CE 6601 DESIGN OF REINFORCED CONCRETE BRICK MASONRY

CE 6601 DESIGN OF REINFORCED CONCRETE & BRICK MASONRY STRUCTURES Prepared by, Mr. S. JEYA UGESH, Asst. Prof, CIVIL DEPT, MSEC – KILAKARAI.

UNIT I RETAINING WALLS

RETAINING WALL • 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 of it. 2

RETAINING WALL TERMINOLOGY 3

TYPES OF RETAINING WALLS • • • Gravity retaining walls. Cantilever retaining walls. Counterfort retaining walls. Buttress wall. Bridge abutment. Box culvert. 4

Gravity retaining wall Cantilevered retaining wall Precast concrete crib retaining wall Sheet pile wall 5

COUNTERFORT RETAINING WALL • In the counterfort retaining wall, the stem 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 retaining wall. • Counterfort retaining walls are economical for height over about 6 m. 6

FORCES ACTING ON RETAINING WALL • Lateral earth pressure • Self weight of retaining wall • Weight of soil above the base slab • Surcharge, i. e. forces due to loads on earth surface. • Soil reaction below base slab • Frictional force at the bottom of base slab 7

Earth Pressure (P) Earth pressure is the pressure exerted by the retaining material on the retaining wall. This pressure tends to deflect the wall outward. • 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 the backfill. • 8

Factors affecting earth pressure • Earth pressure depends on type of backfill, the • height of wall and the soil conditions • Soil conditions: The different soil conditions are • Dry or moist backfill with no surcharge • Submerged backfill • Backfill with uniform surcharge • Backfill with sloping surface • Inclined back and surcharge 9

(1)Analysis for dry back fills with no surcharge • Maximum pressure at any height, p=kaϒh • Total pressure at any height from top, 1 2 �� �� = ϒ�� �� �� 2 10

• �� = Coefficient of active earth pressure �� 2Φ = (1 -sinΦ)/(1+sinΦ)=������ = 1/�� , coefficient of passive earth �� pressure = Angle of internal friction or angle of repose ϒ=Unit weight or density of backfill 11

(2) Submerged backfill • Lateral pressure due to submerged weight of soil = �� ϒH �� • Lateral pressure due to water, =ϒ�� H • Total pressure at base, �� = �� ϒH+ϒ�� H �� �� 12

(3)Backfill with uniform surcharge • The lateral pressure due to surcharge, =�� q �� • The lateral pressure due to backfill, =�� ϒH �� • Lateral pressure intensity at base, �� q+�� ϒH �� =�� �� �� 13

(4) Backfill with sloping surface • The total earth pressure acts at an angle β with horizontal. �� cosβ �� = 2 �� 2∅ ���� − ������ −������ 2∅ ���� + ������ −������ • β=angle of surcharge • If surcharge is horizontal, β=0 1−sin ∅ Therefore, �� �� = 1+sin ∅ 14

(5) Inclined back and surcharge • Resultant of pressure �� 1 and weight of soil wedge W is calculated as 2 2 P= �� + �� 1 where 1 �� 1= 2 2 �� ϒ�� �� 15

Stability Conditions • The retaining wall should satisfy the following stability condition • Stability against overturning • Stability against sliding • Maximum pressure at base should not exceed safe bearing capacity of soil. 16

Stability against overturning • 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 by relation, 0. 9 ��������� F. S= ≥ 1. 4 ����������� �� �� = ��≥ 1. 55 �� 17

Stability against sliding • As per IS: 456 -2000, CI. 20. 2, factor of safety against sliding should not be less than 1. 4. In this case also 0. 9 times characteristic dead load shall be taken into account. 0. 9 ��������� F. S= ≥ 1. 4 ������� 0. 9(�∙∑�� � =) ≥ 1. 4 ��� � ℎ = (�� ∙∑�� +�� ) �� ≥ 1. 55 ��� � ℎ Where, µ=coefficient of friction 18

Maximum pressure at base should not exceed safe bearing capacity of soil. • The intensity of soil pressure at toe, ∑�� 6�� �� [1 + ]………. at toe max= � � �� ∑�� 6�� [1 − ]………. . at heel �� �� �� = • �� should not exceed safe bearing ������ capacity(SBC) of soil. • For no tension, �� should not be negative. ������ 19