erosion control using geosynthetics...erosion control using geosynthetics prof k. rajagopal...

Erosion Control Using GeosyntheticsGeosynthetics

Prof K. RajagopalDepartment of Civil EngineeringDepartment of Civil Engineering

IIT Madras, Chennai 600 036il lk @iit ie-mail: [email protected]

Introduction• Erosion is the process of detaching

soil particles from ground and so pa c es o g ou d a dtransporting it awayErosion forces wind rain currents• Erosion forces – wind, rain, currents, waves, ice

• Soil on slopes is most subject to erosionerosion

• It takes more than 100 years to form 1 f il (H d 1981)1 cm of soil (Hudson 1981)

Problems due to soil erosionProblems due to soil erosion

• Loss of land – loss of shoreline• Loss of land – loss of shoreline• Instability of slopes• Loss of top soil & fertility of soils• Loss of reservoir capacity due to• Loss of reservoir capacity due to

silting• Rubber and tea plantations suffer

most from loss of top soilp

A TNSCB House lost into sea due to coastal erosion in May 2012

Mouth of estuary closed by sand bar frmation

PULICAT LAKE

BAR MOUTHFORMATION

SHAR INLET

Forces causing Soil ErosiongWind, water and gravity forces are theprime agents of erosion Theseprime agents of erosion. Theseelements combine in a variety of waysto perform the dual role of abrading andsimultaneously transporting both soily p gand rock.

Different Types of Erosion Forces• Aeolian erosion: wind blows away fine

cohesionless soil (sand-blasting).( g)• Rain erosion: rain splash and rain wash attack the

land mass so causing surface erosion.• River erosion: hydraulic lifting, scouring,

cavitation and abrasion. River erosion also triggers mass movement in the form of landslips.

• Marine erosion: wind caused waves and currents d tid l d t iand tides may lead to erosion.

• Glacial erosion: ice sheets and glaciers moving l lalong slopes.

Wind Erosion

Wind speed direction cohesionless soilsWind speed, direction, cohesionless soils, large exposed area, lack of protection lead to wind erosionto wind erosion.

Chepil and Woodruff (1963)

Particle Diameter,mm

Wind Susceptibility

< 0.42 Highly erodible0.42 to 0.84 Difficulty erodible0 84 to 6 4 Usually non erodible0,84 to 6.4 Usually non-erodible> 6.4 Non-erodible

Rain water induced ErosionImpact of rain drops dislodges soil particles and surface runoff carries the loosened soil particles

Rainfall Intensity Droplet Kinetic Energy

down the slope

Form (mm/hr) Diameter (mm)

(J/m2/hr)

Drizzle <1 0.9 2Light 1 1.2 10

Moderate 4 1.6 50Heavy 15 2 1 350Heavy 15 2.1 350

Excessive 40 2.4 1000Cloudburst 100 2.9 3000Cloudburst 100 4.0 4000

Factors influencing Rain Water Erosion

• Erosiveness of rainwater - depends on the di i ti f l h d b denergy dissipation of splash and can be assessed

by duration/intensity of rainfall.• Erodability of soil susceptibility of a given soil is• Erodability of soil - susceptibility of a given soil is

guided by the nature of soil (clay content), particle size distribution and soil condition like saturation,size distribution and soil condition like saturation, density, permeability, plasticity etc.

• Physical features e.g., topography and gradient of y g , p g p y gslope, presence of berms, vegetation growth, etc.

Soil Erodability• low in well-graded, coarse gravels.• high in uniform silts and fine sands• high in uniform silts and fine sands.• decreases with increasing clay and organic

t tcontent.• decreases with lower void ratios and higher

moisture contentsMost Erodible Least Erodible

ML > SM > SC > MH > OL >> CL > CH > GM > SW > GP > GW> GM > SW > GP > GW

COASTAL PHENOMENONCOASTAL PHENOMENON• LITTORAL DRIFT: LONGSHORE OR THE LITTORAL DRIFT IS THE TRANSPORTATION OF

SEDIMENTS (SAND MAINLY BUT MAY HAVE COARSER SEDIMENTS) ALONG A COAST ATAN ANGLE TO THE SHORELINE WHICH IS DEPENDENT ON THE WIND DIRECTION SWASHAND BACKWASH. THIS PROCESS TAKES PLACE IN THE ZONE CALLED LITTORAL ZONE.

• IN THE PROCESS SAND IS AFFECTED BY THE OSCILLATORY FORCE OF THE BREAKINGWAVES MOTION OF THE SEDIMENT AND BED SHEAR FROM LONGSHORE CURRENT.

• IT PLAYS AN IMPORTANT ROLE IN THE EVOLUTION OF A SHORELINE.THESE ARE DUETO MANY FACTORS LIKE HYDRODYNAMIC FORCES,SEDIMENT BUDGET AND HUMANINTERVENTION.

COASTAL PHENOMENON• GROYNE: THEY ARE SHORE PROTECTION STRUCTURES PLACED AT EQUAL

INTERVALS ALONG THE COASTLINE IN ORDER TO STOP COASTAL EROSION

COASTAL PHENOMENON

INTERVALS ALONG THE COASTLINE IN ORDER TO STOP COASTAL EROSION.• GROYNE STRUCTURES ARE USUALLY USED ON SHORES WITH HIGH ANNUAL

LITTORAL DRIFT.• THE THREE TYPES OF GROYNES USED ARE:

ZIG ZAG GROYNES WHICH DISSIPATE THE DESTRUCTIVE FLOWS IN WAVE– ZIG ZAG GROYNES WHICH DISSIPATE THE DESTRUCTIVE FLOWS IN WAVEINDUCED CURRENT OR IN BREAKING WAVES.

– T HEAD GROYNES WHICH REDUCE WAVE HEIGHT THROUGH WAVEDIFFRACTION.

– Y HEAD GROYNE SYSTEMY HEAD GROYNE SYSTEM.

Deep gulleys formed on highway slopes by rain water p g y g y p yinduced erosion – notice the fine silty soil which is highly susceptible to erosion

Different protection systems• Rip rap

Concrete lining• Concrete lining• Guniting• Gabion mattresses• Geotextile coveringGeotextile covering• Cellular mattresses & nets

Bitumen & asphalt spray• Bitumen & asphalt spray• Vegetation

Components of the Grass Plant (Hall 1992)

Protective Role of VegetationINTERCEPTION b b i f ll d t ilINTERCEPTION: absorb rainfall energy and prevent soil

detachment by raindrop splash.RESTRAINT: root system bind soil particles & filters theRESTRAINT: root system bind soil particles & filters the

sediments above the ground levelRETARDATION: surface roughness increased and slow

d ff l idown runoff velocityINFILTRATION: higher porosity and permeability thus

delaying onset of runoffdelaying onset of runoff.Deep roots prevent shallow mass movements by:

- Mechanical reinforcement - Soil water depletion through transportation and interception, and

B tt i d il hi ti f b dd d- Buttressing and soil arching action from embedded stems.

Armour Layer provided by VegetationArmour Layer provided by Vegetation(Hall, 1992)

Use of coir mat to promote vegetation growth to prevent surface erosion

Courtesy Garware Wall Ropes, Pune

Irrigation canal lining by using grout filled geocells

Innovative use of geogrids for shore protection at Navi Mumbai

Swaminarayan Temple TithalSwaminarayan Temple, Tithal

• Temple is located on the Arabian Sea GujaratTemple is located on the Arabian Sea, Gujarat.• Severe erosion at the site due to wave and

current actionscurrent actions.• Seabed erosion posed a threat to the structure

i lin long run.

Use of stone filled rope net gabions for shoreline stabilization at Swami Narain Temple, Tithal, Gujarat


Gabion wall at Swami Narayan Temple, Gujarat


Deep cutting in Konkan Railway lineCourtesy Garware Wall Ropes, Pune

Boulder Net installed on the slope to preventBoulder Net installed on the slope to prevent boulders from falling into railway track


Net fixed in the anchor trench at the top of the slope


Vegetation growth within two seasonsCourtesy Garware Wall Ropes, Pune

Basu et al. (2011)

G d t ti th ithi th thGood vegetation growth within three monthsBasu et al. (2011)

River bank after heavy rainfall and floods Basu et al. (2011)

Condition of river bank after floodsBasu et al. (2011)

Basu et al. (2011)

Shore protection using stone filled metallic gabions

Stone filled PP rope net gabions for shore protection

Shore protection work using geotextile bags

Rope net gabions filled with sand bags

Flexible Break water unit made of geosynthetics – beach sand filled bags in rope net gabionsg p g

EFFECTS OF Cyclone THANE

Coast line at INS Adyar, Chennai before cyclone

Coastline at INS Adyar, Chennai after cycloneChennai before cyclone Chennai after cyclone

erosion control using geosynthetics...erosion control using geosynthetics prof k. rajagopal...

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