Strength Parameters. Radek Vašíček

Transcription

1 Strength Parameters Radek Vašíček

2 Requirements / tested properties (geotechnics) Granulometry Particle size distribution (curve) Density (Bulk, Dry, Specific) Water content, Degree of saturation Consistency limits (Atterberg limits) Swelling abilities Swelling pressure, Swell index Permeability Thermophysical properties Strenght properties

3 - uniaxial strength (compressive, tensile) - shear strength - strength on bending - triaxial strength

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5 Börgesson et. al., SKB IC 122

6 Swelling pressure and water content at full saturation

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8 UNIAXIAL COMPRESSIVE STRENGTH

9 No special standard for compacted bentonite blocks - test is performed according to European Standard EN 1926 Natural Stone Test Methods Determination of Compressive Strength

10 Tested samples - cube with edge 50 ± 5 mm or 70 ± 5 mm - cylinder with diameter and height 50 ± 5 mm 70 ± 5 mm - the edge of cube or diameter of cylinder > 10 * size of max grain - size of max grain > 7 mm it is necessary to increase the number of samples

11 Sample preparation

12 Samples differ in the direction of loading Loading in direction of compaction Loading upright the direction of compaction

13 Test description - sample is dried by temperature 70 ± 5 C to stable mass - stable mass = the difference between two weights in interval 24 ± 2 hours is not higher than 0,1 % of sample weight - samples are stored in temperature 20 ± 5 C

14 - samples are measured twice in upper side (l 1 and l 2 ) twice in lower side (l 3 and l 4 ) with accuracy 0,1 mm - dimension l is calculated as l = (l 1 + l 2 + l 3 + l 4 )/ 4 - height h of the sample is measured with accuracy 1 mm - weight m of the sample [ g ]

15 Loading of the sample - sample is placed and centred in pressure machine - centering is very important for test results -sample is continuously loaded up to its breaking - sample is loaded continuously by constant velocity of stress 1±0,5 MPa/s (It corresponds with velocity of 75 kn / min) - maximum force F is recorded [ acc. 1kN ]

16 Samples after testing

17 Test results - loaded sample area A = l *l [mm 2 ] - volume of the sample is calculated V = l * h [mm 3 ] - total density is calculated r = m / V [kg/m 3 ] - strength is calculated R = F / A [MPa] (two valid digits) - tests are performed in sets of 6 samples. - average value of strength is calculated from six measurements.

18 uniaxial strength R [MPa] Influence of direction of testing Bentonite Rokle, grinded non - activated 85 %, sand Provodín 10 %, graphite 5 % in direction of compaction upright to direction of compaction C 1 month 200 C 3 months sample: cube with edge 50 mm 200 C 9 months thermally 2 non - loaded time of thermal loading 200 C [days]

19 uniaxial strength R [MPa] Influence of thermal loading Bentonite Rokle, grinded non - activated 85 %, sand Provodín 10 %, graphite 5 % in direction of compaction upright to direction of compaction C 1 month 200 C 3 months sample: cube with edge 50 mm 200 C 9 months thermally 2 non - loaded time of thermal loading 200 C [days]

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21 SHEAR STRENGTH

22 SHEAR STRENGTH Most destruction of soils have shear character Basic equation is defined by Coulomb = s * tg f + c - tangencial stress on shear surface s - normal stress upright to shear surface c - cohesion of soil φ - friction angle For cohesive soils

23 = s * tg f For sand and gravel, non cohesive soils, c = 0 Shear strength is dependent on: f and c and s

24 Two different types of shear strength parameters: total parameters c u f u not drained test, no consolidation, type UU effective parameters c ef f ef drained test, consolidated, type CU

25 The dependence of tangencial stress on normal stress

26 Shear strength is maximum peak strength f Maximum strength f is reached, drop down to residual strength r

27 SHEAR STRENGTH TESTS laboratory tests on natural samples or on prepared samples (pressing to the shear box), disadvantage sample may be damaged during sampling - in situ tests (only cohesion c) Czech Standard Laboratory direct shear box drained test of soils

28 Sample shape 84 x 84 mm or circle with diameter mm Sample height mm Sample is put into the shear box (it has two parts, one is movable)

29 Shear strength equipment Sample after testing

30 Loading of the sample

31 Each sample is tested in 4 stages. Difference in value of normal stress s [kpa] 50, 100, 150, 200 s= N A N = 352,8; 705,6; 1058,4; 1411,2 [N] A = 7056 [mm 2 ] 1) Sample is loaded and consolidated

32 Time of consolidation sand 0,55 hours loam 1,1 clay 2,5 clay 7 (w L > 50%) 2) After consolidation, sample is loaded also by tangencial stress 3) Tangencial stress [kpa] and displacement of shear box [mm] are recorded. The velocity of tangencial stress is continuous.

33 4) Velocity of tangencial stress [mm*min 1 ] sand 0,25 45 min loam 0, min clay 0, min (w L > 50%) clay 0, min 5) Sample is loaded till displacement of the box is 0,1 * its dimension dimension 84 mm displacement 8,4 mm (maximum value)

34 Results of the test -identification of sample, geotechnical description, sample dimenions,preparation of the sample, water, consolidation c cohesion f friction angle Drained? Undrained? Water content? Density?

35 tangencial stress Results of the test displacement

36 tangencial stress Results of the test displacement If the test curve has no clear peak (the curve is it still rising) the result the peak shear strength - is determined as shear stress for displacement equals 10% of the dimension of sample (e.g. 8,4mm).

37 Dependence of cohesion on water content

38 Dependence of friction angle on water content

39 effective friction angle f ef [ ] Dependence of friction angle on thermal loading Bentonite Rokle grinded non - activated 85 %, sand Provodín 10 %, graphite 5 % 39 compacted material, 1500 kpa powder material C 1 month 200 C 3 months 200 C 6 months 200 C 9 months thermally 27 non - loaded time of thermal loading 200 C [days]

40 Strength on bending

41 Strength on bending Czech standard Special and not very frequent test Samples 40 x 40 x 200 mm

42 Course of the test and results Beam dimensions b x h = 40 x 40 mm l = 200 mm Minimal dimension = at least 10 x max grain Accuracy b, h 0,1 mm l 1 mm Sample is loaded continuously up to its breaking Maximal force is recorded

43 The scheme of placing sample into the testing equipment

44 Sample before testing

45 Equipment for tesing strength on bending

46 Samples after testing

47 Results Different according to the place of breaking 1) In the centre F.l R = 2 b.h F maximal force in N 2) In other place 3F.e R = 2 b. h e distance from support Average of 5 measurements Round strength < 10 MPa to 0,1 MPa > 10 MPa to 1 MPa

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51 And now? 1/ Record all necessary information and break cubes (Rc =?) 2/ Analyse measured data from shear st. test (friction angle, cohesion) Data file will be sent by , but one correct is still missing

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