DHARMLATERAL EARTH PRESSURE AND STABILITY OF RETAINING WALLS 501
Either the trial wedge approach or Culmann’s approach may be used but one has also to
consider the effect of the tensile zone in reducing Ca and C.
However, it must be noted that the Coulomb theory with plane rupture surfaces is not
applicable to the case of passive resistance. Analysis must be carried out, strictly speaking,
using curved rupture surfaces such as logarithmic spirals (Terzaghi, 1943), so as to avoid
overestimation of passive resistance.
A C^1 C^2 zcC 3 C 4Ft
1 ¢2 ¢G¢3 ¢3
2 G
1t– 4 ¢ f-lineBy-line4Fig. 13.42 Culmann’s method adapted to allow for cohesion13.7.9 Use of Tables and Charts for Earth Pressure
To facilitate earth pressure calculations, earth pressure coefficient tables, such as those given
by Caquot and Kerisel (1956) and Jumikis (1962), may be used. These give Ka and Kp coeffi-
cients for various α, β, δ, and φ, values, in reasonable ranges practically possible for each.
Linear interpolation may be used satisfactorily for obtaining values not available directly from
the tables.
For design purposes, even the use of charts may be considered all right. However, most
of the charts available may have φ and δ as variables and consider standard common values for
others, such α= 90° and β = 0°. Therefore, these charts may be useful only for certain simple
situations.
13.7.10 Comparison of Coulomb’s Theory with Rankine’s Theory
The following are the important points of comparison:
(i) Coulomb considers a retaining wall and the backfill as a system; he takes into ac-
count the friction between the wall and the backfill, while Rankine does not.
(ii) The backfill surface may be plane or curved in Coulomb’s theory, but Rankine’s
allows only for a plane surface.
(iii) In Coulomb’s theory, the total earth thrust is first obtained and its position and
direction of the earth pressure are assumed to be known; linear variation of pres-
sure with depth is tacitly assumed and the direction is automatically obtained from