Geotechnical Engineering

DHARM

332 GEOTECHNICAL ENGINEERING

The pore pressure ratio ru can be easily obtained from the flow net for this case, as

shown in Fig. 9.16.

hw

z

Earth

slope

Top flow line

Flow lines

Equipotential lines

Fig. 9.16 Determination of pore water pressure from flow net

The equipotential through the point is traced up to the top of the flow net, so that

piezometric head hw is established.

Since u = γwhw , ru =

γ

γ

wwh
z ...(Eq. 9.21)
The effect of the pore water pressure is to reduce the effective stress and thereby reduce
the stability because the shear strength mobilised would be decreased.

The factor of safety F may be written as:

F =

cr N U

T

′+tan ′Σ( −θφ)
Σ ...(Eq. 9.22)
Here c′ and φ′ are the shear parameters based on effective stress analysis, which may be
got from drained tests in the laboratory.

ΣU = Total force because of pore pressure on the surface.
ΣU can be obtained by obtaining values of u at the points of inter-section of the slip
surface with equipotentials as stated earlier by Eq. 9.21 and showing the respective values as
ordinates normal to the slip surface and getting the area of this U-diagram, similar to the N-
and T-curves.

In the absence of a flow net, the approximate value of F may be given by:

F =

cr N

T

′+tan ′ ′θφΣ
Σ ...(Eq. 9.23)
Here the normal components N′ of the weights of slices have to be obtained using effec-
tive or buoyant unit weight γ′ and the tangential components T using the saturated unit weight.

A value of F ranging from 1.25 to 1.50 is considered to be satisfactory for an earth slope.
For economic reasons, a value greater than 1.50 is not desired. Hence, F = 1.50 may be consid-
ered to be necessary as well as sufficient.

Rapid Drawdown

For the upstream slope of an embankment or an earth dam, the case of rapid or sudden
drawdown represents the critical condition since the seepage force in that condition adds to