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1、Retaining Walls,I. *Introduction 1. *Definition: A retaining wall is a wall, often made of concrete, built for the purpose of retaining, or holding back, a soil mass (or other material). 2. *Types of retaining walls gravity wall: A simple retaining wall depending on its weight to achieve its stabili
2、ty cantilever wall: a taller wall with extended toe and heel to offset the large lateral pressure tending to overturn the wall. A cantilever wall has part of the base extending underneath the backfill, and the weight of the soil above this part of the base helps prevent overturning. Other types: ret
3、aining wall with anchor (锚碇式挡土墙); retaining wall with stepped back (踏步式挡土墙). 1 Wednesday, July 22, 2020,Retaining Walls,I. Introduction 3. * Structure of retaining walls Base heel and toe Stem batter: the outer face of the wall which is built inward to prevent the wall tipping over. backfill: The ma
4、terial placed behind a retaining wall. 4. *Selection of backfill soils It is highly desirable that backfill be a select, free-draining, granular material, such as clean sand, gravel, or broken stones. Clayey soils make extremely objectionable backfill material because they create excessive lateral p
5、ressure. 2 Wednesday, July 22, 2020,Retaining Walls,The designer of a retaining wall should either write the specifications for the backfill, and base the design of the wall on the specified backfill; or be given information on the material to be used as backfill and base the design of the wall on t
6、he indicated backfill. If it is possible that the water table may rise in the backfill, special designing, construction, and monitoring must go into effect. 5. * Earth pressure undertaken by retaining walls Three categories of earth pressure: earth pressure at rest (lateral pressure cause by earth t
7、hat is prevented from lateral movement by an unyielding wall); 3 Wednesday, July 22, 2020,Retaining Walls,active earth pressure (the earth pressure exerted on the wall, when the wall moves away from the backfill); passive earth pressure (the earth pressure exerted on the wall when the wall moves tow
8、ard the soil). (2) A retaining wall must be able to resist sliding along the base, be able to resist overturning, and not introduce a contact pressure on the foundation soil beneath the base of the wall that exceeds the allowable bearing pressure of the foundation soil. Wednesday, July 22, 2020,Reta
9、ining Walls,III. Stability Analysis *Movement of retaining walls: horizontally (by sliding), vertically (by excessive settlement and /or bearing capacity failure of the foundation soil), and by rotation (by overturning). The checks for sliding and for overturning hark back to the basic laws of stati
10、cs. The checks for settlement and bearing capacity of foundation soil are done by settlement analysis and bearing capacity analysis. The factor of safety against overturning is determined by dividing the total righting moment by the total overturning moment. Since overturning tends to occur about th
11、e front base of the wall (at the toe), the righting moments and the overturning moments are computed about the toe of the wall. 6 Wednesday, July 22, 2020,Retaining Walls,The factor of safety against bearing capacity failure is determined by dividing the ultimate bearing capacity by the actual maxim
12、um contact (base) pressure. Some common minimum factors of safety for sufficient stability are as follows: (1) Factor of safety against sliding = 1.5 (if the passive earth pressure of the soil at the toe in front of the wall is neglected) = 2.0 (if the passive earth pressure of the soil at the toe i
13、n front of the wall is included) (2) Factor of safety against overturning = 1.5 (granular backfill soil) = 2.0 (cohesive backfill soil) (3) Factor of safety against bearing capacity failure =3.0 7 Wednesday, July 22, 2020,Retaining Walls,IV. Backfill Drainage The necessity to drain backfill *Methods
14、 of drainage Selection of backfill soil (sand, gravel or crushed stones are highly desirable); Placement of weep holes: 4- to 6-in. in diameter, extending through the wall for every 5 to 10 ft along the wall; Placement of a perforated drain pipe longitudinally along the back of the wall: surrounded
15、by filter material and water drains through the filter material into the pipe and then through the pipe to one end of the wall. 8 Wednesday, July 22, 2020,Retaining Walls,In both cases (weep holes and drain pipes) a filter material must be placed adjacent to the pipe to prevent clogging, and the pip
16、es must be kept clear of debris. Placement of a wedge of pervious material adjacent to the wall or a a “drainage blanket” of pervious material If a less pervious material (silt, granular soil containing clay, etc.) has to be used as backfill. A highly impervious soil (clay) is very undesirable because, in addition to the excessive lateral earth pressure, it also is difficult to drain and may be subject to frost action, and swelling and
