13 September 2019

The historic Delaware and Raritan Canal periodically undergoes dredging in order to keep the waterway active.  As part of this ongoing process, a soil Improvement system from ALLU is being used to mix the dredged material with Portland cement, which enables the dredged material to be reused.

Dredging has resumed on the 96km long Delaware and Raritan Canal, with work expected to run through until the end of October. The canal functions as a means of transporting water and acts as a reservoir, being part of the Raritan basin water supply system, providing water to millions of central New Jersey residents. The canal itself is fed by the Delaware River and smaller streams, channels and other runoff sources. Sediment from road and land runoff is deposited in the canal, decreasing its water storage capacity, which periodically requires the sediment to be removed in order to restore the canal’s original capacity.

450,000,00 litres of water each day is diverted from the Delaware River to the canal, with dredging being used to improve water quality and restore the canal’s original carrying capacity by removing accumulated sediment, aquatic vegetation growth and debris. The canal itself, and many structures along its banks, are considered to be of a ‘historic nature’, with the dredging project designed to protect structures from any adverse impact. For example, to protect sections of the embankment still lined with historic stone armouring, dredging is not allowed within five feet of the embankment walls. 

Beginning the process 
Site preparation began in March 2018, with tree clearing and trimming at the staging area and five access areas, where dredges are placed into the canal with a crane. ‘In-water’ work, which includes the removal of submerged aquatic vegetation, debris and dredging, began in July of 2018, and three years of seasonal dredging activities are expected to be undertaken with full completion of the project by the beginning of 2021.

The logistics of the undertaking require year round work and a great deal of oversight. Following the 2018 dredging season, which ended in November, 76,000 tons of sediment was produced. The dredged material was allowed to dewater for a period of time, with the material needing to be mixed with cement to reduce the moisture content and improve handling for the end user before it could be hauled. Hauling began in December 2018, and continued through the end of March, followed by preparation for the next dredging season. 

The most time consuming part of the work involves hydraulic dredging. It is a slow process, with only approximately 30m of canal per day per dredge accomplished. As part of the work, sediment is mixed with Portland cement and transported to an offsite facility for reuse, with the drained water returned to the canal following treatment. The dredging projects calls for approximately 250,000m3 of material to be dredged from the canal. Cranes are used to place the dredges and floating barges with excavators into the canal, with the large excavators being used ahead of the dredging operation to remove subaqueous vegetation and debris. Dredges are then used to remove the sediment from the canal.

Dewatering and material stabilisation
In order to construct the necessary staging and sediment dewatering area, approximately 30,000m3 of soil and weathered bedrock were placed to create a shallow sloping dewatering area on the side of a hill. This included 27,000 tons of gravel for geo-bags which help convey water to the settling sumps. Geotextile and HDPE liners were used to cover the entire eight acre staging area, which created an impervious barrier to contain the water after draining from the geo-bags that contain the dredged sediment.

Slurry is conveyed via booster pumps and floating pipes to the geo-textile dewatering bags. Before entering the bags, polymer dosages are applied to expedite flocculation and coagulation of the sediments in the geo-bags. All the drain water from the geo-bags is collected in dewatering sumps, with ferric chloride applied to the collected water, so that the rest of the sediment in the drained water can settle out in the sumps. Then the water is pumped back to the canal from the sumps via a 75hp pump and discharge pipe.

It takes four to six weeks for the sediment to dewater, with the geotextile sediment storage bags being then cut open and cement added to the sediment.  An ALLU Soil Improvement system is then used to convey dry Portland cement from the silo to the sediment, and also mix the cement with the sediment at the same time. After mixing in the cement, the sediments are loaded with excavator mounted clam shell buckets into triaxle dump trucks. The winter of 2018 saw approximately 2,700 dump truck loads transport 76,000 ton of such amended sediment off site.

The ALLU Soil Improvement System 
The stabilisation method used on the canal is a quick, mobile and cost effective solution to making the canal dredging reusable, and consists of three parts. The first is the ALLU Processor which is a versatile hydraulic accessory for excavators; the second is the ALLU PF Pressure Feeder, which feeds the binder (in this case, Portland cement) through the hose. The third is the ALLU DAC, Data Acquisition System to measure, control and report the stabilisation. The latter enables the control of all the functions of the PF unit as well as also saving the data during the stabilisation project, thereby providing the facility to transfer data onto other computers for quality control purposes. 

The main part of the system is the ALLU Processor itself; this is an attachment for a conventional excavator, powered by the excavator’s hydraulic system, thereby converting the excavator into a powerful and versatile mixing tool.  The mixing drums penetrate and mix a variety of material in place, or in-situ, meaning that materials can be mixed anywhere the excavator can go, thus capitalizing on the excavator’s mobility and articulation to reach difficult or soft areas. 

In the case of the canal project, the Portland cement is delivered directly to the area of the highest mixing shear by an attached 80mm diameter pipe, injected via a nozzle located between the mixing drums. The Processor penetrates through any hard crust layers and mixes the dredged material with the Portland cement effectively, with penetration and mixing effectiveness being enhanced by the angled position of the drums which minimises the ‘dead’ area at the bottom frame. 
The dredged material consists of varying mixtures of sands, gravels, silts and clays, which are ideal for reuse in a variety of applications. The added Portland cement helps dry out the material and gives the finer material more strength when used to reclaim a brownfield site.