“What do we do with the dredge spoil?”
This question is not always easy to answer. Finding the space to store spoil may be the most difficult problem a designer faces on a dredging project.
The ideal dredge spoil area is immediately adjacent to the impoundment being dredged on a perfectly level site, or with a slight slope away from the inlet. It should be large enough for long-term storage of all the slurry that will be pumped in clearing the impoundment. If this ideal site exists, the designer may simply calculate the total volume that will have to be pumped to clear the site and build a spoil area to hold more than that. On smaller jobs where there is sufficient space, this is the suggested method. On larger projects, available space and money may preclude this method.
This photo shows a “shot glass” spoils site for spot dredging in a short stretch of canal. The site was positioned between two house lots. The siphon pipes at the other side allowed the water to to go back to the canal downhill. At the effluent discharge point silt curtains were used as a second line of defence for turbidity.
On these projects, the designer is forced to adapt the site to his needs and, perhaps, to tailor the dredging project to the spoil area design constraints. Some of the considerations in spoil area design are as follows:
- How many cubic yards of material are to be dredged? Add at least 10 % to this figure for design purposes.
- How much slurry will have to be pumped to remove this much material? As mentioned above, on smaller projects with sufficient space, this is the spoil area design. Build it to hold this volume and start dredging.
- Will the material settle out of the slurry? Sand, ash and coal settle rapidly; clay and silt, slowly; and sludges, often not at all. If the material will separate from the water, this characteristic can be used to advantage in sizing and operating the spoil area. Decanting the water from the settled material and adding new spoil can reduce the volume in the spoil area. Will there be long periods (overnight?) where no slurry is being added and settling can happen? Limit the settling test to this length of time.
- Will the material dry or dewater to the point where it can be removed or pushed aside to make room for more slurry? This depends, not only on the material (sand, ash and coal do this very well), but also on the climate (rainfall, humidity, etc.). Turning the material can enhance drying and dewatering. This, of course, can only be done when no new slurry is being added. If drying is possible, the area should be constructed with a water level as shallow as possible.
- If settling, decanting and, perhaps, dewatering and drying appear viable, consideration might be given to constructing multiple cells — each holding several day’s worth of production. They are used alternatively. While a cell is in service, the other(s) is being decanted and perhaps emptied to make room for another cycle.
Most dredges can pump high enough solids that the sand will settle out fairly close to the discharge point. It isideal to have an excavator near the discharge point to quickly remove the sand for stock piling. The discharge site will become sloped from the accumulation of sand at the discharge point and the water will naturally flow to the other side of the discharge area where a weir gate or siphon pipe can allow the spoils site to dewater rapidly.
There is no foolproof recipe for designing spoil areas. The designer should avoid being merely hopeful. Apply what is known about the material realistically. Test results are always suspect, as it is usually impossible to get a truly representative sample. Be conservative! Probably the worst thing that can happen on a dredging project is to have a full spoil area, with the job only half finished.
- Know as much as possible about the behavior of the spoil material.
- Build the spoil area as large as available space and money permit.
- Be prepared to operate the spoil area. That is, use all the options such as settling and decanting that are economically feasible.