Retaining walls design
Design of cantilever walls Thursday, April 19, 2012 11:00 AM
Failures: Foundation failure • Stability failure • Rotational failure
Design steps 1. Evaluation of soil pressures a. Active pressure b. Passive pressure 2. Check stability of the wall a. Lateral sliding b. Overturning i. Active pressure ii. Bearing pressure on the soil iii. Failure of the soil surrounding the foundation 3. Structural design of wall components a. Shear b. Flexure c. Reinforcement (design + detail)
Basic Information γs
Unit weight of soil
qa
Soil bearing capacity (allowable)
φ
Angle of friction
μ
Coefficient of friction between soil and concrete
Some typical values of γs and φ are
γs (kN/m3) φ° Sandy gravel 17-22
35-45
Loose sand
15-16
30-35
Crushed rock 12-22
35-40
Ashes
35-40
9-10
Broken brick 15-16
35-40
Soil Pressures Rankine equations Active pressure on backfill side passive pressure on other side A uniform surcharge on top of the soil will result in a uniform pressure across the height of the retained material, given by Ca
Pressure exerted by a retained material
Active pressure (wall moves away from soil)
Passive soil (wall moves towards soil)
*To actually develop passive resistance the concrete must have been placed without using forms for the toe and without disturbing the soil * Frequently neglected H: height of wall above foundation level Minimum depth of foundation
Retaining walls Page 1
Rule of thumbs Footing thickness
H/12 to about H/30
Width of footing
L=(0.4 to 0.7) H
Width of toe
L/4 to L/3
Wall stem thickness top
200 to 300 mm
Wall stem thickness at bottom H/30 to H/15
Stability against Sliding and overturning Requirements: 1. Overturning moment (Pa h/3) must be more than balance by the resisting moments due to the weight of the wall stem and the earth retained and the base so that an adequate factor of safety is provided against overturning 2. Sufficient frictional resistance (F) in combination with any reliable passive pressure (Pp) against the toe must provide an adequate factor of safety against sliding cause by Pa. 3. The base width L must be adequate to distribute the load R to the foundation soil without exceeding the allowable bearing pressure and without causing excessive settlement or rotation.
Factor of Safety
Wi=weight of concrete wall, footing and soil resting on the footing
1.
2. a. Pp frequently neglected b. Base key may be place for more resistance i. 3. Resultant R of earth pressure distribution must be a. in the middle third of the footing for sand and gravel
Retaining walls Page 2
a. in the middle third of the footing for sand and gravel
b. In the middle half for rock
Retaining walls Page 3
Failures: Foundation failure • Stability failure • Rotational failure
Design steps 1. Evaluation of soil pressures a. Active pressure b. Passive pressure 2. Check stability of the wall a. Lateral sliding b. Overturning i. Active pressure ii. Bearing pressure on the soil iii. Failure of the soil surrounding the foundation 3. Structural design of wall components a. Shear b. Flexure c. Reinforcement (design + detail)
Basic Information γs
Unit weight of soil
qa
Soil bearing capacity (allowable)
φ
Angle of friction
μ
Coefficient of friction between soil and concrete
Some typical values of γs and φ are
γs (kN/m3) φ° Sandy gravel 17-22
35-45
Loose sand
15-16
30-35
Crushed rock 12-22
35-40
Ashes
35-40
9-10
Broken brick 15-16
35-40
Soil Pressures Rankine equations Active pressure on backfill side passive pressure on other side A uniform surcharge on top of the soil will result in a uniform pressure across the height of the retained material, given by Ca
Pressure exerted by a retained material
Active pressure (wall moves away from soil)
Passive soil (wall moves towards soil)
*To actually develop passive resistance the concrete must have been placed without using forms for the toe and without disturbing the soil * Frequently neglected H: height of wall above foundation level Minimum depth of foundation
Retaining walls Page 1
Rule of thumbs Footing thickness
H/12 to about H/30
Width of footing
L=(0.4 to 0.7) H
Width of toe
L/4 to L/3
Wall stem thickness top
200 to 300 mm
Wall stem thickness at bottom H/30 to H/15
Stability against Sliding and overturning Requirements: 1. Overturning moment (Pa h/3) must be more than balance by the resisting moments due to the weight of the wall stem and the earth retained and the base so that an adequate factor of safety is provided against overturning 2. Sufficient frictional resistance (F) in combination with any reliable passive pressure (Pp) against the toe must provide an adequate factor of safety against sliding cause by Pa. 3. The base width L must be adequate to distribute the load R to the foundation soil without exceeding the allowable bearing pressure and without causing excessive settlement or rotation.
Factor of Safety
Wi=weight of concrete wall, footing and soil resting on the footing
1.
2. a. Pp frequently neglected b. Base key may be place for more resistance i. 3. Resultant R of earth pressure distribution must be a. in the middle third of the footing for sand and gravel
Retaining walls Page 2
a. in the middle third of the footing for sand and gravel
b. In the middle half for rock
Retaining walls Page 3
