REINFORCED EARTH STRUCTURES CONCEPT BASICS OF DESIGN CASE STUDIES

REINFORCED EARTH (R.E.) 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 R.E. SOIL SKIN REINFORCEMENT

SOIL FOR R.E. 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 R.E. 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 precast concrete panels. Should be able to deform without distortion.

DESIGN COMPONENTS SKIN TIES FOUNDATION – Bearing capacity & sliding SETTLEMENT OVERTURNING Drainage

DESIGN : SKIN Calculate the lateral earth pressure. Based on the the spacing of ties. Designed as thin shells. ????

DESIGN : TIES 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.

DSIGN : TIES contd 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.

DESIGN : FOUNDATION 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.

DESIGN : FOUNDATION contd Overturning : Tilting of the upper portion of the wall may occur, if the reinforcing strips are not long enough.

DESIGN : SETTLEMENT 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.

DESIGN : DRAINAGE 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: 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.

MATERIAL SELECTION: 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.

MATERIAL SELECTION: 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.

COST COMPARISON Total :Rs100.00 Clearing the jungle :Rs 0.01 Earthwork in excavation :Rs 0.90 CC 1:2:4 for foundation and coping :Rs 10.24 CC 1:3:6 in the retaining wall :Rs 77.42 Surface reinforcement for the concrete retaining wall :Rs 4.20 Ordinary backfill including compaction :Rs 5.93 RCC parapet wall :Rs 1.32 Total :Rs100.00