Institutional-Repository, University of Moratuwa
Welcome to the University of Moratuwa Digital Repository, which houses postgraduate theses and dissertations, research articles presented at conferences by faculties and departments, university-published journal articles and research publications authored by academic staff. This online repository stores, preserves and distributes the University's scholarly work. This service allows University members to share their research with a larger audience.
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item: Thesis-Abstract
Ultimate shear resistance of rock-sockets in the presence of bentonite filter cake
(2025) Bulathsinhala, AUVB; Puswewala, UGA; Wanniarachchige, PK
In various global regions, and notably in Sri Lanka, Bentonite slurry serves as a prominent drilling fluid and an indispensable agent in the realm of pile and diaphragm wall construction activities. The use of Bentonite slurry in these applications is based on its inherent characteristics as a Bingham fluid, replete with thixotropic properties, which imparts a support for stability enhancement within the bore hole during the intricate phases of drilling, washing, and concreting. Notably, it creates a vital barricade against side wall collapse, thereby safeguarding the structural integrity of the construction site. Moreover, Bentonite serves a dual role, not only as a structural support but also as a versatile coolant and lubricant for drilling tools, ensuring the precision and efficiency of the excavation process. However, despite the numerous advantages conferred by Bentonite slurry in facilitating the pile construction process, some research has identified adverse effects that impact pile performance. This contrast of favourable and unfavourable effects highlights the complex interplay between the unique rheological properties of Bentonite slurry, demanding further investigation to optimize its use in construction activities.
Several researchers actively involved in the oil and construction industries have undertaken extensive investigations into the Bentonite Filter cake (BFC) formation over soil and rock surfaces, as well as its consequences on the performance of piles. However, a substantial proportion of these investigations has been on the development and consequences of filter cakes over sedimentary rocks, including mudstones, siltstones, and sandstones, which are abundant in e.g. North American and Australian regions. These sedimentary rock formations have comparatively higher permeability when contrasted with crystalline rock types. The enhanced permeability of sedimentary rocks facilitates the BFC formation over their surfaces to a greater extent, thereby introducing a unique set of challenges to geotechnical engineering.
The presence of BFCs on the surfaces of sedimentary rocks produces adverse effects, particularly concerning the capacity of rock sockets to provide skin friction. Consequently, construction practices in these geological contexts necessitate the incorporation of higher safety factors to account for the inherent uncertainties that may arise due to the presence of these distinctive infill layers. These uncertainties and associated challenges require a thorough and context-specific approach to pile design and construction in regions of sedimentary rock formations, highlighting the importance of comprehensive research in mitigating potential performance issues and ensuring the structural integrity and safety of construction projects.
Countries like Sri Lanka have used the same conventional pile design practices which are used in other regions, even though it leads to a certain degree of design conservatism. This conservative approach prevails even in the face of the abundant presence of crystalline Metamorphic rocks characterized by higher strength capacities and exceptionally low permeability characteristics. The inherent geotechnical properties of these crystalline Metamorphic rocks, which hinder the formation of BFCs in comparison to sedimentary rock types, warrant a re-evaluation of the prevailing design practices.
Hence, this thesis presents a comprehensive investigation of rock socketed pile design beginning with an examination of Bentonite Filter Cake (BFC) formation over metamorphic rocks. Four distinct metamorphic rock types characterized by their major mineral compositions were collected: Biotite gneiss, Quartzofeldspathic gneiss, Garnet granulite gneiss, and Charnockitic gneiss. These samples were taken to represent three distinct weathering grades: fresh, slightly weathered, and moderately weathered. In order to investigate the formation of BFC over metamorphic rocks, a pressure chamber was constructed to apply a Bentonite slurry pressure of 0.3 MPa over a period of 12 hours, based on data collected via a questionnaire survey providing the basis for the pressure and its duration. The findings revealed that BFC thicknesses averaged 2 mm on fresh metamorphic rocks and 4 mm on moderately weathered rocks. Among the rock types, Garnet granulite gneiss demonstrated the highest BFC thickness, measuring 2.36 mm on fresh samples, while Quartzofeldspathic gneiss exhibited the greatest thickness in both slightly and moderately weathered conditions. Nonlinear logarithmic models were developed to predict the BFC thickness after 12 hours for all rock types and their weathering grades. To complement these findings, X-Ray Diffraction (XRD) tests were conducted to determine the major mineral compositions of the rock samples across weathering grades. The results indicated no direct correlation between mineral content and BFC formation, leading to the conclusion that BFC development is independent of mineral composition.
To assess the impact of BFC on the shear behaviour at the rock-concrete interface, a direct shear apparatus was fabricated at the University of Moratuwa (UOM) after a thorough literature review on similar apparatus. Direct shear tests were conducted under constant normal loading (CNL) conditions for all four rock types under a range of Bentonite infill conditions. Accordingly, Garnet granulite gneiss exhibited the highest shear strength, while Biotite gneiss displayed the lowest. Considering these findings, Biotite gneiss was selected for further shear testing, and samples were transported to the University of Wollongong (UOW), NSW, Australia for additional testing under CNL conditions. The tests confirmed the accuracy of the results given by the apparatus at the University of Moratuwa and established the base friction angle for the Biotite gneiss-concrete interface as 38.53°.
To further explore the effects of rough surface profiles, the least rough Biotite gneiss profile obtained from pile cores was converted into an equivalent triangular profile with a 3.5 mm asperity height and 80° asperity angles. This profile was tested at UOW under constant normal stiffness (CNS) boundary conditions of 8.5 kN/mm. The highest shear strength recorded was 3.53 MPa for a clean joint at initial normal stress of 0.705 MPa. The critical infill thickness where Bentonite began to govern shear behaviour was determined to be 4.5 mm. A new hyperbolic model was developed to estimate shear strength capacity for varying infill thicknesses of between 0 to 5.5 mm. In subsequent testing, a setup involving a thick steel pipe was used to apply higher CNS conditions of 215 kN/mm. Under these conditions, the maximum shear strength capacity of 9.76 MPa was achieved for the clean Biotite gneiss- concrete joint, while the lowest capacity of 2.86 MPa occurred with a 5.5 mm Bentonite infill. A new hyperbolic model was also developed to predict the maximum shear strength capacity under these higher CNS conditions.
Finally, data from 15 field load tests were utilized to analyse mobilized skin friction from the socketed regions of piles. These results were compared to the newly developed model, which was used to optimize skin friction design for pile rock sockets. Based on the findings, it is recommended to adopt the new model with a safety factor of 2.5, which would result in a potential reduction of construction costs for rock sockets by more than half.
item: Thesis-Full-text
Novel modeline methods and evaluation frameworks for assessing and improving the reliability of renewable-rich power systems
(2025) Amarasinghe, PAGM; Abeygunawardane, SK; Prasad , WD
The share of non-conventional renewable energy, such as wind and solar, is gradu ally increasing in many power systems. However, integrating wind and solar power on a large scale would considerably affect the reliability of power systems due to the stochastic nature of renewables. Conventional reliability evaluation methods cannot efficiently and accurately quantify power system reliability when there is a significant amount of renewable power in the system. Therefore, novel reliability evaluation tech niques are required to assess the reliability of modern renewable-rich power systems. This work implements novel modeling methodologies and evaluation frameworks to quantify the reliability of renewable-rich power systems accurately. Different algo rithms and techniques are developed to evaluate the reliability of generation, compos ite generation and transmission, and distribution subsystems because the nature of the problem and its complexity are different in each of the three subsystems. Firstly, a novel method based on Kernel Density Estimation (KDE) is proposed to model inter mittency and both diurnal and seasonal variations of wind and solar power generation using historical renewable power generation data. The proposed KDE-based renewable power models are used with Non-Sequential Monte Carlo Simulation (NSMCS) to evaluate the generation system adequacy of the Institute of Electrical and Electronics Engineers (IEEE) Reliability Test System (RTS) . The diurnal and seasonal varia tions of renewables and the correlation between the load and renewable generation are modeled in the proposed renewable power models. Secondly, the applicability of con ventional MCS for composite system adequacy evaluation is investigated. It is found that a more computationally efficient reliability evaluation methodology is needed to evaluate the adequacy of composite systems. Hence, a novel population-based intelli gent search method called Evolutionary Swarm Algorithm (ESA) incorporated with DC optimal power flow analysis is proposed to evaluate the adequacy of renewable rich composite power systems. The main objective of the ESA is to find out the most probable system failure events that significantly affect the adequacy of composite sys tems. The identified system failure events can be directly used to estimate the system adequacy indices. The random search guiding mechanism of the ESA is based on the underlying philosophies of genetic algorithms and binary particle swarm optimization. The computational efficiency of the proposed ESA is significantly higher than that of traditional simulation methods. Thirdly, the proposed ESA is used to evaluate the reli ability of solar-integrated power distribution systems. AC optimal power flow analysis is used to assess the system failure events while considering the feeder voltage levels. The annual reliability indices of renewable-rich distribution systems can be estimated using this method, which is not addressed in the prevailing literature. Apart from eval uating the power system reliability, the impact of increasing wind and solar integration on the reliability of power systems is investigated at all hierarchical levels. Further, the proposed reliability evaluation frameworks are used to identify possible methods to improve the reliability of power systems. The outcomes of this research allow precise planning and operation of modern power systems in a time-efficient manner.
item: Thesis-Full-text
Road alignment optimization in a hilly region of Sri Lanka using GIS techniques
(2025) Dissanayaka, GDVC; Pushpakumara, TDC
This study focuses on road alignment optimization in a landslide-prone hilly region of Sri Lanka, specifically the Yatiyanthota and Yatiyanthota town areas. The research employs Geographic Information System (GIS) techniques to optimize the design of a proposed bypass road, considering economic, environmental, and societal factors. Traditionally, engineers have used manual drafting techniques to design road alignments, but this method has limitations, particularly in simultaneously analyzing multiple spatial factors. Therefore, GIS techniques, such as the Least Cost Path algorithm in ESRI's ArcGIS, were utilized to generate optimal alignment alternatives for the bypass road. The methodology incorporated digital elevation models, land use data, geological information, and hydrological features to create a comprehensive suitability analysis, with cost surfaces developed by assigning weighted values to each factor based on their relative importance to road construction feasibility and safety. Multiple scenarios were modeled to evaluate different prioritization schemes for environmental protection, construction cost minimization, and social impact reduction. The study compares and assesses these alternatives to bypass landslide-prone areas in Yatiyanthota while considering multiple factors simultaneously. Results indicate that the least-cost optimal alignment better coincides with engineering factors than with other assessment criteria. Additionally, the optimal alignment is shorter and has reduced cut/fill costs compared to the existing alignment. This research provides valuable insights into using GIS techniques to optimize road alignment design in hilly regions with complex geography and geology prone to landslides and earthquakes, with implications for road development projects in Sri Lanka and other similar regions.
item: Thesis-Full-text
Numerical simulation of thin membranes with curved creases
(2025) Kuruppu, KALH; Mallikarachchi, HMYC; Herath, HMST
Deployable membranes are commonly used in the construction of solar sails, solar arrays, and sun shields. These structures are fabricated on the ground and then deployed to their operational configuration in space. These structures are very large in dimensions but need to be compacted into a very small package that can be stowed inside a launch vehicle. Membrane structures are often folded using different crease patterns and wrapped around a central hub to achieve the compacted state. The thickness of the membranes place and important role in selecting a folding pattern as one length of the membrane increases with layers overlapping around the hub. To get a smooth wrap, a viable option is to change the crease pattern geometry to a curved crease with changing curvature to accommodate increased thickness. Changing the geometry into a curved crease pattern needs to be analyzed properly to understand its suitability for use in deployable membrane structures. This study analyzes the effectiveness of curved and straight crease wrapping structures. Two numerical models are used for the analysis of each crease pattern to understand behavior. The curved crease wrapping pattern shows good overall wrapping motion with less stresses in the initial stages when compared to the straight crease wrapping pattern. The stress is reduced up to 26% in the initial stages. Also, a crease idealization technique is introduced in this study to incorporate the self-opening behavior of the membrane creases into the numerical model. This idealization technique is further evaluated for a multiple crease geometry with a Miura-Ori model. The crease modeling technique shows a good overall fit with the experimental results validating the numerical simulations.
item: Thesis-Full-text
Assessment of climate change with special reference to the magnitude and temporal variability of the extreme climate events across Sri Lanka
(2024) Premarathna, SWMJD; Pushpakumara, TDC; Kadupitiya, HK
Climate change has emerged as a critical global challenge, characterized by increasingly frequent and severe extreme weather events. This research focuses on the impacts of climate change on extreme rainfall events in Sri Lanka, a country highly dependent on agriculture and particularly vulnerable to climate-induced natural disasters. Recognizing the limitations of traditional methods for identifying extreme precipitation trends, this study adopts an innovative approach by integrating multiple ETCCDI indices. By combining these indices, the research provides a holistic and nuanced understanding of extreme rainfall patterns, offering valuable insights that are beneficial across sectors such as agriculture, disaster management, and climate policy. The study addresses the evolving challenges of agricultural planning by re-demarcating Sri Lanka’s Agro-Ecological Zones (AEZs). This re-demarcation incorporates a range of critical factors, including yearly catchment runoff, irrigation infrastructure, land use dynamics, and administrative boundaries. By integrating these diverse elements, the research establishes a comprehensive and adaptive zoning framework that better reflects contemporary climatic, hydrological, and socio-economic conditions. The refined AEZs aim to optimize land use, enhance agricultural productivity, and improve disaster risk management. The findings of this study hold significant implications for climate adaptation and sustainable development. They provide actionable recommendations for targeted interventions and evidence-based policy-making, helping to build resilience among vulnerable communities and sectors. Ultimately, this research contributes to a deeper understanding of climate change impacts and offers pathways for addressing its challenges effectively