Geosynthetic Reinforcement on Soft Soils.pdf

27-07-2016

Design and Construction Aspects of Geosynthetic Reinforced Embankment on Soft Soil Seethalakshmi P PhD Research Scholar Civil Engineering IIT Gandhinagar

Short course on Geosynthetics applications in infrastructure development

Introduction – Soft Soil Geology: – Marine sediments – In the vicinity of rivers undergoing meandering and – In areas where runoff restricted and presence of organic matter

Geotechnical properties: – Highly plastic fine grained – High in-situ water content, liquid limit, plastic limit – Low permeability – High compressibility – Low shear strength – Normally consolidated 27-Jul-16

Source: http://www.civil.uq.edu.au/icarus/soft-soil-challenge-1

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Embankments on Soft Soil Problems:

– Stability due to less shear strength – Settlement due to high compressibility

Failure:

– Shear failure (bearing capacity), Sliding, Lateral displacement

Improvement techniques: – – – – – – –

Part or full replacement of soft subsoil Chemical stabilization of foundation soil Stage construction Sand Drains with preloading Use of stone columns Use of reinforcement (metallic- steel or polymeric – Geosynthetics) Use of PVDs

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Geosynthetic Reinforced Embankment Types of Application:

(as per IRC: 113-2013) 27-Jul-16

Source: http://nptel.ac.in/courses/105108075/module8/Lecture34.pdf

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Why Geosynthetic Reinforcement??? Types of material: – Geogrid (uniaxial usually), high/low strength geotextile

Functions of geosynthetic reinforcement: – – – – – –

More bearing capacity and easy movement of machines Accelerated drainage Improved rotational stability Reduced differential settlement Chemically inert and non-bio degradable Greater durability

Benefits: – Prevents lateral spreading, extrusion of the foundation, ensures rotational stability – Accelerates settlement and reduces differential settlement 27-Jul-16

Stability terms

Settlement terms

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How to decide Design Tensile Strength??

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Design Tensile Strength Determination Tal = allowable tensile strength Tult = ultimate tensile strength RFID = Reduction factor for installation damage RFCR = Reduction factor for creep RFCH = Reduction factor against chemical/environmental effects RFW = Reduction factor to allow for weathering fs = global factor of safety

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Design of Geosynthetic Reinforced Embankment Stability checks:

 Rotational stability of embankment  Bearing Capacity failure  Lateral sliding stability

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Rotational Stability

Source: http://nptel.ac.in/courses/105108075/module8/Lecture34.pdf

ΣMr = summation of resisting moment of all slices in kN-m/m ΣMd = summation of driving moment of all slices in kN-m/m Tg = tensile force needed in the reinforcement kN/m R = distance from the centre of slip circle to the reinforcement layer 27-Jul-16

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Bearing Capacity Failure

Source: http://nptel.ac.in/courses/105108075/module8/Lecture34.pdf

Ultimate bearing capacity of the foundation soil:

Cu = Undrained shear strength of the soil in kN/m2 Nc = Bearing capacity factor B = Width of the bottom of the embankment in m D = Depth of the soft soil in m

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Lateral Sliding W = γh γ = Density of embankment fills h = average height of the embankment fill above the reinforcement length (Le) in meter Ls = Horizontal projection of the lateral slope Le = Reinforcement Bond Length α = Interaction coefficient ϕ = Angle of internal friction for embankment fill c = Cohesion of the embankment fill Ka = Active earth pressure coefficient for embankment fill H = Height of embankment in meter Source: http://nptel.ac.in/courses/105108075/module8/Lecture34.pdf

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Lateral sliding continued… •

If FS less than the required value - provide anchorage blocks in the form of sand bags, gabion, concrete blocks

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Other considerations Seismic consideration: – as per IRC: 75 guidelines along with the IS:1893(Part-1)-2002

Allowable strain in the reinforcement – 5% for short term applications and – 5% to 10% for long term applications along with compatibility

Settlement analysis – – – –

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permissible total settlement using conventional methods No reduction in total settlement Decrease in differential settlement Settlement results in increased tension – allowable strain check necessary Short course on Geosynthetics applications in infrastructure development

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Minimum FS requirement

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Selection Of Geosynthetic Material Based on the following parameters:  Strength requirements  Drainage requirements  Environmental considerations  Survivability and fill requirements

Source: IRC: 113 - 2013

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Strength requirement: In terms of material, environmental exposure, Specifications

 Ultimate tensile strength, tensile strength at 2% and 5% strain  Roll length, roll width, long term design strength at 60 years and 100 years Geogrid: – Single strip tensile strength and width (Bonded geogrid) – Strain at max. tensile strength (CMD & MD – knitted or woven geogrid) Geotextile: – tensile strength and strain at max. strength along MD & CMD (wide width tensile strength), puncture strength, apparent opening size, permittivity Geocomposites: – Tests of geogrid for reinforcement, and of geotextile for separation Separation layer: – Grab tensile strength, grab elongation, trapezoidal tear strength, apparent opening size, permittivity, puncture strength 27-Jul-16

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Drainage requirements: – Should not develop excess pore pressure – Should have enough piping resistance and not have clogging problem

Environmental conditions: – Should have very high resistance to chemical and biological attacks – In unusual cases (very low (< 3) or very high (> 9) pH soils, mines, chemically active, acidic soils) – compatibility needs to be checked

Survivability and fill requirements: – Installation damage – major consideration in design tensile strength of geotextiles – Fill requirements as per IRC:36

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Case studies Southern Transport Development Project Seethawaka Industrial Park, Avissawella, Srilanka:

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Case studies Road Project: S.Marco-Argentano Road Italy:

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Case studies Four Laning of NH-5 between Rajahmundry and Eluru, Andhra Pradesh:

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Case studies Restoration of Wharf Road (NH-9) at Vijayawada by Geocell Basal Mattress & Reinforced Soil Wall:

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Case studies Road Project: Visakhapatnam Port Connectivity Road Project, Andhra Pradesh

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Case studies Road Over Bridge near PMC Building, Port Road, Mundra, Gujarat

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References • “Guidelines for the design and construction of geosynthetic reinforced embankments on soft subsoils”, IRC:113-2013 • “Consolidaton By Sand Drains”, (http://nptel.ac.in/courses/105104132/Module5/lecture33.pdf) • “Design of embankments on soft soil using geosynthetics”, Lecture34, (http://nptel.ac.in/courses/105108075/module8/Lecture34.pdf) • “Guidelines on soft soils- stage construction method”, (Guideline no.: ge:g - 5)

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