Geotechnical Engineering

DHARM

SOIL MOISTURE–PERMEABILITY AND CAPILLARITY 129

If the extreme limits z 0 and h at r 0 and R are applied, we get :

k = q

Hh z

e Rr

2 π () 0 0

.log ( / )

−

But (h – z 0 ) = d 0

∴ k =

q

Hd

Rr

q

Hd

e Rr

22 π. 0 0 720 10 0

.log ( / )

.

= .log ( / ) ...(Eq. 5.25)

Since T = kH,

T =

q

d

R

272. 0 10 r 0

.log ...(Eq. 5.26)

The field practice is to determine the average value of the coefficient of transmissibility
from the observation of drawdown values from a number of wells. A convenient procedure for
this is as follows:

A semi-logarithmic graph is plotted with r to the logarithmic scale as abscissa and d to

the natural scale as ordinate, as shown in Fig. 5.8 :

1 10 100 1000

0

1

2

3

4

5

6

DDdd

Radial distance, r (log scale)

Drawdown, d

Fig. 5.8 Determination of T

From Equation 5.24,

T =

q

272..∆d

...(Eq. 5.27)

if r 2 and r 1 are chosen such that r 2 /r 1 = 10 and ∆d is the corresponding value of the difference
in drawdowns, (d 2 – d 1 ).

Thus, from the graph, d may be got for one logarithmic cycle of abscissa and substituted
in Eq. 5.27 to obtain the coefficient of transmissibility, T.

The coefficient of permeability may then be computed by using the relation k = T/H,

where H is the thickness of the confined aquifer.

Pumping-in tests have been devised by the U.S. Bureau of Reclamation (U.S.B.R.) for a

similar purpose.

Field testing, though affording the advantage of obtaining the in-situ behaviour of soil

deposits, is laborious and costly.