1.2 Background to the Conley Terminal Project 



For the first phase of the BHNIP, an in-channel CAD cell was constructed for 

 containment of unsuitable dredged material from shipping berths at Conley Container 

 Terminal in South Boston by Weeks Marine (Camden, NJ). The dredged, fine-grained 

 sediments were disposed into the CAD cell and then capped with sufficient sand to cover 

 the deposit with a 3 ft thick layer of clean, granular material. Dredging and disposal 

 operations are summarized in the following sections. 



1.2.1 Excavation of the CAD CeU 



The CAD cell, located in the main ship channel south of the Inner Confluence near 

 the East Boston shoreline (Figure 1-3), was designated Cell #2 in the FEIR/S for the 

 BHNIP project. The cell was excavated below the maximum chaimel depth anticipated for 

 Boston Harbor (40 ft MLLW) to an average total depth of 57.5 ft. First, the unsuitable 

 maintenance material from the cell area was removed and stored in a barge. Cell 

 excavation continued into Boston Blue Clay (BBC), a homogeneous, high strength greenish 

 gray clay with low water content and low permeability (CDM 1991). Bathy metric surveys 

 were conducted at all phases of cell construction and fill by the dredging contractor (Weeks 

 Marine; ENSR 1997a). Bathy metric data were provided from a survey conducted by 

 Weeks Marine following the dredging of the cell on 29 June 1997, and processed for 

 graphical purposes by SAIC (Figure 1-4). Results showed an irregular topography, with 

 depths of the cell floor that varied from minimum depths along the edges and in the north 

 central part (54-56 ft), to maximum depths in the SW comer (62-64 ft). The approximate 

 dimensions of the CAD cell were 500 ft long (north-south) by 200 ft wide (east- west). 



1.2.2 Dredged Material Disposal Operations 



Following the completion of the cell, the unsuitable maintenance material from both 

 the surface of the cell and from Conley terminal was placed in the cell from 29 June to 

 5 July 1997. Six gravity cores were collected throughout the cell by the NAE on 9 July 

 1997. The recovered cores ranged from 3.3 - 4.5 ft, and results showed that the majority 

 of the cores consisted of dark gray to black silt with a consistent sand component (13- 

 32%), and were relatively watery (moismre content 80-160%). The bottom 3-10 in of 

 each core consisted of gray clay (approximately 95 % silt/clay) with low moismre content 

 (approximately 40%), consistent with the basement BBC. 



Four bathymetric surveys were conducted after disposal of the unsuitable material (6, 7, 

 8, and 14 July). Resujts from the first postdisposal bathymetry survey (6 July) showed a 

 relatively uniform bottom within the cell with an average depth of 48.5 ft, resulting in an 

 average dredged material thickness of 9 ft (NAE 1997). These results indicated that the 

 material was relatively fluid and settled evenly over the cell floor. By the third survey (8 July), 



MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL 



