Effective Soil Pressures - Why?
Effective Soil Pressures - Why?
(OP)
I have a question for the experienced geotechnical engineers out there. I am studying for the PE Exam, and am currently studying the geotechnical portion. Why do engineers use the Effective Soil Pressure when calculating consolidation in clay soils? Does the water pressure have no effect? All the problems/solutions I have are completed by finding the unit weight (density) of the moist soil, then subtracting out the unit weight of water.
A detailed response would be very appreciated.
Thanks in advance.
A detailed response would be very appreciated.
Thanks in advance.





RE: Effective Soil Pressures - Why?
RE: Effective Soil Pressures - Why?
Atmospheric pressure is acting in all directions. So, the 1 tsf is acting sideways, upwards and downwards in every dimension of your body. As a result, it has no affect on your body weight. When you stand on a scale, the reading is your body weight - and just that!
Same is true for hydrostatic forces. When you stand in a pool and hold your girlfriend, she seems light as a feather. If you stood there and they drained the pool, her weight would decrease.
Vertical forces above the water table account for the weight of the soil grains and the weight of the water in the pore space. Vertical forces below the water table include the weight of hte soil grains, but the hydrostatic forces are subtracted.
Let's consider a water table at 10 ft and calculate the vertical effective stress at 15 ft (consider soil with a moist unit weight of 120 pcf). You'd have a soil load of 1,800 psf, but subtract out a hydrostatic force of 288 psf.
Hope this helps.
f-d
¡papá gordo ain't no madre flaca!
RE: Effective Soil Pressures - Why?
As I can remember, when wall subjects to fully saturated soil, the lateral pressure on the wall consisted of two parts - hydrostatic pressure and passive/active soil pressure, the soil pressure is simply the lateral earth cofficient times depth times the difference of saturate soil density and water, that is effective unit weight of soil without influence of water.
RE: Effective Soil Pressures - Why?
To answer EngWade's question:
"Why do engineers use the Effective Soil Pressure when calculating consolidation in clay soils?"
Consolidation is governed by effective stress; not by total stress. Simply, if there is no change is effective stress, there won't be any consolidation. You may want to refer a basic text book on consolidation to understand why it works that way. This is well explained by a spring-piston analogy.
"Does the water pressure have no effect?"
It has. Effective stress is a function of total stress and water pressure
Note that, on the other hand, immdiate (or elastic) settlement is governed by total stress. Generally, change is total stress is instantaneous, whereas, effetive stress changes slowly with drainage. After a long period, the magnitude of changes in total and effective stress must be the same.
RE: Effective Soil Pressures - Why?
RE: Effective Soil Pressures - Why?
effective stress controls (vast majority of "real" world time)...although did see an example once of a clay layer below water table underlain by sand layer underlain by a clay layer...question assumed water table dropped well below the top of the sand layer and you had to consider that the upper clay layer didn't drain as quick as the sand layer so it jacked up the loads and consolidation of bottom clay layer
RE: Effective Soil Pressures - Why?
Good luck!
RE: Effective Soil Pressures - Why?