Effect of Strain Rate on the Strength Characteristics of Soil–Lime Mixture

The effects of rate of strain on strength and deformation characteristics of soil–lime were investigated. Five strain rates (0.1, 0.8, 2.0, 4.0 and 7.0 %/min), five lime contents (0, 3, 6, 9 and 12 %) by dry soil weight and three cell pressures (100, 200 and 340 kN/m²) were carried. Triaxial tests, under unconsolidated condition, were used to study the effect of strain rate on strength and initial modulus of elasticity of soil and soil–lime mixture after two curing periods 7 and 21 days, respectively. A total of 405 triaxial specimens have been tested, where 225 specimens have been tested with first curing period (7 days). The testing program includes nine specimens for each strain rate, and each lime content was carried out, including natural soil with zero lime content. Another set of triaxial tests with a total of 180 specimens for the second curing period (21 days) was prepared at optimum moisture content, and the corresponding maximum dry density was also tested. The effects of strain rate and curing period on each of stress–strain behavior, type of failure, deviator stress at failure, cohesion and angle of internal friction and initial modulus of elasticity were studied thoroughly for the natural soil as well as soil–lime mixtures. For natural soil, the test results showed that the undrained shear strength, the initial modulus of elasticity and the cohesion increase significantly as the strain rate increase, while for soil–lime mixture at different curing periods, the undrained shear strength, initial modulus of elasticity and the cohesion increases to a maximum and then decreases as the strain rate and lime content increase. Also, the same variables and angle of internal friction increase with increasing curing period.     

固化铅污染土的干湿循环耐久性试验研究

在商用高岭土、膨润土与商业黄砂混合物中加入硝酸铅溶液,添加水泥和石灰两种固化剂,采用室内压实制样方法获得固化的铅污染土试样。进行干、湿循环试验,测试固化体的质量损失和无侧限抗压强度等参数随干、湿循环次数的变化规律,评价固化铅污染土的干、湿耐久性。测试结果表明,本试验8种配比的试样都满足干、湿循环的要求;黏土矿物为膨润土的试样干、湿循环耐久性比黏土矿物为高岭土的试样要差;水泥固化土的干、湿循环耐久性要略优于石灰固化土;加入 8 000 mg/kg的铅可略增大土体的抗干、湿循环耐久性。水泥和石灰固化/稳定化重金属污染土时,土体中含水率是保证加固效果的关键参数之一。土体中含水率应能满足固化剂充分水化、水解、火山灰和碳酸化反应之需要。     

Role of smectite-rich shales in frequent foundation failures in southeast Nigeria

This paper investigated the geotechnical properties of smectite-rich shale, and its implications as foundation material. Ten expansive shale samples were collected from foundation materials at Akpugo in Nkanu West L.G.A. of Enugu State, southeast Nigeria. Samples were subjected to grading, Atterberg limits-cum-compaction tests, slake durability, specific gravity, permeability, undrained triaxial tests and x-ray diffraction scan. Fines and sand contents of the soil samples range from 51–97% and 3–49% respectively. Liquid limit, plastic limit and plasticity index have average values of 60.7, 19.1 and 43.3% respectively. Linear shrinkage and free swell showed average of 16.3% and 76%. These results are indicative of predominant clay soil with high plasticity, compressibility and water holding capacity. XRD scan established presence of smectite and illite clay minerals, confirming soil high plasticity, capable of causing instability in foundation soil. The shale achieved maximum dry density range between 1.79 and 1.94 kg/m³ at optimum moisture content range of 6.9–12.8%, indicating poor to fair foundation materials. The shale cohesion ranges from 15 to 30 kPa while the angle of friction ranges between 10° and 18°, signifying an average strength soil material. Samples slake durability index and specific gravity fall within 24–55% and 2.50–2.58 respectively, suggesting non-durable and weak soil. Permeability of the samples ranges between 7.36 ×10^?6 and 4.77 ×10^?8 cm/s which suggested low drainage capable of causing water-log at sites. Therefore, the shale could be generally classified as poor to fair foundation material, which on moisture influx experience reduction in strength due to deterioration of its constituent minerals, especially clay and cement materials during the lifespan of engineering structures. Authors therefore recommend modification of foundation soil, appropriate foundation design and good drainage control as ways of improving stability of engineering structures underlain by expansive shale.     

Mechanical behavior and micro-structure of cement kiln dust-stabilized expensive soil

In this study, the effects of cement kiln dust (CKD) on the swelling properties, strength properties, and microstructures of CKD-stabilized expansive soil were investigated. Samples were prepared and stabilized with different CKD content ratios, ranging from 0 to 18% by dry mass. The results obtained show that the maximum swelling pressures decrease exponentially with increases in CKD content. Both the cohesion and unconfined compressive strength (UCS) increase at ratios below 10% CKD and then decrease slightly, above that ratio. CKD can also improve the strength of saturated, expansive soil. There is no visible change of UCS for soil without CKD when cured, while the UCS of a sample with 10% CKD content after curing for 90 days is higher than that after curing for only 1 day. This indicates that CKD can improve the long-term strength of expansive soil. Finally, microstructure analysis reveals that the addition of CKD reduces the montmorillonite content of expansive soil and decreases its swelling properties. The addition of CKD also changes the pore volume distribution, both the size and amount of macro-pores and micro-pores decrease with increase in CKD content. For saturated samples, the size of macro-pores is obviously reduced, while that of micro-pores is slightly increased for both treated and untreated soils. Hydration and saturation processes make the soil structure become dispersive which results in a lower strength, and adding CKD can restrain this process. The suggested optimal CKD content is between 10 and 14% and with a curing time of more than 27 days.     

Experimental Investigation of Mechanical Properties of Sands Reinforced with Discrete Randomly Distributed Fiber

The addition of cementitious admixtures and/or inclusion of fibers are frequently used in practice to stabilize soils and to improve their mechanical properties. In this study, ring shear tests were conducted to investigate mechanical properties such as shear strength, angle of friction and cohesion values of randomly distributed discrete fiber-reinforced sand mixtures. The length and aspect ratio of the fibers used in the current study were 12 mm and 120, respectively. Specimens were prepared at four different fiber ratios (0.1, 0.3, 0.6, and 0.9 % by weight of sand). A series of ring shear tests were carried out on sand alone and fiber-reinforced sand mixtures at different normal stresses. The test results indicated that the addition of fiber had a significant effect on the shear strength of the sand. Shear stress of the unreinforced sand increases 1.29–2.32, 1.16–1.39, and 1.07–1.5 times at a normal stress of 50, 150, and 250 kPa, respectively with fiber inclusion. Fiber content had positive effects on improving the shear strength parameters (angle of internal friction and cohesion) of the mixtures. The cohesion and angle of internal friction of fiber-reinforced sand prepared at different ratios of fiber increased by 5.3–27.4 kPa and 2.0°–7.3° respectively. The inclusion of fibers improves the ductility of the soil by preventing the loss of post-peak strength.     

Evaluation of Strength Characteristics of Tropical Black Clay Treated with Locust Bean Waste Ash

An expansive tropical black clay (also known as black cotton soil because the cotton plant thrives well on it) was treated with up to 15 % locust bean waste ash (LBWA) to assess its soil improvement potential. Samples were subjected to index, compaction using three energy levels (British Standard light, BSL, West African Standard, WAS or ‘Intermediate’ and British Standard heavy, BSH), shear strength (unconfined compressive strength, UCS), California bearing ratio, CBR and durability tests. Results obtained show that the natural soil is not suitable for road construction. The maximum dry density (MDD) and optimum moisture content (OMC) decreased and increased, respectively. Regardless of the compactive effort and curing period, strength and durability properties increased with higher LBWA content with the BSL effort recording the best improvement. However, based on durability results, the optimal 12.5 % LBWA treatment of black cotton soil did not satisfy criteria for its use in road construction as a stand alone additive. Also, significant improvement in soil properties was obtained using the BSL compactive effort, which is easily achieved in the field. The benefits of the application include reduction in the cost of soil improvement and the adverse environmental impact of locust bean waste.     

Effect of Lime Stabilized Soil Cushion on Strength Behaviour of Expansive Soil

An experimental investigation was undertaken to study the effects of lime-stabilized soil-cushion on the strength behavior of expansive soil. In the present investigation, a series of laboratory tests (Unconfined compression tests and CBR tests) were conducted on both expansive soil alone and expansive soil cushioned with lime-stabilized non-expansive cohesive soil. Lime contents of 2, 4, 6, 8 and 10% by dry weight of cohesive non-swelling soil was used in the stabilized soil cushion. Both expansive soil and lime stabilized soil cushion were compacted to Standard Proctor’s optimum condition with thickness ratio 2:1. Tests on cushioned expansive soils were conducted at different curing and soaking periods i.e., 7, 14, 28 and 56 days. The test results revealed that maximum increase in strength was achieved after 14 days of curing or soaking period with 8% of lime content.     

膨胀土与红黏土石灰改性对比试验研究

为探讨石灰改性膨胀土与红黏土的强度发展规律,以生石灰与消石灰改性的南阳膨胀土与郴州红黏土为研究对象,进行了无侧限抗压、固结快速直剪和固结压缩试验的对比研究。研究发现：在1 a养生龄期内,石灰改性的南阳膨胀土与郴州红黏土无侧限抗压强度与养生龄期的对数基本呈线性关系;按大于最佳含水率3%制样的强度在养生28 d以后高于按最佳含水率制样;生石灰改性效果比消石灰改性效果好,由于矿物成分不同,石灰改性南阳膨胀土的效果比石灰改性郴州红黏土好     

Effects of soil pulverization quality on lime stabilization of an expansive clay

Lime stabilization is an effective way of stabilizing expansive clays, which cause significant environmental problems both as earth and foundation materials. There are considerable environmental benefits in using the in situ lime-stabilized expansive soils in the construction of road pavements, fill or foundations instead of importing valuable granular materials. However, due to high plastic nature of these clays, achieving appropriate pulverization in field applications is a difficult task. This paper presents the results of a laboratory investigation to determine the effects of soil pulverization quality on lime stabilization of a local expansive clay. Effect of mellowing the soil–lime mixtures for 24 h was also studied to find out whether this would compensate for poor pulverization. The clay studied had swelling pressures varying between 300 and 500 kN/m² and free swell potential as high as 19%. In this study, 3, 6 and 9% lime by dry weight were used for lime-stabilized samples. Unconfined compression strength, failure strain and Secant Elasticity Modulus values were measured through unconfined compression strength testing. The results of the study showed that lime stabilization improved plasticity, workability, compressive strength, elastic moduli and swelling and compressibility behavior of the expansive clay. While mellowing did not have a definite effect on the measured strength and moduli values, soil pulverization quality considerably affected the unconfined compression strength and Secant Elasticity Modulus values. The higher the percentage passing No. 4 sieve, the higher the effectiveness of lime treatment. Based on the data obtained in this study, two original equations were derived to assign Secant Elasticity Modulus based on unconfined compression strength, for different soil pulverization qualities. Microfabric investigations conducted by Environmental Scanning Electron Microscope and Mercury Intrusion Porosimetry exposed the effect of lime stabilization on fabric, porosity and pore size distributions. The results of the study clearly demonstrated that if enough time and effort were not given to soil pulverization process in lime stabilization works in field applications, lower performance and therefore increased environmental problems should be expected.     

Assessment of Cement Kiln Dust-Treated Expansive Soil for the Construction of Flexible Pavements

An expansive soil (black cotton soil) treated with up to 10 % cement kiln dust (CKD), a waste obtained from the manufacture of cement, was evaluated for use as a flexible pavement construction material. Laboratory tests were carried out on specimens compacted with British Standard light, British Standard light or standard Proctor (relative compaction = 100 %) energy. Results obtained show that the index properties of the soil improved with CKD treatment. Peak unconfined compressive strength of 357.07 kN/m² and California bearing ratio (CBR) of 7 % as well as resistance to loss in strength of 44 % were recorded at 10 % CKD treatment. Reduction in the particle sizes with curing period was observed when samples were viewed through the scanning electron microscope. The study showed that CKD can be beneficially used to improve the subgrade of lightly trafficked roads and as admixture in lime stabilization during construction of flexible pavements over expansive soil.     

冻融循环影响风化砂改良膨胀土抗剪强度室内试验研究

本文以三峡库区广泛分布的风化砂来改良膨胀土,通过在膨胀土中掺入10%、20%、30%、40%、50%的风化砂,进行1、3、6、9、12次冻融循环,最后对不同冻融循环次数作用后改良膨胀土制备样进行直接剪切试验,测定不同掺砂量及不同冻融次数下,各试样的强度参数,得出冻融循环对风化砂改良膨胀土抗剪强度指标及强度规律的影响.试验结果表明:在同一掺砂比例下,风化砂改良膨胀土的抗剪指标和抗剪强度随冻融循环次数的增大而减小,最后逐渐趋于稳定,其中黏聚力与冻融循环次数呈对数关系; 在同一冻融循环次数下,黏聚力随掺砂比例的增大而减小,内摩擦角随掺砂比例的增大而增大.     

Gypsum Induced Strength Behaviour of Fly Ash-Lime Stabilized Expansive Soil

Physical and engineering properties of soil are improved with various binders and binder combinations. Fly ash and lime are commonly used to improve the properties of expansive soils. An attempt has been made, in this paper, to examine the role of gypsum on the physical and strength behaviour of fly ash-lime stabilized soil. The change in strength behaviour is studied at different curing periods up to 90 days, and the mechanism is elucidated through pH, mineralogical, microstructural and chemical composition study. The strength of soil-fly ash mixture has improved marginally with the addition of lime up to 4 % lime and with curing period for 28 day. Significant increase in strength has been observed with 6 % lime and enhanced significantly after curing for 90 days. The variations in the strength of soil with curing period is due to cation exchange and flocculation initially, and binding of particles with cementitious compounds formed after curing. With addition of 1 % gypsum to soil-fly ash-lime, the strength gain is accelerated as seen at 14 day curing. The accelerated strength early is due to formation of compacted structure with growth of ettringite needles within voids. However, strength at curing for 28 day has been declined due to annoyance of clay matrix with the increase in size of ettringite needle; and again increased after curing for 90 days. The rearrangement of clay matrix and suppression of sulphate effects with formation of cementitious compounds are observed and found to be the main responsible factors for strength recovered.     

干湿循环路径下弱膨胀土峰值及残余强度演化研究

膨胀土具有胀缩性、多裂隙性和超固结性,在自然条件下极易受到降雨和蒸发的干湿循环效应,土的抗剪强度会随着时间的延续而衰减,造成边坡失稳。以荆门弱膨胀土为研究对象,对经历不同干湿循环次数的荆门弱膨胀土开展环剪试验,并探讨分析其峰值强度和残余强度的变化规律。试验结果表明,试样的抗剪强度与法向压力的大小有关,无论是峰值强度还是残余强度均随着法向压力的增大而增大,同时法向压力越大,试样达到残余强度时所需要的剪切位移也会越小;随着干湿循环次数的增加,膨胀土的峰值强度明显衰减,残余强度虽略有变化但并不明显,可近似认为稳定;经历3次干湿循环后膨胀土的峰值黏聚力和残余黏聚力指标已经近乎一致,峰值内摩擦角和残余内摩擦角之间始终保持在2o左右的差异,基本不受循环次数的影响。     

Analysis of a progressive slope failure in the Xiangjiaba reservoir area,Southwest China

The rehabilitation construction in the reservoir area of Xiangjiaba hydropower station in Southwest China has caused many landslides. A shallow progressive failure that occurred on the resettlement site of Xin’an Town of Pingshan County in Sichuan Province was selected as a case study. This landslide occurred in a long and gently inclined area by slope excavation under rainfall conditions. It is about 3.0 m deep with a total length of 35 m and is composed of some subfailures. Undisturbed samples were retrieved, on which the basic properties, shear strength, expansive potential, mineral compositions, and microstructures were tested and analyzed. The results show (1) the landsliding materials belong to medium expansive soil, consisting of the clay minerals of illite–smectite (I/S) and chlorite–smectite (C/S); (2) shear strength of the soil is sensitive to water, which greatly decreases once saturated; and (3) many fractures and relatively large pores are developed in the soils. Back analysis of the landslide shows that the shear strength at failure is less than the residual shear strength obtained from lab tests, indicating that some processes contributing to the slope failure could not be reflected by the shear box test. Based on the above analysis, the progressive process of the slope failure was interpreted, and it is inferred that the rainfall entered into the slope mainly through fractures and relatively large pores in the soil. It caused not only the great decrease in soil strength but also the swelling trend. The latter one would lead to growth, interaction, and coalescence of the fractures. Soon after, these fractures formed the shear planes (zones), which further decreased the resistance of the landslide. Under these favorable conditions, the slope excavation directly triggered the failure.     

Effect of Random Inclusion of Wheat Straw Fibers on Shear Strength Characteristics of Shanghai Cohesive Soil

The effect of random inclusion of wheat straw (fibers) on shear strength characteristics of Shanghai cohesive soil is presented in this paper. 1 year old natural wheat straw (fibers) with four section lengths of 5, 10, 15, 20 mm (aspect ratio: l/d = 1.67, 3.33, 5, 6.67) are used as reinforcement and specific Shanghai cohesive soil is used as medium. Locally available cohesive soil is compacted with standard Proctor’s maximum density with low percentage of reinforcement (0.1–0.4 % of wheat straw sections by weight of oven-dried soil). A series of direct shear tests were conducted on unreinforced as well as reinforced soil to investigate the shear strength characteristics of wheat straw-reinforced soil. The test results show that the inclusion of randomly distributed wheat straw sections (fibers) in soil increases the shear strength to one degree or another. It is noticed that the optimum wheat straw (fiber) content for achieving maximum shear strength is 0.3 % of the weight of oven-dried soil for wheat straw fiber length 15–20 mm (aspect ratio: 5–6.67).     

酸碱环境对水泥土抗剪强度影响的试验研究

以水泥土处于复杂环境中的安全性和耐久性为研究背景,对酸碱环境下水泥土进行了固结不排水剪切试验,得到了水泥土应力偏差（σ₁-σ₃）与轴向应变（ε）关系曲线和抗剪强度指标,研究了酸碱度对水泥土抗剪强度的影响.试验结果表明,随着围压、水泥掺量、pH值的增加,水泥土的抗剪强度逐步增加;水泥土的黏聚力、内摩擦角随着水泥掺量及酸碱度pH值的增大而逐渐增加;得到了不同酸碱度环境下水泥土的抗剪强度方程.其研究结果对在水泥土工程中的安全性和耐久性设计具有一定的指导意义和参考价值.     

Effect of polypropylene fibre and lime admixture on engineering properties of clayey soil

In order to reduce the brittleness of soil stabilized by lime only, a recent study of a newly proposed mixture of polypropylene fibre and lime for ground improvement is described and reported in the paper. To investigate and understand the influence of the mixture of polypropylene fibre and lime on the engineering properties of a clayey soil, nine groups of treated soil specimens were prepared and tested at three different percentages of fibre content (i.e. 0.05%, 0.15%, 0.25% by weight of the parent soil) and three different percentages of lime (i.e. 2%, 5%, 8% by weight of the parent soil). These treated specimens were subjected to unconfined compression, direct shear, swelling and shrinkage tests. Through scanning electron microscopy (SEM) analysis of the specimens after shearing, the improving mechanisms of polypropylene fibre and lime in the soil were discussed and the observed test results were explained. It was found that fibre content, lime content and curing duration had significant influence on the engineering properties of the fibre–lime treated soil. An increase in lime content resulted in an initial increase followed by a slight decrease in unconfined compressive strength, cohesion and angle of internal friction of the clayey soil. On the other hand, an increase in lime content led to a reduction of swelling and shrinkage potential. However, an increase in fibre content caused an increase in strength and shrinkage potential but brought on the reduction of swelling potential. An increase in curing duration improved the unconfined compressive strength and shear strength parameters of the stabilized soil significantly. Based on the SEM analysis, it was found that the presence of fibre contributed to physical interaction between fibre and soil whereas the use of lime produced chemical reaction between lime and soil and changed soil fabric significantly.     

吸力对弱膨胀土强度贡献的试验研究与预测分析

对部分应用土-水特征曲线来预测非饱和土抗剪强度的公式进行了归纳分析。应用压力板仪与非饱和三轴仪,测试了荆门原状弱膨胀土的土-水特征曲线和控制吸力的非饱和三轴抗剪强度参数,并将试验结果与各抗剪强度公式的预测值进行对比,分析了各强度公式的局限性。试验结果表明,非饱和原状膨胀土的净法向应力摩擦角随着吸力的不同而变化,根据双应力变量理论确定的吸力对强度的贡献与围压有关,不同围压下吸力对强度的贡献不同,表观凝聚力 与吸力间符合乘幂函数关系。     

红黏土承载比的土团尺寸效应研究

高液限红黏土用于路基填筑时,因其不良的水理特性,需要掺石灰或水泥改良。但红黏土成团现象十分突出,进行灰土拌和时不易达到均匀状态,影响预期改良效果。通过对不同直径的红黏土土团及石灰改良土土团进行承载比试验,结果表明：干密度随着土团直径尺寸的增大呈现先增大后减小的特征,红黏土和石灰土的最大干密度对应的土团直径处在0.2～ 5 mm范围内;而最大承载比CBR值对应的土团直径分别处在5～10 mm和2～5 mm的范围内。石灰土和红黏土的吸水率、膨胀率均随着土团尺寸的增大,呈现先减小后增大的整体变化趋势。在2～10 mm的范围内,两种土的膨胀量最小。石灰改良只对直径小于5 mm的土团的膨胀特性起到明显的抑制作用。可见,现场施工中严格控制土团的大小对保证土体的强度和水稳定性具有十分重要的意义。     

干湿循环对非饱和膨胀土抗剪强度影响的试验研究

膨胀土是一种气候敏感性土体,研究在干湿循环过程中膨胀土剪切强度的变化,对了解在自然界周期性蒸发和降雨作用下原位膨胀土体工程性质的变化以及由此导致的地质灾害发生过程具有重要意义。文中以重塑非饱和膨胀土为研究对象,模拟了3次干湿循环过程,对每次干燥路径中的试样进行了直剪试验,重点分析含水率、正压力及干湿循环次数对膨胀土剪切强度的影响,得到如下主要结果：（1）在干燥过程中,随着含水率的减小,试样的刚度、脆性、抗剪强度值（峰值剪切应力）、抗剪强度指标（黏聚力、内摩擦角）及抗剪强度损失（峰值强度与残余强度之差）均呈增加趋势;（2）正压力越高,试样的剪切强度和残余强度越大,而破坏后的峰值强度损失越小,破坏韧性增加;（3）在3次干燥过程中,试样的剪切强度及黏聚力呈先增加后减小的趋势,在第二次干燥过程中达到峰值,但内摩擦角受干湿循环的影响无明显规律;（4）试样经历多次干湿循环后,其剪切特性越来越类似于超固结土,脆性显著增加;（5）干燥过程和干湿循环对试样残余剪切强度的影响都不明显,残余剪切强度基本都在100 kPa附近变化;（6）非饱和膨胀土在干湿循环及干燥过程中剪切强度的变化除了与吸力有关外,还与其微观结构调整和裂隙发育状态密切相关,需要综合非饱和土力学和土质学理论对试验现象进行分析。