Fundamental Function of Filters for Dams. James R. Talbot, P.E. Consulting Engineer

Transcription

1 Fundamental Function of Filters for Dams James R. Talbot, P.E. Consulting Engineer

2 Introduction Issues related to the formation, and character of a filter cake that stops progression of a concentrated leak requires some explanation of SCS (NRCS) filter study Small gradation difference of filter between success and failure (no transition) Rapid determination of success and failure soil entering the filter is washed on through Filter function of forming a filter cake on the upstream face of the filter preventing a concentrated leak development some characteristics of the filter cake

3 Introduction Continued Gravelly soil testing differences in response Filters need to be designed for the fine matrix of gravelly soils

4 Most Failures are Seepage/Piping Related New dam first filling Failed with small amount of water stored in the reservoir Foundation and embankment cracks suspected Dispersive clay soils were involved

5 Famous dam Failure Started as seepage through cracks in the rock of the right abutment Dam had no filter-drainage zone Teton Dam, Idaho

6 SCS (NRCS) Performed A Study on Filters 1980 to 1985 the late James L. Sherard worked with SCS to study filters for protection of embankment dams from concentrated leak development caused by erosion in cracks

7 Sand Testing Basic Properties Schematic of test apparatus for testing the basic properties of sand and gravel filters Uniform sand over graded filter high pressure water source No simulated crack or opening

8 Void space along apparatus boundary is largest. We used finer material along the circumference of the cylinder forcing failure in the center of the filter

9 - Performing test for basic properties of filter - High velocity water running through base soil (sand) and filter sieve used to catch particles that get through filter and test for size of these particles

10 Results show D 15 of filter = 9 times d 85 of base soil (sand) with good correlation results found by changing filter with same base and by changing base with same filter

11 Findings Basic Properties of Filters using Sand Small filter gradation difference between success and failure Once base soil starts passing through filter, most of the base soil is eroded through in a short time Very good correlation between D 15 of the filter and d 85 of the base soil same as other researchers For sands, D 15 = 9 d 85 or (d 85 = 0.11 D 15 ). Tergazhi used Max D 15 = 5 d 85 for factor of safety about 2 We found thesis by Lund (U of London) - same results For silt and clay soils, used specimen with simulated crack or opening

12 Grain-size curves of the base soil (sand) used in testing for the basic properties of sand and gravel filters

13 Grain-size curves of the filters used in the testing program of sand soils

14 Testing of silts and clays using simulated crack or other opening Started with a slot formed in the compacted specimen of silt or clay with water under high pressure passed through the slot into filter downstream Several arrangements of slots or holes were tried with variable specimen thickness Tests were run horizontal as well as vertical no difference found for horz. or vert. or for specimen thickness vertical used because less complicated Finally settled on using a specimen approximately 1- inch thick with a 1-mm diameter hole and 40 psi water pressure applied we named it the No Erosion Filter Test because for successful tests, there was no visible erosion

15 Schematic of test apparatus for the slot test. Approximately 40 psi of water pressure was used for high pressure tests. Success or failure of the filter determined very quickly with no doubt

16 No Erosion Filter Test Setup - For soil with no gravel, the specimen is 1-inch thick and 1- mm diameter hole is made through it - High water pressure (40 psi) is used - Success or failure is determined quickly visually

17 The No Erosion Test setup Successful filters seal the opening quickly to a drip or no flow

18 Water under high pressure passes through the simulated crack & the filter Eroded particles of the base soil collect at the filter face and stop flow in the crack Hydraulic Fracturing caused filter cake to extend some distance on each side of the simulated crack Filter beyond no filter cake, open for seepage collection

19 NRCS Filter Study Eroding soil catches at the face of the filter and seals it The filter face seals at the opening and some distance to each side because hydraulic fracturing from high gradients causes further widening of the seal The remaining filter is open to receive normal seepage (between cracks)

20 - The soil particles collected at the filter face penetrate only about 1 to 2 mm into the filter for some width beyond the crack - Any particles that penetrate beyond 2 mm into the filter will pass on through with the water

21 Process for forming filter cake at upstream surface of filter for successful test When pressure is applied to soil specimen with simulated crack, a shot of cloudy water discharges First colloidal particles not caught and pass through Sand (very fine and larger) particles are caught Almost immediately, flow either stops or reduces to a clear drip as Subsequent colloidal particles are caught on the very fine sand and silt. This builds a cake of very low permeability Base soil should have some variance in particle size Filter cake is only about 2 to 3 mm thick with 40 psi on upstream side and no pressure on downstream with very little flow passing through it

22 Results of filter testing for silts and clays plotted points are boundary between success and failure Boundary shown is beyond all plotted points Can test for your soil and filter combination

23 A 10-inch diameter apparatus was used for gravelly base soil containing particles up to 2 inches in diameter Test took a little longer because gravel particles armored the hole so erosion was sometimes slower

24 Filter failure in the large test

25 Successful filter test for gravelly clay soil in large test Test results show that filter design for gravelly soil must be based on the fine matrix of the soil (the portion passing No. 4 Sieve)

26 Filter Criteria Table 26-2 Filtering criteria Maximum D 15 Base soil Filtering Criteria Category 1 9 x d 85 but not less than 0.2 mm mm 3 [40 A / 40 15] [(4 x d 85 ) 0.7 mm] mm A = % passing #200 sieve after regrading (If 4 x d 85 is less than 0.7mm, use 0.7mm) 4 4 x d 85 of base soil after regrading

27 NRCS Filter Design Document Web Site Click on each in sequence Handbooks Title 210 Engineering National Engineering Handbook Part 633 Soil Engineering Chapter 26 Gradation Design of Sand and Gravel Filters

28 Low pressure slot tests made simulating small flood control dams No flow ever came through Crack filled with soft soil at end High pressure tests on sandy soils with no simulated crack for successful filters was also successful in preventing piping

29 Summary Successful filters caught eroding particles and formed a filter cake very quickly at the filter face it required a very small amount of erosion (no visible erosion) Unsuccessful filters allowed eroding particles to pass through the filter rapidly so that most of the base soil could be eroded through in a short time A very slight increase in gradation can change the filter from successful to unsuccessful. A safety factor should be used to avoid designing close to the boundary between success and failure Long-term high pressure tests showed successful filters from simulated crack tests were also successful in preventing piping in tests with no cracks

30 Summary Continued For successful filters, test results indicate the first colloidal particles that reach the filter are not caught and pass through the filter with flowing water Very fine sand and silt particles are caught and subsequent colloidal particles are then caught forming a filter cake with very low permeability The filter cake is formed on the upstream surface of the filter and any particles that penetrate beyond about 2 mm into the filter will pass on through the filter and not be deposited in the filter Filter tests on gravelly soils took slightly longer to form the filter cake because the hole is armored with gravel particles Filter gradation design for gravelly soil must be based on the fine matrix (portion passing the No. 4 sieve)

31 Summary Continued Tests showed properly graded filters are very effective in clogging cracks or other openings as soil particles are caught at the filter face; preventing development of a concentrated leak and failure The filter is clogged over the width of the crack and for some distance on each side of the opening, but is available for receiving seepage through the pores of the soil with no piping at all other locations There is a narrow boundary between filter failure and success, well defined by D 15 /d 85 = 9 or d 85 = 0.11 D 15 for sandy soils, but the traditional criteria used for many years of D 15 /d 85 = 4 is recommended for a factor of safety of about 2 Other criteria are appropriate for silt or clay base soils. Conservative criteria have been developed for all soils based on a margin of safety beyond the limits of the boundary between success and failure for all tests. A test could be made for a given soil to check for using other gradations

32 An example of a two-zone filter-drain in a dam

33 A properly designed filter-drainage zone will prevent development of concentrated leaks through cracks or holes in dams

34 Filter Construction Placement Methods Keep filter higher than adjacent zones Alignment very important Two bin box works well

35 Filter Construction Compaction Vibrating roller Specify density or number of passes from test fill Contamination Control Always keep filter zones higher than adjacent zones No traffic over filter zones allowed

36 Filter Construction Compaction Vibrating roller Specify density or number of passes from test fill Contamination Control Always keep filter zones higher than adjacent zones No traffic over filter zones allowed

37 As part of filter study, the repair measure was tested in the field using dike to create a small reservoir on upstream of dam

38 Cross section of dike used to create small reservoir

39 Piezometers and censors were placed in center trench then it was filled with filter material before filling reservoir with water

40 Water sensor used with alarm to monitor when water entered trench through cracks

41 Reservoirs were filled with water

42 On one test, soil was cleaned off the upstream slope water ran into cracks this picture on similar test in Nebraska

43 After water was in reservoir for 30 days, excavations made into cracked area and filter

44 Cracks and filter inspected No water ever entered the filter cracks were full of soft soil no water or wetness D.S. of filter

45 Filter removed from trench note crack filled with soft soil filter penetrated short distance into crack sensor detected no water

46 Crack filled with soft soil in Nebraska test

47 6-inch wide crack filled with soft soil in Nebraska test

48 One test made where no filter had been installed this test site was excavated after 60 days dam did not fail with no filter

49 Cracks were found, some were nearly closed from swelling of soil

50 Excavation into downstream slope of test where no filter was used

51 In test with no filter, water was found in a crack within 2 feet of downstream slope of the dam, but crack was nearly closed from swelling

52 Root hole and water in crack near the downstream slope

53 Seepage Control Structural antiseep collars used in the past Many failures using this method of seepage control Philosophy of anti-seep collars is to stop flow without pressure reduction Picture Courtesy of Danny McCook, NRCS

54 Filter Diaphragm or Collar For small homogeneous dams, a single filter collar is used instead of structural anti-seep collars If the dam has a chimney drain, that drain can act as the filter collar The collar extends around the pipe and an outlet to the toe is provided

55 NRCS has set some dimensions for filter diaphragms on conduits through embankment dams Filter Diaphragm Placement

56 Seepage Control - Filter Diaphragms Filter diaphragms are used instead of collars - placed where they will intercept any areas of poor compaction, cracking, or potential for concentrated leak development Picture Courtesy of Danny McCook, NRCS

57 Filter Diaphragm Placement Sketch of filter diaphragm in dam Can tie into drainage system of dam if it has one