Master of Engineering in Foundation Engineering & Earth Retaining Systems

Permanent URI for this collectionhttp://192.248.9.226/handle/123/33

Browse

Now showing 1 - 20 of 44

item: Thesis-Abstract
Effect of rectification measures in Kahagolla landside - comparison of monitoring and analytical data
(2022) Thilakarathna PADT; Kulathilaka SAS
This research paper focuses on creep movements of rainfall-induced landslides with their groundwater level fluctuation, to understand the pore water pressure development in saturated/ unsaturated soil layers in relation to the mechanism of failure. A case study was selected at Kahagolla Sri Lanka, which is a massive creep landslide initiated around 1957 and triggering by prolonged rainfall events. The stabilization of the Kahagolla landslide was carried out under the “Landslide Disaster Protection Project” implemented by the Government of Sri Lanka with the support of Japan. Detailed geotechnical investigation along with real-time monitoring data showed mainly four slip surfaces along the landslide axis. The main reason for movement is discovered as the rising of groundwater table and subsequent loss in the slip surface strength. Two-dimensional analyses were carried out with several back analyses and adjusted parameters according to real-time monitoring data. Limit equilibrium analysis coupled with a seepage model was performed to confirm the actual conditions of the landslide occurrence. Thereafter, effects of rectifications were also modeled to access the stability status of the rectified landslide. The performance of the rectification measures was further examined with critical design rainfalls and a threshold for the rectified landslide. The results show an acceptable stabilization of terrain after the construction of counter measures. It can be concluded that the final combination of rectifications has been succeeded in the stabilization of this landslide and the above-mentioned approach is appropriate for use in the simulation of deep-seated landslides.
item: Thesis-Abstract
Importance of detailed consideration of construction sequence in installation of retaining walls with excavation : a case study
(2022) Chathurangani PGDW; Kulathilaka SAS
Earth retaining structures are a key component in most engineering constructions. The present design practice as per the standard procedure is limited to analysis the structure under anticipated permanent design loads. However, in reality, the structure undergoes numerous loading patterns depending on the construction methodology as well as construction sequence that might be vary in type, magnitude, direction and distribution to that of design loads. The recent construction experience of some projects in Colombo indicates that these unaccounted loading patterns in construction stages could lead to failure modes that were not expected during design phase. In this study, a detailed forensic analysis is carried out for one case study by means of numerical modeling of different loadings undergone during various stages of the construction stage and root cause of failure for each case is identified. A gap analysis is carried out to ascertain that why the root causes had not been taken in to consideration during the design phase and recommendations are proposed to the design procedure currently practiced to avoid future such consequences.
item: Thesis-Abstract
A Study of applicability of different facing types in soil nailing
(2021) Kumarage B I; Kulathilaka S A S
Soil nailing has been used in Sri Lanka lately, in a wide range of infrastructure projects, as a cost-effective stabilization technique that can be implemented quickly. However, detailed designs with rigor, for nailhead/ facing are not often carried out in the local practice. Full face shotcrete, grid beams connecting nail heads, isolated nail heads (Pillows), and combinations of all these facings types used in the local practice. When the full face shotcreting is not used vegetation is used as a surface protection cover in between the nail heads with the help of a geotextile and nail heads are combined with a wire mesh of specified tensile strength. The mesh is expected to provide stability against any local shallow failures. The versions without full-face shotcrete blend nicely with the natural environment and have gained greater acceptance. In this research design guidelines for different facing types available in published literature are critically reviewed to assess the suitability under high rainfall intensities in local residual soil formations. A number of sites rectified with soil nailing with different types of facing have now experienced few seasons of rainfall and their performance is assessed. The cost-effectiveness and construction difficulties are also reviewed. Based on these factors some guidelines are developed to decide on the most appropriate type of facing depending on the prevailing local 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: Thesis-Abstract
Compressibility characteristics of municipal solid waste in meethotamulla waste fill site
(2021) Galhena GDWN; Kulathilake SAS
Municipal solid waste (MSW) is defined as solid waste generated from community, commercial and agricultural operations and it includes wastes from households, offices, stores, industries and other non-manufacturing activities. Management of municipal solid waste is a major problem all over the world. Recently, Municipal Solid Waste Management became a growing concern in Sri Lanka with the catastrophic landfill failures occurred at Meethotamulla. The need to ensure the stability of existing landfills which are mostly uncontrolled through appropriate engineering designs is a major task at present. MSW forms the largest portion of the landfill and its strength and stiffness (compressibility) characteristics controls all aspects of landfill designs. In this research compressibility characteristics of MSW at different stages of decomposition were evaluated under both saturated and unsaturated conditions. Considering the highly heterogeneous state of MSW, larger samples were tested using a Rowe Cell of diameter 150mm with loading, unloading and reloading increments. Characteristics such as; Coefficient of volume compressibility, coefficient of consolidation, coefficient of secondary consolidation, compression index, recompression index were determined to assess the applicability of conventional Terzaghi consolidation theory in modelling the MSW behavior. Results were compared with the behavior of residual soil and organic soils. The test results revealed that MSW experienced high primary and secondary consolidation settlements. These could be significantly reduced by preloading. The coefficient of consolidation values were quite high.
item: Thesis-Abstract
Effect of bentonite and polymer drilling fluids on skin friction of bored piles
(2021) Hemadasa MBC; De Silva LIN; Nawagamuwa U
Skin friction is a major component in any pile foundation’s load bearing capacity. In terms of a floating pile, almost entire bearing capacity depends on the skin friction. Skin friction may depend on the soil type and parameters while there are several methods of obtaining the skin frictional resistance of a pile depending on the soil type. Most of the high rise structures in Sri Lanka stand on bored end bearing pile foundations. Construction of bored piles widely involves usage of Bentonite as a drilling liquid and as a borehole soil stabilizer which may result in retention of a considerable amount of Bentonite and soil mix between the pile and the surrounding soil. Hence it is arguable that the used Bentonite slurry has an effect on the skin friction of the pile. Polymer liquid is not a commonly used drilling liquid in Sri Lanka but still it has been used in the current study. Main objective of this research is to investigate the effect of using Bentonite and Polymer liquids on the skin friction of pile foundations in Sri Lankan soils. From the results obtained from the direct shear tests conducted in laboratory for laterite soil and sand, it was evident that polymer liquid had no significant effect on soil strength parameters but with time due to the filter cake formation, bentonite will cause a significant change in soil strength parameters
item: Thesis-Abstract
Evaluation of correlations between SPT N and undrained shear strength for fine grained soils of Sri Lankan geological conditions
(2021) Premathilaka KKW; Thilakasiri H S; De Silva LIN
Undrained shear strength is one of the essential parameters in most of the applications of geotechnical engineering. Unconsolidated undrained (UU) triaxial test is the most commonly practiced method of determining the undrained shear strength which takes a considerable time and effort to produce its results, since the sampling stage. Such correlations seldom exist pertinent to Sri Lankan soils. Hence, it is important to have reliable correlations between easily executable, commonly used test results and undrained shear strength of Sri Lankan soils to easily and promptly predict the soil structure interaction phenomenon, especially in local geotechnical engineering applications. Thus, in this study, an attempt has been made to correlate the standard penetration test (SPT) with experimentally determined unconsolidated undrained triaxial test parameters, and the undrained shear strength estimated from the field vane shear test data. This analysis consists of three main stages, namely analysis of the available correlations, analysis of the available data set based on the soil and sampling properties to develop a correlation and the analysis on a set of reliable data with a defined deviation factor. A correlation between SPT N60 and undrained shear strength has been proposed for Sri Lankan silty soils. Further, possible analysis methods for developing correlations for other different soil types have also been addressed. In addition, existing drawbacks and difficulties associated with developing such correlations related to Sri Lankan context are also described.
item: Thesis-Full-text
Comparison between empirical, numerical and practical compression capacity of rock socketed bored and cast in-situ pile : a case study
(2020) Silva HAM; Puswewala UGA
The development of tall structures as a rapidly developing trend in Colombo-Sri Lanka is evident during the recent past due to the high land prices. These tall structures require to be founded on strong substrata and piling is the most popular method that has been used as the foundation for these tall buildings. In Colombo area having found bed rock at shallow depth around 15m to 20m, always design engineers tend to specify the rock socketed end bearing piles without much considering the load carrying mechanism of the pile. It is evident that Sri Lankan design engineering community has a tendency to disregard the pile shaft skin friction resistance, mostly due to the existence of bentonite slurry within borehole during concreting. Therefore, load carrying capacity of such piles is determined completely based on the end bearing from the bed rock. In addition to that in most standards and codes of practice, the pile load carrying capacity correlations are given for specific soil types i.e. sand, clay, gravel. However in local context it is hard to find such conditions and almost all the soils are residual soils having both 𝑐,∅ values. In this research, different correlations for pile load capacity and its variations are evaluated. A detail comparison is conducted between the compression capacity of piles obtained from different empirical/semi-empirical methods, numerical methods such as FEM and in-situ testing i.e. MLT and HSDLT against the code of practices and local guide lines.
item: Thesis-Full-text
Systematic approach to integrated mine bench optimization in soil and rock of Sri Lankan open pit mines - a case study
(2020) De Mel WDM; Puswewala UGA
Instabilities and failures in rock slopes occur due to numerous factors such as unfavorable slope geometries, geological discontinuities, weak or weathered materials in the slopes, existing weather conditions and environmentally induced external factors such as heavy precipitation, seismic activities and groundwater. Bench optimization is carried out to maintain bench height and dip of the slope within an allowable factor of safety, thus avoiding rock slope failures and instabilities. Therefore, optimum determination of these geometrical features has become a most significant part of soil and rock slope stability analysis in Open Pit Mining where multiple benches of excavation are maintained. Field work related to this research study primarily comprised of observation of structural geological features (dip and strike) and other measurements and observations ( joint spacing, separation, condition of joint) required for analysis work, including Slope Mass Rating analysis, at the selected site of Halbarawa, Sri Lanka. Furthermore, soil and rock samples were collected from the selected site to perform laboratory tests. Proctor compaction test and direct shear test were carried out for selected samples to evaluate the overburden slope stability. Simultaneously, stability of soil and highly weathered rock slope was analyzed by SLOPE W software. In order to analyze rock slopes, initially possible rock failure modes were identified using Georient software. If it indicated some tendency to fail, a detailed analysis of wedge failure was carried out using GEO5 software. Further, Toppling and Planer modes of failure were analyzed via SMR analysis. The study focused on optimizing the bench geometry of mine slopes necessarily consisting soil, highly weathered rock and fractured rock in order to explore ways for safe and economical bench designing. This was achieved by integrating kinematic, empirical and limit equilibrium approaches for slope stability investigation and guidelines were finally developed so that the same methodology can be universally applied for assessing the soil and rock slope stability in similar situations. This procedure was developed through the case study of Halbarawa Mine. Results indicated that the stability is more sensitive to variation in cohesion than variation in friction angle of overburden profile. As far as the bench geometry is considered, multiple benches are seen as the most reliable mining methods for steeply dipping benches. According to RQD of each location, the rocks in the particular area varied from moderately hard rocks to hard rock. The Kinematic analysis disclosed that most of joint planes intersect with each other and produce various potential failure mechanisms. The dip and the dip direction of the slope faces determine the possibility of failure and the mode of failure with respect to the discontinuity plane. For the Halbarawa site, as per the SMR analysis, face 1, 2 and 3 can be categorized into completely unstable (V), partially stable (III) and unstable (IV) rock stability classes respectively. It was also understood that surcharge load is a more critical factor than the static water pressure when a wedge failure is considered. The most successful, economical and rapid remedial measures to enhance the stability of rock slope are reduction of bench height and reduction of bench angle.
item: Thesis-Full-text
Correlation between point load strength index and uniaxial compressive strength for hard crystalline rock
(2020) Jayasinghe JMSTW; Puswewala UGA; Rathnasir M
Large scale constructions such as highways, bridges, high rise buildings, jetties etc. are designed to withstand heavy loads depending on the dead loads and imposed loads on the structure. For this purpose, loads from these super structures are to be transferred to competent rock by adopting pile foundations. For end bearing piles, rock socketing has to be done to the competent rock. To identify competent rock, generally, the Point Load Strength (PLS) index (Is(50)) of a rock sample collected during the rock drilling work is determined and the Unconfined Compressive Strength value of the same sample is estimated. If the Unconfined Compressive Strength value is acceptable, the pile can be terminated. For estimating the Unconfined Compressive Strength value from the Point Load Strength value, there are some correlations which are generally used in the Sri Lankan construction practice. All of these correlations are outcomes of research work done in overseas countries. Therefore, this research is an effort of finding an acceptable correlation between the Point Load Strength value and the Unconfined Compressive Strength value for crystalline metamorphic rocks found in Sri Lanka to be used as a guideline for pile termination criteria. More than 130 rock samples were collected from different rock types and both the Unconfined Compressive Strength value and the Point Load Strength index of the same rock were determined by conducting laboratory tests. In addition to these two tests, the Specific Gravity (SG) value of the same rock was determined and the rock type was also identified visually to group/ classify the data according to the rock type. After analyzing the test data gathered, a correlation between the Unconfined Compressive Strength value and the Point Load Strength index was established for different rock types. Few samples from each rock type were used for validating the correlations established.
item: Thesis-Abstract
Development of a large-scale shear apparatus for the determination of shear strength properties of municipal solid waste
(2020) Jayakody SHS; Kulathilaka SAS
Landfill slope failure became one of the hot topics in Sri Lanka as the infamous Meethotamulla waste fill collapsed, claiming lives of people and infrastructure. Every year all around Sri Lanka, millions of tons of Municipal Solid Waste (MSW) are produced, and no one can predict the variation and heterogeneity of their composition. Although MSW is a difficult material to test, many comprehensive studies have been conducted to determine the likely ranges of waste properties and hence to take them in the design of landfills. In this study, in-situ direct shear test was performed to assess and develop insights about the shear strength properties of MSW. A large-scale direct shear apparatus was fabricated to conduct in-situ test which has dimensions of 300 mm × 300 mm in plan view. The report contains the steps followed to manufacture the large-scale direct shear device. One of the main objectives was to test the MSW at differently aged locations in abandoned Meethotamulla waste fill site. The testing procedure is comprehensively described in the report. Further, density test, moisture content test and composition analysis were also conducted in this study. As stated by many researchers a peak failure state was not observed during in any of the direct shear tests. There was no significant variation in the shear stress parameters of the differently aged samples. However, all the samples are more than 03 years old. The results obtained from the tests were analyzed and compared with the published data in literature. Recommendations were made regarding the further studies needed to develop a relationship between the differently aged MSW and their shear strength properties.
item: Thesis-Full-text
Low strain pile integrity testing for rock socketed bored piles in Sri Lanka
(2020) Kodithuwakku TH; Tilakasiri HS; De Silva LIN
Low strain pile integrity testing has been available over several decades. It is the widely used method of pile testing to detect serious defects in piles. The transient dynamic response (TDR) method of low strain pile integrity testing needs pile top velocity and pile top force generated by a small handheld hammer hit. The velocity and force details are useful to estimate the pile condition near the top and the stiffness of pile-soil system Researchers have proposed that dynamic stiffness at low frequencies associates to the static stiffness of pile head. The linear region of load-settlement behaviour of a pile is described by the static stiffness. However, little attention has been paid to developing a relationship between static stiffness and dynamic stiffness. The carrying capacity of pile is considered as the most important issue in pile foundations. Load Testing is the most reliable approach to evaluate the carrying capacity of piles. However, load tests are rarely performed as it is costly, labour intensive and time dependent, but all the piles are subjected to low strain integrity tests. Following the testing results, this research proposes a relationship between dynamic stiffness and static stiffness of bored piles. It is intended to evaluate the allowable carrying capacity of piles with results of low strain pile integrity testing and high strength dynamic load testing. Finally, this research presents a simple methodology to estimate the allowable carrying capacity of piles using instrumented low strain pile integrity testing. The developed methodology will be verified using field load testing results. In addition to that, the success of implementing the TDR method on bored piles is proved by case studies.
item: Thesis-Full-text
Effectiveness of subsurface drainage and vegetation in enhancing the slope stability : a comprehensive case study on Badulusirigama landslide
(2020) Kankanamge LUM; Kulathilaka SAS
Slope instability, triggered by excessive rainfall, is one of the common geo –hazards that geotechnical engineers are challenged with in tropical countries such as Sri Lanka. Typically, these slope failures are initiated in colluvial layers derived from former landslides or planes of low shear strength in differently weathered zones in the thick soil overburden. Improvement of surface and subsurface drainage has proven to be effective in improving the slope stability by lowering the ground water table as well as preventing near surface perched water table conditions. Badulusirigama Landslide in central highlands of Sri Lanka is an example for a slow moving long rotational slip that activates after heavy rainfall events. The landslide was rectified with over 45 m long individual sub-horizontal drains that are arranged into a network of radial drainage groups at different elevations along the long sliding mass. This site is also well equipped with monitoring instruments and thus provides a great case history to further our understanding on contribution of surface and subsurface drains in mitigating landslides. In this study, the effectiveness of the introduced subsurface drainage measures in enhancing the stability of the Badulusirigama Landslide was investigated using 2D and 3D numerical models. The numerical models were then used to predict the behaviour of the landslide during different anticipated rainfall events. The results show that the subsurface drains system enhances the initial near failure condition of the site to a stable slope with a factor of safety of over 1.25 within one month. After initial drop down of the ground water table, the sub-horizontal drains still remain effective by rapidly draining out any infiltration. The analysis also shows that the width of the influence zone of radial horizontal drains should be carefully selected when simplifying the problem into 2D plane strain models because the influence can be very much localised in a low permeable medium. Possibility of introducing surface vegetation as a hybrid measure along with subsurface drainage was also investigated. A factor of safety improvement of 38% and 16.3% was achieved after the simulation of construction of the drains in 2D plane strain and 3D finite element analyses separately. Also, it was found that, vegetation could result in increasing the hydraulic conductivity of the root zone, leading to development of perched water table conditions.
item: Thesis-Full-text
Soil as a foundation material
Wijesinghe, S; Puswewala, UGA
Cement Stabilized Soil Blocks are now considerably popular in the construction industry as an alternative building material to burnt bricks and cement sand blocks. It provides a timely solution for the over exploitation of clay (for bricks) and sand which has resulted in several sever environmental problems. However, as a foundation material there has been little focus on the use of soil. Apart from concrete, rubble stones with cement and sand are widely used as a foundation material even in construction of one or two storied buildings. However, in some parts of the country burnt bricks are also used as a foundation material especially in construction of single story houses. All these materials used for foundation are transported from sources concentrated in particular areas. In this context if compressed soil (stabilized with cement) could be used as a foundation material it will also provide a solution against over exploitation ofsources ofrock and sand. This dissertation presents the research work carried out to introduce compressed soil blocks stabilized with cement as a foundation material alternative to random rubble masonry and burnt brick work. These blocks are manufactured using lateritic soils and a locally designed and manufactured manually operated soil compressing machine.
item: Thesis-Full-text
Utilization of building debris as aggregates in stone column construction in Sri Lanka
Sanjeewa, KWD; Nawagamuwa, UP
Due to rapid development and population growth, construction industry has emerged with few recent problems. The major problem faced by the construction industry is the scarcity of construction material and disposal of construction waste because of high disposal cost and inadequate land fill area. Due to the remnants of 30 year civil war happened in Sri Lanka, huge amount of building debris are to be disposed during new infrastructure constructions. To curtail the amount of building debris, the possibility of using them (concrete, brick and plaster) for civil engineering applications can provide an attractive way to reduce the wastes to be disposed of and it may also provide fiscal benefits. In this study, the scope for using building debris as the traditional rock aggregate for stone columns was investigated. Experiments were conducted using building debris(concrete, brick and plaster)and stone aggregate passing through a 14 mm and retained on a 10mm British standard (BS) sieves. Where experimental studies were carried out to determine the engineering properties (Durability, Shear strength & Compressive strength) of the recycled construction material and compared with conventional road construction material (aggregates). AIV, ACV and LAAV tests and slake durability index test were carried on selected building debris to find out the suitability to be used in stone columns construction.And uniaxial compressive strength testwas carried out to find the resistance to impact and crushing under loads. Improvement in shear strength was tested using vane shear in radially as well as with depth in several laboratory models with a centred stone column made up of different building debris. It was observed that the model done using concrete wastes exhibited a similar capacity of traditional rock aggregates of same size. Other materials did exhibit the same behavior though their results from slake durability tests were relatively low.
item: Thesis-Full-text
Study on utilization of building debris in road construction in northern province of Sri Lanka
Shankar, P; Nawagamuwa, UP
Due to rapid development and population growth, construction industry has emerged with few new problems. The major problem faced by the industry is the scarcity of construction material and disposal of construction waste because of high disposal cost and inadequate land fill area. Northern region of Sri Lanka is undergoing a massive infrastructure development within a shorter period especially in the road sector development. Roads are major consumers of aggregate and soil and the influence of aggregate cost is more in total construction cost of the roads. The aggregates for these road construction projects are transported from Medawachchiya due to scarcity of local material and the approximated transport distance from Medawachchiya is more than 150 km. Hence, transport cost is approximately 70% of the project cost. In order to curtail the cost of construction of roads and reduce the industrial waste disposal, the possibility of using building waste as road construction material has been studied. Building debris such as concrete, random rubble masonry, concrete block and plaster were selected for this research. Experimental studies were carried out to determine the engineering properties of the recycled construction material and compared with conventional road construction material. Aggregate Impact Value Test, Aggregate Crushing Value Test, Los Angeles Abrasion Test and California Bearing Ratio Test were carried on selected building debris to find out the suitability for road base construction. Crushed samples of selected debris were tested to determine the suitability for replacement of soil in road construction. It has been observed that the Random Rubble Masonry debris can be directly used for road base construction however, crushed debris of block masonry, plaster and concrete can replace the soil for construction of sub base, shoulder, embankment and for surface of ‘D’ & ‘E’ class roads after adding suitable percentage of plastic clay.
item: Thesis-Full-text
Possible use of bottom ash in embankment construction
Chrishanthi, JAC; Kulathilake, SAS
Bottom ash is a solid residue byproduct produced by coal burning for electricity generation. In Sri lanka it has not been developed proper system to dispose of this bottom ash other than using small amount for concrete wall block and paving blocks. Usually 75 tons of bottom ash is produced every day with the operation of three number of power generation units in Lakvijaya Power Station. By product of bottom ash is to be a one of a solution for soil scarcity for filling of embankments. For the testing, it is used 3 set of bottom ash each has different origin. To identify characteristics of bottom ash, several tests were done. Particle size distribution, Specific gravity, plasticity, proctor compaction test and permeability test were done to identify basic characteristics. The test results indicated that granular, permeable, pores structure is available for the bottom ash. Specific gravity and the density are quietly low. The compressibility characteristic of bottom ash was determined by one-dimensional consolidation test by using different loading, unloading and reloading sequences for 3 samples. Low compressibility occurred even at higher loads with higher void ratios. Shear strength parameters were assessed for compacted bottom ash by direct shear test under consolidation drained condition. Cohesion is zero and sufficient friction values are available. Toxicity behaviors were analyzed to identify leach out of toxic materials and radiation risks to the environment. Test results indicated that bottom ash favorably suitable for as an embankment construction.
item: Thesis-Full-text
Enhancement of the stability at the site of an ancient landslide in a road cutting with drainage and reinforcement - case history at Ginigathhena
(2019) Lakruwan SOADM; Kulathilaka SAS
Engineers involved in infrastructure development projects in the hilly terrain of Sri Lanka encounter ancient landslides which could be triggered by rainfall or construction activities. One such landslide was encountered during the widening of a bridge in the main connecting road between central hill country and capital; Avissavella – Hatton – Nuwaraeliya road at bridge no. 48/2 near Ginigathhena. Extensive mitigation measures had to be designed with detailed attention to construction sequence in order to prevent reactivation of the slide. A valley area had been formed by the previous landslide. Morphology of the area is a sloping land with undulating topography towards upper slope. This has led to the formation of a waterlogged marshy area on a flat land at immediate upper slope and a stream flowing through valley. Water table of the area is quite high. The landslide got activated due to a minor excavation at the toe region for the bridge widening. There had been no rain when the slide was activated. Subsequent rain cause further activation of the landslide. Further widening is necessary according to the new highway design. Ground water regime management and geometry modification are the two primary approaches used in enhancing the safety margins of the site. Surface and subsurface drainage improvement by various methods such as; cutoff drains, berm drains, trench drains and horizontal drains were introduced for lowering the ground water table. The stability of the steep cuts necessary to accommodate the increased road width was enhanced further by the use of soil nailing. Top down approach was adopted to ensure the safety of the slope during construction. Drainage measures were very effective in economizing the soil nailing design. The analysis and design of stabilizing measures were done using GeoStudio Seep/W and Slope/W software. Design outcomes were confirmed by monitoring of ground water table and surface movements of the slope.
item: Thesis-Full-text
Application of dynamic and vibro compaction methods for densification of granular fill in reclaimed land in Sri Lanka
(2019) Samarasinghe AI; De Silva LIN
In the recent past, Government of Sri Lanka executed a large-scale reclamation project in Sri Lanka to add a brand-new land of 267-hectare to the Capital, Colombo with strategy of converting Colombo as a commercial hub of South Asia. For this project, 72 Million m3 of sea sand which was dredged by Trailing Suction Hopper Dredgers at 10km off from shore of Colombo was placed mainly by hydraulic methods at lower elevation while applying bulldozers at the top. This reclamation material was noted as clean uniform sand and which was under loose to medium dense condition prior to densification. This sand fill was densified using two methods, namely dynamic compaction and vibro compaction. Dynamic compaction, which is generally considered as one of the most economical sand improving methods, was applied in all areas except vibration sensitive areas at the city end and the areas where deep ground improvement was required for stability of earth retaining structures. Since settlement of subsoil in the seabed is not critical, the considered major geotechnical issues were achieving of required bearing capacity, shear strength and avoiding possible liquefaction. To sort out all geotechnical issues, sand densification was the only solution. Though there is a very long history for dynamic and vibro compaction methods, still reclamation projects are not preplanned to utilize the self-compaction achieves during sand placing very effectively, while designs always follow a very conservative approach. Moreover, designs are carried out using pre-defined energy criterions rather than considering existing fill material properties and its pre-compaction condition. Thus, there was a paramount requirement to assess the dynamic and vibro compaction methods for Sri Lankan fill materials and reclamation methods with the intention of optimization of the above compaction methods. In order to optimize dynamic compaction method, the pre-and post-compaction condition (by CPTs) was evaluated by crater depth, net volume changes, influenced depth and related indices, which assess the degree of improvement based on applied iv energy. Similarly, densification by vibro compaction was evaluated with respect to the factor such as point spacing, amperage and compaction holding time. In addition, effect such as age of the compacted fill was considered for both dynamic and vibro compaction in this reclamation fill of clean sand. Finally, verification of densified ground by selecting CPTs at least compacted points with respect to the compaction grids was assessed for both dynamic and vibro compaction to confirm the optimization has no adverse effect on the final design. Based on the finding of this research, fill material’s index properties of Sri Lankan sea sand were determined while being noted that there is no hesitation for applicability of dynamic and vibro compaction for densification. During the analysis it was suggested to modify some correlations derived based on laboratory test data to achieve more realistic output for actual reclamation condition. In addition, design of dynamic and vibro compaction by performance-based method through trial compaction was discussed.
item: Thesis-Full-text
Investigation on the equivalent modulus of subgrade reaction of layered soil
(2019) Mathuwanthy T; Thilakasiri S; De Silva LIN
‘Modulus of subgrade reaction’ is the ratio between the pressure applied on the soil and the corresponding settlement. There is no theoretical relationship found to obtain equivalent subgrade modulus of layered soil. Top dense sand layer thickness, bottom loose sand layer thickness, strip footing width and thickness are changed and equivalent modulus of subgrade reactions are obtained by equivalent spring theory and weighted average method. These, equivalent subgrade modulus are separately applied in Heteryni method equations in order find vertical settlement, bending moment and shearing force along the medium length footings. PLAXIS 3D numerical models are developed for same footing parameters and soil properties to compare the Heteryni method outputs. Equivalent subgrade modulus using equivalent spring method is constant with top soil layer thickness for a given footing width and footing depth. Weighted average method equivalent subgrade module is non linearly increasing with top dense sand layer thickness for a given footing and bottom loose sand layer thickness. Equivalent subgrade module for thinner footing depth is always greater than the thicker footing for a given footing width and soil profile in both spring theory and weighted average method. Settlement along footing obtained by equivalent spring method equivalent subgrade modulus applied in Heteryni method equation is highly varying from weighted average method equivalent subgrade module applied in Heteryni method equation and PLAXIS 3D model settlement output. Equivalent spring method is considered as unsuitable to calculate the equivalent modulus of subgrade reaction for layered soil stratum. Settlement difference between PLAXIS 3D method and weighted average method equivalent subgrade module applied in Heteryni method equation shows up to 45 percentages and this difference cannot be negligible. This study will shed a light in the theoretical relationship of equivalent subgrade module research field as this would be the first attempt to check the behavior and suitability of equivalent subgrade modulus of layered soil stratum.