Geotechnical Engineering

DHARM

SETTLEMENT ANALYSIS 409

Terzaghi pointed out that the depth, Dn, of the (1/n) (q)-isobar increases in direct pro-

portion to the width of the loaded area for similar shapes of these areas:

D

d

B

b

n

n

= = Constant = f(n) ...(Eq. 11.12)

Terzaghi also observed that direct stresses are considered negligible when they are
smaller than 20% of the contact stress from structural loading and that most of the settlement,
nearly 80% of the total, takes place at a depth less than Dn = 5, (Dn = 5 is the depth of the 0.20q-
isobar). Therefore, the isobar of 0.20q may be taken to define the contour of the pressure bulb,
which is the stressed zone within a homogeneous soil medium. The stress transmitted by the
applied foundation loading on to the surface of this isobar is resisted by the shear strength of
soil at this surface. The region within the 0.20q-isobar is called by Terzaghi the ‘‘seat of settle-
ment’’.
For a homogeneous, elastic, isotropic and semi-infinite soil medium, Dn = 5 ≈ 1.5B is
considered good. (For a uniform and thick sand, Dn = 5 < 1.5B).
The wider the loaded area, the deeper the effect for isobars of the same intensity, as
shown in Fig. 11.10.

q

Dn=5»1.5 B

Stressed

zone

sz= —. q-isobar^1 n

b

Stressed

zone

q

dn

B

sz= —. q-isobar^1 n

Fig. 11.10 Effect of width of foundation on depth of isobars (Jumikis, 1962)
(This will be again referred to in the plate load test in chapter 14).
The depth Dn = 5, to which the 0.20q-isobar extends below the foundation, which gives
the seat of settlement, is termed the ‘active zone’. The thickness of the active zone extends
from the base of the foundation to that depth where the vertical stresses from the structure
are 20% of the magnitude of the over-burden pressure of the soil, which contributes to most of
the settlement. This is shown in Fig. 11.11.
The soil layers below the active zone are considered as being ineffective, small stresses
being ignored. In other words, even if compressible strata exist below the active zone, their
effect on the settlement is negligible.

It may be noted that, while the vertical stress diagram due to self-weight of soil starts
with zero value at ground surface and increases linearly with depth, the stress diagram due to
contact pressure caused by structural loading starts with the value of contact pressure at the