Cyclic shear strain needed for liquefaction triggering and assessment of overburden pressure factor Kσ

R. Dobry, Tarek Abdoun

    Research output: Contribution to journalArticle

    Abstract

    This paper has two objectives. The first is to evaluate the cyclic shear strain needed to trigger liquefaction, γcl, in clean and silty sands in the field during earthquakes as an alternative to the currently used cyclic resistance ratio (CRR). The second objective is to explore the effect of a high effective overburden pressure, σ'v0, on the value of CRR. The first objective is accomplished mainly by using an equation relating γcl and CRR valid for shear wave velocity-based liquefaction charts. This equation is supplemented with laboratory results from undrained cyclic strain-controlled tests as well as large-scale and centrifuge model shaking experiments. It is shown that for recent uncompacted clean and silty sand deposits and earthquake magnitude Mw = 7.5, γcl ≈ 0.03%, with this value increasing to γcl ≈ 0.3% (in some cases to 0.6%) in denser and overconsolidated, preshaken, and compacted sands. These small values of γcl in the field are controlled by two factors: excess pore pressure buildup in the soil due to the cyclic straining and the redistribution of excess pore pressures and upward water flow that occurs during shaking. The available evidence suggests that γcl is either constant with σ'v0 or increases slowly with it. Therefore, in the second objective the simplest assumption is made in the paper that the small γcl, valid for the low confining pressures covered by the liquefaction charts [σ'v0 < 200 kPa (≈ 2 atm)] can be extrapolated without change to confining pressures as high as σ'v0 = 800 or 1,000 kPa (≈ 8 or 10 atm). This assumption allows derivation of a simple expression for the overburden pressure factor, Kσ. The expression predicts a decrease in Kσ with confining pressure that is very similar to some curves of Kσ versus σ'v0 proposed in the literature. Kσ curves are also calculated assuming that γcl is proportional to (σ'v0)β, where 0 ≤ β ≤ 0.5. Additional experimental research is needed to establish the exact variation of γcl with σ'v0 for different sands and different parts of the liquefaction charts and to clarify some of the remaining discrepancies between the different approaches.

    Original languageEnglish (US)
    Article number04015047
    JournalJournal of Geotechnical and Geoenvironmental Engineering
    Volume141
    Issue number11
    DOIs
    StatePublished - Nov 1 2015

    Fingerprint

    Shear strain
    shear strain
    Liquefaction
    overburden
    liquefaction
    confining pressure
    Sand
    sand
    pore pressure
    Pore pressure
    Earthquakes
    earthquake magnitude
    centrifuge
    Shear waves
    wave velocity
    Centrifuges
    low pressure
    S-wave
    water flow
    earthquake

    ASJC Scopus subject areas

    • Environmental Science(all)
    • Geotechnical Engineering and Engineering Geology

    Cite this

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    title = "Cyclic shear strain needed for liquefaction triggering and assessment of overburden pressure factor Kσ",
    abstract = "This paper has two objectives. The first is to evaluate the cyclic shear strain needed to trigger liquefaction, γcl, in clean and silty sands in the field during earthquakes as an alternative to the currently used cyclic resistance ratio (CRR). The second objective is to explore the effect of a high effective overburden pressure, σ'v0, on the value of CRR. The first objective is accomplished mainly by using an equation relating γcl and CRR valid for shear wave velocity-based liquefaction charts. This equation is supplemented with laboratory results from undrained cyclic strain-controlled tests as well as large-scale and centrifuge model shaking experiments. It is shown that for recent uncompacted clean and silty sand deposits and earthquake magnitude Mw = 7.5, γcl ≈ 0.03{\%}, with this value increasing to γcl ≈ 0.3{\%} (in some cases to 0.6{\%}) in denser and overconsolidated, preshaken, and compacted sands. These small values of γcl in the field are controlled by two factors: excess pore pressure buildup in the soil due to the cyclic straining and the redistribution of excess pore pressures and upward water flow that occurs during shaking. The available evidence suggests that γcl is either constant with σ'v0 or increases slowly with it. Therefore, in the second objective the simplest assumption is made in the paper that the small γcl, valid for the low confining pressures covered by the liquefaction charts [σ'v0 < 200 kPa (≈ 2 atm)] can be extrapolated without change to confining pressures as high as σ'v0 = 800 or 1,000 kPa (≈ 8 or 10 atm). This assumption allows derivation of a simple expression for the overburden pressure factor, Kσ. The expression predicts a decrease in Kσ with confining pressure that is very similar to some curves of Kσ versus σ'v0 proposed in the literature. Kσ curves are also calculated assuming that γcl is proportional to (σ'v0)β, where 0 ≤ β ≤ 0.5. Additional experimental research is needed to establish the exact variation of γcl with σ'v0 for different sands and different parts of the liquefaction charts and to clarify some of the remaining discrepancies between the different approaches.",
    author = "R. Dobry and Tarek Abdoun",
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    TY - JOUR

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    AU - Dobry, R.

    AU - Abdoun, Tarek

    PY - 2015/11/1

    Y1 - 2015/11/1

    N2 - This paper has two objectives. The first is to evaluate the cyclic shear strain needed to trigger liquefaction, γcl, in clean and silty sands in the field during earthquakes as an alternative to the currently used cyclic resistance ratio (CRR). The second objective is to explore the effect of a high effective overburden pressure, σ'v0, on the value of CRR. The first objective is accomplished mainly by using an equation relating γcl and CRR valid for shear wave velocity-based liquefaction charts. This equation is supplemented with laboratory results from undrained cyclic strain-controlled tests as well as large-scale and centrifuge model shaking experiments. It is shown that for recent uncompacted clean and silty sand deposits and earthquake magnitude Mw = 7.5, γcl ≈ 0.03%, with this value increasing to γcl ≈ 0.3% (in some cases to 0.6%) in denser and overconsolidated, preshaken, and compacted sands. These small values of γcl in the field are controlled by two factors: excess pore pressure buildup in the soil due to the cyclic straining and the redistribution of excess pore pressures and upward water flow that occurs during shaking. The available evidence suggests that γcl is either constant with σ'v0 or increases slowly with it. Therefore, in the second objective the simplest assumption is made in the paper that the small γcl, valid for the low confining pressures covered by the liquefaction charts [σ'v0 < 200 kPa (≈ 2 atm)] can be extrapolated without change to confining pressures as high as σ'v0 = 800 or 1,000 kPa (≈ 8 or 10 atm). This assumption allows derivation of a simple expression for the overburden pressure factor, Kσ. The expression predicts a decrease in Kσ with confining pressure that is very similar to some curves of Kσ versus σ'v0 proposed in the literature. Kσ curves are also calculated assuming that γcl is proportional to (σ'v0)β, where 0 ≤ β ≤ 0.5. Additional experimental research is needed to establish the exact variation of γcl with σ'v0 for different sands and different parts of the liquefaction charts and to clarify some of the remaining discrepancies between the different approaches.

    AB - This paper has two objectives. The first is to evaluate the cyclic shear strain needed to trigger liquefaction, γcl, in clean and silty sands in the field during earthquakes as an alternative to the currently used cyclic resistance ratio (CRR). The second objective is to explore the effect of a high effective overburden pressure, σ'v0, on the value of CRR. The first objective is accomplished mainly by using an equation relating γcl and CRR valid for shear wave velocity-based liquefaction charts. This equation is supplemented with laboratory results from undrained cyclic strain-controlled tests as well as large-scale and centrifuge model shaking experiments. It is shown that for recent uncompacted clean and silty sand deposits and earthquake magnitude Mw = 7.5, γcl ≈ 0.03%, with this value increasing to γcl ≈ 0.3% (in some cases to 0.6%) in denser and overconsolidated, preshaken, and compacted sands. These small values of γcl in the field are controlled by two factors: excess pore pressure buildup in the soil due to the cyclic straining and the redistribution of excess pore pressures and upward water flow that occurs during shaking. The available evidence suggests that γcl is either constant with σ'v0 or increases slowly with it. Therefore, in the second objective the simplest assumption is made in the paper that the small γcl, valid for the low confining pressures covered by the liquefaction charts [σ'v0 < 200 kPa (≈ 2 atm)] can be extrapolated without change to confining pressures as high as σ'v0 = 800 or 1,000 kPa (≈ 8 or 10 atm). This assumption allows derivation of a simple expression for the overburden pressure factor, Kσ. The expression predicts a decrease in Kσ with confining pressure that is very similar to some curves of Kσ versus σ'v0 proposed in the literature. Kσ curves are also calculated assuming that γcl is proportional to (σ'v0)β, where 0 ≤ β ≤ 0.5. Additional experimental research is needed to establish the exact variation of γcl with σ'v0 for different sands and different parts of the liquefaction charts and to clarify some of the remaining discrepancies between the different approaches.

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