Geosynthetic Encasement for
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- Charles Nicholson
- 5 years ago
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1 Geosynthetic Encasement for Stonger and Stiffer Stone Columns K. Rajagopal Professor, Department of Civil Engineering IIT Madras, Chennai
2 Problems due to soft clay soils Low bearing capacity Excessive settlements Deep seated foundation failure embankment failure wedge soft Clay slip circle Firm Soil Encased Stone Columns 2
3 Solutions to the Problem removal of soft material and replacement with quality fill displacement of soft material piling Pre-consolidation: sand drains, PVDs, vacuum consolidation lightweight fill Stone columns Geosynthetics many possibilities Encased Stone Columns 3
4 Introduction Stone columns/granular piles Popular ground dimprovement ttechnique Flexible structures like Embankment, Storage tank etc. Structures with large loaded areas, Parking garages etc. Liquefaction mitigation Encased Stone Columns 4
5 Stone Column Construction methods Rammed Stone columns Vibro Replacement Vibro Displacement Encased Stone Columns 5
6 Encased Stone Columns 6
7 Encased Stone Columns 7
8 Encased Stone Columns 8
9 Encased Stone Columns 9
10 Encased Stone Columns 10
11 Encased Stone Columns 11
12 42.5 m dia x 14.6 m high external floating roof tanks for storing high/low sulphur diesel for Chochin Refineries Limited Encased Stone Columns 12
13 Historical Development First applied in France in 1830 to improve a native soil Used extensively in Europe since the late 1950's in marginal soils Stone columns are commonly o employed in India. Barksdale and Bachus (1983) Encased Stone Columns 13
14 How does a stone column resist vertical loads? Encased Stone Columns 14
15 Failure mechanism Single SC Encased Stone Columns 15
16 Plan arrangements of stone columns Square pattern Triangular pattern d s d s D D Dia. of unit cell = s Unit cell area = s*s s Dia. of unit cell = 1.05 s Unit cell area = 0.866*s*s * Encased Stone Columns 16
17 Thorburn (1975) Limiting iti bearing pressure on stone columns = 25c u Encased Stone Columns 17
18 Improving the load capacity of the stone column Reinforcing with iron rods Skirting Rao and Ranjan (1985) Layered reinforcement (Geogrids) Sharma (1998) and Sharma et al. (2004) Encasing the Stone Column (ESC) Present study Encased Stone Columns 18
19 Encasing the Stone Column Bearing capacity enhanced by Passive pressure + Additional confinement Geosynthetic encasement Stone column Sectional plan Encased Stone Columns 19
20 From...Clay...to...Pile Clay OSC ESC Pile Homoge eneous Comp posite Semi- rigid Rig gid Encased Stone Columns 20
21 Construction of Encased Stone Column Alexiew et al. (2005) Encased Stone Columns 21
22 Encased Stone Columns 22
23 Construction of Encased Stone Column Alexiew et al. (2005) Encased Stone Columns 23
24 Geotextile encased sand column Encased Stone Columns 24
25 Hamburg, Germany A380 factory site Encased Stone Columns 25
26 Encased Stone Columns 26
27 Pre-fabricated ESC column being lowered into a drill borehole Encased Stone Columns 27
28 Another type of encased stone column Encased Stone Columns 28
29 Courtesy: Dipl.-Ing. Holger Pohlmann Naue Fasertechnik GmbH & Co. KG Present study Encased Stone Columns 29
30 Advantages of Encasement 1. Imparts lateral confinement 2. Increases the load capacity & stiffness by many fold 3. Stresses are transferred to deeper strata. 4. Higher lengths of stone column are possible. Encased Stone Columns 30
31 Advantages of Encasement 5. Lateral squeezing of stones is prevented 6. Higher degree of compaction can be achieved 7. Prevents the clogging of stone columns 8. Strength properties of the aggregate are preserved. Encased Stone Columns 31
32 Construction of Encased Stone Column Encased Stone Columns 32
33 Encased Stone Column Encased Stone Columns 33
34 1 Compression tests t on geosynthetic ti encased stone aggregates Encased Stone Columns 34
35 Geosynthetic Encased Stone aggregate Encased Stone Columns 35
36 Compression test on Encased Stones Encased Stone Columns 36
37 Load (kn N/m) Tensile load-strain behaviour of geosynthetic samples with seam Woven geotextile Non-woven geotextile til Soft grid - 1 Soft grid Strain (%) Encased Stone Columns 37
38 Geosynthetic Confinement Confining i pressure due to membrane 2M c 3 Henkel and Gilbert (1952) d 1 a Axial Capacity, p y, K 1 3 p Encased Stone Columns 38
39 Pr ressure (k kpa) Experiment Vs. Henkel & Gilbert (Non-woven geotextile) mm 75 mm 100 mm 150 mm Experiment Henkel & Gibert Ail Axial strain (%) Encased Stone Columns 39
40 Experiment Vs. Henkel & Gilbert (woven geotextile) Pr ressure (k kpa) mm Experiment 75 mm Henkel & Gibert 100 mm 150 mm Linear strain (%) Encased Stone Columns 40
41 Influence of the diameter on the ultimate load carrying capacity 1500 Ultim mate stres ss (kpa Woven geotextile Non-woven geotextile Soft grid Diameter of the sample (mm) Encased Stone Columns 41
42 2 Load dtests t on single stone column in a Unit cell Encased Stone Columns 42
43 Stone column arrangement s s Square pattern Triangular pattern Influence radius = 0.564s Influence radius = 0.525s Encased Stone Columns 43
44 Schematic of setup Unit Cell Strain controlled loading Proving ring To strain read out unit 500 mm Sof t Cla y 210 mm Geosynthetic encasement Stones Strain gauges Unit cell tank soft clay Sectional plan Encased Stone Columns 44
45 Clay bed undergoing consolidation Encased Stone Columns 45
46 Properties of Clay Sl.No Properties Value 1 Liquid limit 49 % 2 Plastic limit 17 % 3 Specific Gravity Moisture content t after consolidation 47±1% 5 In-situ vane shear strength 2.5 kpa 6 Consistency Index Dry unit weight kn/m 3 8 CBR value 0.11 % 9 IS Classification System CI (Silty clay of medium plasticity) 10 Degree of Saturation 96 % 11 In-situ void ratio Encased Stone Columns 46
47 Strain gauges in encasement Encased Stone Columns 47
48 Stone Column in Unit Cell Encased Stone Columns 48
49 ESC in Unit cell tank Encased Stone Columns 49
50 Load settlement curve for stone columns encased in non-woven geotextile Se ettlement t (mm) Pressure (kpa) ESC - 50 mm ESC - 75 mm ESC mm OSC - 50 mm OSC - 75 mm OSC mm Clay Encased Stone Columns 50
51 Bulging g in stone columns OSC ESC Encased Stone Columns 51
52 Hoop strain variation in the Geosynthetic encasement 0 Hoop strain (%) Depth (m mm) mm Ø 50 mm II Trial 75 mm Ø 100 mm Ø Encased Stone Columns 52
53 Encasement with different geosynthetics (mm) Se ettlement Pressure (kpa) ESC (woven geotextile) ESC (nonwoven geotextile) ESC (soft grid - 1) ESC (soft grid - 2) OSC Clay 50 Encased Stone Columns 53
54 Load settlement curve for stone columns encased din woven geotextile til Sett tlement (mm) Pressure (kpa) ESC- 50 mm ESC - 75 mm ESC mm OSC - 50 mm OSC - 75 mm OSC mm Clay 50 Encased Stone Columns 54
55 Analytical predictions Limiting stress on OSC (ordinary stone column) 4 v ro u col c Kp - IS 15284: Part-1 (2003) Where, ro c u Kp col - Initial effective radial stress - Undrained dcohesion of surrounding clay - Coeff. of passive earth pressure - stone Encased Stone Columns 55
56 Analytical predictions Lateral stress due to encasement in ESC Circumferential strain in the encasement, 2J Additional lateral confining stress, p J c d Where, c 1 1 a 1 a J - Tensile load in the encasement corresponding to strain c d - Diameter of the stone column. Encased Stone Columns 56
57 Experimental Vs. Analytical Pr ressure at 50 mm settlement t ESC(woven)-Experiment ESC(woven)-Analytical ESC(non-woven)-Experiment E ESC(non-woven)-Analytical OSC-Experimental OSC-Analytical Diameter of stone column Encased Stone Columns 57
58 3 Load dtests t on single stone column in large tank Encased Stone Columns 58
59 Load tests on stone column installed in large test tank Vertical loading Dial gauges Stone columns 600 mm Geosynthetic encasement Soft Clay 1200 mm Encased Stone Columns 59
60 Load test on single stone column Encased Stone Columns 60
61 Encased stone column (non-woven) Single stone columns Settl lement (m mm) Pressure (kpa) ESC - 50 mm ESC - 75 mm ESC mm OSC - 50 mm OSC - 75 mm OSC mm Clay 50 Encased Stone Columns 61
62 Experiment Vs. Analytical Limi iting stres ss (kpa) ESC - Experiment ESC - Analytical l OSC - Experiment OSC - Analytical Diameter (mm) Encased Stone Columns 62
63 4 Load dtests t on Group of stone columns in large tank Encased Stone Columns 63
64 Configuration of group of Stone columns - Plan Soft Clay 75 mm 150 mm c/c Loading plate 280 mm Ø 1200 mm 1200 mm Encased Stone Columns 64
65 Group of Stone column Triangular pattern Encased Stone Columns 65
66 Loading plate fitted with pressure cell Group test t Loading To read out unit Pressure cells Encased Stone Columns 66
67 Load test on group of stone columns Encased Stone Columns 67
68 Stress concentration on the stone 12 columns Group test t Stress s concentr ration 8 4 ESC (woven) ESC (non-woven) OSC Clay (ESC - woven) Clay (ESC-nonwoven) Clay (OSC) Settlement (mm) Encased Stone Columns 68
69 Embankment loading exerting a lateral thrust on the stone columns in the extremities Embankment Potential slip circle causing deep seated failure Stone columns Soft clay Encased Stone Columns 69
70 Numerical simulations Encased Stone Columns 70
71 Numerical simulation experiments OSC in Unit cell Set ttlement (mm) Pressure (kpa) mm -Exp 50 mm FEM 75 mm - Exp 75 mm - FEM 100 mm - Exp 100 mm - FEM Clay - Exp Clay - FEM 50 Encased Stone Columns 71
72 Numerical simulation experiments ESC in Unit cell Settl lement (m mm) Pressure (kpa) mm -Exp 50 mm - FEM 75 mm - Exp 75 mm - FEM 100 mm - Exp 100 mm - FEM 50 Encased Stone Columns 72
73 Finite Element Parametric studies Uniform Pressure Modified Hyperbolic model with plasticity c Axisymmetric 8 Node, Continuum Elements Elastic model for geosynthetic Surcharge modeled as 200 kpa pressure Stone column Geosynthetic encasement Soft clay 5m Influence Encased Stone Columns 73 radius
74 Cases considered for analysis 1. Odi Ordinary Stone Column 2. Geogrid Encased Stone Column Pressure on Stone Column only & all over the area Parameters Varied 1. Diameter of the stone Column 2. Pressure on the stone Column 3. Spacing of the stone columns (i.e. Influence radius) 4. Height of encasement 5. Stiffness of the geosynthetic 6. Shear strength of the surrounding clay Encased Stone Columns 74
75 Validation of GEOFEM program with Han and Gabr (2002) Maximum settlement t Maximum m settleme ent (mm) Unreinforced - Present study Unreinforced - Han&Gabr Reinforced - Present Study Reinforced - Han&Gabr Height of embankment (m) Encased Stone Columns 75
76 tlement (%) Norma alised Set Results and Discussions Effect of Geogrid Encasement for Stone Column OSC ESC Influence radius (m) Due to encasement the stone column settlements have reduced up to 20% for all spacing. Encased Stone Columns 76
77 Results and Discussions Variation of settlement in stone column with diameter %) 28 alised set ttlement ( Norm OSC ESC Area ratio (%) Beyond certain diameter the encasement effect is very minimal Encased Stone Columns 77
78 Confining pressure in stone column Confining pressure (kpa) Dep pth (m) m Ø - ESC m Ø - OSC 1 m Ø - ESC 3 2 M 1 1 a d0 1 a 4 5 Bathurst and Rajagopal (1993), Rajagopal et al. (1999) and Latha et al. (2006)] Encased Stone Columns 78
79 Contours of mobilised shear strength (a) OSC (b) ESC with J = 5000 kn/m Encased Stone Columns 79
80 Influence of Stiffness of the encasement (1m Ø Stone column) 0 pressure (kpa) ettlement t (mm) s kn/m 2500 kn/m 500 kn/m 250 kn/m 50 kn/m OSC OSC 150 Encased Stone Columns 80
81 Lateral bulging of stone column (1 m Ø) 0 Normalised lateral bulging ( z/ro) (%) Depth (m m) (vertical pressure =200 kpa) OSC 50 kn/m 250 kn/m 500 kn/m 1000 kn/m 2500 kn/m 5000 kn/m kn/m Encased Stone Columns 81
82 Influence of stiffness of encasement on the lateral confining stress (1 m Ø) 0 Confining pressure (kpa) OSC ESC, J= kn/m Depth (m) OSC 250 kn/m 1000 kn/m 2500 kn/m 5000 kn/m kn/m Encased Stone Columns 82
83 Influence of stiffness of encasement on settlement reduction 100 ent reducti ion (%) Settlem m Ø Stone column 1m Ø Stone column Stiffness of encasement (kn/m) Encased Stone Columns 83
84 Hoop tension in the encasement (1m Ø stone column) 0 Hoop tension (kn/m) Depth (m m) kn/m 250 kn/m 500 kn/m 1000 kn/m 2500 kn/m 5000 kn/m kn/m Encased Stone Columns 84
85 Influence of shear strength of surrounding soil (1 m Ø) 0 Pressure (kpa) Se ettlement in (mm) OSC ESC 250 kn/m ESC 5000 kn/m ESC kn/m Cohesion = 20 kpa Cohesion = 10 kpa Encased Stone Columns 85
86 FE Analysis with embankment loading c Embankment fill Maximum embankmenrt height = 5m Stone column Foundation soil depth = 5 m Geosynthetic encasement Influence radius 3 m Encased Stone Columns 86
87 Variation of stress intensity factor with height ht of embankment stress inte ensity fac tor, SIF kn/m 5000 kn/m 2500 kn/m 1000 kn/m 250 kn/m OSC height ht of ebankment (m) Encased Stone Columns 87
88 Design chart for encased stone columns Encased Stone Columns 88
89 Conclusions Encasement is more effective in lesser diameter stone columns because of mobilisation of larger confining stresses. Significant improvement can be achieved by encasing the top portions to depths of 2 to 3 times the diameter. The load capacity of encased columns is not as sensitive to the shear strength thof fthe surrounding soils as compared to OSCs. The magnitude of loads transferred ed into the encased stone columns from the embankments can be increased by using stiffer encasement. Encased Stone Columns 89
90 Acknowledgements MHRD for sponsoring a research project titled Investigations on Modern Technologies for construction of Road/rail embankments on soft clay soils at IIT Madras Dr. Murugesan, formerly Ph.D. scholar at IIT Madras for doing all this work. Encased Stone Columns 90
91 Thank you