Repeated Load Triaxial Tests Research Articles

Characterisation of stress and moisture-dependent resilient behaviour for compacted clays in South China

In order to predict the resilient modulus of compacted clays, the material indicator tests, repeated load triaxial test, and pressure plate test for compacted clays from South China were carried out. The soil–water characteristic curve (SWCC) was described using the Fredlund and Xing’s model. And a logarithmic function between the soil suction and resilient modulus was built. Then, a new variable named the minimum bulk stress was defined to separate the shear effect of soil samples from the bulk stress, which avoids the bulk stress reflects two contrary effects, namely hardening effect and softening effect. Subsequently, the influences of the degree of compaction, soil suction, minimum bulk stress, and octahedral shear stress on the resilient modulus were analysed. In the following, a new resilient modulus prediction model of compacted clays, which took the soil suction, minimum bulk stress, and octahedral shear stress as the model variables, was developed and verified using the data of different cohesive soils from this study and existing literatures. The results show that the new model matched these data well and have a high prediction accuracy, which indicates that this new model is reasonable and widely applicable.

Variation of Resilient Modulus with Soil Suction for Cohesive Soils in South China

The resilient modulus, MR, of subgrade soils had been used extensively as an important unbound material property in the pavement design and analysis. The stress state and the humidity of cohesive soils have significant impacts on the resilient modulus. To predict the resilient modulus of cohesive soils in South China, the material indicator tests, an improved repeated load triaxial test and the pressure plate test were carried out. Then the SWCC was described using the Fredlund and Xing’s model with a relatively high coefficient of determination. Subsequently, the variation of resilient modulus with the stress state and the humidity characterized by the matric suction for cohesive soils were evaluated. A correlation between the matric suction and resilient modulus of cohesive soils was built, which has high correlation coefficients. In the following, a new resilient modulus estimation model for cohesive soils in South China incorporating the stress state and matric suction was proposed, which has a simple form, less parameters and only requires a universal triaxial testing system. After that, the new model was verified using the data of different cohesive soils from existing literature. The results show that the developed model has a high prediction accuracy.

Permanent deformation behaviour of pavement base and subbase containing recycle concrete aggregate, coarse and fine crumb rubber

Pavement layers must be sufficiently strong to carry all traffic loads and resist the accumulation of permanent deformations such as rutting and avoid premature failures including top-down cracking during their service lives. In recent years, the utilization of recycled construction and demolition (C&D) materials in civil and transportation infrastructure construction has been considered as a significant solution to replace conventional and natural aggregates, therefore to achieve the goal of building low-carbon footprint constructed facilities. Unfortunately, limited data have yet reported on the permanent deformation behaviour of the recycled materials in pavement, especially the effect of crumb rubber and rubber size with C&D aggregates on the permanent deformation behaviour in base and subbase layers. This paper provides unique laboratory information and testing results in this regard. In this study, two different groups of crumb rubber particles with sizes ranged from 400 to 600 µm (fine) and 15–20 mm (coarse) were separately added to 20 mm recycled concrete aggregate (RCA) and 20 mm crushed rock (CR) at 0.5, 1 and 2% by weight of the aggregates to study the effects of crumb rubber and rubber size on permanent deformation behaviour of RCA and CR aggregates. In particular, the permanent deformation behaviour of the RCA/CR with crumb rubber was investigated through the repeated load triaxial tests. It was observed that the CR samples should be avoided for use in base and subbase courses.

Resilient and permanent deformation behaviour of clayey subgrade soil subjected to repeated load triaxial tests

Mechanistic-empirical design of flexible pavement AASHTO practice requires resilient characteristics of sub grade soil. Though the resilient modulus is assumed to account for plastic strain accumulation, some subgrade soils with more fine content undergo excessive plastic deformation under repeated loads even though they have remarkable resilient properties. Hence permanent deformation is also an important parameter which should be considered in the design process. In the present study, permanent deformation behaviour of clayey subgrade soil with intermediate compressibility is studied using repeated load triaxial tests. The influence of confining pressure, moisture content and deviator stress levels on the permanent deformation behaviour is examined. The transition of the soil sample from stable to unstable state can be observed and critical stress levels established with the help of shake down theory. Two regression models reported in the literature, Power law model and VTT model, are used to predict the plastic strains with a number of load cycles and stress levels as the variables.

Permanent deformation parameters of fine – grained tropical soils

In this work, the permanent deformation response of fine-grained soils of different classifications was studied through repeated load triaxial tests under different stress states for a number of load cycles greater than 150.000. Soils were moulded at optimum water content compressed with an amount of energy equivalent to intermediate or normal Proctor. Seven fine-grained soils were selected with the following MCT (Miniature, Compacted, Tropical) classifications: four lateritic clayey, one lateritic sandy, one non lateritic sand and one non lateritic silt. The values of total permanent deformation are presented. The parameters of deformability Ψi were estimated. The permanent deformation calculations were simulated for three different stress levels: low, medium and high.

Unsaturated mechanical behaviour of a granular material

This paper presents the influence of the unsaturated state and especially the suction on the permanent and resilient behaviour of a granular material for roads. In this context, the soil water retention curves (SWRCs) of a compacted clayey sand with two different fine contents are initially obtained to deduce the suction corresponding to a given water content. A parameter s* is defined as the suction value corresponding to the intersection point of wetting and drying paths in the SWRC. The permanent deformation and resilient deformation under repeated loading with different water contents and different fine contents are then studied by repeated load triaxial tests. Based on the experimental results, it can be stated that the hydraulic conditions affect strongly both permanent and resilient deformations of the granular materials. Finally, a series of equations can be proposed to describe the correlations between suction ratio s/s* and permanent and resilient deformations of the granular material at different water contents and different fine contents. Generally, the prediction models can effectively capture the permanent and resilient behaviours based on the suction ratio s/s*. The analytical solutions are unique to estimate permanent and resilient behaviour in the same framework based on the suction value. We recommend the models as improved approaches to reduce the number of tests required to estimate the soil deformation in pavement structures.

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, Sebikotane, 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.

The effect of instrumentation on the determination of the resilient modulus of unbound granular materials using advanced repeated load triaxial testing

Unbound Granular Materials (UGMs) are used in the base/subbase layers of flexible pavement structures for the vast majority of the main roads around the world. The resilient modulus of UGMs is a key input parameter for the design and analysis of flexible pavement structures. In the present study, four road base UGMs with a range of moisture contents are used to evaluate each material’s resilient deformation behaviour using laboratory repeated load triaxial tests. The triaxial system for the tests is instrumented with four axial deformation gauges: an on-specimen axial Hall-Effect transducer, an internal Linear Variable Differential Transformer (LVDT), an external LVDT, and the actuator LVDT. The application of a Hall-Effect transducer directly mounted on the specimen and the three LVDTs permits the comparative study of alternative deformation measurements for the determination of an accurate and reliable resilient modulus value. By comparing tests results obtained with each transducer, the relative capability of each measurement is determined and a reference transducer for deformation measurement is identified. A constitutive model is then used to carry out a regression analysis and to predict the resilient modulus of the four tested materials.

Improved Prediction Model for Dynamic Resilient Modulus of Subgrade Silty Clay in Eastern Hunan and Its Relevant Finite Element Method Implementation

With the enhancement of transportation speed and axle load, dynamic response of subgrade increases significantly. In order to improve the calculation accuracy of subgrade response under complex stress state, it is necessary to use dynamic indicators instead of static indicators in calculative process. For the sake of investigating the influence factor of dynamic resilient modulus of subgrade silty clay in Eastern Hunan, resilient modulus tests were carried out by conducting repeated load tri-axial tests. Based on available model, an improved resilient modulus prediction model considering four parameters was proposed by introducing k4. Corresponding accurate consistent tangent stiffness matrix was derived. Afterward, the improved model was implemented into finite element method software and verification work was put forward both on single element and pavement-subgrade structure. Finally, calculated results were compared with in-site measured results. Study achievements demonstrate that the improved model exhibits a higher precision and efficiency on single element because k4 can better adjust the affecting proportion of octahedral shear stress. When applied to analysis on pavement-subgrade structure, the improved model can reflect subgrade resilient modulus distribution and evolution more factually. In addition, numerical calculated result nearly coincides with measured results, which shows the application value of the improved model.

Effect of Anisotropy on the Resilient Behaviour of a Granular Material in Low Traffic Pavement.

Granular materials are often used in pavement structures. The influence of anisotropy on the mechanical behaviour of granular materials is very important. The coupled effects of water content and fine content usually lead to more complex anisotropic behaviour. With a repeated load triaxial test (RLTT), it is possible to measure the anisotropic deformation behaviour of granular materials. This article initially presents an experimental study of the resilient repeated load response of a compacted clayey natural sand with three fine contents and different water contents. Based on anisotropic behaviour, the non-linear resilient model (Boyce model) is improved by the radial anisotropy coefficient γ3 instead of the axial anisotropy coefficient γ1. The results from both approaches (γ1 and γ3) are compared with the measured volumetric and deviatoric responses. These results confirm the capacity of the improved model to capture the general trend of the experiments. Finally, finite element calculations are performed with CAST3M in order to validate the improvement of the modified Boyce model (from γ1 to γ3). The modelling results indicate that the modified Boyce model with γ3 is more widely available in different water contents and different fine contents for this granular material. Besides, based on the results, the coupled effects of water content and fine content on the deflection of the structures can also be observed.

Characterization and prediction of permanent deformation properties of unbound granular materials for Pavement ME Design

The objective of this study is to characterize and predict the permanent deformation properties of unbound granular materials (UGMs) for Pavement ME Design. First, laboratory repeated load triaxial (RLT) tests are conducted on the UGMs from 11 quarries in Texas to measure the permanent strain curves. The shakedown theory is applied to evaluate the permanent deformation behavior of the selected UGMs. It is found that using Werkmeister’s criteria to define the shakedown range boundaries is not suitable for the selected UGMs. Under this circumstance, new criteria are proposed to redefine the shakedown range boundaries for the flexible base materials in Texas. The new criteria are consistent with the current Texas flexible base specification in terms of aggregate classification. Second, the mechanistic-empirical design guide (MEPDG) model is used to determine the permanent deformation properties of the selected UGMs on the basis of the measured permanent strain curves. The determined permanent deformation properties are assigned as target values for the development of permanent deformation prediction models. Third, a series of performance-related base course properties are used to comprehensively characterize the UGMs, which include the dry density, moisture content, aggregate gradation, morphological properties, percent fines content, and methylene blue value. These performance-related base course properties are assigned as the inputs of the permanent deformation prediction models. Fourth, a multiple regression analysis is conducted to develop the prediction models for permanent deformation properties using these performance-related properties. The developed models are capable of accurately predicting the permanent deformation properties of UGMs. Compared to other prediction models (e.g., simple indicators-based models and Pavement ME Design models), the developed models have the highest prediction accuracy. It is also found that the Pavement ME model-predicted permanent strains are much lower than those measured from the RLT tests. This demonstrates that the current Pavement ME Design software substantially underestimates the rutting that occurs in base course. Finally, the developed prediction models are validated by comparing the predicted and measured permanent strains of other four base materials. The obtained R-squared value of 0.81 indicates that the developed models have a desirable accuracy in the prediction of permanent deformation properties of UGMs.

A new approach for determining resilient moduli of marginal pavement base materials using the staged repeated load CBR test method

The selection of marginal materials to construct workable and durable pavement base materials requires an understanding of their resilient moduli properties. Empirical formulations have been developed previously based on quality quarried material with known performance-based behaviour. However, in recent years, there has been an increasing demand to use locally available marginal materials, which do not necessarily meet road authority specification requirements; hence it is imperative to evaluate the performance of these non-standard materials. In this study, the geotechnical properties of two marginal materials, namely limestone (Ls) and sandstone (Ss), were evaluated and compared with a high-quality rhyolite (Rh) crushed rock. In order to improve the performance of the marginal materials, each of Ls and Ss was blended with 20% Rh and 50% Rh. The geotechnical properties of individual and blended materials which were investigated included: particle size distribution, particle density, Atterberg limits, Los Angeles value, modified compaction test, California bearing ratio (CBR) and repeated load triaxial test. A novel test method termed repeated load CBR (RL-CBR) was used to investigate the performance properties of the individual materials and blends. The RL-CBR test was further developed to study the stress-dependent behaviour of the materials, termed as staged RL-CBR test. An in-depth analysis of the staged RL-CBR test confirmed the capability of this test method to determine the resilient modulus of marginal pavement base materials.

Dense-graded aggregate base gradation influencing rutting model predictions

Abstract This paper presents findings from an ongoing research study at the University of Illinois focused on developing and calibrating an improved permanent deformation model for unbound aggregate materials through laboratory testing and characterization. The project scope included testing sixteen aggregate materials, commonly used in the state of North Carolina for pavement base courses, in the laboratory through monotonic and repeated load triaxial testing. This paper primarily focuses on quantifying effects of aggregate gradation on permanent deformation behavior. To accomplish this, four materials were tested at both: (1) “source gradations,” i.e. original gradations from quarry, and (2) an “engineered gradation,” i.e., standard reference gradation at which aggregate specimens were prepared for testing. Predictive rutting models were developed to consider the influences of shear strength and applied stress states on permanent deformation accumulation. Rutting model parameters obtained from testing aggregate specimen at one gradation could be used to reasonably predict the permanent deformation accumulation in another sample given the shear strength properties did not show notable differences. For specimens corresponding to significantly different amounts and/or plastic nature of fines, the permanent strain levels predicted using one set of model parameters differed significantly from those predicted using another set of model parameters developed for another gradation. Moreover, the effects of gradation on permanent strain accumulation were significantly more pronounced at the higher shear stress ratios (e.g. 0.75), compared to lower shear stress ratios; which is defined as the ratio between the shear stress applied to a specimen during repeated load triaxial testing compared to the corresponding shear strength under the same confinement.

Studies on resilient modulus value from cyclic loading tests for cohesive soil

Abstract In this article the cyclic CBR test as a reference method in determination of resilient modulus (Mr) is confronted with results of cyclic triaxial and unconfined uniaxial cyclic test. The main idea of conducted experiments is establish relationship between cyclic loading tests in testing of natural subsoil and road materials. The article shows results of investigation on cohesive soil, namely sandy silty clay, commonly problematic soil in Poland. The results of repeated loading triaxial test resilient modulus were displayed in order to compare them with cyclic CBR test results by using the Mr–Ө model. Some empirical correlation between factors obtained from triaxial test or uniaxial unconfined cyclic test and cyclic CBR test was introduced here. The behavior of resilient modulus was also examined in this paper.

Evaluating the performance of very weak subgrade soils treated/stabilized with cementitious materials for sustainable pavements

Abstract This paper presents the results of laboratory tests that were conducted to determine the proper treatment/stabilization recipe of very weak subgrade soils at high moisture contents, and to evaluate the corresponding performance-related properties [e.g., the resilient modulus and permanent deformation] for use in the design of sustainable pavement structures. Four different subgrade soil types of different plasticity indices were considered in this study. Three different moisture contents were selected at the wet-side of optimum that correspond to a raw soil strength value of 172 kPa (25 psi) or less. All the soils were treated with different combinations of class C fly ash (or Portland cement type I) and hydrated lime to achieve a 7-day target strength values of 345 kPa (50 psi) to create a working platform and 690 kPa (100 psi) to stabilize the subgrade for subbase application. Repeated load triaxial (RLT) tests were performed on laboratory molded specimens to evaluate their resilient modulus and permanent deformation behavior under cyclic loading. The AASHTO T-307 procedure was followed in this study to conduct the resilient modulus tests. A good correlation was observed between the water/additive ratio and the resilient modulus/permanent deformation, such that the soil specimens prepared at low water/additive ratio showed better performance than those prepared at high water/additive ratio. The results of laboratory tests showed that the use of direct correlation between unconfined compressive strength (UCS) and resilient modulus for cementitiously treated/stabilized soils can be misleading.

Recycled plastic granules and demolition wastes as construction materials: Resilient moduli and strength characteristics

Vast quantities of plastic and demolition wastes are generated annually by municipal and commercial industries in all developed and developing countries. The sustainable usage of recycled plastic and demolition wastes as alternative construction materials has numerous environmental and economic advantages. New opportunities to recycle plastic and demolition wastes into alternative resource materials for construction industries would mitigate landfill issues and significantly reduce global carbon emissions. Infrastructure projects typically consume significant quantities of virgin quarry materials, hence the usage of plastic and demolition wastes as alternative construction materials will divert significant quantities of these wastes from landfills. In this research, three types of recycled plastic waste granules: Linear Low Density Polyethylene filled with Calcium Carbonate (LDCAL), High Density Polyethylene (HDPE) and Low Density Polyethylene (LDPE) were evaluated in blends with Crushed Brick (CB) and Reclaimed Asphalt Pavement (RAP). The blends prepared were evaluated in terms of strength, stiffness and resilient moduli. Resilient moduli prediction models were proposed using Repeated Load Triaxial (RLT) tests to characterize the stiffness properties of the plastic/demolition waste blends. Polyethylene plastic granules with up to 5% content were found to be suitable as a road construction material, when blended in supplementary amounts with demolition wastes. This research is significant, as the usage of plastics as a construction material, in combination with demolition wastes will expedite the adoption of recycled by-products by construction industries. Furthermore, the plastic blends were prepared without the requirement for any further operations, such as reshaping plastic granules into fibers, hence leading to significant energy and costs savings.

Stiffness and deformation properties of spent coffee grounds based geopolymers

Spent Coffee Grounds (CG) are the granular organic waste material from the food and beverage industry resulting from the brewing of coffee, and are often disposed to landfills. Fly Ash (FA) is the primary by-product of coal-combustion power-plants used for energy generation. Slag (S), on the other hand, is an industrial waste generated from iron-ore refineries. In this research, geopolymers comprised of various mix designs of CG, FA, and S were developed via alkaline activation to create a green construction material out of these traditional waste materials. The intended use for this recycled construction material was as a subgrade fill, which if successfully implemented would considerably reduce the need of disposing such waste materials into landfills. In this article, key design parameters including Unconfined Compressive Strength (UCS), resilient moduli (MR) and permanent deformation characteristics of CG-geopolymer remoulded cylindrical specimens are evaluated using simulated long-term Repeated Load Triaxial (RLT) tests. Regression parameters used in two MR prediction models, namely the Bulk Stress Model and 2-Parameter Model, were determined for the CG geopolymers. The developed 3-Parameter Model was found to be valid for predicting the MR of CG geopolymers. When cured at 21°C, the MR for CG+FA geopolymers is generally lower than that of CG+S geopolymer specimens. Curing at 50°C reduces this difference of MR values between FA and S geopolymers. At 50°C curing, the MR of some mixes, notably 70CG:30FA+90Na2SiO3:10NaOH, 70CG:30S+90Na2SiO3:10NaOH, and 70CG:30S+50Na2SiO3:50NaOH decreases after 28days of curing while gaining increased UCS. This indicates that these geopolymers can gain improved ductility while developing high strengths under optimum curing conditions. This article evaluates key design parameters including the MR for CG geopolymer specimens, which enables coffee grounds to be used as green recycled construction materials in roads.

Using Genetic Programming to Predict Plastic Strain in Subgrade Soils Under Repeated Loading

Abstract Accumulated plastic strain in subgrade soils under repeated loading is an important factor for the design and analysis of flexible pavement. Accurate prediction of the plastic strain is dependent on the constitution of a predictive model. This study discusses the plastic strain properties in subgrade soil under repeated loading and develops a predictive model using a genetic programming (GP) method. Repeated load triaxial tests were conducted under various confining pressures, stress levels, and moisture contents. To develop the predictive model, a total of 475 records were randomly divided into three datasets (159, 158, and 158 records) for training, validation, and application purposes. The results showed that the GP method was applied successfully to develop a predictive model that uses stress level, load repetition, dry unit weight, liquid limit, plastic index, and clay content as inputs and has plastic strain as the output. The R2 values for the training, validation, and applied datasets were 0.9551, 0.9494, and 0.9489, respectively. This study demonstrates and confirms the superior performance of GP over traditional prediction methods when applied to predict plastic strain in subgrade soils. The model and algorithms proposed in this study provide a good foundation for further development when more robust datasets of other types of soils are available.

Characterization of Permanent Deformation Behavior of Silty Sand Subgrade Soil Under Repeated Load Triaxial Tests

The performance of pavement structures is highly dependent on the performance of the subgrade layer, because it is the last layer underlying all the other pavement layers. The development of permanent deformation in subgrade material under traffic loads can cause pavement distresses such as fatigue cracking and rutting. This paper presents an evaluation of permanent deformation behavior of a silty sand subgrade material in the laboratory at various stress ratios and stress levels. It was found that the shakedown behavior of the material fell within plastic shakedown and plastic creep for stress ratios below 1.0 and a stress ratio of 1.5, respectively. A statistical prediction model for permanent deformation based on the test results is suggested.

Potential Applicability of Impact Echo Method on Pavement Base Materials as a Nondestructive Testing Technique

The use of both recycled asphalt pavement and recycled concrete aggregates is increasing considerably in pavement construction. These materials are relatively weak and have to be stabilized with cement or other stabilizers. However, because of product variability and lack of strength and stiffness, their applicability has to be evaluated extensively. Traditionally practiced methods of evaluation might be unreasonable in terms of time, cost, reliability, and applicability. Rapid nondestructive methods, such as the spectral analysis of surface wave, impact echo, pulse velocity, and so forth, have the potential to be inexpensive and less time-consuming, as well as offering low variability of the test results. The objective of the study was to assess the potential applicability of the impact echo method in evaluating recycled pavement base materials. Six combinations (0%–100%, 10%–90%, 30%–70%, 50%–50%, 70%–30%, and 100%–0%) of recycled asphalt pavement and recycled concrete aggregates, respectively, treated by four amounts of portland cement (0%, 2%, 4%, and 6%), were evaluated by impact echo, unconfined compression, and repeated-load triaxial test. From the test results, the range of P-wave velocity was between 5,500 in./s and 18,000 in./s, the compressive strength varied from 10 pounds per square inch (psi) and 415 psi, and the tangent modulus range was from 2.8 kips per square inch (ksi) to 41 ksi. Statistical models based on P-wave velocity data were derived for predicting elastic modulus, compressive strength, and resilient modulus. It was found that impact echo has significant potential in characterizing the strength and stiffness properties of cement-treated recycled base materials, which confirms the effectiveness of recycled materials in pavement applications.