The structures which are constructed to retain the earth or other loose material which are unable to stand vertically by itself are called as “retaining walls”. They can also be used to support grounds at different levels on both sides of wall. The material retained by wall is called as “backfill”.
On the basis of shape & mode of resisting pressure due to backfill.1. Gravity retaining wall2. Semi-Gravity retaining wall3. Cantilever retaining wall4. Counterfort retaining wall5. Buttress wall
•The walls which resists the earth pressure due to backfill by its ownweight.•They are constructed with stone masonry or plain concrete.
• The size of section of a gravity retaining wall may be reduced if a small amount of reinforcement is provided near the back face. Such walls are known as Semi-gravity retaining wall.
• Cantilever retaining walls are made of reinforced cement concrete.• The wall consist of a thin stem and a base slab cast monolithically.
• Counterfort retaining walls have thin vertical slabs, known asCounterfort, spaced across the vertical stem at regular intervals.• Counterforts tie vertical stem with the base slab.•The purpose of providing Counterfort is to reduce shear force&bending moment in the vertical stem & the base slab.
• It is similar to the Counterfort retaining wall in which Counterfort,called as buttresses are provided on the opposite side of the backfill &act as compression struts.
• Sliding FailureThere are 5 • Overturning Failure types of modes of • Bearing capacity Failure failure • Shallow shear Failure • Deep shear Failure
Sliding failure is nothing but sliding of wall away from backfill when there is shearing failure at the base of wall. The Factor of safety against sliding is, < 1.5where,µ = coefficient of friction = tan δ = vertical & horizontal components of resultant R of weight of wall & earth pressure
Overturning failure is rotation of wall about its toe due to exceeding of moment caused due to overturning forces to resisting forces. The Factor of safety against overturning is given by, < 1.5 - 2.0where, = sum of resisting moment about toe = sum of overturning moment about toe
The pressure exerted by resultant vertical force at toe of wall must no exceed the allowable bearing capacity of the soil. The pressure distribution is assumed to be linear. The maximum pressure is given by,The Factor of safety against bearing failure is, < 3.0where, = allowable bearing pressure
This type of failure occurs along a cylindrical surface ABC passing through the heel of retaining wall as shown in above fig. The failure takes place because of excessive shear stresses along the cylindrical surface within the soil mass. The FOS against horizontal sliding is lower than that for shallow shear failure. However, FOS against sliding is greater than 1.5, shallow shear failure is not likely to occur.
This type of slope failure occurs along a cylindrical surface ABC as shown in above fig, when there is a weak layer of soil under the wall at a depth of about 1.5 times ht. of wall. The critical failure surface is found by trial & error method. For the backfills having slope i less than 10º, it has been seen that critical failure surface DEF passes through the edge of heel slab as shown in above fig. In this condition possibility of excessive settlement should also be checked.
1. RETAINING WALL COLLAPSE AT CASTLE VILLAGE ON 12TH MAY 2005
The castle village is located in historic Washington heights section of Manhattan. The wall is owned by Castle village owner’s corporation. Five buildings, 575 unit co-operative housing complex is located on the top of hill behind the wall. On Thursday, May 12, 2005 at approx 4:00 pm, a portion of a 65 foot high stone retaining wall on side of bluff rising above Hudson river suddenly collapsed. About 10 minutes later second larger collapse followed as 150 feet section of 800 foot long wall and covered Henry Hudson parkway. Fortunately, there were no deaths or even injuries.
The collapse was cause of structural failure resulting from increased pressure developed behind the wall due to build up of ground water throughout the years. Langan Engg. & Environmental services was the firm inspecting, evaluating and making recommendations about & repair certain areas of retaining walls. The retaining wall has moved about 2 feet from its initial position when it was constructed. Hence this case is remarkable case of sliding of retaining wall.
The landslide triggered due snow melt caused this retaining wall failure. It was due to overturning of wall due to excessive overturning moment caused by pressure generated by snow melt. It fell on the carriageway & Unfortunately it fell onto a truck, killing the driver.