Master of Engineering in Foundation Engineering & Earth Retaining Systems

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Browsing Master of Engineering in Foundation Engineering & Earth Retaining Systems by Author "De Silva LIN"

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    item: Thesis-Full-text
    Application of dynamic and vibro compaction methods for densification of granular fill in reclaimed land in Sri Lanka
    (2019) Samarasinghe AI; De Silva LIN
    In the recent past, Government of Sri Lanka executed a large-scale reclamation project in Sri Lanka to add a brand-new land of 267-hectare to the Capital, Colombo with strategy of converting Colombo as a commercial hub of South Asia. For this project, 72 Million m3 of sea sand which was dredged by Trailing Suction Hopper Dredgers at 10km off from shore of Colombo was placed mainly by hydraulic methods at lower elevation while applying bulldozers at the top. This reclamation material was noted as clean uniform sand and which was under loose to medium dense condition prior to densification. This sand fill was densified using two methods, namely dynamic compaction and vibro compaction. Dynamic compaction, which is generally considered as one of the most economical sand improving methods, was applied in all areas except vibration sensitive areas at the city end and the areas where deep ground improvement was required for stability of earth retaining structures. Since settlement of subsoil in the seabed is not critical, the considered major geotechnical issues were achieving of required bearing capacity, shear strength and avoiding possible liquefaction. To sort out all geotechnical issues, sand densification was the only solution. Though there is a very long history for dynamic and vibro compaction methods, still reclamation projects are not preplanned to utilize the self-compaction achieves during sand placing very effectively, while designs always follow a very conservative approach. Moreover, designs are carried out using pre-defined energy criterions rather than considering existing fill material properties and its pre-compaction condition. Thus, there was a paramount requirement to assess the dynamic and vibro compaction methods for Sri Lankan fill materials and reclamation methods with the intention of optimization of the above compaction methods. In order to optimize dynamic compaction method, the pre-and post-compaction condition (by CPTs) was evaluated by crater depth, net volume changes, influenced depth and related indices, which assess the degree of improvement based on applied iv energy. Similarly, densification by vibro compaction was evaluated with respect to the factor such as point spacing, amperage and compaction holding time. In addition, effect such as age of the compacted fill was considered for both dynamic and vibro compaction in this reclamation fill of clean sand. Finally, verification of densified ground by selecting CPTs at least compacted points with respect to the compaction grids was assessed for both dynamic and vibro compaction to confirm the optimization has no adverse effect on the final design. Based on the finding of this research, fill material’s index properties of Sri Lankan sea sand were determined while being noted that there is no hesitation for applicability of dynamic and vibro compaction for densification. During the analysis it was suggested to modify some correlations derived based on laboratory test data to achieve more realistic output for actual reclamation condition. In addition, design of dynamic and vibro compaction by performance-based method through trial compaction was discussed.
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    item: Thesis-Abstract
    Effect of bentonite and polymer drilling fluids on skin friction of bored piles
    (2021) Hemadasa MBC; De Silva LIN; Nawagamuwa U
    Skin friction is a major component in any pile foundation’s load bearing capacity. In terms of a floating pile, almost entire bearing capacity depends on the skin friction. Skin friction may depend on the soil type and parameters while there are several methods of obtaining the skin frictional resistance of a pile depending on the soil type. Most of the high rise structures in Sri Lanka stand on bored end bearing pile foundations. Construction of bored piles widely involves usage of Bentonite as a drilling liquid and as a borehole soil stabilizer which may result in retention of a considerable amount of Bentonite and soil mix between the pile and the surrounding soil. Hence it is arguable that the used Bentonite slurry has an effect on the skin friction of the pile. Polymer liquid is not a commonly used drilling liquid in Sri Lanka but still it has been used in the current study. Main objective of this research is to investigate the effect of using Bentonite and Polymer liquids on the skin friction of pile foundations in Sri Lankan soils. From the results obtained from the direct shear tests conducted in laboratory for laterite soil and sand, it was evident that polymer liquid had no significant effect on soil strength parameters but with time due to the filter cake formation, bentonite will cause a significant change in soil strength parameters
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    item: Thesis-Abstract
    Evaluation of correlations between SPT N and undrained shear strength for fine grained soils of Sri Lankan geological conditions
    (2021) Premathilaka KKW; Thilakasiri H S; De Silva LIN
    Undrained shear strength is one of the essential parameters in most of the applications of geotechnical engineering. Unconsolidated undrained (UU) triaxial test is the most commonly practiced method of determining the undrained shear strength which takes a considerable time and effort to produce its results, since the sampling stage. Such correlations seldom exist pertinent to Sri Lankan soils. Hence, it is important to have reliable correlations between easily executable, commonly used test results and undrained shear strength of Sri Lankan soils to easily and promptly predict the soil structure interaction phenomenon, especially in local geotechnical engineering applications. Thus, in this study, an attempt has been made to correlate the standard penetration test (SPT) with experimentally determined unconsolidated undrained triaxial test parameters, and the undrained shear strength estimated from the field vane shear test data. This analysis consists of three main stages, namely analysis of the available correlations, analysis of the available data set based on the soil and sampling properties to develop a correlation and the analysis on a set of reliable data with a defined deviation factor. A correlation between SPT N60 and undrained shear strength has been proposed for Sri Lankan silty soils. Further, possible analysis methods for developing correlations for other different soil types have also been addressed. In addition, existing drawbacks and difficulties associated with developing such correlations related to Sri Lankan context are also described.
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    item: Thesis-Full-text
    Investigation on the equivalent modulus of subgrade reaction of layered soil
    (2019) Mathuwanthy T; Thilakasiri S; De Silva LIN
    ‘Modulus of subgrade reaction’ is the ratio between the pressure applied on the soil and the corresponding settlement. There is no theoretical relationship found to obtain equivalent subgrade modulus of layered soil. Top dense sand layer thickness, bottom loose sand layer thickness, strip footing width and thickness are changed and equivalent modulus of subgrade reactions are obtained by equivalent spring theory and weighted average method. These, equivalent subgrade modulus are separately applied in Heteryni method equations in order find vertical settlement, bending moment and shearing force along the medium length footings. PLAXIS 3D numerical models are developed for same footing parameters and soil properties to compare the Heteryni method outputs. Equivalent subgrade modulus using equivalent spring method is constant with top soil layer thickness for a given footing width and footing depth. Weighted average method equivalent subgrade module is non linearly increasing with top dense sand layer thickness for a given footing and bottom loose sand layer thickness. Equivalent subgrade module for thinner footing depth is always greater than the thicker footing for a given footing width and soil profile in both spring theory and weighted average method. Settlement along footing obtained by equivalent spring method equivalent subgrade modulus applied in Heteryni method equation is highly varying from weighted average method equivalent subgrade module applied in Heteryni method equation and PLAXIS 3D model settlement output. Equivalent spring method is considered as unsuitable to calculate the equivalent modulus of subgrade reaction for layered soil stratum. Settlement difference between PLAXIS 3D method and weighted average method equivalent subgrade module applied in Heteryni method equation shows up to 45 percentages and this difference cannot be negligible. This study will shed a light in the theoretical relationship of equivalent subgrade module research field as this would be the first attempt to check the behavior and suitability of equivalent subgrade modulus of layered soil stratum.
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    Low strain pile integrity testing for rock socketed bored piles in Sri Lanka
    (2020) Kodithuwakku TH; Tilakasiri HS; De Silva LIN
    Low strain pile integrity testing has been available over several decades. It is the widely used method of pile testing to detect serious defects in piles. The transient dynamic response (TDR) method of low strain pile integrity testing needs pile top velocity and pile top force generated by a small handheld hammer hit. The velocity and force details are useful to estimate the pile condition near the top and the stiffness of pile-soil system Researchers have proposed that dynamic stiffness at low frequencies associates to the static stiffness of pile head. The linear region of load-settlement behaviour of a pile is described by the static stiffness. However, little attention has been paid to developing a relationship between static stiffness and dynamic stiffness. The carrying capacity of pile is considered as the most important issue in pile foundations. Load Testing is the most reliable approach to evaluate the carrying capacity of piles. However, load tests are rarely performed as it is costly, labour intensive and time dependent, but all the piles are subjected to low strain integrity tests. Following the testing results, this research proposes a relationship between dynamic stiffness and static stiffness of bored piles. It is intended to evaluate the allowable carrying capacity of piles with results of low strain pile integrity testing and high strength dynamic load testing. Finally, this research presents a simple methodology to estimate the allowable carrying capacity of piles using instrumented low strain pile integrity testing. The developed methodology will be verified using field load testing results. In addition to that, the success of implementing the TDR method on bored piles is proved by case studies.