Rock-filled Dams

ENGINEERING AND CONSTRUCTION SPECIFICATOINS

FIG 1: Typical characteristics of a rock-filled dam – centrally located core (http://aitech.ac.jp/~narita/tembankmentdam1.pdf - accessed 2015/03/19)

FIG 2: Rock-filled dam specifications

1.0 Specifics:

1. The height of the dam is the greater of the following (a) The height from the natural bed of the stream at the downstream edge of the

dam wall to the top of the dam, or(b) The height from the lowest elevation of the outside limit of the dam to the top of

the dam (top of the dam means the uppermost part of the dam excluding any settlement chamber or structures as parapets and guard rails that are part of the main water retaining structure.

2. The cut-off trench is required to be taken down a minimum of 600mm into impervious solid and backfilled with good quality clay that is thoroughly compacted.

3. A crest settlement allowance for the total length of the crest of 5% of embankment height is required unless otherwise recommended.

2.0 Design stability considerations and failures

2.1 Stability requirements:

Stability against failure of embankment evaluation of pore-water pressure during and after construction)

Shear strength and deformation characteristic of fill materials Seismic stability Stability at the contact face of dam body and base of foundation (contact clay,

compaction relative displacement, arching, cracking)

2.2 Seepage through embankment and foundation

Seepage analysis (discharge, pore-water pressure, leakage through foundation critical velocity, critical hydraulic gradient, hydraulic fracture

2.3 Foundation treatment

Stability (counterweight fill, relief well) Seepage (drainage facilities, grouting, blanket)

3.0 GENERAL DESIGN CONSIDERATIONS

3.1 Alignment

Axes of embankments that are long with respect to their heights may be straight or of the most economical alignment fitting the topography and foundation conditions. Sharp changes in alignment should be avoided because downstream deformation at these locations would tend to produce tension zones which could cause concentration of seepage and possibly cracking and internal erosion.

The axes of high dams in narrow, steep-sided valleys should be curved upstream so that downstream deflection under water loads will tend to compress the impervious zones longitudinally, providing additional protection against the formation of transverse cracks in the impervious zones. The radius of curvature forming the upstream arching of the dam in narrow valleys generally ranges from 1,000 to 3,000 ft.

3.2 Embankment

Embankment sections adjacent to abutments may be flared to increase stability of sections founded on weak soils. Also, by flaring the core, a longer seepage path is developed beneath and around the embankment.

3.3 Top width (crest)

The top width of the dam within the specified conventional limits has little effect on stability and is governed by whatever functional purpose the top of the dam must serve. The minimum width should be between 25ft and 40ft. if the top is to carry a roadway, road shoulders must be considered in its conformation design.

3.4 Core

The main body of the dam (the core) should have structural resistance against failure, consisting of rock-fill shell and transition zones – core and facing have a role to minimize leakage through embankment. A filter zone should be provided in any type of rock-fill dams to prevent loss of soul particles by erosion due to seepage flow through embankment. The core is separated from the rock shells by a series of transition zones built of properly graded material.

A membrane of concrete, asphalt, or steel plate on the upstream face should be considered, in lieu of an impervious earth core only when sufficient impervious material is not available. NOTE: This means that if the core is not impervious to water, then the upstream face of the rock-fill dam itself should be impervious.

3.5 Rock fill

Rock-fill dams are made up mainly from dumped and compacted rock-fill, help together in a closed or stable system – usually mesh baskets and more lately, geofabrics. Rock filled zones are compacted in layers of 12 or 24 inches thick, by heavy rubber-tired or steel-wheel vibratory rollers. Materials range from sound free draining rock to the more friable materials such as sand stones and silt-shale that break down under handling and compacting to form an impervious to semi-impervious mass. They are often termed random rock and can be used successfully for dam construction but because of stability and seepage considerations, the embankment design using such materials is similar to that of earth dams.

Materials used for the cover (impervious cover, with reference to non-impervious core), include reinforced concrete and asphaltic concrete as mentioned before.

FIG 3: reinforced concrete cover on upstream face of dam (Quoich Dam - http://www.britishdams.org/images/quoich_face.jpg accessed 2015/03/17)

3.6 Freeboard

a. Vertical distance: Freeboard is the vertical distance from the top of the embankment to the level of the spillway. Adequate depth must be provided

Freeboard must not be less than the dimensions specified initially. The absolute minimum is 500mm, with an additional 250mm to cater for any form of wave action.

Note: protection from wave action – additional height required is given by the formula:

Hawksley’s formula: H=0.0135(F)0.5

Where H is the wave height in meters, and F is the fetch distance over the longest exposed water surface expressed in meters.

b. Elevation: In seismic zones, the elevation of the top of the dam should be the maximum determined by either maximum water surface plus conventional freeboard or flood control plus 3% of the height of the dam above stream bed.

3.7 Spillway (stream return)

Consider open crest (ogee) spillway: The purpose of the spillway is to pass flood flows without overtopping the dam wall. Particular attention must be aid to providing adequate width and depth (or free board) of the spillway. The following guidelines apply to spillways:

1. The absolute minimum width of the spillway is three (3) meters

2. The spillway should be cut in solid material (preferably rock) that will resist erosion. The stream return should be channelled back to the original watercourse and stabilised with a suitable sized riprap consisting of rock or other materials such as geofabric.

3. Specific engineering advice must be sought before changing or modifying or obstructing a spillway in any manner.

3.8 Abutments

Works Cited2014. About dams. Accessed 3 17, 2015. http://www.britishdams.org/about_dams/embankment.htm.

Corps, US Arm. 2004. "Engineers Manuals ." publications.usace. Jul 30. Accessed 3 17, 2015. http://www.publications.usace.army.mil/Portals/76/Publications/EngineerManuals/EM_1110-2-2300.pdf.

Tasmania. 2008. "Guidelines for construction of Embankment dams ." dpipwe.tas.gov.au. Accessed 3 17, 2015. http://dpipwe.tas.gov.au/Documents/Guidelines-for-earth-fill-dams.pdf.

n.d. Types of earth embankment Dams. SJ. Accessed 3 19, 2015. http://www.aboutcivil.org/embankment-dams-types.html.