Lateral Soil Load IBC Table 1610.1 Values

[SteveGregory]

I hope this is the right forum for this post. 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?

 

[RFreund]

I'm confused on what your input parameters are.

You are backsolving for the effective friction angle (aka phi) given the lateral soil pressure, right?
If this is correct there are only 3 different values of lateral soil pressures so I would assume that there would only be 3 different friction angles. I think I am missing something here.


EIT

http://www.HowToEngineer.com

 

[SteveGregory]

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

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

Based on these two equations, I get very different numbers for the angle of internal friction for the same backfill.

 

[FixedEarth]

SG;

That Table is more emperical in nature than calculated values. That is why we see rounded numbers. Without knowing the phi angles & moist unit weights that were used, it is impossible to match the Table values. In general, the more cohesive the soils get, the lower their fricion angle, so you would get a higher active and at rest earth pressure values. So why are you computing lower equivalent fluid pressures for CL (19.5 psf/ft & 10 psf/ft) values for example?

http://www.soilstructure.com/

 

[SteveGregory]

Fixed,
The numbers that I calculated are the phi angles in degrees, not the equivalent fluid pressures. On page 1, I used a soil density of 120 pcf and on page 2, I used a soil density of 110 pcf. Plus, I assumed a level backfill.

I used the equivalent fluid pressures in the IBC table and an assumed soil density to solve for the phi (internal friction) angle. Am I missing something?

 

[FixedEarth]

So you backcalculated the friction angle with your own assumed gamma values!. You could have also assumed a friction angle and backcalculated the Gamma. Either way, you are guessing one number and when it doen't match you are saying the Table is wrong.

Based on my experience with geotechnical investigations and many earth retention projects, the IBC Table is conservative. When we do direct shear test & max. dry density and then plug in Rankine equation, we get less conservative values. But that is exactly the point - with a geotechnical report, you can calculate the exact value and often get smaller value.

So let us look at Active EFP for CL soils from the Table. The IBC Table lists a value of 60 psf/ft. If we use friction angle of 14 Degree and Gamma of 114 pcf, we get about 60 pcf. If we were run to lab test, we may get phi angle between 10 to 25 degree for CL soils. But who is to say, we could not use friction angle of 15.9 degree & Gamma of 105.2 pcf to get the sam 60 pcf?

You are not missing something - you are assuming two values and if they don't match to the Table, you can't say the Table is wrong. You just need to keep guessing both phi and gamma. The Table is on the high side.

http://www.soilstructure.com/

 

[SteveGregory]

Thanks Fixed!

I was assuming the Table was correct for both the Active and At-Rest EFP and was trying to figure out what gamma was used to develop the tables.

I assumed that for any specific given soil type, the Table people used the same phi angle for the Active and At-Rest tabulated calculations. My assumptions "seemed" to be right only for GM soil with a gamma of 120 pcf.

 

[CharlesNorris]

Steve,
There's more to it than friction angle. In geotechnical manuals you will find earth pressure charts that have different earth pressure to backfill slope plots based on soil type. As you get more fines, you find the earth pressures go up. Part of this can be explained by strength. Another factor is expansive pressure. You will see in the code that there is a requirement to account soil expansion. For soils with more fines, I believe some allowance for soil expansion is incorporated into the fluid pressures.