Initial Specifications

Table 1 gives initial specifications of the project; while Figure1 shows a computer model of the planned wharf. Conventionally, wharfs are constructed after dredging and removal of the soil from the site. Removal of the dirt after the construction of the dock, however, offers significant economic benefits, but also involves application of new excavation techniques and equipment.

Table 1. Job Specifications

• Length of dock 1700 ft
• Width of dock 120 ft
• Depth at water’s edge 65 ft
• Volume of material to be removed 240,000 cu yards
• Predominate type of material Fine grained silts and clays
• Number of piers 1100
• Diameter of piers 36 in
• East-West clearance between piers 78 to 174 in
• North-South clearance between piers 54 to 87 in
• Expected start July 2005

Fig. 1: Computer model of dock showing deck, piers, water, and soil sections.

Preliminary Design

Starting in early 2004, CEG preformed an initial sizing and design study of equipment for the Bayport excavation job. Preliminary requirements given to us are shown in Table 2. Soil tests results were also provided by Zachry as shown in Table 3.

Table 2. Initial Requirements

• Time 3 months
• Cost $ 5 / cu yd
• Environment Under dock in forest of piers & adjacent to a freeze wall
• Critical feature Safe excavation around piers
• Operating mode Remote
• Green Operation Electric power
• Noise 55 dBA L eq(h) 10 pm to 7 am
  

Table 3. Soil Tests Results

• Predominately fine-grained silts and clays - Classes CH, CL, & ML
• Some coarse grained sands - Classes SP, SM
• Plasticity Index 32 to 63
• Soil strengths 0.4 to 1.2 tons per square foot

The proposed system consists of two main components, a tracked excavating unit and a support trailer. Table 4 gives the specifications for the unit shown in Figure 2. An operator using a remote control box or station moves the digging boom across a working face approximately 6 foot wide by 2 foot high by extending the boom and or advancing the unit into the excavation several inches per cut at a time. Layers of material are thus removed in a benching manner from below the dock. Supersonic air jets on the excavating head dislodge the soil; while vacuum air sucks the loosened material into the boom.

Table 4. Tracked Unit Specifications

• 18 in wide rubber tracks
• 62 in total width
• 20 ft telescoping boom
• 8 supersonic air jets at 150 scfm / 100 psig each
• ½ ft wide by 2 ft high suction head
• Remote operator’s controls
• Dual panoramic camera system
• Laser positioning in horizontal and vertical planes

From the tracked unit, a line of vacuum suction piping extends back to a support trailer located along side of the dock. The support trailer contains a primary separator, a secondary separator, a vacuum air source, a compressed air source, an hydraulic power unit, and miscellaneous equipment. Table 5 gives its specifications for the support trailer shown in Figure 3. Material conveyed to the primary separator is removed from the air stream via impact and settling and discharged through a rotary valve onto a short cross conveyor. Cartridge filters in the secondary separator perform a final clean up of the vacuum air for safe discharge to atmosphere. The filters are pulse cleaned on a time basis and the fines are continuously discharged via a second cross conveyer. Both conveyors feed either trucks or another transport conveyor for final disposal of the material. The support trailer also contains an electrically powered rotary screw air compressor and rotary positive displacement blower to provide the compressed and vacuum air respectively. A silencer is located on the discharge side of the vacuum blower. The compressor is shown as a separate unit since these are readily available industrially; while the vacuum blower, motor, and silencer are shown separately since they generally are made to order for equipment of this size. An integrated package with one electric motor driving both the screw compressor and vacuum blower is also possible and may in the long run be the most economical configuration.

Table 5. Support Trailer Specifications

• 1200 scfm @ 100 psig air compressor
• 4800 scfm vacuum blower and silencer
• 30 in rotary valve for material discharge
• Operator’s remote station
• All electric power
• 10-inch diameter fiberglass suction pipe extendable in 20-foot lengths
• Discharge onto belt conveyor or truck to spoils pile

Fig. 3: Support trailer with material separators, vacuum blower, and air compressor.

Figures 4 shows the tracked unit and support trailer located within the excavation and progressing across the width of the dock from the landside toward the waterside. Preliminary estimates show 4 such units would be needed to perform the excavation in the 3-month time frame.