During 4-9 November 1991 the Acadia Centre for Estuarine Research of Acadia University and the Atlantic Geoscience Centre of the Bedford Institute of Oceanography jointly conducted a preliminary field study of sediment stability at Dump Site B in the inner Miramichi Bay, New Brunswick. The objectives of the study were to: (1) obtain direct in situ measurements of the strength and erodibility of surface sediments; (2) obtain measurements of sediment pore pressures, liquefaction potentials, and short-term consolidation rates; (3) make direct observations as to the presence or absence of fluid mud layers; (4) investigate sub-surface geotechnical and geochemical sediment properties that relate to preexisting events at the disposal site and which affect overall stability of the bed; and (5) investigate biological characteristics that influence or provide information on sediment stability.

A total of 20 stations were sampled. The most extensive sampling was carried out at 12 stations, four located at a control site north of Reach 22, four located at Dump Site B within or near the area of a 1990 spoils mound, and four located within or near the dredged channel. The sampling protocol at each station consisted of obtaining gravity core samples for sediment index properties, Van Veen grab samples for organic content, grain size and settling rate analyses, Eckman grab samples for biological analyses, CTD profiling for salinity and temperature measurements, Sea Carousel deployments for measurements of sediment strength and erodibility, and Lancelot deployments for pore pressure, liquefaction potential and short-term consolidation rate measurements.

The sediments consisted of very loose to loose sandy silts and silty sands with only a small clay-sized fraction. Sediment organic contents were high (3-11 percent) and varied little with depth within the top 10 cm. The high organic content had a strong influence on sediment characteristics. Sediment water content and plastic and liquid limits were much higher than what would be expected on the basis of grain size alone, and sediment bulk densities were very low. Natural water contents were equal to or in excess of the liquid limit and decreased with depth indicating that the seabed was normally consolidated prior to sampling. Plasticity indices ranged from 39 to 106 percent.

Results of the biological survey revealed very little difference between the control, Dump and channel sites. Chlorophyll a and dissolved carbohydrate levels were relatively low at all sites and it is unlikely that benthic diatoms are abundant or active enough to influence sediment stability. Benthic macrofauna samples were dominated by burrowing forms, mainly polycheates, but one suspension feeder, the bar clam (Spisula solidissima), was found to be relatively abundant indicating that chronically high suspended sediment loads are probably not characteristic of the sites studied.

Sea Carousel measurements indicated the most stable sediments to be at the 1990 spoils mound on Dump Site B. This site exhibited the highest critical shear stress for erosion and the lowest erosion rates. The friction angle was also high and there was a linear increase with depth in sediment shear strength suggesting that the sediment was consolidated and not in a fluidized state. A station located within Dump Site B, but at the periphery of the spoils mound, exhibited significantly lower cohesion, higher erosion rates and lower friction angles suggesting that it was much less consolidated The lowest cohesion and highest erosion rates were observed at the channel sites. At both channel sites cohesion was virtually absent and erosion rates were high. Failure took place by aggregate release from the bed rather than as discrete particles. Video observations showed the bed to behave not as a viscous fluid, but as a gel which appeared to be held together by organic fibers within the sediment. The control site exhibited cohesion and erosion characteristics intermediate between the 1990 spoils mound and channel sites. Natural variability at the control site, however, was high.

Lancelot results indicated that the seabed is generally composed of incompressible material with a low clay content. Densities ranged from loose to medium-dense. Excess pore water pressures were in the order of 0 to 3 cm at all sites indicating the presence of a well consolidated soil structure. Penetration pore pressure responses suggested a very loose, variable silty seabed with depth. No natural liquefaction events were detected during the study period, but low-level differential pore pressures were observed as a result of minor wave loading events.

No evidence for the presence of a fluid mud layer was found at any of the sites studied. Based on the combined results of all measurements it appears unlikely that the sediments at the sites studied could ever exist as dense mud suspensions except perhaps for short periods of time during ship passage or storm events.