Cover Image

Not for Sale



View/Hide Left Panel
Click for next page ( 102


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 101
101 based approach is that the amount of movement associated 2. Establish the site peak ground acceleration coefficient kmax with the analysis is estimated, and sometimes this can be an and spectral acceleration at one second, S1 from the new important consideration. Note that both approaches assume AASHTO ground motion maps for a 1,000-year return that liquefaction or porewater pressure effects are not a con- period, including appropriate site soil modification factors. sideration. Section 8.5.3 provides comments on the potential 3. Determine the corresponding PGV from correlation treatment of liquefaction. equations between S1 and PGV (provided in Chapter 5). 4. Modify kmax to account for slope height effects for full slope or embankment height stability analyses (note that 8.3.1 Limit Equilibrium Approach factors described in Chapter 7 for retaining walls appear The limit equilibrium approach involves the following steps: compatible with those for slopes based on comparison with analysis methods described above). 1. Conduct static slope stability analyses using appropriate 5. Determine the yield acceleration (ky) using a pseudo-static resistance factors to confirm that performance meets static stability analysis for the slope (that is, the seismic coeffi- loading requirements. Typically these will be a C/D ratio cient corresponding to a factor of safety equal to 1.0). Note of 1.3 to 1.5 for natural slopes and 1.5 for engineered that these stability analyses should normally be conducted slopes. A variety of factors should be considered when se- using the undrained strength of the soil because of the lecting the C/D ratio including the quality of the site char- short-term loading from the earthquake. acterization and the implications of failure. Both short- 6. Establish the earthquake slope displacement potential cor- term, undrained stability, and long-term drained stability responding to the value of ky/kmax using the Newmark dis- should normally be considered in this evaluation. placement chart recommendations given in Chapter 5. 2. Establish the site peak ground acceleration coefficient 7. Evaluate the acceptability of the displacement based on kmax and spectral acceleration at one second, S1 from the performance criteria established by the owner for the spe- new AASHTO ground motions maps for a 1,000-year re- cific project site. turn period, including appropriate site soil modification factors. 8.4 Example Application 3. Determine the corresponding PGV from correlation equations between S1 and PGV (provided in Chapter 5). The proposed displacement-based methodology is illus- 4. Modify kmax to account for slope height effects for full trated by considering an existing slope located in the State of slope or embankment height stability analyses (note that Washington. This slope is next to a heavily traveled roadway. factors described in Chapter 7 for retaining walls appear The roadway is being widened to accommodate projected in- compatible with those for slopes based on comparison creases in traffic. Stability analyses were required to deter- with analysis methods described above). mine the potential effects of seismic loading to the slopes lo- 5. Reduce the resulting kmax by a factor of 0.5, as long as 1 to cated above and below the roadway. 2 inches of permanent displacement are permissible. If larger amounts of deformation are acceptable, further re- 8.4.1 Problem Description ductions in kmax are possible, but these would have to be determined by conducting separate calibration studies Seismic stability of the natural slopes was evaluated for the between displacement and the ratio of the yield accelera- following conditions: tion (ky) and kmax. 6. Conduct a conventional slope stability analysis using 0.5 Slope angles ranging from 2H:1V up to 1H:1V. kmax. If the factor of safety is at least 1.1, the slope meets Soils comprised of glacial till and fill. Till is a dense silty seismic loading requirements. sand with gravel. Standard penetration test (SPT) blow- counts range from 30 blows per foot to refusal. Soil strength values were interpreted from SPT blowcounts. 8.3.2 Displacement-Based Approach (See Appendix J for sections and assigned properties). The following displacement-based methodology is recom- Groundwater located at the base of the slope. mended for slopes and embankments, where the static strength The firm-ground values of PGA, Ss, and S1 for site are esti- parameters can reasonably be assumed for seismic analyses: mated to be 0.41g, 0.92g, and 0.30g, respectively, for the 1,000-year earthquake based on the USGS deaggregation 1. Conduct static slope stability analyses using appropriate website. (Note that at the time the example was developed, resistance factors to confirm that performance meets static the new AASHTO ground motion hazard maps and im- loading requirements. plementation CD were not available to the NCHRP 12-70