Question

11. Bonus (10%) Explain your understanding of effective Stress concepts. Why effective stress is considered the most important concept in geotechnical engineering including but not limited to slope engineering, design of earth structures and foundation engineering, etc.?  How did you make the best understanding and application of effective stress to soil engineering? (Note: You have calculated them in your homework jobs assignment and have been applying them in settlement analysis as in the tests).

Answer 1

Effective stress is defined as the stress which keeps soil particles together and act as a rigid structure. In the absence of effective stresses soil particles may loose contact with each other and tend to flow. This is why effective stress is important for any structure or any slope.

In case of earth retaining structures in the absence of effective stresses, soil behind the wall acts like water causing heavy stresses on the retaining structure which may then fail in sliding or overturning.

In case of foundation the load of the structure is balanced by upthrust which is a function of effective stress. The absence of effective stresses may cause sinking of the structure into the soil.

Effective stress depends on total stress and pore water pressure. Pore water pressure is also called as neutral stress and it decreases effective stresses. Effective stress is equal to total stress minis pore water pressure.

Increase in water table decreases effective stress because below the water table effective stress depends on the submerged unit weight of soil. Change in water level above the surface of earth has no effect on the effective stress.