42 



The initial combination of bathymetric 

 and SCUBA diver observations indicated 

 that the STNH capping exercise was an 

 operational success. Formation of a 

 discrete mound and cap was proved to be 

 feasible under controlled conditions. 

 Physical monitoring next approached the 

 problem of the stability of capped mounds. 



3.1.3 Long-Term Cap Stability: 

 Consolidation vs. Erosion 



The passage of Hurricane David in 

 September 1979 provided an opportunity 

 to document the response of the relatively 

 new STNH capped mounds to storm- 

 associated disturbance. Reviews of the 

 bathymetric profiles obtained in November 

 1979 indicated a significant change in the 

 contours of STNH-S and the apparent loss 

 of approximately 10,000 m 3 of volume 

 (Figure 3-2A and 3-4 A). Although located 

 in shallower water, STNH-N displayed 

 contours essentially similar to those 

 measured in the June 1979 and August 

 1979 bathymetric surveys (Morton 1980a). 



Earlier studies of the factors 

 responsible for the observed volume losses 

 at STNH-S concluded that the most 

 probable cause was erosion due to 

 increased boundary shear stress associated 

 with the storm- induced surface wave-field. 

 It was postulated that the effects were 

 more pronounced at STNH-S because of a 

 substantial difference in boundary 

 roughness, the northern sand cap being 

 significantly smoother than the southern 

 silt-clay cap (Morton and Karp 1980). 

 Geotechnical factors potentially associated 

 with mound stability, including water 



content and pore pressure characteristics, 

 were not measured at the time. As a 

 result, it was not possible to evaluate 

 whether the observed volume loss was the 

 result of erosion, rapid consolidation, or 

 slumping. 



A recent modeling study by 

 Poindexter-Rollings (1990) may shed some 

 light on the apparent loss of material from 

 STNH-S. Her study used results of 

 geotechnical measurements of dredged 

 material to predict consolidation of capped 

 and uncapped mounds. The model 

 predictions compared favorably with 

 measured change in bathymetric height 

 over time (Figure 3-5). These results 

 suggested that the apparent loss of material 

 at STNH-S between August and November 

 1979 may represent rapid consolidation 

 (Figure 3-5B) rather than effects of the 

 hurricane. However, the initial 

 consolidation should have begun, 

 according to her results, before the August 

 1979 bathymetric survey. The addition of 

 110,000 m 3 of cap material to STNH-S in 

 the spring of 1980 was not included as part 

 of her work; therefore the settlement curve 

 for STNH-S predicts more rapid 

 consolidation than actually occurred 

 (Figure 3-5B). 



Poindexter-Rollings (1990) used a one- 

 dimensional finite strain consolidation 

 model (MOUND) to predict consolidation. 

 The inputs to this model were based on 

 parameters derived from laboratory 

 measurements, reported disposal volumes, 

 and bathymetric surveys. She 

 acknowledged that clamshell dredging does 

 not necessarily change the properties of 



Sediment Capping of Subaqueous Dredged Material Disposal Mounds 



