Consolidation models

consolidation of subaqueous capped mounds was done with MOUND

(Poindexter 1989; Poindexter-Rollings 1990). This program incorporated

capabilities for analyzing deposits that were subjected to surcharge (cap)

loads and included an empirical relationship between shear strength and

void ratio, plasticity index, and activity of the sediment particles. Most re-

cently, PCDDF89 has been updated to include secondary compression; this

version is known as PSDDF (Primary Consolidation, Secondary Compres-

sion, and Desiccation of Dredged Fill) and is likely the most user-friendly

version (Stark, in preparation). Each of these computer programs is based

on the same 1-D theory of consolidation and is capable of predicting the

consolidation of multiple compressible layers. Computational details and

processing speeds vary among the programs, but similar consolidation esti-

mates should be obtained from each.

In evaluating consolidation, both the rate and the magnitude of consoli-

dation should be determined separately for the contaminated sediment, the

capping material, and the foundation layers, as appropriate. Then for any

given time of interest, the individual settlement values for the foundation,

contaminated sediment, and capping sediment should be summed to pro-

vide an estimate of the total amount of settlement to be expected at that

particular time. This information can be used in conjunction with field-

monitoring data in the ongoing assessment of cap integrity. The change in

thickness of the capping layer is of primary concern from an environ-

mental containment perspective. However, the total amount of consolida-

tion settlement, or decrease in elevation, of the cap surface over time is

necessary to delineate between mound height changes caused by erosion

and those accounted for by consolidation of constituent materials.

Because consolidation settlement of capped mounds can be mistaken for

erosion of the cap, estimates of consolidation of capped mounds should be

made when mound geometry is established and should be routinely com-

pared with field-monitoring data thereafter. Estimating consolidation of

capped mounds requires collection of appropriate samples, conducting

necessary geotechnical testing (as described in Chapter 3), and conducting

a consolidation analysis for each compressible material (foundation,

contaminated sediment, and/or capping material).

The MOUND model and another consolidation model, CONSOL (Gib-

son, Schiffman, and Cargill 1981; Wong and Duncan 1984), were used to

predict consolidation of three capped dredged material mounds in Long

Island Sound (Silva et al. 1994). Bathymetry of these sites showed reduc-

tions in mound elevations of up to 3.5 m over time periods of 10 to 13 years

after cap placement. Comparisons between consolidation and bathymetry

estimates were made to show that the reductions in mound elevation could

be attributed to consolidation rather than cap erosion. These results com-

pare favorably with earlier analyses of the same capped mounds in which

the predictions were also validated by field measurements (Poindexter

1989). Results showed the two models used in the recent study were rea-

sonably accurate in predicting consolidation, that consolidation of the

base (native) sediments can constitute a majority of the observed consoli-

dation, and that the caps had not experienced erosion losses. The work also

pointed out the need to obtain more accurate geotechnical information on

the void ratios and initial effective stress of the contaminated materials.

Chapter 8 Long-Term Cap Stability