Browsing by Author "De Silva, LIN"

Now showing 1 - 20 of 54

item: Conference-Abstract
Analysis of stability enhancement of soldier pile retaining wall
Athmarajah, G; De Silva, LIN
In construction industry, Soldier pile retaining wall with timber laggings is one of the most common retaining walls. It is relatively fastest and cheapest method. Stability of the soldier pile retaining wall is the main consideration during the design and construction period. To predict the lateral wall deformation for excavations, it is important to perform a numerical analysis, because the deflection of the soldier pile retaining wall is critical in practice. In the present study, theoretical analysis and numerical analysis were carried out to analyse the stability of the soldier pile retaining wall with the presence of surcharge. This research mainly focusing on the stability enhancement of the soldier pile retaining wall considering the factors such as, depth of embedment of the steel H-sections, spacing between steel H-sections, width of the steel H-section and soil parameters. Also, this paper presents the results of theoretical study using limit equilibrium method to assess the stability of the wall by considering the factor of safety. And, two-dimensional finite element model (FEM) of soldier pile retaining wall with horizontal timber laggings was created in Plaxis 2D software to estimate the lateral displacement of the wall and bending moment capacity.
item: Conference-Abstract
Behaviour of rock socketed pile groups
(Department of Civil Engineering, University of Moratuwa, 2024) Gunasekara, HWP; De Silva, LIN; Pasindu, HR; Damruwan, H; Weerasinghe, P; Fernando, L; Rajapakse, C
Rock socketed pile groups are extensively employed in the construction industry due to their effectiveness in maximizing space utilization while supporting substantial structural loads, thereby enabling efficient vertical construction. This technique is particularly relevant in Sri Lanka, where bedrock is frequently encountered at shallow depths. However, despite the widespread use of rock socketed pile groups, the accurate determination of their bearing capacity remains insufficiently explored in existing literature. The conventional method, which involves simply aggregating the bearing capacities of individual piles, often lacks precision and necessitates further investigation. This study seeks to characterize the behaviour of rock socketed pile groups by estimating their bearing capacity, taking into account key parameters of pile groups such as pile spacing, socket length, and bedrock properties, as well as evaluating the overall efficiency of these pile groups. This study aims to achieve objectives through a thorough three-dimensional finite element (FE) analysis. This analysis involves developing a detailed model to assess the impact of key parameters of the pile group on both the bearing capacity and efficiency. This study seeks to confirm the accuracy and relevance of the finite element analysis results by cross-referencing them with experimental data from local sources. The study employs instrumental pile load test (IPT) data from a 1200 mm diameter, 10 m long test pile, implemented in the Port Access Elevated Highway project in Sri Lanka, to validate the FE model. The initial elastic modulus achieved 67% accuracy, which improved to 70% when doubled, and reached a peak of 75% with a tripled elastic modulus. Following model validation, a comprehensive FE analysis was conducted, adjusting bedrock properties and pile group parameters such as spacing and socket length to assess bearing capacity and group efficiency. The study focused on 2x2 pile groups, each with a 1200 mm diameter, assessing group efficiency by comparing individual pile performance to behaviour of the entire group. Increasing the spacing between piles does not affect the settlement for pile groups embedded in very strong bedrock. Similarly, extending the socket length for pile groups in very strong bedrock also does not alter settlement. In such cases, group efficiency remains constant regardless of changes in pile spacing or socket length, and the efficiency typically exceeds 1. In contrast, for pile groups installed in weak bedrock, increasing the spacing between piles leads to greater settlement, while extending the socket length results in reduced settlement. For these weaker bedrock conditions, group efficiency improves with increased socket length but declines with greater pile spacing. Despite these variations, group efficiency remains below unity for weak bedrock, though it approaches unity as socket length increases. The study shows that current practice method is effective for pile groups in strong bedrock but less reliable in weak bedrock. It emphasizes the need to account for group parameters and bedrock conditions when selecting rock socketed pile groups for effective structural load support.
item: Conference-Abstract
Cellular pile raft foundations for lightweight multi-storey buildings
(Department of Civil Engineering, University of Moratuwa, 2024) Sandamal, NGTM; Jayasinghe, MTR; De Silva, LIN; Pasindu, HR; Damruwan, H; Weerasinghe, P; Fernando, L; Rajapakse, C
The global demand for housing and urban land scarcity has driven the need for multistorey buildings. The substructure design plays a crucial role in ensuring the stability of these structures, as traditional foundation methods, like piled or piled raft foundations, are essential for distributing the substantial loads. However, the high costs associated with these systems have prompted the e ploration of alternative foundation designs This study’s approach seeks to optimize foundation construction by reducing costs without compromising structural integrity, making it a viable solution for sustainable urban development. This study investigates the feasibility of employing a raft foundation, particularly a weight-compensated cellular raft design for multistorey buildings exceeding 10 floors which typically require costly pile foundations. Unlike traditional piles, Backhoe loaders are proposed for constructing piles filled with Aggregate Base Course (ABC) with cement and inserting reinforced columns for anchoring the cellular raft. The strategy involves settling the building slightly to mobilize the soil capacity, particularly for sandy clay soil conditions. Furthermore, the study explores the potential of lightweight superstructures to significantly reduce construction costs by optimizing structural weight and eliminating the need for pile foundations. Specifically, it explores the utilization of Expanded Polystyrene (EPS) based lightweight panels and precast prestressed concrete beam systems with precast prestressed concrete slabs. Investigating a 10-story reinforced concrete moment resisting frame (MRF) supported by a cellular piled raft foundation, the research employs a direct approach considering soil-structure (SSI) interaction effects. Through construction stage analysis using finite element software (Midas GEN, Midas GTS NX), the study determines optimal gap sizes for the cellular raft and assesses the maximum number of storeys feasible without pile foundations. Overall, this study suggests that on sandy clay soil, constructing taller buildings with a maximum of 14 floors, in addition to the cellular basement, is feasible using lightweight superstructures in conjunction with cellular rafts. Moreover, the research recommends increasing pile spacing beyond the current 5m x 5m grid configuration to fully mobilize soil capacity. Future studies should also investigate the effectiveness of these foundation systems across various soil types, including silty clay, loamy soil, and sandy loam, to further validate the design's applicability in different geological conditions.
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: Article-Abstract
A density- and stress-dependent elasto-plastic model for sands subjected to monotonic undrained torsional shear loading
(2015-08-20) Chiaro, G; Koseki, J; De Silva, LIN
A density- and stress-dependent elasto-plastic model for saturated sands undergoing monotonic undrained torsional shear loading is presented in this paper. The model is developed under an extended general hyperbolic equation (GHE) approach, in which the void ratio and stress level dependence upon stress-strain response of sand is incorporated. Most importantly, a state-dependent stress-dilatancy relationship is introduced to account for the effect of density on the stress ratio. Such a stress-dilatancy relation is used for modeling the excess pore water pressure generation in undrained shear conditions as the mirror effect of volumetric change in drained shear conditions. In this paper, details of the model formulation and soil parameters calibration are described. By using the proposed model, numerical simulation of monotonic undrained torsional shear tests have been carried out on Toyoura sand. The model predictions show that undrained shear behavior, described in terms of stress-strain relationship and effective stress path for both loose and dense sands can be modeled satisfactorily by using a single set of soil parameters.
item: Thesis-Full-text
Developing a risk assessment framework for safety evaluation of earth dams in Sri Lanka
(2014-06-02) Sothilingam, P; De Silva, LIN; Kulathilaka, SAS
Sri Lanka has a rich history of earth dam construction with over 300 large and medium scale dams and over 12000 small scale earth dams currently in service. According to ICOLD (International Commission of Large Dams) classification, there are 76 large dams in Sri Lanka. A vast majority of those earth dams were built several centuries ago and limited scientific investigations have been conducted on the performance of such ancient earth dams from a geotechnical point of view. After serving the nation for centuries, a large numbers of ancient earth dams are suffering partial failures due to excessive seepage, piping, slope instability, and excessive lateral deformations and cracking due to vibrations caused by heavy vehicles and tremors. No regular monitoring schemes were implemented to investigate the mechanisms of above failures. The quantitative risk assessment seeks to enumerate the risk in terms of likelihood (probability) and consequences. The probability of failure for each mode involves engineering assessment of the particular failure mechanisms, and looking for solutions that can reduce the probability of those failure modes or minimize the consequences of a failure. There is no standard framework adopted in Sri Lanka for the risk assessment process of earth dams. The main objectives of this report are to propose a quantitative risk assessment framework for safety evaluation of earth dams in Sri Lanka and to apply the developed risk assessment framework to an ancient earth dam of Sri Lanka to investigate its performance under different conditions. Here, as a case study, initial level risk assessment has been done for Nachchaduwa dam, using the developed framework. The critical loading conditions which are relevant to Sri Lanka were included in the study. Nachchaduwa is an ancient tank, which was built 17 centuries ago to supply water for irrigation purposes. It was restored in 1906 and improved in 1917 by the Irrigation Department of Sri Lanka. According to an investigation carried out by Dam Safety and Water Resource Planning Project (DSWRPP), Nachchaduwa dam is selected as one of the dams with a higher risk of failure with some signs of excessive seepage and slope instability along the dam embankment. Risk assessment can provide valuable information on the risk reduction measures and benefits of structural and non-structural risk reduction options. In addition, risk assessment outcomes can strengthen the case for funding capital improvements, additional investigations, and on-going dam safety activities, such as monitoring and surveillance and emergency management. This report produces a quantitative risk assessment framework to be used for any type of earth dams in Sri Lanka and summarizes the risk assessment process, results, findings and recommendations for Nachchaduwa dam.
item: Conference-Full-text
Development of an earth pressure cell to evaluate the total and effective stresses of soil
(IEEE, 2018-05) Bandara, AMDC; De Silva, LIN; Amarasinghe, YWR; Chathuranga, D
This study focuses on development of an earth pressure cell to identify the effective soil pressure and pore water pressure in a single degree of freedom sensor system. A strain gauge based four cross-beam metal structural element is used to identify the maximum earth pressure of 150 kN/m2. For that, pressure transferring mechanisms are developed to transfer the earth loads to sensing elements via deformation of cross-beam structure as a total load and as an effective load. Finite element analysis is done to select the optimum dimension for the sensor element. Other dimensions and the shape of the sensor are selected by referring the literature. Accuracy of the system will be verified by using methods which are currently available for measuring soil pressure and pore water pressure under laboratory and field conditions.
item: Article-Abstract
Development of high capacity torsional shear apparatus for the measurement of small strain deformation properties of soils
De Silva, LIN
Recently developed medium-sized torsional shear apparatus at Institute of Industrial Science. University of Tokyo, Japan and results from tests on dry Toyoura sand using a modified version of pin-typed local deformation transducer (PLDT) system with other conventional strain measurement techniques are presented. Loading system of this apparatus consists of servo motors coupled with reduction gear systems for controlling both vertical and torsional loadings precisely. This apparatus is capable of controlling both vertical and torsional cyclic loading either by stress amplitude or strain amplitude. Cell pressure can be controlled by an electro-pneumatic transducer. The three stress components (axial, torsional and confining stress) can be controlled independently by using a personal computer. High loading capacity of the apparatus and its capability of testing specimens of various sizes up to 20 em in outer diameter, 12 em in inner diameter and 30 cm in height arc helpful in understanding the properties ofgeomaterials with large particle sizes such as gravel. Axial and torsional loads are measured precisely with a twocomponent load cell, which has a negligible coupling effect. Cell pressure is measured by using a high capacity differential pressure transducer. In order to measure thc local strains, two sets of PLDT's arranged in a triangular form using separate hinges are used at opposite sides of a diameter of the specimen. Based on cyclic torsional shear and triaxial tests on dry Toyoura sand using this apparatus, it was confirmed that quasi-elastic deformation properties such as Young's modulus and shear modulus can be measured at various stress states by conducting small amplitude cyclic loading in axial and torsional directions. In addition, it was verified that this apparatus is capable of conducting large cyclic drained and undrained loadings and hence liquefaction behavior of sand can be investigated.
item: Conference-Abstract
Effect of a shear key on the behaviour and stability of cantilever type retaining walls
(Department of Civil Engineering, University of Moratuwa, Sri Lanka., 2021-11) Gowshikan, A; De Silva, LIN; Hettiarachchi, P
Retaining walls are categorised into several types, of which cantilever retaining walls are the commonly used retaining wall type. The stability of these walls should be ensured for its longterm use without any anticipated failures. A retaining wall can fail due to four main failure mechanisms: sliding, overturning, bearing capacity, and deep-seated failure. Shear keys are the structures incorporated in the cantilever retaining walls to increase their resistance to sliding, thus, increasing the Factor of Safety against sliding. The development of passive earth pressure due to the soil in front of the shear key will generate an additional resistance against sliding. This study aims to identify the optimal location and depth of the shear key to yield maximum use from it. Both the theoretical approach based on limit equilibrium and numerical modelling have been adopted in the analysis. Limit equilibrium analysis was carried out using the Excel spreadsheet application, and two different scenarios were considered based on the distribution of lateral loads due to active soil conditions. Rankine's method was used for active and passive earth pressure computation. For the shear key, three different locations were considered: at the toe, middle of the base, and heel, and five different depths were considered: 0.4 m, 0.6 m, 0.8 m, 1.0 m, and 1.2 m. Soil strength properties were taken by referring to the commonly used backfill soil materials in Sri Lanka. The design soil parameters considered for the analysis were calculated using BS 8002:1994. From the Limit Equilibrium approach, the values of Factor of Safety (FOS) against sliding and overturning, varying with the shear key's depth and location, were obtained as graphical representations. Finite Element Analysis was carried out using PLAXIS 2D software to analyse the variation of the overall stability with the increasing depth of the shear key and validate the location of the point of rotation assumed in the limit equilibrium approach. Both the retaining wall and shear key were modelled as plate elements in PLAXIS 2D. The results of limit equilibrium analysis suggested that the use of a shear key enhances the stability of the retaining wall against sliding. The location of the shear key does not influence the stability of the retaining wall against sliding. It was also found that the increased depth of the shear key reduces the stability of the retaining wall against overturning, and the optimum location of the shear key is at the heel of the wall base. Results from the Finite Element Analysis show that the overall stability of the retaining wall increases with an increase in depth of the shear key. The point of rotation is assumed to be located underneath the toe of the wall and at a depth of the shear key. From the Finite Element Analysis, changes in the direction of displacement were visible around the assumed location of the point of rotation for all three locations of the shear key. Hence, the assumed location of the point of rotation is reasonable in this study.
item: Conference-Full-text
Effect of construction sequence on the behaviour of gravity type retaining wall
(Department of Civil Engineering, University of Moratuwa, 2015-10) Sanjei, C; De Silva, LIN; Hettiarachchi, MTP
Gravity retaining walls derive their capacity to resist lateral movement through the dead weight of the wall. The design methodologies proposed by standards do not take into account the construction sequences that simulate the process by which the soil and retaining wall are brought together. However, in reality, at least during the backfilling process, the retaining wall undergoes many displacements that are not so far considered in the design. In this investigation, effect of construction sequences in the gravity retaining walls with different shapes is investigated with the help of finite element method. Two different construction sequences, namely the backfilling after wall construction and the backfilling parallel to wall construction, are compared for different wall shape models. Lateral displacement of the bottom and the top of the wall is plotted for each model and construction sequence with construction stages. Bearing pressure distribution, lateral earth pressure and failure wedge angle are summarized and compared with design values. Each wall showed different behavior for each of the construction sequences. Back filling after wall construction minimizes the sliding failure and bearing pressure. Overturning failure could be reduced by backfilling parallel to wall construction. However, it was observed that, comparatively, backfilling after wall construction is more effective than backfilling parallel to wall construction, suggesting that proper selection of construction method also may reduce negative effects on the wall stability.
item: Article-Abstract
Effect of static shear stress on undrained cyclic behavior of saturated sand
Chiaro, G; Kiyota, T; De Silva, LIN; Sato, T; Koseki
To investigate the effect of static shear stress on the undrained cyclic behavior of saturated sand, a series of torsional shear re t was conducted on saturated Toyoura sand specimens up to extremely large strain level of about 100%. After being isotropieally consolidated, the specimens were subjected to drained monotonic torsional shear stress, and then, undrained cyclic torsional shear stress was applied. The amplitude of combined static and cyclic shear stress was kept constant by correcting the measured value for the effect of membrane force. The test results revealed that the effective stress path and the stress- train curve during the cyclic shear loading were affected by the initial static shear stress. Accumulation of shear strain was clearly noticed in the same direction where previously static shear stress was applied. Progressive localization of specimen deformation was observed.
item: Conference-Abstract
Effect of tie beams on the behavior of isolated foundation systems
(Department of Civil Engineering, University of Moratuwa, 2024) Gunathilaka, LTD; De Silva, LIN; Pasindu, HR; Damruwan, H; Weerasinghe, P; Fernando, L; Rajapakse, C
The stability of isolated foundation systems is crucial to the structural integrity and safety of buildings, especially in regions with varying load conditions. Tie beams, widely used to connect isolated footings, play an integral role in this process. However, their impact on bearing capacity and settlement characteristics is often overlooked in standard analyses. This research primarily aimed to explore the influence of tie beams on the settlement characteristics and overall performance of isolated foundation systems. A comprehensive methodology combining both model experiments and finite element analysis (FEA) was employed to achieve these objectives. Initially, two model experiments were conducted using small-scale prototype structures: one with a single isolated footing and another with two footings connected by a tie beam. These experiments aimed to validate the FEA results by comparing the experimental settlement data with the simulated outcomes. Two distinct models of the foundation system were developed, each based on a typical three-story building with 16 isolated footings. This setup was designed to simulate a realistic structural scenario and evaluate the impact of tie beams under varying loading conditions. The footings were loaded with varied loadings in the range of 350 kN to 1300 kN to simulate real-world scenarios. One model included tie beams connecting the footings, while the other omitted them, allowing for a comparative analysis of their effects on settlement and structural integrity. The findings reveal that incorporating tie beams significantly reduces the maximum individual settlement, with a decrease of up to 22 mm (43%). Furthermore, the inclusion of tie beams narrowed the variation in settlement across individual footings, resulting in a more uniform distribution of settlements. Differential settlements were notably reduced, with all values staying under 2 mm, reflecting a 92% reduction compared to the model without tie beams. In conclusion, the inclusion of tie beams significantly reduces both settlements and differential settlements, contributing to a more uniform distribution of loads across isolated foundation systems. While positioning tie beams at the footing level may further reduce settlements, it also increases the forces acting on the tie beams, necessitating higher reinforcement and potentially leading to increased construction costs. The research recommends maintaining tie beams at ground level in general construction practices, except in scenarios where minimizing settlements is critically important. This study underscores the significance of tie beams in enhancing the performance and stability of isolated foundation systems, highlighting their essential role in mitigating settlement-related issues.
item: Conference-Full-text
Effective applications of poker vibrator for compacting quarry dust as a ground improvement technique
(Department of Civil Engineering, University of Moratuwa, 2016-12) Sampath, KHSM; De Silva, LIN; Hettiarachchi, MTP
Recently, quarry dust has been widely used as a ground improvement technique to replace the weak, incompetent soil under shallow foundations when there is a high groundwater table or weak soil. Practical approaches suggest that a poker vibrator can be used easily, to achieve a higher Degree of Compaction (DOC) in quarry dust, under saturated condition. However, as this technique is still novel to the industry, the expected results cannot be guaranteed. It is therefore essential to eliminate inappropriate practice by carrying out laboratory experiments on optimum poker vibration application techniques. The aim of this study was to optimize the effectiveness of poker vibrator in shallow foundation design by studying the variation of factors affecting it, including time of application, shape of the foundation, preferable layer thicknesses and application patterns of vibrators. According to the test results, the optimum period of compaction for poker vibration is around 35s/point. DOC increases with increasing application points and reduces with increasing initial layer thickness, regardless of pattern.
item: Conference-Abstract
An elasto-plastic model to describe the undrained cyclic behavior of saturated sand with initial static shear
Chiaro, G; De Silva, LIN; Kiyota, T; Koseki, J
With the aim of simulating the behavior of saturated sand with initial static shear (i.e., sloped ground) undergoing undrained cyclic loading, which leads to liquefaction and large cyclic shear strain development, an elastoplastic constitutive model which can describe both monotonic and cyclic torsional shear behaviors of saturated sand under drained and/or undrained conditions is presented. It can simulate qualitatively the stress-strain relationship and the effective stress path, even after the specimen enters fully liquefied state. To verify its effectiveness, the proposed model is employed to simulate the results of a series of hollow-cylindrical torsional shear tests on loose Toyoura sand specimens with initial static shear stress under stress-reversal and non-reversal loading conditions.
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.
item: Conference-Abstract
Evaluation of liquefaction potential of Western and Eastern Coastal Areas in Sri Lanka
Premkumar, S; De Silva, LIN
There has been an increasing concern on earthquake related disasters in Sri Lanka after 2004 tsunami. It is partly due to the fact that that most of the important structures in Sri Lanka are founded on loose sandy soils along the coastal areas. During a strong earthquake, there is a huge possibility that these loose sand deposits may liquefy causing significant damage to the structures founded on them. In addition, some of the coastal areas suffer frequent floods, which may magnify the damage due to liquefaction. Standard Penetration Test (SPT) and Cone Penetration Test (CPT) are widely used for the site specific evaluation of liquefaction potential of sandy soils. In this paper, results of the analysis of liquefaction potential based on SPT resistance values are presented. Here, 46 bore holes from Colombo and east- coast were analyzed, and the liquefaction potential was evaluated by means of a factor of safety against different earthquake magnitudes and different ground water levels. From the analysis results, liquefiable areas and the depths of liquefiable areas were identified. It was observed from the analysis of 24 boreholes in Colombo area that, 3-13 m depths from ground surface are liquefiable during a 6.5 magnitude earthquake, while on average 4-10 m depths are liquefiable in Batticaloa, Mutur, and Ampara areas in the east coast under normal ground water conditions.
item: Thesis-Abstract
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: Conference-Full-text
Evaluation of soil liquefaction in Sri Lanka - a dynamic approach using geotechnical investigation data
(Department of Civil Engineering, University of Moratuwa, Sri Lanka., 2012-12) Kumarasiri, C; de Silva, N; Abayakoon, S; Nawagamuwa, U; De Silva, LIN
Soil Liquefaction is a process, where granular soils below the ground water table temporary lose its strength and behaved as a viscous fluid rather than a solid. The porewater pressure is suddenly increased during an earthquake due to the cyclic loading. The increased porewater pressure is forced the soil particles to suspend in water. As a result, the buildings, utility services, natural substances and other structures are collapsed causing severe damage to the people and the nature. In Sri Lanka, it is rare to find the historical data of liquefaction or related incidents. Recent studies demonstrated that there is a potential for liquefaction in some places of the island. It could be evaluated by using the basic geotechnical investigation data, according to the simplified procedure proposed by Seed and Idriss (1971). This paper is intended to evaluate the liquefaction potential in Sri Lanka by identifying the liquefiable layer thickness, using an extensive geotechnical investigation data base. As per the analysis 384 locations were identified as susceptible for liquefaction with various layer thicknesses out of 3282 locations analysed. Further, the analysis has extended for varied ground water table.
item: Conference-Abstract
An Experimental investigation on shaft resistance of cast in-situ bored piles in intact rock
Suloshini, S; De Silva, LIN
Pile foundations socketed into rock are used to transfer the loads from the large structures to the underlying bedrock when the surface soils are unable to provide the adequate bearing capacity. The bearing capacity of the pile foundation depends on the base resistance and the shaft resistance given by the underlying rock. The shaft resistance of the rock socketed pile is a very sensitive parameter and governed by several parameters including the mechanical properties of the surrounding rock, rock mass classification, pile diameter, roughness of rock – concrete interface, discontinuities of the rock, the radial force induced by the load applied on the pile and the rock types. The performance of rock-socketed piles at serviceability limit mainly depends on the shaft resistance induced at the rock – concrete interface. The shaft resistance should be estimated accurately for safe and economic design of the structure. This research is focused on the experimental investigation on shaft resistance of cast in situ bored piles in intact rock. The samples were prepared to get the concrete – rock interface and tests were conducted in the laboratory for different types of rock using tri-axial apparatus and unconfined compressive strength apparatus. The ultimate shaft resistance was calculated from the experimental results for different rocks commonly found in Sri Lanka. Biotite hornblende gneiss, marble rock and charnokitic hornblende gneiss are the types of rock used in this study. From the gradient of shear stress–shear strain graph, shear modulus of the interface was evaluated. It was found that these rocks have ultimate shaft resistance in the range of 3.5 - 4.5 MPa and Shear modulus to be in the range of 60 – 70 MPa. Confining stresses in the range of 0 kPa to 400 kPa do not seem to have any effect on the shaft resistance.
item: Conference-Abstract
Extremely large post-liquefaction deformations of saturated sand under cyclic torsional shear loading
Chiaro, G; Kiyota, T; De Silva, LIN; Sato, T; Koseki, J
The effect of static shear stress on the undrained cyclic behavior of saturated Toyoura sand was studied by conducting a series of torsional shear tests up to double amplitude shear strain of about 100%. After being isotropically consolidated, the specimens were subjected to drained monotonic torsional shear loading, and then, cyclic torsional shear stress was applied under undrained condition. The amplitude of combined static and cyclic shear stress was kept constant by correcting the measured value for the effect of membrane force. Based on these test results, it was found that the effective stress path and the stress-strain curve were affected by the initial static shear stress. Accumulation of shear strain was clearly noticed in the same direction where the static shear stress was applied. Localization of specimen deformation, which increases with the shear strain level, was observed.