Compatibility of Capping and Contaminated Material Placement Options

location of discharge (surface or submerged), frequency and scheduling of

discharges, physical characteristics of discharge material, and other fac-

tors influence the tendency of the material to mound or flow and the ten-

dency to displace or mix with material already placed. The primary

concern with compatibility relates to geotechnical considerations and the

ability of the contaminated material to support the cap, considering the ma-

terial characteristics and dredging and placement techniques.

In general, if the contaminated material were mechanically dredged and

released from barges, the capping material can be similarly placed or

could be placed hydraulically. However, if the fine-grained contaminated

material were hydraulically placed, then only hydraulic placement of the

capping material is appropriate due to the potentially low shear strength of

the contaminated material. An exception may be the slow controlled

placement of a sand cap. The exposure of the contaminated material to

the environment and need to allow consolidation of the contaminated mate-

rial to occur prior to cap placement must be balanced in scheduling both

placement operations.

The flow characteristics data in Table 2 plus the field experience with

capping operations to date were used to develop the compatibility informa-

tion shown in Table 3 (Palermo 1994). This table may be used as an initial

guideline in selecting compatible equipment and placement operations. It

is anticipated that the table will be updated as more field experience and

monitoring data become available for a wider range of project conditions.

Exposure Time Between Placement of

Contaminated Material and Cap

Scheduling of the contaminated material placement and capping opera-

tion must satisfy environmental and engineering/operational constraints.

Following the placement of contaminated material, there is necessarily

some time lag prior to completion of the capping operation. This results in

some degree of unavoidable exposure of colonizing benthic organisms to

surficial portions of the contaminated material deposit. Placement of the

cap material must begin as soon as practicable following completion of

the placement of contaminated material to minimize this exposure time.

However, a delay of 1 to 2 weeks is desirable from an engineering stand-

point to allow initial consolidation of the contaminated material to occur,

with an accompanying increase in shear strength, prior to placement of the

cap.

Factors to consider in arriving at an appropriate exposure time are as

follows:

a. Potential effects due to exposure prior to capping.

b. Estimates of time required for initial colonization of the site by

benthic organisms.

Chapter 5 Equipment and Placement Techniques