39 2.6 Permeability Tests 2.6.1 General - To determine te coefficient of permeability (or coefficient of ydraulic conductivity) k. - General metod for determining k directly. 1) Constant-ead metod (for coesionless materials) 2) falling-ead metod (for coesive materials) - Darcy law or v = ki q = kia were q = flow quantity in a unit time. v = flow velocity i = ydraulic gradient = /L = total ead difference L = flow pat A = cross-sectional area of soil mass - It is difficult to get a reliable value of k wit conventional laboratory testing metods. (Its variation can be one order of magnitude.)
40 - Te major reasons for te variation of te measured k (Te reason for te difference between in-situ k values and k values obtained from lab test) : 1) Cannot duplicate te same state as in te field (density, structure and orientation of te in situ stratum, degree of saturation ) 2) Conditions at te boundary. (Te smoot wall of permeability mold in te laboratory makes for better flow pat tan if tey were roug.) 3) Te effect of te applied ydraulic gradient i (i in te lab is usually 5 10 times larger tan in te field.) 4) Darcy s law can be nonlinear (at least at large values of i). 5) Size effect of sample (Usually k in te field is muc larger (more tan 10 times) tan k obtained wit small specimens in te lab.) - Influence factors on te coefficient of permeability of a soil. 1) Te viscosity of te pore fluid depending on te type of pore fluid and temperature. As te temperature increases, viscosity decreases, and k increases. (4 o C cange in temperature of te pore water results in about 10% increase of k). Practically, te temperature correction is not necessary but is required in most standard test procedure. And k is standardized at 20 C. k 20 = T k η T η20
41 2) Te void ratio of te soil k = f(e) k e 3 /(1+e) log k e 3) Te size and sape of te soil particles k in angular and platy particles is larger tan k in rounded and sperical particles. k increases wit increase of particle size. Hazen s formula (for clean sands and gravels) k = 100D 2 10 cm/s (in te range of particle size, 0.01 < D 10 < 0.3 cm) 4) Te degree of saturation S. As te degree of saturation increases, te apparent coefficient of permeability also increases (due to te breakage of surface tension).
42 2.6.2 Te Constant Head Permeability Test Figure 11-2 - Use te constant ead Employ te overflow weir in bot inlet (at te bottom) and outlet of flows (on te top). q q = kia, k = ( i = ) ia L A large amount of water is wasted unless te test is of sort duration Apply coesionless soils only. - Te standard compaction mold is widely used (te base wit porous stone and te cap wit valve) as sown in Fig 11-2. 1) Advantages: Easy to control and to make a sample. 2) Disadvantages: No potential of observation and possible ead loss across te porous stone. - Modified device by Bowles Employ te transparent cylinder for permeameter and #200 sieve screen instead of porous stone.
43 2.6.3 Falling Head Permeability Test q = kia = k A - 1 L and d q = a - 2 dt d k A = a L dt t 2 al d dt = ( ) 0 1 Ak al t = Ak 2 (ln ) 1 al 1 k = (ln At 2 ) Figure 12-2 - For coesive soils wit low permeability, flow quantity is very small. Ex) For soils wit k = 1 10-6 cm/min Flow quantity =.0972 cc/r for i=20 and A = 81 cm 2 (small amount of flow quantity and longer duration time) Accurate measurement of flow quantity wit some provisions to control evaporation is required.
44 - Te equation applicable to falling ead permeability test k = al ln 1 AΔt 2 Were A = cross-sectional area of sample a = cross-sectional area of burette 1 = ydraulic ead across sample at beginning of test (t=0). 2 = ydraulic ead across sample at end of test (t=t test ) - k from a consolidation test 1) Estimate k using te coefficient of consolidation c v, from te equation below, avγ wc k = 1 + e v 2) Perform te falling ead test on te loaded sample in te oedometer at te end of primary loading - k from a triaxial cell