geosynthetics engineering: in theory and practice · 2017-08-04 · substantially reduce the...

GEOSYNTHETICS ENGINEERING: IN THEORY AND PRACTICE

Prof. J. N. Mandal

Department of civil engineering, IIT Bombay, Powai , Mumbai 400076, India. Tel.022-25767328email: [email protected]

Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay

Module-5LECTURE- 20

GEOSYNTHETICS IN PAVEMENTS


OUTLINE

Introduction Mechanisms and concepts of pavement Design of unpaved roads Design charts of U.S. Forest Service (USFS) for

unpaved roads Modified California Bearing Ratio (CBR) testsDesign of pavement in unreinforced and reinforced

conditions Development of design methods for geosynthetic

reinforced flexible airfield pavements Pavement overlays Geosynthetics in railroads Geosynthetics in roadway repair and extension Stabilization of pavement using NANO material


Basic concepts

Embankments

Interaction Polymer properties

Applications

Steep slops

Soil mechanics

Reinforced fill applicationsSoft soil applications

Unpaved roads Retaining walls

(Short term reinforcement strength required) (Long term reinforcement strength required)


Roads and highway are the backbone of any country in theworld for development. India has the second largest roadnetwork and fastest growing airport infrastructures. Indianrailway has a network of 63,465 route kilometers.

Rut formation under heavy vehicle loads is a major concernfor unpaved roads over soft subgrade. It is required tominimize the rut formation to prevent local shear failure.

It can be achieved by increasing the thickness of the baselayer with good quality of fill materials. In that case, the groundpressure on subgrade gets reduced due to wider loaddistribution. However, it will increase the cost of construction.

The subgrade soil can also be improved by stabilizationsystems using lime, cement and chemicals. This will also leadto a higher construction cost.


Placement of geosynthetics over subgrade soil cansubstantially reduce the required fill thickness. It can providethe following benefits: Separation layer

Reduce vertical and lateral deformation

Reduce construction and operational costs

Increase the bearing capacity of soft soil

Increase lifetime of the road

Better drainage

Rapid consolidation

Less periodical maintenance

Saving of construction costProf. J. N. Mandal, Department of Civil Engineering, IIT Bombay

Geotextiles and geogrids are generally used inpavements.

Hybrid geosynthetics can be used as good drainage,filtration and reinforcement materials.

Geosynthetics should properly be selected, specified andinstalled

Use of geosynthetics ensures 36% improvement inhighway quality and 10% saving in cost.

The beauty of geosynthetic is that it not only acts forseparation but also acts for filtration, drainage andreinforcement. That is why it is called multi functional.


MECHANISMS AND CONCEPTS OF PAVEMENT

Stone aggregates enter into the fine subgrade soil andloose its strength


Soil pumps up into the voids of stone aggregates causingthe stone aggregate to loose its drainage capacity.


Geosynthetics prevent granular materials from penetratinginto the soft underlying subgrade as well as prevent fine-grained subgrade soil from being pumped up into permeablegranular materials.

Separation Mechanism

The geosynthetics may tear off due to sharp edged grains ofthe ballast under the dynamic loads of railways. Therefore,sandy gravel as a protective layer is placed under the ballast.


Filtration and Drainage Mechanism

Separation, Filtration and drainage mechanism of geotextile


Mechanism of Reinforcement

Soil: Good in compression and poor in tension

Geosynthetic: Good in tension and poor in compression

Combination of geosynthetic and soil ensures an excellentbond and form a composite material.

Geosynthetic has three main reinforcement mechanisms:

Lateral restrain

Bearing capacity

Tension Membrane


Lateral restrain: Mobilization of friction and/or interlockingbetween base or subgrade course and geosynthetic


Bearing capacity: Due to geosynthetic, failure zonepossesses higher volume and hence, increase in bearingcapacity.


Tension Membrane: Development of tension in geosyntheticdue to wheel loads resulted in vertical thrust.


Problem of Unpaved road


Unpaved Roads


Applications:

Temporary roadways

Haul roads

Access roads

Permanent roads

Airport taxiways

Railway tracks

Parking lotsProf. J. N. Mandal, Department of Civil Engineering, IIT Bombay

Type of Geosynthetics used in pavement:

Woven and non-woven geotextile

Extruded geogrid (PP, PE)

Welded geogrid (PET, PP, PE)

Knitted or woven geogrid (PET, PP)

Bonded geogrid-geotextile composite (GG-GT composite)

Unbonded geogrid-geotextile composite (GG is laid over GT)Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay

Type of subgrades

1. Firm subgrade:CBR ≥ 8Shear strength ≥ 240 kPaResilient modulus > 80 MPa

2. Moderate subgrade:3 ≤ CBR ≤ 8Shear strength lies between 90 kPa and 240 kPaResilient modulus lies between 30 MPa and 80 Mpa

3. Low subgrade:CBR ≤ 3Shear strength ≤ 90 kPaResilient modulus < 30 MPa


Design of Geosynthetics reinforced pavement requires anoverall knowledge of the following areas.

Unreinforced pavement design Reinforced pavement design

Life-cycle cost analysis

Value added cost benefit

Specification

Product- specific specification or Generic material

property specification

20%-50% subbase or base course thickness reduction

(Maximum 75-125 mm is used)Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay

Design of unpaved roads

Giroud and Noiray (1981) developed an analytical methodfor designing the geotextile reinforced unpaved road on softsub-grade soil. The following assumptions are taken intoconsideration:

Soil sub-grade is saturated fine-grained clay and silt. Soil sub-grade is under undrained condition. Shear strength of the soil sub-grade is represented by

cohesion. The bearing pressure can be transferred to the elastic

limit range for unreinforced soil. The bearing pressure can be transferred to the plastic

limit range for reinforced soil. The wheel pressure transfers like a pyramidal shape.


Without geotextile With geotextile

Wheel load distribution without geotextile (left) and with geotextile (right) (Giroud and Noiray, 1981)


000000ec hptanh2Ltanh2BLBp

hptanh2Ltanh2BLBpec

Without geotextile:

With geotextile:h = aggregate thickness with geotextile, = angle of load distribution with geotextile (assumed equal to 0), andp = stress on the soil sub-grade with geotextile,

h0 = aggregate thickness without geotextile,0 = angle of load distribution = (45 - /2), or =26°,po = stress on the soil sub-grade without geotextile,γ = unit weight of stone aggregate,

Equivalent tire contact pressure:

LB2Pp ec P = Axle load, Wheel load = P/2

B x L = contact area of the wheel


From the previous equations, the following equations canbe written,

Stress on the soil sub-grade with geotextile (p)

Stress on the soil sub-grade without geotextile (po)

h)tanh2L)(tanh2B(2

Pp

00000

0 h)tanh2L)(tanh2B(2

Pp


When there is no geotextile, stress on the sub-grade soil(p0) should not exceed the elastic bearing capacity of thesub-grade soil (qe). So we can write,

00000

0 h)tanh2L)(tanh2B(2

Pp

u0 0 0 0

Pc2 B 2 h tan L 2 h tan

0u0e hcpq

cu = undrained shear strength of the sub-grade

Unreinforced roads:


uc 0 0 c 0 0

Pc2 P/p 2 h tan P/2p 2 h tan

u

c 0 0 c 0 0

Pc2 (P 2)/p 2 h tan P/( 2p ) 2 h tan

For on-highway vehicles,

For off-highway vehicles,

BL = 2

PB = Pc

Again, For on highway vehicles:

BL = 2

P 2B = Pc

For off highway vehicles:


If we know undrained shear strength of the sub-grade (cu),axle load (P), tire pressure (pc) and angle of load distribution(αo) = 26°, the required theoretical thickness of granular layerfill (h0) can be determined.

If the CBR value is known, cu = 30 x CBR% kN/m2


Please let us hear from you

Any question?


Prof. J. N. Mandal

Department of civil engineering, IIT Bombay, Powai , Mumbai 400076, India. Tel.022-25767328email: [email protected]


geosynthetics engineering: in theory and practice · 2017-08-04 · substantially reduce the...

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