2. DESIGN OF RCC DAMS
2.7. Instrumentation
2.7.1. General
In terms of general dam safety, seepage, pressures in the dam and foundation, structural behaviour/response, foundation bearing loads, water levels, ground motions and water and air temperatures, the monitoring requirements for an RCC dam do not differ from those typically applicable for a CVC dam. Two specific monitoring differences, however, relate to the induced contraction joints and the development and dissipation of temperatures, which typically require more instrumentation in an RCC dam. Due to the more rapid construction of RCC dams and the general use of induced, rather than formed joints, more emphasis is placed on monitoring temperatures and thermal gradients and the consequential response of the induced joints. For this purpose, more thermistors/thermocouples will typically be installed in an RCC dam than in a comparable CVC dam, while strain measurement on the induced joints generally requires a longer instrument to accommodate the greater alignment tolerance variations of an induced joint, compared to a formed joint.
Permeability of RCC and particularly the joints between lifts and layers remains a particular sensitivity and requires the design of specific seepage monitoring and measurement systems.
Instrumentation data relating to the early thermal and creep behaviour of fresh RCC remains of particular interest and importance in respect of the continuing development of the technology and consequently, instrumentation and monitoring system design should be given particular attention in the case of all RCC dams.
The installation of instrumentation in an RCC dam is generally more difficult than is the case in
surface of an RCC dam. For this reason, duplication and an adequate level of redundancy are important considerations in the development of an appropriate monitoring system.
2.7.2. Appropriate instruments and instrumentation configurations
To limit interference with RCC placement, an obvious advantage exists for instruments that can be installed after construction, such as geodetic survey targets, extensometers and pendulums drilled from and between the galleries and 3-dimensional crack meters installed in the galleries. Certain instruments, however, are required to monitor the behaviour of the RCC material itself and these must be installed during the process of RCC placement.
As a consequence of the movement of heavy equipment on an RCC placement surface, robustness is a key requirement of monitoring instrumentation and cabling to be embedded in RCC.
To allow for alignment deviations caused by the movement of heavy equipment and RCC compaction close to formed (rather than drilled) pendulum shafts, precast circular concrete elements with an inner diameter of not less than 800 mm are generally required.
Long-base-strain-gauge-temperature-meters (LBSGTMs) have proved particularly successful for the measurement of strain and induced joint openings in RCC, with gauge lengths as long as 1 m taking cognisance of the typical induced joint alignment tolerances and the realities of induced crack propagation. In modern practice, it is considered that a gauge length of 500 to 600 mm is generally quite adequate for a LBSGTM. Thermocouples and thermistors are commonly used to measure temperature distributions and gradients on a number of sections within an RCC dam. Although these instruments are not always consistently reliable, they are simple and relatively low cost, which usually allows an adequate degree of redundancy to be provided. Although significantly higher cost, fibre optic cables have successfully been used for temperature and strain measurement in RCC dams.
A standard arrangement of embedded stress-strain measurement instrumentation for RCC dams has been proposed, as a means to develop the understanding of early behaviour and particularly stress-relaxation creep in different RCC types (Conrad, Shaw & Dunstan, 2012). The proposed system comprises a specific arrangement of LBSGTMs and effective concrete stress meters designed to provide consistent data in the research of RCC stress-strain behaviour during the early part of the hydration heat development and dissipation cycle.
2.7.3. Instrument installation
To minimise interference with RCC placement, the installation of embedded instruments such as strain gauges, thermocouples and piezometers is often planned for a specific elevation at which an interruption in RCC placement or a construction break is scheduled. While this approach allows for good control during installation, it generally requires that instruments are actually embedded in a CVC concrete mix with a smaller maximum aggregate size and installed in a trench excavated in the RCC, the sides of which will often comprise poorly compacted RCC. Furthermore, the embedded instrument’s first measurements realistically occur when the fresh RCC is placed for the layer above. In the case of temperature measurement, this situation is of little importance, but in the case of strain measurement, consequential issues often compromise the value of the important early measurements.
While success has been achieved embedding instrumentation and cabling into the surface of super-retarded high-workability RCC before initial set (Shaw, 2010), instruments installed into this RCC type in this manner are susceptible to subsequent damage during the setting period through the passage of trucks, particularly in areas of restricted space where the trucks must repeatedly follow the same path.
Significant advantage is perceived in installing embedded stress and strain measurement instruments into RCC before first set and during the uninterrupted placement process, as these instruments measure the actual behaviour of the RCC into which they are placed, without the distortions associated with a significant placement interruption, etc. The value of the associated data is generally considered worth
the additional care required in the management of the transportation and the placement processes for the subsequent RCC layer.