Liquefaction Response of Soils by CPT
Seismic Piezocone testing on Mud Island, Memphis, TN (March 2000)
GRA: James Schneider
The project is funded by the MidAmerica Earthquake Center (Dr. Dan Abrams, Univ. of Illinois, Urbana Champaign) that was recently established by NSF; with matching funds by Georgia Institute of Technology (Dr. JeanLou Chameau, Dean, College of Engineering)with complementary program is underway at UIUC by Prof. Tim Stark.
Liquefaction is the result of excess porewater pressure generated in saturated granular soils from rapid loading, such as earthquake shaking. A study of liquefaction and related soil properties can lead to an understanding of a specific deposit's resistance to earthquake vibrations. Liquefaction evaluation can include laboratory as well as in-situ methods. Due to the difficulty and expense associated with obtaining undisturbed field samples of sandy and silty soils, in-situ tests have become popular for evaluating how a soil deposit will respond under earthquake loading. Empirical relations between in-situ test resistance parameters are commonly compared to liquefaction resistance of soil deposits that were subjected to historic earthquakes. Liquefaction analyses are then based on simplified curves generated from large databases of sites which have surface evidence of liquefaction, as well as those where no obvious liquefaction has occurred. This can lead to additional uncertainty in a liquefaction analysis when dealing with Mid-American soils, since databases predominantly contain sites from China, Japan, and the western United States. The new Mid-America Earthquake Center (MAEC) established by the National Science Foundation (NSF) has been developed to focus on seismic hazards associated with the New Madrid earthquake region and Charleston, South Carolina earthquake region. This poster will summarize tests performed to analyze the liquefaction response of Mid-American soils.
In-situ tests have been performed at a number of test sites in the heart of the Mid-America earthquake regions (Fig. 1). Testing areas include Charleston, SC, Memphis, TN, West Memphis, AR, Blytheville, AR, and Steele, MO. Predominantly seismic piezocone penetration tests (SCPTu) have been performed, but other tests performed include u1 and u2 piezocones (CPTum), flat plate dilatometer tests (DMT), and piezovibrocone penetration tests (PVCPT). Soundings in excess of 30 meters have repeatedly been performed so that site amplification analyses may be performed to accompany liquefaction evaluation.
The seismic piezocone penetration test provides four independent channels of data, making it an efficient and economical exploration tool for evaluating a soils liquefaction potential, postcyclic undrained residual shear strength, and site amplification properties, Gmax. Continuous readings of tip resistance, qt, sleeve friction, fs, penetration pore water pressure, um, are recorded, while downhole shear wave velocity, Vs, tests are performed at discrete intervals. When compared to conventional soil borings, the SCPTu is much less disruptive since it only creates a 36 to 44 mm diameter hole with no auger cuttings or spoils produced. The method is considerably quicker, in that the cone penetration portion of the test is conducted at 20 mm/sec, and the downhole shear wave velocity tests are conduced at one-meter intervals. A complete 30-meter sounding may be completed in less than two-hours. If desired, pore pressure dissipation readings can be monitored to provide evaluations of permeability and time rate parameters. The SCPTu provides very detailed stratigraphic information, as well as reliable means of interpreting soils properties such as strength and stiffness. Both qt and Vs can be used to independently assess liquefaction potential by current methods, thus providing redundancy in the evaluation. The small strain stiffness, Gmax, can directly be obtained from shear wave velocities, and methods are available to determine damping characteristics of the soil as well. SCPTu tests have been performed at prior geologically-mapped paleo-liquefaction sites having sand dikes, evidence of lateral spreading, subsidence features, and other liquefaction evidence. The features have been dated by geologists and archeologists using carbon-dating techniques and characteristic features, such as pottery types. Tests have also been performed at sites with companion SPT borings, as well as at key lifeline facilities. Figure 2 displays a SCPTu sounding at a key bridge in the New Madrid earthquake region. December 2, 1998