Integrated Management of (Construction process) River Training Work (IMRTW)

Md. Balayet Hossain (Ifat)
Field Engineer (Consultant) RTW

Padma Multipurpose Bridge Project
B.Sc. in Civil Engineering (AUST)

M.Sc. in WRE (BUET)

Introduction:

River training work is very important to protect the hydraulic structure. River erosion is a continuous process, especially in flooding time which is dangerous for the hydraulic structure.

On the other hand, it is important to control the river flow, sediment movement, bank protection, scouring, river morphology etc. The whole construction process is conducting at a certain period without any obstruction of river morphology changed. It is very much sensitive during the construction work. The working process is followed as per specification although some challenges is appeared. Working scheduled is depends on some major factors. All working items should be controlled to get a better and safe working site and more efficient. Each of the item depends on each other. Sometime the working schedule should be changed by keeping in mind to the site condition.

Although the design and the construction site are fixed, but it is a matter of great sorrow that often the bank slide is occurred due to improper management which is hampered our valuable time and increase the total cost. It should be maintained by observed the practical situation such as some work ignore and some work added or some time combinedly work. The main focused of this article is to described some factors that must be followed to maintain the work and more efficient. This article is written in based of practical experience. The whole process should be combined and managed together which is called the integrated management process. The construction work is influenced by the seasonal variation. The whole process divided by:

1. Pre-monsoon period for river training work.

2. During the monsoon period for river training work.

Major challenges/ Problems for river training work:

1. Seasonal variation.
2. Identify the critical point of erosion / bank failure along the river and construction
   site.
3. Inland water transport vessel movement.
4. Surcharge load condition.
5. Soil formation of construction site area.
6. Improper use of reclaimed area.
7. Transport the dredging material at a certain location.
8. Improper drainage facilities during the intervention of bank construction in local area.
9. Do not follow the construction process.

Possible Solution for river training work:

➢ Pre- monsoon period/ dry season:
Before construction work the morphological analysis should be implemented and this is the continuous process. The river dynamics and sediment movement should be analyzed. The easiest way to use Arch GIS software and Delft 3d software. The previous land formation analysis and river pattern should be measured. This information gives us the critical point of the construction site area. After monsoon, this critical area may be sliding or slope failure. Before starting dredging, it is mandatory to do the protection work at that location first.

This failure is generally occurred by highly toe erosion, surcharge load, seepage, rapid water level fall down, high water velocity etc. It is very important to know the soil information and select the dredging cutting thickness. The cutting thickness is depending on the soil properties like angle of internal friction, slope angle and the soil shear strength. The large cutting thickness increased the high torque and shear strength and may be possible failure is occurred like as slit failure. Permeability of soil is influenced the slope failure during the dredging work. The monitoring survey is an important thing to observe the cutting layer and if any failure is occurred, should stopped the dredging work and filled the overcutting area by geo-bag dumping. Monitoring survey is observed by the Global Mapper Software.

When the cutter suction dredger (CSD) reaches the design elevation safely then the protective element should be dumped as soon as possible.

The design elevation is identified from the pre-monitoring survey and post-monitoring survey. The Global Mapper software showed the design elevation and the cutting elevation where the dredger reached. This process is done by three processes firstly the bulk dredging at maximum thickness, secondly the minimum thickness not more than 3m thickness and lastly the final trimming. The selection of thickness is depending on the soil stability. It is suggested that when the cutter suction dredger (CSD) is nearly reached the design elevation level then the thickness should not take more than 2.0 m because the sliding possibility may be increased. During trimming the thickness must be taken from 0.5 to 1.5 m. The major challenge of disposal of dredged material at a certain place. Reclaimed areas can be used for disposal. But this reclaimed area may be influenced by the river morphology which hampered the construction site.

The total morphology is mainly depending on the seasonal variation.

In the pre-monsoon period usually deposition and hardly any erosion are seen. This dredging material disposed of that area where the navigation channel is not used because this dredged material influenced the lower draft. Inland water vessel movement was also hampered. Also, this material may be used in the upper back slope area of the construction site if the area is available. So, the location of the disposal area is overall depending on the reclaimed area, and the reclaimed area is selected by the morphological analysis. It is not wise that the dredging material is disposed of in an open river. Now after completing the dredging and should properly follow the construction sequence.

Generally, the apron is taking 2.5 to 3 times more carrying load capacity than the upper slope area. The factor of safety of soil stability should be greater than 1.5 with the surcharge load consideration. It is often observed that the slope is failing although the stability is high. This is happened because of the surcharge load and it is high when the dry excavation is continuing. This excavated soil is kept on the top bank and also kept the construction material. If the total load of the upper slope is over the apron or toe level strength it may be failing. The protective element is dumped from the apron and then the lower slope and finally the lower water transition. This is the dumping process that should be strictly maintained.

The density of geo-bag dumping is measured by the pool test. This density is measured in volume of per (m3) area.

Another challenge arises to dump the geo-bag at a certain location of the apron.

The multi-layer geo-bag should be placed at apron/lower slope and gained the proper thickness of the geo-bag. This thickness is gained from the density calculation. Long steel cage should be used to maintain the dumping at certain location otherwise the protective element cannot reach the certain placed due to high water velocity and current. Monitoring survey gives the proper information of actual thickness. The multi beam echo sounder is used to survey. This echo sounder gives the underwater data and plot a profile of actual scenario. This survey data is analyzed by the Cloud Computing and Global Mapper software, 2nd one is more comfortable and easier to visualized the scenario. This survey system saves the time and take necessary step quickly when any unusual situation observed. If the proper thickness is not gained then the additional dumping should be required.

The lower slope followed the same process.

Lower slope is divided by two portion such as filter layer and protective layer.
Filter layer may be used long geo-textile placement or multilayer geo-bag dumping which is totally considered by the designer. Normally when the lower slope is very high elevation, it is very much tough to place the filter layer (geo textile) at the toe level of river. So, the multilayer geo-bag dumping is more efficient but high cost. Multi-layer is used because of reducing soil erosion below the protective layer. After that the protective layer is dumped on the filter layer. At the same time low water transition work should be continued and meet the lower slope work.
Now this is the time to place the filter layer and block to the upper slope area. The upper slope protection work and road construction work should continue. Same process is followed the offtake work. The dredged material may also use to the upper back slope and road construction area. This is the way to proper use of dredged material and remaining material is disposed in reclaimed area. The whole process is given below by a diagram:

➢ During Monsoon period:

During monsoon period there are two challenges are observed such as heavy rainfall and flash flood. The upper slope stability will increase than the pre-monsoon period causes the water level is high which is support with acting the passive pressure (Water column pressure). The influence of seepage is reduced. In sandy soil the seepage force and the water column pressure reached at a balanced, that’s why slope failure is hardly seen in this time.
During heavy rainfall and poor drainage facility also caused the slope failure. When upper slope is continued it may be installed the water discharge pipe at a certain distance.
Sometimes the fine sand also coming out with discharge water and caused the failure. So before installed the discharged pipe, it is important to know the soil characteristics like porosity, void ratio, seepage rate etc. Another challenge is flash flood in river and always should updated the water level rise. Improper drainage facilities also caused the water logging in the upper backslope area which is increased the seepage rate. Not only increased seepage rate but also creating water logging in local area like houses, local roads etc.

In pre-monsoon season when the water level fall down then the surface water is decreased specially in offtake area. Sometimes the offtake mouth is closed due to construction work which caused water logging in local area and water shortage in dry season. So, the internal discharge pipe should be installed to supply the water during dry season and local people can use the available
water in dry season. The main work is considered in this season is to dumping the protective element to keep strength at river toe level. If any vetiver plantation is included in design, then it is high time to planted the vetiver.

The vetiver plantation is increased the soil stability.

The inland water transport vessel is easily move due to high water level. Only bulk dredging is continuing in this season. After immediate monsoon period, usually observed the frequent slope failure. This is caused due to fall down water level and reduced the passive water pressure. The sandy soil is coming out with high seepage pressure and the previous surcharge load is also influenced. At this time the river also shifting and change the morphology. The main priority is to keep the safety in construction site. If any major changes are seen in site area like huge erosion, a necessary step should be taken. External support may be taken to protect the site area.

A spur or temporary dam is constructed at a certain place so that the river dynamics are automatically changed and safe for the construction site. This spur and temporary dam can be constructed by dumping geo bag or local rice bag filled with sand. Sometimes the spur can be constructed by the local bamboo or wood or the tree logs. If the water velocity is very high, the sheet piling can be constructed which is used as a barrier and dumping the geo bag or local rice bag filled with sand. Sometimes we reduced the cost by dredging the artificial cut-off the river. These types of remedy can apply when we have enough knowledge about the river change pattern. To know this pattern always observed the river morphology in
every year.

Conclusion:

Here some critical situation has been discussed but have some more factor that may cause the hampered of river training work. This is long term process and need more study about the real causes of slope failure during construction work. This article paper has discussed some possible solution and trying to connected the each working item which is depend on each other. Proper management should be reduced the high risk and it is important to apply integrated management during river training work. It is very much necessary to complete the work in time and low cost.