1. Retaining Walls I. *Introduction
*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).
*Types of retaining walls
gravity wall: A simple retaining wall depending on its weight to achieve its stability
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: retaining wall with anchor (锚碇式挡土墙); retaining wall with stepped back (踏步式挡土墙).
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2. 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 material 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 pressure.
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3. 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 the 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 that is prevented from lateral movement by an unyielding wall);
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4. 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 toward 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.
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5. 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);
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;
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.
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6. Retaining 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 statics.
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 the front base of the wall (at the toe), the righting moments and the overturning moments are computed about the toe of the wall.
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7. Retaining Walls
The factor of safety against bearing capacity failure is determined by dividing the ultimate bearing capacity by the actual maximum 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 in 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
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8. Retaining Walls IV. Backfill Drainage The necessity to drain backfill
*Methods 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 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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9. 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 pipes 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 shrinking.
V. *Settlement and Tilting
1. Settlement by retaining walls is inevitable, just as by any other structure resting on footings or piles.
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10. Retaining Walls V. Settlement and Tilting
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.
Retaining walls on cohesive soils: Settlement will occur slowly and for a long period of time after construction has been completed.
Retaining walls on spread footings: The amount of settlement can be determined using the principles of settlement analysis for footings.
Retaining walls on pile foundations: The amount of settlement can be determined using the principles of settlement analysis for pile foundations.
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11. Retaining Walls
Non-uniformity of settlement (differential settlement) resulting in vertical cracks in walls
It occurs when the bearing capacity of the foundation soil along the wall is not uniform and the wall itself fails to bridge across poor material.
Possible remedies: a. improving the foundation soil (e.g., by replacement, compaction, or stabilization of the soil); b. changing the width of the footing.
Tilting
Tilting is commonly caused by eccentric pressure on the base of the wall. Tilting can be reduced by keeping the resultant force near the middle of the base. The amount of tilting may be expected to be in the order of magnitude one-tenth of 1% of the height of wall or less.
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