Design, specification and construction of filters - Geotechnical Engineering of Dams - page 345

Table 9.1. Critical and non-critical filters (adapted from ICOLD, 1986).

Type of flow Significance Access for

Filter location Purpose of filter or loading of filter repair

a. Downstream Control of erosion P - Occasional Non-critical Easy

slope protection by rainfall surface flow.

b. Downstream Removal of surface P - Continuous or Non-critical. Local Easy,

surface drains seepage occasional local wet areas may reappear possible

seepage

c. Upstream Control of erosion P - Cyclic flow Usually non-critical Possible, but

slope protection by wave action and during wave action. may be

by outward flow N 2 - Small flow difficult

during drawdown during drawdown

d. Temporary Dissipation of N 2 - Temporary Non-critical. Failure None

internal drainage excess pore pressure flow, limited may lead to instability

during construction during construction quantity. Some during construction,

of wet fills migration of fines or delays

allowable if drains
not blocked

e. Upstream Prevention of N 2 - Transient and Non-critical. Only None

internal fill unacceptable small flows during significant if migration

boundary migration of fines in drawdown is large and continuous

upstream direction

f. Downstream Prevention of N 2 - Flow only due Critical, but only if None

internal unacceptable to infiltration of erosion is large and

interface migration of fines rainfall, not from continuous

into filter-drains reservoir or

foundation

g. Downstream Prevention of N 1 - Continuous Critical None

interface, e.g. internal erosion of flow from reservoir,

downstream core core including potentially large

boundary or effects of and increasing if

foundation inter- concentrated flow erosion occurs

face near core in cracks, etc.

h. Upstream Prevention of N 1 - Continuous Critical None

interface between internal erosion of flow from reservoir,

embankment core into foundation potentially large and

and foundation increasing if erosion

occurs

i. Upstream Prevention of N 2 - usually Critical, but only if None

internal interface unacceptable approaching N 1 for erosion is large and

migration of fines pumped storage continuous

from core into dams

upstream fill

the foundation can occur. If it can, the filters are critical. As shown, “d” is a construction
drainage system, so would probably be non-critical.
Much of the discussion in this chapter is about critical filters designed to control inter-
nal erosion under N 1 flow conditions.

9.1.4 Filter design notation and concepts

9.1.4.1 Notation
The notation used in this topic is described in the following examples:
D 15F

Particle size of the filter material for which 15% by weight is finer.