ASSESSMENT OF LIQUEFACTION SUSCEPTIBILITY OF HOLOCENE SEDIMENTS AND ARTIFICIAL FILL IN BOSTON, MASSACHUSETTS

Much of the downtown Boston, Massachusetts urban area is underlain by unconsolidated Holocene sediments and unengineered artificial fill placed over the last 200 years. These types of sediments are often associated with high susceptibility of earthquake-induced liquefaction. The occurrence of several large earthquakes in the Boston area in the 18th century, most notably the 1727 and 1755 Cape Ann events (estimated at M6.0–6.5), calls attention to the question of whether these sediments are susceptible to liquefaction during future large earthquakes. We present preliminary results of our detailed characterization of sediments for liquefaction susceptibility mapping in two areas of Boston: the Cambridge riverfront and the Back Bay areas. We collected and compiled subsurface boring logs from these areas to assess the location, lithology, density, grain size distribution, and 3-D geometry of the deposits. The data were entered into a digital database and exported to a Geographic Information System (GIS) for mapping. Additional data came from historic geologic maps and consulting reports. Zones corresponding to areas of low to high susceptibility were defined based on a liquefaction triggering threshold analysis using SPT data in areas with borehole data, and with a criteria matrix based on the deposit’s age, texture, and groundwater condition for areas lacking borehole data. Geostatistical analyses were performed to assess spatial variability of the deposits, and to quantify the uncertainty in areas of poor data coverage. Our results indicate that portions of the study areas pose moderate susceptibility to liquefaction, and that these areas are concentrated in regions of artificial fill and sand and silty sand natural sediments. The heterogeneous character of the fill may restrict large-scale liquefaction of fill units, though the more homogeneous underlying natural deposits may be prone to larger areas of liquefaction. Most large modern structures in the susceptible areas were constructed by excavating fill and unconsolidated sediments from below foundations, and thus are not vulnerable to damage by liquefaction. However, liquefaction may affect surface and shallow subsurface structures and lifelines such as water and gas pipelines, roadways, and older residential and commercial buildings.