Performance of polypropylene textile encased stone columns

https://doi.org/10.1016/j.geotexmem.2020.10.025Get rights and content

Highlights

  • Potential utilization of a novel woven polypropylene material in soft ground is highlighted.

  • Effect of various encapsulation conditions on performance of stone column is investigated.

  • Influence of area replacement ratio on geosynthetic encased stone column is studied.

  • Effect of different encasing materials and encapsulation conditions on stone column is highlighted.

  • Influence of group arrangement of stone columns is also studied.

Abstract

This paper explores the potential use of a woven polypropylene textile for encapsulating stone columns and improving performance of a local soft soil in Warangal city of India. A series of axial load tests were performed on stone columns of various diameters and under various encapsulation conditions that include single and double layers and other combinations. Load carrying capacity of stone column increased twice its original capacity when encapsulated with different geofabric materials. Performance enhancement strongly correlated to the tensile strength of encasement material and encapsulation condition. In addition, the influence of lateral thrust on group of stone columns arranged in square and triangular patterns were investigated. Irrespective of the material used, lateral displacement reduced by half for encased stone columns. Apart from tensile strength of encasing material, the amount of material used for encasement in the form of additional encasement layer was found to be crucial. The cost of using the polypropylene encasing material is only a third of the commercial geotextiles; however, the performance is inferior to woven geotextiles but far superior to non-woven geotextiles.

Introduction

Geotechnical engineers widely use natural resources for ground modifications to improve the stability and deformation behavior of soft soils. For example, stabilization using lime and cement are used in soft soils even for achieving a moderate increase in the bearing capacity. Such ground improvement techniques are expensive. In recent years, several researchers have promoted using alternative materials that are cheaper but still effective in improving the soft soil behavior such as the coir, jute, bamboo and polypropylene fibers (Ghosh et al., 2005; Punthutaecha et al., 2006; Hegde and Sitharam, 2015a, 2015b; A. Hegde and Sitharam, 2015; Lal et al., 2017; Burragadda and Thyagaraj, 2019). The use of sustainable and recycled materials has been steadily increasing in the 21st century in the construction industry for addressing challenging problems associated with the improvement of soft ground in various countries A. Hegde and Sitharam, 2015; Soleimanbeigi, 2015; Fu. Et al., 2018; Meguid and Youssef, 2018; Chai et al., 2019). In recent years, the focus of using recycled materials has been directed towards developing innovative methods that contribute to savings in the construction projects. For example, Zhou et al. (2002) adopted lime and lime - fly ash columns to improve weak fly ash ground, Santos et al. (2013) constructed a wrapped face geosynthetic wall using recycled construction and demolished waste as a backfill material, Mazumder et al. (2018) replaced granular material with building debris and shredded tire chips partially or totally for construction of stone columns, Terzi et al. (2018) encased stone columns using waste tires replacing commercial geosynthetics, Alkhorshid et l. (2019) simulated conventional geosynthetic encased stone columns with column constructed using recycled construction and demolished waste and encased with geosynthetic, Selvakumar and Soundara (2019) reused expanded polystyrene (EPS) beads for the formation of geofoams granules column to arrest swelling behavior of expansive clays. In addition, several researchers during the last three decades have used various sustainable alternatives to improve soft soils behavior to a depth of 30 m that were cost efficient (Balaam and Booker, 1981, 1985; Bergado and Lam, 1987; Ayadat and Hanna, 2005; Guetif, 2007; Castro and Karstunen, 2010; Deb et al., 2010; Babu et al., 2012; Najjar, 2012; Zhang et al., 2019).

Several studies suggest that stone columns can be effectively used in improving soft soils behavior whose undrained shear strength lies between 15 kPa and 50 kPa (Huges. et al., 1975; Murugesan and Rajagopal 2006; 2007; Shahu and Reddy 2011). The confining support that is provided with stone columns is a key factor in the improvement of the soil behavior. Soft soils with undrained shear strength less than 15 kPa are not capable of providing the requisite confining support to the stone columns; they typically fail by bulging, punching or in shear. Inadequacy of the surrounding confining pressure also leads to the displacement of aggregate material into the surrounding soils (Murugesan and Rajagopal, 2010; Indraratna et al., 2013; Miranda et al., 2015; Almeida et al., 2015; Fattah et al., 2016; Tai et al., 2018). A geosynthetic encasement to stone columns enhances the circumferential confinement and avoids displacement of aggregates in the surrounding soil. Zhang and Zhao (2013, 2015) studied the deformation characteristics of stone columns under axial loads. Their results suggest that there is an increase in the stiffness with the use of the encasing material that contributes in the reduction in bulging and settlement of the stone columns. Studies of several researchers suggest that stone columns with geosynthetic encasement contribute to an increase in stiffness of column and a decrease of excess pore pressures and stress concentration on the surrounding soil (Murugesan and Rajagopal, 2007; Yoo, 2010, 2015; Yang et al., 2016; Castro 2012, 2014a, 2014b, 2017).

Various materials were used by many researchers for encasing stone columns. Several researchers (Gniel and Bouazza, 2009; Deb et al., 2011; Gu et al., 2016) conducted laboratory tests on stone columns encased with geogrid and studied methods of overlap. Their results suggest that a ratio of 1:2 can be used for the length of the stone column to that of its depth of encasement. The stone columns are strengthened with vertical circumferential nails and random fibers, exhibited greater load carrying capacity and stiffness in comparison to stone columns that were not encased (Shivashankar et al., 2010; Basu et al., 2016; Rezaei et al., 2019). The effective depth of encasement was observed to be three times to that of diameter of the stone column from these test results. Black et al. (2007) reported enhanced performance of weak deposits modified with stone columns that were reinforced with tubular wire mesh, concrete plug and metal rod, with respect to load carrying capacity and settlement behavior. The load carrying capacity and stiffness characteristics of laterally reinforced stone columns significantly increased by using geotextiles (Hasan and Samadhiya, 2016a, 2016b; Ghazavi et al., 2018). In addition, reduction of lateral bulging, which was attributed to effective interlocking between the column and reinforcing materials was reported from these studies. Lateral movements frequently occur in field loading conditions that can cause displacement of column material in surrounding soil and shear deformations in the stone columns. Such problems observed in engineering practice applications can be significantly reduced or alleviated with geosynthetic encasement to stone columns that contribute towards the shear strength of the stone column until the material ruptures. (Sivakumar et al., 2004; Murugesan and Rajagopal, 2008; Basack et al., 2017; Mohapatra et al., 2016a, 2016b; Miranda et al., 2016).

The summarized literature encourages the use of geotextiles in encased stone columns, which promote their behavior to function as rigid piles. Geosynthetic encased stone columns can be effectively and economically replace pile foundations, where a modest increase in bearing capacity is required. In this paper, woven polypropylene textiles were used to encase stone columns for enhancing the stone columns performance. Encasing numerous stone columns in field requires a massive quantity of geotextiles, which has to be specifically manufactured, based upon the design requirements without having a longitudinal joint, which significantly increases the project cost. The woven polypropylene textile, which is widely used for packing cement and agricultural products, can serve as a sustainable and economical alternative for low and medium cost projects. The recycled polypropylene textiles can be used for reinforcement and separation purposes in highway embankments, stabilization of slopes etc. These textiles also have a special capability to form into composites with various natural and commercial materials. The cost comparison and the properties of the commercial synthetic textiles and woven polypropylene textile used in the current study is summarized in Table 1. The woven polypropylene textiles with adequate tensile strength that are available in the local market of Warangal, India, were chosen for the encasement of stone column, in the current study. Their performance is compared with stone column encased with woven and non-woven geotextiles. Investigations were carried out on encased stone columns with different encapsulation conditions by using an additional encasement layer to the existing one. In addition, investigations were also carried to study the impact of lateral thrust, which usually occurs in the field due to lateral movements of soil or due to earthquakes. Finally, group of stone columns arranged in different patterns (square and triangular), were subjected to the induced lateral forces and their effect on stone columns were also examined. A comprehensive comparison is provided between conventional geotextiles and woven polypropylene textile both from their performance and associated economics. The recycled woven polypropylene can be used for different applications in soft ground engineering instead of conventional geotextiles for low and moderate costs projects.

Extensive experimental studies were carried out in this study to investigate the use of woven polypropylene material for encasing stone columns in place of conventional geotextiles. The woven polypropylene material used in this study is widely manufactured in India and can be purchased from local markets; for example, in Warangal city of India where the present study has been undertaken. These materials are easily available in comparison to commercial geotextiles, which are not available in local markets and are expensive. In this study, both commercial woven geotextiles and non-woven geotextiles were used for encasing stone columns to investigate their technical performance and for cost comparisons. Three different encapsulating conditions were chosen for testing, which include: (i) encasement with single layer; (ii) encasement with double layer; and (iii) encasement with halfway length double and halfway length single layer as depicted in Fig. 1. A series of axial load tests were carried out on soil samples constructed using soil collected from Bhadrakali lakebed of Warangal city, India and modified with stone column in a specially designed test tank made of cast iron with dimensions 650 mm × 650 mm x 650 mm and thickness 8 mm. One side of the tank has a provision of detachable wall, with a nut-bolt mechanism, which was helpful for removal of soil after conduction of the test with relative ease. Stone column with three different diameters of 50 mm, 75 mm and 100 mm with an area replacement ratio Ar, which is defined as the ratio of area of stone column (Asc) to that of area of unit cell (Au)Ar=AscAuof 11, 25 and 44% respectively were tested under vertical loads. Ratio of length of the stone column (l) to that of its diameter (dsc) i.e. l/dsc was maintained at a constant value of 4, in all the tests. Columns shorter than critical length (i.e. 4 times the diameter of the stone column) fails in shear and punching (IS. 2003). The critical length defined above is relevant for single columns subjected to smaller footing size. Whereas, the critical length varies with an increase in the size of footing. Moreover, the stress due to the applied load transfers to a depth of 2.5–3 times the diameter of the stone column (Dash and Bora, 2013a, 2013b). The detailed program of axial and lateral force tests conducted on stone columns is summarized in Table 2. A circular steel plate of 150 mm diameter (D) and thickness 20 mm was used as a model footing. A mechanical jack consisting of rotating wheel to facilitate its forward and backwards movement was mounted against the self-reacting frame for loading the footing. A thin layer of sand was placed under the footing such that the footing base is rough. A proving ring of capacity 20 kN was fastened to the mechanical jack to measure the applied load on to the footing. A special ball bearing arrangement was made to place the spindle of another dial gauge to measure the corresponding displacements. The schematic view of axial load test setup is shown in Fig. 2.

Locally procured crushed stone aggregates of size 2–10 mm were used for the construction of stone column (Ghazavi and Afshar, 2013; Murugesan and Rajagopal, 2010). Angular shape aggregates were preferred for better interlocking and strength. The soil used in the study is classified as clay with intermediate plasticity (CI) as per ASTM D2487. Fig. 3 shows the grain size distribution of the soil along with crushed stone aggregates used for construction of clay bed and stone column. The properties of the soil and stone aggregates used in the experimental study are summarized in Table 3. Commercial geotextiles used for encasing stone columns were obtained from Strata Geosystems Pvt Ltd., India. Unused polypropylene textiles of mass 125 g/m2 formed by warp and weft process and with melting point of 167 °C was procured from a local manufacturer. Scanning electron microscope (SEM) and Energy dispersive X-ray analysis (EDAX) were carried to investigate the microstructure of the woven polypropylene textile and quantify the concentration of elements present. Fig. 4 shows the SEM image and EDAX results of the woven polypropylene material. Results from EDAX shows presence of carbon (C) content of 79% and oxygen (O) content of 21% in the polypropylene textile. SEM image of polypropylene textile displays its distinct texture with uneven and irregular surface that contributes to roughness of the material, which was an imperative property of the material for choosing it. The surface roughness of the material contributes to the interfacial shear strength and mobilization of confining pressure (Sudarsanan et al., 2018). The mechanical interlock between the soil and encasing material contributes to surface roughness which resists displacements (Tang et al., 2010). The surface roughness of the polypropylene material was found to be greater than commercial geotextiles and hence can be used as a candidate for encasing material to enhance the confining support. The cost of polypropylene material was 3.5 and 6 times less in comparison to that of non-woven and woven geotextiles, respectively. To encase stone columns, a longitudinal joint with an overlap of 20 mm is been catered using an epoxy adhesive in the encasing material (Miranda and Costa, 2016). It is acknowledged that creating a longitudinal joint in the encasing material develops weaker section in the encasing material. For this reason, it is suggested that these encasing materials are to be specially manufactured without any joints to improve their performance in the field. The tensile strength of materials used for encasement with an overlap joint was determined by standard width tension tests (ASTM D4595 2001). Fig. 5 shows the tensile load-strain behavior of the materials used for encasement with and without longitudinal joint. It is evident from the figure that tensile strength of the encasing material decreases up to 50%, when catered with a longitudinal joint. The failure in commercial geotextiles may be attributed to the split in the longitudinal joint. The polypropylene textile behaved linearly elastic and demonstrated a significant increase in the stress with elongation without rupture at longitudinal joint.

The undrained shear strength Su, of soil was maintained at 15 kPa in the testing tank to represent typical soft soil conditions in the field (Murugesan and Rajagopal, 2010). A series of unconfined compressive tests were conducted on cylindrical specimens of height 76 mm and diameter 38 mm, to determine the moisture content at which soil exhibits an undrained shear strength Su, of 15 kPa. The variation of undrained shear strength with respect to water content is plotted in Fig. 6. The results summarized in this figure suggest that a water content of about 31% has to be used in the test tank to achieve an undrained shear strength of 15 kPa. Soil was mixed with required water content in addition to the existing moisture content achieving a total moisture content of 31%. The soil after mixing with water was placed and spread on a plastic sheet and covered with another plastic sheet for 72 h to achieve uniform moisture content conditions throughout the sample (Ghazavi and Afshar, 2013). Later, the soil was filled in a test tank in three layers of 200 mm acquiring a total height of 650 mm, while retaining bulk unit weight of 19 kN/m3. The inner faces of the walls of tank were coated with lubricant to curtail friction between the wall and soil. A uniform compactive effort of 300 J was applied using a rectangular face rammer of weight 10 kgs and height of fall of 300 mm. The water content, density and undrained shear strength of soil in the tank was monitored at regular intervals by collecting undisturbed samples from each layer and conducting unconfined compressive tests. No significant variation in properties of the soil in the tank was observed from their original soil properties. The settlement of the soil was monitored using dial gauges with an accuracy value of 0.01 mm. The recorded settlement of the soil was close to zero. The discussed method was followed for performing all the tests in the test tank of the present study.

The stone columns were installed by replacement method. Stone column were designed maintaining constant l/dsc ratio of 4 to avoid bulging failure, as excessive bulging breaks the interlocking between stone aggregates in addition to contributing to a decrease in the strength and stiffness of the column (Barksdale and Bachus, 1983). A clearance distance of 2.5dsc was maintained from center of stone column to the walls of the tank, as well as at the bottom, to nullify the effect of wall on to the stone column (Dash and Bora, 2013a, Dash and Bora, 2013b). The same clearance distance was maintained from one column to the other in case of group of columns, which were tested under lateral thrust. A PVC pipe of outer diameter equal to the diameter of the stone column was used as a casing pipe. The outer and the inner surface of casing pipe were coated with lubricant to facilitate ease in penetration and extraction from the soil. The casing pipe was driven vertically into the soil gradually until it penetrates to the required depth, with a minimal interruption to the surrounding soil. The soil inside the casing pipe was extracted with the help of helical augers. At a time, 50 mm depth of soil was extracted to minimize the disruption to the surrounding soil (Ghazavi and Afshar, 2018). The quantity of coarse aggregates required for the installation of stone column was pre-measured and charged into the casing pipe in five equal layers, building required height of stone column. The stone aggregates were compacted in five equal layers, using 25 blows with a tamping rod of 10 mm diameter made of steel under a drop height of 200 mm for each layer. This method of light compaction was helpful in avoiding lateral bulging and displacement of aggregates into the surrounding soil. After compaction of the aggregates, the PVC pipe was simultaneously lifted slowly, maintaining an overlap of 25 mm with the existing layer of aggregates, to minimize the disturbance from the surrounding soil. This procedure was followed until the required height of stone column is formed. The deformation of stone column after installation was monitored for 24 h by a dial gauge with a sensitivity of 0.01 mm and was observed close to zero in all the tests.

In case of encased stone column, encasing material was wrapped over a circular wooden block of diameter slightly lesser than the diameter of casing pipe. A wooden block inserted into the casing pipe was extracted back leaving the encasing material inside the casing pipe. Reuse of materials for formation of soil bed, construction and encasing stone column were avoided for all the consequent tests, for alleviating errors in the results. Load on model footing was applied using mechanical jack and corresponding load-displacement values were read from proving ring and dial gauge. The axial loading was continued with a constant displacement rate of 1 mm/min on model footing until a displacement of 50 mm was attained (Ghazavi and Afshar, 2018).

The test setup was devised accordingly to stimulate the stone columns subjected to lateral soil movements under embankments. The diameter and length of stone column were 75 mm and 300 mm maintaining a constant l/dsc ratio of 4 in all the tests. Lateral forces were created using the dead weights of 10 kPa. An incremental vertical loading of 10 kPa was applied through a 10 mm thick loading plate of size 150 mm × 650 mm, on the surface adjacent to the group of stone columns with a clear gap of 100 mm. Fig. 7 shows the schematic for the lateral thrust tests on group of stone columns. Tests were conducted on a group of four stone columns arranged in a square pattern with replacement ratio As, of 25%. The calculated replacement ratio is confined to the column group. In addition, in a different set up, three stone columns arranged in triangular pattern with replacement ratio As, of 44%, with a center-to-center distance of 2.5dsc, were used as shown in Fig. 8. The vertical load applied on the soil adjacent to the stone column, impacts lateral movements in the soil due to lateral thrust that exerts on stone column. Comparisons were made between group of stone columns without encasement to those encased with woven polypropylene textile and woven geotextile. Similar methodology for construction of soil sample and stone column were followed for axial load tests. Based on results achieved from axial load tests, lateral thrust tests were carried on stone columns without encasement and encased with halfway double and halfway single encasement. Incremental load of 2 kPa were applied each time, and displacements were measured at hourly intervals until steady displacement reading of 1 mm/h were observed, followed by further stage of loading such that the total applied load for all the tests was 10 kPa.

Section snippets

Load-settlement behavior of axially loaded stone columns

Fig. 9, Fig. 10, Fig. 11 summarize the axial load-settlement response of stone column with diameters 50 mm, 75 mm and 100 mm encased with various materials under different encapsulation conditions. There is a strong relationship between the load carrying capacity of soil modified with stone column, tensile resistance of encasing material and number of encasing layers. The loading on unmodified clay had shown a well-defined failure before significant raise in pressure, where as a soil modified

Conclusions

Performance of stone column encased with local woven polypropylene material, woven geotextile and non-woven geotextiles, under different encapsulation conditions, subjected to axial and lateral forces were studied.

The load carrying capacity and stiffness of stone column increased to a significant extent, when encased with different encasing materials compared to un-encased stone column that displayed softer response with greater radial displacements. Enhancement in load carrying capacity and

Acknowledgements

This work was assisted by Science and Engineering Research Board (SERB), a statutory body of Department of Science & Technology (DST), Government of India, under Grant No. TAR/2019/000274. The financial supports are gratefully acknowledged.

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