Contents preface (VII) introduction 1—37

Table 15.4 Thickness and size of dumped rock riprap

Maximum wave	Minimum average	Layer thickness	Minimum filter
height (m)	rock size D50 (cm)	(cm)	layer thickness (cm)
0-0.6	25	30	15
0.6-1.2	30	45	15
1.2-1.8	38	60	22.5
1.8-2.4	45	75	22.5
2.4-3.0	52	90	30

If the surface of the downstream slope of an earth dam consists of fine-grained soil, considerable erosion may be caused by either surface water runoff during rainstorms or wind during windstorms in arid regions. The erosion of the surface results in deep gullies (as deep as 3 m in severe cases) on the downstream slope at the abutment contacts as well as in the central portion of the dam.

A good cover of grass on the surface of the downstream slope holds the surface soil in place and provides most satisfactory and economical slope protection. However, in very dry areas, it may not be possible to spare enough water to grow and maintain the grass cover. In such situations, dumped rock riprap (used for upstream slope protection) can be used for the downstream slope as well.

15.6. INSTRUMENTATION
Instruments are installed in an embankment dam to measure pore pressures at different locations, and also the settlement and horizontal meovement of the dam. The measurements from these instruments enable the concerned authorities to compare the measured quantities with the corresponding values used by the designer. These measurements also provide a reliable basis for analysing the performance of the dam and for taking suitable steps to overcome a problem which may develop. Besides, these instruments provide valuable data for use in future designs of dams. The instruments installed in an embankment dam are piezometeres (for measuring pore water pressure), and instruments for measuring horizontal movements, foundation settlement, and embankment compressions. Instruments of the second type can be either internal instruments which are installed within the embankment during construction,

EMBANKMENT DAMS

533

or surface monuments consisting of concrete-embedded steel rods installed accuraterly along straight lines. The installed instruments should be simple in design with minimum moving parts, and should require no maintenance as most of the instruments would be embedded. The installation procedure should cause least interference in construction activities. The method of observation should also be simple.