This paper presents the instrumentation planned and implemented during construction and evaluation and study of the behavior of the dam and the underground structures post construction with respect to performance and safety monitored by various instruments. Based on the analysis of the instrumentation data the 92m high concrete gravity dam is found to be working satisfactorily. The underground Power House cavern is still undergoing stabilization probably in view of it being situated near the main dislocation features of Main Central Thrust MCT and Main Boundary Thrust MBT of western Bhutan which is indicated by instrumentation data and manifested by some rock bolt failures from the walls of the cavern.
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The cavern is being closely monitored by analysis of data collected from existing geotechnical instruments. The project involved construction of a 92m high concrete gravity dam with peaking live storage capacity of 3. The project taken up for execution from the zero level of infrastructure development on 1st October and the first unit was commissioned on 31st July, and started generating revenue to the Government exchequer. The bed rock was found below 17m thick overburden and weathered rock mass during construction.
The purpose of dam instrumentation and monitoring in general is to maintain and improve safety by providing information to evaluate whether the structure is performing as expected and warn of any changes that could endanger the safety of dam. It has seven concrete blocks with five sluice spillway, one overflow and non-overflow blocks. The sluice spillway is designed to pass standard project flood of cumecs and PMF of about 10,cumecs. Most of the instruments are embedded in blocks 2 and 5 which represents non-overflow and maximum over-flow sections of the dam respectively.
Multi point borehole extensometers have been installed at right and left abutments. Four numbers of strong motion accelerometers have been installed at different locations. The details of instruments being monitored during operation stage is shown in Table 1 below.
Table: 1 Sl. Table: 2 Sl. Average values Name of Instrument Parameter No.
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Construction of these underground structures requires involving instrumentation in order to study the deformation of rock mass during excavation and assess stability during construction. It is also used to assess the adequacy of support measures and strengthening measures required to be implemented to ensure safety.
Instruments were installed as per the drawings provided by the designers i.
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Central Water Commission. The total instruments installed at various underground structures are as shown in Table 3 below. Instrumentation data was utilized to assess the displacements inside the rock mass so as to take decisions on the support system and other strengthening measures. The instrumentation in desilting chambers started during July A typical section of instrumentation in desilting chamber is shown below in Figure 4. MPBX installed during in chamber 3 indicated significant movement of the surface and displacements up to depths of 10m in the crown and side.
It was therefore decided to support the crown from RD 80 to These ribs were above EL for the full width. The legs of the ribs were resting on the horizontal RCC wall beam which was bolted on the side wall. The ribs were backfilled with concrete.
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Similarly in chamber-2 steel ribs were provided from RD MPBX were installed on the right wall of chamber 3 which was situated in adverse geological condition due to the presence of shear zones. At the time of construction of the RCC cladding wall, the portion where the chamber was already excavated, it was backfilled with muck. The excavation of the hopper portion or removal of the muck could result in movement of the side walls.
Convergence of the side walls was monitored by a total station with the reflective targets installed on the cladding wall at EL and at 13 locations in chamber 3 and 6 locations at chamber 1 and 2 respectively. Although this was a post excavation exercise, it helped in ascertaining the stability of the cladding walls over a period of time.
The shape of the HRT is modified horse shoe shape with a diameter 6.
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The construction of the HRT was undertaken by 5nos of intermediate Adits and one Adit at the Dam inlet with 11 working faces. Instrumentation in HRT was planned basically with a view to understand the rock mass behaviour and magnitude of displacements and loads acting on the support system.
A section showing different types of instruments installed is shown schematically in Figure 7. The pore water pressure was measured using vibrating wire piezometers.
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Instrumentation was planned and executed during the excavation through m of very adverse ground conditions. Load cells of vibrating wire type were installed on the ribs on either side at springing level and reflective targets were installed at the same location to monitor the convergence of the ribs.
In AGO Reach 25 sections were instrumented and the observations were carried out at regular intervals. Instrumentation in HRT was discontinued after filling of the water conductor system. Four sections along the length of machine hall at RD 15m, 65m, m and m at different elevations were selected for instrumentation in walls. MPBX of magnetic type had been installed at the crown and walls for measuring the rock deformation. Anchor load cells were installed on rock bolts and on ribs to measure the load.
Piezometers were installed on the walls to measure pore water pressure. Instrumented bolts also known as vibrating wire stress meters were installed on the walls to measure stress impacted on rock bolts. The convergence of side walls was monitored using reflective paper targets with total station at elevations m, m m and m at four sections. An extensive instrumentation program has been implemented to monitor the time dependent behavior of the invert of the turbine pit. The condition was monitored till the turbine installation works.
The two perpendicular horizontal alignments are being measured. Additional paper targets are also installed at several sections to monitor convergence more closely.
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The total instruments monitored during operational stage are as shown in Table 4 below. Table: 4 Sl. There is no increase in load in all load cells which shows a stabilized trend of the structures. It can be seen that there is increasing trend of convergence. The maximum cumulative convergence has reached The numerical modeling study carried out by NIRM in the year has predicted wall deformation variation from mm to mm with the maximum deformation at elevation m.
Thus, the maximum predicted limit of convergence has already exceeded at this elevation. Instruments were installed at RD14m, 60m, m and m at different elevations. MPBX was installed to measure the deformation of rock mass during excavation. Load cells were installed to measure load on the ribs used to support the crown of the cavern.
Load cells were also installed on the rock bolts to monitor the load at both upstream and downstream walls. The success of the commissions gives confidence that implementing applications that combine monitoring and construction progress data alongside the use of data analytics can offer significant savings for monitoring programmes and reduce the risks of significant ground or structural movements being missed.
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