Typical River Channel Dredging Case

2026-04-09 15:53:40     Category：Technical Resources     Browse number：207     Release time：2026-04-09 15:53:40

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River Channel Dredging Typical Case 

I. Main Content and Project Volume

The main work of river channel dredging includes widening and deepening the existing channel. According to the designed dredging scope and requirements, river channel dredging mainly involves silt removal and general soil dredging.

Total project dredging volume: approximately 85,000 m³

• Silt: approximately 35,000 m³ (disposed at designated spoil yard)

• General soil: 50,000 m³ (disposed at designated spoil yard)

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II. River Channel Dredging Construction Flowchart

III. Proposed Mechanical Equipment

1. Mechanical Equipment Configuration

(1) Silt Dredging

• Maximum dredging depth: 16.0 m

• Maximum dredging width: 65.0 m

• Maximum discharge distance: 6.0 km

(2) General Soil Dredging

Onshore soil: dredging with 1.0–2.0 m³ backhoe, 2.2–3.0 m³ loader; transported by 10–20 t dump trucks; leveled by D80/D85 bulldozers.

Bank soil: jointly dredged by 0.5–1.0 m³ ship-mounted backhoe and 1.0–2.0 m³ onshore backhoe;
transported by 150–200 m³ self-propelled mud barge and belt ship.

Underwater general soil: dredged by 1.0–4.0 m³ grab dredger and 0.6–1.0 m³ pontoon backhoe;

transported by 200–300 m³ self-propelled bottom-opening mud barge.

Cover layer at Dongshazhou Spoil Yard: dredged by IHC-3800 cutter suction dredger; slope finishing by pump-ship backhoe and long-arm backhoe.

(3) Structure Demolition

Table 8.3.1 River Dredging & Transportation Equipment Schedule

IV. Silt Dredging, Transportation and Disposal

1. Construction Preparation

1. Drill sampling holes in river bottom silt to confirm distribution and volume for disposal planning.

2. Set survey traverses, protect control points, re-survey underwater topography, and draw cross-sections at 25 m intervals.

3. Install temporary water gauges (1 per km) with zero at design riverbed elevation; assign dedicated staff to record water levels for dredging depth control.

4. Deploy dredgers and auxiliary vessels in accordance with SL176-2007 (Water Conservancy and Hydropower Standard).

5. Erect slurry pipelines, construct spoil yard cofferdams, water outlets, and leachate treatment facilities.

6. Build a small docking terminal downstream of Xintingbu Wharf for mud barge loading/unloading.
   
   

2. Silt Dredging

1. Set central line and edge flags at 25 m intervals along the dredging axis; densify flags at curves.

2. Dredger is positioned under survey guidance; dredging methods vary by silt distribution.

3. Pre-calculate depth, width, and swing angle for each section.

4. Excavate bank silt at high tide; some general soil is removed with silt.

5. Re-survey topography and cross-sections after completion to verify volume.
   
   

3. Silt Transportation

All silt is transported by vessels. Auto-floating submerged pipeline is used to maintain navigation.

1. Main pipeline erected along Zhuji side; supported by steel frames at critical sections; connected by elbows/hoses; land-water joints every 3 m.

2. Shore pipes installed by skilled workers with small crane and chain hoist.

3. Land-water joints assembled with crane and anchor boat.

4. Floating pipelines installed by dredger crew.

5. Submerged pipeline ensures uninterrupted Puyang River navigation.
   
   
   
   
   

4. Silt Disposal

(1) Cofferdam Construction

(2) Water Outlet Design

1. Use approved water outlets; cofferdam compacted in layers; bottom paved with two layers of sandbags; overflow slope 1:4–1:5.

2. Retaining walls on both sides: top width 0.5 m, slope 1:0.5, built with sandbags.

3. Low earth bunds: divide fill area into cells to extend retention time and improve water quality.

4. Filter layers: crushed stone and sand transported by water; laid by loader, dump truck, bulldozer and manual work.

(3) Silt Dredge Fill

1. Arrange discharge outlets far from water outlets to extend sedimentation time.

2. Discharge pipe extends ≥5.0 m above cofferdam; protect slope with sandbags; rotate outlets to avoid accumulation.

3. Lay geotextile and reed mats at discharge points to prevent scouring.

4. Use multi-stage sedimentation and alternate zones for full consolidation.

5. Add quicklime if suspended solids (SS) exceed standards to accelerate sedimentation.

6. Set booms to collect floating debris.

（4）Silt Discharge & Filling

① Reasonably arrange the discharge outlets during silt discharge and filling. Position the discharge outlets relatively far from the return water inlets to extend the slurry flow path and sedimentation time, ensuring the quality of return water.

② The discharge pipe shall extend more than 5.0m beyond the cofferdam to prevent backflow of filling slurry from scouring the cofferdam and causing collapse. The slope inside the cofferdam at the discharge outlet shall be protected with sandbags. During silt excavation, part of the general soil is removed, which has high sand content and is easy to accumulate at the outlet, so the discharge outlets shall be used alternately to make the filled surface flat.

③ For the second and third layers of silt filling, lay filter geotextile on the gravel and sand filter layer near the discharge outlet, and place reed mats at the direct impact point of the slurry to disperse the slurry impact force and ensure the filter layer at the discharge outlet is not washed away.

④ Adopt the method of multi-stage sedimentation and inspection in zones during filling, and use each filling area alternately to allow sufficient time for silt sedimentation, improve water quality, and facilitate the laying of filter layers.

⑤ During silt dredging and filling, some fine-grained silt easily forms colloidal suspended solids, which takes a long time to settle under natural conditions. During construction, according to the SS value of water quality testing, if there is an excess, we will take measures such as putting chemical agents (quicklime, etc.) into the filling area to solve the problem of suspended solids.

（5）Silt Covering, Grid Greening and Drainage Facilities at Spoil Yard

① Silt Covering

② Drainage Facilities at Spoil Yard

③Grid Greening

V. General Soil Dredging, Transportation and Disposal

1. Construction Preparation

1. Survey & Volume Check: Establish 3rd-order traverses and 4th-order leveling networks; measure cross-sections at 25 m intervals.

2. Equipment Mobilization: Comply with environmental and SL17-90 standards; water vessels by waterway, land equipment by road.

3. Site Clearing: Remove obstacles, drain water, clean silt; dispose waste at Dongshazhou Spoil Yard.

4. Navigation Lights: Install ≥4 floating flashing beacons at spoil yard.

2. General Soil Dredging

NOTE:   
This schematic clearly divides the river channel into two dredging zones for dredging construction:

Upper Slope Zone: Excavated jointly by onshore backhoe excavators and floating pontoon barges, covering the bank area above the river surface.

Underwater Zone: Dredged by marine grab dredgers, pontoon backhoes and cutter suction dredgers, covering the area from the river surface down to the designed riverbed elevation.

This zoning ensures efficient, safe and compliant excavation of both bank slope and underwater riverbed sections.

(1) Onshore Earthwork Excavation for River Channel

(2) Underwater Earthwork Dredging for River Channel

Underwater earthwork dredging adopts three types of dredging machinery and equipment: cutter suction dredgers, grab dredgers and pontoon backhoes. Dredging is carried out in sections of 100m as required by construction conditions, in conjunction with revetment construction.

The pipelines in the river channel section are all supported by floating pontoons, anchored and erected manually.

The land-water joints are connected by multi-section hoses. The onshore pipelines are erected along the original soil of the bank slope, and the filling and discharging are controlled through variable-direction valves. The discharge outlet and water return outlet are set at relatively far positions, adopting overflow type water return outlet, and the position of the discharge outlet is replaced in time according to the filling situation. According to the bottom elevation of the filling area and the designed filling height, filling is carried out from far to near.

The cofferdam mainly utilizes the original soil dike. For sections without original soil dike, small pontoon backhoes are used in conjunction with manual repair and connection to seal the cofferdam of the filling area.

To prevent the spread of pollution sources during filling, a method combining ebb tide construction and regular drainage is adopted during construction. If necessary, quicklime and other chemical agents are added to the filling area to accelerate the sedimentation of suspended particles, so as to meet environmental protection requirements.

d. First, surveyors set out stakes according to the survey. According to the working performance of grab dredgers and pontoon backhoes and construction conditions, a stake is set every 20m as a sampling section, and a group of sample stakes are set every 5m to accurately measure and set out the excavation line mark. Each dredging section shall have at least two groups of stakes. For groove dredging, the bottom elevation mark and slope excavation starting point mark of the dredging section shall be set out to facilitate dredging control.

e. Before dredging of each section or each time, surveyors shall recheck and position the stakes.

f. Each grab of the grab dredger is 5~10m. A method combining layered dredging and retention of stepped berms is adopted. When closing the excavation, the dredging speed is appropriately slowed down to improve the dredging accuracy (the dredging part shall be trimmed).

According to the on-site side slope line mark, dredging is controlled by positioning instruments, and layered dredging is carried out to form an open stepped shape; the main body forms a natural slope, and finally the slope is trimmed to the designed slope by a slope trimmer.

g. When each grab dredger retracts the cable and shifts, it shall strictly operate in accordance with the specifications. Control the shifting distance; measure the dredging section in time according to water level changes. After each section meets the design requirements, shift to carry out dredging of the next section.

h. Pontoon backhoes shall cooperate with grab dredgers for trench dredging and trim the tooth groove side slopes; to ensure the safety of the vertical retaining wall, dredging within 5.0m outside the vertical retaining wall shall be carried out by backhoes.

During construction, adjust the excavation depth in time with water level changes; use the rotation angle and survey setting out to control the depth and width of each layer of dredging. Measure the completed section in time, and only after passing the inspection can the vessel sail forward and dredge the next section.

(3) Slope Trimming

(4) Earthwork Disposal

Except for the general dredging earthwork that is disposed at the spoil yard or reserved for dam filling, all the remaining earthwork shall be disposed at the spoil yard.

According to the specifications, construction can only be carried out during the day from 7:00 to 19:00. However, the mud barges need to be transported to a long-distance spoil yard (about 10km), so each barge can only make one trip per day. According to the on-site working face layout and water level requirements, try to use mud barges above 200m³ with sufficient quantity to ensure a balanced coordination between dredging and transportation.

The soil transported by mud barges must be disposed at the spoil yard within the fixed range of the guide marks, and disposed in zones from far to near.

(5) Methods to Control Disturbance to River Channel Siltation

During the river channel dredging process, we adopt the following measures to control disturbance to river channel siltation:

2) When constructing with pontoon backhoes and grab dredgers, adopt a method combining segmented and zoned layered dredging with staggered dredging; slowly lift the grab during construction, and the mud barges should be as close to the dredgers as possible to shorten the mud loading distance and prevent soil from scattering into the river.

3) During the finishing of the completed surface, the soil trimmed by the pontoon backhoe shall be directly loaded into the mud barge.

4) Assign dedicated personnel to patrol the spoil discharge yard and slurry pipelines during construction, and handle any problems found in a timely manner. When transporting by mud barge, leave a certain margin in the mud hold or install baffles and other measures to strictly prohibit slurry leakage into the river.

5) Strictly dredge in accordance with the survey-set stakes during dredging construction. For underwater dredging, correct the vessel position in a timely manner. Each dredging pass shall be positioned under the command of on-site surveyors to ensure the accuracy of the dredging section and avoid rework.

6) During the spring and neap tide periods, arrange underwater dredging as much as possible during small neap tides according to the construction schedule to ensure that underwater river channel dredging does not cause water quality to exceed standards.

7) Prohibit underwater dredging during typhoons and floods.

8) Adopt combined manual and mechanical operations during construction. Construct manually in special sections to avoid inefficient disturbance by machinery. Timely remove soil from steep slopes on both banks to prevent collapse into the river.

9) Strengthen water quality monitoring during construction, and have divers inspect underwater dredging. Timely improve the construction process according to the construction requirements and actual conditions of different sections, and reduce the disturbance to river channel siltation to within the scope required by specifications.