CVE 515 EMBANKMENT DAM ENGINEERING ENGR S.O ODUNFA DEPT. OF CIVIL ENGINEERING UNIVERSITY OF AGRICULTURE, ABEOKUTA

Introduction A dam is an obstruction placed across a river or stream to perform certain function

Types of Dams Storage dam Diversion dam Detension dam Multipurpose dam

Classification of Dams Based on constructional materials we have Earthfill dam – made up of locally material Rockfill dam – composed of rocks of all sizes to provide stability and impervious membrane for water seal. Concrete gravity dam Concrete arch dam

Earth and Rockfill Dams Further classifications Homogeneous – Constructed of only one materials usually of sufficiently impervious materials Zoned – Composed of a central impervious core flanked by zones of considerably pervious materials called shells. Diaphram – Involves the use of a pervious materials to form a water barrier.

Principles of analysis and design for earth and rockfill dams Site investigation Reconnaissance study – To identify the needed data. Site apprasial – Functional suitability of a site must be governed by the balance between its natural physical characteristics and the purpose of the reserviour catchment hydrology Study of certain parameters of the environment so that their normal behaviour can be built into the dam. E.g

Stream flow records Flood records Wind records Ground water data Evaporation rates Rainfall and temperature records Water quality records,etc

Dam Site Evaluation The nature of the soil and rock formations present,critical to formations integrity is to be proved by subsurface exploration. Based on all these imformation, analysis and design would follow

Feasibility Report It is prepared at the conclusion of the reconnaissance survey and interpretes all available information and data.

Materials for Construction Earthfill dam – Locally materials Rockfill dam – Rocks of all sizes are used for stability and impervious membrane for water seal.

Methods of Constructions Rockfill Dam Rock placed in layers – Materials are dumped on the embankment surface and pushed over the advancing construction face with a buldozer

Dumped in high sluiced lift – The material is dumped down the sloping face of the construction lift and sluiced with high pressure water jets from monitors. The fines and smaller rocks are deposited at the top of the lift and the larger rocks slide and rolled down the face of the slope to the lower part of the lift. Earthfill Dam Excavate the materials Hauling to the dam,

Mixe to predetermine water content and unformity of properties Sread it in layers and Compact to desire density

Advantages of construction in layers Rock with a higher percentage of fines can be placed, there is less danger segregation of fines accumulations Embankment obtained is more denser, uniform and less compressible It can be built with any slide slope inclination but rockfill dumped in high lift is placed on the angle of repose of the materials etc

STABILITY The stability of an embankment lies in its ability to resist shearing stresses created by external applied loads such as reservoiur water pressure and internal forces caused by the soil mass and embankment slope

Stability of Downstream slope during steady seepage Examine the most critical condition i.e when the reservouir is full and the seepage is taking place at full rate Draw the flow net and determine the points of intersections of equipotentiallineswith failure arc Get the critical arc Calculate the shear strength developed on the slices and find the net shear strength

Seepage and Drainage Seepage is the ease at which water flows through the soil and this is possible due to the presence of voids within the soil particles. Permeability is the property that allows the flow of water/fluid through the soil. The flow of water through the soils is assumed to follow Darcy’s law: Q/t = KAH/l

Where Q = Quantity of water flowing t = time for quantity Q to flow; K = coefficient of permeability for the soil A = area of cross section through which the water flows; H = hydraulic head across soil l = length of flow path through soil.

Seepage can be effectively study by the use of flow nets Flow net is a pictorial representation of the path of flow through a soil. Flow net comprises of flow lines and equipotential lines Construction of flow net

Control of Seepage Through embankment:- Rock Toe/filter and Horizontal blanket methods Through foundation:- Impervious cut offs and Chimmy drain methods

Maintenance – periodic and Daily routing maintenace Performing monitoring

Reservoiur Design Studies Types of reservoiurs Storage reservoiurs – To retain excess water flow in a natural sream or river during raining season Service/Distribution reservoiur – used in water supply system where water stored in it is used to supplement the water supply when the demand exceed the water pumping rate