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Geotechnical Engineering: Coefficient of Permeability Testing, Slides of Geotechnical Engineering

University of Kerbala (UK)Geotechnical Engineering

An in-depth analysis of the methods and techniques used to determine the coefficient of permeability (k) in geotechnical engineering. both constant-head and falling-head permeability tests, explaining their principles, advantages, and disadvantages. Additionally, it discusses the factors influencing k, such as temperature, void ratio, particle size, and degree of saturation.

Typology: Slides

2021/2022

Uploaded on 09/07/2022

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adnan_95 🇮🇶

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Site Investigation and Soil Testing
SNU Geotechnical engineering lab.
39
2.6 Permeability Tests
2.6.1 General
- To determine the coefficient of permeability (or coefficient of hydraulic
conductivity) k
.
- General method for determining k directly.
1) Constant-head method (for cohesionless materials)
2) falling-head method (for cohesive materials)
- Darcy’ law
v = ki
or q = kiA
where q = flow quantity in a unit time.
v = flow velocity
i = hydraulic gradient = h/L
h = total head difference
L = flow path
A = cross-sectional area of soil mass
- It is difficult to get a reliable value of k with conventional laboratory testing
methods. (Its variation can be one order of magnitude.)
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Site Investigation and Soil Testing

2.6 Permeability Tests

2.6.1 General

  • To determine the coefficient of permeability (or coefficient of hydraulic conductivity) k .
  • General method for determining k directly.
    1. Constant-head method (for cohesionless materials)
    2. falling-head method (for cohesive materials)
  • Darcy’ law v = ki or q = kiA

where q = flow quantity in a unit time. v = flow velocity i = hydraulic gradient = h/L h = total head difference L = flow path A = cross-sectional area of soil mass

  • It is difficult to get a reliable value of k with conventional laboratory testing methods. (Its variation can be one order of magnitude.)

Site Investigation and Soil Testing

  • The major reasons for the variation of the measured k (The reason for the difference between in-situ k values and k values obtained from lab test) : 1) Cannot duplicate the same state as in the field (density, structure and orientation of the in situ stratum, degree of saturation…) 2) Conditions at the boundary. (The smooth wall of permeability mold in the laboratory makes for better flow path than if they were rough.) 3) The effect of the applied hydraulic gradient i ( i in the lab is usually 5 – 10 times larger than in the field.) 4) Darcy’s law can be nonlinear (at least at large values of i ). 5) Size effect of sample (Usually k in the field is much larger (more than 10 times) than k obtained with small specimens in the lab.)
  • Influence factors on the coefficient of permeability of a soil.
    1. The viscosity of the pore fluid ⇒ depending on the type of pore fluid and temperature. As the temperature increases, viscosity decreases, and k increases. (4o^ C change in temperature of the pore water results in about 10% increase of k ). Practically, the temperature correction is not necessary but is required in most “standard” test procedure. And k is standardized at 20 ° C.

20 20

k kT^ η^ T

η

Site Investigation and Soil Testing

2.6.2 The Constant Head Permeability Test

Figure 11-

  • Use the constant head ⇒ Employ the overflow weir in both inlet (at the bottom) and outlet of flows (on the top). q = kiA, iA k = q ( ) L i = h

⇒ A large amount of water is wasted unless the test is of short duration ⇒ Apply cohesionless soils only.

  • The standard compaction mold is widely used (the base with porous stone and the cap with valve) as shown in Fig 11-2.
  1. Advantages: Easy to control and to make a sample.
  2. Disadvantages: No potential of observation and possible head loss across the porous stone.
  • Modified device by Bowles Employ the transparent cylinder for permeameter and #200 sieve screen instead of porous stone.

Site Investigation and Soil Testing

2.6.3 Falling Head Permeability Test

Figure 12-

  • For cohesive soils with low permeability, flow quantity is very small. Ex) For soils with k = 1× 10 -6^ cm/min ⇒ Flow quantity = .0972 cc/hr for i=20 and A = 81 cm^2 (small amount of flow quantity and longer duration time) ⇒ Accurate measurement of flow quantity with some provisions to control evaporation is required.

A

L

q = kiA = kh - ①

and

dt

q = − adh - ②

dt

A adh L

k h =−

2 (^0 )

t h h dt aL^ dh Ak h

2 1

t aL (ln^ h ) Ak h

(ln ) 2

1 h

h At k = aL