Application of CPT data for evaluation of ground liquefaction in ChiChi Earthquake, Taiwan

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Application of CPT data for evaluation of ground
liquefaction in Chi-Chi Earthquake, Taiwan

• D.H. Lee1 , C.S. Ku2, C.S. Chen1, C.H. Juang3
  J.H. Wu3
• 1National Cheng Kung University, Taiwan
• 2I-Shou University, Taiwan
• 3Clemson University, USA

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Acknowledgment

• This study was sponsored by the National Science
  Council of Taiwan
• (Grant No.NSC 91-2211-E-214-006)

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Evaluation of ground liquefaction in the Chi-Chi,
Taiwan earthquake using CPT data

• CPT equipment and test procedure
• CPT data of the liquefied areas
• Soil resistance change of the liquefied area
• Post earthquake assessment of liquefaction
• CPT-based liquefaction curve
• Summary and Conclusion

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CPT equipment and procedure

• CPT is a relatively rapid and reliable in-situ
  test that can provide an almost continuous soil
  profile.
• Cone apex angle60o ,
• base area10cm2 ,
• friction sleeve area 150cm2.
• Penetration rate is about 20 5 mm/sec, readings
  are taken at every 50mm.

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CPT sounding Raw dataCone resistance (qc),
Local friction (fs), Porewater pressure (Pw)
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Profile of CPTU sounding
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CPT data of the liquefied area

• The liquefaction manifestations in Yuanlin,
  Nantou, Wufeng and Changhua coastal industrial
  park are more apparent and more extent.
• 275 CPT soundings were collected from these
  liquefied sites.
• Most of the CPT explorations in land were
  conducted by MAA for TNCREE.
• In the newly reclaimed area, CPT sounding were
  conducted by the writers.

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Sand boiling occurred at lane in Nantou
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Widely liquefactions were observed at a newly
reclaimed land
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Profiles of SCPT at WF4 (liquefied)
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Profiles of SCPT at NT1 (liquefied)
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Profiles of SCPT at YL2 (liquefied)
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Profiles of SCPT at LW-C1 (liquefied)
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Geotechnical Characteristic of the liquefied area

• According to these SCPT soundings at the
  liquefied area, the continuous sandy soil (SP,
  SP-SM, SM) with low cone resistance and slow Vs
  were observed, it could be the most susceptible
  liquefaction layer.

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Soil resistance change of the liquefied area

• It is a interesting that the alteration of the
  soil characteristics with time at the liquefied
  site after an earthquake.
• The CPT are employed at the same location but in
  different time (November, 1999, July, 2000, and
  May, 2002), and the results were used to check
  the soil resistance change of the liquefied area.

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CPT profile at LW-C1 liquefied
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CPT profile at LW-C2 non-liquefied
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Soil resistance change of the liquefied area

• The CPT profiles of LW- C1, wherein the sand boil
  occurred, the changes of qc data were relatively
  small.
• The CPT profiles of LW- C2, non-liquefied
  evidence observed therein, the qc, fs and
  porewater pressure are no obvious difference
  between the two stages soundings.

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Post earthquake assessment of liquefaction

• Simplified methods
• Vs-based method
• SPT-based method
• CPT- based method
• Olsen (19811997)
• Robertson (19831998)
• Juang (19992003) probabilistic methods

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Assessment of liquefaction potential at LW-C1
(liquefied)

• A thick layer with high probability of
  liquefaction can be observed.

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Assessment of liquefaction potential at LW-C2
(non-liquefied)

• Most of the soil are with small to medium
  probability of liquefaction

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CPT-based liquefaction curve
?Liquefaction or non-liquefaction The
sounding could be judged as the occurrence of
liquefaction when the liquefaction phenomena such
as sand boils, ground subsidence, lateral spread
and building settlement are observed on the
ground. ?The liquefaction related phenomena only
indicate that the liquefactions occur in the
place but not the precise location or depth.
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Evaluation Criteria

• The liquefied soil could be disturbed to a loose
  state and has lower cone resistance value.
• The resistance of the non-liquefaction soil have
  insignificant changes and are almost the same as
  their original values. The sandy soil with
  normal high cone resistance values could be
  considered as non-liquefaction.

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This soil is defined as non-liquefied layer
because of it high cone resistance value.
?
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Proposed CPT-based liquefaction curve
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Summary and Conclusion

• A thick continuous loose sandy soil with low
  resistance and slow shear wave velocity were
  measured, and this layer could be the most
  susceptible to liquefaction.
• From the liquefied and not liquefied CPT
  soundings of the different periods after the
  Chi-Chi earthquake, the soil resistances in the
  liquefied zone almost dont change.

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Summary and Conclusion (cont.)

• A site with liquefaction experiences in an
  earthquake will not necessarily reduce its
  liquefaction potential in the future seismic
  events.
• According to the proposed judgment criteria of
  liquefaction or non-liquefaction in this study,
  46 liquefaction cases and 88 non-liquefaction
  cases could be derived from the 43 CPT soundings
  in the liquefied areas during Chi-Chi earthquake.

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The endThanks for your attention