INCORPORATION OF ENGINEERING PROPERTIES IN QUATERNARY MAPPING OF ST. CROIX COUNTY, WISCONSIN

There is currently tremendous development in the western part of the St Croix County due to expansion of the Minneapolis-St. Paul metropolitan area. Between 1970 and 1990 most incorporated villages in the county increased in population by 50 – 200%. This rapid growth has continued with a countywide increase in housing units of 20% between 1990 and 1995, in contrast to a Wisconsin-wide increase of 8% over the same time period.

Mapping and characterization of surficial Quaternary deposits can assist development of an intelligent and resourceful zoning and development plan for the future. The three main Pleistocene deposits in the county are Copper Falls, River Falls and Pierce Formations. Each formation contains glacial diamicton (most considered to be till) and associated proglacial sand and gravel, and most units also contain associated silty lacustrine sediments. These Quaternary deposits were derived from several glaciations spanning time from at least 430,000 years ago for Pierce Formations deposits to 15,000 years ago for the youngest glacial deposits (Copper Falls Formation) in addition to Holocene alluvial and organic sedimentation. The deposits of each formation are subdivided into members and subdivided further by landform type on the 1:100,000 scale map and cross sections.

The till of each unit has a relatively narrow range of sediment properties. These properties are a function of the source of the material, glacial flow path, mode of deposition, age, and stress history. Each unit also has a characteristic range of geotechnical properties. Copper Falls and River Falls Formation deposits have similar provenance and were deposited in similar environments. The till of these Superior-basin-derived formations have hydraulic conductivity on the order of 10^-3 cm/s, and they are fairly strong, with an effective angle of internal friction of 30-36 degrees. Cohesion is almost lacking, but there is some minor cementation. Till of the Pierce Formation is very cohesive (200 kPa), and contains significant amounts of high activity clay, with plasticity indices of between 17 and 54. Pierce till is extremely old, was deposited in a period of reversed magnetic polarity, provenance is from the northwest, and it has a very low in-situ saturated hydraulic conductivity, on the order of 10^-7 cm/s.