The Importance of
Obtaining the Proper Information To Conduct a Dredging Project
The following are
quotations from the Standard Handbook for Civil Engineers.
"Unanticipated subsurface conditions
encountered during dredging are by far the largest source of dredging
related claims for additional payment by contractors and cost overruns."
"It is extremely important that owners
and design professionals recognize that cost saving which reduce the
quality of geotechnical services may purchase liabilities several orders
of magnitude greater than their initial "savings."
The following sample recovery and tests
should be conducted to classify the soils on a dredged project.
- The number of samples and the
locations should provide a reasonable representation of the material.
The location and the depth at which the samples were taken should be
- Samples should be taken using a 2"
split spoon sampler, driven by a 140-pound hammer that is dropped 30".
- The number of blows to drive the
sampler 12" should be recorded. This is called the N number.
- If only soft lake sediments or
industrial sludge are to be dredged, there are sampling methods other
than the split spoon sampler that are expectable. Depending on the
consistency of the material, Dredging Specialists has several samplers
and methods for recovering samples.
- In soft lake sediments and
industrial sludge, a plastic tape measure with a 1/8” thick by 4”
diameter steel plate on the end can be used to determine the top of
- A 1/2” steel rod can be used as a
probe to determine the hard bottom.
- Data from split spoon sampling or
from items 5 and 6, along with dimensioned drawings are used to
determine the volume of material that is to be dredged. Note: There
are 1,600 cubic yards per acre foot.
- Soils should be classified, using
the Unified Soil Classification System.
- The material should be processed in
a shaker screen and grain size analysis should be reported.
- Pocket Penetrometer and Torvane
tests should be made on cohesive soils.
- Test for the presence of toxic
substances should be make if their presence is suspected or required
by regulatory agencies.
- Samples showing the general
consistency of the soil and samples showing the worst case should be
retained and available for bidders to inspect. This is important, as
many experienced contractors base their bids on their own personal
tests, of seeing, feeling, squeezing and pushing a finger into the
material. They then compare the material on this project to other
material that they have dredged.
Additional information needed by
1) Drawings as follows: [With the
a) The area to be dredged and the
topography of the surrounding area.
b) The containment area where the
material will be pumped, [spoil area], with surrounding topography
and cross sections of the levees.
c) Elevation changes between the
water surface in the cut area and the spoil area.
d) Route and topography to the
2) The volume of material to be
dredged. The depth of material to be dredged. The depth from the water
surface to the bottom of the cut.
3) Pictures of the area to be
dredged, route to the spoil area, and the spoil area.
4) Note: As a rule of thumb, the
spoil area should be 1.3 times the volume of material to be dredged
plus required freeboard.
Many owners and Civil Engineering firms
want to avoid the time and expense of providing the information outlined
herein. Consequently, they give vague descriptions of the material based
on assumptions and guesses. It is unreasonable to expect each bidder to
go to the expense of obtaining the proper data themselves. Without the
proper data, to prepare an informed bid, foolish dredging contractors
then make their own assumptions and guesses about the material. Bad
guesses can cause production to be as low as 10% of what was estimated.
When contractors discover that their production is far below their
assumptions and guesses, there are outcries of changed conditions and
pleadings for additional compensation. On occasion, contractors just
walk off the job. Contractors that bid, based on guesses, do not have a
long life in the dredging business.
Projects that go sour cost all the
parties involved, time, money, and emotional pain.
It is in the owner and their
engineering firm's long range best interest, to provide the data needed
for dredging contractors to submit informed bids.
By: Don Searles