lec 6 behaviour of re wall

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DESIGN AND BEHAVIOUR OF REINFORCED EARTH WALL

LECTURE -6

Faculty: Prof. Samirsinh.P.ParmarDepartment of Civil EngineeringFaculty of Technology, Dharmasinh Desai University,NadiadMail Add: [email protected]

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DESIGN OF REINFORCED EARTH RETAINING WALLS Divided in three parts: Internal Stability is first addressed to

determine lift thickness, fabric length and overlap.

External Stability against overturning, sliding and foundation failure is verified.

Miscellaneous considerations, including wall facing details.

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BEHAVIOUR OF RE WALL: INTERNAL FAILURE MODE

Internal failure modes of geosynthetic-reinforced soil retaining walls:(a) geosynthetic rupture; (b) geosynthetic pullout; (c) connection (and/or facing elements) failure.

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INTERNAL STABILITY:

It cover internal mechanism ( tension and pull out failure) such as shear within the structure , arrangement and behavior of the reinforcement and backfill. It checks the stability for each reinforcement layers and stability of wedges within the reinforced fill

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BEHAVIOUR OF RE WALL: EXTERNAL FAILURE MODE

External failure modes of geosynthetic-reinforced soil retaining walls:(a) sliding;(b) overturning; (c) load-bearing capacity failure; (d) deep-seated slope failure.

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EXTERNAL STABILITY :

It consider the reinforced structure as whole and check the stability for sliding, overturning, bearing/tilt and slip as shown in

7(a) Geosynthetic-reinforced retaining wall without surcharge and live load

INTERNAL STABILITY :

8(b) Geosynthetic reinforced retaining wall with surcharge and live load

INTERNAL STABILITY :

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LATERAL EARTH PRESSURE DISTRIBUTION

(c) lateral earth pressure distribution.

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DESIGN METHODOLOGIES OF RE WALL

1. Design-by-experience.2. Design-by-cost-and-availability.3. Design-by-specification.4. Design-by-function.

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DESIGN-BY-FUNCTION This method consists of the following steps:a) Assessing the particular application, define the primary

function of the geosynthetic, which can be reinforcement, separation, filtration, drainage, fluid barrier or protection.

b) Make the inventory of loads and constraints imposed by the application.

c) Define the design life of the geosynthetic.d) Calculate, estimate or otherwise determine the required

functional property of the geosynthetic (e.g. strength, permittivity, transmissivity, etc.) for the primary function.

e) Test for or otherwise obtain the allowable property (available property at the end of the design life) of the geosynthetic.

f) Calculate the factor of safety, FS, using Equation

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g) If this factor of safety is not acceptable, check into geosynthetics with more appropriate properties.

h) If acceptable, check if any other function of the geosynthetic is also critical, and repeat the above steps.

i) If several geosynthetics are found to meet the required factor of safety, select the geosynthetic on the basis of cost–benefit ratio, including the value of available experience and product documentation.

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GEOSYNTHETIC FAILURE MECHANISMS

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DESIGN PRINCIPLE

Working stress design approach Limit state design approach - Ultimate limit state - Serviceability limit states

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ULTIMATE LIMIT STATE

Associate with collapse or other similar forms of structure failure.

Margins of safety against attaining limit state of collapse is provided by use of partial material factor and partial load factor.

Disturbing forces are increased by multiplying by prescribed load factor to produce design loads.

Restoring forces are reduced by dividing by prescribed by factor to produce design strength.

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SERVICEABILITY LIMIT STATE

Serviceability limit states are attained if the magnitudes of deformation occurring within the deigns life exceed prescribed limits.

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SERVICE LIFE

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THE DESIGN PROCEDURE FOR GEOSYNTHETIC-REINFORCED RETAINING WALLS WITH WRAPAROUND VERTICAL FACE AND WITHOUT ANY SURCHARGE IS GIVEN IN THE FOLLOWING STEPS: Step 1: Establish wall height (H). Step 2: Determine the properties of granular

backfill soil, such as unit weight (γb) and angle of shearing resistance (Φb)

Step 3: Determine the properties of foundation soil, such as unit weight (γ) and shear strength parameters (c and Φ)

Step 4: Determine the angle of shearing resistance of the soil–geosynthetic interface (Φr)

Step 5: Estimate the Rankine earth pressure coefficient from Equation (5.7).

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DESIGN PROCEDURE CONT....

Step 6: Select a geotextile that has allowable fabric strength of σG.

Step 7: Determine the vertical spacing of the geotextile layers at various levels from Equation (5.9).

Step 8: Determine the length of geotextile layer, l, at various levels from Equation (5.13).

Step 9: Determine the lap length, ll, at any depth z from Equation (5.14).

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Step 10: Check the factors of safety against External stability including sliding, overturning, load-bearing capacity failure and deep-seated

slope failure as carried out for conventional retaining wall designs.

Assuming that the geotextile-reinforced soil mass acts as a rigid body in spite of the fact that it is really quite flexible.

The minimum values of factors of safety against sliding, overturning, load bearing failure and deep-seated failure are generally taken to be 1.5, 2.0, 2.0 and 1.5, respectively.

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Step 11: Check the requirements for backfill drainage and surface runoff control.

Step 12: Check both total and differential settlements of the retaining wall along the wall

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RETAINING WALL BACKFILL (AFTER NCMA, 1997)

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INITIAL SIZING OF STRUCTURES

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DIMENSIONS OF WALL AND ABUTMENTS

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DESIGN BY FUNCTION

Typical allowable (or test) value and required (design) value of a functional property as a function of time: Technical Handbook: Geosynthetic.

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TYPICAL LOAD-SETTLEMENT CURVES FOR A SOIL WITH AND WITHOUT REINFORCEMENT

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Height of the retaining wall, H 8 m For the granular backfill Unit weight, γb = 17 kN/m3 Angle of internal friction, Φb =35 Allowable strength of geotextile, σG = 20

kN/m Factor of safety against geotextile rupture

1.5 Factor of safety against geotextile pullout

1.5 Calculate the length of the geotextile layers,

spacing of layers and lap lengths at depth z 2 m, 4 m, and 8 m.