Kalehiwot Nega Manahiloh

X-Ray Computed Tomography and Nondestructive Evaluation of Clogging in Porous Concrete Field Samples

This study aims to investigate and quantify the fraction of clogging in pervious concrete samples cored from parking lots. Image observation, analysis, and processing of representative cores retrieved from parking lots enabled qualitative relation of porosity with clogging. Porosity profiles obtained from processing of computed tomography (CT) scanned images were used to assess the nature and extent of clogging. Significantly lower porosity values were computed for old cores compared to newer ones. It was also observed that samples manifested clogged behavior because of high cement paste (mortar) content. Porosities of samples were calculated, for comparison using a gravimetric method and the clogged fraction in the cores was quantified by recalculating porosities of specimens scanned after vacuum cleaning. X-ray CT scanning proved a useful tool to study clogging, and the clogged fraction varied from 1.48% in sample PL 12-1 to 11.15% in sample QP 45-3.

Sustainable Improvement of Tropical Residual Soil Using an Environmentally Friendly Additive

Many tropical residual laterites have relatively poor engineering properties due to the significant percentage of fine-grained soil particles that they contain, which are formed by the soil weathering process. The widespread presence of laterite soils in tropical regions often requires that some form of soil improvement be performed to allow for their use in various civil engineering applications, such as for road base or subbase construction. One of the most commonly utilized stabilization techniques for laterite soils is the application of additives that chemically react with the minerals that are present in soil to enhance its overall strength; effective soil stabilization can allow for the use of site-specific soils, and can consequently result in significant cost savings for a given project. With an increasing focus on the use of more environmentally friendly and sustainable materials in the built and natural environments, there is an emerging interest in eco-friendly additives that are an alternative to traditional chemical stabilizers. The current study examines the viability of xanthan gum as an environmentally friendly stabilizer that can improve the engineering properties of tropical residual laterite soil. Unconfined compressive strength (UCS) tests, standard direct shear tests, Brunauer, Emmett, and Teller (N2-BET) surface area analysis tests and field emission scanning electron microscopy (FESEM) tests were used to investigate the effectiveness of xanthan gum for stabilization of a tropical laterite soil. The UCS test results showed that addition of 1.5% xanthan gum by weight yielded optimum stabilization, increasing the unconfined compressive strength of the laterite soil noticeably. Similarly, direct shear testing of 1.5% xanthan gum stabilized laterite specimens showed increasing Mohr–Coulomb shear strength parameters with increases in curing time. From the FESEM results, it was observed that the stabilization process modified the pore-network morphology of the laterite soil, while also forming new white layers on the surface of the clay particles. Analysis of the test results indicated that xanthan gum stabilization was effective for use on a tropical residual laterite soil, providing an eco-friendly and sustainable alternative to traditional soil stabilization additives such as cement or lime.

Characterizing Liquid Phase Fabric of Unsaturated Specimens from X-Ray Computed Tomography Images

Pore-water in the funicular and pendular saturation regimes of the SWCC (Sr < 90%) assumes a complex fabric consisting of saturated pockets of water under negative pressure and a network of liquid bridges formed from menisci at the contact points of particles. Measurement and characterization of this liquid fabric for unsaturated soil assemblies over a range of saturation, stress, and deformation plays a pivotal role in improving our fundamental understanding of unsaturated soil behavior. However, lack of microstructural visualization techniques has hindered the consideration of liquid fabric distribution and evolution in macroscale geotechnical formulations. In this study X-ray CT scanning was used to monitor the changes in the liquid fabric of unsaturated glass beads. Images showing the three distinct phases of unsaturated specimens were successfully obtained. A computer code that automatically analyzes multiple images to quantify the components of a second-order fabric tensor was developed and applied to CT images obtained along the drying and wetting paths of a SWCC determined by digital image processing. Principal values, principal directions and invariants are quantified and implications of the changes to better description of unsaturated soil behavior are discussed.

Influence of Recycled Asphalt Pavement Content on Air Void Distribution, Permeability, and Modulus of Base Layer

The use of recycled materials for construction is beneficial to both the environment and the economy. Reclaimed asphalt pavement (RAP) is one of the most commonly used recycled materials. State departments of transportation allow its use in unbound base materials at different percentages. The modulus and the permeability are recognized as important parameters for base course materials because of their effect on pavement service life. Determining the effects of the percentage of RAP on the modulus and permeability of base materials that contain RAP is important for proper pavement design. This study evaluates the effects of RAP percentage on the resilient modulus and the permeability of unbound base materials. Increasing the RAP percentage increased the resilient modulus. Constant-head permeability test results indicated that increasing the RAP percentage decreased the coefficient of permeability. High-resolution X-ray computed tomography was used to study the internal structure of the test specimens. RAP mixtures had lower void ratios than virgin crushed aggregate, which explained the high resilient modulus and low permeability of RAP mixtures.

Microstructure-Based Effective Stress Formulation for Unsaturated Granular Soils

AbstractThe principle of effective stress states that the strength and volume change behaviors of soil are governed by intergranular forces expressed in terms of a continuum quantity called effective stress. Although the principle of effective stress is regarded as one of the most fundamental concepts in soil mechanics, its applicability to unsaturated soil has been questioned. The central issue is whether a measure can be developed for three-phase soils that plays an equivalent role as the effective stress does for two-phase soils. Combining the techniques of microstructural analysis and image processing, this study formulated the effective stress for unsaturated granular soils. A novel suction-controlled experimental setup was integrated with an X-ray computed tomography (CT) scanning system and used to image and model microstructural features. A tensorial quantity, called the fabric tensor of the liquid phase, that characterized the complex fabric resulting from saturated pockets and networks of liquid...

Determining the Soil Water Characteristic Curve and Interfacial Contact Angle from Microstructural Analysis of X-Ray CT Images

AbstractThe complex behavior of unsaturated soils can be partly attributed to the co-existence of networks of liquid bridges and saturated pockets in the soil void space. Past studies have examined...

Evolution of Interphase Contact Angle in Partially Saturated Granular Soils Using Digital Analysis of X-ray Computed Tomography Images

The co-existence of networks of liquid bridges and saturated pockets in partially saturated soils yields complex soil behavior. Attempts have been made to understand the evolution of liquid bridges and pockets under changing suction conditions in soils. Analyzing the evolution of inter-phase boundaries and quantifying interfacial contact angle can be cited as one example. However, to date, the lack of technological developments that enable microstructural observation has impeded the results of such efforts. This study utilizes advancements in the field of nondestructive testing and applies X-ray micro Computed Tomography (X-ray micro-CT) to investigate the evolution of contact angle. A partially saturated granular specimen is prepared and scanned under controlled suction conditions. Three dimensional images of interphase microstructure are acquired and processed with digital algorithms. The evolution of the liquid-air and solid-liquid interfaces is tracked from 3D images obtained with phase-based segmentation. Interfacial contact angles are measured on orthogonally projected images and values are reported.

Assessment of Dynamic Load Allowance for Buried Culverts

In the past, research on buried culverts has focused on looking at live load distribution through soil onto buried culverts and given little attention to the dynamic amplification of moving loads. The few studies that looked at the dynamic amplification of buried culverts idealized the load-soilculvert problem using a plane-strain assumption. Under such conditions, the finite area axle loads are erroneously modeled as strip loads acting over the entire width of the culvert. In this study, the plane-strain assumption is removed and the load-soil-culvert system is modeled and analyzed a three-dimensional problem. Dynamic load allowance (DAF) was determined from field measurements, two-dimensional finite element method (2-D FEM), and three-dimensional finite element method (3-D FEM). The 2-D and 3-D FEM models resulted in an average DAF of 1.099 and 1.048, respectively. The average DAF calculated from field measurements and the American Association of State Highway and Transportation Officials (AASHTO) recommended formula are 0.963 and 1.295, respectively. Overall, the AASHTO DAFs are the highest and the field DAFs are the lowest. The plain strain assumption adopted in the 2-D FEM models resulted in DAF values that are higher than the 3-D FEM and field evaluated DAFs. The DAF calculated from threedimensional models is the closest to the field measured DAFs. Citation information: please cite this work as follows Kadivar, M., N. Manahiloh Kalehiwot, N. Kaliakin Victor, and W. Shenton Harry, Assessment of Dynamic Load Allowance for Buried Culverts, in IFCEE 2018. Link to published article: https://ascelibrary.org/doi/abs/10.1061/9780784481608.018

Global Segmentation Algorithm for Partially Saturated Granular Geomaterials

A new global segmentation technique, namely refined statistics-based method is proposed. X-ray computed tomography (CT) images of partially saturated granular geomaterials are segmented using this technique. Index properties of the geomaterials are quantified using the new technique and compared with those of the Otsuand iterative-Otsu techniques. For the tested silica sand specimen, the new-technique-estimated void ratio and the degree of saturation were 0.67 and 39.35%. The estimates for the glass bead specimen yielded 0.64 and 43.49%, respectively. The true void ratio (0.66) and degree of saturation (37.71%) were determined with user controlled Image-Pro software package. It was found that the proposed method estimated the void ratio and the degree of saturation with 1.52 and 4.35 percent errors for the silica sand and with 15.63 and 0.34 percent errors for the glass bead, respectively. The computational time of the proposed method was found to be shorter than other methods considered. Overall, it is concluded that the proposed technique performed better in segmenting three-phase granular geomaterials. Citation information: please cite this work as follows Abera Kokeb, A., N. Manahiloh Kalehiwot, and M. Nejad Mohammad, Global Segmentation Algorithm for Partially Saturated Granular Geomaterials., in IFCEE 2018. Link to published article: https://ascelibrary.org/doi/10.1061/9780784481585.046

Investigating the Role of Soil Fabric in Unsaturated Soils

The principle of effective stress states that the strength and volume change behaviors of soil are governed by intergranular forces expressed in terms of a continuum quantity called effective stress. Past research on effective stress formulations has identified a tensorial quantity that characterizes the liquid phase of unsaturated granular geomaterials. This quantity was named fabric tensor of the liquid phase and was shown to be anisotropic and to have an intrinsic association with the evolution of the effective stress tensor. It was also shown that its variation is random and can be depicted with microstructural image analysis. In this study, two past micromechanical effective stress formulations are discussed in comparison with Bishop’s effective stress. The extended Mohr-Coulomb and effective stress approaches are used in interpreting shear strength parameters and effective stress parameter for partially saturated granular soils. Correlations are identified for some material variables. The nonlinearity observed in the angle of friction associated with the matric suction was discussed in relation to the fabric tensor of the liquid phase. Citation information: please cite this work as follows Motalleb Nejad, M. and Manahiloh K.N., Investigating the Role of Soil Fabric in Unsaturated Soils, in Geotechnical Frontiers 2017: Geotechnical Materials, Modeling, and Testing, GSP 280. Link to published article: https://ascelibrary.org/doi/10.1061/9780784480472.063