See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/263274348
Experimental and numerical studies on geocell reinforced sand beds Conference Paper · January 2013
CITATIONS
READS
0
58
2 authors: Amarnath Hegde
Sitharam T G
Indian Institute of Technology Patna
Indian Institute of Science
26 PUBLICATIONS 72 CITATIONS
264 PUBLICATIONS 1,751 CITATIONS
SEE PROFILE
All in-text references underlined in blue are linked to publications on ResearchGate, letting you access and read them immediately.
SEE PROFILE
Available from: Amarnath Hegde Retrieved on: 15 September 2016
Geosynthetics 2013 April 1-4, Long Beach, California
Experimental and Numerical Studies on Geocell Reinforced Sand Beds A. Hegde, Research Scholar, Department of Civil Engineering, Indian Institute of Science, Bangalore, India560012; Email:
[email protected] , T.G.Sitharam, Professor, Department of Civil Engineering, Indian Institute of Science, Bangalore, India560012; Email:
[email protected]
ABSTRACT Indian Railway has taken up the ambitious project of constructing 3300 km of new track across eastern and western corridors. The majority of these tracks will pass through the regions with poor soils including loose sands of the IndoGangetic belts. Hence, the experimental and numerical studies are undertaken to evaluate the suitability of geocell cellular confinement as a foundation technique for this project. This paper presents the results of the 1-g model tests performed on geocell reinforced sand bed supporting the square footing. Results show that the provision of geocell increases the ultimate bearing capacity of the sand bed by 4 times. Introduction of planar geogrid at the base of the geocell mattress not only enhances the load carrying capacity but also arrests the surface heave and prevents the footing from undergoing the rotational failure. In addition to the experimental studies, numerical study was also carried to demonstrate the simplistic approach of modeling of the geocells in FLAC2D.
1.
INTRODUCTION
Indian economy is one of the largest as well as the fastest growing economy in the world. In a last decade or so, India’s growing economy has experienced the rise in the demand for additional transportation, infrastructure, and services. Until these days, Indian railway was playing a pivotal role in the transportation sector and was contributing to the economic success of the country. However, since last few years, Indian Railway has been facing the serious challenge to keep up the pace with country’s continued economic growth and the rising demand, since all the country’s high-density rail corridors are facing severe capacity constraints. Hence, to enhance the capacity, Railway ministry has undertaken the construction of Dedicated Freight Corridors across the heavy traffic networks in India. As part of this initiative, DFCCIL (Dedicated Freight Corridor Corporation of India) is constructing two corridors along western and eastern routes spanning a total length of about 3300 km (Courtesy, http://dfccil.org). Much of the railway embankments and railway lines have to be constructed on the loose deposits of the Indo-Gangetic belt. In the present situation, ground improvement using vibro techniques seems to be the feasible option for the DFCCIL to strengthen the loose deposits along the proposed route. As vibro stone columns are more complex to install, it is very high time to demonstrate a new technique as an alternate to the vibro techniques. Reinforcing the soil with geocells could be one such alternative option as they distribute the load laterally. In addition, geocells are more amenable to the field applications as it is cheaper and better compared to vibro techniques. Hence, research work has been taken up with the objective of evaluating the suitability of geocell reinforcement for the construction of foundations along these railway lines. Since last two decades, geocell reinforcement has been showing its efficacy in the field of highways and embankments. Due to its three dimensional nature, geocell offers all-round confinement to the encapsulated soil and hence leads to the improvement in the overall performance of the foundation bed. Several researchers have reported the beneficial use of 3D-reinforcements in the construction of foundations. “Broms and Massarach (1977)” adopted the metallic grid mat consisting of rectangular and triangular cells for different offshore structure. “Rea and Mitchell (1978)” and “Mitchell et al. (1979)” carried out the study on footings supported on sand beds reinforced with square shaped paper grid cells through a series of small-scale laboratory tests and observed the different modes of failure. “Guido et al. (1989)” studied the influence of number of layers of geoweb cells on bearing capacity of sand bed using laboratory model tests. Similar studies were carried out “Dash et al. (2001a)” on strip footing, “Sitharam and Sireesh (2005)” on circular footing and “Madhavi Latha and Somwanshi (2009)” on square footing by using laboratory prepared geogrid cells. “EI Sawwaf and Nazer (2005)” used Unplasticized Polyvinyl Chloride (UPVC) cylinders of different diameters to study the soil confinement on circular footing. “Tafreshi and Dawson (2010)” used strips of thermo welded non-woven geotextiles as 3D cells to study the response of sand bed under static and dynamic loading. Review of the literature reveals that the majority of the researchers have used the laboratory prepared geogrid cells instead of commercially available geocells. Since commercially available geocells are the ones, going to be used in the field, the experimental findings cannot be directly correlated with the field conditions. In the present investigation, new commercially available geocells made of polyethylene are used; which is known for its high strength and durability. Sand
119
collected from the Indo-Gangetic belt itself was used in the experimental program to prepare the foundation bed. The test results of the static plate load tests and the numerical simulations are discussed in this paper.
2. 2.1
LABORATORY MODEL TESTS Design of Experiments
Footing sizes were arrived as per the scaling laws suggested by “Wood (2004)”. The prototype footing was a square raft with thickness 0.5 m and width 4.5 m made up of concrete with Young’s modulus 25 GPa. For the sake of convenience, the steel footing with Young’s modulus 200 GPa was selected for the experimental studies. Considering the scaling down factor of 10, the flexural rigidities of the model and prototype footings were equated to obtain the dimensions of the model footing. The arrived size of the model footing was 150 mm width and 20 mm thickness. The sufficiently bigger size of the tank was chosen, i.e. 6 times the size of the footing (900 mm x900 mm in plan area) to keep the boundary effect minimal. 2.2
Experimental Setup
The experiments were conducted in the cast iron test tank of size 900 mm length x900 mm width x 600 mm height. The tank was fitted to the loading frame and which was connected to manually operated hydraulic jack. A square shaped steel plate with 20 mm thickness and 150mm sides was used as the model footing. The bottom of the footing was made rough by a coating thin layer of sand with epoxy glue. The load was applied on the footing through the hand operated hydraulic jack and a pre-calibrated proving ring was placed between the footing and hydraulic jack to measure the applied load. To prevent the eccentric application of the load, the ball bearing arrangement was used. Figure 1a-b represents the photographic and schematic view of the test setup.
(a)
(b)
Figure 1. Test setup: (a) photographic view; (b) schematic view 2.3
Materials Used
Sand used in the investigation was dry sand with specific gravity 2.64, effective particle size (D10) 0.26mm, coefficient of uniformity (Cu) 3.08, coefficient of uniformity (Cc) 1.05, maximum void ratio (emax) 0.81, minimum void ratio (emin) of 0.51 and angle of internal friction was (φ) 30o. According to Indian Standard Soil Classification System (ISSCS) the soil can be classified as poorly graded sand with symbol SP. The geocell used in the study was made up of polyethylene with a density of 0.95 g/cm3. The cells are 250 mm in length, 210mm in width and 150mm in depth. The thickness of the strip is 1.53 mm with cell-to-cell seam strength is 2150 N. Biaxial geogrid made up Polypropylene with aperture size 35 mm x 35 mm was used. Ultimate tensile strength of the geogrid was 20 kN/m. Figure 2 represents the tensile load-strain behavior of both geocell and geogrid used in the experiments. 2.4
Preparation of Test Bed
Firstly, the sides of the tank were coated with Polythene sheets to avoid the side friction. Pluviation technique was used to prepare the sand bed of 500 mm thickness. Before the start of the actual test, a series of trials were conducted to determine the height of fall required to achieve the desired relative density. In each trail, small aluminum cups with
120
known volume were placed at the different locations of the tank. A calibration chart was prepared by knowing the maximum and minimum void ratios of the sand. All the tests were conducted at a constant relative density of 65%. The height of fall required to achieve 65% relative density was directly obtained from the chart. The average density achieved in the sand bed during the pluviation was 16.08 kN/m3 and the less than 1% difference was observed in the densities measured at the different locations of the bed. Reinforcement was placed at the predetermined depth and sand was used as the infill material. Sand was filled into the geocell pockets using the pluviation technique itself to maintain the relative density same as that of foundation bed.
Tensile load (kN/m)
25
20 15 10 Geocell
5
Geogrid
0 0
20
40 60 Axial strain (%)
80
100
Figure 2. Tensile load-strain behavior of geocell and geogrid 2.5
Testing Procedure
Reinforcements were placed at the predetermined depth during the pluviation of sand into test tank. After preparing the sand bed of 500 mm thickness, the fill was leveled using a trowel without disturbing the density of the bed. Footing was placed at the pre-determined alignment at the surface of the sand bed. A recess was made on the top surface of the footing and a ball bearing arrangement was placed on it. The concentrated vertical load was applied on the footing with the help of ball bearing arrangement. Through the precise measurements, the footing was placed exactly at the center of the test tank in order to avoid the eccentric loading. The load applied to sand bed was measured through the precalibrated proving placed between the footing and the hydraulic jack. Two dial gauges (D1 and D2) were placed on the either side of the centerline of the footing to record the footing settlements. Another set of dial gauges (S1 and S2) was placed at the distance of 1.5B (B is the width of the footing) from the centerline of the footing to measure the deformation underwent by the fill surface. The footing settlement (S) and the surface deformation (δ) were normalized by footing width (B) to express them in non-dimensional form as S/B (%) and δ/B (%). In all the plots, settlements are reported with the negative sign and heave with the positive sign.
3. 3.1
RESULTS AND DISCUSSION Bearing Pressure-settlement Curve
Figure 3 represents the pressure settlement response for the different test conditions. For unreinforced case, failure was occurred at footing settlement of 8% of footing width. Clearly defined failure was observed in case of unreinforced bed. In case of geocell reinforcement, no clear-cut failure was observed in the pressure settlement behavior even up to the large settlement of 35% of footing width. There was a slight reduction in the slope of the pressure settlement curve at the settlement range of 22% to 24% of the footing width and after which again the slope has become constant. This nature of the curve can be attributed to the beam effect of the geocell mattress; due to its high bending and shear stiffness, geocell mattress can support the footing even after the failure of soil. The interconnected cells form a panel that acts like a large mat that spreads the applied load over an extended area, leading to an overall improvement in the performance of the foundation beds. Post test exhume of geocell has shown the deformation in the vertical and horizontal ribs. The reason for this could be, once the soil below footing underwent the shear failure, the footing might have rested directly on the ribs of the geocell. Figure 4a-b represents photograph of the geocell before and after the tests. It was observed that the provision of the additional geogrid layer at the base of the geocell mattress further
121
increase the load carrying capacity as well as the stiffness of the sand bed.(i.e. flattened pressure settlement curve). Similar trends were also observed by “Dash et al. (2000 b)”.
Bearing pressure (kPa)
Footing settlement, S/B (%)
0
200
400
600
800
1000
0 -5 -10 -15
-20 -25
Unreinforced
-30
Only Geocell
-35
Geocell+Geogrid
-40 Figure 3. Variation of bearing pressure with footing settlement
(a)
(b)
Figure 4a-b. Photograph of the geocell: (a) before the test; (b) after the test 3.2
Surface Deformation
Surface deformation (settlement/heave) profiles of sand bed reinforced with different forms of reinforcement are discussed in this section. Surface deformation measurements were made through the dial gauges placed at the distance of 1.5B from the centerline of the footing. “Chummar (1972)” observed that the surface heaving extends up to 2B from the centerline of the footing for unreinforced case and with maximum heaving at a distance of 1.5B. Generally, surface heaving can be attributed to the general shear failure of the soil mass. Figure 5 quantifies the maximum surface deformation for the different combination of reinforcements. The solid and dotted lines represent the deformation measured at the left and right side of the footing centerline. One can observe from the figure that the large difference between the solid and dotted lines. This difference is due to the rotational failure of the footing. The extent of rotation and amount of heaving was reduced in the case of the geocell reinforcement and further reduction in rotation was observed in the presence of the planar geogrid below the geocell. Interestingly, no surface heaving was observed with additional basal geogrid. From the above observation, it is evident that the combination of geocell and basal geogrid completely arrest the surface heaving and reduces the footing rotation. In addition, the planar geogrid contributes to improve the overall performance of the foundation bed by resisting the downward movement of soil due to the footing penetration. Hence, it is always beneficial to use planar geogrid layer at the base of the geocell mattress.
122
Surface deformation, δ/B (%)
5.5 Unreinforced
Left ....... Right
4.5
Only Geocell Geocell+Geogrid
3.5 2.5 1.5 0.5
-0.5 -1.5 0
10
20 30 40 Footing settlement, S/B (%)
50
Figure 5. Variation surface deformation with footing settlement 3.3
Bearing Capacity Improvement Factor (If)
The increase in the bearing capacity due to the provision of the reinforcement can be measured through a nondimensional parameter called bearing capacity improvement factor (If), which is defined as, If =
Where, qr is the bearing pressure of the unreinforced soil at the same settlement. bearing capacity ratio. When the ratio is bearing capacity (qult) is used instead of q0.
qr qo
[1]
reinforced soil at the given settlement and q0 is the bearing pressure of “Binquet and Lee (1975)” reported the improvement factor is similar to the beyond the ultimate bearing capacity of the unreinforced soil, the ultimate Bearing capacity improvement values for different tests are listed in Table 1.
Table 1. Bearing capacity improvement factors Avg.S/B
2% 4% 6% 8% 10% 12% 14% 16% 18% 20% 3.4
Bearing capacity improvement factors (If) Only Geocell Geocell+Geogrid 1.10 1.12 1.14 1.25 1.28 1.37 1.49 1.63 1.78 1.92
1.48 1.55 1.57 1.83 2.12 2.43 2.67 2.88 2.96 3.08
Percentage Reduction in Footing Settlement (PRS)
The performance improvement of the foundation bed due to geocell reinforcement can also be quantified in terms of the reduction in the settlement of the footing using the parameter called percentage reduction in settlement(PRS), which can be defined as, PRS = (
S0 - Sr )x100 S0
[2]
123
Where, S0 is settlement of the unreinforced foundation bed corresponding to its ultimate bearing capacity (which is determined using the double tangent method), was found out to be 15% of the footing width (S/D=15%). Sr is settlement of reinforced foundation bed corresponding to the footing pressure equal to the ultimate bearing pressure of unreinforced foundation bed. In the present investigation, the PRS value of 53 % was observed with only geocell case and 73% was observed with additional planar geogrid at the base of the geocell.
4.
NUMERICAL STUDY
The aim of this section is to demonstrate the simplistic approach of modeling the 3-dimensional nature of geocell in 2D finite difference software called FLAC2D (Fast Lagrangian Analysis of Continua in 2D). In this approach, the geocell filled with sand is modeled as an equivalent composite layer with improved strength and stiffness parameters (“Madhavi Latha and Somwanshi, 2009”). Various investigators have reported that the geocell confinement of sand induces the apparent cohesion while the friction angle remains constant (“Bathurst and Karpurapu, 1993”; “Rajagopal et al., 1999”). The induced apparent cohesion of the equivalent composite layer was calculated using the Equations (3) and (4) given by “Rajagopal et al. (1999)”. The Equation (3) is originated from the rubber membrane theory proposed by “Henkel and Gilbert (1952)”. The increase in the confining pressure (Δσ3) on the soil due to the presence of geocell is given by,
Δσ 3 =
2Mξ c 1 2M 1- 1- ξ a = d 1- ξ a d0 1- ξ a
[3]
Where, M is the secant modulus of the geocell material at axial strain εa, which is obtained from the tensile load-strain response of the geocell material at 2% axial strain and do is the equivalent diameter of the geocell pocket. The increment in the apparent cohesion (Cr) due to the increase of the confining pressure is given by,
Cr =
Δσ 3 Kp 2
[4]
Where, Kp is the coefficient of passive earth pressure. In the present study, the equivalent diameter of the geocell pocket is 0.258 m and the secant modulus (M) corresponding to 2% strain obtained from the load–strain response of the geocell material (Figure 2) was 435 kN/m. The additional confining pressure (Δσ3) was calculated from Equation (3) as 34.4 kPa. Then from the Equation (4), the apparent cohesive strength of the soil composite was obtained as 30 kPa. The Cr value calculated is valid only for a single cell. Due the presence of many interconnected cells, the apparent cohesion value reduces. Rajgopal et al. (1999) observed that the reduction in Cr value is in the range of 50% to 72% of the calculated value depending upon the number of interconnected cells. In the present study, the 70% reduction in Cr value i.e Cr=21 kPa was used for numerical simulations. Same value of the friction angle (φ=30o) as that of the sand used in the experimental study was used in the simulation of sand bed and the dilation angle value of 2/3φ was considered. Elastic-perfectly plastic Mohr Coulomb model was used to model the behavior of soil. Analyses were carried out under controlled velocity loading of 2.5 x E-5 m/step. Only half portion of the test bed was modeled using symmetry to reduce the computational effort and the time. The FLAC2D model with the details of loading and the boundary conditions are shown in Figure 6. The size of the mesh was kept same as that of the test bed used in the experimental studies. The displacement along the bottom boundary was restricted in both horizontal as well as vertical direction. The side boundaries were restrained only in the horizontal direction, while the displacements were allowed in the vertical direction. Roughness of the footing was simulated by restraining the surface nodes representing the base of the footing in the horizontal direction. The elastic properties were obtained from the experimental pressure settlement behavior using the method proposed in the “Appendix D of the Engineering Manual: 1110-1-1904, U.S Army Corps of Engineers (1990)”. The method uses the relationship involving the slope of the pressure settlement curve, Poisson’s ratio, and equivalent diameter of the footing to determine the elastic modulus of the bed. The values found to be underestimating the ultimate bearing pressure by large extent and hence the sensitivity analyses were carried out to determine a suitable value of the elastic modulus. Table 2 presents the details of the various material properties used in the analysis. Comparisons of the pressure settlement behavior of experimental and numerical studies are presented in the Figure 7. There exist a good match between the experimental and the numerical results. However, it was found that the FLAC2D overestimates the bearing pressure at higher settlements.
124
Velocity load=2.5 x E-5 m/step
Geocell soil composite
150 mm
500mm
Sand Bed
450mm Figure 6. FLAC2D model with details of loading and boundary condition Table 2: Material properties used in the analysis Properties
Unreinforced
Shear modulus, G(MPa) Bulk modulus, K (MPa) Poisson's ratio, μ Cohesion, C (kPa) Friction angle, φ(o) Dilation angle, Ψ(o)
3.1 6.6 0.3 0 30 20
Geocell reinforced 1.25 2.6 0.3 21 30 20
Geocell+Geogrid reinforced 6 13 0.3 21 30 20
Footing Settlement, S/B (%)
Bearing Pressure (kPa) 0
200
400
0
600
800
1000
1200
Unreinforced
-5
Only Geocell
-10
Geocell+Geogrid
-15
-20 -25 -30
-35
Experiment FLAC
-40
Figure 7. Comparison of experimental and numerical pressure settlement behavior
125
5.
CONCLUSIONS
This paper has investigated the suitability of new commercially available geocells in the constructions of foundations subjected to heavy loads through stress controlled static plate load tests. In addition, the study has demonstrated the simplistic approach of modeling the 3D nature of geocell in 2D finite difference package called FLAC2D. Test results suggest that the provision of geocell increases the ultimate bearing capacity of the sand bed by 4 times. Due to its high bending and shear stiffness, geocell mattress can support the footing at larger settlements and even after the failure of soil. Provision of the additional planar geogrid at the base of the geocell arrests the surface heaving and prevents the footing from undergoing the rotation failure. Hence, it is always beneficial to use planar geogrid layer at the base of the geocell mattress. By using the scaling laws suggested by “Butterfield (1999)” the results of the small-scale models tests can be extrapolated to full-scale cases. Thus, the initial studies in static condition suggest that the combination of the geocell and basal geogrid can be successfully used for the construction of foundations subjected to heavy loads. Moreover, geocells are cheaper and faster compared to vibro stone columns and hence more amenable for field applications. However, further studies are necessary to understand the behavior of geocells under repeated dynamic loadings to ascertain the suitability of geocells in the construction of foundations for railway lines. In this direction, the process of establishing the dynamic plate load test setup is in progress in the Department of Civil Engineering, Indian Institute of Science, Bangalore.
REFERENCES Bathurst, R.J. and Rajgopal, K. (1993). Large scale triaxial compression testing of geocell reinforced granular soils, Geotechnical Testing Journal 16 (3), 296-303. Binquet, J. and Lee, L. K. (1975). Bearing capacity tests on reinforced earth slabs, Journal of Geotechnical Engineering Division, ASCE 101 (12), 1241–1255. Broms, B.B. and Massarach, K.R. (1977). Grids mat a new foundation method. Proceedings of 9th International Conference of Soil Mechanics and Foundation Engineering, Tokyo, 1,433-438. Butterfield, R., (1999). Dimensional analysis for geotechnical engineers, Geotechnique 49(3), 357–366. Chumma,r A.V. (1972). Bearing capacity theory from experimental results, Journal of the Geotechnical Engineering Division, ASCE 98 (12), 1257–1276. Dash, S.K., Krishnaswamy, N.R. and Rajagopal, K. (2001a). Bearing capacity of strip footings supported on geocellreinforced sand, Geotextiles and Geomembranes 19, 235–256. Dash, S.K., Krishnaswamy, N.R., Rajagopal, K. (2001b). Strip footing on geocell reinforced sand beds with additional planar reinforcement, Geotextiles and Geomembranes 19, 529–538. EI Sawwaf, M. and Nazer, A. (2005). Bearing capacity of circular footing resting on confined granular soil, Journal of Geotechnical and Geoenvironmental engineering, ASCE, 131 (3), 359-366. EM 1110-1-1904. Settlement Analysis, Appendix-D, U.S Army Corps of Engineers (1990), D6 –D8. Guido, V.A., Sobiech, J.P. and Christou, S.N. (1989). A comparison of texturized and non-texturized geoweb reinforced earth slabs, Proceedings of Geosynthetics 1989, 215-230. Henkel, D.J., and Gilbert, G. D. (1952). The effect of rubber membrane on the measured triaxial compression strength of clay samples, Geotechnique 3(1), 20-29. Madhavi Latha, G. and Somwanshi, A. (2009). Effect of reinforcement form on the bearing capacity of square footing on sand, Geotextiles and Geomembranes 27, 409-422. Mitchell, J.K., Kao, T.C. and Kavazanjiam, Jr E. (1979). Analysis of grid cell reinforced pavement bases, Technical Report No GL-79-8, U S Army Waterways Experiment Station, July, 1979. Rajagopal, K., Krishnaswamy, N.R. and Madhavi Latha, G. (1999). Behavior of sand confined with single and multiple geocells, Geotextiles and Geomembranes, 17, 171–181 Rea, C., and Mitchell, J.K. (1978). Sand reinforcement using paper grid cells, ASCE Spring Convention and Exhibit, Preprint 3130, Pittsburgh, PA, April, 24- 28. Sitharam, T.G., and Sireesh, S. (2005). Behavior of Embedded Footings Supported on Geocell Reinforced Foundation Beds, Geotechnical testing Journal, ASTM, Vol. 28, No. 5, 452-463. Tafreshi Moghaddas, S.N. and Dawson, A.R. (2010). Behavior of footings on reinforced sand subjected to repeated loading comparing use of 3D and planar geotextile, Geotextiles and Geomembranes 28, 434-447. Wood, D.M., (2004). Geotechnical modeling, Version 2.2. Taylor & Francis, U.K., 258-259.
126
