Page 38 - Water and DSİ

Basic HTML Version

37
CHAPTER III - HYDROELECTRIC ENERGY
In order to protect fine material from wave action and from scouring, riprap is applied on the filter
material. Generally riprap material is composed of 0.50-1 m diameter spalls. Mostly thickness of
riprap layer is 1 m. Beneath the riprap material, there is a gravel filter layer, thickness of which is
0.30-0.50 m. Generally clay core slopes may be 2/1-1/0.3 (horizontal/vertical). More steep slopes
may be formed. Upstream face slope of embankment may be 1/3 and downstream face slope of
embankment may be 1/3-1/5 the slope depending on height of dam foundation condition and
specification of embankment material.
Downstream face slope of embankment is subject to erosion of rain water, hence riprap thickness
of which is 0.50-1 m. or gravel layer or gravel layer’s thickness of which is 0.30-0.50 m. may be
covered on the downstream slope. Likewise, grass or shrub can serve the same function if the
climate is suitable. Minimum crest width is to be 6 m. It will be 10 m if it is used as road. For small
dams, crest width can be calculated by formula of (h/5)+3 (h=height of embankment).
3.5.2. Dams with Impervious Upstream Face
3.5.2.1. Concrete Face Rockfill Dams (CFRDs)
They are used when impervious material is too far to be carried to the construction site. Even if
impervious material is near, it will be detrimental to extract on the grounds of environmental
reasons. They are 25-30% cheaper than the cost of Earth Core Rockfill Dams (ECRDs). Their
constructions are faster than those of ECRDs. Using lesser quality rock in the construction of
CFRDs is possible. They are regarded as the best dam type in seismic regions. The embankments
are built by well dumped and compacted rock fill. So as to ensure the impermeability, upstream
face of CFRDs is lined with concrete. Settlements in rock fill and the foundation are studied so
well that impervious concrete cover would not impair. Embankment stability is to be analyzed
without considering impervious concrete face cover. Upstream and downstream slopes are
generally designed as 1.3/1.0 or 1.4/1.0 (horizontal/vertical). Concrete face cover is to be so
flexible that it is compatible with the deformation of the embankment. No stress is allowed on
anywhere of concrete face cover. Minimum thickness is 0.3 m. Various calculations based on the
water load are used to determine the thickness of the concrete face. t=0,30+0,002xH (H=meter,
water height. Concrete reinforcing steel in the concrete face are to be laid down both horizontal
and vertical constituting 0.5%-%0.8% of concrete cross-section so as to prevent shrinkage
fractures. Concrete lining is to be up to phlint in order to connect the pillow concrete. The width
of the pit is to be 2-12 m. Impervious earth material is to be lined on the pillow concrete up to the
certain level. The earth material is to be protected by riprap material. Curtain and gate grouting is
to be executed over the pillow concrete. Amount of rockfill and concrete is to be reduced by
constructing parapet wall. Rock spalls to be used in the rock fill constitutesvarious sizes to be used
in different zones.
3.5.2.2. Asphalt Faced Rockfill Dams
These dams are constructed in the similar way as CFRDs. Since asphalt cover is more flexible, they
are more resistant against fractures. Some expected attributes of asphalt cover material are;
Durability, Impermeability, Resistant to wave action and water pressure, Resistant to hydrostatic
bellow pressure, and Resistant to and compatible with local deformations and flexibilities.
Upstream slope may be between 1/1 and 1/2.5 depending on the material properties of the
embankment. Their upstream face covers are mainly designed as porous asphalt cover lined
bitumen or a cover system formed more than one asphalt cover.