Soil Liquefaction
Will the sand under the site turn to liquid in an earthquake? From the SPT log of a borehole.
Applies the Idriss & Boulanger SPT procedure to every test of a borehole: it corrects the blow counts for hammer energy, rods, borehole and sampler, overburden and fines, compares the cyclic stress of the design earthquake with the cyclic resistance of the sand, and gives the factor of safety at each depth. It adds the liquefaction potential index, the thickness that liquefies and the settlement of the ground after shaking, and places every test on the classic chart of case histories.
Overview
Loose, saturated sands and silty sands near the water table, the common ground beside the Tigris, the Euphrates and the Shatt al-Arab, can lose their strength during strong shaking. Buildings tilt, buried pipes float and the ground settles and cracks. This check is required by seismic codes for important structures on such ground.
Every correction factor is shown in the results table and the CSV file, so that the calculation can be checked line by line in a report or a thesis.
What you enter · What you get
What you enter
- Design magnitude, peak ground acceleration and water table
- Hammer energy ratio, borehole diameter and sampler type
- SPT log: depth, blow count, fines content, unit weight and clay-like behaviour
What you get
- Factor of safety at each depth and the lowest value
- Liquefaction potential index and liquefiable thickness
- Post-liquefaction ground settlement
- Depth profile, cyclic resistance chart and full table (CSV)
How it works in the field
Method and equations
Corrections of the blow count
N60 = N·(ER/60)·CB·CR·CS with rod-length factors 0.75–1.0; CN = (Pa/σ′v)^m ≤ 1.7 with m = 0.784 − 0.0768√N1,60cs solved by iteration; fines correction ΔN = exp(1.63 + 9.7/(FC+0.01) − (15.7/(FC+0.01))²).
Demand and resistance
CSR = 0.65 (σv/σ′v) amax rd with the depth reduction rd of Idriss (1999); CRR for M 7.5 and 1 atm from the Idriss & Boulanger (2008) curve, multiplied by MSF = 6.9 exp(−M/4) − 0.058 ≤ 1.8 and Kσ = 1 − Cσ ln(σ′v/Pa) ≤ 1.1.
Consequences
Volumetric strain from the maximum shear strain and relative density (Ishihara & Yoshimine 1992, as formulated by Idriss & Boulanger 2008) summed to 20 m; LPI of Iwasaki et al. (1978) with the weight 10 − 0.5z.
Accuracy and error limits
- The CRR curve separates liquefied and non-liquefied case histories with about 15–20 % probability of liquefaction on the curve itself; a factor of safety of 1.0 is therefore a boundary, not a guarantee. The SPT blow count varies ±20–30 % with equipment and practice, which changes FS by a similar amount.
- Settlement estimates are within a factor of about 2; LPI classes were calibrated in Japan and are indicative. The method does not cover lateral spreading on sloping ground or near river banks, or the cyclic softening of clays.
- To reduce the error: measure the hammer energy, test every 1–1.5 m in sandy layers, measure fines content and plasticity on the SPT samples, use the highest water level, and confirm with CPT soundings or shear-wave velocity where the decision is critical.
Who uses it
Standards and references
- Idriss, I.M. & Boulanger, R.W. (2008). Soil Liquefaction during Earthquakes. EERI Monograph MNO-12, Earthquake Engineering Research Institute, Oakland.
- Boulanger, R.W. & Idriss, I.M. (2014). CPT and SPT based liquefaction triggering procedures. Report UCD/CGM-14/01, University of California, Davis.
- Youd, T.L. et al. (2001). Liquefaction resistance of soils: summary report from the 1996 NCEER and 1998 NCEER/NSF workshops. Journal of Geotechnical and Geoenvironmental Engineering, 127, 817–833.
- Ishihara, K. & Yoshimine, M. (1992). Evaluation of settlements in sand deposits following liquefaction during earthquakes. Soils and Foundations, 32, 173–188.
- Iwasaki, T., Tatsuoka, F., Tokida, K. & Yasuda, S. (1978). A practical method for assessing soil liquefaction potential based on case studies at various sites in Japan. 2nd International Conference on Microzonation, San Francisco, 885–896.
At a glance
- Category
- Engineering & geotechnics
- Runs on
- Phone, tablet and desktop browser
- Export
- Depth profile and resistance chart as PNG, JPG and PDF; every correction as CSV; full PDF report
- Access
- Free
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