Preserving Monks Mound's East Slope and the Fourth Terrace (2007)

by Bill Iseminger, John Kelly, T. R. Kidder, Tim Schilling and Mark Esarey

If you drove by Monks Mound in August, you may have noticed heavy machinery and large areas where digging was in progress. What you were seeing was the repair to slumping problems on the east side and the northwest corner of the mound. A slump occurred on the northwest corner in 2004 and the east and west sides slumped again in 2005. The site and its parent agency, IHPA, requested a state capital project to study the problem. The engineering consultants advised us that the problem was quite serious. The scars created by the northwest and east slumps were steadily getting worse due to erosion from rainstorms and were threatening to eat into the top of the mound, so we decided to conduct repairs in order to preserve the mound's summit.

There is evidence from testing done by the Illinois State Museum after the 1984 slump, that these slope failures were an ancient problem, and 1971 excavations by the University of Wisconsin-Milwaukee in one of the "East Lobes" also had revealed evidence of ancient slumping. The modern history of slumping goes back to the mid-1980s, when large sections of the east and west sides of the mound slumped following three very wet years. Apparently, as water percolates down into the mound it tends to accumulate in several perched water tables over hard clay layers that may be older mound surfaces. As the water migrates to the sides of the mound, it weakens those areas and with all the weight of the mound above, sections eventually collapse, creating the slumps. Slumps are not surface erosions. The slip face along which the slump moves starts nearly vertically down into the mound and moves out towards the slope edge in a curving arc and the slump moves as if on the end of a pendulum, almost like an earthen glacier.

We initially repaired the east slump in 1988, when IDOT had a surplus of bluff loess soil they had to dispose of from a temporary highway project. They donated that soil to us and rebuilt the four mounds along the entrance road to the Interpretive Center and also bulldozed the soil up the slope of the east slump to fill in the scar there and also built up the toe of the slump to help anchor the repair. However, the bulldozer was not able to get completely up to the very top of the scar. Over time, this repair settled and moved a little and eventually failed again. The soil engineers said that the technique and the loess soil used were not the best choices for a repair and contributed to the failure.

Through the Capital Development Board (CDB), the Illinois Historic Preservation Agency (IHPA), which administers our site, and with funds left over from previous mound repair projects, professional engineering and geotechnical firms were selected to examine the slumping problem and recommend solutions. Oates and Associates, Inc. was hired to do the engineering study. They also surveyed and made a new precise contour map of Monks Mound. Shively Geotechnical, Inc. was hired to do the studies and monitoring of the slope failures and to come up with recommendations for repairs and stabilization. For nearly two years there were in-depth studies of the problem and soil cores and test probes were made of the slump areas on Monks Mound by Shively and also a geomorphologist, Ed Hajic.