Underwater Capping

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Sub-Aqueous Sand Cap Surcharge Placement
 Computerized QA/QC System


Sub-aqueous capping is performed on environmental remediation sites to isolate contaminated sediments from the surface environment.  This construction method can also be employed on projects involving construction on submerged soft sediments to build up a firm and uniform underwater work surface. 

Sub-aqueous caps are essentially "sprinkled" into place underwater with minimal disturbance to the underlying soft sediments and can be placed evenly from floating platforms either hydraulically or mechanically.  Mr. Wells's QA/QC system has been used with the two hydraulic placement barges, as pictured below.  Sand is mixed with water and pumped to a placement barge where baffling and screening controls the sand's outflow.  The system could also be used with a mechanical placement system, provided that an electronic scale can monitor the flow of capping material.

In 2004, on behalf of environmental contractor Envirocon, Tom was asked to propose a QA/QC system to help guide and document a sub-aqueous sand capping operation at the St. Louis River Interlake Duluth Tar Superfund site in Duluth, Minnesota.  Envirocon was awarded the general construction contract for placing a sand surcharge on impacted sediments in Stryker Bay, Minnesota in 2006.  Tom designed, built, implemented and operated the original sub-aqueous capping QA/QC system that year.

The 2006 environmental construction task placed 58,755 tons of man-made sand under water on 11 acres of tar-contaminated sediments. The sand was pumped 2,600 feet from shore to a placement barge operating in shallow water.  Once the sub-aqueous cap was completed, additional sand was placed temporarily using conventional land-based methods to compress the contaminated sediments.  Starting in 2009, that temporary sand layer was removed to recreate the original bathemetry.

Some of the equipment and programming involved is discussed below.  Note: The sub-aqueous capping system was also utilized in 2009 and 2010 to help guide and document capping in Slip 6 near Stryker Bay, MN.  (Slip 6 has become the CAD; the Contained Aquatic Disposal area.)  The main QA/QC program was modified in 2009 primarily to deal with a different conveyor belt and more importantly, a different brand belt scale (with a similar but different integrator / controller).

2006 Placement Barge

The 2006 Placement Barge Operating in Stryker Bay, Duluth, MN.
 MCM Marine, Inc. was the Marine Contractor.

2009 & 2010 Placement Barge

The 2009 / 2010 Placement Barge Operating in Slip 6, Duluth, MN.
 MARINE TECH, LLC was the Marine Contractor.

Crushed Dolomite on the Slip 6 Dock in 2006

Some of the sand placed on site.  Reddish sand was used too.
The partially hidden 60 ton off-road truck looks tiny compared to the pile.

Crushed Dolomite with a high ange of repose.


As shown by the connectivity diagram below, the QA/QC system utilizes two wireless links: one connects the QA/QC PC to the placement barge; the other connects the QA/QC PC to the stationary conveyor belt scale controller / integrator.   The sand feed rate is obtained from the conveyor belt scale and used to calculate the target placement velocity for the desired thickness.  This target velocity can be transmitted for visual display to a PC on the placement barge or elsewhere. 

Both the scale and GPS data streams are stored in a multi-user database on the QA/QC PC by the main "Cap QC" program discussed below.  Microsoft (MS)  Excel and Visio were used to generate tabular and graphical reports from the shared MS Access database.  (See REPORTING below.)  The two-way wireless link to the placement barge was also used to simultaneously track the barge's location in real-time at the QA/QC PC Trailer.  (See REAL-TIME TRACKING below.)

QA/QC System Diagram


In 2006 the GPS data was converted to state plain coordinates by HyPack, a hydrographic surveying program, running on the Placement Barge PC.  (The wireless link from the Placement Barge to the QA/QC PC was a two-way Ethernet based link in 2006.)  In 2009 and 2010, the GPS data was converted to state plain coordinates by CableVision, a commercial dredging package, running on an additional PC in the QA/QC trailer.  (The wireless link from the GPS to the CableVision PC was a one-way RS-232 based link in 2009 and 2010.)  The CableVision PC was also used for real-time placement barge tracking.

In both cases, the processed GPS data was transmitted via a Local Area Network in the QA/QC trailer to the main QC program for storage in a MS Access based multi-user database.


In 2006 a Belt-Way Scales, Inc. brand conveyor belt scale was used with their standard controller / integrator.  In 2009 & 2010 a ConveyWeigh, LLC brand conveyor belt scale was used with their standard controller / integrator on a different conveyor.  Both controllers utilize an RS-232 serial interfaces for communication with 3rd party systems such as the Sub-Aqueous Capping QA/QC System. The Belt-Way controller can be set to output scale data every minute or scale data output can be requested by transmitting a code to the controller.  The ConveyWeigh controller simply outputs scale data approximately every second.

During all three sub-aqueous capping construction seasons, both scale and GPS data was captured approximately every 9-10 seconds.  Consequently, the capping QA/QC program was programmed to request a reading from the Belt-Way controller every 9 seconds and allow 1 second for the response.  The communication approach with the ConveyWeigh controller is to simply look for data every 9 seconds and if the data is incomprehensible due to the lack of data flow control, then the next data set would be captured one second later.


During capping operations, the main Cap QC program runs continuously on the QA/QC PC capturing and storing two data streams; placement barge location and capping material conveyor belt scale readings.  Both raw text data streams are shown on the main program form below (including some unprintable ASCII characters as vertical bars).  The following screen shot was captured 8/27/2010 at 10:09:32.781 according to the GPS data stream.  According to the Scale Input Stream, the scale had totaled 62,712.656 tons of capping material at this point.  The 2010 version of the Cap QC program calculates the actual placement barge speed as well as the calculated target velocity based on the capping material feed rate.  (The actual velocity calculation and display was added because the GPS used in 2009 & 2010 did not supply the actual velocity within the GPS data stream.)  The target velocity is the placement speed that will produce the desired lift thickness based on the feed rate, placement width and the capping material unit weight.

QA Program main form

The target velocity can also be transmitted to the placement barge for display on a PC running the QC Monitor program (and other programs such as HyPack, a hydrographic mapping program).  An example of the main QC Monitor form and the QC Monitor Details form are shown below.  The QC Monitor program displays the target velocity after the use specified "Current Soil Delay".  In the 2006 example shown, target velocities are calculated every 10 seconds and displayed after a 60 second delay to account for the travel time from the scale to the placement barge.  Hence the 11:57:09 target velocity was received last but the 11:56:09 target velocity is displayed to the operator on the main QC Monitor form.

QC Monitor main form   QC Monitor details form


The main QC program stores the GPS and scale data in a MS Access database that can be accessed by multiple PCs simultaneously even while new data is being added to the database file.  This multi-user capability allowed for the creation of a MS Visio based real-time placement barge tracking system.  The following partial screen shot shows the system in action drawing the placement barge's path in the CAD on August 28th, 2010.  The red placement barge tracks show how the crossed out area was avoided (once the cables were marked).

Real Time tracking with Visio


Daily sand placement reports were generated during each 2006, 2009 & 2010 construction season.   Microsoft Visio was also used to produce, weekly reports, tracking reports, and lift reports based on the captured data.  An example lift summary report is shown below from 2006. 

Lift Summary Report

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