Civil Engineering Reference
In-Depth Information
By plotting the load-time relationship, the time t
1
can be found (Fig.
12.9)
, the time t
2
being taken as
the time in which the net foundation load is applied. The settlement curve, assuming instantaneous appli-
cation of the load at time t
1
, is now plotted and a correction is made to the curve by assuming that the
actual consolidation settlement at the end of time t
2
has the same value as the settlement on the instan-
taneous curve at time t
2
/2. Point A, corresponding to t
2
/2, is obtained on the instantaneous curve, and
point B is established on the corrected curve by drawing a horizontal from A to meet the ordinate of time
t
2
at point B. To establish other points on the corrected curve the procedure is to:
(i) select a time, t;
(ii) determine the settlement on the instantaneous curve for t/2 (point C);
(iii) draw a horizontal from C to meet the ordinate for t
2
at D, and
(iv) join OD.
Where OD cuts the ordinate for time, t gives the point E on the corrected curve, the procedure being
repeated with different values of t until sufficient points are established for the curve to be drawn. Points
beyond B on the corrected curve are displaced horizontally by the distance AB from the corresponding
points on the instantaneous curve.
Example 12.4:
Settlement versus time relationship
If in Example 11.7 the excavation will take 6 months and the structure will be completed
in a further 18 months, determine the settlement to time relationship for the central
point of the raft during the first 5 years. The clay has a c
v
value of 1.86 m
2
/year and the
sandstone may be considered permeable.
Solution:
The initial excess pore water pressure distribution will be roughly trapezoidal. The first
step is to determine the values of excess pore pressures at the top and bottom of the
clay layer (use Fig.
3.10)
.
B
Z
L
Z
Depth below
foundation (m)
I
σ
4I
σ
Δ
σ
1
(kPa)
Top of clay
6.1
1.5
4.5
0.229
0.916
295
Bottom of clay
36.6
0.25
0.75
0.06
0.24
77.3
36 6 6 1
2
.
−
.
Drainage path length
=
=
15 25
.
m
295
77 3
m
3 82
.
;
values of U are obtained from Fig
.
12 4
.
.
=
=
.
Plotting the values of consolidation against time gives the settlement curve for instan-
taneous loading, which can be corrected to allow for the construction period (Fig.
12.10,
which also shows the immediate settlement to time plot). The summation of these two
plots gives the total settlement to time relationship.
