INSTRUMENTATION 295
of time, possibly several decades. As guidelines underwriting sound design
it is therefore desirable that instruments be
- as simple in concept as is consistent with their function,
- robust and reliable,
- durable under adverse environmental and operating conditions and
- acceptable in terms of ‘through-life’ costs (i.e. the sum of purchase,
installation, support and monitoring costs).
A sound principle is to retain the sophisticated and vulnerable
sensing elements, e.g. electronic components and transducers, above
ground level wherever possible. In such instances it may also be advanta-
geous to make the above-ground elements readily transportable, e.g. by
use of compact portable transducer units to monitor porewater pressures
from piezometers. Additional advantages associated with the use of a
transportable sensing element lie in greater physical security and avoid-
ance of the need to construct large and costly instrument houses for fixed
measuring equipment.
Instrument capability has developed significantly over recent years,
and reliable and robust equipment is now readily available. More recent
developments have tended to concentrate upon provision for automatic or
semi-continuous interrogation of instruments, with a facility for storage
and/or automatic transmission of data to a central location. The enhanced
capability and complexity are reflected in high costs and a greater risk of
component, and thus system, malfunction or failure. Such sophistication is
therefore generally justifiable only in more exceptional circumstances. For
most dams instrumentation at a relatively unsophisticated and basic level
will prove adequate for routine monitoring and surveillance.
7.2.4 Instruments: types and operating principlesA brief review of the more common instruments is provided below to
demonstrate important principles of operation and measurement. Reference
should be made to Dunnicliff (1988) and Penman, Saxena and Sharma
(1999), or manufacturers’ literature, for comprehensive details of instru-
ments, their operating principles and characteristics.
(a) Collimation, settlement and external deformation
Precise survey techniques, using optical or electronic distance measuring
(EDM) equipment or lasers, are employed to determine the relative verti-
cal and horizontal movement of securely established surface stations. Col-
limation and levelling to check crest alignment on concrete dams may be