Effect of Sand column with and without encasement of Geosynthetics in Black cotton soil

Transcription

1 International Researc Journal of Engineering and Tecnology (IRJET) e-issn: - Volume: Issue: July- p-issn: - Effect of Sand column wit and witout encasement of Geosyntetics in Black cotton soil Haris C, M S Nagaraja Gupta, Radika K N and ManjunatItagi Assistant Professor, Department of Civil Engineering, East West Institute of Tecnology, Bengaluru, Karnataka, India. Associate Professor, Department of Civil Engineering, East West Institute of Tecnology, Bengaluru, Karnataka, India. Assistant Professor, Department of Civil Engineering, East West Institute of Tecnology, Bengaluru, Karnataka, India. Assistant professor, Department of Civil Engineering, East West Institute of Tecnology, Bengaluru, Karnataka, India *** Abstract Soil reinforcement can be an ideal solution for improvement of clay. Out of oter conventional metod, Sand columns are effectively being used for ground improvement, particularly for flexible structures suc as road embankments, oil storage tanks, etc. Te load capacity of te sand columns mainly depends on te sear strengt of te surrounding Black cotton soil. Te sand column is found useful in improving load capacity and reducing te settlement of clay deposit. In addition to tis, te encasement of geosyntetics all-round te sand columns is suggested for enancing te load carrying capacity of te sand column in treated ground wic also ensures te easy formation of columns in weak strata. Te present study investigates te effect of diameters of geosyntetics encased sand columns in Black cotton soil deposit during loading. Te load responses of sand columns are also investigated wit te variation of encasement lengt of te column and compare te response of sand column wit and witout encasement. Key Words: Encasement, Improvement, Bearing Capacity, Black Cotton Soil, sand Column, Geosyntetics etc.. INTRODUCTION India as a large coastline area exceeding more tan kms. Te large numbers of industries and ports were being built in te coastal area; te soil wic is available in tis area is of weak strata, were in geotecnical engineers faces a presence of different problematic soils wit varied engineering caracteristics. Among all tese we considered te soft soil is one of te biggest problematic soil for geotecnical engineers because of low bearing capacity, ig compressibility, low permeability etc. Wen te soft clay indicates strengt sensitivity, te problem may be even severe. Sensitive soft clays often contain meta-stable structure and tis structure may provide a ig degree of compressibility to te soil. As te settlement of te sensitive soft clay is significantly ig, ground improvement metods are needed. Tese metods include prefabricated vertical drain (PVD), surface and deep compaction, vacuum drainage, preloading, stone/sand columns etc. Among tese metods, stone/sand columns metod is one of te efficient ways of reducing bot settlement and consolidation time. Ground improvement tecniques are normally preferred for economic considerations. Out of several tecniques available stone/sand columns ave been widely used. Tis ground improvement tecnique as been successfully applied to increase te bearing capacity and to reduce te settlement for foundation of structures. Te stone/sand columns are vertical columnar elements formed below te ground level wit compacted and uncemented stone fragments or gravels or sand. Te stone/sand columns are increasingly being used as ground reinforcement for supporting a wide variety of structures including buildings and flexible structures. Tus, in case of group pile, bulging is primary mode of failure. Tis drawback can be overcome by wrapping te individual sand columns wit suitable geosyntetics. Te geosyntetics encasement elps in easy formation of sand column and improves te strengt and stiffness of columns. By reinforcing te sand column by geosyntetics, te ultimate bearing capacity of tat column can be increased to a considerable amount.. Failure mecanisms:.. Single sand columns: Sand columns may be constructed as eiter end bearing on a firm stratum underlying soft soil, or as floating columns wit te tip of te column embedded witin te soft layer. In practice, owever, end bearing sandcolumns ave almost always been used in te past. Consider a sand column loaded over just te area of te column as sown in Fig.. Eiter end bearing or free floating sand columns greater tan about tree diameters in lengt fail in bulging as illustrated in Fig.a. A very sort, IRJET Impact Factor value:. ISO : Certified Journal Page

2 International Researc Journal of Engineering and Tecnology (IRJET) e-issn: - Volume: Issue: July- p-issn: - column bearing on a firm support will undergo eiter a general or local bearing capacity type failure at te surface (Fig.b). Finally, a floating stone column less tan about to diameters in lengt may fail in end bearing in te weak underlying layer before a bulging failure can develop (Fig. C). For te subsurface conditions generally encountered in practice, owever, bulging is usually te controlling failure mecanism.. To study te load bearing capacity of Geosyntetics encased Sand columns wit group columns.. To predict te settlement reduction factor of reinforced ground wit te unreinforced ground. Te goal of tis tesis work is to increase sear strengt and provide required minimum bearing capacity, restrict total and differential settlements to accept magnitude of proposed loading and to provide acceptable long term performance.. SCOPE OF WORK: Figure: Failure mecanisms of a single column in a omogenous soft layer... Sand column groups: A group of sand columns in a soft soil probably undergoes a combined bulging and local bearing type failure as illustrated in Fig..a. A local bearing failure is te puncing of a relatively rigid sand column (or group) into te surrounding soft soil. Sand column groups aving sort column lengts can fail in end bearing (Fig..b) or peraps undergo a bearing capacity failure of individual sand columns similar to te failure mode of sort, single sand columns. Te purpose of tis researc is to investigate te performance of sand column encased wit Geosyntetics, wic is a new concept and to study te strengt caracteristics of Black cotton soil wen used as a foundation material for floating, embedded dept, under different saturation condition. Te researc presents results of observations obtained from tests using group of columns for floating and embedded footing on a Black cotton soil wic is essentially reinforced. Reinforced soil performs satisfactorily under monotonic loading, wellestablised literatures are available wit respect to geosyntetics encasement. Te Geosyntetics enwrapped sand columns subjected to static loads finds a tremendous potential in restoring te column for practical usage as te static loads comes into play. Studies and observations sow tat te settlements are greatly reduced and te load carrying capacity of encased columns is increased appreciably. Tese results triggered a series of studies to determine te load-settlement beavior of bot unreinforced and reinforced soil under different conditions. Any decision to effectively use an improvement tecnique requires a clear understanding of material beavior subjected to static loading. Hence in te present investigation te efficacy of using te Geosyntetics as encasement to te sand column is investigated and experiments are conducted to know te load-settlement beavior of group of columns wit different lengt reinforcement and different diameter of columns under different parameters.. Materials:. Black cotton soil: Figure : Failure modes of sand column groups. OBJECTIVE: Need for engineering ground improvement arises wen a project encounters difficult foundation conditions. Te main objective of te study is as follows. To study te effect of sand column encased wit Geosyntetics in improving te strengt caracteristics of Black cotton soil. Te Black cotton soil is collected from te new railway station site of bypass road, near Hiremagalore village, Cikmagalore. Te soil sample was collected by open excavation, from a dept of one meter below natural ground level leaving te surface soil. Te soil is oven dried and used after sieving troug IS. sieve, properties of te Black cotton soil is suarized in table., IRJET Impact Factor value:. ISO : Certified Journal Page

3 International Researc Journal of Engineering and Tecnology (IRJET) e-issn: - Volume: Issue: July- p-issn: - Table:.. Properties of sand Specific gravity. Coefficient of Uniformity. Coefficient of curvature.. Lime: Te lime is used wic will directly get in te market.and te lime is stored in place to prevent its carbonation. Cart.: Sieve Analysis of black cotton soil Table.: Properties of black cotton soil A.Index properties of soil Water content. Specific gravity. Liquid Limit. Plastic Limit. Plasticity Index. Srinkage Limit. Free swell index Te soil classification based on Index properties belongs to CH group in wic Inorganic clay wit ig compressibility.. Geosyntetics For our project we ave used different grades of geotextiles and geogrids suc as. a) Geogrids b) Geotextiles Geotextiles (Non oven) of different grades. PE-EF. PE-EF. PE-EF B. Engineering properties of soil OMC MDD.kn/m Undrained coesion.kn/m. Sand as Fine Aggregate: Te sand sieved troug IS. used for experimental work. Te sand is collected from te construction site near East West Institute of Tecnology in Bangalore. Figure : Geotextiles Table.: Properties of geotextiles as per company details. PROPERTY PE- PE- PE- EF EF EF UNIT Mass per unit area g/m Tickness... Mm Grab tensile strengt N Grab elongation at break Trapezoidal tear N CBR puncture KN Cart.: Sieve analysis of sand, IRJET Impact Factor value:. ISO : Certified Journal Page

4 International Researc Journal of Engineering and Tecnology (IRJET) e-issn: - Volume: Issue: July- p-issn: -. Spacing of sand column: Figure : Geogrids Table.: Properties of geogrids as per company details Weigt g/m Mes size.*. Coating content Tensile strengt (MD) N/cm As per IS standards, spacings are provided in between columns ranges from.d to d were d represents te diameter of sand column. According to IS: (Part me /Sec)-. Te spacing of columns is followed ere by providing single & group columns at centre and along peripery. Te four numbers of columns are arranged along te peripery wit spacing of d c/c and five numbers along centre & peripery by providing appropriate spacing of.d c/c.. Experimental work and metodology. Metodology: ) Te Model footing test is conducted for different lengts of sand columns wit geosyntetics as encasement for various D/B ratios were D denotes te diameter of test tank and B denotes te diameter of te circular model footing. Te results are observed for different diameter columns. ) Te model footing test is conducted for group of columns encased wit geosyntetics for embedded dept, floating dept & intermediate depts. ) Load settlement caracteristics of black cotton soil using sand columns wrapped wit geosyntetics is analyzed for group of arrangement and encased columns using model footing tests. ) Based on te experimental results conclusions are drawn for D/B ratios of,, and for different lengt and for different pattern of arrangement of sand columns.. Preparation of black cotton soil deposit: Oil is applied to inside wall of tank to reduce any friction between soil and tank wall. Required quantity of soil is mixed wit water content to maintain te density of.kn/m and torougly mixed soil is filled in te tank in layers by compacting eac layer torougly wit te elp of tamping aer design for te work.. Preparation and construction of Sand column: By mixing te different percentage of lime and sand wit Black cotton soil te UCS and direct sear test is conducted te maximum percentage is selected as a sand column material. Figure.: Spacing of encased columns. Testing Procedure:. A circular small steel test tank of diameter and eigt is used as model test tank. Te side walls of te tank were made smoot by coating wit a lubricating gel to reduce te boundary effects. Te black cotton soil wic is used for work is kept for r to reac its maturity period.. Te required weigt of soil sample is calculated for te lowest density of.kn/m as obtained from te compaction test results is mixed torougly wit water content and filled in layers into te test tank using rainfall tecnique.. Te soil sample is filled into te test tank wit maintaining its density trougout te procedure.. Te different diameter of oles is created wit different pattern witout disturbing te density of soil in te tank.. A cylindrical ole enwrapped wit te geosyntetics formed in te test tank.. Te sand column material is ten filled into te oles troug a cone made of paper.. Te circular model footing of dia is placed exactly on top of te constructed sand column.. Te footing was placed exactly at te centre of te loading jack to avoid eccentric loading. Calibrated proving ring is used to measure te load transferred to te footing. Te load was applied in small increments at constant rate of strain. Eac load increment is maintained constant until te footing settlement stabilized.. Tree dial gauges are fixed on footing and deformations are measured troug dial gauges (Dg, Dg, and Dg ). Te Load improvement ratio and settlement reduction factor are calculated for different settlement levels.. Te above procedure is repeated for different lengts of encased sand columns and for different pattern of arrangement of sand columns., IRJET Impact Factor value:. ISO : Certified Journal Page

5 load in KN International Researc Journal of Engineering and Tecnology (IRJET) e-issn: - Volume: Issue: July- p-issn: -. RESULTS AND DISCUSSIONS Figure : Preparation of mould and inserting diameter To evaluate te beneficial effects of providing sand columns encased wit geosyntetics in Black cotton soil, experiments are conducted on embedded; floating columns constructed in black cotton soil subjected to monotonic loads. Te effects of group of sand columns of different lengt in different pattern of arrangement are found out. Black cotton soil is mixed wit varying percentage of lime and sand and unconfined compressive and direct sear tests are conducted and for all te combination te direct sear test is conducted wit encasement of te geosyntetics.. Model footing test results: Figure : Group of columns in mould Figure : Inserting geosyntetics Te load deformation beavior of te encased sand column is studied by loading it in a Triaxial loading frame at a strain rate of./minute. To load te stone column area alone a circular model footing of diameter is placed exactly at te center of te column and te load is applied. Settlement is observed for equal intervals of load up to an average deformation of. Model footing test is carried out on black cotton soil wic is compacted to lower density witout reinforcement, te load-settlement caracteristics of soil are plotted in te different graps above. Te curve sows tat te soil undergoes settlement at lower points of load wic is not appreciable and ence requires reinforcement. Pattern(no Lengt() Diameter() Load(KN) of column).... Figure : Filling te sand column materials settlement in Figure : Final setup mould Cart.: Load - Settlement caracteristics grap of reinforced black cotton soil. Load - Settlement caracteristics of encased reinforced (PE-EF lengt) sand columns for different lengts and different diameters for settlement., IRJET Impact Factor value:. ISO : Certified Journal Page

6 Load improvement ratio load in KN International Researc Journal of Engineering and Tecnology (IRJET) e-issn: - Volume: Issue: July- p-issn: - Lengt() Diameter() Pattern(no of Load(K column) N).... Cart. Load - Settlement caracteristics graps of reinforced black cotton soil. Improvement wit respect to Geogrids. Lengt N o diamete r Failure load(kn) Wit out Contribution (KN) Improvement wit respect to PE-EF. lengt N o settlement in diamete r Failure load(kn) Wit out Contribution (KN) Improvement strengt Improvement strengt Improvement wit respect to PE-EF. lengt N o diamete r Failure load(kn) Wit out Contribution (KN) Improvement strengt..... Improvement wit respect to PE-EF. lengt No diame ter Failure load(kn) Wit out Contribution (KN) Improvement strengt Load improvement ratio: It is te ratio of load of reinforced to te load of unreinforced soil. Te effect of lengt of sand column on load carrying capacity is measured in terms of load ratio. Te load improvement ratio is analyzed for two different patterns of columns for settlement. Load improvement ratio is given by equation, LIR = Qur/Qr Were, Qur = Load of encased reinforced soil in KN. Qr = Load of reinforced soil in KN... Lengt of encased column in LIR for no.s LIR for no.s Load improvement V/S lengt for encasement of geogrids., IRJET Impact Factor value:. ISO : Certified Journal Page

7 Settlement reduction factor International Researc Journal of Engineering and Tecnology (IRJET) e-issn: - Volume: Issue: July- p-issn: - Settlement analysis for group encased columns. A suitable foundation system sould satisfy te settlement criteria. A week ground treated wit sand column will settle less tan an untreated ground ence te settlement reduction factor is te main objective in te ground improvement tecnique. Settlement reduction factor (β) can be calculated by te relation β = (S O-S r) /S O Were, So = Settlement of unreinforced soil in compared to unreinforced soil. Sr = Settlement of reinforced soil in compared to unreinforced soil. Te settlement analysis for columns of different lengt and different Numbers. From te above table grap is plotted wit respect to SRF V/S lengt of encasement. Te settlement reduction factor for five numbers is less compare to four numbers for settlement. Effect of lengt of encase (PE-EF) column in settlement reduction for settlement for dia. Settlement reduction factor V/S lengt for encasement of PE-EF. CONCLUSIONS From above discussions following conclusions are drawn... Lengt of encased column in SRF for no.s SRF for no.s It is concluded tat te lime and sand combination, i.e. lime and sand will give a better sear strengt. Te reinforced sand column dia of number columns will give a better strengt of. Kn. Geogrids encased wit will give a iger strengt of.kn for dia of number columns. As te different material is used as encasement, in tat geogrids will give a better result in all percentage of encasement. Tere is a marginal reduction in settlement wen te sand column is encased wit geosyntetics. Tere is an improvement in LIR wit te numbers of sand columns wit encasement of geosyntetics. Te group of column in wic five numbers diameter will give better result compare to number of column in larger lengt and larger diameter. More number of closely spaced layer affect omogeneity of soil mass and ultimately te band between soil and reinforcement tis results in decreasing rate improvement of black cotton soil. REFERENCES [] Amet Demir, et al.,(), An experimental study on beaviour of geosyntetic reinforced stone columns. nd International Balkans Conference Of Civil Engineering,BCCCE,- May,Epoka University. [] Ali, K et..al (), Beaviour of reinforced stone columns in soft soils: an experimental study.indian GeoTecnical conference, Geotrends [] Cungsik Yoo, A.M.ASCE (), Performance of Geosyntetic-Encased Stone Columns in Embankment Construction:Numerical Investigation.A.M.ASCEJ.Geotec.Geoenviron.Eng..:- [] Gupta.R and A. Trivedi (), Beavior of model circular footings on silty soils wit cellular supports. [] Hamed Niroumand, et al.(), Soil Improvement by Reinforced Stone Columns Based on Experimental Work. EJGE VOL.[],BUND,L [] Joel Gneil,Abdelemalek Bouazza (), Improvement of soft soils using geogrid encased stone columns. [] J.A Black, Sivakumar, et al.,(), Reinforced Stone Columns in Weak Deposits: Laboratory Model Study. [] Kameswar Rao et al., (), Comparative study of experimental and teoretical load carrying capacity of stone column wit and witout encasement of geosyntetics IJAET ISSN:- [] Murugesan.S and Rajagopal.K (), Sear load tests on stone columns wit and witout geosyntetic encasement.geotextiles and Geomembranes () - ASCE(), [] S. Murugesan and K.Rajagopal (), Model tests on geosyntetic-encased stone columns.asce(),- [] Moaed Y. et.al., (), Improvement of soft clays by end bearing stone columns encased wit geogrids. [] Mereena K.P. et.al., (), Triaxial compression of clay reinforced wit quarry dust fibre column. [] Sujit kumar Das, Mukul Candra Bora (), Influence of geosyntetic encasement on te performance of stone columns floating in soft clay., IRJET Impact Factor value:. ISO : Certified Journal Page

8 International Researc Journal of Engineering and Tecnology (IRJET) e-issn: - Volume: Issue: July- p-issn: - [] Y.F. Leung, A, Klar(), Teoretical Study on pile Lengt Optimization of Pile Groups and Piled Rafts Geotec.Geoenviron, Engg,.:- [] [] Zamatkes. A & Coobbasti.A.J (), Settlement evaluation of soft clay reinforced by stone columns, considering te effect of soil compaction, IRJET Impact Factor value:. ISO : Certified Journal Page