Southeastern Minnesota, as well as other large portions of the Midwestern United States, is largely covered with a blanket of loess. This blanket is so extensive that entire counties have almost no available soils other than silt loams. These silty soils are poor highway materials and create a problem for the engineers. This problem is compounded by the lack of granular material which may require long hauls or similar expensive procedures.
One way to alleviate the problem is to improve or upgrade the native soil sufficiently to permit its use as a subbase and reduce the thickness of granular material required. This improvement requires some form of stabilization and the purpose of this study is to investigate the possibilities of bituminous stabilization.
This report covers two phases of the study. Chapter 2 presents data from tests on five silty soils from various locations in southeastern Minnesota. These data demonstrate the similarity and also the differences between these soils. These soils were tested in stabilized mixtures as well as in an unstabilized condition. Three other small samples were also obtained and the limited data on these soils are also presented for information.
The second phase of the study was concerned with setting up a laboratory procedure for conditioning and for testing stabilized mixtures. For this part of the investigation only one soil was tested. A large supply of Soil No. 1 from Houston County was obtained and used for all tests relative to this approach
During the summer of 1965, a test road was constructed in southeastern Minnesota. The subgrade of this road is a silty soil typical of the area and similar to soils which have been extensively studied in the University of Minnesota laboratories. The test road is 8000 feet in length and contains eight 1000-foot test sections,, Two of the sections are for control and were not stabilized while the other six were stabilized with 6% or 9% of RT-6 or with 9% of MC-2. In all cases, the upper six inches of the subgrade was stabilized and then covered with three or five inches of base and two inches of surfacing.
A comprehensive sampling program was carried out to permit more detailed analysis of the test road performance. This began with sampling of the unstabilized subgrade for determination of soil properties. These samples indicated soil variations which must be considered in future evaluation. As the subgrade was stabilized, stabilometer specimens were molded in the field and tested after appropriate curing and conditioning treatments. Other specimens were stabilized in the laboratory at comparable moisture and bitumen contents. The results of these sampling and testing programs are used to compare the various test sections. These comparisons show tendencies but final conclusions must be withheld until they are substantiated through service.
The final evaluation of the test road will require additional testing over a period of years. This evaluation will have to consider effects of traffic, frost action, soil moisture, etc. Provisions have been made for the Materials and Research Division of the Minnesota Department of Highways to make field measurements and observations at periodic intervals for this purpose. A final report after this more extensive investigation should prove of great value in future stabilization projects.
