Effective Soil Pressures - Why? 1

[EngWade]

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.

 

[dgillette]

The explanation of effective stress is best obtained from Chapter Two of an intro soil-mechanics textbook.

 

[fattdad]

atmospheric pressure (i.e., the weight of the air above your head) imparts a uniform stress of 1 tsf on your head. Let's say your head is one square foot in plan dimension (i.e., blockhead - ha). The air pressure would then make you "weigh" ALOT! Well, that's not right. You may only weigh 220# like me.

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!

 

[cntw1953]

Your Cinderella girl friend might crush you in the event of a pool suddenly drained

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.

 

[Jalthi]

fattdad, I like your analogy.

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.

 

[cntw1953]

Wow, I misread the question. Apologize!

 

[msucog]

and if you've ever wondered what happens during a severe drought, run the numbers and see what happens if the groundwater drops say 10 feet...is like adding several feet of new fill on top(ie. could act like fill induced settlement!).

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

 

[geohec]

Effective stress in a main concept in geotechnical engineering. Is a form of describing indirectly soil particle interaction. You cannot measure effective stress directly. You can calculate total pressure and water pressure, if you subtract them you get effective stress. This is the best way of describing it.
Good luck!

 

[paganel]

The effect stess is what really acts on the soil skelecton.For the strength and deformation property,that is really matter,assuming there is no deformation of soil particle itself.