锚碇式挡土墙.pptx

= 2.0 (cohesive backfill soil)
(3) Factor of safety against bearing capacity failure =3.0
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IV. Backfill DRraeintagaeining Walls
1. The necessity to drain backfill
2. *Methods of drainage
1) Selection of backfill soil (sand, gravel or crushed stones are highly desirable);
2) Placement of weep holes: 4- to 6-in. in diameter, extending through the wall for every 5 to 10 ft along the wall;
Retaining Walls
(3) The designer of a retaining wall should
a. either write the specifications for the backfill, and base the design of the wall on the specified backfill;
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Retaining Walls b. active earth pressure (the earth pressure exerted
on the wall, when the wall moves away from the backfill);
V. * S e t t l e m en t a n d T i l t i n g
1. Settlement by retaining walls is inevitable, just as by any other structure resting on footings or piles.
2) Retaining walls on cohesive soils: Settlement will occur slowly and for a long period of time after construction has been completed.
3) Retaining walls on spread footings: The amount of settlement can be determined using the principles of settlement analysis for footings.
5. * Earth pressure undertaken by retaining walls
(1) Three categories of earth pressure:
a. earth pressure at rest (lateral pressure cause by earth that is prevented from lateral movement by an unyielding wall);
c. passive earth pressure (the earth pressure exerted on the wall when the wall moves toward the soil).
(2) A retaining wall must
a. be able to resist sliding along the base,
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V. SettlemenRt eantdaTiilntinigng Walls
1) Retaining walls on granular soils: Most of the expected settlement will occur by the time the construction of the wall and placement of backfill have been completed.
coarse-grained soil of low permeability, owing to admixture of particles of silt size;
fine silty sand; granular materials with conspicuous clay content; or residual soil with stone;
= 2.0 (if the passive earth pressure of the soil at the toe in front of the wall is included)
(2)
Factor of safety against overturning = 1.5 (granular
backfill soil)
2. The checks for sliding and for overturning hark back to the basic laws of statics.
3. The checks for settlement and bearing capacity of foundation soil are done by settlement analysis and bearing capacity analysis.
3) Placement of a perforated drain pipe longitudinally along the back of the wall: surrounded by filter material and water drains through the filter material into the pipe and then through the pipe to one end of the wall.
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Retaining Walls II. * Types of backfill materials for retaining walls
coarse-grained soil without admixture of fine soil particles, very free-draining (clean sand, gravel, or broken stone);
6) 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 shrinking.
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Re t a i n i n g Wa l l s 4) In both cases (weep holes and drain pipes) a filter
material must be placed adjacent to the pipe to prevent clogging, and the pipes must be kept clear of debris.
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Re t a i n i n g Wa l l s 5. The factor of safety against bearing capacity failure is
determined by dividing the ultimate bearing capacity by the actual maximum contact (base) pressure.
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III. Retaining Stability Analysis Walls
1. *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).
4. 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 the front base of the wall (at the toe), the righting moments and the overturning moments are computed about the toe of the wall.
6. 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)
5) Placement of a wedge of pervious material adjacent to the wall or a a “drainage blanket” of per vious material If a less pervious material (silt, granular soil containing clay, etc.) has to be used as backfill.
b. or be given information on the material to be used as backfill and base the design of the wall on the indicated backfill.
c. If it is possible that the water table may rise in the backfill, special designing, construction, and monitoring must go into effect.
soft or very soft clay; organic silt; or soft silty clay;
medium of stiff clay that may be placed in such a way that a negligible amount of water will enter the spaces between the chunks during floods or heavy rains.
b. be able to resist overturning, and
c. 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.