Geotechnical Engineering

DHARM

400 GEOTECHNICAL ENGINEERING

line, as shown in Fig. 11.4(a). (Although some rebound and some recompression occur during
the period of excavation and replacement of an equivalent load, their combined effect is con-
sidered negligible and hence is ignored).

An approximate method for the prediction of settlements during construction, advanced
by Terzaghi and extended by Gilboy, is presented in Fig. 11.4 (b), as given by Taylor (1948).
This method is based on the assumption that at the end of construction the settlement is the
same as that which would have resulted in half as much time had the entire load been active
throughout.

When any specified percentage of the effective loading period has elapsed, the load act-
ing is approximately equal to this percentage of the total load; at this time the settlement is
taken as this percentage times the settlement at one-half of this time from the curve of time
versus settlement under instantaneous loading.

Effective loading

period (assumed)

Excavation

period

O

+

Load

Construction period

P

Q

tl

Time

O —t^1

1

2 —tl

1

2 t 1 tl

Time

G

B

A

Settlement curve

for instantaneous loading

Settlement

C

D

E

F

(a) Loading pattern during construction period (b) Prediction of settlements during construction

Fig. 11.4 Graphical method for determination of settlements

during loading period (After Taylor, 1948)

Let OAB be the time-settlement curve for the given case for instantaneous loading. The

settlement at the end of the effective loading period tl is equal to that at^12 tl on the curve OAB.

Point C is obtained on the curve by projecting A horizontally onto the vertical through tl.

For any time t 1 < tl, the curve OAB shows a settlement FD at time 21 t 1. Since the load at
time t 1 is t 1 /tl times the total load, the settlement at time t 1 is obtained by multiplying FD by
this ratio. This is done graphically by joining O to D and projecting the vertical through t 1 to
meet OD in E. Then E represents the corresponding point on the corrected time-settlement
curve.
Repeating this procedure, any number of points on the curve may be obtained; the thick
curve is got in this manner. Beyond point C, the curve is assumed to be the instantaneous
curve AB, offset to the right by one-half of the loading period (for example, BG = AC). Thus,
after the construction is completed, the elapsed time from the start of loading until any given
settlement is reached is greater than it would be under instantaneous loading by one-half of
the loading period.