Master of Science in Geotechnical Engineering

Permanent URI for this collectionhttp://192.248.9.226/handle/123/16123

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item: Thesis-Abstract
Correlation between standard penetration resistance (SPT N) and cone resistance (Qc) in cone penetration test (CPT) for residual soils
(2024) Sashikala, JRM; De Silva, LIN
"Corelation between Standard Penetration Resistance (SPT N) and Cone resistance (qc) in Cone Penetration Test (CPT) for Residual Soils" examines the complex relationship between these two key geotechnical engineering parameters. The basis of the research is a set of in-depth field tests carried out on residual soils, which are remarkable for their specific qualities and common occurrence. This study's main goal is to establish the numerical relationship between SPT N and CPT qc values, which is useful for foundation design and soil characterization. The study begins by providing a thorough analysis of the existing literature of research on the topic, which is followed by a thorough justification of the approach used for the field experiments. Cone Penetration Test (CPT), an appreciated in-situ testing method, is used to quantify qc, and Standard Penetration Test (SPT) is used to compute SPT N values. Statistical techniques are then employed to examine the test findings and determine if SPT N and qc are correlated. The results show that these parameters for residual soils have a high association, which gives geotechnical engineers important information. A correlation between SPT N and qc is then determined by statistically analysing the test data. Geotechnical engineers may learn a great deal from the results, which show a high link between these parameters for residual soils. For the planning and building of foundations on residual soils, the findings have important contributions. Finally, this study has identified a solid framework on further research which can be performed to and emphasizes the importance of comprehending the link between SPT N and qc in residual soils. Key words: standard penetration resistance (SPT N), cone resistance (qc), cone penetration test (CPT), residual soils, correlation
item: Thesis-Abstract
Improvement of uplift capacity of transmission tower foundation
(2024) Amarakoon, DVT; De Silva, LIN
Electricity transmission is carried out by conductors attached to the lattice tower structures, mostly supported by a shallow spread footing foundation, associated with around 40-45 percent of the total project cost. The uplift capacity is one of the main factors when selecting the size of a transmission tower foundation. The current design practice considers the weight of the concrete foundation and the soil as an inverted frustum that contributes to resisting forces. However, it is noted through literature that the actual behaviour of the uplift capacity of the foundation does not exactly depend only on the weight of the soil inverted frustum shape and concrete. Instead, it depends on the soil properties, such as friction and cohesion. Global research has been carried out on cement soil stabilization, insertion of fiber to the cement sand backfills, and use of geo-grids as improvement techniques for uplift capacity. This study aims to assess different techniques and propose suitable techniques to improve the uplift capacity of transmission tower foundation construction. To commence this process, soil samples with low SPT values were collected from five tower locations of ongoing transmission lines in the western province of Sri Lanka. Most of the remoulded samples are tested and identified as Clayey Sand according to the USCS classification. The tri-axial test on remoulded soil resulted in an undrained cohesion value of 5.6 kN/m2 to 10.4 kN/m2. Then, modified soil samples are prepared, adding 1 percent and 3 percent cement by soil weight, cured for 7 days, and tested. Uniaxial compressive tests (UCS) are performed on the mixed soil samples. The UCS on samples of 3 percent cement shows a significant improvement of cohesion, resulting in values between 20 to 60 kN/m2. Then a 3D finite element model is developed and verified by using the values of the research carried out by (Consoli, Ruver, & Schnaid, Uplift Performance of Anchor Plates Embedded in Cement-Stabilized Backfill, 2013). Then, the actual foundation is analyzed for various area configurations. Hence, a series of models are developed incorporating different configurations of foundation depth, improvement angle, and cohesion for full-depth, layer-wise, and partialdepth types of improvement. In conclusion, the uplift capacity of the transmission tower foundation can be increased significantly by adding 3 percent cement to the existing clayey sand backfill. The uplift capacity increases with the angle of improvement and cohesion of the backfill material. The uplift capacities were compared as a percentage of the remoulded soil backfill at a foundation with a depth of 3.0 m. When considering full-depth improvement cases, a minimum of 100 and 120 percentages resulted for the foundation depths of 2.0 and 2.5, respectively, with a cohesion value of 40 kPa and a 25-degree angle. However, it is 150 percent for the foundation depth of 3.0 m, even with a cohesion of 20 kPa. Also, the uplift capacities of layer-wise depth improvement and the total layer improvement of the practical excavation are similar. Further, improvement as a partial depth with a layer thickness of 1.5 m (starting from the bottom of the foundation) results in 120 and 150 percentages for depth of foundation 2.5 and 3.0 m, respectively, with a cohesion value of 40 kPa and 25-degree angle. Keywords: - Improvement, Uplift capacity, Foundation, Cement, Cohesion
item: Thesis-Abstract
Study on the applicability of finite element analysis on predicting the behavior of axially loaded piles
(2024) Fernando, GNP; De Silva, LIN
The performance of axially loaded piles is a key factor in ensuring the stability and safety of various civil engineering structures, including buildings and bridges, within the field of geotechnical engineering. Accurate prediction of pile behavior is essential during the pile design stage. This master's thesis presents a comprehensive investigation into the applicability of Finite Element Analysis (FEA) as a predictive tool for assessing the behavior of axially loaded piles, with a focus on the Sri Lankan context. The research methodology involves a comparative analysis of field test data with numerical and manual calculation data. A dataset comprising 12 pile tests and borehole data was collected from the Colombo suburb area and Mannar Island in Sri Lanka. This dataset includes five PDA tests and one MLT test at each location. Simultaneously, a 3D finite element model is developed using PLAXIS 3D software to represent the pile-soil interaction. The finite element model is precisely calibrated and validated against the field test data to ensure its accuracy and reliability. In addition to FEA, manual calculations of pile capacities and settlements were performed using the collected borehole data and compared with the field test data. The findings of this study suggest that FEA can effectively predict the behavior of axially loaded piles with a high degree of precision when calibrated appropriately. Notably, the results show strong agreement with experimental test data when the Elastic Modulus of soil (E50) is increased by a factor of five in both Colombo and Mannar areas. The implications of this study are highly relevant to geotechnical engineers, designers, and construction professionals, as it serves as a valuable resource for designing axially loaded piles in the specific context of Sri Lanka. In conclusion, this master's thesis demonstrates that Finite Element Analysis is a powerful and reliable tool for predicting the behavior of axially loaded piles. It has the potential to significantly enhance the efficiency and reliability of pile design and construction in geotechnical engineering projects.
item: Thesis-Abstract
Uplift capacity of helical piles on residual soil
(2024) Herath, HMUS; De Silva, LIN
Utilizing helical piles in foundation construction associated with compressive, tensile and lateral loads of power transmission towers are increasingly used by many countries all over the world. Lesser installation time, lesser manpower, lesser involvement of machinery and ability to use just after installation, can be identified as certain reasons for the popularity for helical piles compared to other foundation types. Further, helical piles which are more versatile and environmentally friendly, can be removed, reused and recycled as and when necessary. The installation torque required to install helical piles correlates with their load-bearing capacity, resistance to uplift forces, and ability to withstand lateral loads. In this research, ultimate uplift values of helical piles were calculated and the correlation between installation torque and uplift capacity of helical piles in residual soil were investigated. Residual soil subsurface was selected for this research, since such terrains are more common in Sri Lanka. Accordingly, three transmission tower locations (AP14, AP27 & AP47) consisting of residual soil, along Monaragala-Wellawaya power transmission line were selected and three helical piles were driven with measured depth and torque values. Also, three uplift load tests were conducted measuring the load and the deflection. For these tests, SS175 lead w/200 mm, 250 mm & 300 mm helices and RS3500.300 (88.9 mm diameter x 7.6 mm wall) extensions were used manufactured by CHANCE under Hubbell Power Systems, inc. USA were used. The findings reveal that the correlation (Kt) between installation torque and uplift capacity of helical piles on residual soil is 29 m-1. The FEM analysis was conducted with the help of PLAXIS 3D software, and the results were validated through the aforementioned tests. Accordingly, PLAXIS 3D FEM can be used to calculate the uplift capacity of helical piles on residual soil, and the mechanism of failure of helical piles on residual soil during uplift was identified as cylindrical shear. Furthermore, it is revealed that there is a certain association between SPT N and the installation Torque of helical piles on residual soil

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