % cement. The reduction in the PI of the clay–cement mix was found to be an efficient tool to represent the improvement in the strength of the clay after mixing with cement.     

Bio-mediated soil improvement of loose sand with fungus

This article presents an innovative method of bio-mediated soil improvement for increasing the shear strength of loose sand.The improvement is realized by mixing the loose sand with the inoculum of Rhizopus oligosporus,a kind of fungus widely used in food industry for making Indonesian tempeh.The objective of this article is to investigate the performance and mechanism of mixing tempeh inoculum as a binding agent of loose sand particles.The inoculum dosage,water content of loose sand,and curing time were examined for identifying the increment of unconfined compressive strength(q_u) of the samples.The results showed that q_u of the treated samples increased when the inoculum dosage was elevated.It shows that 5.24% inoculum could yield 68 kPa of q_u,and 5% water content and 3 d curing time produced the maximum q_u.Moreover,the mechanism of hypha and mycelium in binding the soil particles was clearly observed using a digital microscope and scanning electron microscope.     

复合固化剂固化某淤泥质土的试验研究

为了改善青弋江分洪道工程淤泥质土地基的物理力学性能,选用普通硅酸盐水泥、粉煤灰、水玻璃以及木质素磺酸钠组成的水泥基复合固化剂,以青弋江芜湖段典型淤泥质土样作为试验土样,进行了室内固化试验研究,分析了固化剂掺量、淤泥质土初始含水率以及养护龄期的改变对固化土无侧限抗压强度和抗剪强度参数的影响关系。研究结果表明:对于提高青弋江淤泥质土强度,试验所用固化剂作用效果明显,90d龄期养护条件下,掺入复合固化剂处理的固化淤泥质土无侧限抗压强度最高为单掺水泥条件下固化土无侧限抗压强度的4.2倍,同时前者抗剪强度也明显大于后者;固化土无侧限抗压强度随固化剂掺量增加而提高,但增长速率逐渐减缓,同时还随着养护龄期的增加而提高,两者呈明显的对数关系。     

纳米黏土改良黄土力学性能试验研究

在陕西安塞地区黄土中掺入两种不同纳米黏土材料,设计不同掺量、含水率、养护龄期等多因素影响下的正交试验,并对改良后黄土进行三轴试验,分析凹凸棒土和纳米蒙脱土改善黄土无侧限抗压强度指标以及抗剪强度指标的变化特征,并采用SEM对两种纳米黏土改良黄土进行改善机理微观分析。结果表明:掺量为1%的凹凸棒土、掺量为2%的纳米蒙脱土对黄土抗压强度影响最为显著,在一定含水率下可使素黄土的抗压强度分别提高21%和42.3%; 两种纳米黏土改良黄土应力-应变曲线受试样干密度、围压及掺量的影响,干密度为1.35 g?cm^-3、围压为50 kPa的改良黄土有应变软化现象,干密度达到1.65 g?cm^-3时,4种不同围压下改良黄土应力-应变曲线均有应变硬化的趋势; 两种纳米黏土对改良黄土的抗剪强度指标影响效果显著,其中1%掺量的凹凸棒土、2%掺量的纳米蒙脱土可以使改良黄土的黏聚力分别提升61.02%和81.70%,内摩擦角分别提升27.86%和21.31%。     

湿排粉煤灰固化强度试验研究

通过对海水湿排粉煤灰、水泥固化粉煤灰进行室内无侧限抗压强度试验和三轴压缩试验,以及对海水湿排固化粉煤灰材料强度形成机理分析,得到了水泥固化海水湿排粉煤灰基本力学性质及指标．通过设计试验对不同龄期、水泥固化剂掺量及养护条件影响因素进行分析,得到水泥固化海水湿排粉煤灰在不同条件下的强度变化规律．试验结果表明,水泥固化海水湿排粉煤灰具有良好的稳定性和耐久性,拌和固化混合料强度随龄期稳定增长,随水泥掺量增加强度提高．该试验研究对挡土结构、路基加固及地基处理等工程应用有重要的理论与实践研究价值．     

An investigation into the effects of lime on compressive and shear strength characteristics of fiber-reinforced clays

To meet the ever-increasing construction demands around the world during recent years,reinforcement and stabilization methods have been widely used by geotechnical engineers to improve the performances and behavior of fine-grained soils.Although lime stabilization increases the compressive strength of soils,it reduces the soil ductility at the same time.Recent research shows that random fiber inclusion modifies the brittleness of soils.In the current research,the effects of lime and polypropylene(PP) fiber additions on such characteristics as compressive and shear strengths,failure strain,secant modulus of elasticity(E_(50)) and shear strength parameters of mixtures were investigated.Kaolinite was treated with 1%,3% and 5% lime by dry weight of soil and reinforced with 0.1% monovalent PP fibers with the length of 6 mm.Samples were prepared at optimum conditions and cured at 35℃ for 1 d,7 d and28 d at 90% relative humidity and subsequently subjected to uniaxial and triaxial compression tests(UCT and TCT) under cell pressures of 25 kPa,50 kPa and 100 kPa.Results showed that inclusion of random PP fibers to clay-lime mixtures increases both compressive and shear strengths as well as the ductility.Lime content and curing period were found to be the most influential factors.Scanning electron microscopy(SEM) analysis showed that lime addition and the formation of cementitious compounds bind soil particles and increase soil/fiber interactions at interface,leading to enhanced shear strength.The more ductile the stabilized and reinforced composition,the less the cracks in roads and waste landfill covers.     

水泥粉煤灰搅拌桩复合地基的离心模型试验及现场实测效果研究

在水泥桩体中掺入一定量的粉煤灰不仅可以节约资源,还可以增强桩体在酸性地基中的抗腐蚀性能。为了研究水泥粉煤灰搅拌桩处治饱和黄土地基的适用性,以蒲渭高速公路为工程依托,以水泥与粉煤灰不同重量比组成的固化剂及其不同掺入比配制的试件进行的无侧限抗压强度试验、直剪试验和固结压缩试验为基础,选择最佳配比后进行水泥粉煤灰搅拌桩的离心模型试验,并与现场实测结果相对比。结果表明:当水泥与粉煤灰的重量比不小于2∶1时,粉煤灰的添加对试件的抗剪强度影响不大;固化剂掺入比达到15%以后,水泥粉煤灰土与水泥土试件的强度相差值减小;离心模型试验所得工后沉降量小于0.3m;现场实测工后沉降量和桩体强度均符合设计要求。水泥粉煤灰搅拌桩处理饱和黄土地基是经济可靠的。     

Accumulated Deformation of Sand with Initial Shear Stress and Effective Stress State Lying Near Failure Conditions

This study concerns the development of residual deformations in soil subjected to static shear and cyclic stress components. This problem plays an important role in such situation as natural slopes, slopes of embankments and dams, quay walls. Among them, the present study focuses in particular on cyclic undrained behavior with static shear stress. Furthermore, in the case of structures with static shear stress and subjected to the earthquake shaking, the forces acting in the shear plane are generally one-way cyclic loading without reversing its direction. Experimental research was focused on the accumulation of shear strain in the soil element during one-way cyclic loading (torsional stress loading only on positive or negative side). Series of torsional shear tests on hollow cylindrical specimens of Toyoura sand with relative density of 40% and 60%, with isotropic stress condition under effective pressure of 98 kPa, 196 kPa and 294 kPa as well as anisotropic conditions were made in order to investigate the incremental shear strain per cycle during cyclic loading. The level of static shear stress was set equal to half of the ultimate stress under drained conditions (factor of safety equal to two). The experimental results showed that the presence of static shear stress increases the resistance of sandy soil. The experimental results of the shear strain increment per cycle and the number of cycles were plotted in a logarithmic scale to show a linear relationship with each other. Based on these findings a simple model for prediction of shear strain was developed.     

Peak and residual strength characteristics of cement-treated soil cured under different consolidation conditions

The shear strength of cement-treated soil can be changed by both cementation and consolidation during the early stages of hardening because of cement hydration. Based on the results of triaxial and unconfined compression tests, this paper describes the effects of isotropic and one-dimensional consolidation stress, applied during the curing period, on the undrained peak and residual shear strengths of cement-treated soil. The sample used was a mixture of fine-grained sand and ordinary Portland cement. A consolidated undrained triaxial compression test (ICU) was conducted on the specimens immediately after the cement treatment. Each test was conducted under different consolidation pressures, curing times and delayed loading times. The following conclusions were developed from the results and discussions: (1) the undrained peak shear strength of cement-treated soil, cured under different consolidation conditions, increases with an increase in either the consolidation pressure or the curing time, whereas it gradually decreases with an increase in the delayed loading time. (2) The rate of undrained strength increase resulting from consolidation differs significantly between isotropic and one-dimensional consolidations. (3) For a curing time of between one and seven days, the rate of strength increase by isotropic consolidation exceeds that by one-dimensional consolidation. The simultaneous volumetric change of cement-treated soil during consolidation depends on the stress conditions of the specimen, that is, the difference between isotropic and one-dimensional consolidations. (4) When the test is not conducted under nearly in-situ conditions, the undrained shear strength may be underestimated, depending on the time interval between the cement treatment and the start of consolidation. (5) The shear strength in the residual state is influenced by the consolidation pressure during curing. (6) As the consolidation pressure during curing increases, the specimens exhibit a higher residual strength.     

The compressibility characteristics of a compacted laterite soil

Laterite soils occupy large parts of the tropical regions of the world. They are used, in the compacted state, as bases for roads and air-field pavements, as fills for foundations, embankments and earth dams. Although the response to load with decreasing void ratio (compressibility) is important under the conditions of use, it has not been the subject of considerable study. Engineering studies in laterite soils have been limited to classification, compaction characteristics and sometimes, shear strength. The compressibility characteristics of a compacted laterite soil from Paraiba State, North-East Brazil has been examined in the oedometer. The results indicate the presence of collapsible soil grains which comminute under load. The behaviour of soil with collapsible soil grains is described.     

Shear strength of a fibre-reinforced clay at large shear displacement when subjected to different stress histories

The topic of fibre-reinforced soil has been introduced and studied increasingly in the past few decades. However, the shear strength response of fibre-reinforced clay soils with different initial void ratio values when subjected to large shear displacement has not been explored in the literature. The purpose of this study is to evaluate the shear strength responses of fibre-reinforced clay soils when remoulded with relatively small and large initial void ratio and subjected to large shear displacement. In order to exclude the composition variability of the fibre-reinforced samples when subjected to various normal effective stresses, a series of multi-stage reverse drained direct shear test was undertaken with four reverse cycles of ±7 mm, ±7 mm, ±7 mm and +14 mm to achieve an accumulative horizontal shear displacement up to 56 mm that is 93% of the sample dimension. The first stage of the testing programme was carried out on soil samples consolidated at normal effective stress of 600 kPa and unloaded to 50 kPa, followed by 4 shear cycles at normal effective stresses of 50, 100 and 200 kPa, respectively. The results of these tests confirmed significant effective stress ratio improvement with fibre reinforcement, even at large shear displacement to the fourth cycle. However, the rate of improvement decreased with normal effective stress and initial void ratio. Based on the experiments carried out in this study, the optimum fibre content to increase the shear strength of the clay soil with initial void ratio of 0.64 was found to be 0.25% with 140%, 81% and 23% increase in the stress ratio over that of the unreinforced soil at normal effective stresses of 50, 100 and 200 kPa, respectively. The second stage of the testing programme was conducted on a set of samples consolidated and sheared at normal effective stresses of 50, 100 and 200 kPa, respectively. The optimum fibre content was found to be related only to the initial void ratio of the soil, irrespective of the stress history of the soil and the applied normal effective stress. The shear stress ratio of the fibre-reinforced clay soils at large shear displacement was found to be relatively independent of the stress history of the soil. For all soil samples tested in this study, the stress ratio at 200 kPa normal effective stress was found to remain between 0.45 and 0.60.     

Geotechnical characterization of collapsible salty sands subjected to monotonic and cyclic loadings – A case study for areas with high seismicity

Collapsible soils are typically found in arid regions and often have an aeolian or alluvial origin. In their natural states, they may have a low moisture content and cemented structure that can contribute to high strength and stiffness; however, wetting or saturation can reduce the strength and stiffness due to loss of the cementation. This paper presents a geotechnical characterization of collapsible salty sands in the highly seismic southern coast of Peru, which makes the characterization of their dynamic properties and expected response to earthquake-induced demands (e.g., liquefaction) of primary importance. The geotechnical characterization was performed on intact and remolded samples utilizing various field and laboratory tests, including oedometer, direct shear, static triaxial, cyclic simple shear, torsional resonant column, plate loading, and MASW tests. The results revealed insights on the geotechnical properties and mechanical response of collapsible soils and the effects of salt cementation. The results indicated: 1) a decreasing brittle and collapsible behaviors with decreasing cementation while maintaining consistent post-collapse residual strength; 2) oedometer and in situ plate loading tests showed a sudden increase in deformations once cementation is broken; 3) higher dilation potential of collapsible soils as compared to natural sands; 4) decrease in the maximum shear modulus due to the loss of cementation; 5) increase in the stress dependence of the maximum shear modulus with loss of cementation; and 6) a higher resistance to liquefaction for the collapsible soils, even after washing, as compared to natural sands, which may be ascribed to the more pronounced dilation potential.     

Behavior and characteristics of compacted expansive unsaturated bentonite-sand mixture

Limited studies dealt with the expansive unsaturated soils in the case of large-scale model close to the field conditions and therefore,there is much more room for improvement.In this study,expansive(bentonite-sand(B-S) mixture) and non-expansive(kaolin) soils were tested in different water contents and dry unit weights chosen from the compaction curve to examine the effect of water content change on soil properties(swelling pressure,expansion indices,shear strength(soil cohesion) and soil suction)for the small soil samples.Large-scale model was also used to show the effect of water content change on different relations(swelling and suction with elapsed time).The study reveals that the initial soil conditions(water content and dry unit weight) affect the soil cohesion,suction and swelling,where all these parameters slightly decrease with the increase in soil water content especially on the wet side of optimum water content.The settlement of each soil at failure increases with the increase in soil degrees of saturation since the matric suction reduces the soil ability to deform.The settlement observed in B-S mixture is higher than that in kaolin due to the effect of higher swelling observed in B-S mixture and the huge amount of water absorbed which transformed the soil to highly compressible soil.The matric suction seems to decrease with elapsed time from top to bottom of tensiometers due to the effect of water flowing from top of the specimen.The tensiometer reading at first of the saturation process is lower than that at later period of saturation(for soil sample B-S3,the tensiometer #1 took 3 d to drop from 93 kPa to 80 kPa at early stage,while the same tensiometer took 2 d to drop from 60 kPa to 20 kPa).     

铁路路基湿陷性黄土改良试验研究

对铁路路基湿陷性黄土掺加水泥、石灰后物理指标进行了测试,并通过研究确定了水泥和石灰的最优掺量,为确定湿陷性黄土路基技术方案提供依据。     

电石渣改良膨胀土试验研究

膨胀土吸水体积膨胀,失水体积收缩的特性,给工程建设带来的危害屡见不鲜。因此如何改良膨胀土,显得尤为突出。化学改良法是膨胀土改良的常用手段之一。电石渣是工业制乙炔的主要废弃物,其堆放不仅占用大量土地,而且会造成环境及地下水污染等。通过系统的室内试验,探讨利用电石渣改良膨胀土的方法,掌握电石渣改良膨胀土的物理、力学性质及胀缩性特征。试验结果表明:随电石渣掺量的增大,改良土的最优含水率逐渐增大,最大干密度逐渐降低;塑性指数随电石渣掺量的增大先增大后降低;改良土的自由膨胀率、膨胀量、膨胀力与线缩率均随着养护龄期的增长呈减小趋势;随养护龄期的增长,改良土的压缩模量呈增大趋势,压缩系数呈减小趋势;改良土的抗剪强度随养护龄期增长主要体现在黏聚力、内摩擦角及无侧限抗压强度均随养护龄期的增长而增大;通过试验得到电石渣改良膨胀土的最优掺量为10%。扫描电镜的结果也验证了随养护龄期的增长,改良土的强度增大,胀缩性减弱。     

Strength Development in Cement Admixed Bangkok Clay: Laboratory and Field Investigations

The in-situ deep mixing technique has been established as an effective means to effect columnar inclusions into soft Bangkok clay to enhance bearing capacity and reduce settlement. In this paper, an attempt is made to identify the critical factors governing the strength development in cement admixed Bangkok clay in both the laboratory and the field. It is found that clay-water/cement ratio, w_c/C is the prime parameter controlling the laboratory strength development when the liquidity index varies between 1 and 2. Based on this parameter and Abrams' law, the strength prediction equation for various curing times and combinations of clay water content and cement content is proposed and verified. This will help minimize the number of trials necessary to arrive at the quantity of cement to be admixed. Besides the w_c/C, the strength of deep mixing column is controlled by the execution and curing conditions. For low strength improvement (laboratory 28-day strength less than 1,500 kPa), the field strength of the deep mixing columns, q_uf, made up from both dry and wet mixing methods is higher than 0.6 times the laboratory strength, q_ul. The q_uf/q_ul ratios for the wet mixing columns are generally higher than those for the dry mixing columns. This higher strength ratio is due to the dissipation of the excess water in the column (consolidation) caused by the field stress. The water to cement ratio, W/C, of 1.0 is recommended for the wet mixing method of the soft Bangkok clay. A fast installation rate was shown to provide high quality for low strength columns. Suggestions are made for improving the deep mixing of soft Bangkok clay, which are very useful both from economic and engineering viewpoints.     

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。     

灰土挤密桩在大厚度湿陷性黄土场地中的应用

以具体工程为研究背景,结合工程的地质条件,对灰土挤密桩在大厚度湿陷性黄土场地中的应用进行了探讨,着重介绍了试桩的设计和检测要点,指出当建筑物基底荷载不大于350 kPa时,采用灰土挤密桩法处理地基效果显著,取得了较好的社会效益和经济效益。