Unit 1

CE8703 STRUCTURAL DESIGN AND DRAWING UNIT I RETAINING WALLS Reinforced concrete Cantilever and

Counter

fort Retaining Walls –Horizontal Backfill with Surcharge–Design of Shear Key-Design and Drawing.  

RETAINING WALL •The retaining walls are structures used to provide stability of earth masses (or) other loose materials

•when field condition do not allow the earth masses to assume its natural slope(or)shape (or)when abrupt changes in the ground surface elevation are needed.

APPLICATION OF RETAINING WALL

• • • • • • • •

Retaining a rail-road (or) a highway in a hilly areas. For underground water tanks. Depressed roads Elevated roads Erosion protection Flood wall Box culvert Basement wall

The retaining wall are usually made up of • stone masonry, • mass concrete • reinforced cement concrete.

TYPES OF RETAINING WALL • • • • •

Gravity Retaining wall Cantilever Retaining wall Counterfort retaining wall. Buttress wall Retaining Wall Bridge abutment

GRAVITY RETAINING WALL

GRAVITY RETAINING WALL • Gravity Retaining wall retains earth (or) other materials entirely by its own weight and generally contains no reinforcement. Gravitational walls are bulky and made of bricks, stone or concrete masonry. • Gravity walls are generally trapezoidal in section. A mild taper is given on the earth side to improve the stability. A key in the stem & base are provided in order to resist the shear.

GRAVITY RETAINING WALL • Gravity wall are recommended when a high degree of performance under unfavourable climatic environment exists. • A semi-gravity walls is also in use which is slender and the wall are provided with since reinforcement in the face of wall contact with the soil. • The exposed face is provided with Nominal Reinforcement to prevent the surface Cracking.

CANTILEVER RETAINING WALL

CANTILEVER RETAINING WALL • Cantilever Retaining wall is a Reinforced concrete wall which consists of vertical arm that retains the earth and is held in position by a footing base slab. • Here the weight of the fill material on the top of the heel, in addition to weight of wall contributes to the stability of the structure. • The thin stem and base slab are fully reinforced to resist the moment and shear.

CANTILEVER RETAINING WALL • For a clean sand (or) gravel the Ratio of Base(B)/Height(H) is less. In case of weak foundation soil, backfill sloping (or) backfill is of cohesive material, then B/H Ratio should be adopted higher. • The base Width should be adequate such that the resultant force should fall within of base. • The Cantilever Retaining wall is usually adopted to height of 6m.

COUNTERFORT RETAINING WALL

COUNTERFORT RETAINING WALL • In the Wall greater than 6m height bending moment in the vertical wall occurs, which is very high. • Inorder to reduce the bending moment, wall perpendicular the stem are provided at a specific spacing which are called as Counterforts. • These Counterforts are placed in the backfill side and space between counterforts are filled with backfills and thus it is subjected to tensile forces.

COUNTERFORT RETAINING WALL • The counterforts are not provided at the ends. The unsupported end part is 0.41L generally, where L is the spacing between the counterforts. • When the counterforts are placed on the exposed face of the wall and not on the backfill side it is called Buttressed Retaining wall. Buttress Reduce the clearance in front of the wall. • In this case the counterforts are subjected to compression.

BUTTRESS WALL RETAINING WALL

Transverse stem support provided on front side

PARTS OF A BRIDGE

BRIDGE ABUTMENTS

PARTS OF A RETAINING WALL

VARIOUS FORCES SUBJECTED TO A CANTILEVER RETAINING WALL • Earth pressure due to backfill • Vertical forces including weight of soil, stem, heel, toe, and soil fill above the toe. • The soil pressure developed to resist the earth pressure and other vertical forces acting on the heel and toe.

LATERAL PRESSURE • If the Retaining wall is built in contact with solid surface such as rock face, No pressure is exerted. • Instead if wall is built to retain water (or) oil (or) any other liquid Retaining structure, then Hydrostatic Pressure is exerted on a wall in case of liquid, (or)

• earth pressure in case of soil Retaining wall.

LATERAL EARTH PRESSURE • If a Vertical Retaining wall retains soil the later earth pressure Ph increases proportionally with the depth whose magnitude is,

Ph=k0 γh • Where γ = unit Weight of soil N/m3 • h = height from the top. • k0= Coefficient of earth pressure at rest. In General, • For Cohesive Soils => K0 = 0.7 to 1.0 • For Non- Cohesive Soils => K0 = 0.4 to 0.8

ACTIVE EARTH PRESSURE

• If the Retaining wall moves away from the backfill a sliding plane forms in the soil mass. • The inclination of the sliding plane is [45o + ɸ/2], Where ɸ=angle of internal friction. And the pressure exerted on the wall is known as ACTIVE EARTH PRESSURE.

PASSIVE EARTH PRESSURE • If the Retaining wall pushed against the backfill a sliding plane is formed and inclined at [45o - ɸ/2] to the horizontal. • The soil wedge is pushed upwards by the wall along the plane.

EARTH PRESSURE FOR COMMON CONDITION OF LOADING There are 4 common conditions of loading, 1. Horizontal surface of fill at the top of the wall 2. Inclined surface of fill sloping up 3. Horizontal surface of fill carrying UDL(surcharge) 4. Backfill is saturated by ground water.

FACTORS TO BE CONSIDERED WHILE DESIGNING THE RETAINING WALLS a)Safety against Overturning b) Safety against Sliding c) The soil on which the wall rests mustn’t be overloaded d) The materials used in construction are not overstressed.

STABILITIES HAS TO BE CHECKED, DESIGNING RETAINING WALLS (a) Stability against over turning

(b) Stability against sliding

(c) Stability of foundation base

SURCHARGE

SLIDING  • The backfill exerts a lateral pressure against the wall. • This sliding force is resisted by the friction between the underlying soil and the footing, and by the passive pressure at the front of the wall. • When more sliding resistance is required, a shear key may be provided. • The factor of safety against sliding equals the resisting force divided by the driving force, and the minimum value should be 1.50.

OVERTURNING  • The overturning moment from the applied forces must be resisted by an opposite moment produced by the vertical forces, including the wall selfweight and the weight of the backfill over the heel. • The factor of safety against overturning is defined as the resisting moment divided by the overturning moment, and the minimum value should be 1.50.

SOIL BEARING • The allowable soil bearing pressure should be provided by the soils report, which already includes a safety factor of about 3.0. • The resultant of the bearing pressure should fall within the middle third to avoid negative soil pressures at the heel.

Wing wall