CONCEPT OF REINFORCED EARTH STRUCTURE DESIGN

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Reinforced earth is a composite material formed by the friction between the earth and the reinforcement. By means of friction the soil transfers to the reinforcement the forces built up in the earth mass. The reinforcement thus develops tension and the earth behaves as if it has cohesion. COMPONENTS OF REINFORCED EARTH

• SOIL
• SKIN
• REINFORCEMENT

SOIL FOR REINFORCED EARTH

• Development of sufficient friction between earth and reinforcement.
  • No interstitial pore water pressure develops within the reinforced earth structures
  • The placing and compaction of the earth fill layers can be accomplished easily
  • The soil must conform to certain electro-chemical conditions to avoid corrosion

REINFORCEMENT FOR REINFORCED EARTH

• Reinforced members are composed of thin wide strips also called ties.
• Should be flexible to ease placement.
• Should have adequate tensile strengths.
• Should have adequate service life taking in to account corrosion and weathering.

FACING ELEMENTS

• Should retain the back fill between the layers of reinforcements.
• Made of either metal units or pre-cast concrete panels.
• Should be able to deform without distortion.

DESIGN COMPONENTS

• SKIN
• TIES
• FOUNDATION – Bearing capacity & sliding
• SETTLEMENT
• OVERTURNING
• Drainage

SKIN DESIGN:

• Calculate the lateral earth pressure.
• Based on the spacing of ties.
• Designed as thin shells.

TIES DESIGN: Two boundary conditions –

• The wall is perfectly flexible & frictionless.
• The wall will move laterally during construction by sufficient amount to mobilize a state of active earth pressure.
• The tensile force increases from zero at the free end to maximum at the face of the wall.
• The tensile force also increases linearly with depth.
• Tension mode of failure.
• Bond mode of failure.
• Factor of safety – 1.5 to 2.5.

FOUNDATION DESIGN:

• Resistance to Bearing Capacity Failure: Bearing capacity of foundation soil must withstand the substantial settlement without damage to the structure.
• Sliding at the base of the structure: Active earth pressure acts on the face wall and the friction at the bottom. This can be improved by help of passive earth pressure.
• Overturning: Tilting of the upper portion of the wall may occur, if the reinforcing strips are not long enough.

SETTLEMENT DESIGN:

• Not much design issues involved.
• Limiting values depend upon usage.
• Calculations based on normal settlement of any structures.
• Settlements within the reinforced earth mass itself and settlement of the foundation soil.

DRAINAGE DESIGN:

• Aim to avoid interstitial pore pressure.
• Choose backfill soil with good drainage characteristics.
• Provide boulder/sand column drain in case the R.E. is supporting embankment slope.

MATERIAL SELECTION:

1. 1. SOIL

• Must be able to develop friction.
• The backfill should be from non-organic soils such as sand and Gravel which are not affected by biological activity.
• Have good drainage.
• Primarily cohesion less soils are used.

1. 2. FACING ELEMENTS

• Can be chosen from metal and concrete.
• Based on durability and stability criteria.
• Aesthetics can be equally important when used in urban areas.

1. 3. REINFORCING STRIPS

• Based on electro-chemical properties of back fill.
• Can be chosen from metal, geo-synthetics etc based on durability criteria.
• Select fixtures accordingly used to connect strips to the facing elements.