lec 10 reinforced earth in roads

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LECTURE-10GEOTEXTILE AND REINFORCED EARTH FOR ROADS

Prof. SAMIRSINH.P.PARMARDEPARTMENT OF CIVIL ENGINEERINGDHARMASINH DESAI UNIVERSITY, NADIAD, GUJARAT, INDIAMail: samirddu@gmail.com

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Design Philosophy

• Divided in Two Parts1. Unpaved Roads2. Paved Roads

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Unpaved roads

• Some of the manufacturers have developed their own unpaved road design charts for use with their particular geosynthetic.

• A design method based on the specific, well-defined geosynthetic property, such as geosynthetic modulus, is generally acceptable by all.

• Such a design method is described as a reinforcement function design method. (RFDM)

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Reinforcement function design method (RFDM)

• This method evaluates the risk of failure of the foundation soil and of the geotextile.

• The geotextile is considered to function as only reinforcement.

• The failure of the granular layer is not considered; thus it is assumed that:

1. The friction coefficient of the granular layer is large enough to ensure the mechanical stability of the layer.

2. The friction angle of the geotextile in contact with the granular layer under the wheels is large enough to prevent the sliding of the granular layer on the geotextile.

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It is also assumed that…..

1. Thickness of the granular layer is not significantly affected by the subgrade soil deflection.

2. The granular layer provides a pyramidal distribution with depth of the equivalent tyre contact pressure, pec, applied on its surface

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Load diffusion model

equivalent tyre contact pressure =

Where: L and B are the length dimensions of the equivalent rectangular tyre contact area.

h0 is the thickness of granular layer in the absence of geotextile h is the thickness of granular layer in the presence of geotextile α0 is the load diffusion angle in the absence of geotextile α is the load diffusion angle in the presence of geotextile p0 is the pressure at the base of the granular layer in the absence of geotextile p is the pressure at the base of the granular layer in the presence of geotextile

eq-1

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In presence of Geotextile, Effective tyre contact pressure =

The equivalent tire contact pressure is given as

where, P is the axle load.

--- eq-2

--- eq-3

From Equations (1), (2), and (3), the following equations are obtained:

In the absence of geotextile

In the presence of geotextile

--- eq-4

--- eq-5

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Behaviour after load transmission

• On the application of the wheel load, the geotextile exhibits a wavy shape; consequently, it is stretched.

• the volume of soil subgrade displaced downwards by settlement must be equal to the volume of soil displaced upwards by heave.

• which may be called volume conservation of the undrained soil subgrade.

• In the stretched position of the geotextile, the pressure against its concave face is higher than the pressure against its convex face.

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Behaviour after load transmission

• This reinforcing mechanism is known as the membrane effect of the geotextile,

• which provides the following two beneficial effects:

1.Confinement of the soil subgrade between and beyond the wheels;

2. Reduction of the pressure applied by the wheels on the soil subgrade.

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kinematics of subgrade deformation

The pressure applied on the subgrade soil by the portion AB of the geotextile is

Where: pg is the reduction of pressure resulting from the use of a geotextile.

The pressure reduction, pg, is a function of the mobilized tension in the geotextile, which depends on its elongation; thus its deflected shape is significant.

--- eq-6

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Calculation for Bearing Pressure

Since the soil subgrade confinement provided by the geotextile helps in keeping the deflection small for all applied pressures less than the ultimate load-bearing capacity, qu, of the soil subgrade, as given by Equation (7) below, the pressure p* can be as large as qu.

--- eq-7

Where: cu is the undrained cohesion or shear strength of the soil subgrade.

--- eq-8

From Equations (6) and (8), one gets

--- eq-9

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Calculation for Bearing Pressure

which is applicable in the absence of geotextile

In order to avoid large deflection under the wheel… Thus,

In the absence of the geotextile --- eq-10

--- eq-11

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shape of the deformed geotextile

The shape of the deformed geotextile is assumed to consist of portions of parabolas connected at A and B, points located on the initial plane of the geotextile

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Calculation for Bearing Pressurepg is a uniform pressure applied on AB and is equivalent to the vertical projection of the tension T of the geotextile at points A and B:

--- eq-12

According to the property of parabolas: --- eq-13

From the definition of secant modulus, E (in N/m), of the geotextile, one gets:

--- eq-13

where, ε is the per cent elongation

Combining Equations (12), (13), and (14), one gets

--- eq-14

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Calculation for Bearing PressureEquations (5), (9), and (14) lead to

which is applicable in the presence of geotextile.

--- eq-15

--- eq-16

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Calculation for Layer Thickness

Solving Equation (11) for h0, and Equation (15) for h allows us to determine thereduction of granular layer thickness, h, due to reinforcement function of geotextile as per quasi-static analyses. Thus,

--- eq-17

A further assumption is that the value of h remains unchanged under repeated traffic loading, thus allowing it to uncouple the reinforcement effect and its analysis from the cyclic nature of loading. Therefore,

--- eq-18

Where:h is the required granular layer thickness of the unpaved road in the presence of the geotextile and under traffic loading, h0 is the required granular layer thickness of the unpaved road in the absence of the geotextile and under traffic loading

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Calculation for Layer Thickness

where Ns is the number of passages of standard axle with a load Ps 80 kN; and CBR is the California Bearing Ratio of soil subgrade.

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Calculation for Layer Thickness

This formula is based on extrapolation and therefore, it should not be used when the number of passages exceeds 10,000.

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Design chart for the geotextile-reinforced unpaved road related to on-highway truck with standard axle load (after Giroud and Noiray, 1981)

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The following two features of this chart are noteworthy:

1 Δh can never be higher than h0.2 No granular layer is needed on top of the geotextile when Δh versus cu

curve is above the h0 versus cu curve.

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Separation function design method (SFDM)

• Steward et al. (1977) presented a design method for the geosynthetic-reinforced unpaved roads, considering mainly the separation function of the geosynthetic.

• This design method is based on theoretical analysis and empirical (laboratory and full-scale field) tests.

• It allows the designer to consider vehicle passes, equivalent axle loads, axle configurations, tyre pressures, soil subgrade strengths and rut depths.

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Limitations of Method

1 The granular layer must be (a) cohesionless (non-plastic), and (b) compacted to CBR 80.

2 Vehicle passes less than 10,000.3 Geotextile survivability criteria must be considered.4 Soil subgrade undrained shear strength less than about

90 kPa (CBR < 3).

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ConceptSteward et al. presented design charts to determine the required thickness of the granular layer

ultimate bearing capacity, qu

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Chart for Single Wheel Load

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Chart for Dual Wheel Load

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Tandem wheel load

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Bearing capacity factors for different ruts and traffic conditions both with and without geotextile

separators

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Richardson (1997a) presented a simple separation function design method (SFDM) for geosynthetic-reinforced unpaved roads,

described in the following steps:

• Step 1: Use a granular layer thickness that produces a subgrade pressure p 4cu. This results in sufficient granular material being placed initially to fill the ruts that will develop as the geotextile/granular layer deforms.

• Step 2: Determine the geotextiles’s minimum survivability requirements.

• Step 3: Determine the geotextile’s minimum hydraulic requirements.

• Step 4: Select a suitable geotextile that meets the criteria in steps 2 and 3. Almost any woven and nonwoven geotextile can be used if it meets the requirements in steps 2 and 3.

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Modified CBR design method

• This method uses a multiplier to the in situ CBR of the soil subgrade, in order to get an equivalent CBR when using a geosynthetic layer at the interface of soft soil subgrade and the granular fill layer.

• The multiplier is assumed to be equal to the reinforcement ratio, which is the ratio of loads at the specified deflection, as determined from load versus deflection test in the CBR mould both with and without geosynthetic at the interface of soft soil subgrade and granular fill layer.

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Modified CBR design method

• It has been observed that the reinforcement ratio increases as both the deflection and the water content in soil subgrade increase.

• The thickness of the granular fill layer is calculated using the following equation for both the cases, without and with geosynthetic, taking in situ CBR, and modified CBR, respectively at the specified deflection

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where, h is the design thickness of the granular layer in inches;C is the anticipated number of vehicle passes;P is the single or equivalent single wheel loads in pounds; and A is the tyre contact area in square inches.

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PAVED ROAD

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Geosynthetic layer at the subgrade level

• Ruts with depth in excess of approximately 25 mm are generally not acceptable in paved Roadways.

• The paved roadways with geosynthetic layers are usually designed for structural support using normal pavement design methods.

• As described by various agencies (AASHTO, 1993; IRC: 37-2001; IRC: 58-2002), without providing any allowance for the geosynthetic layers.

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Geosynthetic layer at the subgrade level

• In the presence of a geosynthetic layer, especially a nonwoven geotextile, at the interface of granular subbase/base layer and the soil subgrade, the required additional granular layer thickness can be reduced by approximately 50% keeping the project cost effective (Holtz et al., 1997).

• Savings of granular material can also be made by placing a geosynthetic layer in the granular stabilizer lift that can tolerate even 75 mm of rutting under construction equipments.

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Effect of water on soil subgrade at asphalt overlay

• Water coming from rain, surface drainage or irrigation near pavements, if allowed to infiltrate into the base and subgrade, can cause pavement deterioration by following processes:

• Softening the soil subgrade• Mobilizing the soil subgrade into the road base stone,

especially if a separation/filtration geosynthetic is not used at the road base and subgrade interface

• Hydraulically breaking down the base structures, including stripping bitumen-treated bases and breaking down chemically stabilized bases

• Freeze/thaw cycles.

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Geosynthetic layer at the asphalt overlay base level

• Prior to laying the paving fabric, the tack coat should be applied uniformly to the prepared dry pavement surface at the rate governed by the following equation (IRC: SP:59-2002):

where :• Qd is the design tack coat quantity (kg/m2);• Qs is the saturation content of the geotextile being used

(kg/m2) to be provided by the manufacturer; and • Qc is the correction based on tack coat demand of the

existing pavement surface (kg/m2).

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Importance of tack coat

• The quantity of tack coat is critical to the final membrane system.

• Too much tack coat between the fabric and the new overlay resulting in a potential sliding failure surface and potential bleeding problems,

• While too little will fail to complete the bond and create the impermeable membrane.

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Tack coat quantity

• For typical paving fabrics in the 120–135 g/m2 mass per unit area range,

• most manufacturers recommend fabric–bitumen absorption of about 900 g/m2, or application rates of about 1125 g/m2.

• For the full waterproofing and stress-relieving benefits, the paving fabric must absorb at least 725 g/m2 of bitumen.

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Laboratory-prepared paving fabric tests, demonstrating the permeability’s sensitivity tothe amount of tack coat on the paving fabric (after Marienfeld and Baker, 1998).

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THANKS

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