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MOUND REPAIR
CAHOKIA MOUNDS HISTORIC SITE
MADISON COUNTY, ILLINOIS
1 PAGE
COPYRIGHT JULY 31, 2007 PETER A. BOSTROM
Monks Mounds abstract image.
MONKS MOUND IS THE LARGEST ANCIENT EARTHEN
MOUND STRUCTURE IN NORTH AMERICA

ABSTRACT
SLUMP REPAIR TO MONKS MOUND

   The largest prehistoric earthen mound in North America underwent a major engineering repair project during the month of September. A large area on the eastern slope was removed down to the slip line. The procedure was done in an attempt to prevent further slumping. It's hoped that by refilling and compacting the slumping area with a more stable material the problem may be held in check.

     "---there is a steep scar at the top of the slump and the slump moves as a whole unit, like the end of a pendulum or an earthen glacier, with the slip face going deep into the mound."---------2007, by William R. Iseminger, Assistant Site Manager Public Relations, e-mail--, "Monks Mound Slump History."

     "Over time, this upper scar had begun to drop even more, eating back into the upper part of the mound (Monks Mound) and it was just a matter of time before it would get to the summit and eat into the flat surface there."---------2007, by William R. Iseminger, Assistant Site Manager Public Relations, e-mail--, "Monks Mound Slump History."

Monks Mound abstract image.
MONKS MOUND
SLUMP REPAIR
CAHOKIA MOUNDS SITE
MADISON COUNTY, ILLINOIS

   Monks Mound is an earthen mound that has been plagued by erosion for many years. In fact, the mound was also repaired for similar reasons anciently by the Mississippian people who built it. The term that archaeologists and engineers use to describe the phenomenon is slumping. Slumping occurs when the soil in the mound becomes saturated with rain water and a portion slips downward along a fault line or slip zone. If the situation was left to the natural elements of wind and rain, the largest prehistoric earthen mound in the western hemisphere might eventually become unrecognizable. In the distant future, the impressive multi-terraced structure of today  might become more rounded and eventually begin to lose some of its height.

Monks Mound area of slumping.
MONKS MOUND SLUMP AREA
CAHOKIA MOUNDS STATE HISTORIC SITE
MADISON COUNTY, ILLINOIS

    This picture was taken in the early nineties. The doted line shows the area in question where it was first repaired in 1987. A bulldozer pushed bluff top loess up the slope from below but could not reach the steepest area at the top. The area held for several years but slumped again about two years ago.

   This area of the mound first became damaged from slumping in 1984. The event left a massive and steep scar that began at the top and traveled down to the lower area of the mound. The damaged area was first repaired in 1987 when it was filled with bluff top loess that was provided by the Illinois Department of Transportation. They pushed the soil up from the bottom with a bulldozer as far as they could but because of the steepness of the slope the upper portion of the mound could not be repaired. The lower area of the mound was also provided with additional soil that acted like a buttress to hold back further movement.

Slump repair of Monks Mound.
CLICK ON PICTURE FOR LARGER IMAGE
MONKS MOUND SLUMP REPAIR
CAHOKIA MOUNDS STATE HISTORIC SITE
MADISON COUNTY, ILLINOIS

    This picture shows heavy equipment beginning to refill the large stair step excavation where the large slump feature was located on the eastern side of Monks Mound. A large track hoe can be seen on the top of the mound. Other equipment used was a front end loader, compacting roller and dump trucks.

     Soil engineers later agreed that the bluff top loess that was used in the first repair was not a good repair material. The patch held for many years. But a couple of years ago the area once again slipped away leaving a larger scar than before. The slippage occurred when heavy rains saturated the soil.

Stair-step slump repair on Monks Mound.
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MONKS MOUND SLUMP REPAIR
CAHOKIA MOUNDS STATE HISTORIC SITE
MADISON COUNTY, ILLINOIS

   This picture shows the area that was excavated by a track hoe down to and below the main slip zone. The stair step structure is believed to be the best way to stabilize the slump damage. The area was later back filled with a different type of soil than was taken out. This picture also shows archaeologists mapping the soil features in the side and step walls. The soil features showed ancient erosion repair and the mound construction process.

     This most recent repair project was again conducted in order to try and save the mound structure from further damage. It was thought that doing nothing would eventually cause the top surface of the mound to begin to erode away. This repair project is the most aggressive engineering plan done to the mound to date. It involved large earth moving equipment such as track hoes, front end loader, compacting roller and dump trucks.

Monks Mound wall profile showing mound construction.
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MONKS MOUND SLUMP REPAIR
SOIL FEATURE
CAHOKIA MOUNDS STATE HISTORIC SITE
MADISON COUNTY, ILLINOIS

    The only archaeological excavation during the slump repair project was straightening and scrapping the side and step walls then mapping the soil features. The mound was constructed with alternating basket loads of black gumbo and sandy soil. Areas of ancient erosion repair was also recorded.

      This September, 2007 repair to Monks Mound involved cutting large stair steps, approximately seventy feet long, up the steep slope. The soil was excavated behind the slip zone so that portion of the erosion feature was no longer an issue. The area was then back filled with a more stable soil material. The stair step structure was thought to be the best plan of action to fix the problem in a more permanent way.

Water seepage at bottom of excavation.
MONKS MOUND SLUMP REPAIR
CAHOKIA MOUNDS STATE HISTORIC SITE
MADISON COUNTY, ILLINOIS

    This picture shows water seeping out of the mound at the lowest step in the excavated area. Even though the area is undergoing a drought the large mound still exhibits an active movement of water within the mound.

     The only archaeological feature found within the eastern slump area, besides the earthen structure itself, is a cypress post and some limestone slabs that were found near the top of the mound. These materials had become partially moved out of their original location from the two slumping events. No other information has been provided about these artifacts. The steps and side walls of the excavation were scraped straight and smooth so the original mound construction and ancient repair features could be recorded. Some of these features indicate erosion damage that was anciently repaired. The mound construction technique was done with alternating basket loads of black gumbo and sandy soil.

Filling in slump repair area on Monks Mound.
MONKS MOUND SLUMP REPAIR
CAHOKIA MOUNDS STATE HISTORIC SITE
MADISON COUNTY, ILLINOIS

    This picture shows the final refilling process of the slump repair project. The soil is dumped by one operator, spread out by another and compacted by yet another.

      Monks Mound was constructed approximately one thousand years ago. It has survived because people in the area have always wanted to preserve it. The Ramey family, who once owned the farm, protected the mound until the state of Illinois purchased the property. The only enemy Monks Mound has now is mother nature. Ancient engineers built and repaired it and today's engineers will continue the same process.

"REFERENCES"

2007, Isminger, William R., e-mail--, "Monks Mound Slump History."
2007, Personal communications with Larry Kinsella.

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