Geotechnical Engineering

DHARM

SETTLEMENT ANALYSIS 407

In order to make large settlements harmless, structures may be designed as statically

determinate systems; the structures and their foundations may be designed as rigid units;

and, long structures may be subdivided into separate units.

Sometimes, structures such as bridges and water towers are supported at three points;

this facilitates jacking up the structure at any support so that the structure may be raised and

levelled as settlement occurs.

The need for thorough soil exploration and soil testing is obvious in the context of achiev-

ing these objectives.

11.7 SETTLEMENT RECORDS

Settlement records offer an excellent test of the accuracy of settlement analysis; as such, the

maintenance of such records, wherever possible, is considered very valuable. However, differ-

ent factors make it difficult to maintain such records; for example, very slow progress of settle-

ments, winding up of construction organisations and the waning interest after the completion

of construction. Careful comparison of settlement records with predicted values of settlements

goes a long way in the development of better methods of analysis for future use.

The most common method of observing settlements uses periodic lines of levels and

observing a representative group of reference points. Special levelling devices, such as the one

described by Terzaghi (1938), may be used for more accurate records. In any case, a reliable

and dependable benchmark must be available and in a locality where there is a deeply buried

clay layer, benchmarks that are not disturbed by settlement are difficult to be obtained. Bench-

marks are to be founded on firm ground, preferably on ledge or hard rock, for them to be

satisfactory, even if this means extending to a depth of more than 30 m.

11 .8 Contact Pressure and Active Zone From Pressure Bulb Concept

The concepts of contact pressure and active zone in soil based on the pressure bulb concept are

relevant to settlement computation and hence are treated in the following sub-sections.

11.8.1Contact Pressure
‘Contact pressure’ is the actual pressure transmitted from the foundation to the soil. It may
also be looked upon as the pressure, by way of reaction, exerted by the soil on the underside of
the footing or foundation. A uniformly loaded foundation will not necessarily transmit a uni-
form contact pressure to the soil. This is possible only if the foundation is perfectly ‘flexible’;
the contact pressure is uniform for a flexible foundation irrespective of the nature of the
foundation soil.
If the foundation is ‘rigid’, the contact pressure distribution depends upon the type of
the soil below the foundation as shown in Fig. 11.8.
On the assumption of a uniform vertical settlement of the rigid foundation, the theoreti-
cal value of the contact pressure at the edges of the foundation is found to be infinite from the
theory of elasticity, in the case of perfectly elastic material such as saturated clay (φ = 0).
However, local yielding of the soil makes the pressure at the edges finite, as shown in Fig. 11.8(a).
Under incipient failure conditions the pressure distribution, tends to be practically uniform.