M. R.Kahyaoglu & M. Vanicek: A numerical study of reinforced embankments supported by encased floating columns A NUMERICAL STUDY OF REINFORCED EMBANKMENT-SUPPORTED BY ENCASED FLOATING COLUMNS NUMERICNA ŠTUDIJA OJAČENIH NASIPOV, PODPRTIH Z OBLOŽENIMI GRUŠČNATIMI STEBRI Mehmet Rifat Kahyaoglu (corresponding author) Martin Vaniček Mugla Sitki Kocman University, Engineering Faculty, Geosyntetika Ltd. Department of Civil Engineering N.Tesla str. 3, 160 00 Praha 6, The Czech Republic 48000, Mentese-Mugla, Turkey E-mail: mvanicek@geosyntetika.cz E-mail: rkahyaoglu@mu.edu.tr https://doi.org/10.18690/actageotechslov.16.2.25-38.2019 DOI Keywords geogrid reinforcement, geotextile encasement, surcharge, soil settlement, column bulging, sand mat Ključne besede geomreže, geotekstilna obloga, preobremenitev, posedanje zemljine, izbočenje stebra, peščena podlaga Abstract This paper presents a three-dimensional, finite-element, parametric study of a base-reinforced embankment supported by encased floating columns on soft soil. A 3D numerical model is made to study the effects of geogrid basal reinforcement and geotextile encasement on the displacement behavior of the columns. The numerical model was initially verified using measured data from a real case study. Then, parametric studies were subsequently performed, considering the effect of the encasement stiffness, the basal reinforcement stiffness and the embankment fill height, together with an examination of the effective length of the encasement. The results from this parametric study are presented here in the form of comparative graphs. The objective of this paper is to present the behavior of the embankment on floating encased columns after the soft soil consolidation for different embankment heights, basal reinforcement and column-encasement stiffnesses. Izvleček V prispevku je predstavljena tridimenzionalna parametrična študija končnih elementov ojačenega nasipa, podprtega z geotekstilom obloženimi gruščnatimi stebri na mehkih tleh. Izdelan je 3D numerični model za proučevanje učinkov osnovne ojačitve z geomrežo in geotekstilnih oblog na deformacijsko obnašanje gruščnatih stebrov. Numerični model smo sprva preverili s pomočjo študije izmerjenih podatkov na realnem primeru. Nato so bile naknadno izvedene parametrične študije ob upoštevanju učinka togosti geotekstilnih oblog, togosti osnovne ojačitve z geomrežo in višine polnilnega nasipa vzdolž raziskovane efektivne dolžine geotekstilnih oblog. Rezultati iz te parametrične študije so predstavljeni v obliki primerjalnih grafov. Cilj tega prispevka je predstaviti obnašanje nasipa ležečega na z geotekstilom obloženih gruščnatih stebrih po konsolidaciji mehkih tal za različne višine nasipov, osnovne ojačitve in togosti z geotekstilom obloženih stebrov. 1 INTRODUCTION The construction of embankments on soft soils, as part of the efforts to reclaim new areas for the construction of highways, railways, airport runways and urban infrastructure, faces several hurdles with regard to the low load-bearing capacity and high compressibility of the subsoil, as well as the tendency for excessive lateral deformations. Among the various available techniques, such as surcharging, excavation and replacement, vertical drainage, vacuum consolidation and column- Acta Geotechnica Slovenica, 2019/2 25. M. R.Kahyaoglu & M. Vanicek: A numerical study of reinforced embankments supported by encased floating columns -supported embankments, the use of column-supported embankments (CSEs) allows for a rapid construction, total and differential settlement reduction, and adjacent facility protection [1-3]. However, it is impossible to construct CSEs in very soft clays (cu < 15 kN/m2) due to the insufficient columns material lateral confinement and excessive lateral bulging of the columns [4-6]. In such soils, the required lateral confinement can be induced through the encasement of individual columns with geosynthetics [7-11]. In 1995 the first project utilizing a seamless geotextile-encased column was successfully implemented in Germany, and later, Kempfert et al. [5], Raithel and Kempfert [6] and Raithel et al. [7] tested the performance of geosynthetic-encased stone columns (GECs) using numerical and analytical models. The technique detailed in the above-mentioned projects has been adopted in Europe [8, 9] and more recently in South America [11], but with growth in the construction sector and improvements in geosynthetic production technologies, new design procedures have been developed. The performance of geosynthetic encasement on the capacity and settlement behavior of soft soils has been studied in both laboratory and field tests [12-17], while numerical studies of encased granular columns have been conducted successfully in the literature [18-27]. The cited studies investigated the influence of the geometry and material properties of encased and non-encased stone columns (SCs) on vertical stresses, excess pore-water pressures and tangential strains in the geotextile, with a focus on the effect of encasement length and stiffness, the strength of the soft ground and surcharge from the embankment fill. The benefit of encasing stone columns in terms of settlement, lateral deformation and load-carrying capacity has been underlined in the above studies, and design charts for an estimation of the maximum settlement in soil and column strain during the preliminary design are presented. In recent years, in the event of high embankment loads, one layer of geogrid has been used at the base of the embankment in combination with GECs over soft clay soils to form a geosynthetic reinforced and column-supported embankment (GRCSE) [28-30]. The application of a geogrid layer over the columns and the soft soil enhances the efficiency of the load transfer from the embankment to the columns, provides controllable deformation, minimizes soil yield, enhances global stability and eliminates the need for inclined columns to resist the horizontal thrust at the sides of the embankment [31-34]. The complicated mechanism of load transfer in GRCSEs combines with the arching effects, tension in the geosynthetic reinforcement and stress transfer from the soft soil to the column due to the different stiffness values. Over the past few years, both experimental and numerical investigations into the behavior of GRCSE have been carried out by many researchers [35, 37]. Previous studies have analyzed the performance of GECs and the time-dependent behaviors of geosynthetic-reinforced embankments supported on end-bearing columns. In some instances, when the column does not reach a hard stratum, the construction of floating columns is found to be more economical and technically feasible. The frictional force along the floating column, based on the relative deformation between the column and the surrounding soil, affects the behavior of GECs [36, 38, 39]. Although previous research has contributed valuable information to the knowledge of end-bearing columns, information about the group behavior of floating columns is still lacking, and so further research is required into the design of embankments on encased floating columns [40-42]. This paper explores the time-dependent behavior of geogrid-reinforced embankments supported by floating columns encased in geotextiles. Firstly, a real case study of GRCSE in thick soft soil was modeled numerically. Then, the numerical results and the measured data were compared, and some calibrations on the numerical model were made for the verification. Finally, parametric studies including variations of the embankment height, the stiffness of the column encasement and the base reinforcement were performed. Many of the recent studies mentioned above have dealt with the load-carrying capacities and settlement of unreinforced embankments supported with GECs; however, the effects of reinforcement to the base of the embankment have not been considered to date, nor have the load-transfer mechanism and the lateral bulging deformation patterns associated with GECs. The published literature focusing on the long-term effects of these parameters on the vertical and lateral displacement behaviors of the GECs is limited, and so in order to enhance the performance of the GECs to contribute to the above-mentioned issues, the objectives of this paper are as follows: (1) to examine the long-term behavior (100% consolidation) of a floating, column-supported embankment under different surcharges; (2) to investigate the performance of basal geogrid reinforcement; (3) to consider the effects of geotextile encasement on the lateral and vertical displacement of columns; and (4) to determine the effective length of the geotextile encasement of floating columns. 26. Acta Geotechnica Slovenica, 2019/2 25. M. R.Kahyaoglu & M. Vanicek: A numerical study of reinforced embankments supported by encased floating columns 2 NUMERICAL MODELLING 2.1 Model verification A case study of a stone-column-supported embankment constructed in Kebun-Malaysia, the details of which can be found in Raju (1997) [43], was modelled numerically. The soil profile for the Kebun Interchange project contained marine clay where th e CPT tip resistance values for the top 11 m are 0.1-0.3 MPa (Fig 1). Stone columns with a 1.1-m diameter were installed at a 2.2-m rectangular spacing to a depth of 12 m under the 2.6-m-high embankment. Settlement gauges were placed on the top of the stone columns and the total settlement was read as 0.4 m. A 1-m settlement was observed for untreated ground under the same circumstances. The results of the settlement in the soft soil and the encased column after the completion of the embankment construction from our numericalm odel were compared with measured settlements from the Kebun project. This comparison presented in Fig. 2 shows that the numerical model followed the trends of the measured data. The vertical stress transmitted to both the stone column and the soft soil was verified with measured values, and this consistency indicates that the numerical model is appropriate for a parametric study. /i s / --- // n / X * J ✓ / ✓ jf H = 2.6m LC = 12m —•—Measured (Soil) --Calculated (Soil) —■—Measured (Column) --Calculated (Column) - // // It r '} ✓ / // / t lt St 6t 8t 1tt 12t 1st 16t 18t Itt Time (day) Figure 2. Comparison between the calculated and measured settlements of the column and the soil. 2.2 Parametric study GRCSE in 40-m-thick soft soil lying on a rigid and firm layer were modeled and studied numerically. The water level was modelled at the original ground surface. Floating columns having a diameter of 1 m (D) were arran- 11 it si st 0) 31 3t 11 lt Acta Geotechnica Slovenica, 2019/2 25. M. R.Kahyaoglu & M. Vanicek: A numerical study of reinforced embankments supported by encased floating columns ged in a square-grid pattern with a 3-m center-to-center spacing, giving an area replacement ratio of 8.7 percent. All the stone columns were encased with geotextiles of the best geosynthetic type for the encasement of the floating columns [20]. A 1-m-thick sand mat, acting as a working platform below the embankment (2V:1H side slopes), was established on top of the natural clay soil prior to the embankment fill to allow equipment access and to provide drainage for the columns. Furthermore, one layer of geogrid was laid to provide a basal reinforcement for the embankment. The numerical analyses were carried out using an available PTAXIS 3D Foundation package [44], The displacements and the vertical stresses on the column and the surrounding soil, as well as the tensile strains and tangential tensile forces acting on the geosynthetics, were calculated. The details of the cross-section of the Figure 3a. Cross-section of the model. model and the finite-element mesh are shown in Fig. 3, representing the right half of the domain on account of the symmetry. In the analyses, the model limits were 50 m in the vertical direction and 220 m in the horizontal direction, being five times the width of half of the embankment base, so as to minimize the boundary effect. Fig. 4 shows the finite-element mesh used in the 3D numerical simulations. The soil clusters were modeled using 10-noded tetrahedral elements, whereas the geosynthetic elements are represented by 6-noded triangle surface elements. A horizontal displacement was not permitted on the vertical boundaries of the model; however, the bottom boundary was fixed securely in both the vertical and horizontal directions. The embankment fill construction to the top surface was simulated in four stages. For each stage 20 days was envisaged for the construction of a 2-m layer and 90 days for the consolidation from its surcharge. The consolidation analyses were carried out during and after each construction stage. After the completion of the embankment construction, the calculations were continued until the excess pore-water pressure dissipation at mid-depth of the clay layer had reached 1 kPa. A closed consolidation boundary was applied to the sides of the model parallel to the embankment axis to prevent lateral drainage. Both the embankment fill and the sand mat (assumed to be Sacramento River sand) were modeled using the Mohr-Coulomb failure criterion under a drained condition. Kaliakin et al. [45] discussed the determination of the values from experimental data for Sacramento River sand based on the tests carried out by Lee and Seed [46]. The column material was modeled as granular soil, in Figure 3b. Cross-section of the finite-element model. 28. Acta Geotechnica Slovenica, 2019/2 25. M. R.Kahyaoglu & M. Vanicek: A numerical study of reinforced embankments supported by encased floating columns Figure 4. 3D finite-element model. line with the suggestions of Ambily and Gandhi [47]. The soft soil was idealized using the modified Cam Clay (MCC) model. The MCC parameters considered in this study were adopted from the geotechnical parameters of soft Kebun clay soil encountered in a recent soft-ground improvement project [43]. Khabbazian et al. [37] stated that the use of the MCC model is preferable over the Mohr-Coulomb or linear elastic models, in that it allows a more accurate modeling of the behavior of the soft soil. The geosynthetics used for both the reinforcement and the encasement were modeled as linear elastic material with no bending stiffness, as recommended by Muru-gesan and Rajagopal [19] and Liu et al. 2007 [13]. The stiffness of the geosynthetic reinforcement (J=EA) was determined as the tensile force at 3% elongation divided by that elongation (3%). Perfect adhesion between the stone and the surrounding soil were assumed, and thus interface elements with a rigid interface were used at the interfaces of either the stone column and the encasement, or the encasement and the soft clay [22, 23]. In fact, a large number of researchers have been investigating so much to characterize the interface working mechanism and propose fruitful achievements on the constitute models of the soil-geosynthetic interface. The parameters used in the numerical analyses are summarized in Table 1. Stone columns are installed using vibro-displacement and vibro-replacement methods. The stone material is laterally expanded, which is accompanied by an increase in the horizontal earth pressure and the excess pore-water pressure in the soft soil during and after the column's installation. However, any influence related to the installation of the columns was disregarded in this study. Table 1. Material parameters used in the numerical analyses. Parameter Column Material Stone Soil (Ambily and Gandhi 2007) Embankment Fill Sacramento River Sand (Kaliakin 2012) Working Platform Sacramento River Sand (Kaliakin 2012) Soft Clay Kebun Clay (Raju 1997) Model Type Mohr-Coulomb Mohr-Coulomb Mohr-Coulomb Modified Cam Clay Unit Weight, y(kN/m3) 24 22.5 20 15 Effective Friction Angle, ^>'(°) 42 36 32 - Effective Cohesion, c'(kPa) 1 1 1 - Dilation Angle, f'(°) 10 4 3 - Elastic Modulus, E(kPa) 55000 20000 15000 - Poisson's Ratio, v 0.3 0.3 0.3 0.3 Slope of Swelling Line, K - - - 0.02 Slope of the Virgin Consolidation Line, X - - - 0.4 Void Ratio at Unit Pressure, e - - - 1.0 Slope of the Critical State Line, M - - - 1.0 Permeability, k(m/s) 1x10-2 1x10-3 1x10-3 1x10-6 Acta Geotechnica Slovenica, 2019/2 25. M. R.Kahyaoglu & M. Vanicek: A numerical study of reinforced embankments supported by encased floating columns In order to cover all the cases in the embankment-construction scenarios, parameters such as the embankment height (H), column-encasement stiffness (Je), and basal reinforcement stiffness (Jr) were varied, as summarized in Table 2. Table 2. Parameters evaluated in the parametric analyses. Parameter Embankment Height, 2 4 6 8 H (m) Geogrid Reinforcement Stiffness, JR (kN/m) 1000 2000 3500 5000 6500 Column Length, LC (m) 16 Geotextile Encasement 500 1000 1500 2000 2500 Stiffness, JE (kN/m) For the case of the 8-m-high embankment, the critical length of a floating column according to the analytical equation developed by Satibi [40] was determined as 15 m. Based on this critical length, the lengths of the columns are determined to be 16 m (Lc=16m) for the whole parametric study. A comparison is made of the surface settlement of the column and the soft soil, the column bulging, the vertical stresses on the floating column (Lc=16m) and the soft soil, and the tangential force in the geogrid reinforcement. A similar parametric study with several variables for reinforced shallow foundations was performed by Jelusic and Zlender [48, 49]. 3. RESULTS OF THE PARAMETRIC STUDY The results of the parametric study evaluating the variation of the embankment height, column encasement and basal reinforcement stiffnesses were categorized according to the effects on the stress strain behavior of the GRCSE in the following subsections. 3.1 Surface settlement Fig. 5 shows the surface-settlement behavior of the encased columns and the soft soil for different unreinforced embankment cases. The results reveal a significant decrease in the settlement with the encasement, which is thought to be a direct consequence of the column bulging reduction by additional confining pressure produced by the geotextile encasement along the column length. It is also clear that an increase in the stiffness of the encasement improves the performance of the GEC. The settlement curves (Fig. 5) also indicate that geotextile encasement reduces the total settlement, but generates some differential settlement. The soft soil closer to the embankment centerline is subjected to greater vertical stresses when compared to the soil near the embankment edges, leading the settlement values to decrease with the distance from the centerline of the embankment. The value of the maximum settlement of the column close to the middle is about 30 percent greater than that of the column near the edge. The settlement response of the GECs also depends strongly on the surcharge from the embankment's self-weight. When the embankment height is less than 4.0 m (H < 4 m), the surface settlements are small. 60 Ï 1 ¿4 5 Q. ~ ~ 4 5 Vi\ \ / ti % tj ' \ H=6m Lc=16m 6 /5 6 JR= 0 kN/m ? 7 1 f / i1