COMPARISON BETWEEN DIFFERENT ANALYSIS IN PREDICTION OF LIQUEFACTION-INDUCED LATERAL DISPLACEMENTS

1. Piero Pampanin – EAP student Advanced Soil Mechanics COMPARISON BETWEEN DIFFERENT ANALYSIS IN PREDICTION OF LIQUEFACTION-INDUCED LATERAL DISPLACEMENTS • TARGETS • Explain the most widely used procedure to compute lateral displacements due to lateral spreading • Analyze different approaches using 2 research studies as examples • Present which will be the field of my master thesis for the next months

2. BRIEF INFORMATION ON LIQUEFACTION • phenomena occuring during an earthquake on saturated cohesionless soil (gravels, sands and very low plasticity silts) unable to drain during shaking earthquake increase of water pore pressure decrease in effective normal stress sand boils, cracks, quick-sands loss of strength and stiffness • General definition of Liquefaction (no unambigous one) Unisotropically consolidated undrained cyclic test ru<100% FLOW LIQUEFACTION Isotropically consolidated undrained cyclic test ru=u/σ’vc=100% INITIAL LIQUEFACTION

3. LATERAL SPREADING DISPLACEMENTS DUE TO LIQUEFACTION • Liquefaction induced deformations predicted from the STRESS-BASED METHOD STRESSES CSR obtained by: • Characteristics of design earthquake • Type of soil and depth STRENGTH-RESISTANCE CRR obtained by: • Correlations with in situ CPT SPT data FSliq = CRR/CSR Accuracy is limited by: • Soil characteristic • Site stratigraphy • Complicating phenomena (3D effects, void redistribution) max Dr% Lateral Displacement Index FS

4. FOCUS ON 2 EXAMPLES • Understand how different assumptions and procedures can have different results and accuracy. 1) Estimated liquefaction-induced lateral displacements using the Standard Penetration Test or Cone Penetration Test (by G. Zhang; P.K. Robertson, M. ASCE and R.W.I. Brchman). • main porpuses of the study: • to find general correlations between geometric characteristics of ground and lateral displacements • to compare measured lateral displacements to the calculated LDI The case histories SPT and CPT from different locations and earthquakes a- Gently sloping ground without free face b- Nearly level ground with a free face c- Gently sloping ground with a free face • ASSUMPTIONS • effects of earthquake characteristics on lateral spreading displacements quantified by their effects on FS and thickness of liquefied soils • effect on fine content on lateral spreading considered similar to that on liquefaction triggering

5. Results from CPT and SPT considered together to find correlations between Slope S LDI Ratio L/H a) Gently sloping ground without a free face b) Level ground and free face b+c) Level and gently sloping ground and free face

6. COMPARISON:calculated lateral displacements VS measured ones • 90% of calculated LDI shows variations between 50 and 200% of the measured values: factor 2 • relations are recommended for use within ranges of earthquake and ground conditions considered as case history • To improve accuracy and applicability of this approach • additional case history data are required • further evaluation of the influence of earthquake characteristics and other parameters is needed This accuracy may be reasonable and acceptable for low to medium risk projects

7. 2) Evaluation of CPT liquefaction analysis methods against inclinometer data from Moss Landing (Kulansingam-Boulanger-Idriss) Comparison between measured lateral displacements on the site (using three slope inclinometers on the shoreline of Mass Landing before Loma Prieta Earthquake) to results computed with 3 different methods analyze the influence of : • different correlations used to get CRRliq and relative density (Dr%) starting from CPT results (qc) • Type of interpretation for the soil profile Roberts and Write: any soil samples needed CRRliq versus qc correlations Idriss et al.: soil samples needed Tatsuoka et al. Dr% versus qc1 correlations Kulhawy and Mayne Automated: point-by-point calculations, Interpretation of soil profile Manual: attention given to the influence of interfaces and thin layers on the tip resistance value

8. Method B1 and B2:consistent results in comparison to observed lateral movements along the road. The results on the shoreline are less precise: they have a better prediction than Method A but accuracy must be improved • Method A: unreliable results for all the cases, always over-predicting the displacements. It tends to over predict the strains in fine grained layers and interfaces and to slightly under-predict strains in the sand (while the observed deformations were actually greater).

9. CONCLUSIONS • manual interpretation of the data is fundamental to consider the influence of thin layers and interfaces, to better predict the real behavior of soil BUT None of the CPT analysis methods could accurately predict the observed displacements at all six locations or the distribution of deformations with depth at all three inclinometers In order to improve the accuracy of these approaches, NEW DATASET AND CASE HISTORIES MUST BE CONSIDERED AND INCLUDED IN UPDATING THE METHODS