Master of Engineering in Foundation Engineering & Earth Retaining Systems

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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.
item: Thesis-Full-text
back analysis of slope failure at walipanna, southern express way
Idirimanna, IAND; Kulathilake, SAS
Slope failures due to excessive rainfall are a common geotechnical hazard in tropical countries where residual soils are abundant. These soils possess significant matric suctions in dry seasons and are in a stable state. Heavy infiltration of rainwater causes destruction of matric suctions, development of perched water table conditions and rise of ground water table. Thus shear strength is reduced causing slopes to fail. In order to understand the mechanism of rainfall induced slope stability it is necessary to model this process with a reasonable accuracy. Sri Lankan residual soil formations are formed by weathering of the metamorphic parent rock and have inherited significant abrupt variations in engineering characteristics as; soil water characteristic curves (SWCCs), variation of permeability with water content and unsaturated shear strength parameters. Cut slope at chainage of 42+340 to 42+400 in Walipanna at southern expressway failed after few days of rain. The back analysis of failure indicated that the safety margin is less than unity when saturated shear strength parameters of soil were used in the analysis. Infiltration of the rainfall that was recorded in nearby rain gauges was modeled using the SWCC and permeability function derived from the tests conducted on undisturbed samples recovered from the site. The presence of relict joints was confirmed during the rectification work and the combination of the relict joints and failed surface drainage system would have contributed to the failure. The results of the analysis also revealed that if the drainage measures are in position in perfect working order this failure would not have occurred. Those measures were found to be capable of tolerating even a rainfall of much higher intensity than that actually occurred. The modeling of infiltration revealed that the rise of ground water table is quite significant at the toe of the wall. Therefore when natural slopes are excavated into steeper profiles it is recommended to have a series of sub horizontal drains at the toe level even if the ground water table is found to be lower than the toe level. Also, the importance of routine maintenance of the drainage systems of all slopes is highlighted very strongly
item: Thesis-Abstract
Behaviour of Sri Lankan peaty soils under surcharging and vacuum loading
Thavakumar, T; Thilakasiri, HS
The composition of natural deposits of peaty soil may vary considerably among different sites, as do their engineering properties. Peaty soils have distinctive properties of low shear strength, high compressibility, high secondary compressibility and high initial permeability. This results excessive settlements and may continue for a long period of time due to the dominant creep behaviour. In addition, low shear strength often causes stability problems in peat soils when they are subjected to external loading. This criterion makes construction highway embankments over very soft highly compressible peat layers difficult. Therefore, in order to prevent differential settlements and subsequent potential failures, it is necessary to improve the strength and the stiffness properties of natural peat deposits using appropriate ground-improvement techniques. In this process, it is important to understand and to quantify the consolidation behaviour of peaty soils when employing ground improvement techniques and to predict the short term and long term settlements. Since the consolidation behaviour of the peat may be region-specific, it important to study the index properties and engineering properties of peaty soils. The present research study focuses on the engineering properties and index properties of peaty soils found in Southern Transport Development Project. The first objective of this study is to establish empirical correlations between some important properties such as Ce vs moisture content, C, vs eOt Ca vs Ce, C, vs Ce, C, vs organic matter content and ea vs organic matter content for STDP soils. Thereby update the empirical relationships for Sri Lankan peaty soils developed by Karunawardena (2000) using Southern Transport Development Project peaty soils empirical relationships. Second phase of this research study is to concentrate on consolidation behaviour of peat under surcharges. It is important to identify and separate the components of settlement mainly immediate settlement, primary settlement and secondary settlement. For this purpose graphical methods can be used. The objective of this study is to analyse the applicability of these graphical methods for Sri Lankan peaty soils and thereby predicting the degree of consolidation & hydraulic parameters of peaty soils from field monitoring data obtainedfrom surface settlement plates, subsurface settlement plates, and piezometers. Preloading can be used to improve the engineering properties of the peaty soils due to their high initial permeability. Successfulness of the pre loading depends on the stability of the embankment. This can be achieved by proper control of the pore pressure under embankment and the lateral displacement of foundation soil at the toe of the embankment. Third and final objective of this research study is to study about prediction and control the pore pressure under embankment and the lateral displacement of foundation soil at the toe of the embankment. Vacuum preloading method was applied first time in Sri Lanka at Southern Transport Development Project. The present study focuses on effectiveness of the vacuum pressure in controlling the pore pressures and the lateral displacements during the construction period.
item: Thesis-Abstract
Capacity estimation of piles using dynamic methods
Jayaweera, MSR; Thilakasiri, HS; Puswewala, UGA
The foundation is the most important part of any type of structure. Pile foundations are used to transfer very high loads from supper structure to the ground. Therefore, design and construction of piles must be carried out with a high confidence. Engineers are using various methods and equations for testing pile foundations and those methods are improving when new technologies come. Most reliable way of testing of pile is to have a full scale maintain load test on that pile. But this is become critical when large piles encountered. Therefore, application of High Strain Dynamic Test (HSDT) is getting popular. The use of pile foundations is increased and new driving techniques, as well as new sophisticated stress wave measurement equipments have led researchers to look for better understanding of dynamic and static behavior of the hammer-pile-soil system and to develop more reliable methods of pile analysis. The reliability of pile dynamic test is mainly dependent on the accuracy ofthe dynamic soil parameters used in the data analysis. The scope of this study is to find reliable driving equations for different situations by comparing the capacities taken from driving equations and HSDT measurements. Some piles were subject to both HSDT and Static load test to increase the reliability of readings.
item: Thesis-Full-text
Comparison between empirical, numerical and practical compression capacity of rock socketed bored and cast in-situ pile : a case study
(2020) Silva HAM; Puswewala UGA
The development of tall structures as a rapidly developing trend in Colombo-Sri Lanka is evident during the recent past due to the high land prices. These tall structures require to be founded on strong substrata and piling is the most popular method that has been used as the foundation for these tall buildings. In Colombo area having found bed rock at shallow depth around 15m to 20m, always design engineers tend to specify the rock socketed end bearing piles without much considering the load carrying mechanism of the pile. It is evident that Sri Lankan design engineering community has a tendency to disregard the pile shaft skin friction resistance, mostly due to the existence of bentonite slurry within borehole during concreting. Therefore, load carrying capacity of such piles is determined completely based on the end bearing from the bed rock. In addition to that in most standards and codes of practice, the pile load carrying capacity correlations are given for specific soil types i.e. sand, clay, gravel. However in local context it is hard to find such conditions and almost all the soils are residual soils having both 𝑐,∅ values. In this research, different correlations for pile load capacity and its variations are evaluated. A detail comparison is conducted between the compression capacity of piles obtained from different empirical/semi-empirical methods, numerical methods such as FEM and in-situ testing i.e. MLT and HSDLT against the code of practices and local guide lines.
item: Thesis-Abstract
Compressibility characteristics of municipal solid waste in meethotamulla waste fill site
(2021) Galhena GDWN; Kulathilake SAS
Municipal solid waste (MSW) is defined as solid waste generated from community, commercial and agricultural operations and it includes wastes from households, offices, stores, industries and other non-manufacturing activities. Management of municipal solid waste is a major problem all over the world. Recently, Municipal Solid Waste Management became a growing concern in Sri Lanka with the catastrophic landfill failures occurred at Meethotamulla. The need to ensure the stability of existing landfills which are mostly uncontrolled through appropriate engineering designs is a major task at present. MSW forms the largest portion of the landfill and its strength and stiffness (compressibility) characteristics controls all aspects of landfill designs. In this research compressibility characteristics of MSW at different stages of decomposition were evaluated under both saturated and unsaturated conditions. Considering the highly heterogeneous state of MSW, larger samples were tested using a Rowe Cell of diameter 150mm with loading, unloading and reloading increments. Characteristics such as; Coefficient of volume compressibility, coefficient of consolidation, coefficient of secondary consolidation, compression index, recompression index were determined to assess the applicability of conventional Terzaghi consolidation theory in modelling the MSW behavior. Results were compared with the behavior of residual soil and organic soils. The test results revealed that MSW experienced high primary and secondary consolidation settlements. These could be significantly reduced by preloading. The coefficient of consolidation values were quite high.
item: Thesis-Abstract
Construction of a factory building with ground improvement by pre loading
Madunishanthi, KA; Kulathilaka, SAS
Due the scarcity of land underlain with good subsoil conditions, Planners and Engineers are compelled to , use land underlain by poor subsoil conditions for new developments. \ factory complex for Star Packaging Pvt Ltd is to be constructed at Nawagamuwa. Ranala, Kaduwela manufacturing of card board boxes used in packaging. The site is accessible through a connecting road leading from Colombo Awisawella road. The site had been a low lying land in the past and a fill had been placed a few years back making it to a flat ground. significant settlements and bearing failures could be anticipated in the proposed building owing to the prevailing subsoil condition. As such, it is necessary to propose a suitable methodology considering the safty and economy issues.
item: Thesis-Full-text
Correlation between point load strength index and uniaxial compressive strength for hard crystalline rock
(2020) Jayasinghe JMSTW; Puswewala UGA; Rathnasir M
Large scale constructions such as highways, bridges, high rise buildings, jetties etc. are designed to withstand heavy loads depending on the dead loads and imposed loads on the structure. For this purpose, loads from these super structures are to be transferred to competent rock by adopting pile foundations. For end bearing piles, rock socketing has to be done to the competent rock. To identify competent rock, generally, the Point Load Strength (PLS) index (Is(50)) of a rock sample collected during the rock drilling work is determined and the Unconfined Compressive Strength value of the same sample is estimated. If the Unconfined Compressive Strength value is acceptable, the pile can be terminated. For estimating the Unconfined Compressive Strength value from the Point Load Strength value, there are some correlations which are generally used in the Sri Lankan construction practice. All of these correlations are outcomes of research work done in overseas countries. Therefore, this research is an effort of finding an acceptable correlation between the Point Load Strength value and the Unconfined Compressive Strength value for crystalline metamorphic rocks found in Sri Lanka to be used as a guideline for pile termination criteria. More than 130 rock samples were collected from different rock types and both the Unconfined Compressive Strength value and the Point Load Strength index of the same rock were determined by conducting laboratory tests. In addition to these two tests, the Specific Gravity (SG) value of the same rock was determined and the rock type was also identified visually to group/ classify the data according to the rock type. After analyzing the test data gathered, a correlation between the Unconfined Compressive Strength value and the Point Load Strength index was established for different rock types. Few samples from each rock type were used for validating the correlations established.
item: Thesis-Abstract
Densification of hydraulic sand fill by vibro hammer and quality control by cone penetration test : a case study
(2015-02-23) Atapattu, C; Hewage, ST
Densification of Hydraulic Sand Fill by Vibro Hammer and Quality Control by Cone Penetration Test Ground improvement techniques are widely adopted in Sri Lanka and are increasingly being used for a vast variety of projects with particular emphasis on infrastructure projects such as roads, ports and harbors. Ground improvement schemes work with the existing ground rather than bypassing it. They can be tailored to meet specific bearing capacity and/or settlement requirements and are generally much more cost-effective than other traditional methods such as piling. The process:  Reduces foundation settlement  Increases bearing capacity  Mitigates liquefaction potential  Provides slope stabilization  Prevents earthquake-induced lateral spreading Layer by layer compaction by means of roller or portable compactor is a widely used compaction method. However this method has limitations to undertake to a greater depths due to ground water problems that may be encountered. It is time consuming too. Under such circumstances the deep vibro compaction method is superior to the roller compaction methods. Vibro techniques consist of large depth vibrators, typically about 300 to 500mm in diameters, which penetrate the ground to suitable depth. Vibrocompaction known also as Vibroflotation in some references and countries consist on the in-situ densification of loose granular soils, such as loose sand, gravel and hydraulic fills, using vibro compaction equipment but without the addition of granular material.
item: Thesis-Abstract
Development of a large-scale shear apparatus for the determination of shear strength properties of municipal solid waste
(2020) Jayakody SHS; Kulathilaka SAS
Landfill slope failure became one of the hot topics in Sri Lanka as the infamous Meethotamulla waste fill collapsed, claiming lives of people and infrastructure. Every year all around Sri Lanka, millions of tons of Municipal Solid Waste (MSW) are produced, and no one can predict the variation and heterogeneity of their composition. Although MSW is a difficult material to test, many comprehensive studies have been conducted to determine the likely ranges of waste properties and hence to take them in the design of landfills. In this study, in-situ direct shear test was performed to assess and develop insights about the shear strength properties of MSW. A large-scale direct shear apparatus was fabricated to conduct in-situ test which has dimensions of 300 mm × 300 mm in plan view. The report contains the steps followed to manufacture the large-scale direct shear device. One of the main objectives was to test the MSW at differently aged locations in abandoned Meethotamulla waste fill site. The testing procedure is comprehensively described in the report. Further, density test, moisture content test and composition analysis were also conducted in this study. As stated by many researchers a peak failure state was not observed during in any of the direct shear tests. There was no significant variation in the shear stress parameters of the differently aged samples. However, all the samples are more than 03 years old. The results obtained from the tests were analyzed and compared with the published data in literature. Recommendations were made regarding the further studies needed to develop a relationship between the differently aged MSW and their shear strength properties.
item: Thesis-Abstract
Development of empirical correlations between California bearing ratio (CBR) and soil index properties.
(2015-02-28) Jayamali, KVSD; Nawagamuwa, UP
The California Bearing Ratio is a penetration test for evaluation of the mechanical strength of road sub-grades and base-courses. This can be used as a mean of designing the road pavement required for a particular strength of sub-grade by comparing the strength of different sub-grade materials. However; civil engineers always encounter difficulties in obtaining representative CBR value for design of pavement. Over the years, many correlations had been proposed by various researchers in which the soil index properties were used to develop these correlations. A study was carried out to find correlations between CBR value with soil index properties those best suit the type of soils in Sri Lanka. Analyses were carried out based on the published correlations and soil data obtained from several Sri Lankan project sites. Based on the results, it is observed that the current published correlations are not in good agreement with Sri Lanka soils. In addition, no typical range could be found based on the soil index properties. Mechanical Strength of soil depends not only on the soil type but also on the observable physical characteristics which significantly influence on a soil’s behavior. Therefore, a method is proposed for correlating soaked CBR value and compaction parameters with such index properties, for Sri Lankan soils. This research covers the entire soil types according to Unified Soil Classification System which are generally used as sub-grades and base-courses. Among the several soil index properties, Atterberg Limits and grain size distribution data are used in this regard as these tests are much more economical and rapid than Compaction and CBR tests. The correlations are established in the form of an equation as a function of different soil properties by the method of regression analysis. Finally, results of the laboratory test are used to compare with the results of regression equation for the compiled data for the validation of the correlation. Key Words : California Bearing ratio, Compaction Parameters, Index Properties, Regression Analysis
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
item: Thesis-Abstract
Effect of rectification measures in Kahagolla landside - comparison of monitoring and analytical data
(2022) Thilakarathna PADT; Kulathilaka SAS
This research paper focuses on creep movements of rainfall-induced landslides with their groundwater level fluctuation, to understand the pore water pressure development in saturated/ unsaturated soil layers in relation to the mechanism of failure. A case study was selected at Kahagolla Sri Lanka, which is a massive creep landslide initiated around 1957 and triggering by prolonged rainfall events. The stabilization of the Kahagolla landslide was carried out under the “Landslide Disaster Protection Project” implemented by the Government of Sri Lanka with the support of Japan. Detailed geotechnical investigation along with real-time monitoring data showed mainly four slip surfaces along the landslide axis. The main reason for movement is discovered as the rising of groundwater table and subsequent loss in the slip surface strength. Two-dimensional analyses were carried out with several back analyses and adjusted parameters according to real-time monitoring data. Limit equilibrium analysis coupled with a seepage model was performed to confirm the actual conditions of the landslide occurrence. Thereafter, effects of rectifications were also modeled to access the stability status of the rectified landslide. The performance of the rectification measures was further examined with critical design rainfalls and a threshold for the rectified landslide. The results show an acceptable stabilization of terrain after the construction of counter measures. It can be concluded that the final combination of rectifications has been succeeded in the stabilization of this landslide and the above-mentioned approach is appropriate for use in the simulation of deep-seated landslides.
item: Thesis-Full-text
Effectiveness of subsurface drainage and vegetation in enhancing the slope stability : a comprehensive case study on Badulusirigama landslide
(2020) Kankanamge LUM; Kulathilaka SAS
Slope instability, triggered by excessive rainfall, is one of the common geo –hazards that geotechnical engineers are challenged with in tropical countries such as Sri Lanka. Typically, these slope failures are initiated in colluvial layers derived from former landslides or planes of low shear strength in differently weathered zones in the thick soil overburden. Improvement of surface and subsurface drainage has proven to be effective in improving the slope stability by lowering the ground water table as well as preventing near surface perched water table conditions. Badulusirigama Landslide in central highlands of Sri Lanka is an example for a slow moving long rotational slip that activates after heavy rainfall events. The landslide was rectified with over 45 m long individual sub-horizontal drains that are arranged into a network of radial drainage groups at different elevations along the long sliding mass. This site is also well equipped with monitoring instruments and thus provides a great case history to further our understanding on contribution of surface and subsurface drains in mitigating landslides. In this study, the effectiveness of the introduced subsurface drainage measures in enhancing the stability of the Badulusirigama Landslide was investigated using 2D and 3D numerical models. The numerical models were then used to predict the behaviour of the landslide during different anticipated rainfall events. The results show that the subsurface drains system enhances the initial near failure condition of the site to a stable slope with a factor of safety of over 1.25 within one month. After initial drop down of the ground water table, the sub-horizontal drains still remain effective by rapidly draining out any infiltration. The analysis also shows that the width of the influence zone of radial horizontal drains should be carefully selected when simplifying the problem into 2D plane strain models because the influence can be very much localised in a low permeable medium. Possibility of introducing surface vegetation as a hybrid measure along with subsurface drainage was also investigated. A factor of safety improvement of 38% and 16.3% was achieved after the simulation of construction of the drains in 2D plane strain and 3D finite element analyses separately. Also, it was found that, vegetation could result in increasing the hydraulic conductivity of the root zone, leading to development of perched water table conditions.
item: Thesis-Full-text
Enhancement of the stability at the site of an ancient landslide in a road cutting with drainage and reinforcement - case history at Ginigathhena
(2019) Lakruwan SOADM; Kulathilaka SAS
Engineers involved in infrastructure development projects in the hilly terrain of Sri Lanka encounter ancient landslides which could be triggered by rainfall or construction activities. One such landslide was encountered during the widening of a bridge in the main connecting road between central hill country and capital; Avissavella – Hatton – Nuwaraeliya road at bridge no. 48/2 near Ginigathhena. Extensive mitigation measures had to be designed with detailed attention to construction sequence in order to prevent reactivation of the slide. A valley area had been formed by the previous landslide. Morphology of the area is a sloping land with undulating topography towards upper slope. This has led to the formation of a waterlogged marshy area on a flat land at immediate upper slope and a stream flowing through valley. Water table of the area is quite high. The landslide got activated due to a minor excavation at the toe region for the bridge widening. There had been no rain when the slide was activated. Subsequent rain cause further activation of the landslide. Further widening is necessary according to the new highway design. Ground water regime management and geometry modification are the two primary approaches used in enhancing the safety margins of the site. Surface and subsurface drainage improvement by various methods such as; cutoff drains, berm drains, trench drains and horizontal drains were introduced for lowering the ground water table. The stability of the steep cuts necessary to accommodate the increased road width was enhanced further by the use of soil nailing. Top down approach was adopted to ensure the safety of the slope during construction. Drainage measures were very effective in economizing the soil nailing design. The analysis and design of stabilizing measures were done using GeoStudio Seep/W and Slope/W software. Design outcomes were confirmed by monitoring of ground water table and surface movements of the slope.
item: Thesis-Full-text
Establishment of fundamental characteristics of some unsaturated Sri Lankan residual soils
Vasanthan, N; Kulathilaka, SAS
Slope failure in tropical climates frequently occurs due to excessive rainfall. Heavy infiltration causes destruction of matric suctions, development of perched water table conditions and rise of ground water table. Severe erosion and surface destruction will also be caused by the heavy prolonged rainfall. In order to understand the threshold values of rainfall leading to instability it is necessary to model this process with a reasonable accuracy. Sri Lankan residual soil formations are formed by weathering of the metamorphic parent rock and have inherited significant abrupt variations in engineering characteristics. Basic characteristics of these soil formations such as soil water characteristic curves (SWCCs), variation of permeability with water content and unsaturated shear strength parameters are essential parameters in these analyses. These characteristics have not been established for typical residual soils forming slopes in Sri Lanka. This thesis highlights the need for detailed experimental studies and presents comprehensive studies that have been conducted at the University of Moratuwa and National Building Research Organization (NBRO) laboratories to establish the fundamental characteristics of unsaturated Sri Lankan residual soils. Undisturbed samples of soil obtained from the failed slope at Welipenna in the Southern Expressway were used in this study. Direct shear tests were done by modifying the conventional apparatus by incorporating a miniature tensiometer which allows for the simple and direct measurement of soil matric suction during shearing. Soil water characteristic curves (SWCCs) were also established using these apparatus. Alternatively, pressure plate apparatus was also used for this purpose. In addition to that, soil water characteristic curves (SWCCs) were developed from gradation curve also.Permeability of an unsaturated soil varies considerably with the level of saturation and will make a very significance influence on the infiltration process. Permeability function which defines the variation of permeability with matric suction was investigated on undisturbed samples. The method is based on continuously drying and wetting the soil sample while continuously monitoring the suction gradient and the change in soil mass. The thesis highlights the importance of these studies and presents the procedures that are being used.
item: Thesis-Full-text
Estimation of the stability of embankment slopes using field monitoring data
Nawarathna, THK; De Silva, LIN
In Sri Lanka lands underlain by soft, weak and problematic soil are being use for various constructions due to rapid development and the lack of suitable lands. Construction of an embankment over soft soil is challenging due to its low shear strength and high compressibility nature. As a result of that embankment can be subjected to a shear failure or excessive settlement. Assessment of the stability of the embankment is vital to ensure a safe embankment and stability can be evaluated by using available analytical methods or field monitoring data. Use of field monitoring data to evaluate the stability is easy and more practicable. In Sri Lanka Matsuo chart which is based on the field monitoring data was used for the prediction of stability of the embankments in the Colombo Katuanayaka highway project. However applicability of Matsuo chart for various embankment conditions is still questionable. This study was carried out to investigate the applicability of Matsuo chart for various embankment conditions using advanced numerical tools. Two test embankments published in the literature and three embankments, which belong to the Colombo - Katunayaka expressway were analyzed by using Finite Element Method, Limit Equilibrium Method and Matsuo chart and compared with the field data. Further, the effect of embankment width, height and the subsoil parameters on the prediction of stability using Matsuo method was investigated. Research finding verify that the Finite Element Method, Limit Equilibrium Method and Matsuo chart can predict the stability of the embankment accurately and Finite Element Method can be used to predict the deformation characteristics. Stability of the embankment is directly proportional to the un-drained shear strength of the sub soil and Factor of Safety values decrease with the increase of the embankment height. However it was found that the embankment width has little influence on the factor of safety.
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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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Evaluation of shansep parameters for Sri Lankan cohesive soils
(2021) Senewimala EH; Thilakasiri HS; Nawagamuwa UP; De Silva, LIN
This thesis contains a study on SHANSEP parameters, evaluated for Sri Lankan cohesive soils. In Sri Lanka, various correlations are used for the evaluation of shear strength parameters and the settlement. Most of such correlations are developed in overseas countries, which may not accurately model the behavior of Sri Lankan soils as they are developed from other geological conditions. Though the undrained shear strength is a function of both stress history and stress path, most of the time, they are not considered, which may lead to large errors. The SHANSEP model proposed by Prof. Charles C. Ladd, shows the normalize behavior of the cohesive soils which consider both stress path and stress history in determination of the Undrained Shear Strength of Soils. Therefore, this research is an effort to see the applicability of SHANSEP model for Sri Lankan cohesive soils, using the test data provided by major projects in Sri Lanka. Since CK0U Triaxial testing facilities are not available in Sri Lanka, data from field vane shear test have been used for the estimation of Undrained Shear Strength in this study. Finally, a SHANSEP equation has been proposed in this thesis for selected alluvial clay soils along with two more conservative equations for the estimation of undrained shear strength and over consolidation ratio respectively.
item: Thesis-Abstract
Extrapolation of plate load test results to foundations on sand
Priyantha, S.M.
Estimation of the settlement of shallow foundations in sand is generally carried out using the settlement estimation equations, empirical correlations based on insitu tests or by extrapolation of plate load test results. Plate load test is a common field test used to estimate allowable bearing capacity, expected settlement and/or the modulus of subgrade reaction of soil. The results obtained from a plate load test are extrapolated to prototype foundation using different extrapolation methods with unknown accuracy levels. There is no standard way to extrapolate the plate load test results and Terzaghi and Peck method is very widely used for most engineering applications. In most practical cases, layered soil mediums are present below foundations. In such situations the effective zone of test plate lies within the top layer whereas that of the prototype foundations is spread to the underlying layers. The behaviour of the plate is based on the properties of the top most layers while the stiffness of the layers below may affect the behaviour of the prototype foundation. In such situations, the reliability of the extrapolation techniques becomes highly questionable. Due to the uncertainties associated with the estimation of the settlement of shallow foundations in sand, this research is aimed at investigation of the settlement estimation methods of shallow foundations in sand using the finite element method. , A special attention is paid to investigate the reliability of the extrapolation of plate load test results. The scope of the study: 1. To investigate the ·reliability of settlement estimation methods in relation to the finite element method. 2. To study the effect of foundation width, foundation depth, elastic parameters (E, 0) and plastic parameters (c, ~) of homogeneous soil on the estimated settlement of shallow foundations. 3. Investigation ofthe accuracy of the extrapolation of the plate load test results in homogeneous and layered soil mediums, compared to the same estimated from the finite element method. 4. To develop a reliable method to estimate the settlement of a foundation .on layered soil using plate load test results.