Browsing by Author "Thilakasiri, HS"

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item: Conference-Full-text
Analysis of settlement monitoring data to assess the state of primary and secondary consolidation of soft organic soil deposits
(Department of Civil Engineering, University of Moratuwa, 2012-12) Premalal, RPDS; Jayasinghe, TWASL; Indrachapa, KRTH; Thilakasiri, HS; Nawagamuwa, U; De SIlva, LIN
Organic soil deposits under road embankments may cause consolidation settlement problems during the serviceability stage. It is not feasible to design road traces on such soil deposits using the traditionally used most unfavourable ground conditions. Instead the most probable ground conditions are used in the initial design and this approach is referred to as the observational approach. The risks associated with using most probable soil properties are managed by reviewing the initial design through the analysis of the observational data obtained during the construction stage. In this regards, analysis tools to assess the stability of the embankment during construction stage and the estimation of the degree of consolidation during ground improvement phase are extremely important. Even though there are large number of analytical methods available to use for the data analysis, not much research is done to identify the tools relevant to the organic soft soils found in Sri Lanka. This research is aimed at investigation of the analytical tools that can be used in the analysis of settlement monitoring data obtained during the implementation of the observational approach for construction of the road embankments over the soft soil deposits in Sri Lanka.
item: Conference-Full-text
Analysis of Settlement Monitoring Data to Assess the State of Primary and Secondary Consolidation of Soft Organic Soil Deposits
(2014-05-07) Premalal, RPDS; Jayasinghe, TWASL; Indrachapa, KRTH; Thilakasiri, HS
Organic soil deposits under road embankments may cause consolidation settlement problems during the serviceability stage. It is not feasible to design road traces on such soil deposits using the traditionally used most unfavourable ground conditions. Instead the most probable ground conditions are used in the initial design and this approach is referred to as the observational approach. The risks associated with using most probable soil properties are managed by reviewing the initial design through the analysis of the observational data obtained during the construction stage. In this regards, analysis tools to assess the stability of the embankment during construction stage and the estimation of the degree of consolidation during ground improvement phase are extremely important. Even though there are large number of analytical methods available to use for the data analysis, not much research is done to identify the tools relevant to the organic soft soils found in Sri Lanka. This research is aimed at investigation of the analytical tools that can be used in the analysis of settlement monitoring data obtained during the implementation of the observational approach for construction of the road embankments over the soft soil deposits in Sri Lanka.
item: Conference-Extended-Abstract
Application of GIS in 3G modelling
(2002) Wijesekara, GN; Thilakasiri, HS; Wickramasinghe, AM; Wickramanayaka, AN
The research primarily was based on application of principles of GIS (Geographical Information System) technology in the field of Geotechnical Engineering. With the objectives being laid towards a model to be applied in Geotechnical Engineering, the authors were able to make plans in phases while solving the requirements in steps, until the whole idea was finalised towards the end of the research. With this project, the objectives, feasibility and foundation of a large-scale implementation were established with an introduction to a new concept - 3G Modelling.
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: Conference-Extended-Abstract
Consolidation properties of soft organic soils in Sri Lanka
(2007) Thilakasiri, HS
Most of the new coming highways in Sri Lanka are run through low-lying areas mainly to minimize the damages to the existing structures and to the livelihood of the people living in the buildup areas. The shallow subsurface of these low-lying areas consists mainly of very soft highly compressible organic soils or soft alluvial deposits. Due to the high flood levels above the existing ground surface in the low-lying areas, high embankments are needed to avoid flooding of the highway. As a result, geotechnical engineers are faced with a tremendous challenge of construction of high embankments over thick very soft highly compressible soil deposits. Due to shear failure and large settlement of the soft soil deposits under the embankment loading, these soft soil deposits must be improved before the placing of the embankment fill or during placing the fill. There are different types of treatment methods used in Sri Lanka such as preloading, preloading with vertical drains, sand columns, stone columns, piled embankment, vacuum consolidation etc. The actual method of treatment depends mainly on the thickness of the soft deposit, consolidation and shear strength properties of the soft deposit, height of the embankment etc. Therefore, it is essential that the properties of the soft soil are determined during the initial stage of the design to decide the type of the soft ground treatment method as well as during the design stage. Determination of the strength and compressibility properties of commonly found soft soil deposits using undisturbed samples is a no easy task due to the inability to recover undisturbed samples and difficulties associated with preparation of test samples and testing.
item: Conference-Extended-Abstract
Development of negative skin friction on pilesinstalled through soft compressible soils in Sri Lanka
(2006) Thilakasiri, HS
Use of grounds with soft compressible layers at shallow depths for development projects requires the use of pile foundations to support the structural loads. Piles installed through soft compressible soil layers are subjected to Negative Skin Friction (NSF) as a result of consolidation of the soft compressible layers due to placing fills, lowering of the ground water table, pile driving etc. The NSF, or commonly referred to as the drag force, is generally estimated from the skin frictional resistance due to the consolidating layer and layers above it. However, it is shown by the researchers [Bjerrum et al. (1969) and Endo et al. (1969)] that the NSF is not developed up to the bottom of the consolidating layer but only upto a point, referred as the neutral point, above the bottom of the consolidating layer. Moreover, the results of full scale field tests conducted by researchers clearly indicates that the development ofNSF is not due to the total stress but due to the effective stress acting on the pile shaft. Moreover, it is experimentally shown that the development of the ultimate NSF requires very small relative displacement between the pile and the soil. Furthermore, it is experimentally shown that the action of short-term live loads reduces the developed NSF. Therefore, the NSF and short-term live loads are not simultaneously acting on the pile.
item: Article-Abstract
Earthquake induced kinematic forces on pile foundations in fayered medium
Thilakasiri, HS; Potts, DM; Kontoe, S; Zdravkovic, L
The kinematic forces induced on piles in two-layered soil medium are investigated using finite element method. Typical layered subsurface condition with the soil properties relevant to Sri Lanka are considered in the present study. Hyperbolic nonlinear constitutive model is used for the soil medium and the soil medium is modelled as a fully coupled system. The effect of the earthquake was simulated by lateral ground cceleration applied to the bedrock. The validity of the results of the proposed model was established by comparing with the trends observed in similar studies reported in the literature. The effects of the stiffness of the soil layers, pile diameter, fixity at the pile head, and a defect present at the layer interface on the kinematic forces are investigated in the present study. Finally, the use of the results of the present study in the design and construction of bored and cast insitu concrete in Sri Lanka are highlighted.
item: Article-Abstract
Empirical correlations for Sri Lankan peaty soils
Thavasuthan, T; Thilakasiri, HS
The composition of natural deposits of peaty soil may vary considerably among different sites, as do their engineering properties. Peat, generally formed in marshy and water logged areas, is usually dark brown to black in colour, has distinctive odour of decaying vegetation, is spongy in consistency without exhibiting distinct plasticity and has amorphous and fibrous texture. As a result, engineering properties of peaty soils are significantly different from most of the inorganic soils. The important engineering properties of the peat are permeability, compression index, coefficient of secondary compression and shear strength. These properties may vary due to chemical and biological conditions. Humification of organic constituents alters the compressibility, shear strength and hydraulic conductivity. There are useful relationships found between index properties and mechanical properties for peaty soils. The preliminary design of structures to be built on peats can profit from these relationships, sometimes even more so than when dealing with clays, due to the difficulty of obtaining high quality peat samples and to the extreme variability of peat deposits. Since the engineering properties of peaty soils are region specific, the empirical correlations for peaty soils found in Southern Transport Development project (STOP) and the updated correlations for Sri Lankan peaty soils are presented in this study.
item: Conference-Extended-Abstract
Estimation of skin friction on bored and cast In-situ concrete Piles in sand
(2009) Mohotti, D; Thilakasiri, HS
Bored and cast in-situ piles in Sri Lanka are design as end bearing piles and the skin friction contribution is generally I neglected. Results of the high strain dynamic load tests are presented to show that a significant skin friction capacity is developed on bored and cast in-situ piles. The distribution of the skin friction along the pile shaft, obtained from high strain dynamic testing of piles, is used to investigate the accuracy of the commonly used skin friction capacity estimation methods.
item: Conference-Extended-Abstract
Estimation of the ultimate carrying capacity of bored piles using the static load settlement curve
(2007) Thilakasiri, HS
Testing of bored piles is carried out to investigate the load-settlement behavior of the tested pile. The pile is loaded upto about l.5 times the working load to establish the loadsettlement behavior. The termination of the load testing at 1.5 times the working load is mainly due to the cost associated with providing the reaction load required to load the pile. If the pile doesn't reach the ultimate failure before the final test load, the ultimate carrying capacity of the pile cannot be obtained directly from static load testing. Therefore, it is not possible to obtain the Factor of Safety (FoS) of the pile against the shear failure of the soil surrounding the pile.
item: Conference-Abstract
Improvement of Sri Lankan peaty soil by vacuum consolidation
Ariyarathna, PRC; Thilakasiri, HS
Vacuum consolidation was used to improve highly compressible soft organic soil encountered in the Southern Transport Development Project in Sri Lanka. As this method of soil improvement is used for the first time in Sri Lanka, several trial embankments were built to study the feasibility and the effectiveness of the vacuum consolidation method in improving the Sri Lankan peaty soils prior to adopting the same in main construction work. At the trial embankment sites the founding soil mainly consisted of very soft highly compressible peaty or organic soil layers of thickness varying from 3 to 8 m. The soft layers were improved by a combined vacuum consolidation and surcharging with 5 to 9m high stage constructed embankments to compensate the primary consolidation settlements and to minimize the secondary settlements that can take place during the service life of the road. The improvement in the peaty soil due to the vacuum consolidation was assessed from the data obtained through the field monitoring program and the results obtained from the laboratory and field tests carried out on improved and unimproved ground areas. The obtained data was analyzed to investigate the effectiveness of vacuum consolidation process in improving the properties of Sri Lankan peaty soils. Furthermore, the obtained data was analyzed to quantify the improvement in the strength gain due to vacuum consolidation and compressibility properties of Sri Lankan peaty soils as a results of the over consolidation due to vacuum consolidation process. In addition, some of the already published empirical correlations for Sri Lankan peaty soils were improved by including the data obtained from the field and laboratory testing programme carried out under the present study.
item: Article-Abstract
Investigation of the accuracy of the dynamic methods of capacity estimation of piles
Thilakasiri, HS; Jayaweera and, R; Abeyasinghe, RM
Capacity estimation of piles using dynamic methods is considered in this paper. Commonly used dynamic methods in Sri Lanka, high strain dynamic load testing and pile driving equations are considered in detail. The accuracy of the CAPWAP method of capacity prediction was discussed considering the results of the available comparison studies. Considering the CAPWAP predicted capacities and the capacities estimated from the Case method, the applicability, of the commonly used Case damping factor of 0.5 was investigated. Furthermore, Case damping factors to be used in the field high strain dynamic load testing are also suggested. The capacities estimated from the CAPWAP analysis of the dynamic load test data and the measured set of the pile during high strain dynamic load testing are used to evaluate the accuracy of the capacity predicted by pile driving equations. Based on the data collected, the accuracy of each pile driving method is discussed. Furthermore, the factors affecting the accuracy of the pile driving equations are also investigated. Moreover, the factors of safety to be used with different methods are also presented.
item: Article-Abstract
Investigation of the accuracy of the elastic settlement estimation methods and the extrapolation techniques of plate load test results
Thilakasiri, HS; Priyantha, SM
Accurate estimation of the settlement of shallow foundations is an important aspect in the design of such foundations. Most often elastic settlement estimation methods are used in the estimation of settlement of shallow foundations. In addition, plate load tests are also performed to estimate the settlement of shallow foundations. Since a smaller size plate is used in the plate load test, the settlement obtained from such tests should be xtrapolated to the actual size footings. Even though there are large number of settlement estimation methods and extrapolation techniques for plate load test results, the verification studies of the accuracy of the methods are very rare. It is very often observed that the settlement of shallow foundations are estimated based on the plate load tests carried out in layered soil medium, where a stiff fill soil layer overlying a soft soil. Extrapolation techniques of the plate load test results are highly questionable under such situations. A finite element model was developed using the software called PLAXIS to simulate the settlement of footings. The developed model was used to model some case studies presented in the literature to investigate the accuracy of the model predictions. Thereafter, parametric studies were carried out to investigate the accuracy of: (i) commonly used elastic settlement estimation methods; (ii) the commonly used extrapolation techniques in the homogeneous subsurface conditions; and (iii) the commonly used extrapolation techniques in the two layered medium with a hard layer overlying a soft layer.
item: Conference-Extended-Abstract
Investigation of the dynamic soil properties to be used in interpretation of dynamic load test results
(2006) Thilakasiri, HS
Testing of piles using high strain dynamic load testing using Pile Driving Analyzer (PDA) has become very popular among Sri Lankan geotechnical engineers. In the high strain dynamic pile load testing, a pile instrumented at the top with accelerometers and strain gauges, is impacted with a heavy hammer to generate a large strain compression wave, which propagates along the pile shaft. The measured strain is converted to force using cross sectional area and elastic modulus of the pile, and acceleration is integrated into velocity. There are mainly two methods used in the analysis of the velocity and the force measurements at the pile top: (i) Case method - using the wave propagation theory and assuming linear elastic pile material, and dynamic soil properties; and (ii) CAPW AP method - using a combined wave equation soil model and a continuous pile model to iteratively determine the unknown soil parameters by matching the field recorded velocity and force measurements at the pile top. The PDA operator in the field will use the Case method, with assumed Case damping factor (Jc) and wave velocity (C) to check the mobilized capacity under a given hammer blow and the integrity of the pile. When the mobilized capacity estimated from the Case method exceeds the required capacity, the PDA operator will terminate the test. Thereafter, measured velocity and force records are down loaded to a computer and a more rigorous matching process is used to obtain more accurate strength and deformation characteristics of the pile tested. In this research, the soil material parameters used in the field with the Case method, namely Case damping factor and the wave velocity are investigated.
item: Article-Abstract
Kinematic and inertial effects of earthquakes on rock socketed single piles in a two-layered medium
Thilakasiri, HS
The behaviour of piles in a two-layered soil medium subjected to earthquake ground accelerations are investigated using finite element method. The kinematic and inertial effects on rock socketed piles in a two-layered soil medium are investigated in this study considering the soil properties relevant to Sri Lanka. Hyperbolic nonlinear constitutive model is used for modelling the behaviour of the fully coupled soil medium. The effect of the earthquake was simulated by lateral ground acceleration applied to the bedrock and in this respect two earthquake records measured at a considerable distance away from the epicentre are used in the numerical simulation to take into account large epicentral distance to an anticipated earthquake that might affect Sri Lanka. The validity of the results of the proposed model was established by comparing with the trends observed in similar studies reported in the literature. A detailed parametric study of the kinematic bending moments developed in piles at the layer interface of the two-layered medium is carried out using the developed model. Furthermore, the effects of the inertial forces on the kinematic bending moment developed at the layer interfaces are investigated by varying the pile diameter and the effective masses at the pile head. Moreover, the variation of the bending moments, developed in the pile due to the combined effects of the kinematic and inertial responses of the pile-soil system, with the effective mass at the pile head for different pile diameters is also investigated.
item: Thesis-Full-text
Optimization of uplift capacity of transmission tower foundations
Gunasena, UPP; Thilakasiri, HS
Transmission tower lines using lattice towers and concrete foundations are the common practice in most of the countries all over the world. A significant amount of the cost of the transmission tower line is spent for the tower foundation construction. Uplift capacity of transmission tower foundation is determined based on the assumption that, the uplift force is resisted by the weight of the footing and the weight of the soil inside the volume of inverted frustum. Hence, the uplift capacity of the foundation is a major aspect of determining the dimensions of the tower foundation. However, the values of the frustum angle seem to be arbitrary and the failure pattern is more likely to be varied from the assumed pattern. Apart from that, the frustum angle is unsymmetrical for the inclined loadings. Further, the estimated uplift capacity shall also be reassessed according to the failure plane variations. As the first step of this study, a research survey was conducted on the available empirical methods of estimating the uplift capacity. Data were collected on transmission tower types and their foundation types based on different soil categories. This report will use PLAXIS; a finite element software to analyze the uplift capacity of transmission tower foundation. A comparison between the results from the software analysis and the capacities given by empirical methods is included in the report. An evaluation on the assumptions made on frustum angle, composition of uplift capacity to determine the uplift capacity of the transmission tower foundations is also given in the report. This analysis also includes the failure patterns, uplift capacity, composition variation of uplift capacity for different types of foundations used for transmission towers. Conclusively, this will evaluate and make a recommendation on determination the uplift capacity of transmission tower foundation, assumption of the frustum angle and the composition of the uplift capacity.
item: Thesis-Abstract
PDA Test as a method of pile testing for bored and cast in-situ R.C. piles end bearing on rock
Abeysinghe, RM; Thilakasiri, HS; Tennakoon, BL
The Pile Driving Analyzer (PDA) is a powerful tool to assess pile driving and High Strain Dynamic Pile (HSDP) load testing which may supplement or replace static testing. The PDA is seeing increasing usage in the testing of the various types of cast-in-place piles. In some parts of the world, this type of testing is the most common use for the PDA. This solution is particularly attractive for very large, high capacity piles where the large capacity makes a static test extremely costly. The rapidly increased use of pile foundations and the appearance of the new driving techniques, as well as stress wave measurement equipment have led researchers to look for better understanding of the dynamic and static behavior of the hammer-pile -soil system and to develop more reliable methods of pile analysis. The reliability of the pile driving analysis is primarily a function of the accuracy of the soil parameters and the theological model of the soil. The application of stress wave theory on piles is critically revived in literature. The scope of this study is an attempt to find a reliable range for dynamic soil parameters for end bearing bored piles in residual formation and to improve the accuracy of the wave equation analysis methods. The investigation has been focused on the comparison of the soil parameters used in practice from those used in conventional soil mechanics and soil dynamics. The solution of the stress wave equation is based on Smith's approach. Dynamic analysis of the field-recorded data was performed using CAPW AP (Case Pile Wave Analysis Program) and based on the Case Method. Ground investigation is done for evaluation of static soil properties before installation of piles. Static pile load tests were also performed on the same pile which were subjected to dynamic load test to study the load settlement behavior of the bored cast insitu piles. Further more, the static' piles capacity variation of driven piles with time termed set-up effect was studied in details for piles driven in residual formations.
item: Conference-Full-text
Prediction settlements of foundations on soft soils - a case study
(1999) Thilakasiri, HS; Peiris, TA; Tennakoon, BL
Excessive settlement of engineering structures and subsequent correction procedure have become a major concern for both the public and the engineering community. The severity of the above problem has increased due to the use of more filled marshy lands for construction. If one studies the problem of excessive settlement of most domestic structures constructed on fills, it could be clearly attributed to the violation of fundamental engineering principles. Lack of engineering knowledge and judgement or sheer negligence of basic engineering principles could be main reasons. If a structure is to be constructed on soft grounds it is clear that the ground should be improved before the construction or the construction procedure should be phased out in such a way that the final settlement, more importantly differential settlement, is within allowable limits for that particular structure. Pre-loading (with or without sand drains), replacement of the soft soil with good material, dynamic replacement and electro-osmosis are some of the methods used for improvement of clayey or peaty deposits. Of the above methods, pre-loading is widely used due to its economy and simplicity. Important considerations in pre-loading are: duration of pre-loading; height of pre-load heap (surcharge load) and the monitoring of settlement experienced. In this paper a case study of construction of a building in marshy lands violating some basic engineering principles is presented. Use of some basic soil properties together with in-situ testing (e.g. Standard Cone Penetration Test) to predict the ultimate settlement under the pre-loading is illustrated. Furthermore, the rate of settlement, an essential component of the pre-loading design, is obtained by curve fitting the settlement data while the pre-loading is in progress. This method of using basic soil properties together with in-situ tests to predict ultimate settlement and curve fitting data to predict the rate of settlement does not require the almost impossible process of obtaining undisturbed soft soil samples for laboratory testing.
item: Conference-Abstract
Reliability of statnamic load testing of rock socketed end bearing bored piles
Thilakasiri, HS
The pile load testing methods could be broadly classified into three categories: static, rapid and dynamic depending on the rate of loading. In this paper, the rapid load testing method referred to as the Statnamic test is discussed. The commonly used analysis method of the statnamic testing referred to as the Unloading Point (UP) method is used successfully for the floating piles but validity of some of the assumptions of the unloading point method to end bearing bored piles is questionable. Due to this problem, other analytical methods such as: Modified Unloading Point (MUP) method, Segmental Unloading Point (SUP) method and other signal matching techniques are introduced by some researches. Therefore, the validity of the unloading point method to rock socketed end bearing bored piles in Sri Lanka is investigated in this paper. This investigation is carried out using the commonly used wave number. Furthermore, the wave equation method, commonly used numerical procedure to model dynamic behavior of piles, is used by the author to investigate the validity of the assumptions associated with the unloading point method to rock socketed end bearing bored piles.