Consolidation (Overview)

Transcription

1 Ch. 8 - Consolidation Page 1 Consolidation (Overview)

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50 Ch. 8 - Consolidation Page 50 Definitions

51 Ch. 8 - Consolidation Page 51 Definitions (cont.)

52 Ch. 8 - Consolidation Page 52 Stress - Strain Relations for 1D Compression

53 Ch. 8 - Consolidation Page 53 Components of Settlement and Immediate Settlement

54 Ch. 8 - Consolidation Page 54 Components of Settlement (cont.) and Consolidation Settlement

55 Ch. 8 - Consolidation Page 55 Consolidation Settlement - Spring Analogy The rate at which the excess pore water pressure dissipates, u, is a function of the permeability of the soil and the thickness of the layer undergoing consolidation. When consolidation is complete, u approaches zero once again.

56 Ch. 8 - Consolidation Page 56 Components of Settlement and Secondary Settlement Immediate Settlement (re-compresssion) Consolidation Settlement (primary compression or virgin compression) Secondary consolidation or creep settlement Effects of surcharging on the rate of secondary settlement

57 Ch. 8 - Consolidation Page 57 1D Consolidation Testing

58 Ch. 8 - Consolidation Page 58 1D Consolidation Testing (cont.) Consolidation curve on arithmetic plot Cr Consolidation curve on semi log plot

59 Ch. 8 - Consolidation Page 59 Consolidation Calculations How do we calculate the initial void ratio of the sample?

60 Ch. 8 - Consolidation Page 60 Consolidation Calcs (cont.) H s = M s 4*D 2 G s w ] (SI units) W s = weight of solids = wt. of oven dry specimen = g D = diameter of specimen = 2.5 inches = 6.35 cm G s = specific gravity of solids = 2.72 W = 1 g / cm^3 (selected to be consistent with g and cm units above) H s = /(Pi/4*6.35^2x2.72x1)=1.355 Units check g/(cm^2*g/cm^3) = cm Alternatively from the phase diagram Vs = Ms / Gs Vs = /2.72= cm^3 Hs = Vs / As Hs = /2.72/(Pi*(6.35/2)^2=1.355 Where As is the cross-sectional area of the specimen

61 Ch. 8 - Consolidation Page 61 Consolidation Calcs (cont.)

62 Ch. 8 - Consolidation Page 62 Preconsolidation Stress and Overconsolidation

63 Ch. 8 - Consolidation Page 63 Preconsolidation Stress and Overconsolidation (cont.)

64 Ch. 8 - Consolidation Page 64 Preconsolidation Stress - Casagrande's Method

65 Ch. 8 - Consolidation Page 65 Schmertmann's correction for disturbance Void ratio versus log effective consolidation stress curve illustrating deposition, sampling (unloading) and reconsolidation in the consolidation test apparatus.

66 Ch. 8 - Consolidation Page 66 Schmertmann's correction for disturbance (cont.) Illustration of the Schmertmann (1955) procedure to obtain the field virgin compression curve: (a) normally consolidated soil; (b) overconsolidated soil.

67 Ch. 8 - Consolidation Page 67 Schmertmann's technique notes Notes for use of Schmertmann's technique for overconsolidated clay (i.e., refer to Fig. b. on previous page Perform the Casagrande (1936) construction and evaluate the preconsolidation pressure ' p Calculate the initial void ratio e o. Draw a horizontal line from e o, parallel to the log effective stress axis, to the existing vertical overburden pressure ' vo. This establishes control point 1, illustrated by triangle 1 in Fig. b. From control point 1, draw a line parallel to the rebound-reload curve to the preconsolidation pressure ' p. This will establish control point 2, as shown by triangle 2 in Fig. b. From a point on the void ratio axis equal to 0.42 e o, draw a horizontal line, and where the line meets the extension of the laboratory virgin compression curve L, establish a third control point, as shown by triangle 3. The coefficient of e o is not a magic number, but is a result of many observations on different clays. Connect control points 1 and 2, and 2 and 3 by straight lines. The slope of the line F joining control points 2 and 3 defines the compression index C c for the field virgin compression curve. The slope of the line joining control points 1 and 2 of course represents the recompression index C r. Pasted from <file:///c:\users\sfbartlett\documents\my%20papers\udot%20preconsolidation%20stress \UDOTREPORT.doc>

68 Ch. 8 - Consolidation Page 68 Settlement Calculations

69 Ch. 8 - Consolidation Page 69 Settlement Calculations (cont.) Note: Because the preconsolidation stress is equal to the vertical effective stress, the specimen is normally consolidated.

70 Settlement Calculations (cont.) Note that the stresses for the settlement calculation are usually calculated at the mid-point of the layer or sublayer. Ch. 8 - Consolidation Page 70

71 Ch. 8 - Consolidation Page 71 Settlement Calculations (cont.)

72 Ch. 8 - Consolidation Page 72 Settlement Calculations (cont.)

73 Ch. 8 - Consolidation Page 73 Settlement Calculations (cont.) Note: In this case, all of the consolidation will occur in recompression. Note: In this case, some of the consolidation will occur in recompression and some will occur in virgin compression.

74 Ch. 8 - Consolidation Page 74 Settlement Calculations (cont.) Note: If the clay is normally consolidated, then the first half of this equation is zero and is not needed, and p ' is set equal to V1 '

75 Ch. 8 - Consolidation Page 75 Example 1 - Overconsolidated Clay A 6-foot thick clay layer is subjected to an increase in vertical stress of 700 psf at the center of the layer from a very wide embankment fill. Laboratory consolidation tests for the clay layer produced an initial void ratio, e 0, of 1.50, an preconsolidation stress 1500 psf, and C c and C r values of 0.30 and 0.05, respectively. Calculate the consolidation settlement for this clay layer. Note that v1 ' is less than the preconsolidation stress, p '; hence the clay is overconsolidated. The overconsolidation ratio (OCR) is: 1500/1000 = 1.5. Settlement in recompression Settlement in virgin compression

76 Ch. 8 - Consolidation Page 76 Example 2 Wednesday, April 24, :43 PM Cr can also be obtained from the slope of this curve Use this information to calculate the consolidation settlement given on the next page

77 Ch. 8 - Consolidation Page 77 Example 2 (cont.) Wednesday, April 24, :43 PM Settlement in recompression due to the change in stress Settlement in virgin compression due to the change in stress

78 Ch. 8 - Consolidation Page 78 Secondary Consolidation (Creep) Settlement tp = time to end of primary consolidation ( EOP) e C 1 log cycle H o is thickness of layer T = elapsed time since load application T p = time at end of primary consolidation

79 Ch. 8 - Consolidation Page 79 Secondary Consolidation (Creep) Settlement Creep strain Creep strain in expanded polystyrene (EPS) geofoam from the I-15 Reconstruction project. Fine-grained soils have similar creep behavior. Cae = (1.5/ /100 )/ log (10/1)= Note that we use C instead of C because the y-axis is expressed in percent strain and not void ratio Ss = *8*log(10/1)= or 0.04 m or 4 cm for EPS embankment = 8 m thick = Ho

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