Design of earth and rockfill dams
Considering the variety of potential dam locations, and the number and variability of potential construction materials, it is obvious that each dam is a unique structure. Accordingly, the design is individual for each dam, and it is not easy to define general design criteria. An attempt is made here, whereby the background of each of the criteria must, necessarily, be generalized, expressing rules of engineering which do not apply only to dam engineering but also to other engineering disciplines:
– All water-retaining dams must be impervious in a practical sense. Inevitable seepage through dam and foundation must not endanger the safety and the durability of the structure. In addition, seepage must be considered from the viewpoint of reservoir economy and environmental impact,
– the permeability of the foundation and of the abutments must be low. Otherwise those involved must establish favourable flow conditions by proven means of impermeabilization,
– the foundation area must be made regular and smooth. The shape and the properties of the foundation should not cause excessive deformation of the structure on top,
– dam zoning allows the use of different materials, each of them at the place where the properties fit best with the function,
– the incorporation of transition layers between different zones serves to bridge differential settlements between parts of the dam. Regular lines of equal stresses and deformations indicate compatible conditions,
– deformations must not lead to cracks in the sealing element. This is the more important the shorter the seepage path across the element,
– internal earth pressures in the sealing element must exceed the hydrostatic pressure of the retained water. This applies to pressures in all directions and in the center as well as in the outer zones of the element. Otherwise hydraulic fracturing and piping cannot be excluded,
the construction materials should not be overstressed. Plastified zones are acceptable to some extent, but this is subject to individual approval.
106A ‘Committee on Earthquakes’ (ICOLD 1975) elaborated recommendations for the earthquake resistant design of embankment dams, reflecting experiences with dams and earthquakes at that time. These recommendations are still relevant. It is noted that in the last decades earthquakes have been registered in areas which had formerly been considered as ‘safe’. Necessarily, these recommendations are generalized, but it is worthwhile to present these ideas here with a short comment.
– For soft foundations the dam body and the foundation should be considered as an interacting system. Additional material at the toe of the dam will be effective as a stabilizing berm.
This recommendation refers to the non-elastic behaviour of such foundations and related excessive deformations due to earthquakes. It is noted that sealing blankets are prone to shear failure because of their geometry. They tend to develop tension cracks given large horizontal displacements.
– Sufficient foundation treatment is requested.
– Measures in this respect are taken mainly to safeguard proper bond between the sealing element and the foundation. Such a bond will help to exclude piping.
– It is considered effective to make the axis of the dam slightly curved and to make the width of the dam somewhat greater towards the abutments.
This recommendation aims to protect the interface of dam and abutment from excessive leakage, which might develop due to the different natural motions and related shear movements of the two media. In addition, the curvature is deemed to create a slight arching effect which helps to compensate for potential tensile stresses at the downstream side of the crest due to the horizontal forces.
– The dam crest should be made wide.
This refers to the magnified acceleration and amplitude at the crest triggering local sliding along shallow failure planes. Shallow planes will not cut through the whole crest with the risk of overtopping if the crest is widened.
– An extra allowance of freeboard should be provided for settlement of the dam and its foundation.
This recalls the vertical component of earthquakes and the related risk of settlements leading to unacceptable loss of freeboard and subsequent overtopping.
– The construction materials must be selected very carefully.
Careful material selection will help to compose the dam of zones which are homogeneous. It is noted that maximum compaction gives maximum strength to the shells and minimum permeability to the sealing element.
– The slopes of the dam should be made gentle.
107Irregularities in the outer slopes or steep and abrupt slopes might lead to unfavourable stress and strain conditions with the risk of failure.
– The thickness of the impervious zone should be increased.
The wide sealing offers additional safety against penetrating cracks. It will reduce the quantity of seepage due to the decrease of the hydraulic gradient.
– The utmost care is required in the work of construction.