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What Engineers Need To Know

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.

  1. 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 logged.
  2. Samples should be taken using a 2" split spoon sampler, driven by a 140-pound hammer that is dropped 30".
  3. The number of blows to drive the sampler 12" should be recorded. This is called the N number.
  4. 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.
  5. 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 the material.
  6. A 1/2 steel rod can be used as a probe to determine the hard bottom.
  7. 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.
  8. Soils should be classified, using the Unified Soil Classification System.
  9. The material should be processed in a shaker screen and grain size analysis should be reported.
  10. Pocket Penetrometer and Torvane tests should be made on cohesive soils.
  11. Test for the presence of toxic substances should be make if their presence is suspected or required by regulatory agencies.
  12. 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 bidders.

1) Drawings as follows: [With the scale shown]

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 spoil area.

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

Dredging Specialists