Research Focus and Philosophy

The overarching goal of my research is to develop and integrate sustainable geomaterials and their characterization schemes in today’s infrastructure, and to make them more durable and resilient when faced with today’s natural hazards. Resilient geomaterials can reduce maintenance efforts and slow down foundation, embankment, and backfill deterioration, ultimately improve infrastructure performance, and reduce overall lifecycle costs. Realization of this goal warrants systematic research approaches from the fundamental and applied dimensions. At the fundamental level, I conduct research related to (1) experimental unsaturated soil mechanics, (2) micro-geomechanics, and (3) nondestructive characterization of geomaterials. These three areas have huge potential in helping us understand and better characterize the constitutive relationships governing interactions between the hydraulic and mechanical processes in unsaturated soils. At the applied research level, I have been conducting research related to (1) field, laboratory, and numerical assessment of geo-structural systems, (2) Geo-environmental applications of Biochar, (3) eco-friendly ground improvement methods, and (4) applications of machine learning in Geotechnical systems..

I follow a research philosophy in which I maintain open-mindedness, determination, and diligence and celebration of outcomes. Open-mindedness promotes new ideas and innovation. Determination drives problem-solving and goal achievement. Diligence helps me stay on task and continue with care and discipline. Celebration rewards hard work and rejuvenates the spirit after goals are reached. I am also a strong proponent of collaboration. In fact, the research project that funded my Ph.D. studies was a collaborative NSF project. Collaboration involves working in multi-disciplinary teams, which provide valuable support networks and ensure fresh critical outside views. I have collaborated with other researchers as well as DOT bridge and geotechnical engineers on three funded research projects, serving as the principal investigator. My ultimate goal and highest achievement will be attained once my research is recognized and referred to by other researchers and implemented into real-world tools by practicing engineers. Motivated by this goal I have been cultivating strong and collaborative relationships with researchers in my discipline as well as structural, environmental, and material engineering, food-biosciences, and computer science; with consulting engineers; and with infrastructure owners.
N.B. * indicates current or past student; J – Journal Paper, C – Archived and Peer Reviewed Conference Paper

Research Activities

 Characterization of Geomaterials

+ Application I - Unsaturated and saturated soils

Microstructural Characterization of 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. 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 debated. The central issue is whether a measure can be developed for three-phase soils that plays an equivalent role as the effective stress does in two-phase soils. Under this research dimension, it is attempted to forward such a measure by incorporating soil microstructure in the effective stress formulation. A novel suction-controlled experimental setup was developed and integrated with an X-ray computed tomography (X-ray CT) scanning system to image and model microstructural features. A tensorial quantity, called fabric tensor of the liquid phase, that characterizes the complex fabric resulting from saturated pockets and networks of liquid bridges is identified and introduced in the proposed formulation. It is shown that fabric tensor of the liquid phase varies randomly, in both the wetting and drying phases of the partial saturation, and has an intrinsic association with the evolution of the effective stress tensor. It is also shown that this random variation can be depicted by applying techniques of digital image processing. It is concluded that, for partially saturated granular soils, the consideration of fabric tensor of the liquid phase in effective stress formulations is imperative.


J1- Manahiloh K.N., Muhunthan B., and Likos W.J. (2016). “Microstructure based effective stress formulation for partially saturated granular soils.”  ASCE International Journal of Geomechanics: Special Issues on Experimental and Computational Geomechanics for Unsaturated soils. 16(6): 1-13. (Link)

C1 – Motalleb Nejad M.*, and Manahiloh K.N. (2017).  “Investigating the role of soil fabric in unsaturated soils.” Proceedings of Geotechnical Frontiers 2017 conference, pp. 595-605. (Link)

C2 – Manahiloh K.N., Muhunthan B., and Likos W.J. (2014). “Effective Stress and the Role of Liquid Fabric in Unsaturated Granular Soils.” Proceedings of the International Symposium on Geomechanics from Micro to Macro (IS-Cambridge 2014), Cambridge, UK, 863-868. (Link)

C3 – Manahiloh K.N., and Muhunthan B. (2012). “Characterizing Liquid Phase Fabric of Unsaturated Specimens from X-Ray Computed Tomography Images.” Unsaturated Soils: Research and Applications, C. Mancuso, C. Jommi, and F. D’Onza, eds., Springer, Berlin Heidelberg, 71-80. (Link)

C4 – Manahiloh K.N., Jaafar R., Muhunthan B., and Likos W. (2011). “Imaging and modeling the microstructure of unsaturated soils for improved prediction of macroscale response.” NSF-CMMI Research and Innovation Conference, Atlanta, Georgia.

Nondestructive characterization of Soils

X-ray CT imaging and techniques of digital image processing can be implemented in the microstructural characterization of saturated and unsaturated geomaterials. The co-existence of networks of liquid bridges and saturated pockets in unsaturated soils yields complex soil behavior. Attempts have been made to understand the evolution of liquid bridges and pockets under changing suction conditions in soils. This area of research 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. Partially saturated granular specimens are prepared and scanned under controlled suction conditions. Three dimensional images of the interphase microstructure are acquired and processed by developing image processing algorithms. The evolution of the liquid-air and solid-liquid interfaces was tracked from 3-D images obtained with phase-based segmentation. Interfacial contact angles were measured on orthogonally projected images and values are reported. The applicability of existing thresholding-based image segmentation techniques is also scrutinized in this research. Global segmentation techniques were applied and evaluated on their performance. A refined iterative and recursive type global segmentation technique has also been proposed in a study that fall under this area of research.


J1 – Manahiloh K.N., and Meehan C.L. (2017). “Determining the Soil Water Characteristic Curve and interfacial contact angle from microstructural analysis of X-ray CT images.” ASCE Journal of Geotechnical and Geoenvironmental Engineering. 143(8). (Link)

J2 – Abera K.*, Manahiloh K.N., and Motalleb Nejad M. (2017). “The Effectiveness of Global Thresholding Techniques in Segmenting Two-Phase Porous Media.” Journal of Construction and Building Materials. 142:256-267. (Link)

C1 – Abera, K.*, Manahiloh, K.N., and Motalleb Nejad, M.* (2018). “Global Segmentation Algorithm for Partially Saturated Granular Geomaterials.” IFCEE 2018, Orlando, FL.

C2 – Motalleb Nejad M.,* Manahiloh K.N., and Meehan C.L. (2017).  “Applying the techniques of microstructural image processing towards measuring interface angles in unsaturated geomaterials.” Proceedings of Geotechnical Frontiers 2017 conference, pp. 659-668. (Link)

C3 – Manahiloh K.N., and Muhunthan B. (2012). “Characterizing Liquid Phase Fabric of Unsaturated Specimens from X-Ray Computed Tomography Images.” Unsaturated Soils: Research and Applications, C. Mancuso, C. Jommi, and F. D’Onza, eds., Springer, Berlin Heidelberg, 71-80. (Link)

C4 – Manahiloh K.N., Jaafar R., Muhunthan B., and Likos W. (2011). “Imaging and modeling the microstructure of unsaturated soils for improved prediction of macroscale response.” NSF-CMMI Research and Innovation Conference, Atlanta, Georgia.

+ Application II - Biochar Amended Soils

Biochar is one of the innovative “construction materials” that has gained popularity in recent years. It is a carbon rich product obtained when plant-based or poultry-based biomass is heated in a closed container with little or no oxygen. Its positive attributes include carbon dioxide removal (sequestration) from the atmosphere, increasing plant fertility, decreasing nutrient leaching, enhancing moisture retention capacity in soils (i.e. reduced irrigation needs). More or less, all of the listed benefits circle around improving our environment and are all related to the effects biochar impose on the flow-related behavior of soils. It should be noted that, the moisture retention and/or conduction of biochar-amended soil affects not only their flow-related behavior but also their inherently coupled shear strength- and volume change-behaviors. No robust study has been done to look at the all-round effects of biochar addition on soils. Past studies singled out and investigated the flow-related aspects of biochar addition and a very few studies looked at how other geotechnical engineering properties got affected.

In this research area it is attempted to look at the interaction between flow-, deformation-, and strength-properties of different soil types as they are amended with different types and sizes of biochar. Such an investigation is imperative to the advancement of fundamental and mechanistic understanding of soil-biochar mixtures. This goal is especially important to be addressed on those soil types that are commonly used in the geotechnical and transportation engineering fields. The research also aims at studying the short- and long-term behavioral manifestations with due consideration to time and temperature effects.


 C1 – Jin J., Abera K.*, Imhoff P., and Manahiloh K.N. (2017).  “Experimental investigation of the effects of biochar on the hydraulic conductivity of soils.” Proceedings of Geotechnical Frontiers 2017 conference, pp. 549-558. (Link)

+ Application III - Transition (non-textbook) soils

This research investigates the geotechnical engineering behavior of intermediate (transition, also called non-textbook soils) soils in their unsaturated state. This is an ongoing research with lots of potentials for furthering the understanding we have on clay-silt and silt-sand transition soils. A laboratory experimental investigation in which the suction, confining pressure, strain rate, and soil composition are varied is underway.

Field, laboratory, and Numerical Characterization of Geo-structural Infrastructure

+ Dynamic Load Amplification in Buried Culverts

This research seeks to better understand the factors that influence the dynamic amplification of reinforced concrete buried culverts (RCBC) resulting from moving traffic loads. Due to large discrepancies between observations made during inspections and the results of load rating many RCBC in the State of Delaware, it is believed that the dynamic amplification factor (DAF) is overconservative.

According to American Association of State Highway and Transportation Officials (AASHTO) provisions, DAF for RCBC is specified as 1.33 when zero meters of fill is present between the culvert and pavement, and decreases linearly to 1.00 when 2.44 m of fill is present. Two approaches are taken to investigate the adequacy of this specification. First, a parametric finite element analyses (two- and three-dimensional) are conducted to examine the influence of fill depth, soil elastic modulus, span length, slab thickness and asphalt pavement thickness on DAF. Second, culverts representative of the Delaware Department of Transportation’s inventory are instrumented and field tested to investigate the behavior of in-service RCBCs.

Contrary to the numerical results, field test results show fill depth to have a negative relation with DAF and slab thickness to have a positive relation. These differences may be due to the disparity between the limited number of field tests conducted and the number of model configurations analyzed. Additionally, the maximum fill depth of any culvert instrumented and tested is 0.5 m while the finite element analysis examined model configurations with up to 1.83 m of fill. Depending on the statistic used, field tests do appear to corroborate trends suggested by AASHTO. However, based on the limited number of tests conducted and the contradictory results presented in the parametric finite element study it is difficult to elaborate on the extent to which that is true. The scope of field tests was limited to RCBC with less than 0.5 m of fill and with road surfaces in good condition. For culverts within the range of these and other parameters tested in the field, a maximum DAF of 1.20 is recommended.


J1 – Wells A.*, Shenton H.W., Manahiloh K.N., and Wenczel G. (2018). “Dynamic load allowance provisions for box culverts with low fill depth.” Journal of Transportation infrastructure geotechnology, 5(1): 42-58. (Link)

J2 – Kadivar M.*, Manahiloh K.N., Kaliakin V., and Shenton H.W. (2018). “Numerical Investigation of Dynamic Load Amplification in Buried Culverts.” Journal of Transportation infrastructure geotechnology, 5(1): 24-41. (Link)

C1 – Kadivar, M.*, Manahiloh, K.N., Kaliakin, V.N., and Shenton, H. (2018). “Assessment of Dynamic Load Allowance for Buried Culverts.” IFCEE 2018, Orlando, FL.

C2 – Wells A.*, Shenton H., and Manahiloh K.N. (2017). “Experimental Evaluation of Dynamic Amplification Factor for Box Culverts.” 2017 TRB annual meeting, January 8-12, Washington D.C. (Link)

C3 – Wells A.*, Shenton H., and Manahiloh K.N. (2016). “Parametric investigation of factors influencing the dynamic response of buried reinforced concrete culverts.” Geotechnical and Structural Engineering Congress, pp. 648-659. (Link)

Machine Learning/Artificial Intelligence in Geotechnical Engineering

+ Application I – Neural Network, Global Information System, and Random Effects Regression

In most geoscience studies, the direct measurement of parameters imposes a huge cost on projects. On one hand, field tests are expensive, time-consuming, and require specific high-level expertise. Laboratory methods, on the other hand, are faced with difficulties in perfect sampling. These limitations foster the need for the development of new numerical techniques that correlate simple-accessible data with parameters that can be used as inputs for site characterization. In this research area it was attempted to develop a microzonation algorithm that combined neural networks (NNs) and geographic information system (GIS). Data collected from the field (i.e., Standard penetration and downhole tests), and the lab (i.e., Atterberg limit test and sieve analysis) are used as inputs, in the integrated NNs-GIS algorithm, for developing the microzonation of shear wave velocity and soil type of a selected site. The resulting algorithm was capable of automatically updating the microzonation maps upon addition of new data.

Within the same research umbrella, it was attempted to apply the techniques of Random-effects regression to correlate simple lab and filed data (e.g., standard penetrating resistance, effective overburden pressure, plasticity index, fines content) with variables such as the shear wave velocity that are crucial inputs in the seismic characterization of sites. A unified equation was proposed to estimate the shear wave velocity of different soil types. The performance of the proposed model was assessed by examining “residuals”.


J1 – Motalleb Nejad M.*, Momeni M.S., and Manahiloh K.N. (2018). “Shear wave velocity and soil type microzonation using Neural Networks and Geographic Information System.” Soil dynamics and earthquake engineering. 104, 54-63. (Link)

J2 – Motalleb Nejad M.*, Manahiloh K.N., and Momeni M.S. (2017). “Random-effects regression model for shear wave velocity as a function of standard penetration test resistance, vertical effective stress, fines content and plasticity index.” Soil dynamics and earthquake engineering. 103(2017): 95-104. (Link)

J3 – Motalleb Nejad M.*, Momeni M.S., and Manahiloh K.N. “Shear wave velocity and soil type microzonation using Neural Networks and Geographic Information System: Reply.” Soil dynamics and earthquake engineering. (Link)

+ Application II – Metaheuristic Optimization

Traditionally, the analysis and design of earth structures are done following the working stress design principles. Recent developments have led to the application of Load and Resistance Factor Design (LRFD) in the analysis and design of such structures. In the broader engineering world, there are many optimization techniques that can be adopted in the analysis and design of earth structures. Once such technique namely, harmony search algorithm, falls under the category of metaheuristic optimization. Harmony search algorithm is adopted and successfully applied in the design optimization of the mechanically stabilized earth (MSE) walls reinforced with geosynthetic. The effects of using non-uniform length and spacing of reinforcement layers, on cost and design of the MSE walls, is investigated. Results showed that harmony search algorithm is successful in optimally reducing the cost of construction of geosynthetic-reinforced MSE walls.


J1- Motalleb Nejad M.*, and Manahiloh K.N. (2015). “A modified harmony search algorithm for the optimal design of earth walls reinforced with non-uniform geosynthetic layers.” Int. Journal of Geosynthetics and Ground Engineering, 1(4): 1-15. (Link)

J2 – Manahiloh K.N., Motalleb Nejad M.*, and Momeni M.S. (2015). “Optimization of design parameters and cost of geosynthetic-reinforced earth walls by Harmony Search Algorithm.” Int. Jour. of Geosynthetics and Ground Engineering, 1(2):1-12. (Link)

Eco-friendly Ground Improvement Methods

+ Eco-friendly and Sustainable Improvement of Tropical Residual Soils

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 foundation, road base, and subbase soils. 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 the soil. Effective soil stabilization can allow for the use of indigenous 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. In the collaborative study, we examined the viability of Xanthan gum as an eco-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.


J1 – Rashid A.S.A., Latifi N., Meehan C.L., and Manahiloh K.N. (2017). “Sustainable improvement of tropical residual soil using an environmentally friendly additive.” Geotechnical and Geological Engineering. 31(4). (Link)

+ Nanoclay Stablized Loess: A Sustainable Construction Material

This collaborative study explored the potentials of nanoclay -an engineered nanomaterial- as an eco-friendly additive to improve the engineering properties of Loess soils. Laboratory and field scale experiments were designed and executed to evaluate the effects of nanoclay stabilization. Varying fractions of nanoclay (ranging from 0.2% to 3% by mass) were added to the natural loess soil. In the laboratory, representative specimens were prepared and subjected to a variety of tests, including Atterberg limits, standard Proctor compaction, unconfined compressive strength, unconsolidated undrained Triaxial compression, wetting-induced collapse, and pinhole dispersivity. Results from the laboratory tests revealed that the addition of nanoclay altered the plasticity, strength, and stiffness behavior of the specimens. Also, the dispersivity and relative wetting-induced collapse behavior of the natural loess were impacted by the addition of nanoclay. In-situ investigations indicated that loess soils that had been stabilized with 2% nanoclay showed notable improvement..


J1 – Tabarsa A., Latifi N., Meehan C.L., and Manahiloh K.N. (2018). “Laboratory investigation and field evaluation of loess improvement using nanoclay – A sustainable material for construction.” Journal of Construction and Building Materials. 158, 454-463. (Link)

Laboratory and Field Characterization of Construction Materials

+ Evaluation of Clogging in Pervious Concrete Parking Lots

This study quantifies 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 to relate changes in porosity with clogging. Porosity profiles obtained from processing of CT scanned images were used to assess the nature and extent of clogging. Porosities of scanned samples were calculated, for comparison, using a gravimetric method and the clogged fraction was quantified for specimens scanned after vacuum cleaning. X-ray CT scanning was found to be a useful tool to study clogging of pavement cores.


J1 – Manahiloh K.N., Muhunthan B. Kayhanian M., Gebremariam S.Y. (2012). “X-ray Computed Tomography and Nondestructive Evaluation of Clogging in Porous Concrete Field Samples.” Journal of Materials in Civil Engineering, 24(8), 1103-1109. (Link)

+ Geotechnical Properties of Recycled Asphalt Pavement Base Layers

This study evaluates the effects of RAP percentage on the resilient modulus and permeability of base materials. It is found that increasing the RAP percentage increases the resilient modulus, as expected. Constant-head permeability test results indicate that increasing the RAP percentage decreases the coefficient of permeability. High resolution X-ray computed tomography (X-ray CT) is used to study the internal structure of the test specimens. It is found that RAP mixtures have lower void ratios than virgin crushed aggregate, which explains the observed high resilient modulus and low permeability of RAP mixtures.


J1 – Wu M., Wen H., Muhunthan B., Manahiloh K.N. (2012). “Influence of RAP Content on the Air Void Distribution, Permeability and Modulus of the Base Layer in Recycled Asphalt Pavements.” Journal of the transportation research board, 2267, 65-71. (Link)