Large-scale Field Test

Transcription

1 Vaskinn 1 Large-scale Field Test Kjetil Arne Vaskinn kav@trh.statkraftgroner.no Statkraft Grøner AS SUMMARY The objective the controlled failure of large-scale embankment is to monitor and record the failure process and mechanisms in detail. This will provide valuable data to assist in understanding the fundamental failure process, for developing predictive models and for assessing the validity of smaller scale laboratory testing. The tests will take place in Nordland in Norway. The paper gives a short description of the test site and the plans for the tests in 2002 and the preliminary plan for the instrumentation. 1 INTRODUCTION Breach formation is the term used to cover the factors that lead to an uncontrolled release of water from a dam structure. It is well known that the most common causes of failure of an embankment are water overtopping the crest or internal erosion (so-called piping). Although there have been tests and analysis of homogeneous structures of non-cohesive material, the failure of multi-element structures incorporating impervious cores remain poorly understood. Experimental tests will be undertaken to support the theoretical development of models of the failure modes and rates of failure. Tests will include both large-scale fieldwork (failure of 6m high embankments) and smaller laboratory modelling (0.8m high). The large-scale experimental facilities will allow studies of the factors that govern the initiation and growth of breaches under controlled conditions. Also, the issue of scaling from the laboratory to the prototype scale will also be addressed. 2 OBJECTIVES The objective of this part of IMPACT is to undertake experiments of controlled failure of large-scale embankments in order to monitor and record the failure process and mechanisms in detail. This will provide valuable data to assist in understanding the fundamental failure process, for developing predictive models and for assessing the validity of smaller scale laboratory testing. 3 DESCRIPTION OF THE TEST SITE 3.1 Location The test site is located in the middle of Norway (Nordland), downstream of the reservoir Røssvatnet and upstream of the reservoir Stormyrsmagasinet. See map (Figure 1) and picture (Figure 2). The test embankments may be constructed to a height of 6m and subsequently force to failure under controlled conditions. The picture in Figure 2 is taken during a testing period of the gates at the dam Røssvassdammen. There is normally no water flowing in this part of the river.

2 Vaskinn 2 Pictures in Figures 3 and 4 show the test-dam used during the 2001 field-test in the Norwegian project: Stability and Breaching of Rockfill Dams. Figure 5 gives a view of the river reach downstream from the test-site. Figure 1 Test-site in Nordland Figure 2 Arial photo of the test-site

3 Vaskinn 3 Figure 3 The test-dam used in the Norwegian project Stability and Breaching of Rockfill Dams (2001) Figure 4 The test-dam used in the Norwegian project Stability and Breaching of Rockfill Dams (2001)

4 Vaskinn 4 Figure 5 The river reach downstream of test-site Water-level/discharge The gates at the dam Røssvassdammen have a total capacity up to 450m3/s. The gates are new and the operation of them is easy and flexible. The high capacity through the gates gives us the opportunity to simulate breaching in large reservoirs (slow reduction in the water level in the reservoir as a function of time) and small reservoirs. The release of water from the upstream-reservoir will be done in close co-operation with the dam-owner, Statkraft SF. The site is located such that there will be minimal impacts on the environment and no impacts on local infrastructure. 4 TEST-PROGRAM 4.1 Test dams There will be 5 tests in total. 3 of the tests will be done in 2002: Test 1: Homogeneous earthfill (minimal cohesive) dam / embankment with failure by overtopping. Test 2 Homogeneous (maximum cohesive) dam / embankment with failure by overtopping. Test 3 Composite (2 layers) embankment with failure by overtopping. Moraine core, rockfill support/protection layer. In addition to tests 1-3, a test of toe stability will be made at the test-site. The proposed geometry and layout of the test-dams are shown in Figures 6 9.

5 Vaskinn 5 Figure 6 Test-dam no.1 Figure 7 Test-dam no. 2 There is an option of two different tests with test-dam no. 1: First run a test with a protective layer downstream, then run the same test without the protective layer. The test planned in IMPACT is without the protective layer.

6 Vaskinn 6 Figure 8 Test-dam no. 3 Figure 9 Test-dam 4 (for dam-toe stability)

7 Vaskinn Field work-plan In outline, the plan for the field test is: 1. Preparing the site for the test dam 1.1 Building of transport road to the riverbed 1.2 Work at the riverbed/prepare the foundation of the test-dams. 1.3 Preparing the side-slopes 2. Selection and transport of the materials for dam building. 3. Building of test-dam #1 4. Test #1 5. Cleaning up at dam-site and preparing for test #2. Step 2-5 will be repeated for each test. A local contracting firm will undertake building of the dam. A call for tenders has already been made to select the firm to do the job. This is done to make ensure that the building/construction of the test-dam will be done according to our-plans. 5 INSTRUMENTATION FOR THE FIELD TEST 5.1 Data collection Based on discussions in the working group, the IMPACT meeting in Wallingford and during the workshop in Trondheim 17th and 18th of April 02 it is clear that the following data should be collected: 1. Breach formation geometry: 3D surface of breach at any time 2. Water levels: record level variation with time at: - Upstream reservoir 2 locations minimum - 3D surface through breach - Downstream water levels along the river 3. Gate Operation and Water level in Røssvassdammen (flow into test area) 4. Water pressure through embankment/dam 5. Flow / velocity patterns - Flow through the embankment/dam in the initial phase and at through the toe during testing of toe-stability - Flow through the embankment/dam during braking of the dam. - Flow in the river downstream of the test-dam 6. Sediment movement (materials from the test-dams) 7. Material properties: - Particle size distribution - Liquid / plastic limit - Type of clay - Compaction - Rock properties (interlocking effect) - (Clay will require further tests e.g. chemistry) - Permeability of the test-dam - Porosity of the test-dam

8 Vaskinn Instrumentation for data collection There are a number of approaches that could be taken for collecting data. The methods that are chosen are considered to be simple, well tested and documented and cost effective D surface of the breach at any time A grid will be painted across whole of embankment, including crest and upstream face to aid video and photography. The following methods will be used: Video Photo Photogrammerty Sonar upstream Point measurement within the dam-body either through wires or balls. Measurement in the dam is done to provide information to calibrate and verify data from the other methods. Video will be taken from: Downstream: 3 camera stations (2001-2) Above: 1 camera Upstream: 1 camera Still camera shots can be taken from video footage as the qualities of the photos are high enough, but as a security additional camera will be used. Photogrammetry is a possible method for identifying movement of embankment material. This requires at least two cameras, firing simultaneously, at a fixed spacing. Possible camera-positions are shown in figure 10. Figure 10 Possible positions for the cameras

9 Vaskinn 9 Sonar offers a possible means of monitoring breach growth underwater. Sonar is however not appropriate for downstream conditions, but could be placed underwater upstream to show growth of breach through upstream face. Sonar has been used for monitoring/mapping of dams in Norway. Figure 11 show an example of a Norwegian dam. Figure 11 Sonar picture of a dam A possible solution to monitor the growth of the breach is to use movement sensors e.g. floating balls. These can be buried within the dam and are released as erosion occurs. These sensors need to be uniquely identifiable, unrestricted by cables, traceable or disposable Water levels Water level will be recorded through automatic recording (pressure sensors): 2 points upstream of the test-dam several points downstream to monitor the flood wave Automated system will be complemented with some manual readings of staff-gauges Pressure sensors in the dam. There will be place at least 10 pressure sensors in the test-dam. See Figures Inflow rate into the test-reservoir Discharge through the gates at Røssvassdammen is equal to the inflow to the reservoir upstream of the test-dam. The discharge will be measured through measuring the gate opening at every minute. The waterlevel in the lake Røssvassdammen will be recorded.

10 Vaskinn Discharge There are two different situations in which monitoring of the discharge is necessary: During testing of the toe-stability During the breach The discharge may be monitored through readings of calibrated staff-gauges in addition to automatic sampling of water pressure/water levels. Discharge will also be recorded through an Acoustic Doppler Current Profiler (ADCP). The ADCP is either mounted on the bottom looking upward (will be used for monitor the discharges during the breach or the ADCP can be floating on the surface looking downwards. (EasyQ) Sediment movements The sediment movement will be recorded by survey only. Based on the changes in bed surface elevation, calculation of sediment transport will be done Properties of the Materials Details still to be decided.