DAMSDAMS

DAMS:Dams are structures that block the flow of a river, stream, or other waterway.Purpose of a Dam:Dams are built for the following purposes:

1. Irrigation and drinking water2. Power generation (hydroelectric)3. Navigation4. Flood Control5. Multi purposes

TYPES OF DAMS:1. Gravity Dams – use only the force of gravity to resist water pressure – that is, they hold back water by the sheer force of their weight pushing downward.

>>> Grand Coulee Dam, >>> Grand Coulee Dam, Grand Coulee, Grand Coulee, Washington, USAWashington, USA

TYPES OF DAMS:2. Embankment Dam – is a gravity dam formed out of loose rock, earth, or a combination of these materials.

>>> Ambuklao Dam, Bokod, Benguet, >>> Ambuklao Dam, Bokod, Benguet, PhilippinesPhilippines

TYPES OF DAMS:2. Embankment Dam – is a gravity dam formed out of loose rock, earth, or a combination of these materials.

>>> Magat Dam, Cagayan, Isabela, >>> Magat Dam, Cagayan, Isabela, PhilippinesPhilippines

TYPES OF DAMS:2. Embankment Dam – is a gravity dam formed out of loose rock, earth, or a combination of these materials.>>> San Roque >>> San Roque Dam, Dam, Pangasinan, Pangasinan, PhilippinesPhilippines

TYPES OF DAMS:2. Embankment Dam – is a gravity dam formed out of loose rock, earth, or a combination of these materials.

>>> San Roque Dam, Pangasinan, >>> San Roque Dam, Pangasinan, PhilippinesPhilippines

TYPES OF DAMS:3. Arch Dams – are concrete masonry structures that curve upstream into a reservoir, stretching from one wall of a river canyon to the other.

>>> Glen Canyon Dam, Colorado River, >>> Glen Canyon Dam, Colorado River, Arizona, USAArizona, USA

TYPES OF DAMS:4. Buttress Dam – consists of a wall, or face, supported by several buttresses on the downstream side.>>> Buttress Dam, France>>> Buttress Dam, France

ANALYSIS OF GRAVITY DAM:Dams are structures whose purpose is to raise the water level on the upstream side of river, stream, or other waterway. The rising water will cause hydrostatic force which will tend the dam to slide horizontally and overturn about its downstream edge or toe.

ANALYSIS OF DAMS:The raised water level on the upstream edge or heel will also cause the water to seep under the dam. The pressure due to this seepage is commonly called hydrostatic uplift and will reduce the stability of the dam against sliding and against overturning.

ANALYSIS OF DAMS:Step 1: Consider 1 unit length (1 m length) of dam perpendicular to the cross section.With reference to the figure :

ANALYSIS OF DAMS:Step 2: Determine all the forces acting:A.Vertical Forces1. Weight of the damW1=γcV1; W2 =γc V2; W3 =γc V3

2. Weight of water in the upstream side (if any) W4=γV43. Weight of permanent structures on the dam4. Hydrostatic Uplift U1=γVu1 U2=γVu2

Where:γ = unit wt. of waterγc = unit wt. of concreteγc = 2.4γ (usually taken as 23.5 kN/m3 )

With reference to the figure:

ANALYSIS OF DAMS:Step 2: Determine all the forces acting:B.Horizontal Force

1. Total Hydrostatic Force acting at the vertical projection of the submerged portion of the dam.2. Wind Pressure3. Wave Action4. Floating Bodies5. Earthquake Load

With reference to the figure:

ANALYSIS OF DAMS:Step 3: Solve for the ReactionA.Vertical Reaction, RyRy= ΣFv Ry= W1+ W2 + W3 + W4 - U1 - U2•Horizontal Reaction, Rx Rx= ΣFh Rx= F

With reference to the figure:

ANALYSIS OF DAMS:Step 4: Solve for the ReactionA.Righting Moment, RM (rotation towards the upstream side)

RM=W1x1 +W2x2 +W3x3 +W4x4B.Overturning Moment, OM (rotation towards the downstream side)

OM = F(y)+ U1z1 +U2z2

With reference to the figure:

ANALYSIS OF DAMS:With reference to the figure:Step 5: Location of Ry, ( )

FACTORS OF SAFETY:1. Factor of safety against sliding, FSs:

2. Factor of safety against overturning, FSo:

Where: µ = coefficient of friction between the base of the dam and the foundation

FOUNDATION PRESSURE:1. For eccentricity, e ≤ B/6:

Note: Use (+) to get the stress at point where Ry is nearest.In this figure, use (+) to get qT and (-) to get qHA negative stress indicates compressive stress and a positive stress indicates tensile stress.

FOUNDATION PRESSURE:2. For eccentricity, e > B/6:

SAMPLE PROBLEMS:A gravity dam of trapezoidal cross-section with one face vertical and horizontal base id 22 m high and has a thickness of 4m at the top. Water upstream stands 2m below the crest of the dam. The unit weight of concrete is equal to 23.5 kN/m3 and the coefficient of friction between the base of the dam and the foundation is 0.5.

SAMPLE PROBLEMS:Neglecting hydrostatic uplift, determine the following:1.Factor of safety against sliding2.Factor of safety against overturning3.Overturning moment acting against the dam4.Foundation Pressure

SAMPLE PROBLEMS:Considering uplift pressure to vary uniformly from full hydrostatic pressure at the heel to zero at the toe.1.Factor of safety against sliding2.Factor of safety against overturning3.Overturning moment acting against the dam