Brief Discussion on the Construction Method of River Dredging Project

The main purpose of the dredging project is to dig a shallow section of the river or bay, which can greatly improve the flood discharge capacity of the river, improve the irrigation guarantee rate of the river network, improve the water supply conditions and water ecological environment, and improve the navigation capacity. This paper mainly discusses how to carry out reasonable river dredging construction.

With the progress of science and technique, the dredging technique changes with each passing day. Various new and advanced special dredging equipment for different soil quality and working conditions have appeared one after another, greatly improving the construction efficiency and the dredging effect. In short, according to different operating conditions and construction requirements, the construction method with advanced technique, convenient construction, and low cost should be flexibly selected in practice to achieve the expected dredging effect.

1. Selection of dredging measures

 1.1 Selection of construction method for cutter suction dredger

(1) Main pay pile transverse excavation

The key points of construction are: take one steel pile as the main positioning pile, and the other steel pile as the supporting pile. When the main pile moves forward, it always stays on the centerline of the excavation channel. This construction method is different from projects of different soil quality.

(2) Double main pile excavation

This construction method takes two steel piles in turn as the swing center, which is suitable for digging loose soil. It is not suitable for those projects with higher digging quality. The advantage is that the operation is relatively simple, but due to the inconsistency of the swing center, it is easy to cause two disadvantages, such as digging and leakage.

(3) Anchor cable construction technique

Concrete can be divided into four anchors horizontal excavation methods, five anchors horizontal excavation methods, and six anchors horizontal excavation methods. Concrete construction technique for dredger cast the main bow anchor. Side anchors are set at the front and back respectively, with the bow anchor as the swing center. This construction technique is suitable for the dredger with main and side cable winch, and the flow rate is large, the direction of flow is basically consistent with the trenching direction. The advantages are wind-wave resistance, water flow adaptability, and the width of excavation is larger than the steel pile positioning method.

(4) Shallow area piling method

First, the steel pile positioning method is adopted to excavate the production forward. When half a ship’s position is put in, half a ship’s position is put back, and then the excavation is carried forward. In such a reciprocating cycle, the rotation pendulum method can be used to reverse the position. This construction method is suitable for the projects that need to catch the tide, as well as the projects where the excavation area is weak and the designed excavation depth cannot be reached due to the leakage of piles. And the designed excavation depth exceeds the maximum excavation depth of the dredging vessel and the water depth varies to a certain extent.

 1.2 Grab dredger

This construction method can include row bucket dredging measure, quince-shaped dredging measure, angle cutting dredging measure, and stem retaining dredging measure. The scope of application of bucket dredging measure is the project of soil compactness, the soil layer is thicker, and the quality requirement is higher. The plum-shaped dredging is suitable for soft soil and small thickness of the mud layer. The method of cutting angle and retaining stem dredging is suitable for hard soil.

 1.3 Bucket dredger

The commonly used construction methods are inclined transverse excavation method, more special parallel transverse excavation method, sector transverse excavation method, and cross-shaped transverse excavation method. The oblique cross-section method is suitable for water area and hydrologic conditions. And the advantages of higher quality excavation projects are less excavation resistance, less mud flushing, and easy to meet the quality requirements when digging the edge.

The parallel cross-cutting method is suitable for water areas with large flow velocity and has the advantage of good adaptability to flow. The scalloped method is suitable for the situation where the water depth at the boundary is less than the draft of the dredger. The crosswalk method is suitable for the situation where the depth at the edge of the trench is smaller than the draft of the dredger and the width of the trench is smaller than the length of the dredger.

 1.4 Dipper dredger

In this case, the back dredging method and the horizontal excavation method can be adopted. The main construction points of the back dredging measure are: After lowering the bucket, use the back rope to pull the bucket as far as possible to the hull, push the bucket with the weight of the hull, cut into the river bottom for digging. This is suitable for a thicker soil layer with a thickness of 3~4m, which has the advantage of higher efficiency. The main point of the horizontal excavation method is that the rotary bench dredger pushes and presses the bucket at any time during the cutting process to keep the bucket track level.

 1.5 Pneumatic pump

It includes three construction methods: hole excavation, cross excavation, and drag excavation. Among them, the hole excavation method adopts the plum-shaped arrangement of the hole side, and the air pump is lowered at the same time to move after reaching a certain depth. It is suitable for loose flow plastic silt, low compacting soil, and obstacles in the soil layer, and has the advantages of convenient operation and high production efficiency. Cross excavation method means that digging another hole after digging a hole and dredge until the cave been soft and collapse. This method is suitable for sandy clay. The main technical points of the drag method are to install a shovel knife and traction rope at the pump mouth. The shovel knife excavates the soil layer by traction, which is suitable for soil, sand, and clay with a high compacting degree and good excavation quality.

2. Selection of construction machines and tools

Different types of dredging machine have different soil and have their respective adaptability and limitation, so in the process of construction should be based on dredging soil can be dug and transportability chooses different dredging machines. Thus achieve the purpose of improving the dredger construction progress. Certain principles should be followed in the selection of dredger tools for cutter suction dredger and grab dredger.

In some dredging and hydraulic reclamation projects often encounter excavation above the water, such as excavation canal, dock, and channel. There are anchoring, transverse movement of difficult operations, and high slope collapse safety problems.  Too much collapsed soil will not only bury the machine and tools but also produce adverse shock waves and produce destructive power. The main technical measures adopted in the construction are as follows:

(1) When the water above exceeds 4m, measures shall be taken to lower its height before excavation to ensure safety. Common methods are: Reduce the height by land mechanical excavation and loose blasting reduces the height of the landslide in advance.

(2) The delamination thickness of excavation should be reasonable, and the excavation thickness of the first layer should be reduced as far as possible under the condition of ensuring the dredger’s draft and minimum excavation depth.

(3) The distance and thickness of the dredger should be less than the normal value. Soil collapse can be reduced by reducing the distance forward and the thickness of excavation.

(4) Flexible strip excavation is a short strip excavation to reduce the impact of soil collapse on both sides of the dredger and to reduce the lateral tension.

(5) In the area affected by damp, the upper layer should be excavated by high tide, and the lower layer should be excavated by low tide. Excavation should be arranged as far as possible when the conditions are better.

(6) It is necessary to strengthen the observation of prow and bank slope, master the law of soil collapse, find out the problem, and take timely measures to avoid it.

3. Environmental dredging

Compared with ordinary dredging techniques, the environmental protection dredging technique has the following features: It can effectively remove polluted sediment from rivers and lakes and create conditions for the recovery of aquatic life. In the process of environmental protection dredging construction, the appropriate environmental protection vessel should be used to automate the equipment configuration and achieve a high degree of accuracy. And the dredging process has higher requirements. The corresponding sludge will be different degrees of special treatment. At the same time, there are also special analysis and strict control. Environmental dredging projects usually use the cutter suction dredger construction.

4. Conclusion

This paper discusses the construction technique of different river dredging. According to different soil conditions, river conditions, and other environmental conditions, it is necessary to select the river construction technique scientifically and reasonably. Reasonable dredging methods should also be used to maximize the effectiveness of the corresponding mechanical equipment. In this way, good social and economic benefits can be achieved and corresponding project quality requirements can be met.