Behavior of Geosynthetic-Reinforced Earth Presented by Kousik Deb Assistant Professor Department of Civil Engineering IIT Kharagpur Problems for Foundations on Weak Soils Experience excessive settlement Possible bearing capacity failure under load Possible Solutions Geosynthetic/fiber reinforcement Metallic strip Stone columns Soil-cement columns Columnar systems Lime columns Grout columns Vibro concrete columns Micro piles Placement of Geosynthetic Reinforcement Method of Installation Stone Columns Typical Applications of Columnar Support Combined Use of Geosynthetics and Pile/Columnar System Tank foundations Highway embankments Widening of existing roads Retaining walls Railway embankments Reinforced earth walls Others Industrial structures Individual footings Bridge approaches Runways Subgrade improvement Storage tank 1
Other Ground Improvement Techniques preloading with or without the use of sand drains removal and replacement compaction by means of explosion dynamic compaction grouting thermal stabilization ground freezing chemical stabilization Major Applications of Geosynthetics Transportation and Geotechnical Geoenvironmental Hydraulic Engineering Private Development Major Functions of Geosynthetics Reinforcement Separation Filtration Drainage Moisture barrier Geosynthetic (GS) Materials geotextiles (GT) geogrids (GG) geonets (GN) geomembranes (GM) geosynthetic clay liners (GCL) geopipe (GP) geofoam (GF) geocomposites (G C) Geotextiles (GT) majority are made from polypropylene,, polyester, polyethylenes and polyamides fibers standard textile manufacturing woven nonwoven very versatile in their primary function 2
GEOPIPE GEOFOAM 3
Function vs. Geosynthetic Type Type of Geosynthetic geotextile geogrid geonet geomembrane geosynthetic clay liner geopipe geofoam geocomposite Separation Reinforcement Filtration Drainage Containment Design Methods Application Areas (a) Cost -based on experience/availability (b) Specification for common applications (c) Function for specialty, critical and/or permanent applications Transportation/Geotechnical Geoenvironmental Hydraulic Systems Private Development Transportation and Geotechnical Applications GTs as filters GTs and GGs as wall reinforcement GTs and GGs as slope reinforcement GC Wick Drains (also called PVDs) GC Erosion Control Systems Geotextile Filtration GT is acting as a filter not as a drain three design requirements: 1. adequate flow 2. proper soil retention 3. long-term flow equilibrium many applications, e.g., behind retaining walls under erosion control systems around pavement underdrains (follows) 4
Stone base Pavement Soil subgrade (GT Filter in Excavated Trench) Geotextile Filtration 400 mm Topsoil 300 mm 450 mm GT Crushed stone/ perforated pipe (Crushed Stone & Perforated Pipe) Wall Reinforcement Design Concepts internal design results in: spacing of GT or GG length of GT or GG external design used to assess: overturning stability sliding stability bearing capacity reduction factors on reinforcement put on laboratory values for allowable strength factor-of of-safety on each design aspect to resist the unknown Various Types of Reinforced Retaining Wall Elements of a GT or GG Wall Design P 1 Surcharge P 2 (live loads) D z H L R L E s v σ hs + σ hq + σ ht = σ h 45+φ/2 L 0 L Soil Surcharge Live load Total lateral (With Concrete Facing) (Green Wall with Vegetated Facing) Reinforcement for Soil Slopes Placement patterns for reinforcement most soil slopes become unstable steeper than 2(H)-to to-1(v) (26.5 ) use GT or GG reinforcement to increase either the slope angle or height essentially no limit, except for erosion various placement patterns are possible (a) Even spaced-even length (c) Even spaced-even length with short facing layers (b) Uneven spaced-even length (d) Even spaced-uneven length with short facing layers (One that Failed)! (With Reinforcement-Steep & Stable) 5
Geocomposite Wick Drains also called prefabricated vertical drains (PVDs( PVDs) used for rapid consolidation of saturated fine grained soils consists of a drainage core with a GT filter/separator wrapped completely around it typically 100 mm wide, by 2 to 10 mm thick, by 100 m long (in roll or coil form) Geocomposite Erosion Control Systems huge array of products slope protection channel protection increase shear stress (Driving Wick Drains) (Ready for Surcharge Fill) Nature of Waste Problem moisture within and precipitation on the waste generates leachate leachate takes the characteristics of the waste thus leachate is very variable and is site-specific specific flows gravitationally downward enters groundwater unless a suitable barrier layer and collection system is provided GG Double Liner System (with leak detection layer) GT GN GCL GM perforated pipe Final Cover System GG (Geonet Leak Detection) Cover Soil GC or GN GT GCL GM GP or GC (Nine Layers of Geosynthetics) 6
Hydraulic Engineering Applications (Seven Layers of Geosynthetics) (Sequential Placement of GSs) Possible Geosynthetic Layers in a Waste Containment System in Final Cover - 7 in Waste Itself - 2 in Base Liner - 9 18 Layers! Waterproofing of Dams Waterproofing of Canals Reservoir Liners Tunnel Waterproofing & Rehabilitation Pipe Rehabilitation & Remediation Waterproofing of Dams (Lining a Concrete Dam) masonry, concrete, earth and RCC dams GM is not a structural element, its waterproofing many dams over 50-years old often have leakage; sometimes excessive leakage (Concrete Dam Leaking!) (Completed Concrete Dam Lining) (Lined Earth Dam: Before Rip-Rap) Waterproofing of Canals conveyance of all liquids; however, water is the most common distances and quantities vary greatly fundamental issue is leakage (i.e., how much, if any, is allowable) some type of liner (GM or GCL) is necessary (Lining a Canal: Before Soil Covering) (GCL Lining of a Canal) (GM Canal 18 years after GM Lined) 7
Reservoir Liners used to contain all types of liquids potable water industrial waters process waste waters sewage sludge industrial sludge agricultural wastes hazardous liquids Common Characteristics generally shallow liquid depths typically 2 to 7 m side slopes from 4(H)-to to-1(v) to 1(H)-to to-1(v), i.e., β = 14 to 45 both exposed and covered exposed GM durability issue covered soil stability issue Private Development Applications Selected Areas of Focus (Lined Potable Water Reservoir) various dwellings industrial buildings storage areas parks and playgrounds pools and lakes sport fields golf courses airfields agriculture aquaculture liquid transportation (Huge GM Bag Transporting Potable Water) New Tunnel Waterproofing many old tunnels without GMs are leaking key is to use a GT and GM behind the permanent concrete surfacing in turn, this requires a GP drainage system Pools, Ponds and Lakes sites vary from small-to to-huge usually access is limited liners required for leakage control covers sometimes required for contamination control and for safety 8
Soil-Geosynthetics Interaction Confinement Effect String Effect Design of Geosynthetic-Reinforced Earth Types of Failure of Reinforced Earth under Footing Load Failure above top reinforcement layer u > 0.5B Failure between reinforcement layers h > 0.5B Failure on a two-layer soil system Sharma et. al., 2009 Failure within reinforced zone Das et. al., 1998 Improvement in Settlement and Bearing Failures of Reinforced Embankment Bearing Optimum Thickness of Sand Bed Rotational Internal Spreading Lee et. al., 1999 Optimum Width of Geosynthetic Layer 9
Reinforced Retaining Wall Design of Geosynthetic-Reinforced Wall P 1 Surcharge P 2 (live loads) D z H L R L E s v σ hs + σ hq + σ ht = σ h 45+φ/2 L 0 L Soil Surcharge Live load Total lateral Cai and Bathurst, 1996 Types of Failure Internal Stability: Step 1 Determine the active distribution (σ a )on the wall σa = K a σ v = Ka γ z Where K a = Rankine earth oressure coefficient = tan 2 (45 - φ/2) Step 2 Select a geotextile that has an allowable strength of σ g Step 3 Determine the vertical spacing of the layer at any depth z S v = σg/(ka γ z FS) Where FS is the factor safety. The magnitude of FS is generally 1.3 to 1.5 Step 4 Determine the length of each layer of geotextile L = L R + L E L R = (H z)/tan(45 + φ/2) and L E = (S v σa FS)/(2 σ v tan φ F ) 1m Where φ F = 0.67 φ Step 5 Determine the lap length, L 0 = (S v σa FS)/(4 σ v tan φ F ) The minimum lap length should be 1m Design Example A geotextile-reinforced retaining wall 6 m high. For the backfill, γ = 19 KN/m3 and φ = 36. For the Geotextile, σg = 16 KN/m. K a = tan 2 (45-36 /2) = 0.26 At 2 m depth Sv = σg/(ka γ z FS) = 16/(0.26 x 19 x 2 x 1.5) = 1.08 m At 4 m depth Sv = 16/(0.26 x 19 x 4 x 1.5) = 0.54 m At 6 m depth Sv = 16/(0.26 x 19 x 6 x 1.5) = 0.36 m So, use Sv = 0.5 m for z = 0 to 4 m and Sv = 0.33 for z > 4m Determination of L L = (H z)/tan(45 + φ/2) + (S v σ a FS)/(2 σv tan φ F ) = 0.51 (H z) + 0.435 Sv At z = 0.5, L = (2.8 + 1) = 3.80 m At z = 1.0m, L = (2.55 + 1) = 3.55 m At z = 1.5m, L = (2.3 + 1) = 3.3 m At z = 2.0m, L = (2.0 + 1) = 3.0 m At z = 2.5m, L = (1.8 + 1) = 2.8 m At z = 3.0m, L = (1.5 + 1) = 2.5 m At z = 3.5m, L = (1.3 + 1) = 2.3 m At z = 4.0m, L = (1.0 + 1) = 2.0 m At z = 4.33m, L = (0.85 + 1) = 1.85 m At z = 4.67m, L = (0.68 + 1) = 1.68 m At z = 5.0m, L = (0.51 + 1) = 1.51 m At z = 5.33m, L = (0.34 + 1) = 1.34 m At z = 5.67m, L = (0.17 + 1) = 1.17 m At z = 6.0m, L = (0 + 1) = 1.0 m L = 4 m L = 3 m L = 2 m Determination of L 0 L 0 = (S v σ a FS)/(4 σ v tan φ F ) = 0.218 Sv = 0.218 x 0.5 = 0.1 m < 1 m Take L 0 = 1 m External Stability: (i) Check for overturning F.S > 3 (ii) Check for Sliding F. S > 3 (i) Check for bearing capacity F.S > 3 to 5 10
Performance of Reinforced Retaining Wall under Seismic Condition Izmit Earthquake, Turkey, 1999 Bridge has been collapsed The Reinforced Earth walls were designed for a ground acceleration of 0.10 g. This resulted in increase in the amount of reinforcing strips compared to static design. The actual ground acceleration was measured at 0.4 g. Forces acting on single horizontal elemental slice containing reinforcement Nimbalkar et. al., 2006 Effect of Earthquake loading on Factor of Safety Effect of Earthquake loading on Required reinforcement Length Choudhury et. al., 2008 Cai and Bathurst, 1996 At present, the available codes for seismic design of reinforced soil wall systems (e.g., FHWA 1996, AASHTO 1998 Summary Geosynthetics are bona fide engineering materials Test methods and designs are available Basic advantage of geosynthetics is quality control of factory manufactured products Field performance has been excellent Geosynthetics potential is awesome! 11