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Geology of Dam Spillways

The importance of the dam spillway geology in many dam projects stems from the fact that the most economic (cheapest) dam is often one which is built with rock fill obtained from the adjacent spillway excavation. The two important questions that the engineering geologist must answer are therefore;

  • is the spillway rock acceptable for use as rock fill in the dam embankment construction;
  • is the spillway rock sufficiently erosion resistant that concrete lining and energy dissipation structures can be omitted from the spillway.

The above questions require a detailed study of the geology of each dam site on an individual basis, so it is difficult to give general rules which will apply to any dam site. Some rules of thumb which I have found useful are given below but it must be emphasised that they are not intended to replace a detailed study of each site.

Criteria for acceptability as rock fill: The two properties that are useful here are Unconfined Compressive Strength (UCS) in the saturated state and Water Absorption (WA).

UCS (saturated)
> 50 MPa Good to excellent quality rock fill. Durability OK
20-50 MPaAcceptable but rather weak rock fill
< 20 MPa Durability testing required before acceptance

Water Absorption
< 1%Good to excellent quality, highly durable rock fill
1-3% Fair to good quality rock fill, acceptable durability
3-5% Durability testing required before acceptance
> 5% Not suitable for riprap, durability testing essential before use as rock fill

Rock fill in service should be durable (ie its properties should not change over time) and should have a high shear strength and low compressibility.

Criteria for provision of concrete lining and energy dissipators: A useful property here is the Rock Quality Designation (RQD) which, taken in conjunction with the state of stress of the rock mass and the presence or absence of erodible seams, gives the following criteria:

Full energy dissipator < 50 Yes
Flip bucket > 50 No
Unlined or partially lined:
moderate conditions* > 75 No
severe conditions* > 90 No
* these spillways require durable, lowly stressed rock

As an example of what can happen if the spillway rock is under high in-situ stresses, see Copeton Dam Spillway.


Woodward, R.C. (1985), Geological Factors in Spillway Terminal Structure Design, Engineering Geology, Vol. 22, pp. 61-70.

Woodward, R.C. (1992), The Geology of Dam Spillways, Engineering Geology, Vol. 32, pp. 243-254.

Cooke, J.B. and Sherard, J.L. (1987), Concrete-face rockfill dam: II Design, ASCE J. Geotech. Eng., Vol. 113, pp. 1113-1132.

Annandale, G.W. (1995), Erodibility, Journal of Hydraulic Research, Vol. 33, No. 4, pp. 471-493

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