Tuesday Feb 11th, MnDOT presented their 90% plan for the redesign of the HWY. 55/62 interchange.
They have stuck to the design concept that was presented and approved in September 2002. click here to see the design concept.
The main changes are in it's improvement
over the initial design.
The liner will have a geotextile layer at the bottom that is about 1/4 inch thick. This layer will have some slight leakage. So on top of it will be about a foot of gravel that will transfer water evenly across the liner to equalize pressure so the road will not heave unevenly. On top of that will be a 1/4 inch thick goecomposit drain that is itself three layers thick, to form a stable base for the 1/8 in thick plastic liner, 1/8 in bentonite clay liner, with another 1/8 inch thick plastic liner on top of that. Then there will be another geotextile layer, with another layer of gravel on top of that to protect the liners below from construction activities when building the road, and on top of that will be a final fabric liner to keep sand and silt from getting into it. See the cross section below.
Together, the liner is expected to leak about one gallon a DAY.
There will be two main holes in the liner, for the Hwy. 55 and LRT bridge footing. All the layers of the liner will be brought up to and sealed against the concrete footing. Some will be physically anchored to it for stability. These are welded into place to prevent leakage as much as possible. They will leak about 15 gallons a DAY. See the cross sections below.
The edges of the liners will be wrapped
and brought above the July 31st 2002 water levels (historic highest
precipitation summer in 111 years)
There are drains that will capture and dewater any water that goes above this level. If the water levels are at or below the July 31st 2002 water levels, no dewatering will take place. Except for the 15-20 gallons a DAY leakage (compared to the 80 gallons a MINUTE flow of Coldwater Spring). Then consider that there is only about a 30% connection between the interchange and spring areas.
Due to the isolation that the road will have from the groundwater, and that all construction (liner and all) is well above the bedrock that contains the fractures that naturally transfer water, there should be no impact to the Coldwater Spring. Both Minnehaha Creek, and Lower Minnesota River watersheds have approved the plan.
MnDOT will bid out and begin construction
One drawback to expect this summer. When
installing the liner, it will be necessary to once again temporarily
dewater the interchange.
The liner should last at least 50 to 100
years (Much longer than the road will). After that time something
will need to change. Maybe the height restrictions will be lifted
by the airport, maybe not. Maybe a new liner will be needed, maybe
not. But in any case, because of all the work that citizens, legislators,
MnDOT, The Federal Highway Administration, Watersheds, Board of
Soil and Water Resources, and
Without your work, the spring would already be gone. THANK YOU, for helping to save the Spring now, and into the future!!