Nonlinear Mechanical Behavior Analysis of Flexible Lateritic Pavements of Senegal (West Africa) by FEM for M.-E. Pavement Design

Pavement design in Senegal is based on a linear elastic behavior of pavement materials and the hypothesis of a static loading. However, previous works on the mechanical behavior of road materials showed that this one is reversible after several loading cycles and depends on the applied stress. The described behavior is from then on, purely nonlinear. One of the objectives of this research is to determine the parameters of response of lateritic pavement materials submitted to road traffic by using FEM. Therefore, experiments were made on gravel lateritic soils from Dougar, Sébikotane, Mont-Rolland, Pâ Lo and Ngoundiane. The Young’s modulus of the materials was defined in unconfined compression test while repeated load triaxial test was performed to determine the resilient modulus of the gravels and the appropriate model (Uzan model). An implementation was realized with Cast3M©. The importance of the nonlinearity was revealed in a very clear way and was crucial in the construction of the calculation algorithm. The observations for certain conditions showed that the values of the critical responses are more important for the linear model than for the nonlinear model. However, this trend should be validated by further studies.     

A new approach for stabilization of lateritic soil with Portland cement and sand: strength and durability

The presence of lateritic soils occurs in tropical and subtropical regions. The improvement of lateritic soils that are not suitable for a particular purpose through techniques that combine modification of grain size through the insertion of sand, incorporation of Portland cement and densification through compaction is seen as an alternative. In this context, a dosage method to use a local lateritic soil as construction material in a most rational way reducing the economic and environmental impacts related to this activity is still missing. Therefore, the current research aims to evaluate the performance of a lateritic soil via modification of grain size through the insertion of sand, incorporation of Portland cement and densification through compaction. For this, unconfined compression, and durability (wetting and drying) tests were carried out on specimens of compacted clayey gravel lateritic soil, whose granulometry was modified by the insertion of distinct amounts (from zero to 45%) of weathered sand, treated with distinct Portland cement contents (from 4 to 10%), molded at different dry unit weights (from 16.8 to 20.1 kN/m³) and cured for 7 and 28 days. Results of the mechanical tests have shown the significant influence exerted by cement content and dry unit weight of the blend, followed by curing time and finally sand insertion. Satisfactory correlations between the response variables (q_u and ALM) and the adjusted porosity/cement index (η/C_v) were obtained. Furthermore, an innovative approach to replacing the laborious durability test is proposed.

     

Properties of Ironstone Lateritic Gravels in Relation to Gravel Road Pavement Construction

A laboratory investigation was conducted to determine properties of five samples of ‘oolitic ironstone’ lateritic gravel aggregates which are relevant to their use as road construction materials. Measured values of physico-chemical, physico-mechanical as well as index properties and compaction characteristics are similar to those of other lateritic, gravels occurring in West Africa which are used in road pavement applications. These lateritic clayey gravels have good workability as engineering construction materials and are rated fair to good as road aggregates in terms of probable in-situ behaviour based on water absorption values. Relationships between maximum dry unit weight and optimum moisture content as well as those between California bearing ratio and some derived soil parameters such as grading modulus, plasticity product or plasticity modulus can be described using a third-order polynomial function. The maximum dry unit weight has a bimodal frequency distribution and can be predicted from logarithm to base 10 of compaction energy with fairly good accuracy.     

Laboratory Comparison of Crushed Aggregate and Recycled Pavement Material With and Without High Carbon Fly Ash

In-place recycling of asphalt pavement materials is a sustainable rehabilitation method. Existing hot-mix asphalt (HMA) layer is pulverized and blended with some or the entire base course and possibly some subgrade to form a broadly graded material referred to as recycled pavement material (RPM). The RPM is then compacted as the new base course and overlaid by a new layer of HMA. In some occasions, additives are added to increase the strength of RPM base course, such as cement, emulsion, fly ash. It is plausible to utilize high calcium high carbon fly ash, as the high level of carbon prevents fly ash from being used in concrete. A series of laboratory tests were conducted to evaluate the performance of these materials, including crushed aggregate, untreated RPM, and treated RPM with high carbon fly ash. The tests included compaction, California Bearing Ratio, resilient modulus, and unconfined compressive strength for treated RPM. The engineering properties of these materials were compared.     

压实黄土回弹模量试验研究

土基回弹模量是路面结构设计中一个非常重要的参数,直接关系到路面结构的安全性和经济性。室内振动压实是室内击实成型试件的一种新方法,能较好地模拟振动压实的特点。研究了压实黄土回弹模量与其干密度、含水率、稠度和压实度以及龄期等物理指标之间的关系,并分析了振动压实黄土的回弹模量与击实黄土的回弹摸量。研究结果表明,压实黄土的回弹模量与其干密度、含水率、稠度和压实度以及龄期之间有一定的相关关系。在推荐参数条件下,室内振动压实黄土回弹模量比室内击实的要大。     

Effects of water and fines contents on the resilient modulus of the interlayer soil of railway substructure

This paper deals with the resilient behavior of the interlayer soil which is created mainly by the interpenetration of ballast and subgrade soils. The interlayer soil studied was taken from a site in the southeast of France. Large-scale cyclic triaxial tests were carried out at three water contents (w = 4, 6 and 12 %) and three fines contents corresponding to 5, 10 % subgrade added to the natural interlayer soil and 10 % fine particles (<80 μm) removed from the natural interlayer soil. Soil specimens underwent various deviator stresses, and for each deviator stress, a large number of cycles was applied. The effects of deviator stress, number of cycles, water content and fines content on the resilient modulus (M _r) were analyzed. It appears that the effects of water content and fines content must be analyzed together because the two effects are closely linked. Under unsaturated conditions, the soil containing high fines content has higher resilient modulus due to the contribution of suction. When the soil approaches the saturated state, it loses its mechanical enhancement with a sharp decrease in resilient modulus.     

Stabilization of lateritic soils with phosphoric acid

Summary This paper describes a laboratory study on the stabilization of lateritic soils with phosphoric acid-H₃PO₄. This method is most promising for road and airport pavement construction in tropical regions where fine textured lateritic soils (red clays and silts) occur over large areas. The iron and aluminum phosphates formed are hard and insoluble. The main source of iron is free iron oxide, and the aluminum sources are free aluminum oxide, exchangeable cations and clay minerals (hydrated aluminum silicates). Four different soil samples were studied, but the most comprehensive study was carried out with a lateritic soil evolved from weathered basaltic bedrock. The variables of the test specimens were: percentage of acid, moulding water content, compaction energy, and curing time. Strength tests performed were the axial or unconfined compression test and the indirect tensile or diametrical compression test. With 5% of phosphoric acid to dry weight of soil, values of compressive strength around 4.0 MPa were obtained after 28 days curing.     

水泥改良高液限黏土动态回弹模量试验研究

利用动三轴试验,研究了水泥改良高液限黏土动态回弹模量的影响因素及其变化规律。研究表明,动态回弹模量随围压、压实度和水泥含量的提高而增大,随循环偏应力和含水率的增大而减小。为分析动态回弹模量的应力依赖性,研究采用了双因素方差分析。分析表明,偏应力与围压均对动态回弹模量有显著性影响,但偏应力的影响更为显著。鉴于动态回弹模量是偏应力和体应力的函数,在分析现有动态回弹模量本构模型适应性的基础上,采用偏应力和体应力为变量的3参数复合模型对试验数据进行回归分析,结果表明,所选模型具有较高的决定系数,证明所选模型具有较高的合理性与可靠性。研究获得了不同含水率、压实度和水泥剂量下水泥改良高液限黏土的动态回弹模量预估模型,为基于动力学的路面结构设计提供了参数。     

Small-Strain Shear Modulus of Cement-Admixed Kaolinite

An experimental study is conducted to measure small-strain shear modulus of clay-cement mixture using bender element apparatus setup in a triaxial cell. Bender element tests were conducted on cement-treated soils and the results were analyzed to study the variation of shear modulus properties of soil specimens at different cement contents, confining pressures, curing times, and compaction moisture contents. Based on the obtained results increasing the cement ratio has a significant effect on the small-strain shear modulus of the treated soils, and this effect signifies with increasing the moisture content and curing time. Rates of shear modulus enhancements due to cement content, curing time, and compaction moisture content are quantified and presented. In this study, a clay–cement–water ratio formulation is proposed that enables one to calculate cement and water contents required to obtain specific small-strain shear modulus.     

高聚物堆石料回弹特性试验

基于中型三轴仪开展了高聚物堆石料回弹模量试验,分析了高聚物堆石料卸载－再加载的力学特性和回弹模量变化规律。结果表明：围压和应力水平对体积变化规律有明显影响,随着试验围压的增大,试样由初始状态的剪胀逐渐变为剪缩;高围压下,随着应力水平的增大,试样的卸载体缩量呈非线性增大;而围压在100 kPa下,卸载时表现为体胀,不同应力水平下的卸载体缩量相近且较小。平均回弹模量与初始模量的比值大约在3.9～4.2之间,应力水平为0.7条件下对应的回弹模量与平均回弹模量较为接近。高聚物堆石料的回弹模量满足邓肯-张模型关系,数值计算过程中回弹模量系数近似为初始模量系数的4.0～4.2倍。     

Determination of unconfined compressive strength and Young's modulus of porous materials by indentation tests

The formation of a compacted zone under the indenter seems to be the major factor controlling the indentation process in porous rocks. In the case of very porous materials, where the pore structure fails and deformation (by structural collapse) proceeds with almost no increase in the applied load and with very limited damage to the surrounding material, no chipping is observed. The extent of the compacted zone is controlled by the porosity of the material and by the strength of its porous structure. This paper presents an interpretation model developed by the authors to obtain the uniaxial compressive strength of porous materials from the results of indentation tests. It is based on the model proposed by Wilson et al. (Int. J. Mech. Sci., 17, 1975, 457) for the interpretation of indentation tests on compressible foams and on an estimation by the authors of the extent of the compacted zone under the indenter. The results of indentation tests can also be used to obtain the Young's modulus of the material with a model proposed by Gill et al. (Proceedings of the 13th Canadian Symposium on Rock Mechanics, 1980, 1103). Uniaxial compression and indentation tests have been performed on artificial porous materials showing porosities varying between 44 and 68%. The uniaxial compressive strength values obtained from both types of test show a very good agreement. For the Young's modulus, the values obtained from the two types of test are different but the variation of the moduli with porosity is the same. Finally, a parameter called permanent penetration modulus is proposed as a means of characterizing the uniaxial compressive strength of porous materials.     

考虑吸力效应的非饱和黏土回弹模量预估模型

回弹模量是反映路基弹性支撑能力的重要指标,并受运营期间基质吸力变化的显著影响。通过3种压实黏土试样的土-水特征曲线试验和重复动三轴试验,研究了回弹模量与基质吸力的相关性,并基于非饱和土有效应力原理,提出了考虑吸力效应的回弹模量预估模型。结果表明：基质吸力与回弹模量之间具有较强的非线性相关性,可采用指数形式进行两者的拟合;含水率由最佳含水率-4%提升至最佳含水率+4%时,受基质吸力减小的影响,回弹模量降幅达到29.1%～39.0%;在 土-水特征曲线过渡区,采用以饱和度表征的有效应力参数,可较好地反映基质吸力对于回弹模量的贡献,预估效果优于采用残余含水率或进气值相关的有效应力参数;在相同饱和度的情况下,黏粒含量越多、塑性指数越高,基质吸力对有效应力的贡献比例越小。     

Wave-Based Techniques for Evaluating Elastic Modulus and Poisson’s Ratio of Laboratory Compacted Lateritic Soils

A rapid, low-cost evaluation methodology using the wave-based techniques is proposed in this study in order to determine the design parameters e.g., elastic modulus and Poisson’s ratio of the laboratory compacted lateritic soils. Knowing the elastic wave velocities as measured with the wave propagation technique (i.e., a small-strain non-destructive test) and total mass density of the specimens, the elastic moduli and Poisson’s ratio of the soil specimens can be determined. In addition, the unconfined compression test (i.e., a large-strain destructive test) is also performed on the same specimens under the same unconfined testing condition in order to compare the moduli corresponding to different strain levels. The experimental results showed the potentials and limitations of using impulse signal for the determination of the elastic moduli and Poisson’s ratio for laboratory compacted soil specimens from elastic wave propagation techniques.     

Repeated Load Triaxial Testing of Recycled Concrete Aggregate for Pavement Base Construction

This paper presents the results and analysis of Repeated Load Triaxial Testing (RLTT) conducted on three basecourse products; two crushed concrete, or Recycled Concrete Aggregate (RCA) materials, and a local Virgin Aggregate (VA). The objective of this testing program was to study the impact of applied stress states on resilient modulus and permanent deformation of the materials using the AUSTROADS RLTT approach (Vuong and Brimble 2000). A series of RLTT was performed under drained conditions on cylindrical specimens statically compacted at different levels of initial moisture content. The RLTT specimens were tested at 60, 80 and 90 % of Optimum Moisture Content (OMC) and to a single target dry density ratio of 98 % of Maximum Dry Density from Modified Proctor compaction testing. Duplicate specimens were tested for each moisture level. On sample measurements were made of sample deformation. It was found that the resilient moduli of the two RCA products ranged between 340 and 715 MPa, while corresponding moduli from tests on VA varied from 270 to 450 MPa. Resilient modulus was found to be dependent on both moisture content and applied stress. The two specimens of VA prepared at 90 % OMC failed during the permanent strain testing stages of the AUSTROADS test protocol. Moreover the recycled materials could be used as basecourses according to interpretation of the AUSTROADS RLTT data by Vuong and Arnold (2006).     

Geotechnical characterisation of lateritic soils from south-western Nigeria as materials for cost-effective and energy-efficient building bricks

Lateritic soils which have been described as highly weathered tropical or subtropical residual soils were studied with an attempt to establish its suitability or otherwise as sustainable material in building bricks and housing development that will meet the present challenge of sustaining the environment without costing too much and maintaining a high standard of strength, durability and aesthetics. Index properties of the tested lateritic soils revealed them as mostly well graded, comprising both cohesive (silt and clay) and cohesionless (sand and gravel) soil fraction. The geotechnical analyses on the studied lateritic soil revealed a strong compressive strength with a relatively sound dry density which could guarantee a good durability in resulting bricks made from these soil materials. Further test on the strength and durability of the compressed earth bricks (CEBs) made from these lateritic soils revealed a brick with compressive strength ranging between 6.33 and 15.57 MPa which is considered to be of good strength coupled with its sound durability strength established over a period of more than one year under a complete cycle of weather and seasonal conditions. In conclusion, lateritic soils from the study area were found to be suitable as materials for bricks (CEB) with good compressive and durability strength which qualifies them as sustainable and cost-effective materials for low-cost housing development.     

Use of lateritic soils for road construction in North Dahomey

Due to the deterioration of the Parakou—Malanville highway, the preparation of a rehabilitation program was necessary. The related geotechnical exploration provided an excellent opportunity to study the properties of the encountered lateritic soils. The mineralogical analysis proved that the lateritic overburden originates from the local rocks. However, the analyses also indicated that the soils can only be identified as lateritic by virtue of their hematite content. The soils were classified according to the AASHO-system and their geotechnical properties were thoroughly investigated. The results revealed that the overwhelming majority of the available soils was unsuitable for highway construction due to their high plasticity. In order to improve them, cement stabilization was suggested. The laboratory tests resulted in an economically feasible solution.     

Impact of Carpet Waste Fibre Addition on Swelling Properties of Compacted Clays

Municipalities and recycling and environmental authorities are concerned about the growing amount of carpet waste produced by household, commercial and industrial sectors. It is reported that 500,000 tonnes of carpet waste fibre are plunged into landfills annually in the UK. In the United States of America, around 10 million tonnes of textile waste was generated in 2003. In geotechnical engineering, expansive clay soils are categorised as problematic soils due to their swelling behaviour upon increase in the moisture content. The problematic nature of such soils is intensified with the increase in the plasticity index. This paper presents results of a comprehensive investigation into utilisation of carpet waste fibres in order to improve the swelling characteristics of compacted cohesive soils. Therefore, two different clay soils with markedly different plasticity indices (i.e. 17.0 and 31.5 %) were treated with two different types of carpet waste fibre. Waste fibres were added to prepare specimens with fibre content of 1, 3 and 5 % by dry weight of soil. Soil specimens with different dry unit weights and moisture contents were prepared so as to the swelling behaviour of fibre reinforced compacted clays is completely attained under various scenarios. The results indicated that the behaviour of the fibre reinforced soils seems highly dependent on the initial compaction state and secondary on the moisture content. It was found that the swelling pressure drops rapidly as the percentage of fibre increases in samples prepared at the maximum dry unit weight and optimum moisture content. Reducing the dry unit weight, while maintaining constant moisture content or increasing the moisture content at constant dry unit weight was found to reduce the swelling pressure.     

Stability analysis of lateritic waste deposits

In open-pit mines covered with lateritic soil, the sterile ground is stored in specially prepared sites. The waste area must then be enhanced in order to extend storage capacity.

To better understand the effect of an elevated dump, typical laboratory tests (triaxial and oedometer tests) have been performed on the lateritic material. The waste disposal site has been modeled using both limit equilibrium and finite element methods. Different types of finite element models have been run; they take the variation of Young's modulus with depth into account and simulate the effect of construction over various phases. Modeling results are then compared with in situ measurements.     

Effect of freeze—thaw cycles on resilient properties of fine-grained soils

Stress-deformation data for silt and clay subgrade soils were obtained from in-situ tests and laboratory tests, for use in mechanistic models for design of pavements that will experience freezing and thawing. Plate-bearing tests were run on in-service allbituminous-concrete (ABC) pavements constructed directly on silt subgrade, and on an experimental ABC pavement constructed on clay subgrade, applying repeated loads to the pavement surfaces while the subgrade was frozen, thawing, thawed, and fully recovered. The in-service pavement had experienced several seasons of natural cyclic freezing and thawing, while the experimental pavement was artificially frozen and thawed twice. Repeated-load laboratory triaxial compression tests were performed on the same soils in the frozen and thawed states.

Analysis of deflection data from the in-situ tests showed resilient moduli of the subgrade soils up to more than 10 GPa when frozen, as low as 2 MPa during the thawing period, and up to more than 100 MPa when fully recovered. Analysis of the laboratory tests, which gave moduli comparable to the latter values, showed that resilient modulus and Poisson's ratio in the thawed and recovering conditions can be expressed as a function of the stress state, the moisture content, and the dry density.     

Deformability characteristics of brazilian laterites

This paper summarizes the resilient and permanent deformation characteristics of laterite gravels, determined by cyclic loading in triaxial tests of compacted specimens. A study of laterite pavements was carried out on eight paved road sections in the state of Mato Grosso. Also reported is the influence of fine sand admixture, specimen size and soaking in two laterite samples from Roraima and Rondônia in the North of Brazil. The experimental relationships of resilient moduli against applied stresses are interpreted empirically for models what may be incorporated in a structural analysis of full depth of laterite pavement.