1

CIVE 554/CIVE650Design of Embankments

on Soil Ground

Design CriteriaPrevent shear failure

STAGE CONSTRUCTION

Account for settlement in designAbility to construct within required time frameCost effective design

2

STABILITY

DURING CONSTRUCTIONMETHOD OF ANALYSIS:

•SLICES (BISHOP, MORGENSTERN PRICE)

•TOTAL STRESS ANALYSIS IN CLAYS (SHANSEP)

•UNDRAINED SHEAR STRENGTH: Su = f(p’)

•Su/p’= f(LIQUIDITY INDEX) = 0.1 TO 0.3

•NOTE: p’ = σ’0v FOR N/C CLAYS

How High can you Build?

3

Embankment Shear Failure

Shear Equation

For φ = 0 Nc = 5.14, Nq=1, Nγ =0 c=CuFor embankment sitting on ground surface

Df = 0

γγ BNNDCNq qfcultimate 5.0++=

CuNCuq cultimate == 5

4

Typical Soft soil Stress-Strain Reponseq= applied stress

εv = vertical strain

qultqallowable

Fqq ultimate

allowable =

Total Stress Design Analysis

ee

c

HCuNF

γ=

where: F= factor of safety typically 1.2Cu = undrained strength of soilγe = embankment unit weight He = height of embankment

5

Cu Design ConsiderationsCu is the average undrained strength to a depth approximately equal to the height of the embankmentSoil will fail at the weakest point…

Use average Cu values in the soft zone

Cu are corrected values..see Mitchell notes

Nc Design Considerations

)4.0(452

1 −+=ddNc

Varies from 5 to 8

6

Embankment with BermsAddition of Berm can be use to increase height of embankment

Embankment with BermsUse of berms can reduce lateral spreadMore material efficient than flattening the slope angle

)( dHCuNFee

c

−=

γDmax must satisfy

dCuNFe

c

γ=

7

Other Embankment Design OptionsUse of light weight fill or geo-foamStage construction

Allow for consolidation and increase in Cu

Note:To increase Cu σ’v must be greater than the soil

preconsolidation pressure

Stage Construction Process1. Construct embankment to H1

2. Allow 90% of the excess pwp to dissipateEstimate using consolidation theoryMonitor peizometer installed in the soft zone

3. Determine increase in Cu due to increase in vertical effective stress

4. Determine the magnitude of settlement5. Increase height to H2

6. Repeat steps 2 to 5 to get to design height

8

Normally Consolidated Clay

100(%)37.011.0

'PIC

v

u +=σ

Skempton 1957

Embankment Height at any time

⎟⎟⎠

⎞⎜⎜⎝

⎛+⎟⎟

⎠

⎞⎜⎜⎝

⎛= 1

0''

UHCuFNH

v

c

γσ

σ

Where:

U is the average degree of consolidation over the potential failure zone

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Degree of Consolidation

STABILITY

DURING CONSTRUCTION

BEFORE CONSTRUCTION:

HYDROSTATIC PORE PRESSURES

DURING CONSTRUCTION:

HYDROSTATIC PLUS EXCESS PORE PRESSURES

PROBLEM ASSOCIATED WITH FAST CONSTRUCTION: FAST PORE PRESSURE GENERATION AND SLOW DISSIPATION IN FINE SOILS

DURING CONSTRUCTION:

HYDROSTATIC PLUS EXCESS PORE PRESSURES

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Time Rate of Consolidation

v

v

CHTt

2

=

Figure 1.21 (a) Derivation of Eq. (1.60); (b) nature of variation of Δu with time

© 2004 Brooks/Cole Publishing / Thomson Learning™

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Figure 1.22 Drainage condition for consolidation: (a) two-way drainage; (b) one-way drainage; (c) plot of Δu/Δu0 with Tv and H/Hc

© 2004 Brooks/Cole Publishing / Thomson Learning™

© 2004 Brooks/Cole Publishing / Thomson Learning™

Figure 1.23 Range of Cv(after U.S. Dept. of Navy)

©20

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Thom

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Figure 1.24 Plot of time factor against average degree of consolidation (Δu0 = constant)

©20

04 B

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Stress Under Embankment

13

OsterbergChart

Corner of triangular Load

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Figure 1.25 One-dimensional consolidation due to single ramp loading

© 2004 Brooks/Cole Publishing / Thomson Learning™

© 2004 Brooks/Cole Publishing / Thomson Learning™

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Figure 1.26 Olson’s ramp-loading solution: plot of U vs. Tv (Eqs. 1.69 and 1.70)

© 2004 Brooks/Cole Publishing / Thomson Learning™