Lateral Earth Resistance On Thrust Block

[fmathis]

In evaluating an existing reinforced concrete thrust block for a steam line, I am wondering if there is a way to determine a value for the magnitude of resistance on the thrust block from the earth behind it. The thrust block extends approximately 20' below grade in soil that has an allowable bearing capacity of 8,000psf at a depth of 10' (determined by soils testing). The face of the thrust block has an area of 48sf (3'x16' mat at 20' depth)and it seems to me that in addition to the sliding resistance between the mat and the earth, there should be some allowance for what I call "lateral earth resistance". The geotechnical report (and the geotechnical engineer) will only say that an "equivalent fluid" having a unit weight of 360psf can be considered in resisting lateral movement. That's far less than the allowable bearing capacity, and I'm wondering if there is some method of determining what portion of the allowable bearing capacity could be considered as "lateral bearing capacity" on something this deep?

 

[CarlB]

The concept "equivalent fluid" is often used to express earth pressures. The units are actuall pcf, not psf. The equivalent fluid method assumes a linearly increasing pressure with depth. So at 20 feet the allowable passive pressure is 20 x 360 or 7200 psf. And yes you could include contribution from sliding resistance although you probably won't need to.
HTH,
Carl

 

[fmathis]

CarlB:

Thanks for the info. The geotechnical report didn't say the equivalent fluid weight could be used that way, but it makes sense. Now I'm wondering why the equivalent fluid wt. is 360pcf for this soil when the acutual unit weight is about 120pcf? It would make more sense to me to use the soil unit weight and multiply that times the depth. do you know any more about how (or if) the equivalent fluid weight correlates to the unit weight of the soil?

 

[Rjeffery]

Note:

When refering to Unit Weight of soil many assume DRY unit weight. Soil can contain a large amount of water without a perceptable change in volume.

Just a hint, 'cause I knew you know that.

 

[Focht3]

fmathis:

The (apparent) factor of 3 difference between the soils' (likely) total unit weight of 115 to 125 pcf is the passive pressure coefficient, K_P. A K_P of 3 indicates that your geotechnical engineer assumed the soil had an angle of internal friction of about 30 to 32 degrees. These are reasonable values for a fine sand or similar soil.

You need to be aware of two factors that may influence your use of this equivalent fluid pressure:

1. The recommended equivalent fluid pressure assumes that the water table remains below the bottom of the thrust block.

2. YOU will apply a factor of safety to this value before you use it. (I would suggest a minimum FS = 2; you might want to use FS = 3 if the pipe is critical and/or hard to repair if the thrust block moves and the pipe cracks.)

Do verify that your geotechnical engineer meant 360 pcf, not psf. And have him/her confirm the above design assumptions.