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Warren’s sewer issues

January 26, 2013

Issues currently plaguing the Oakland and Macomb area :

Problem Area Macomb Interceptor

Problem Area Macomb Interceptor

High Water Table

The water table is only 5 to 20 feet below the ground’s surface;
while the sewer lines are buried 35 feet deep or more.
This results in significant water pressure,
acting on the outside of the plain concrete sewer pipes.
Inflow and Infiltration are constant threats.

Fine Sands and Silts

Both counties are located along the Mt. Clemens Moraine system.
The soils in this area were deposited as layers of silts and fine sands
at the depths commonly used by municipalities to bury sewer lines.
The water pressure at relatively shallow depths (only 5 ft underground)
can already force silt and fine sand through very fine cracks.
The end result is continually “clogged” pipes.

Old Monolithic Sewer Liners

Almost the entire old sewer pipe system was built with
no steel reinforcement in the concrete pipes.
The pipes merely rely on compression from soil support.

Step 1 – Fine Cracks Develop.
All concrete sewer liners crack as the liner flexes to
support the ground, and as the ground relaxes and
supports the concrete liner.
The action results in the liner forming a compression
ring to support the soil loads.
Such cracking typically occurs at the crown, invert and springlines;
and is typically not visible to the naked eye.
If construction joints are not installed
(which was not the practice for the old OMID system construction),
cracks also occur circumferentially, in response to shrinkage of
the concrete. These cracks may be visible to the naked eye.

Cracks in Concrete Sewer Pipes

Cracks in Concrete Sewer Pipes

Step 2 – Ground Water Pressure
Where groundwater is more than about 5 feet above a crack in a sewer,
seepage will begin. Where silt or silty fine sand is also present
immediately around the sewer lining, the silt-sized particles will
be carried through the cracks by the seeping water.
This may begin with a very slow rate of soil loss.

Water Pressure in Concrete Pipes

Water Pressure in Concrete Pipes

Step 3 – Voids and Loose Zones Form Around the Liner.
Over time, water seeping through relatively small
cracks will carry enough soil through the liner
to create loose zones and eventually voids around the lining.
This is a problem, because the monolithic lining is designed
to function as a perfect compression ring, and relies on the soil
surrounding the lining to support the lining under uniform pressure.
As the loose zones form, the monolithic lining flexes to maintain
the compression ring, and as it flexes, wider cracks open up,
in turn carrying more soil and larger particles of soil.

Voids in Concrete Sewer Pipe

Voids in Concrete Sewer Pipe

Step 4 – Large Voids Develop Around and Under the Sewer.
This step may or may not occur prior to
failure, depending on soil conditions.
As larger voids and loose zones develop, part of the barrel of the
sewer may lose support below the invert, and the sewer barrel
begins to “span” and deform downward across the loose zone or void.
As a result, diagonal stress cracking will begin to form,
bringing more soil into the sewer and further reducing support
from the surrounding soil, resulting in more deformation.

Large Voids in Concrete Sewer Pipes

Large Voids in Concrete Sewer Pipes

Step 5 – Extensive Deformation Occurs and Sewer Collapses.
This step may occur relatively quickly after Steps 3 or 4.
As larger and larger cracks open up following deformation
in Steps 3 and 4, larger sand particles and even gravel,
will flow into the sewer through the cracked and broken lining.
Catastrophic collapse and complete failure of the sewer lining occurs.

Collapse of Concrete Sewer Pipes

Collapse of Concrete Sewer Pipes

Source:
(Taken from NTH Consultants memorandums)
http://www.oakgov.com/water/documents/omi_final/appendix_e_public_doublesided.pdf

OMID Public Hearing Signatures

OMID Public Hearing Signatures

The people shown above signed a document explaining :
“”It is the intent of this project to perform extensive rehabilitation
of the OMID to return the facilities to a condition commensurate with
a minimum 20-year life span. The project will rehabilitate approximately
21.4 miles of sanitary sewer, repair an existing gate structure and
construct new control gates.””

The City of Warren, Michigan will eventually be hooked into
the newly renovated Oakland Macomb Interceptor Project .

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