Phd Defense | Ricardo Correia dos Santos
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PhD Defence Experimental Investigation on limiting the progression of the erosion in zoned dams
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Phd Defense | Ricardo Correia dos Santos
- 1. Experimental Investigation on Limitation of the Progression of Internal Erosion in Zoned Dams by Ricardo N. Correia dos Santos ricardos@lnec.pt Under scientific supervision of Dra. Laura Caldeira Dr. Emanuel Maranha das Neves PhD thesis prepared at PhD thesis defence | 29th October, 2014
- 2. Question | Objective Research question: ‘What is the influence of the upstream zone limiting the progression of internal erosion through a crack in the core ? ‘ Sandy gravel Sandy gravel Upstream Upsztorenaem zone Core Filter > Flow-limiting action > Crack-filling action Objective: Experimentally investigate what are the upstream materials that may provide these two actions
- 3. Previous laboratory testing on erosion in soils by others Sample Ø ~ 10 cm h ~ 12 cm Core Point gauge Cylindrical cell Ø ~ 0.5 m Sample Sample Pea gravel Ø ~ 2.5 cm h ~ 3.8 cm Core Ø ~ 10 cm h ~ 12 cm Core > Erosion along a concentrated leak Hole Erosion Test (HET) Wan and Fell (2004) > Erosion of an immersed soil JET Erosion test Hanson and Cook (2004) > Identification of dispersive clays Pinhole test Sherard, Dunningan and Decker (1976)
- 4. Previous laboratory testing on erosion in soils by others > Susceptibility to suffusion of a soil Upward Flow (UF) seepage test Wan and Fell 2008 Sample Ø = 30 cm h = 25 cm Downward Flow (DF) seepage test Wan and Fell 2008 Sample Ø = 30 cm h = 30 cm Drainage layer
- 5. Previous Previous llaabboorraattoorryy tteessttiinngg oonn eerroossiioonn iinn ssooiillss bbyy ootthheerrss Core Filter Sample Ø=20.5 cm Core: h = 10 cm Filter: h = 20 cm Section A-A' A A' Filter Crack Pea gravel Core Perspex Crack Sample from Proctor test Core > Filtering in a hole Continuing Erosion Filter test Foster and Fell 2000 > Filtering in a crack Crack Erosion Test Maranha das Neves (1989, 1991) Crack filling by upstream uniform sand
- 6. New test cell developed at LNEC | Two laboratory tests 63 cm 40 cm > Flow Limitation Erosion Test > Crack Filling Erosion Test Upstream material Core Core Upstream material Filter Ø 30 cm 1 cm thk 12 17 25 > Test Cell F L E T C F E T
- 7. Specimen preparation & cell assembly | Tests setup > Specimen preparation in FLET > Specimen preparation in CFET > FLET setup > CFET setup
- 8. Experimental study in the FLET/CFET (Soils tested) > Upstream material A 5 broadly-graded soils N1, N2 and N3 – Rb Grande Dam P1 and P2 – Odelouca Dam 2 uniform granular soils Sand A0 and Gravel A 6 gap-graded granular soils GA1, GA2, GA3 and GA4 GN and GP (5% fines) 13 Upstream soils
- 9. Experimental study in the FLET/CFET (Soils tested) > Core and Filter 2 Filters 2 Cores
- 10. Experimental study in the FLET/CFET (Soils tested) > Characterisation of soils used in the FLET/CFET Standard laboratory testing • Standard compaction tests • Maximum/minimum density tests • Permeability tests Theoretical analysis • Susceptibility of soils to internal instability • Ability of the soils to support an open pipe Internal erosion tests • 9 Upward Flow (UF) tests on gap-graded soils • 25 Hole Erosion Tests (HET) on core soils
- 11. Experimental study in the FLET/CFET (Soils tested) > Erosion behaviour of gap-graded soils in the simpler UF test UF test on soil GN 25% sand | 5% fines (NP) 4 24 3.5 21 Hydraulic gradient, i Flow rate, Q 3 18 2.5 15 2 12 1.5 9 1 6 iboil 0.5 3 0 0 Time (min) uf test_sufusion_time_gn.grf 1E-3 1E-2 5E-4 5E-3 icr 2E-4 2E-3 1E-4 1E-3 i i start boil 5E-5 5E-4 Velocity, V Permeability, k 2E-5 2E-4 1E-5 1E-4 5E-6 5E-5 2E-6 2E-5 1E-6 1E-5 Hydraulic gradient, i Average velocity, V (m/s) Permeability, k (m/s) 0 0.5 1 1.5 2 2.5 3 3.5 4 Hydraulic gradient, i Flow rate, Q (cm3/s) 0 40 80 120 160 200
- 12. > Erosion behaviour of the core soils in the simpler HET Water content, w (%) Dry unit weight, d (kN/m3) 4.61 12 13 14 15 16 17 18 19 2020.5 20 19.5 19 18.5 18 17.5 17 16.5 4.11 4.6 Sr = 100% 4.24 4.49 4.39 4.7 4.48 3.76 3.39 3.78 4.25 ...... Erosion rate Index, I Modified (core#4) No erosion (core#4) Standard (core#4) 'Reduced' (core#4) Standard (core#20) Core#4 Core#20 Experimental study in the FLET/CFET (Soils tested)
- 13. Experimental study in the FLET > Testing conditions 19 FLETs on broadly-graded upstream soils • DH = 2 m → i = 4 • Di = 12 mm (few with 10 and 16 mm) • Core#4 (IHET = 4.1) • Upstream soil: wopt or wopt ± 2% | 95 or 98% 14 FLETs on granular upstream soils • DH = 2, 1.5 or 1 m | Di = 12 mm • Core#4 (IHET = 4.1) • Upstream soil: low w | Dr of 100% 33 FLETs 1 year | ~ 3 tons of soil
- 14. Experimental study in the FLET > Observed behaviour types | Type F1– Flow limitation (self-healing)
- 15. Experimental study in the FLET > Observed behaviour types | Type F2– Flow limitation (non-erodible) Q = 650 l/h Q = 750 l/h Q = 800 l/h Q = 803 l/h
- 16. Experimental study in the FLET > Observed behaviour types | Type F3– Slow down of erosion process
- 17. Experimental study in the FLET > Observed behaviour types | Type F4 – Strong progression of erosion
- 18. Experimental study in the FLET > Observed behaviour types | Type F4 – Strong progression of erosion
- 19. Experimental study in the FLET (Conclusions) > Critical parameters influencing the flow-limiting action N1 N1wet dry N1opt N2dry N2opt N2wet 20% Near optimum Dry side Wet side dw vs pf200_broadly.grf w wopt (%) Fines content, FC (%) 40 30 20 10 P1opt P2dry P2opt98;D10 P2wet98;D16 N3dry N3dry;98% N3wet P2opt P2wet C#4 P1dry N3opt 12% 5% N1wet N soils | P soils Type 1 . Type 3 Type 4 N3wet Type 1 Type 2 . Type 4 P2wet98;D16 N2wet 50% dw vs pc4_broadly.grf C#4 N1dry N1opt N2opt N3opt P1opt N2dry N3dry N3dry;98% P1dry P2dry P2opt98;D10 P2opt P2wet w wopt (%) Gravel content, GC (%) 0 10 20 30 40 50 60 70 80 -2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 Broadly graded soils • Fines content • Fines plasticity • Gravel content • Water content Gap-graded soils • Unlikely to limit flow • Low Hydraulic head, DH • Internal stability Gravel content (%) Fines content (%) w –wopt (%)
- 20. Experimental study in the FLET > Proposed rules for estimation of likelihood of flow limiting action Upstream material Upstream material is cracked Upstream material is not cracked Water content Internally unstable soil with iU Internally stable soil with kU (m/s) Zone type Fines content Fines type dry side near opt wet side > 1 < 1 > 10-2 ‡ < 10-5 ‡ I NA NA 0 0 0 II <5% Any ** ** ** VU U VU *** III 5 to 12% SPF L* N* N* U N U L HPF‡ N* U* N* IV >12% SPF L* N* N* N L *** VL HPF‡ U* N* L* V 5 to12% NP ** ** ** VU N VU *** VI >12 to 20%‡ NP L* N* N* U N U L VII >20 to 30% NP N U U N L *** VL VIII >30% NP U VU VU N L *** VL Symbol /colour Qualitative descriptor Example of likelihood interval VL Very Likely 0.98–0.999 L Likely 0.70–0.98 N Neutral 0.30–0.70 U Unlikely 0.02–0.3 VU Very Unlikely 0.001–0.02
- 21. Experimental study in the CFET > Testing conditions 4 CFETs on broadly-graded upstream soil • DH = 2 m | Di = 12 mm or 16 mm • Core#4 (IHET = 4.1) and Filter G (Dr = 100%) • Upstream soil N1: wopt or wopt ± 2% | 95% 37 CFETs on granular soils • DH = 2m | Di = 12 or 16 mm • Core#4 or Core#20 (IHET = 4.1) • Filter S or Filter G (Dr about 60%) • Upstream soil: low w | Target Dr of 100% 41 CFETs 1 ½ year | ~ 4.5 tons of soil
- 22. Experimental study in the CFET > Observed behaviour types | Type C1– rapid ‘crack-filling’
- 23. Experimental study in the CFET > Observed behaviour types | Type C2a – Filtering after ‘some’ erosion
- 24. Experimental study in the CFET > Observed behaviour types | Type C2b – Filtering after ‘excessive’ erosion
- 25. Experimental study in the CFET > Observed behaviour types | Type C3 – Continuing erosion
- 26. Experimental study in CFET (Conclusions) > Critical parameters influencing the crack-filling action Broadly graded soils • Highly unlikely to fill cracks Gap-graded soils • pA0 versus D15F • 5% fines may decrease capability • Core soil with I > 4 has no influence
- 27. Experimental study in the CFET > Proposed rules for estimation of likelihood of crack-filling action Key features of upstream zone Embankment zoning in the erosion path at downstream of the core Rapid crack-filling Formation of a self-filtering action layer No crack filling nor filtering mechanism Fines content Effectiveness of upstream soil Key feature of the filter Estimate erosion zone by Foster and Fell (2001) D15F < Transi D15F ≥ No Some Excessive 2.9 mm tion 5.1 mm erosion erosion Erosion Continuing Erosion <5% psand > 30% and no fines content VL L U VL L N U Transition L L – U* U L N U U psand ≤ 20% and 5% of fines U U VU N U U VU ≥12% NA VU VL L N VU Symbol /colour Qualitative descriptor Example of likelihood interval VL Very Likely 0.98–0.999 L Likely 0.70–0.98 N Neutral 0.30–0.70 U Unlikely 0.02–0.3 VU Very Unlikely 0.001–0.02
- 28. Final conclusions Initial question ‘What is the influence of upstream zones? ’ • Answer: Some soils compacted in certain conditions, when located upstream of a crack in the core, may effectively provide the flow limiting action or the crack-filling action. Proposed objective Experimental study of both actions • Development of a new test device and of 2 laboratory tests (FLET and CFET), and their test procedures. • Identification of the potential behaviour types and of the influence of critical parameters for each action. • Proposed rules to aid practical engineers in the design phase or to estimate probabilities for dams in operation.
- 29. Future research FLET Flow limiting action • Extend testing to greater variety of upstream soils prepared to a wider range of test conditions. • Evaluate if self-healing ability of very dry upstream materials also occurs in soils with highly plastic fines. • Investigate the influence of the permeability of coarse grained (stable) soils. CFET Crack-filling action • Extend testing using materials from existing dams. • Evaluate the relation pA0 versus D15F for other soils. Both • Evaluate influence of the erosion rate of the core (for I < 4). • Evaluate influence of the flow orientation.
- 30. Progression of internal erosion in the embankment Tunbridge Dam, Australia Zoned earth dam (28 m) Source: Jeffery Farrar (2005) Hanson e Hunt (2007) Homogeneous dam (25 m) Source: USDA
- 31. Experimental Investigation on Limitation of the Progression of Internal Erosion in Zoned Dams by Ricardo N. Correia dos Santos ricardos@lnec.pt Under scientific supervision of Dra. Laura Caldeira Dr. Emanuel Maranha das Neves PhD thesis prepared at PhD thesis defence | 29th October, 2014
Hinweis der Redaktion
- Good morning to all. I will start my presentation with the research question of the thesis, which is “What is the influence of the upstream zone limiting the progression of erosion through a crack in the core?” It is believed that the upstream zone may provide the ‘flow limitation action’ or the ‘crack-filling action’ or even both. The flow limitation is related with the capability of the upstream zone to impose hydraulic head losses that are high enough to limit the flow converging to the crack. To obtain the isolated influence of this mechanism the best way is to assume that the crack occurs in the core and also the filter. The worse-case scenario should occur when the upstream zone also cracks. The crack-filling action occurs when the material eroded from the upstream zone is transported through the crack filling it. The presence of a filter is, in this case, an absolute requirement. However there are very few studies on these mechanisms. With that said, The main objective of the thesis was to experimentally investigate what are the materials that may provide these two actions.
- The next obvious step was to search in literature for laboratory tests that could be used to model the flow-limiting and the crack-filling actions. However, I found that majority of the available tests do not consider the upstream zone. In fact, many of the tests only consider one material, usually the core. They are focused on the evaluation of the erodibility of the core, like the Hole Erosion Test or the JET, or on the identification of dispersive clays, like the pinhole test,
- Or in the study of the susceptibility to suffusion of soils, like the Upward flow seepage test or the downward flow seepage test.
- More complex tests consider the core and the filter. They are used to evaluate the performance of filters, due to erosion along a hole in the core, like the continuing erosion filter test, or along a crack, like in the crack erosion test. Professor Maranha das Neves performed a few tests in this device using a uniform fine sand as crack-filler, and concluded that such material was very effective at filling cracks and stopping erosion. In fact, these were the only tests that I have found, focused on the influence of an upstream zone. However, to be able to examine coarser soils I felt the need of a larger test device.
- It was therefore developed a new test cell, which was completely built at LNEC. It allows the compaction of up to 3 zones (upstream material, core and filter), and its dimension copes with soils up to the gravel size. The cell is composed by several aluminum pieces and a Perspex front panel. The cell was developed to allow performing two distinct tests. The FLET to study the isolated influence of the flow limitation action, and the CFET to study the crack-filling action. In the FLET there is no filter, and usually a hole is drilled in the core and in the upstream material. In the CFET there is a filter and usually the hole is only drilled in the core.
- The compaction and drilling of the specimen and the assembly of the cell is made sequentially. The preparation of the specimen in the CFET is slightly different than in the FLET, because there is the need to consider the filter. The test setup is similar in both tests. Water is forced to pass through the specimen, applying a differential between the upstream head and the downstream head. During test we measure the flow rate and the water level in three piezometers. The turbidity and the erosion in the outlet chamber are recorded with a digital camera.
- After construction of the cell, it was necessary to choose the soil to be tested. 5 broadly-graded soils were selected as upstream material. These soils were collected during construction of two dams in Portugal. Soils N1, N2 and N3 (in red) are from Ribeiro Grande dam and have non-plastic fines. Soils P1 and P2 (in blue) are from Odelouca Dam and have fewer fines but with some plasticity. They were selected in accordance with guidelines available in literature related with the likelihood of occurrence of the flow limitation action. There were also selected 2 uniform granular soils and 6 gap-graded granular soils. Soil A0 is similar to the material tested by prof. Maranha das Neves that proved to be highly effective at filling cracks. Soils GA1 to GA4 are gap-graded soils formed by mixing soil different percentages of soil A0 with a uniform gravel. Soil GN has also 5% of non-plastic soils and soil GP 5% of plastic fines. A total of 13 soils were examined in the upstream zone.
- 2 soils were selected as Core Core#4 and a finer one Core#20. And 2 soils were selected as Filter Filter S and a coarser Filter G These filters were intentionally selected to be excessively coarser to be unable to retain effectively both cores.
- The next step was to characterize the soils selected. For that, there were used standard laboratory testing and some theoretical analysis, for example, to evaluate da susceptibility of soils to internal instability. There were also performed 9 Upward flow seepage tests on the gap-graded soils, and 25 Hole Erosion tests on the core soils to observe the behaviour of these soils in more simpler test configurations.
- In this slide, I show the results of one Upward Seepage test, in particular, on soil GN. In this type of test, the specimen is subjected to vertical flow (from the bottom to the top), and the hydraulic gradient is slowly increased in steps. the flow rate is measured, and the average velocity of the flow and the permeability of the soil are estimated. In some soils it was observed the development of sand boiling, visible on the top of the specimen When this phenomenon starts there is an increase of the flow rate, velocity and permeability of the soil. I was expecting that this material could fill the pipe in the core, in the CFET.
- Here I show the pieces of the Hole Erosion Test, built at LNEC, for the study of the erodibility of the core soils along a preformed hole. .. the look of the specimen after assembly of the cell and before starting the test, and during the test, where we can observe the widening of the pipe. This is how a typical specimen looks like at the end of a test, and this the paraffin mold of the resulting pipe. These tests allowed to investigate the erosion proprieties of core#4 and of core#20 for different compaction water contents and compaction efforts. The goal was to compact the core soils in the larger cell with an erosion rate index of about 4 (which corresponds to a moderate erosion behaviour).
- There were performed 19 FLETs using the broadly graded upstream materials, for various compaction properties. and 14 FLETs on the gap-graded In total there were performed 33 tests, which took about 1 year to perform and involved the compaction of about 3 tons of soil.
- These tests allowed the identification of four behaviour types. In tests showing Type F1, the upstream material erodes at fast rate, but when its coarser particles start being transported they are retained at the interface with the core, resulting in a decrease of the flow rate. Eventually the pipe in the upstream material collapses and self-heals.
- In behaviour Type 2, the flow rate increases initially but then stabilizes. This occurs when the upstream material in non-erodible for the testing conditions. In this time lapse, we see that the flow rate, turbidity and deposition of eroded material all decrease during the test. After 40 minutes we have 800 liters/hours and after 54 minutes flow is practically the same and there is no evidence of erosion.
- Behaviour Type 3 is initially similar to Type 1. Strong progression of erosion, then coarser particles eroded from the upstream material become blocked at the interface with the core, reducing the flow rate However, erosion in the core does not stop completely, and suddenly there is a blowout of the coarser particles. This results in the rapid increase of the flow, which doesn’t decrease anymore. I have here a small video showing this blowout phenomenon.
- Finally, Behaviour type F4 is associated to fast and continuous increase of the flow rate, and strong progression of erosion in the core and mainly in the upstream material.
- In this small video, I show an example of a test showing behaviour Type F4. Flow is always increasing and there is a continuous deposition of material in the outlet chamber. Ater dissasembly of the cell, usually it can be seen an open pipe in both materials. This is the parafin mould of the pipe. In this case the pipe in the upstream material is much larger than in the Core.
- The results of the FLETs on the broadly graded soils showed that the critical parameters for the flow-limiting action are: -fines content, gravel content and compaction water content of the upstream soil. The plasticity of the fines is also an important parameter. Tests on upstream soils with non-plastic fines are in red and with plastic fines in blue. Solid symbols correspond to tests in which the flow practically stopped (Type F1), These have low water content, fines content up to 20% and gravel content higher than 50%. Hollow symbols are tests with no flow limitation (Type F4), These typically occur in soils with non-plastic fines, with fines content higher than 30%, and low gravel content. ________________________________________ The results of the FLETs on the gap graded soils showed that these soils are unlikely to limit flow. Limitation may occur for low hydraulic head losses, or in internally stable soils.
- Based on the results of the tests I have proposed in the thesis some rules for preliminary estimation of the occurrence of the flow limiting action. These rules depend on the critical parameter identified in testing (fines content, type of plasticity of the fines and water content). For example, an upstream material with more than 30% of non-plastic fines compacted on the wet side is very unlikely to be able to provide the flow limiting action considering that the upstream material is cracked. If there is no crack and the material is internally stable then it is very likely to limit flows.
- In relation to the tests conducted to evaluate the crack-filling action, There were performed 4 tests using the broadly graded upstream materials. and 37 tests on the gap-graded soils In total there were performed 41 tests, which took about 1 year and a half to perform, and involved the compaction of about 4,5 tons of soil.
- The tests allowed the identification of four behaviour types. In tests showing behaviour Type C1, the flow rate decreases fast to a practically null value. There is a formation of a ‘jet of sand’ that is retained by the filter and fills in the pipe.
- In behaviour Type C2a, the flow initially increases because the filter is unable to filter the washed in material from the upstream zone (which is spread all over the filter), but then the filter ends up sealing the flow after some erosion of the core, I
- Behaviour type C2b is similar to C2a, However, the filtering mechanism occurs only after an excessive erosion of the core. The filter becomes more contaminated with particles from both materials.
- Finally, behaviour Type C3 corresponds to the case where the flow rate never decreases because the filter is unable to retain the soils eroded from the upstream material and from the core.
- The results of the tests on the gap-graded soils showed that the occurrence of the crack filling action depends on the relation between sand fraction of the upstream material that can be eroded and of the D15 of the filter. The higher the sand content of the upstream material and the lower the D15 of the filter more likely is crack-filling to occur. Fines content up to 5% may decrease potential of soils to provide the crack-filling action. Core soils with index higher than 4 should not have great influence on the crack filling action, because this phenomenon is very rapid. The tests on the broadly graded soils showed that upstream soils that are able to support an open pipe are highly unlikely to fill cracks.
- Based on the results of the tests on the CFET I have proposed some rules for preliminary estimation of the occurrence of the crack action. These rules use the critical parameter identified in the tests. For example: If we have an upstream soil with sand content higher than 30% without fines, and a filter with D15 lower than 2,9 mm, Then crack-filling action is very likely to occur.
- Final conclusions. In relation to the initial question about “what is the influence of upstream zones”? The answer is that, in fact, some soils compacted in certain conditions, when located upstream of a crack in the core, may effectively provide the flow limiting action or the crack-filling action. I believe that the initial proposed objective was fully achieved With the Development of a new test device and of 2 laboratory tests, and their test procedures With the identification of the potential behaviour types and of the critical parameters influencing each action With the proposed rules to aid pratical engineer in the design phase or to estimate probabilities for dams in operation.
- There are multiple possibilities to extend this research, For example, using the FLET, - Extend testing to greater variety of upstream soils prepared to wider range of test conditions. - Evaluate if self-healing ability of very dry upstream materials also occurs in soils with highly plastic fines. - Investigate the influence of the permeability of coarse grained (stable) soils Using the CFET, - Extend testing using materials from existing dams - Evaluate the relation between sand content and D15 for other filters Using both, Evaluate the influence of the erosion rate of the core (for soils more erodible than those tested) And evaluate the influence of the flow orientation of the flow.
- Thank you all for your attention.