Lateral Soil Load IBC Tabole 1610.1 Values

[SteveGregory]

I did a little reverse engineering on the equivalent fluid pressures for presumptive values given in this table. If I did my math right, the table doesn't make a whole lot of sense. For soil density of 120 pcf, the phi angle values match for soil classified as GM. For a density of 110 pcf, the phi values are close for GM. For all other soil types, the numbers go wild.

1. Is my math right?
2. If so, do you know why the IBC table was set up this way?

 

[DST148]

For simplification, let us assume back fill is horizontal, wall face is vertical, and wall friction is zero and the soils
can be broadly classified as granular and non-granular or clayey.....

For clayey soils, the angle of internal friction phi is close to zero.
The active earth pressure is predominantly governed by the cohesion of soil.
For granular soils, cohesion is zero.
The active earth pressure is governed by angle of internal friction.
Similarly for passive pressures....

 

[SteveGregory]

For the Active Pressure, I used the Rankine formula, Gamma(soil density) x Tan²(45-phi/2) [phi = angle of internal friction]

For At-Rest Pressure, I used the Jaky formula, Gamma x (1-Sin(phi))

Based on these two equations, I get very different numbers when I backsolve for the angle of internal friction for the same backfill. Are you saying that these formulas don't apply to soils with clay in the mix?

 

[DST148]

I don't have soils book at home, but I have old CERM....
Coeff of active earth pressure Ka and passive earth pressure Kp in the most general form depend on
four parameters: angle of internal friction, angle of external friction, slope of backfill,
and rake angle of retaining wall face.

Active earth pressure, Pa = Pv * Ka - 2 * c * sqrt(Ka)
Passive earth pressure, Pp = Pv * Kp + 2 * c * sqrt(Kp)

Pv is vertical soil pressure, c is cohesion of soil

For saturated clays, phi is zero.
For granular soils, c is zero.

 

[dcarr82775]

The Rankine equations assume cohesionless soils. Cohesion complicates. The cohesion can indicate essentially free standing cuts so long as they never ever dry out. I think many use a Phi in the high teens with no cohesion to represent cohesive soils.

Depending on your conditions maybe the soil will not dry out in which case you may be more comfortable using a higher Phi angle to account for cohesion. Many textbooks have equations that take into account both Phi and C.