Loading...

Loading
26
27
Primary loads
1.  Water load. This is a hydrostatic distribution of pressure with horizontal
resultant force P1. (Note that a vertical component of load will also exist
in the case of an upstream face batter, and that equivalent tailwater loads
may operate on the downstream face.)
2.  Self-weight load. This is determined with respect to an appropriate unit
weight for the material. For simple elastic analysis the resultant, P2, is
considered to operate through the centroid of the section.
3.  Seepage loads. Equilibrium seepage patterns will develop within and
under a dam, e.g. in pores and discontinuities, with resultant vertical
loads identified as internal and external uplift, P3 and P4, respectively.
(Note that the seepage process will generate porewater pressures in
pervious materials, and is considered in this light as a derivative of the
water load for the embankment dam)
28
Secondary loads
1.  Sediment load. Accumulated silt etc. generates a horizontal thrust,
considered as an equivalent additional hydrostatic load with horizontal
resultant P5.
2.  Hydrodynamic wave load. This is a transient and random local load,
generated by wave action against the dam (not normally significant).
3.  Ice load. Ice thrust from thermal effects and wind drag, may develop in
more extreme climatic conditions (not normally significant).
4.  Thermal load (concrete dams). This is an internal load generated by
temperature differentials associated with changes in ambient conditions
and with cement hydration and cooling.
5.  Interactive effects. These are internal, arising from differential
deformations of dam and foundation attributable to local variations in
foundation stiffness and other factors, e.g. tectonic
6.  Abutment hydrostatic load. This is an internal seepage load in the
29
abutment rock mass (It is of particular concern to arch or cupola dams.)
Exceptional loads
1.  Seismic load. Oscillatory horizontal and vertical inertia loads are
generated with respect to the dam and the retained water by seismic
disturbance. For the dam they are shown symbolically to act through the
section centroid. For the water inertia forces the simplified equivalent
static thrust, P8 (slide 27), is shown
2.  Tectonic effects. Saturation, or disturbance following deep excavation in
rock, may generate loading as a result of slow tectonic movements.
30
Predimensioning of a gravity dam
Water load
h
1
2
S = ! wh
2
S
h/3
P
Sp b/3
Self-weight load
1
P = ! d bh
2
Uplift load
1
S p = µ! w hb
2
31
To calculate the unknown width b impose that the section of the
foundation, supposedly rectangular with unit deep, is subject only to
compression, it follows that the reaction of the soil must be applied within
the middle third; as limit condition it can passes in the limit of the core.
central core of inertia, is defined as the area in which the load must fall
for the homogeneous type of stress, for which the section is the whole or
any compression or traction
b/3
central core of inertia
b
middle third
32
Equilibrium of moments
h
b
b
S + SP ! P = 0
3
3
3
1
1
1
3
2
! w h ! ! d b h + µ! w hb 2
6
6
6
b 2 (µ! w ! ! d )h = !! w h 3
!w
b=h
! d ! µ! w
Pre-dimensioning for gravity dams with vertical upstream
face (h<30-35 m)
33
By comparison with other possible profiles you realize that the
triangle minimizes the volume.
34
Sediment load The gradual accumulation of significant deposits of fine
sediment, notably silts, against the face of the dam generates a resultant
horizontal force, Ps. The magnitude of Ps, which is additional to water load
Pwh, is a function of the sediment depth, h3, the submerged unit weight !s
and the active lateral pressure coefficient, Ka,
It is active at h/3
1 2
PS = K a ! s h3
2
1! sin ! s
Ka =
1+ sin ! s
where "s is the angle of shearing resistance of the sediment
Values of "s=18–20 kNm-3 and "s=30° are representative.
Accumulated depth h3 is a complex time-dependent function of suspended
sediment concentration, reservoir characteristics, river hydrograph and other
factors .
35
Hydrodynamic wave load The transient hydrodynamic thrust generated by
wave action against the face of the dam, Pwave, is considered only in
exceptional cases. It is of relatively small magnitude and, by its nature,
random and local in its influence. An empirical allowance for wave load may
be made by adjusting the static reservoir level used in determining Pwh.
Where a specific value for Pwave is necessary a conservative estimate of
additional hydrostatic load at the reservoir surface is provided by
Pwave = 2! w H
2
s
Hs is the significant wave height, i.e. the mean height of the highest third
of waves in a sample, and is reflected at double amplitude on striking a
vertical face
36
As a basis for a wave height computation, H (m) (crest to trough) can be
estimated from:
H = 0.34F 1 2 + 0, 76 ! 0.26F 1 4
F (km) is the fetch (the maximum free distance which wind can travel
over the reservoir). For large values of fetch (F > 20 km) the last two
terms may be neglected.
Using the concept of significant wave height, Hs (the mean height of the
highest third of the waves in a train with about 14% of waves higher than Hs)
the use for the design wave height, Hd, is recommended.
Hd is a multiple of Hs ranging from 0.75Hs for concrete dams to 1.3Hs for
earth dams with a grassed crest and downstream slope and 1.67Hs for dams
with no water carryover permitted. Hs (m) can be determined quickly from
figure as a function of the wind velocity (m/s) and fetch (m) based on the
simplified Donelan/JONSWAP equation:
UF 1 2
H=
1760
37
38
Il Regolamento Italiano Dighe (R.I.D.) [D.P.R. n. 1363 del 1/11/59
(parte I) e D.M.LL.PP del 24/3/82] stabilisce precise norme per la
progettazione e la verifica statica di ogni tipologia di dighe.
In particolare, per le dighe a gravità ordinaria essa prescrive due
verifiche di sicurezza:
a)  Verifica a scorrimento
b)  Verifica di resistenza
Da effettuare sia in condizioni di lago vuoto, che di lago pieno alla
quota di massimo invaso, tenendo conto di tutte le forze che
agiscono sulla struttura.
39
La verifica di resistenza deve essere effettuata per le seguenti
condizioni di carico:
1) A serbatoio vuoto, considerando le azioni del peso proprio ed
eventualmente le azioni sismiche;
2) A serbatoio pieno, considerando le azioni del peso proprio, la spinta
idrostatica, spinta del ghiaccio ed eventualmente le azioni sismiche.
La verifica allo scorrimento deve essere effettuata per la condizione di
lago pieno.
40
fixed
Earth
Brickwork
Concrete
mobile
Plain
Radial
Drum
Roller
Flap
Weirs
Barrages
embankment
Earthfills
rockfills
gravity
Dams
Concrete
Arch
Gravity
butress
Arch
Arch-gravuty
Cupola
41
Weirs and barrages are relatively low-level dams constructed across
a river to raise the river level sufficiently or to divert the flow in full, or
in part, into a supply canal or conduit for the purposes of irrigation,
power generation, navigation, flood control, domestic and industrial
uses, etc. These diversion structures usually provide a small storage
capacity. In general, weirs (with or without gates) are bulkier than
barrages, whereas barrages are always gate controlled. Barrages
generally include canal regulators, low-level sluices to maintain a
proper approach flow to the regulators, silt excluder tunnels to control
silt entry into the canal and fish ladders for migratory fish movements.
42
43
44
The fixed barrages can be constructed in masonry, concrete, clay and
many other materials (stone, wood) and they have a suitable profile to be
overflowed by excess capacities and are often equipped with gates to
evacuate gravel
The movable barrages consist normally of a work fixed in masonry or
concrete (threshold and driving piles) and real moving parts (gates) that
can be of various types.
45
Plain gates
Le paratoie piane sono costituite
da pareti piane in legno o in
acciaio, scorrevoli in guide
(gargami) con esse complanari;
quelle in legno sono adatte per luci
con larghezza massima di 3 metri
e altezza di ritenuta di 2 o 3 metri,
quelle in acciaio possono
raggiungere valori più elevati (luce
di 20 metri e ritenuta di 10 metri)
perché vengono rinforzate con
travi in profilati di acciaio che
sopportano la spinta idrostatica.
Le tenute sul fondo e sui lati vengono realizzate per mezzo di travi
con guarnizioni di gomma. Il problema delle paratoie piane, che ne
limita le dimensioni, è quello del sollevamento a causa dei notevoli
attriti in gioco.
46
Sector gates
Le paratoie a settore hanno la
forma di un settore cilindrico
girevole attorno ad un perno
coincidente con l’asse del
cilindro; esse sono costituite da
una robusta ossatura a traliccio
rivestita da una lamiera
metallica; la tenuta sulla soglia e
sulle pareti laterali è realizzata
con strisce di gomma.
The advantages of radial over vertical lift gates are smaller hoist,
higher stiffness, lower (but longer) piers, absence of gate slots,
easier automation and better winter performance.
47
Roller gates
Le paratoie cilindriche sono indicate per luci fino a 40 metri ma con piccoli
battenti. Sono costituite da un cilindro in lamiera di acciaio opportunamente
irrigidito da appositi profilati. Il cilindro è disposto orizzontalmente ed il moto
di sollevamento avviene per rotazione su una apposita cremagliera. Per
aumentare l’altezza di ritenuta, che normalmente è pari al diametro del
cilindro, si può dotare la paratoia di un becco inferiore e di uno scudo. La
tenuta è realizzata con una trave di legno sulla generatrice di appoggio
inferiore e con lamierino o gomma sui fianchi.
48
Flap gates
Le paratoie a ventola sono costituite da una
struttura piana in ferro, ricoperta di lamiera,
girevole intorno ad un asse orizzontale
coincidente con il bordo inferiore; esse si
prestano bene per luci fino a 15 metri e
altezza di ritenuta non superiore a 5 metri.
Le paratoie a ventola e a settore si prestano
assai bene al comando automatico.
Flap gates provide fine level regulation, easy flushing of debris and ice, and
are cost effective and often environmentally more acceptable than other
types of gates; they require protection against freezing and are particularly
sensitive to aeration demand and vibration
49
Reservoir storage zone and uses of reservoir
50
Full Reservoir Level (FRL): It is
the level corresponding to the storage
which includes both inactive and
active storages and also the flood
storage, if provided for. In fact, this is
the highest reservoir level that can be
maintained without spillway discharge
or without passing water downstream
through sluice ways.
Minimum Drawdown Level (MDDL): It is the level below which the
reservoir will not be drawn down so as to maintain a minimum head
required in power projects.
Dead Storage Level (DSL): Below the level, there are no outlets to
drain the water in the reservoir by gravity.
Maximum Water Level (MWL): This is the water level that is ever likely
to be attained during the passage of the design flood. It depends upon
the specified initial reservoir level and the spillway gate operation rule.
This level is also called sometimes as the Highest Reservoir Level or
the Highest Flood Level.
51
Live storage: This is the storage
available for the intended purpose
between Full Supply Level and the
Invert Level of the lowest discharge
outlet. The Full Supply Level is
normally that level above which over
spill to waste would take place. The
minimum operating level must be
sufficiently above the lowest
discharge outlet to avoid vortex
formation and air entrainment. This
may also be termed as the volume of
water actually available at any time
between the Dead Storage Level and
the lower of the actual water level
and Full Reservoir Level.
Dead storage: It is the total storage below the invert level of the lowest
discharge outlet from the reservoir. It may be available to contain
sedimentation, provided the sediment does not adversely affect the
lowest discharge.
Outlet Surcharge or Flood storage: This is required as a reserve
between Full Reservoir Level and the Maximum Water level to contain the
peaks of floods that might occur when there is insufficient storage
capacity for them below Full Reservoir Level.
52