Geotechnical Compatibility of Operations

production rate. There are also considerable engineering problems associ-

ated with successfully deploying the GFCs without having them rupture.

The decision to use GFCs for a capping project should be made based on

the benefits versus costs rather than a blanket decision based solely on the

desire to reduce losses to the water column. Data collected from a 1996

demonstration of GFCs conducted jointly by New York District and the

Port of New York and New Jersey should provide additional data on GFC

viability. However, additional research is needed to better define GFC

abilities to reduce water column losses of contaminants and to refine engi-

neering aspects associated with deployment. Clausner et al. (1996) sum-

marizes the present state of the art on using GFCs with contaminated

sediments.

Geotechnical Compatibility of Operations

Geotechnical considerations are important in capping because of the

fact that most contaminated sediments are fine-grained silts and clays and

usually have high water contents and low shear strengths in situ. Once

sediments are dredged and placed at a subaqueous site, the water contents

may be initially higher and the shear strengths initially lower than in situ.

Capping involves the placement of a layer of clean sediment of perhaps

3 ft or more in thickness over such low-shear-strength material. Field-

monitoring data have definitively shown that contaminated sediments with

low strength have been successfully capped with slow placement of sandy

material. The geotechnical considerations involved can be described in

terms of the ability of a capped deposit with given shear strength to sup-

port a cap from the standpoint of slope stability and/or bearing capacity

(Ling et al. 1996).

Only limited geotechnical evaluations have been considered in past cap-

ping projects. In virtually all of past capping projects the design was em-

pirical, i.e., prior field experience showed that it worked, but actual

geotechnical design calculations were not conducted. Limited research on

this topic is now underway, and more detailed guidance on this aspect of

capping design will be provided in the future. Additional research is also

planned to define geotechnical design for bearing capacity, slope failure,

loading rate, impact penetration, etc. For the present time, geotechnical

aspects of capping-project design are limited to the evaluation of compati-

bility of equipment and placement technique for contaminated and cap-

ping sediments with sediment properties. An acceptable match of

equipment and placement techniques for contaminated and capping mate-

rial is essential to avoid displacement of the previously placed contami-

nated material or excessive mixing of capping and contaminated material.

The availability of certain types of equipment and the distance between

dredging and placement sites may also influence selection of compatible

equipment types.

The nature of the materials (cohesive versus noncohesive), the dredging

method (mechanical versus hydraulic), the method of discharge (instanta-

neous dump from hopper dredge or barge versus continuous pipeline), the

Chapter 5 Equipment and Placement Techniques