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  • ssm institute of engineering and technology - ssmiet
  • Civil Engineering
  • B.Tech
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Karthik M P
Karthik M P
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30-12-2016 Backfill & Surcharge Gravity Wall Types of retaining walls: • The material retained or supported by a retaining wall is called backfill. • Backfill may have its top surface horizontal or inclined. • The position of the backfill lying above the horizontal plane at the elevation of top of wall is called surcharge & its inclination to the horizontal is called as Surcharge angle. • The “gravity wall” resist the earth pressure used by backfill by • Gravity Retaining Wall its own self weight (dead load) . • It is usually built in stone masonry, and occasionally in plain • Semi-Gravity Retaining Wall concrete. • The thickness of wall is also governed by need to eliminate or • Cantilever Retaining Wall limit the resulting tensile stress to its permissible limit . • Plain concrete gravity walls are not used for heights exceeding • Counter fort Retaining Wall about 3m, for obvious economic reasons Stress developed is very low. 7 8 Cantilever Wall Semi Gravity Retaining Walls The “Cantilever wall” is the most common type of retaining structure and is generally economical for heights up to about 5m. Semi-gravity walls resist external forces by the combined action of self weight, weight of soil The structure consists of vertical stem, and a base slab, made up of two distinct regions, viz., a heel slab and a toe slab. above footing and the flexural resistance of the wall components. 10 9 11 12 2

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30-12-2016 • “Stem” acts as a vertical cantilever under the lateral earth pressure • “Heel slab” acts as a horizontal cantilever under the action of weight of the retained earth (min soil pressure acting upwards from below) • “Toe slab” acts as a cantilever under the action of resulting soil pressure acting upward. 13 14 • Counterfort wall are placed at regular intervals of about 1/3 to ½ of the wall height, interconnecting the stem with the heel slab Counterfort Retaining Wall • For large heights, in a cantilever retaining wall, the bending moments developed in the stem, heel slab and toe slab become very large and require large thickness. • The counterforts are covered within the retained earth on the rear side of the wall. • Can be constructed of reinforced or pre-stressed concrete and Suitable above 5m. • The bending moments can be considerably reduced by introducing transverse supports, called counterfort. • The counterforts subdivide the vertical slab (stem) into rectangular panels and support them on two sides(suspender-style), and themselves behave essentially as vertical cantilever beams of T-section and varying depth. 16 17 15 Alternatives To Retaining Walls CELLULAR CONFINEMENT • Cellular confinement systems have become increasingly popular for earth retention applications. • They can be constructed as a gravity wall or a "geogrid" wall which consists of vertical layers of geocells with geogrid reinforcement installed behind the face of the wall every few layers of the geocell depending on design. 18 3

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30-12-2016 • • Cellular confinement systems (geocells) are also used for steep earth stabilization in gravity and reinforced retaining walls with geogrids. • Geocell retaining walls are structurally stable under self- weight and externally imposed loads, while the flexibility of the structure offers very high seismic resistance. • The outer fascia cells of the wall can be planted with vegetation to create a green wall. Soil nailing is a technique in which soil slopes, excavations or retaining walls are reinforced by the insertion of relatively slender elements - normally steel reinforcing bars. • The bars are usually installed into a pre-drilled hole and then grouted into place or drilled and grouted simultaneously. • They are usually installed untensioned at a slight downward inclination. • A rigid or flexible facing (often sprayed concrete) or isolated soil nail heads may be used at the surface. 19 Forces acting on the retaining wall: wall: Buttress Wall 20 Lateral Earth Pressure • Vertical forces: Acting downwards: Self weight of the retaining wall ; Weight of soil above heel slab. 1. Earth Pressure at rest 2. Active Earth Pressure 3. Passive Earth Pressure Acting upwards: Force due to soil pressure underneath the base slab. 22 21 Active Earth Pressure • In 1929, Terzaghi conducted experiments on the retaining wall and showed the relation of pressure on the wall if wall changes its position i.e., to move inwards to the backfill, outwards of it or remain at its place. There are three types of earth pressures on the basis of the movement of the wall. • Lateral forces: Earth pressure due to backfill and surcharge. • It is similar to counterfort wall, except that the transverse stem supports Called buttress, are located in the front side, interconnecting the stem with the toe slab (and not with heel slab, as with counterforts) • Although the buttresses are structurally more efficient (and more economical) counterforts, the counterfort wall is generally preferred to the buttress wall as it provides free usable space (and better aesthetics) in front of the wall. When the wall moves away from the backfill, there is a decrease in the pressure on the wall and this decrease continues until a minimum value is reach after which their is no reduction in the pressure and the value will become constant. This kind of pressure is known as active earth pressure. 23 24 4

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30-12-2016 Active Earth Pressure Passive Earth Pressure Passive Earth Pressure When the wall moves towards the backfill, there is an increase in the pressure on the wall and this increase continues until a maximum value is reach after which their is no increase in the pressure and the value will become constant. This kind of pressure is known as passive earth pressure. 25 Rankine's Theory 26 Cohesion less Backfills Assumptions • The soil mass is semi-infinite, homogeneous, dry and cohesion less. • The ground surface is plane which may be horizontal or inclined. • The back of the wall is vertical and • The wall yields about the base and satisfies the deformation conditions for plastic equilibrium. 28 27 Dry or moist backfill with no surcharge • Dry or moist backfill with no surcharge • Submerged backfill • Backfill with uniform surcharge • Backfill with sloping surfaces 29 30 5

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