7.2.0 Computations of lateral earth pressures shall comply with the provisions of Sections 7.2.1 through 7.2.6. Wall movements set forth in Table 7.1 shall be considered the magnitude required for active and passive conditions to exist. Soil permeability characteristics, boundary drainage and loading conditions, and time shall be considered in selection of strength parameters. In soils where partial drainage occurs during the time of construction, analysis shall be performed for short-term and long-term conditions, and the wall shall be designed for the worse conditions.
7.2.1 Wall friction. Wall friction and vertical movement, slope of the wall in the backside and sloping backfill shall be considered in determining the lateral pressures applied against the wall. Unless data to substantiate the use of other values are submitted and approved by a registered design professional, the values set forth in Tables 7-2 and 7-3 shall be used in computations that include effects of wall friction.
TABLE 7.1
MAGNITUDE OF ROTATION TO REACH FAILURE
SOIL TYPE AND CONDITION ROTATION (δ H)a
ACTIVE PASSIVE
Dense cohesionless soil 0.0005 0.002
Loose cohesionless soil 0.002 0.006
Stiff cohesive soil 0.01 0.02
Soft cohesive soil 0.02 0.04
a.δ = Horizontal translation at the top of the wall.
H = Height of the wall
TABLE 7.2
ULTIMATE FRICTION FACTORS FOR DISSIMILAR MATERIALS
INTERFACE MATERIALS FRICTION FACTOR, tan δ a
Clean sound rock 0.7
Clean gravel, gravel-sand mixtures, coarse sand 0.55 - 0.60 Clean fine to medium sand, silty medium to coarse sand,
silty or clayey gravel
0.45 – 0.55 Clean fine sand, silty or clayey fine to medium sand 0.35 – 0.45
Fine sandy silt, nonplastic silt 0.30 – 0.35
Very stiff and hard residual or preconsolidated clay 0.40 - 0.50 Medium stiff and stiff clay and silty clay 0.30 – 0.35
a. Values for δ shall not exceed one-half the angle of internal friction of the backfill soils for steel and precast concrete and two-third the angle of internal friction of the backfill soils for cast-in place concrete.
RETAINING WALLS
SBC 303 2007 7/2
7.2.2 Wall movement. The effect of wall movement on the earth pressure coefficients shall conform to the provisions of Sections 7.2.2.1 and 7.2.2.2.
7.2.2.1 Rotation. If the wall is free at the top and there are no other structures associated with, wall tilting shall not exceed 0.1 times the height of the wall. Where the actual estimated wall rotation is less than the value required to fully mobilize active or passive conditions set forth in Table 7-1, the earth pressure coefficient shall be adjusted in accordance with Figure 7.1.
TABLE 7.3
ULTIMATE ADHESION FOR DISSIMILAR MATERIALS
INTERFACE MATERIALS COHESION (kPa)
ADHESION (kPa)
Very soft cohesive soil 0 - 10 0 - 10
Soft cohesive soil 10 - 25 10 - 25
Medium stiff cohesive soil 25 - 50 25 - 35
Stiff cohesive soil 50 - 100 35 - 45
Very stiff cohesive soil 100 - 200 45 - 60
7.2.2.2 Translation. It shall be permitted to consider uniform translation required to mobilize ultimate passive resistance or active pressure equivalent to movement of top of wall based on rotation given in Table 7.1.
FIGURE 7.1
EFFECT OF WALL MOVEMENT ON WALL PRESSURES (NAFAC, 1986)
RETAINING WALLS
SBC 303 2007 7/3
7.2.2.3 Restrained wall. Where wall is prevented from even slight movement, the earth pressure shall be considered to remain at rest conditions.
7.2.2.4 Basement and other below grade walls. Pressures on walls below grade shall be computed based on restrained conditions that prevail, type of backfill, and the amount of compaction. The provisions of Chapter 6 shall apply where applicable.
7.2.2.5 Wall on rock. Where the wall is founded on rock, sufficient rotation of the base and wall so that active pressure is developed, shall be accomplished by placing 150 to 300 mm thick earth pad beneath the base and by constructing the stem with sufficient flexibility to yield with the soil pressure.
7.2.3 Groundwater conditions. Pressure computations shall include uplift pressures and the effect of the greatest unbalanced water head anticipated to act across the wall. For cohesionless materials, increase in lateral force on wall due to rainfall shall be considered and walls shall be designed to support the weight of the full hydrostatic pressure of undrained backfill unless a drainage system is installed in accordance with Sections 13.4.2 and 13.4.3.
7.2.4 Surcharge. Stability shall be checked with and without surcharge. Lateral pressure on wall due to point and line loads shall be computed based on the assumption of an unyielding rigid wall and the lateral pressures are set equal to double the values obtained by elastic equations. The applicability of the assumption of an unyielding rigid wall shall be evaluated for each specific wall.
For uniform surcharge loading it shall be permitted to compute lateral stress by treating the surcharge as if it were backfill and multiplying the vertical stress at any depth by the appropriate earth pressure coefficient. It shall be permitted for design purposes to considerer a distributed surface load surcharge on the order of 15 kPa to account for construction materials and equipment stored within 5 to 10 meters from the wall. Where construction equipment is anticipated within 2 meters of the wall, it must be accounted for separately.
7.2.5 Compaction. For backfill of granular soils compacted in a confined wedge behind the wall, the horizontal pressure beyond those represented by active or at-rest values shall be computed in accordance with Figure 7.2.
Compaction-induced pressures shall not be considered in bearing, overturning and sliding analyses and need only be considered for structural design. Backfill shall be brought up equally on both sides until the lower side finished grade is reached and precautions shall be taken to prevent overcompaction which will cause excessive lateral forces to be applied to the wall.
Clays and other fine-grained soils, as well as granular soils, with amount of clay and silt greater than 15 percent shall not be used as a backfill behind retaining wall. Where they must be used, the lateral earth pressure shall be calculated based on at-rest conditions, with due consideration to potential poor drainage conditions and swelling. Where loose hydraulic fill is used it shall be placed by procedures which permit runoff of wash water and prevent building up of large hydrostatic pressures.
RETAINING WALLS
SBC 303 2007 7/4
FIGURE 7.2
HORIZONTAL PRESSURE ON WALLS FROM COMPACTION EFFORT (NAFAC, 1986)
7.2.6 Earthquake loading. For retaining walls assigned to Seismic Design Category C or D, provisions of SBC 301 and SBC 304 shall apply when not in conflict with the provisions of Chapter 7.
The combined resultant active force due to initial static pressure and increase in pressure from ground motion shall be computed from the following formula
θ ψ θ θ
β
γ 2 * * 2 *2
cos cos ) cos 1
)(
, 2 (
1
v A
AE H k k
P = − (Equation 7-1)
where:
PAE= Combined resultant active force.
H = Wall height.
θ = Slope of wall back with respect to vertical
β = Inclination of soil surface (upward slopes away from the wall are positives).
θ* = (θ+ψ)= Modified slope of wall back.
β* = (β+ψ)= Modified inclination of soil surface.
γ = unit weight of soil.
ψ = seismic inertia angle given as follows
⎟⎟⎠
⎜⎜ ⎞
⎝
⎛
= − −
v h
k k tan 1 1
ψ (Equation 7-2) where:
RETAINING WALLS
SBC 303 2007 7/5
k = Horizontal ground acceleration in g’s. h
k = Vertical ground acceleration in g’s. v
For modified slope angle β*and θ*, the modified coefficient of earth pressures kA(β*,θ*) shall be calculated from the Coulomb theory. Dynamic pressure increment shall be obtained by subtracting static active force (to be determined from Coulomb theory for given β and θ) from combined active force given by Equation 7-1. Location of resultant shall be obtained by considering the earth pressure to be composed of a static and dynamic component with the static component acts at the lowest third point, whereas the dynamic component acts above the base at 0.6 times the height of the wall. Under the combined effect of static and earthquake load the factor of safety shall not be less than 1.2.
Where soil is below water, the hydrodynamic pressure computed from the following formula shall be added
2 /
)1
2 (
3k h z
pwz = hγw w (Equation 7-3) where:
p = Hydrodynamic pressure at depth z below water surface wz
h = Height of water w
z = Depth below the water surface.
γw = Unit weight of water.
SECTION 7.3