notes1-5-1 - Soil Mechanics-I(CENG-2202) Chapter 5 :...

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Soil Mechanics-I(CENG-2202) Chapter 5 : Seepage through soils Lecture notes for 2008/2009 Academic Year Instructor-Bruk Melaku - 59 - Department of Civil Engineering, Faculty of Technology Addis Ababa University 5. TWO-DIMENSIONAL FLOW OF WATER THROUGH SOILS 5.1 INTRODUCTION In many instances the flow of water through soils is neither one-dimensional nor uniform over the area perpendicular to flow. It is often necessary to know events associated with two dimensional flow of water through soil media especially in hydraulic and earth retaining structures. Seepage is closely associated with effective stress. Many catastrophic failures happened due to seepage stresses. 5.2 SEEPAGE STRESSES If water is seeping through soil, the effective stress in a soil mass will differ from that in the static case. Two cases shall be considered: a. Upward seepage, and b. Downward seepage The figure below shows a granular soil mass in a permeability setup where an upward seepage is maintained. In reference to the figure above, the total stress at a point located z distance from the top of the soil specimen is sat w z H γ σ + = 1 At this same location the pore water pressure is
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Chapter 5 : Seepage through soils Lecture notes for 2008/2009 Academic Year Instructor-Bruk Melaku - 60 - Department of Civil Engineering, Faculty of Technology Addis Ababa University w z H h z H u γ + + = 2 1 Thus the effective pressure will be given by w z H h z u σ 2 ' ' = = The value h/H 2 is the hydraulic gradient of the flow and thus w iz z = ' ' This equation shows that in the case of an upward seepage the value of the effective stress decreases by an amount iz w . Increasing the rate of seepage, i.e. the hydraulic gradient, will at some point result in a critical state where e G i z i z s w cr w cr + = = = 1 1 ' 0 ' This value of the hydraulic gradient is known as the critical hydraulic gradient. At this stage the soil will have no resistance, i.e. ’=0 and acts like a fluid. This phenomenon is known as boiling , or a quick condition and it usually results in failures of structures like dams and retaining walls. It should be taken into account when designing water and earth retaining structures. It is also important to consider boiling when planning excavations in soil strata underlain by artesian aquifers. The critical hydraulic gradient is approximately 1.0 for most soils. Let us now consider a case of downward seepage as shown below.
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This note was uploaded on 11/29/2011 for the course 209 5303 taught by Professor Andre during the Spring '10 term at Iowa Lakes.

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notes1-5-1 - Soil Mechanics-I(CENG-2202) Chapter 5 :...

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