Studies on Modulus of Resilience of Lateritic Soil Blends Using CBR, DCP, PFWD, and Cyclic Tri-Axial Tests, With Fem Based Analysis on Dynamic Loading

Abstract

Design and construction of highway embankments constitute a major component of highway engineering science. Poor sub-grade strength, overloading due to traffic loads, and seismic vibrations can cause distress to pavement sub-grades and embankments. Inadequate compaction and poor sub soil drainage, in addition to low bearing strength of soils cause failure of embankments especially in submersible regions. The coastal, and peninsular regions of India, and the District of Dakshina Kannada, in particular, are characterized by the presence of lateritic and lithomargic or silty soils (locally known as Shedi soil). These soils are characterized by the presence of more of silty fines and lesser clay content. In the process of expansion and widening of highways in this region, engineers are often required to design embankments using soils of predominantly lateritic and lithomargic origin. The effective use of these soils is therefore often hindered by difficulty in handling particularly under moist and wet conditions typical of tropical regions soils that present such problems during construction processes are termed problematic. Lateritic and lithomargic soils are not expansive in nature. However, it is required to perform a comprehensive study on the type of soil, the strength and stiffness of the soil in order to understand the phenomenon of pavement and embankment failure. The design of pavements require details on soil stiffness which in turn depends upon the density, soil-moisture content, degree of saturation, the drainage condition, the confining pressure, the homogeneity, and the load applied. Due to this reason, the evaluation of sub-grade strength and stiffness is of immense importance in pavement design. Vehicular loads applied on pavement surfaces are dynamic in nature. Due to this reason, the study of pavement sub-grades necessitates the analysis of elastic behaviour of underlying soil layers when subjected to repeated loads. Traditional and qualitative methods for subgrade evaluations include visual inspection, proof rolling (passing over subgrade with heavy roller to locate soft spots), time-consuming moisture density tests, and observations of settlements due to heavy construction equipment in addition to other destructive and non-destructive approaches. In the recent years, non-destructive approaches to pavement evaluation using the FWD, PFWD, Geo-Gauge, DCP and test using cyclic tri-axial test have gained popularity. Additionally, present sub-grade evaluation, and pavement design procedures give more importance to the use of modulus of resilience as a measure of pavement strength and stiffness that can be easily determined using non-destructive approaches. Moreover, there has been a change in approach in the design of flexible pavements with the introduction of mechanistic empirical method that necessitates the use of the modulus of resilience in place of soil stiffness measured using the CBR and the DCP. Field engineers in developing countries need to at times depend on conventional methods to assess the soil-stiffness due to non availability of the PFWD, or FWD, or the cyclic tri-axial test equipment. Additionally, the cost of purchase and installation of the FWD and the cyclic tri-axial test equipment is quite high. The use of such equipment is also quite difficult considering the need to employ additional skilled personnel. Recent pavement design procedures such as the Mechanistic-Empirical design approaches incorporate use of modulus of resilience or resilient modulus. Here, the design of the pavement is modeled based on the structural response measured in terms of stress, strain or deflection and the physical parameters of the pavement material. The resilient modulus or modulus of resilience (Mr) is defined as the ratio of cyclic deviator stress to the recoverable strain. This research work deals with a parametric study on laterite sub-grades using standard soil-testing procedures, and performing investigations on the engineering properties of lateritic and lithomargic soils for the District of Dakshina Kannada, Karnataka State, India. The study encompasses experimental investigations on static properties of soil such as, the Atterberg’s limits, grain-size distribution, specific gravity, maximum dry density (MDD), optimum moisture content (OMC), tests for California Bearing Ratio (CBR), tests for unconfined compressive strength (UCS), and tri-axial tests. This is followed by a study on the soil strength using the dynamic cone penetrometer (DCP) and the portable falling weight deflectometer (PFWD). Further tests were performed for determining the modulus of resilience (Mr) using the cyclic tri-axial testing equipment. Further studies were performed on strength and stiffness characteristics of laterite soil-samples with varying percentages of fines and moisture contents close to optimum moisture conditions (OMC), for standard proctor density conditions. The results obtained using the PFWD, CBR, and DCP tests conducted in the laboratory, and correlations between the results for various tests were analyzed using suitable statistical techniques. Also, the test results for various equipments were correlated to the observations on various soil parameters such as the percentage of fines, percentage of sand, percentage of gravel MDD and OMC. Investigations on this phase of study was conducted on remolded soils at MDD and at three moisture contents (OMC-3%, OMC and OMC + 3%) for soaked and un-soaked conditions using special test-boxes designed for the purpose. A number of regression models were then developed correlating the important observations made in the experimental studies. Additionally, the results of the experimental studies were further used in developing FEM-based models in PLAXIS-2D to study the impact of vehicular loads on soil embankments. It is felt that the findings of this study will provide a strong basis for further research in pavement engineering. The correlations developed as part of this study relating the results obtained using the triaxial, PFWD, CBR, DCP and the modulus of resilience obtained by cyclic tri-axial test can be used effectively by field engineers in assessing the subgrade strength of laterite sub-grades. The results of this study can further provide the basis for future investigations on the effect of the intrusion of fines in laterite soils.