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Operations in Embankment Dam Construction

Construction of Embankment Dam

The construction operation of embankment dams is divided into four major groups of construction activities which include:

• Material source development activities
• Foundation preparation activities
• Fill construction operation
• Ancillary works construction activities

Fig.1: Embankment Dam Construction

Material Source Development Activities

The material source development activities include opening out quarries from which necessary materials are obtained, installation of fixed plants for instance crushers, and conveyor which is employed to transfer crushed materials to a specified location.

Construction of roads between various areas of quarries and the embankment dam is another material source development activity. These roads are commonly utilized to haul and transport materials to the embankment dam, in addition to move transportation and haulage plant between various parts of the quarry.

Fig.2: Installation of plants and conveyor and road construction to haul materials

Foundation Preparation Activities

One of the first works needs to be tackled properly is drying up the foundation area. This usually can be done through the construction of temporary diversion tunnel at the sides of the embankment dam.

Fig.3: Inlet portal of diversion tunnel used in the construction of hoover dam in the united states

Such tunnel may host outlet works as well. Sometimes, the outlet culvert through or under the embankment dam is built instead of diversion tunnel and this outlet culvert would be used for river diversion.

Earth fill source development is another construction activity at this stage. This can be carried out along with river diversion works. After the previous works finished, weathered materials, which migrated due to water and weak soil at the top surface would be removed at the project site.

There are cases in which soil foundation strength needs improvement. In addition to consolidate the area in advance and install sand drains. Necessary instruments and tools are also installed at this phase.

Not only do these devices used to measure pore water pressure but also observe the performance of cutoff point. Finally, the foundation construction stage is finished with the drainage blanket placement beneath downstream shoulder.

Fig.4: Drainage blanket placement in embankment dam

Fill Construction Operation

It is another dam construction activity that could be easily carried out. The material used should be conformed to the requirements of applicable code. As far as fill placement is concerned, it could be affected by weather conditions in addition to possible variations in material characteristics.

Fig.5: Placement of earth fill material

The quality of the construction is governed by number of factors for instance water content, thickness of soil layer being compacted, and compaction effort.

So, fill placement cannot be carried out properly unless the works is supervised and monitored adequately. The quality and uniformity of compacted core fill is considerably significant and need to be taken into consideration.

Then, the placement of horizontal drain layer in both shoulders at a vertical distance of 3-6m is proceeded. This construction activity is required to control construction pore water pressure and increase speed of consolidation in cohesive materials of low permeability.

While fill material are placed, devices and instruments needed to be placed in the core and shoulders are installed. The final activity of this stage is the construction of protection measures for instance placement of armoring layer at the upstream of the embankment dam.

Ancillary Works Construction Activities

It may involve the construction of spillway, stilling basin, tunnel or culvert for outlet works, valve towers, drainage works, wave wall, road way, and grassing of downstream face of embankment dam in the locations where weather condition would not spoil it.

Fig.6: Spillway of Darbandikhan Dam In Kurdistan Region-Iraq

Fig.7: Removing excess water using spillway of Darbandikhan dam in Kurdistan region-Iraq

Fig.8: Grassing downstream of an embankment dam

  

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Cost of Reinforced Concrete FRP Strengthening System and other Methods

There are various techniques which are used to enhance both ultimate load capacity and serviceability of reinforced concrete structures. Strengthening reinforced concrete beams with FRP composite is the most novel method which has been developed. This method is widely accepted and has proven to be effective in improving capacity of reinforced concrete elements.

In this article, the economy of FRP strengthening systems in comparison with the cost of other conventional methods based on the conducted studies and projects will be discussed.

Fig.1: Reinforced concrete member strengthening using FRP composite (left side) versus strength using traditional methods (right side)

The economy of strengthening systems will be discussed in two stages which include:

• Cost of Installation of FRP systems
• Cost of Entire life service of strengthening systems installation of FRP systems

Cost of Installation of FRP Systems

The economy of FRP composite strengthening system at installation stage is based on the working conditions and factors that may involve the task.

Moreover, it is required to take the time frame allocated for completing the project and the available space at site when the cost of the work is evaluated. This is because these factors could have a substantial influence on the cost.

Furthermore, it is claimed that the cost of strengthening using FRP composite is around 70% of the total cost using other methods for strengthening. This claim is based on the study that conducted on the strengthening of rail track structure.

Installation cost estimation and studies on strengthening technique using FRP composite compare with other conventional methods

In certain projects such as high ways, closing of the structure for traffics would lead to penalty and hence increase the cost.

For instance, when main high way in New York City in the United States had to be upgraded, it was needed to do the work at night only because the high way needed to be open during the day otherwise a penalty of around thirty thousand US dollars would have been imposed for hour of high way closure in day.

There are number of projects for cost estimation has been carried out and all of them showed the superiority of strengthening technique using FRP over other ordinary or traditional methods.

For example a beam to column joint of parking garage structure enhanced by bonding carbine fiber sheet. It is found that, the use of such method needed around 35% of ordinary techniques which may involve the installation of additional reinforcement bars and pouring concrete as well.

In another project which located in Canada, the cost of shear strengthening of the entire bridge was estimated FRP sheet and steel stirrup. It was demonstrated that the cost of using FRP sheet was cheaper by about 30%.

As far as the installation time frame is concerned, there are cases in which the time needed to complete the strengthening project using FRP composite is substantially lesser that that needed when conventional method is employed.

For instance, the duration needed to strengthen a chimney in Japan was a month whereas the conventional method required six months. Finally, from the above examples, it can be concluded that, the cost of installation of FRP composite is considerable smaller than other traditional methods.

Cost of Strengthening Systems Installation of FRP Systems

The entire life service cost of strengthening technique is a major factor that plays significant role while a decision is made on how and when strengthens reinforced concrete structure.

Commonly, the enter life service cost of any strengthen system is composed of installation cost and maintenance cost that may be required during the life span of the structure.

However, since the installation cost has been discussed and accounted for, so the maintenance cost will be explored in this section.

When the entire life cost of strengthened structure is evaluated, it is necessary to take number of factors into account. For example, durability of strengthening technique, costs due to loss of service, requirements of the owner for serviceability and life service of the structure, and maintenance cost.

It is proven that, the cost of maintenance of reinforced concrete elements strengthened with FRP composite is smaller than that of reinforce concrete members strengthened with conventional methods.

Finally, it can be claimed that, the economy of using FRP composite systems to strengthen reinforced concrete structure is superior to other conventional method.

Underwater Concreting

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Underwater concreting is same as conventional concreting except for one difference. It is done under water.

There are mainly four methods for placing concrete under water which are listed below

• Tremie method

• Pump method

• Hydrovalve method

• Skip Method

• Preplaced-Aggregate Method

 Among these five methods Tremie method is the most common method used for underwater concrete placement. This article only covers tremie method however there will be tutorials on other methods soon.

Tremie Method:

Tremie method is a way of placing concrete using gravity as the main driving force. Tremie method mainly consists of an impermeable, rigid pipe (also known as tremie) which is suspended vertically through water.

Tremie method has been a reliable method for engineers of placing high-quality concrete. Its main advantage is that concreting can be done in a flow with very little disturbance or turbulence.

Placement technique:

• A Tremie is placed upto a point where concreting is required.

• Tremie pipe generally is 250mm in diameter.

• Funnel shaped hopperat its upper end so that concrete can be poured with ease.

• A loose plug or seal is present at the bottom which prevent concrete to move out before pipe is full. It also prevent water to enter from bottom. After pipe becomes full with conrete a slight jerk is
given to break the seal to let concrete out.

Placement Scheme:

Area where tremie placement is needed dictates us to use certain scehme for placement. There are two general schemes used for Tremie placement.

• Continuous concrete scheme
• Advance Slope scheme

Continuous concrete method:

In the scehem concrete is fed into several tremie pipes at same time. The benifit of doing so that concrete rise everywhiere at approximatly the same rate.

This placement technique is very useful in small areas but for large concrete placement it becomes almost impossible to meet concrete demand. Furthermore cold joints can also come into play. To counter this negative aspect it is advisable to divide large area into several smaller areas.

Advancing Slope Method:

In advaning slope scheme, Concrete placement is starts at one location and then with time it is moved to cover entire area.

When tremie location reaches a particular elevation , with an adjacent tremie is immersed in concrete by about 1m. The tremie placement will proceed to the adjacent tremie.

Tremie concrete flows with an advancing slope and it ranges from 1:5 to about 1:40 which depends on concrete slump (flowability) and also placement rate. The main advantage of this scehem is that it demands less concrete production as compared to first scheme. Secondly this scehem eliminates the potential of cold joints between adjacent concrete pours.

Slump Range:

Slump Range Used for tremie method is on higher size which is understandable as it needs to flow down and settle without and disturbance. Self compacting concrete system is a very good option.

Cement Washout and Laitance Problem

cement washout problem is always a concern in underwater concrete placement. In order to minimize it, placement operation should cause very little disturbance to concrete underwater. Sources of disturbance can be

• Starting and restarting of placement.
• Loss of seal before time.
• Dragging the tremie pipe horizontally while embedded in concrete. It is advisable that tremie pipe should be embedded to a minimum depth of about 2 ft in fresh concrete.

Cold Joint Problem:

Cold Joints can form as described earlier between two adjacent layers of pour. Cold joints as we all know generally decrease the quality of concrete so it becomes important to continuously produce and supply concrete at required placement rate.

It is also essential that necessary material quantities are supplied to batch plant at required rate. There should be an alternative concrete source so that in case if concrete supply is stopped from one source, concreting goes on from other sourcce.

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