Basis of Payment

ERDC TN-DOER-T7

October 2004

operations were included in the plant design, what are the impacts if 25-percent sand is

encountered? What costs would be anticipated in connection with this, including equipment

changes and dredging or processing interruptions? These are the uncertainties that drive costs of

remediation activities up. Defining the necessary actions and remedies in advance, to the extent

possible, allows better definition of the risk to all parties. It is to be hoped that this will have the

ultimate effect of reducing the cost of processing for mainstream materials treatment.

Sampling and testing to determine compliance with materials specifications should also be

carefully developed. The scale of material heterogeneity should be considered. Sampling

volume, location, frequency, and sampling and analytical methods must be explicitly specified.

Uncertainty associated with the sampling and analysis should be estimated and factored into

considerations. Material variations resulting from changes in operating conditions should also be

considered in specifying performance testing. For example, percent solids of the cake is a

commonly tested parameter for process comparison. However, percent solids in the cake is a

function of the compaction and dryness of the materials (% solids = mass solids/total mass), and

the specific gravity of the materials in the sample tested. Percent solids may be increased in

organic sediment by coarsening the cut at the front of the treatment train and allowing more sand

to report with the fines. The density of the cake would be higher, and permeability may be

improved, but the volume may also increase due to the higher mass reporting with the fines and

decreased material compressibility. Higher percent solids for materials of the same specific

gravity would indicate greater volume reduction. Testing of the cake should therefore also

include not only percent solids, but bulk density of the cake, specific gravity of the solids, grain

size distribution, and water content in order to fully determine the fate of materials and the

effectiveness of the process. Use of these factors in evaluating overall volume reduction was

discussed earlier in this document. Weight reduction can be estimated using the same

parameters.

Basis of Payment. Payment is typically specified in terms of cost per unit volume dredged.

Alternatively, payment might be specified in terms of cost per unit volume processed, or cost per

unit volume cake produced. The obvious disadvantage of the latter specification is that the

contractor will profit from maximizing the cake volume, which will likely be contradictory to

processing and cost reduction objectives. Chemical costs, however, should be correlated and

charged based on the volume or mass of fines processed, rather than the total volume of sediment

processed. For example, if sand content is higher than expected, chemical demand will be lower.

Chemical costs based on total sediment volume would not reflect this.

Process water normally requires treatment prior to discharge. If the dewatering contractor is

responsible for water treatment costs, this may motivate conservative water use and minimize the

volume of water to be treated. Emissions control, decontamination, and other site management

considerations will normally be encompassed in the agreement as well. Payment basis is

therefore an area which should be given careful consideration when developing performance

specifications.

COST: Unit treatment costs are a function of character and volume of the material to be

processed. Mobilization and demobilization costs are relatively insensitive to treatment volume;

small projects will therefore have a higher associated unit cost. Mechanical dewatering is