design and construction of pavements usingusing ... · geogrid reinforced section b. unreinforced...

GEOSYNTHETICS AND REINFORCED SOIL STRUCTURESREINFORCED SOIL STRUCTURES

Design and Construction of Pavements Using Geosynthetics-IIUsing Geosynthetics II

P f K R j lProf K. RajagopalDepartment of Civil EngineeringIIT Madras, Chennai 600 036e-mail: [email protected]

OutlineOutline

• Worked Examples• Use of geosynthetics in pavementsUse of geosynthetics in pavements• Construction aspects

Field Identification of CBR valueField Identification of CBR value

Field observation Approximate CBR Value

E il t t d ith th b 2Easily penetrated with thumb

Moderate effort to penetrate with thumb

< 2

2 – 3Moderate effort to penetrate with thumb

Indented with thumb

2 3

3 – 6

Indented with thumb nail 6 – 16

Difficult to indent with thumb nail > 16

US ARMY CORPS of Engineers generalized equation forUS ARMY CORPS of Engineers generalized equation for any rut depth and loading (N<10000):

34.228301.271log98.470log24.119o

rPNh 63.0

uo

Ch

Where, ho = base course thickness under traffic (m)h o base course thickness under traffic (m)N = Number of traffic passes P = axle load in NewtonsP axle load in Newtonsr = rut depth (m)c = undrained cohesive strength (Pa)cu undrained cohesive strength (Pa)

Worked ExampleWorked ExampleEstimate the granular base thickness for the gfollowing cases:

i) N 1000 P 80 000 N 0 3 C 30 000 Pi) N=1000, P=80,000 N, r=0.3m, Cuu = 30,000 PaSubstituting these values in the above equation, we get the pavement thickness as 0.46m0.46m

ii) N=10,000, P=80,000 N, r=0.3m, Cuu=30,000PaThickness of base = 0.64 m

Comparison of base thicknessfor different cases

N P (N) r (m) Cuu (Pa) Thickness (m)

1000 80 000 0 30 30 000 0 461000 80,000 0.30 30,000 0.4610000 80,000 0.30 30,000 0.6410000 80,000 0.075 30,000 0.7310000 80,000 0.30 60,000 0.4110000 120,000 0.30 30,000 0.76

P = 80 kNPc = 480 kPar = 300 mm

h depends on CBR &ho depends on CBR & no. of passes

h depends on preinforcement modulus

Example on the use of Design chart:

Design the thickness of the base layer for the following data:No. of load passes = 10000Standard axle load = 80 kNCBR value of subgrade = 1CBR value of subgrade = 1Allowable rut depth = 0.3mModulus of the geosynthetic = 200 kN/mg yTyre inflation pressure = 480 kPa

From the chart for CBR=2, N=10,000: ho 0.57 m

Reduction in thickness due to reinforcement h = 0 25mReduction in thickness due to reinforcement, h = 0.25m

Thickness to be provided for reinforced case p= 0.57 – 0.25 = 0.32 m 350 mm

Modified CBR method of design adapted by Koerner (1999) g p y ( )

The laboratory CBR test is modified by introducing the th ti l d th difi d CBR ith i f tgeosynthetic layer and the modified CBR with reinforcement

is determined. Using the empirical equation, the thickness of base layer is estimatedbase layer is estimated

AP

20303621.2log24.3 A

CBRPCh

h = base layer thickness in mmC = no. of load passesP = equivalent single wheel load (N)A = tyre contact area (mm2)CBR l ith d ith t i f t iCBR value with and without reinforcement is used in the above equation

Example:Design the thickness of pavement for subgrade soakedCBR=1.0, N=10000, ESWL = 40 kNTyre contact area is 300450 mm The CBR value withTyre contact area is 300450 mm. The CBR value withreinforcement layer = 3

h = (3.24 log10000 + 2.21)*(40000/(36*1)-(300*450/2030)1/2

= 15.17 * 32.32 = 490 mm

Pavement thickness with geosynthetic layer (use CBR=3)

h = (3.24 log10000 + 2.21)*(40000/(36*3)-(300*450/2030)1/2

= 15.17 * 17.43 = 265 mm

Reduction in thickness = 490 – 265 = 225 mm

Use of GeosyntheticsUse of Geosynthetics

h i h l i l f llGeosynthetics help in several ways as follows:o Reinforcement – helps in reducing subgrade stresses and prevents cracking of pavement due to swelling of foundation soil

o Separator – prevents mixing up of layerso Filter layer – prevents piping phenomenono Drainage layer – provides for safe disposal of water

o Asphalt reinforcement – helps in preventing the reflection cracks

Geotextile Function as function of CBR (Koerner 1999)

Function Unsoaked CBR Soaked CBRSeparation 8 3Stabilisation 8 – 3 3 -1 Reinforcement & 3 1separation

Geosynthetic Stabilisation of Roads on Expansive clays

• Identification – based on liquid limit (LL > 50%) Shrinkage limit (<12%) Differential50%), Shrinkage limit (<12%) Differential Free Swell Index (>50%)

• Stabilisation – lime mixing, lime columns, lime columns, cement stabilisation, soil , ,reinforcement

Desirable geosynthetic propertiesproperties

• Biaxial geogrids g g• % Open area > 70%• High stiffness at low strain

l l (200 t 300 kN/ )

Longitudinal crack on pavements

levels (200 to 300 kN/m)• Good junction strength

Longitudinal crack on pavements over expansive clays

Zornberg and Gupta (2009)

Mechanisms of pavement deflection over expansive claysa: shrinkage during dry season b. Heaving during wet season

Zornberg and Gupta (2009)

a. Geogrid reinforced section b. unreinforced section gb. c. Geogrid reinforced section Zornberg and Gupta (2009)

Repair of Asphalt PavementsRelatively thin asphalt overlay is constructed on existing pavement (with surface cracks) in order g p ( )to reduce water intrusion, reduce surface roughness, improve skid resistance and increase g , pthe structural capacity. In short time the cracks from the old pavementIn short time, the cracks from the old pavement propagate to the surface of overlay.Thi i ll d fl ti kiThis process is called reflection cracking phenomenon

Pavement surface with overlay fabric

Typical cracks on the pavement surface

Surface without overlay fabric

P f f h lt t ith d ith tPerformance of asphalt pavements with and without overlay fabrics (Techfab India Industries Ltd. Mumbai)

U f l f b i f t ti tUse of overlay fabrics for protecting pavements(Techfab India Industries Ltd. Mumbai)

Reflection CrackingPropagation of vertical cracks from old pavement to the new overlays

Khodaii et al. (2009)

Laboratory repeated load test to investigate reflection cracks pressure = 690 kPareflection cracks pressure = 690 kPa


Rate of crack growth lower for reinforced specimensRate of crack growth least for reinforcement in bottom third

Crack growth with number of loading cycles

g


N 1 ld t t & N 2 ld h lt t

Service life with different crack widths & position

No. 1 - old concrete pavement & No.2 - old asphalt pavement

of reinforcement Khodaii et al. (2009)

No. 1 - old concrete pavement & No.2 - old asphalt pavement


Typical MORTH Specifications for Paver bFabrics

Property ASTM Standard Specification

Grab tensile strength

ASTM D4632 360 N

Grab Elongation ASTM D4632 50%

Asphalt Retention Texas DOT 3099 10 kg/m2

Melting point ASTM D276 150CMelting point ASTM D276 150 C

aggregate layer geotextile separator

b dsubgrade soil

Pre ents the intermi ing Prevents the intermixing, prevents piping, strength of aggregate is preserved

Separation Function in a pavement layer

aggregate is preserved

p p y

Separation function of geosyntheticsSeparation function of geosynthetics

Loss of aggregate into soft ground

Stable aggregate layer due to separation

Typical Specifications for separation fabric (AASHTO 1990)

Class of loading

Grab strength < 50%

Grab strength > 50%

Punctureresistance

Puncture resistance

Tear strength

Tearstrength

< 50%ance

> 50%

Class I 1400 N 900 N 500 N 350 N 500 N 350 NClass-I 1400 N 900 N 500 N 350 N 500 N 350 NClass-II 1100 700 400 250 400 250 NClass III 800 500 300 180 300 180 NClass-III 800 500 300 180 300 180 N

Drainage function of a geotextile layer

ingress of water into pavement layersflow of waterflow of water

drain drain

Thick geotextile with good drainage properties is used at interface

Filtration FunctionFiltration Function

Geotextile layer acting as a filter

Water coming out leaving behind the fine soil particles intact

Installation of GeosyntheticInstallation of Geosynthetic

L l & P th d• Level & Prepare the ground• Spread the geosyntheticSpread the geosynthetic• Dump the aggregate on geosynthetic• Spread the aggregate• Compaction by suitable meansCompaction by suitable means

Prepare the Groundp

• remove the stumps boulders etc ;remove the stumps, boulders, etc.; fill in low spots.

Spread the Geosynthetic on prepared ground

Use suitable pins to hold down the geosynthetic – no p g ywrinkles in the geosynthetic

Forming a Curve Using Foldsg g

Forming a Curve Using Cut Piecesg g

Back Dump Aggregatep gg g• onto previously placed aggregate.

Do not drive on the geosynthetic. Maintain 150 mm to 300 mm cover between truck150 mm to 300 mm cover between truck tires and geosynthetic.

Back Dumping Aggregatep g gg g

Minimum cover of 150 to 200 mm soil above geosynthetic

Spreading Aggregatep g gg g

Compact the AggregateCompact the Aggregate• using suitable compaction equipment.

design and construction of pavements usingusing ... · geogrid reinforced section b. unreinforced...

Documents