GEOTECHNICAL CONSTRAINTS IMPOSED BY LOCALIZED LACUSTRINE DEPOSITS—ARE THEY MISUNDERESTIMATED?

During interglacial, and post-glacial time, large glacial lakes developed in many areas in New England, depositing interbedded clay, silt, and sand deposits. Thick varved sequences have been associated with large, extensive glacial lakes that were sustained in the pre-existing major drainage valleys (e.g. Connecticut River). However, thinly-bedded, less extensive lacustrine deposits may overlie, or be interbedded in till and drift deposits in the proximity of modern, natural lakes, which are often of glacial origin. Because of their localized occurrence, these sediments may not be identified or described on surficial geological maps. Lacustrine deposits have long been characterized as being poor substrates for building foundations, and under saturated conditions and seismic activity, may be prone to liquefaction. Standard geotechnical evaluations of soils often do not include continuous sampling, thus thinly-bedded clay layers could easily be missed during site characterization. To demonstrate this geologic scenario, a case study from a controversial school building site in the central Lakes Region of New Hampshire will be presented. The blow counts from soil borings at the site indicated a weak layer approximately 10 feet below land surface, which was interpreted by the building engineer as a previously disturbed layer (construction floor for the existing building). However, the foundation excavation for the new building exposed a 1-foot layer of near pure clay of lacustrine origin. Although warned of the poor site conditions, a new building is being constructed on the footprint of the previous building, which suffered significant damage from differential settlement.

Because of elevated seismic risk, surficial geological maps published by state surveys in the Pacific northwest often contain descriptions of the engineering properties of the deposits depicted on the map, but this type of information is largely absent in the northeast. New England state surveys could benefit the public by including more information on the engineering characteristics of surficial deposits on their published maps. Moreover, it may benefit public safety and the investment of public funds if continuous sampling was routine when geotechnical site evaluations are being conducted for foundation design in areas suspected of containing discontinuous lacustrine deposits.