CE6601 DESIGN OF REINFORCED CONCRETE & BRICK MASONRY STRUCTURES

1. CE6601 DESIGN OF REINFORCED CONCRETE & BRICK MASONRY STRUCTURES Prepared by, Mr. S. JEYA UGESH, Asst.Prof, CIVIL DEPT, MSEC – KILAKARAI.

2. UNIT I RETAINING WALLS

3. RETAININGWALL • A Retaining wall is a structure used to retain the earth or other materials and to maintain ground surface at different elevations on either side ofit. 2

4. RETAINING WALLTERMINOLOGY 3

5. TYPES OF RETAININGWALLS • Gravity retainingwalls. • Cantilever retainingwalls. • Counterfort retainingwalls. • Buttress wall. • Bridge abutment. • Box culvert. 4

6. Gravity retainingwall Cantilevered retaining wall Precastconcrete retainingwall Precast concrete crib retainingwall Sheet pilewall 5

7. COUNTERFORT RETAININGWALL • In the counterfort retaining wall, the stem and the base and the base slab are tied together by counterforts, at suitable intervals. Because of provision of counterforts, the vertical stem as well as the heel slab acts as a continuous slab, in contrast to the cantilevers of cantilever retainingwall. • Counterfort retaining walls are economical for height overabout 6m. 6

8. FORCES ACTING ON RETAININGWALL • Lateral earth pressure • Self weight of retainingwall • Weight of soil above the base slab • Surcharge, i.e. forces due to loads on earthsurface. • Soil reaction below baseslab • Frictional force at the bottom of baseslab 7

9. Earth Pressure(P) • Earth pressure is the pressure exerted by the retaining material on the retaining wall. This pressure tends to deflect the walloutward. • Types of earth pressure: • Active earth pressure or earth pressure (Pa) and • Passive earth pressure(Pp). • Active earth pressure tends to deflect the wall away from thebackfill. 8

10. Factors affecting earthpressure • Earth pressure depends on type of backfill,the • height of wall and the soilconditions • Soil conditions: The different soil conditionsare • Dry or moist backfill with nosurcharge • Submergedbackfill • Backfill with uniformsurcharge • Backfill with slopingsurface • Inclined back andsurcharge 9

11. (1)Analysis for dry back fills with no surcharge • Maximumpressure at any height, p=kaϒh • Total pressure at any height fromtop, 1 𝑃 = ϒ𝑘 𝐻2 𝑎 𝑎 2 10

12. 𝑘𝑎= Coefficient of active earthpressure • =(1-sinΦ)/(1+sinΦ)=𝑡𝑎𝑛2Φ • = 1/𝑘𝑝, coefficient of passive earth pressure • = Angle of internal friction or angle of repose • ϒ=Unit weight or density ofbackfill 11

13. (2) Submergedbackfill • Lateral pressure due to submerged weight ofsoil • = 𝑘𝑎ϒH • Lateral pressure duetowater, =ϒ𝑤H • Total pressure atbase, • 𝑃𝑎 =𝑘𝑎ϒH+ϒ𝑤H 12

14. (3)Backfill with uniformsurcharge • The lateral pressure due to surcharge, • =𝑘𝑎q • The lateral pressure due to backfill, • =𝑘𝑎ϒH • Lateral pressure intensity atbase, • 𝑃𝑎=𝑘𝑎q+𝑘𝑎ϒH 13

15. (4) Backfill with slopingsurface • The total earth pressure acts at an angle β with horizontal. 𝑘 cosβ𝑐𝑜𝑠𝛽− 𝑐𝑜𝑠2𝛽−𝑐𝑜𝑠2∅ 𝑐𝑜𝑠𝛽+ 𝑐𝑜𝑠2𝛽−𝑐𝑜𝑠2∅ 𝑎= • β=angle ofsurcharge • If surcharge is horizontal,β=0 1−sin∅ Therefore, 𝑘𝑎= 1+sin∅ 14

16. (5) Inclined back andsurcharge • Resultant ofpressure • 𝑃1 and weight of soil wedge W iscalculated as • P= 𝑃12 +𝑊2 𝐻2 where 𝑃 1 ϒ𝑘 1= 𝑎 2 15

17. StabilityConditions • The retaining wall should satisfy the following stabilitycondition- • •Stability against overturning • •Stability againstsliding • •Maximum pressure at base should not exceed safe bearing capacity of soil. 16

18. Stability againstoverturning • As per IS:456-2000, CI.20.1, factor of safety against overturning should not be less than 1.4.In case where dead load provides the restoring moment, only 0.9 times the characteristic dead load shall be considered. • Hence ,the factor of safety is given byrelation, 0.9 𝑟𝑒𝑠𝑡𝑜𝑟𝑖𝑛𝑔𝑚𝑜𝑚𝑒𝑛𝑡 F.S= ≥1.4 𝑜𝑣𝑒𝑟𝑡𝑢𝑟𝑛𝑖𝑛𝑔𝑚𝑜𝑚𝑒𝑛𝑡 𝑀𝑟 = ≥1.55 𝑀𝑜 17

19. Stability againstsliding • As per IS:456-2000, CI.20.2, factor of safety against sliding should not be less than 1.4.In this casealso • 0.9 times characteristic dead load shall be taken intoaccount. 0.9 𝑟𝑒𝑠𝑡𝑜𝑟𝑖𝑛𝑔𝑓𝑜𝑟𝑐𝑒 F.S= ≥1.4 𝑠𝑙𝑖𝑑𝑖𝑛𝑔𝑓𝑜𝑟𝑐𝑒 0.9(𝜇∙∑𝑊) = ≥1.4 𝑃𝑎ℎ (𝜇∙∑𝑊+𝑃𝑝) = ≥1.55 𝑃𝑎ℎ Where,µ=coefficient offriction 18

20. Maximum pressure at base should not exceed safe bearing capacity ofsoil. • The intensity of soil pressure attoe, 𝑃 ∑𝑊 [1 + 6𝑒]……….attoe max= 𝑏 𝑏 𝑃 ∑𝑊 [1 − 6𝑒]………..atheel 𝑏 𝑏 𝑚𝑖𝑛= • 𝑝𝑚𝑎𝑥 should not exceed safe bearing capacity(SBC) ofsoil. • For no tension, 𝑝𝑚𝑖𝑛 should not benegative. 19