GSP 160 Dynamic Response and Soil Properties
A SITE-SPECIFIC COMPARISON OF SIMPLIFIED PROCEDURES FOR EVALUATING CYCLIC RESISTANCE OF NON-PLASTIC SILT A.S. Bradshaw1, C.D.P. Baxter2, and R.A. Green3 1
Assistant Professor, Department of Civil Engineering, Merrimack College, North Andover, MA 01845; e-mail:
[email protected] 2 Assistant Professor, Departments of Ocean/Civil and Environmental Engineering, University of Rhode Island, Narragansett, RI 02882; e-mail:
[email protected] 3 Assistant Professor, Department of Civil and Environmental Engineering, University of Michigan, Ann Arbor, MI, 48109; e-mail:
[email protected] ABSTRACT
Assessing the liquefaction potential of silts is a challenging problem for practicing engineers. A common approach to performing such assessments is to apply a fines content "correction" factor to field-based correlations that were developed primarily for sands that relate cyclic resistance and in-situ indices (i.e., standard penetration test (SPT) N-value, cone penetration test (CPT) qc-value, or shear wave velocity (VS)). This paper compares the cyclic resistance estimated using these methods for a site in Providence, Rhode Island which is characterized by deep deposits of non-plastic silt. Standard penetration tests with energy measurements were performed in conjunction with seismic cone penetration tests, and representative samples were collected for laboratory testing. A site-specific VS-based cyclic resistance correlation was developed from cyclic triaxial tests with shear wave velocity measurements and was used as a baseline for comparing the results of current SPT- and CPT-based simplified approaches. The analysis suggests that the existing SPT- and CPT-based methods, as described in Youd et al. (2001), provide reasonable predictions of cyclic resistance of the non-plastic silt when the recommended fines content corrections are applied. INTRODUCTION Soil liquefaction potential is typically evaluated by comparing the cyclic demand of an earthquake to the cyclic resistance of the soil. One approach for evaluating cyclic resistance is to perform laboratory cyclic tests on undisturbed samples recovered from the field. High quality, undisturbed samples of non-plastic soils are difficult, if not impossible, to obtain without the use of ground freezing techniques. For this reason, 1
GSP 160 Dynamic Response and Soil Properties
the focus in recent decades has been to develop empirical correlations based on insitu test indicies and field observations. These correlations, as described in Youd et al. (2001), form the basis of the current standard-of-practice for evaluating liquefaction potential. Large areas of Providence, Rhode Island are underlain by thick deposits of very loose to dense deposits of inorganic silt. Evaluating the liquefaction potential of these soils is a challenging problem for practicing engineers. In an effort to assess the insitu cyclic resistance of the Providence silts, Baxter et al. (2006) used cyclic triaxial tests in combination with shear wave velocity measurements to develop a correlation between cyclic resistance ratio (CRR) and shear wave velocity (VS). They showed that the method of sample preparation did not affect the correlation between CRR and VS. This relationship, which was developed using reconstituted samples, was also in very good agreement with the data obtained from cyclic triaxial tests performed on specimens trimmed from an undisturbed block sample, thereby indicating that the correlation was applicable to in-situ conditions. This paper presents a comparison of current simplified approaches that are used to estimate the cyclic resistance of non-plastic silt encountered at a site in Providence, Rhode Island. A site investigation program is described that includes geotechnical borings and seismic cone penetration tests. Laboratory cyclic tests are used to develop a correlation between CRR and VS that in turn is used to assess the in-situ cyclic resistance at the site. The site-specific analysis is used as a baseline for comparing cyclic resistance predicted using current SPT- and CPT-based simplified approaches. The objective of this paper is to compare the results of each method and discuss the practical implications of any possible differences. GEOTECHNICAL SITE INVESTIGATION A geotechnical site investigation was performed at a study site in Providence that included rotary wash borings with standard penetration tests performed adjacent to seismic cone penetration tests. The standard penetration tests were performed with a standard split-spoon sampler with the inside liner removed. The efficiency of the donut hammer system was measured using a Pile Driving Analyzer, PAK Model manufactured by Pile Dynamics, Inc., and ranged from 30% to 40% with an average of 37%. Representative samples of silt were recovered between SPT intervals using a 3-inch diameter split spoon sampler with a core catcher. These samples were reconstituted and used for the laboratory testing program. Seismic cone penetration tests included near continuous measurements of tip resistance, sleeve resistance, and porewater pressure, as well as shear wave velocity measurements at 1-meter intervals. A typical profile of the soil conditions, including blow counts (N), cone tip resistance corrected for pore pressure effects (qt), and shear wave velocity (VS), is shown in Figure 1. The site consists of approximately 4 to 10 m of sand and gravel fill underlain by a thick layer of non-plastic silt. The silt was deposited as glacial lake sediments during the last glacial retreat and is therefore characterized by seasonal varves. Grain size analyses of bulk samples indicate that the silt is composed of about 95 percent fines (
