Browsing by Author "Kulathilaka, SAS"

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item: Thesis-Abstract
Assessment of slope stability and stabilization techniques through probabilistic approach
Mettananda, DCA; Kulathilaka, SAS
Uncertainty and randomness of data is a major Issue associated in geotechnical engineering. It is therefore desirable. to use methods and concepts in engineering planning and design that can facilitate the evaluation and analysis of uncertainty. Formal probabilistic approach provides a useful framework to incorporate these uncertainties in slope instability problems. Under the research described in this thesis, number of probabilistic models have been developed to evaluate the stability of slopes and their use in the evaluation of stability of slopes and the evaluation of effectiveness of various stabilization techniques have been discussed. Two probabilistic analytical models that can be used to evaluate the stability of slopes that can fail either along circular failure surfaces or non-circular failure surfaces have been developed. These models formally recognize the uncertainties associated with various geotechnical parameters and provide means to quantify their effects on the stability. The result is given in the form of probability of unsatisfactory performance of the slope. Each mode! developed was facilitated to perform computations in five different ways ranging from theoretically sound methods to some approximate methods, and the results obtained were compared with each other. In addition, the results were compared with the results obtained by a commercial software, wherever applicable. In addition, two other probabilistic analytical models were developed to analyze the slopes stabilized by soil nailing. It is seen that the behavior of the probabilistic models developed under this research perform satisfactorily, and it can be recommended to use these models in routine problems of slope instability to provide more realistic results incorporating the uncertainties associated with various geotechnical parameters. Analyses of a number of examples and case histories discuss the uses of the probability of failure in decision making to evaluate the stability of slopes as well as the effectiveness of various stabilization techniques. It also emphasizes the importance of the appropriate failure mechanism and the appropriate deterministic model in the probabilistic analysis. Analysis of case histories provides an important discussion showing the inadequacy of the conventional factor of safety alone in evaluating the stability of slopes.
item: Conference-Extended-Abstract
Assessment of slope stabilization techniques through a probabilistic approach
(2002) Kulathilaka, SAS; Mettananda, DCA
Stabilization of slopes that are unstable, or that do not possess a sufficient safety margin, is a major challenge faced by geotechnical engineers. Different techniques are adopted for the stabilization purpose. In central Sri Lanka where a number of slope failure are reported in periods of heavy rain, provision of effective surface and sub surface drainage would be the main method for stabilization. In slopes where some movements have already taken place and strength values are somewhat reduced towards the residual values, other means of support in the form of soil nailing, or the use of retaining structures may also be needed.
item: Conference-Abstract
Back analysis and rectification of a failed cut slope in the Southern Expressway
Dharmasena, UKNP; Bandara, KN; Karunawardena, WA; Kulathilaka, SAS
A failure occurred in a cut slope of the southern expressway where all the surface drainage measures had been implemented. The failure surface was shallow but it has disrupted three levels of berms. The debris of failure were quite dry. A close examination indicated that a cascade drain has cracked and water is leaking to the slope. There were five systems of joints in the rock and adversely oriented relict joints were identified during rectification work . The failure could have been triggered by infiltration of rainwater through relict joints , reduction of matric suction and development of pore pressure . Rectification process involved installation of soil nailing and cable anchoring together with long sub horizontal drains.
item: Conference-Full-text
Comparison of design of stone columns by limit equilibrium and finite element methods
(Department of Civil Engineering, University of Moratuwa, 2016-12) Perera, WLDHA; Kulathilaka, SAS; Hettiarachchi, MTP
Use of stone columns is a ground improvement technique that can be applied when high embankments are to be constructed on thick layers of soft soils. Stone columns installed in an appropriate pattern, reinforce the soft soil enhancing the shear strength and reducing the settlements. There are several methods for the analysis and design of stone columns. These designs come up with a factor of safety, which cannot be verified in the field. Comparison of Factor of Safety computed with different approaches will be useful in optimizing the design procedures. Designs done with limit equilibrium approach and finite element approach are compared in this research. GEOSLOPE - SLOPE/W software was used for the limit equilibrium approach and PLAXIS 2D software was used for the finite element analysis.
item: Conference-Extended-Abstract
Compressibility characteristics of electrically treated peaty clay
(2003) Kulathilaka, SAS; Sagarika, DKNS
Finding cost effective, and relatively quick solution to handle the thick layers of soft peaty clays is a major challenge faced by Sri Lankan Geotechnical Engineers. Number of . different methods had been tried out to improve the engineering qualities of peaty clays. There were problems associated with preloading in the case of extremely soft peaty clays. due to their almost water like consistency. Methods such as deep mixing need special machinery.
item: Conference-Abstract
Compressibility characteristics of unsaturated soil
(Department of Civil Engineering, University of Moratuwa, 2021) Krichikeshan, I; Kulathilaka, SAS; Hettiarachchi, P
Conventional Terzaghi's consolidation theory is developed for saturated soil. However, in practice the concept is used with unsaturated soils as well. The behaviour of unsaturated soils is quite different due to the presence of the air phase and the contractile skin- the boundary between air and water phases. In many places in the country residual soils formed by the weathering of parent metamorphic rock are present at the surface level and there is a zone of unsaturated soils just below the ground surface. Loads from many structures are applied on this zone and the understanding of the compressibility of this zone is very important. Also, there is high variability in this zone. The objective of this research is to identify the variation of compressibility characteristics of different soils with the saturation level. Due to the existence of air phase in unsaturated soil, excess pore water pressure is not equal to the load applied when the soil is compressed under an undrained loading condition. To get an accurate reliable equation we need to consider both pore water pressure and pore air pressure. If the excess pore air pressure and excess pore water pressure are allowed to dissipate, the excess pore water and air pressure will dissipate gradually with time. The volume change of the unsaturated soil is not equal to the water drainage in unsaturated condition. And also, it is necessary to consider the saturation level. The main parameter that causes the difference from saturated soils is matric suction. As such in this research a controlled uniform saturated sample is obtained, and it was brought to equilibrium under different matric suctions. The compressibility characteristics of the same soil under different matric suctions are evaluated. The parameters are evaluated under the framework of Terzaghi model. An attempt was made to establish a relationship between the matric suction and the consolidation parameters. Identical samples were tested on conventional Oedometer and a Rowe Cell of diameter 75 mm. The void ratio, natural moisture content, compressibility index are the features which can be obtained from standard Oedometer consolidation test and Rowe cell consolidation test. Available data states that the compressibility characteristics of unsaturated soils are usually very nonlinear and cannot be considered in some conditions. Compressibility of unsaturated soil depends on many factors. Some of them include particle size distribution, saturation level, temperature, permeability. Compression index (Cc) and Recompression index (Cr) decrease in the initial stage and then increase with matric suction. mv value for the saturated sample was greater than unsaturated soil and with the increase of stress level, the mv value of all the samples decreased.
item: Thesis-Full-text
Consolidation analysis of Sri Lankan peaty clay using elasto-viscoplastic theory
(7/16/2011) Karunawardena, WA; Kulathilaka, SAS
is necessary to take these properties into account in order to obtain better predictions from peat consolidation analyses. In the present study, the consolidation behavior of peaty clay found in Sri Lanka is extensively studied using a model based on the elasto-viscoplastic theory. The model can describe the prominent creep behavior of peaty soil as a continuous process. In addition, the model can accommodate the effect of structural degradation on the consolidation process. The analysis takes into account all the main features involved in the peat consolidation process, namely, finite strain, variable permeability, and the effect of secondary compression. Also, it considers the variable compressibility for stage constructed embankments which exert high levels of pressure on the peaty subsoil. The constitutive equations used in the model and the procedure adapted to account for the above-mentioned features of the analysis are described. The constitutive model is based on Perzyna's type viscoplastic theory and the Cambridge elasto-plastic theory combined with empirical evidence. In the finite element formulations, which are based on the finite deformation theory, an updated Lagrangian method is adopted. A description of the material parameters used in the model and the procedures applied to evaluate them, with standard laboratory and field tests, are explained. In addition,. a performance of the model incorporating the original and the modified Cam-clay theory is evaluated by simulating triaxial test results. A comparison shows that with the present definition of the parameters, the original model yields more representative results than the model based on the modified Cam-clay theory. Initially, the capability of the constitutive model to capture the consolidation behavior is verified using the consolidation model test data on peaty clay found in Sri Lanka. It is confirmed that the constitutive model is able to predict the observed creep characteristics and the effect of sample thickness on settlement predictions for the material under consideration. The performance of the model in predicting the consolidation behavior under field conditions is studied using field data on instrumented earth fill constructed on peaty clay. One-dimensional compression is assumed for the peaty clay due to the large plane area of the fill. Separate analyses are carried out by the model considering the infinitesimal strain theory, the finite strain theory, and the finite strain theory together with the effect of structural degradation in order to explore how these features describe the observed field behavior. Analyses reveal that it is necessary to consider finite deformation together with the effect of structural degradation in order to successfully simulate the resultant large strain and the stagnated pore water pressure observed in the field. The construction of road embankments over peat deposits is quite problematic, and thus, it is often done after first improving the properties of the peaty soil through the utilization of appropriate ground-improvement techniques. Understanding the field response of peaty clay during this improvement process is naturally of great importance. A constitutive model is applied to predict the field performance of embankments constructed on peaty clay using different ground-improvement techniques. The back analysis of embankments constructed with the preloading method indicates that the model can be successfully applied to predict both the deformation and the stability of structures constructed on peaty clays. The stability of the embankment during and after construction is verified by investigating the stress-strain characteristics of the subsoil. The model applications used to predict the consolidation behavior of embankments constructed by the preloading method, combined with other ground-improvement techniques, are then discussed. Embankments constructed with prefabricated vertical drains (PVDs) and sand compaction piles (SCPs) are considered, and finite element analyses are carried out in all cases by converting the actual three-dimensional conditions that exist around the drains into simplified two-dimensional plane strain conditions. The field behavior when PVDs are installed in the peaty clay is simulated using the equivalent vertical permeability for the PVD-improved subsoil. In the case of SCPs, a conversion scheme is used to transform the axisymmetric nature of sand columns into equivalent plane strain conditions. A comparison of the predicted results with the field observations shows a reasonable agreement. An analysis of the PVD-improved foundation indicates that the installation of PVDs not only accelerates the rate of consolidation, but influences the deformation pattern of the subsoil due to embankment loading. The analysis also shows that the use of PVDs can significantly increase embankment stability. The model prediction for the SCP-improved foundation reveals that the stiffness and the area replacement ratio used in the conversion scheme play vital roles in predicting the behavior of SCP-improved soft grounds. The observed improvements in the bearing capacity of the subsoil and in the stability of the embankment, brought about by the installation of SCPs, can be simulated by the model.
item: Conference-Extended-Abstract
Consolidation testing of peaty clay
(2003) Kugan, R; Puswewala, UGA; Kulathilaka, SAS; Peiris, TA
The first rational theory to explain the settlements occurring in saturated clays was the 1- D theory of consolidation proposed by Terzaghi (1925). One of the main advantages of the theoretical modelling of consolidation is that it becomes possible to carryout laboratory tests on thin samples of clay. a few centimetres thick (with one increment of load normally maintained for 24 hours). and then from the laboratory results predict the settlements in the field where the clay layer may be several meters thick and where the settlements may take place over a long period of time.
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: Conference-Full-text
Determination of permeability characteristics and SWCC of an unsaturated residual soils in Sri Lanka
(Department of Civil Engineering, University of Moratuwa, 2016-12) Karunasena, GKGM; Kulathilaka, SAS; Hettiarachchi, MTP
Sri Lankan residual soils are formed by weathering of the metamorphic parent rock. Failure in slopes made of such residual soils frequently occurs due to excessive rainfall. With the rain water infiltration, soil will gradually get into a saturated state loosing the matric suction. Sometimes perched water table could also develop. Therefore, with the rainfall, slopes that are stable in dry periods may be subjected to failure. Soil Water Characteristic Curves (SWCC) and knowledge of variation of permeability in unsaturated soil are essential to model this behavior and to devise a reliable early warning system which is in most demand in the present day context. This research project focuses on the development of the permeability function of an unsaturated residual soil in Sri Lanka. Permeability function for both wetting and drying phases were investigated. The method is based on continuously drying and wetting the soil sample while continuously monitoring the suction gradient and the change in soil mass.
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
Effect of Preloading on Secondary Consolidation of Peaty Clay
(2016-04-26) Fernando, RMS; Kulathilaka, SAS
Special consideration should be given to secondary consolidation settlements during service in the construction of high road embankments in lands underlain by thick layers of soft peaty clay. Usually a preloading design will be done to ensure that the peaty clay will remain in an over consolidated state during the operation of the road. At a stage of surcharge removal the in service settlement has to be estimated. In a preloading design the practically achievable over-consolidation ratios are in the range of 1.1 to 1.2. Effectiveness of such over-consolidation ratios (OCR) in keeping the long term in service secondary consolidation settlements within acceptable limits was studied in this research. Oedometer tests were done simulating the process of loading-unloading-reloading. Further tests were done on undisturbed samples obtained from preloaded peaty clay layers in two different projects. Results illustrated that the level of reduction of Cu is related to the achieved OCR.
item: Conference-Abstract
Effect of relict joint in rain induced slope failures in residual soil
(2011) Kulathilaka, SAS; Kumarasinghe, R; Lakshamana, NWH; Neethimappiriya, T
Rain induced slope failures i s a common geotechnical hazards in most parts of the world. Particularly in the tropicaI regions, which are covered with residual soil, the temporal frequency of rainfall induced slope failures is very high. These problems are encountered in the construction of highway with the very high cut slopes. So the special focus should consider in the highway construction. Residual soils are formed due to weathering of rocks. The residual soils are heterogeneous due to variable weathering of the jointed mass rock. Joints are formed due to decomposition of rock Due to the presence of relic joints, the behavior of the residual soil is quite complex. Generally the ground water table is low during the dry season and these soils are in an unsaturated state. The shear strength of a soil is enhanced by matric suction. As a result of infdtration of rain water, the matric suction will be destroyed and ground water table rise.. Due to the loss of matric suction the slope failures can induce. This aspect is studied in this research with the infiltration model by using SEEPW, SLOPE Wsoft ware.
item: Conference-Full-text
Effect of relict joints in rain induced slope failures in residual soil
(Department of Civil Engineering, University of Moratuwa, 2011-12) Tharmalingam, N; Lakshamana, NWH; Kumarasinghe, RDTB; Kulathilaka, SAS; Ratnayake, N
Rain inducted slope failures are a common geotechnical hazard in the most parts of the world. particularly in the tropical reagon, which are covered with residual, soil, the temporal frequency of rainfall inducted slope failures is very high. These problems are encountered frequently in the construction of highway where very high cuts are made in the slopes. Residual soils are formed due to weathering of rocks and the joints remain as relict joint even after the weathering process. The residual soils are heterogeneous due to variable weathering of the jointed mass rock. Generally the ground water table is low during the dry season and these soils are in an unsaturated state with negative pore water pressure above the water table. As a result of infiltration of rain water, the matric suction will be distroued and ground water table will rise. Relict joints present in the system will further complicate the pattern of infiltration. Changes in the pore pressure regime due to infiltration and its effect on safety was studied in a previous research. The objective of this research is to investigate the effects of different systems of relict joints in the infiltration and stability. This aspect was studied in this research using SEEPW, SLOPEW software.
item: Article-Abstract
Effectiveness of Surface drainage in enhancing the stability of cut slopes during the periods of heavy rain
Kulathilaka, SAS; Kumara, LM
Rain induced failures in slopes made of residual soils are a major geotechnical hazards in Sri Lanka. These soils are characterized by the heterogeneous nature inherited from the variability in the parent rock and the process of variable weathering. Safety margins of these slopes are high during the periods of dry weather due to the prevailing matric suctions. Rainwater infiltration causes a loss of matric suction and builds up of positive pore water pressures. With the use of sealed drains in the berms, cascade drains to allow rapid passage of water down the slope and appropriate vegetation cover over the slope surface, the infiltration of rainwater can be reduced. The effect of rainwater infiltration on the pore pressure regime and the effectiveness of the said surface drainage measures were studied using the software SEEPW [4]. The stability of the slopes after a rainfall event was analyzed using the software SLOPEW [3]. The complex nature of the residual soil was idealized with zones of different levels of weathering. The results illustrated that the destabilizing effects of rainfall could be minimized with the usage of appropriate surface drainage measures.
item: Conference-Abstract
Effects of rainfall on stability of cut slopes in residual soils
Kulathilaka, SAS; Sujeevan, V
Rain induced slope failures are a common geotechnical problem in tropics where residual soils are abundant. Residual soils are characterized by significant differences in the levels of weathering within short distances. Two typical cut slopes from the Southern Transport Development Project were used in theanalysis. The complex geological situation in an unsaturated cut slope is idealized by two cases; a uniform soil and a soil overlying moderately weathered rock. The changes to the pore pressure regime with the progression of the rainfall and consequent reduction of the safety margin of the slope was modeled with SEEP/W and SLOPE/W software. Both the deterministic approach and the probabilistic approach were used in the evaluation of stability. With the continued rainfall, matric suctions depleted, perched water table developed and the safety margins reduced. The presence of a moderately weathered rock layer at a shallow depth was found to be more critical.
item: Conference-Extended-Abstract
Engineering properties of tyre chips and Sri Lankan lateritic soil mixtures as a lightweight fill material
(2004) Muhunthan, A; Kulathilaka, SAS
A number of proposed highways in Sri Lanka are to be constructed over sites underlain by soft peaty clays, due to scarcity of lands. In order to ensure that the in service settlements of these roads are _4 small enough and the roads could functions satisfactorily, number of special ground improvement techniques are to be adopted. Another, alternate approach that could be considered is the use of a lightweight fill material in the construction. Extremely lightweight fill materials such as expanded and extruded polystyrene blocks were used in a number of developed countries in the construction of road embankments over soft ground and in landslide repair. However, these material are to be imported to the country and would be very expensive. As such, from a local point of view a process involving the use of such materials would not be economically competitive. In order to find an economically feasible solution, the lightweight fill materials should be developed with the locally available inexpensive raw material.
item: Conference-Abstract
Enhancement of engineering characteristics of peaty clay due to mixing with cement
Saputhantiri, DR; Kulathilaka, SAS
Thick layers of soft peaty clay encountered in recent infrastructure development projects in Sri Lanka had been improved mainly by different techniques of pre-consolidation. Alternatively, if peaty clay is mixed in situ with a binder, such as cement, apozzolonic reaction can be induced causing a fundamental change in microstructure. Greater improvement of strength and stiffness would be achieved more rapidly than with pre-consolidation. Samples of peaty clay obtained from the Southern Transport Development Project were mixed with 15% and 20% of cement by wet weight and the mix was allowed to harden. A surcharge was applied by a special loading arrangement to simulate the overburden pressure under which it will harden in the field. Improvements achieved in strength and stiffness of treated peat were assessed by conducting consolidation tests and unconsolidated undrained triaxial shear tests. The results indicated that significant improvements were achieved in the undrained shear strength and primary and secondary consolidation characteristics. Improvements achieved in secondary consolidation ,characteristics were more significant. Micrographs obtained through Scanning Electron Microscopes (SEM) illustrated the micro structural changes taken place in the peaty clay due to mixing with peat.
item: Conference-Abstract
Establishment of a relationship between landslide susceptibility zonation and threshold rainfall intensities
(Department of Civil Engineering, 2023-09-27) Jayasundara, GM; Kulathilaka, SAS; Mallikarachchi, C; Hettiarachchi, P; Herath, S; Fernando, L
Landslides induced by heavy rainfall pose a common geotechnical hazard in Sri Lanka. Sloping grounds in Sri Lanka are made of rocks at different levels of weathering, residual and colluvial soils. The parent rock's mineralogical composition influences the heterogeneous nature of these weathering profiles. Slopes in the region have deep groundwater tables and unsaturated soil profiles with high matric suction during dry periods. However, prolonged rainfall leads to slope instability due to the loss of matric suction and the potential formation of a perched water table or the rise of the groundwater table. National Building Research Organisation (NBRO) has developed the landslide hazard zonation maps of Sri Lanka at 1:50000 and 1:10000 for districts where landslide risk is high based on the overall hazard rating (score) determined by terrain factors. The terrain factors are bedrock geology and geological structures, type and natural soil cover and thickness, slope range and category, hydrology and drainage, land use and management and landform. In landslide hazard zonation maps, sloping grounds are divided into regions of different hazard levels: Safe slope, landslide not likely to occur, a modest level of landslide hazard and landslides are to be expected. To supplement that zonation, it is essential to identify the threshold rainfall intensities causing a disastrous situation in zones of different landslide susceptibility. Currently, the threshold rainfall values are determined by previous experience. Researchers have developed different empirical statistical relationships to identify threshold intensities that vary with local landslide factors and site-specific. Given the significant material variations in Sri Lankan slopes, relying solely on statistical approaches is inappropriate. Landslide susceptibility level-specific thresholds based on numerical studies would be necessary to make reliable decisions regarding early warning and remedial measures. In this research, landslides where the triggering rainfall data is available are back analysed to establish a relationship between landslide susceptibility zonation and threshold rainfall intensities. The SEEP/W 2018 software was used to model rainwater infiltration and the consequent changes in the pore water pressure. The results of the above seepage analysis were then transferred to SLOPE/W software to analyse the slope stability. Initially, recent landslides (Pinnawala landslide and Kithulgala landslide) where rainfall records are available was back analysed and threshold rainfall values for these two slopes was established. The level of hazard of these two landslide areas was identified using the landslide hazard zonation maps of Sri Lanka that developed by National Building Research Organisation (NBRO). A relationship between landslide susceptibility zonation and threshold rainfall intensities can be established by doing this analysis on several landslides that rainfall data was available.
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.