Civil Engineering Reference
In-Depth Information
(Notice that the hydraulic gradient at the phreatic surface is strictly d
P
/d
s
, but d
P
/d
r
is a sufficiently good approximation.) Integrating Eq. (17.3) between
P
1
at
r
1
and
P
2
at
r
2
we have
ln
r
2
r
1
P
2
−
P
1
=
π
k
q
(17.4)
Hence
k
can be obtained from observations of the pumping rate
q
and water levels in
standpipes at a number of different radii.
For fine-grained soils steady state seepage will not be reached quickly and during a
reasonable test period there will be simultaneous steady state flow and consolidation
or swelling. Figure 17.6(b) illustrates a flow from a spherical cavity radius
r
with a
constant excess pore pressure
w
h
w
. The rate of flow at any time
t
is given by
u
¯
=
γ
rkh
w
1
r
√
π
q
=
4
π
+
(17.5)
c
s
t
where
c
s
is the coefficient of consolidation for spherical consolidation. (This is similar
to
c
v
for one-dimensional flow, discussed in Chapter 15) A condition of steady state
flow would be reached after infinite time and, with
t
=∞
in Eq. (17.5),
rkh
w
(17.6)
A value of
q
∞
can be found by plotting
q
against 1/
√
t
, as shown in Fig. 17.5(c), and
extrapolating. Hence a value for
k
can be obtained from Eq. (17.6). If the cavity is not
spherical the term 4
q
∞
=
4
π
π
r
must be replaced by an intake factor
F
which depends on the
geometry.
17.7 Ground investigation reports
The findings of ground investigations are recorded in two different kinds of reports.
(a) Factual reports
These simply describe the procedures and findings without comment or interpretation.
The report will contain text describing what was done, how, where and by whom.
It will summarize the factual findings of the desk study, the field investigations and the
in situ
and laboratory tests.
The basic information from the drilling and sampling operations is contained in
borehole logs. (Similar logs contain information from test pits.) A typical borehole log
is shown in Fig. 17.7, this is idealized and simplified to illustrate the principle features
which should be recorded. The top panel gives the date, time, place, method of drilling
and other basic information. The legend is a pictorial representation of the principal
strata with a word description alongside. To the left are depths and levels. To the right
are columns for sample recovery, groundwater observations and
in situ
tests. Borehole
logs prepared by different ground investigation companies differ in detail but should
contain at least this basic information. The borehole log in Fig. 17.7 is for a borehole
