The overall minimum thickness specified for the rock armour layer is
determined by two requirements:
- layer thickness, normal to the face, of not less than 1.5 D 50 ;
- layer thickness to be sufficient to contain the largest rock.
An additional important requirement is that the rock armouring should be
well graded to maximize stone interlocking and ensure stability. The rock
armouring is protected from undermining and subsidence by the introduc-
tion of one or more underlying filter layers designed to allow free drainage
beneath the armouring while preventing removal of fill or bedding mater-
ial. Filter design criteria and recommended grading envelope limits for
rock armouring are set out in Thompson and Shuttler (1976).
The thickness of a layer of rock armouring is generally of the order
of 0.7–0.9 m for fetches up to c. 5 km, with maximum-size stones of
c. 2000 kg (USBR (1987)).
Concrete blockwork face protection is generally designed by estab-
lished empirical relationships. The conventional rule is that given the provi-
sion of an appropriate underlayer system and properly executed joints, a
block thickness of Dwill be satisfactory for a significant wave height
Hs 6 D. Later research has indicated that the permissible Hs/Dratio is
dependent upon many factors, and it has been suggested (Hydraulics
Research (1988)) that the Hs/Drelationship is expressed by a function of the
formHs/ D, where is the density of the blockwork relative to water
(approximately 2.5 for concrete blocks) and has a value dependent upon a
number of factors including slope, wave characteristics, joint width and
drainage (permeability) of the under-layer system. Stability of the block-
work is improved by narrow joint widths (10 mm) to enhance interlocking.
Concrete slab face protection is constructed in large panels of the
order of 6 m6 m, the narrow gaps between panels providing necessary
articulation and allowing some transfer of hydraulic pressures into the
under-layer. The smooth surface of such panels allows significant wave
run-up (see Section 4.4), requiring an increase in overall design freeboard.
A monolithic asphaltic concrete facing, generally in the form of an
upstream deck serving a dual purpose as both watertight element and face
protection, offers a further alternative. It has the advantage that asphaltic
concrete has the flexibility to accept local deformation without distress and
therefore no articulation is necessary.
A soil-cement protective facing has been used on some dams in the
USA, but long-term performance has not always proved satisfactory.
For further information on face protection see also Chapter 14 and
Section 15.6
Face protection is discussed in depth in Thomas (1976), Thompson
and Shuttler (1976), ICOLD (1993a) and in Besley et al. (1999). A com-
prehensive treatment of rock armouring and rip-rap is given in CIRIA,
CUR, CETMEF (2005).
72 EMBANKMENT DAM ENGINEERING
