Report - Oregon State Library: State Employee Information Center ...

5.0 MITIGATION OF LIQUEFACTION HAZARDS
5.1 INTRODUCTION
If a seismic hazard assessment demonstrates that liquefaction is likely adjacent to a bridge and
approach structures, and geotechnical/structural limit states may be exceeded, mitigation
strategies should be evaluated. Generally, seismic strengthening can be achieved by soil
improvement and/or structural enhancement. Only soil improvement techniques are addressed
here. The goal of remedial soil improvement is to limit soil displacements and settlements to
acceptable levels.
Guidance on the seismic performance of bridge foundations in liquefiable soils and tolerable
movement criteria for highway bridges can be found in several Federal Highway Administration
reports (FHWA 1985; Lam and Martin 1986a, 1986b, 1986c) and professional papers (Youd
1993; Zelinski et al. 1995). This chapter summarizes a review of the literature on soil
improvement for mitigating seismic hazards. It provides an introduction to ground treatment
methods utilized to mitigate seismic hazards at bridge sites and contains references pertaining to
the analysis, design, and seismic performance of ground treatment applications.
Remedial strategies for improving the stability of slopes and embankments have been well
developed for both onshore and submarine slopes. Common techniques for stabilizing slopes
include: (1) modifying the geometry of the slope; (2) utilization of berms; (3) soil replacement
(key trenches with engineered fill); (4) soil improvement; and (5) structural techniques such as
the installation of piles adjacent to the toe of the slope. Constraints imposed by existing
structures will often dictate which methods, or combination of methods, should be used
(Koelling and Dickenson 1998). The report, Guide to Remedial Measures for Liquefaction
Mitigation at Existing Highway Bridge Sites by Cooke and Mitchell (1999) provides thorough
and practice-oriented guidelines for the application of soil improvement. This reference, along
with the reports Screening Guide for Rapid Assessment of Liquefaction Hazard at Highway
Bridge Sites (Youd 1998) and Handbook on Liquefaction Remediation of Reclaimed Land (PHRI
1997) are highly recommended.
5.2 TECHNIQUES FOR MITIGATING LIQUEFACTION HAZARDS
Remediation objectives include increasing the soil’s liquefaction resistance through
densification, increasing its strength, and/or improving its drainage. Table 5.1 presents the most
common remediation measures. The use of these measures has limited the occurrence of
liquefaction during recent earthquakes.
101