NATM: Application in MRT-1 Subway construction

MD Abdullah Al Naim B.Sc in Civil Engineering (Appearing) Ahsanullah University Of Science and Technology

Dhaka, the capital of Bangladesh.

It is one of the world’s most densely populated cities. It has a population of more than 21 million people. The city’s transportation system faces every day challenges due to high population density, poor infrastructure, and a lack of effective public transportation. Road transit is the most popular means of transportation in Dhaka. It comprises buses, minibusses, automobiles, and motorcycles. However, the city’s roads are frequently crowded, with traffic delays typical, especially during peak hours. This traffic bottleneck is mostly caused by the large number of vehicles on the roads and the insufficient road network.

To reduce traffic congestion, the government has implemented a number of efforts, including the construction of flyovers, the expansion of road networks, and the implementation of dedicated bus lanes.

The public transportation system in Dhaka needs to be improved,

With a limited supply of buses and trains that are frequently overcrowded and unreliable. Many commuters rely on private cars due to a lack of an efficient public transportation infrastructure, increasing the congestion problem.

Another big concern with Dhaka’s transportation system is the lack of adequate pedestrian infrastructure. Due to the restricted availability of public transportation, walking is frequently the only practical choice for many individuals. Nonetheless, the city’s sidewalks frequently require improved maintenance and safety, making it difficult for people to stroll around the city safely.

In conclusion, the transportation scenario in Dhaka is a significant challenge for the city’s residents,

and there is a need for comprehensive and sustainable solutions to address the problems. The government must improve the public transportation system and provide safe pedestrian infrastructure. Only then can the transportation scenario in Dhaka be improved, making it easier for residents to move around the city.

Subway construction or,Mass Rapid Transit (MRT) in Dhaka,the capital city of Bangladesh, is of utmost importance due to the numerous benefits it can provide to the city and its residents.

First and foremost, an MRT system can significantly alleviate the traffic congestion problem currently plagues the city.

With a population of over 21 million people, the amount of private vehicles on the roads is overwhelming, resulting in traffic jams and lengthy commutes. An effective and dependable MRT system can minimize the number of cars on the road while also making commuter mobility faster and more comfortable.

Second, an MRT system can provide citizens with a more environmentally responsible form of transportation. With Dhaka’s high levels of pollution, boosting sustainable transportation choices is critical. An MRT system can assist the city cut carbon emissions and build a cleaner, greener environment.

Furthermore, an MRT system can help the city’s economic development. Improved transportation infrastructure can help the city’s economy by providing jobs and attracting investments.

In conclusion, the construction of an MRT system in Dhaka is essential for the city’s sustainable development and the well-being of its residents.

While it may involve significant investment and planning, its benefits are far-reaching and can positively impact the city’s transportation, environment, economy, and safety.

The main part of the topic is NATM for Subway construction for the MRT-1. Subway construction is a complex process involving various methods with advantages and disadvantages; choosing the right method is critical to the project’s success.

There are several methods used for subway construction.

Some of them are:

1. Cut-and-Cover Method.

2. Tunnel Boring Machine Method (TMB).

3. NATM for subway construction.

4. Sequential Excavation Method.

5. Immersed Tube Method.

They are the most famous subway construction methods. Each with its own advantages and disadvantages. The choice of construction method depends on various factors including the surface level, soil or rock conditions, environmental considerations, and capital. The cut-and-cover method suits shallow areas, while the TBM method is suitable for deeper areas. The NATM and SEM are used for unstable ground conditions, and the ITM is used for underwater construction. Each method has its unique challenges.

The New Austrian Tunneling Method/NATM for subway construction

NATM for subway construction is a popular technique through unstable ground conditions.

This method involves excavating the tunnel by spraying a mixture of concrete and water onto the ground, which forms a lining around the tunnel and stabilizes the surrounding soil or rock. NATM is a suitable method for constructing subways in unstable soil or rock areas. The benefits are-

a. Low cost 

b. No Uncertainty disturbances to total schedule 

c. Ease of application 

d. In-place solutions 

e. No need for the above access 

f. No at grade diversion 

g. No utility or underground facilities disturbance 

h. No expensive machinery or technology 

i. Suitable for long or short, deep or shallow projects 

j. Full independence of foreign technology and labor 

k. Several cutting heads

The results found from the previous study on The Application of NATM for construction

 of Metro System in Dhaka City:

SL	Description	NATM	Cut and Cover	TBM
1	Profile construction	Yes	Yes	Yes
2	Stations construction	Yes	Yes	Yes
3	Earthwork excavation volume	Small	Large	Small
4	Earthwork Fill back volume	No	Needed	No
5	Transportation of earthwork	Small	Large	Small
6	Utility lines relocation need	No Need	No Need	No Need
7	Earth retention systems	Shorter but heavy	Longer and heavier	Short and Heavy
8	Deep excavation costs	Small	Very large	Small
9	Alternative route aspect	Applicable	Not Applicable	Applicable
10	Cost in Increase By length of the project	Length does not affect the cost per km	Length does increase cost per km very large	Length reduces cost per km
11	Limitation to road alignment	No	Yes	No

Based on different studies on the methods of construction, the TBM method is a much more costly method as compared to the two others despite of distinct advantages that it has got. Although the cut and cover method and NATM for subway construction are cheaper as the long route of MRT 5 is concerned, selecting between the two methods will be narrowed based on limitations and challenges. Following is the list of findings and applicability of the Cut and Cover and NATM method in the selected route:

 As the cut and cover method is executed through the full section excavation of soil, cut and cover need access from the top, which limits the cut and cover only to the at-grade route roads and opens spaces layout. There is no possibility of passing below the existing structures.

But NATM does not need that access as it excavates through the shafts constructed.

By increasing the depth, NATM shows little change in excavation volume nor much cost for profile construction. As the profile goes deeper, only shafts are constructed a bit deeper than before. In Cut and Cover, by every height of the profile adding to the depth desired, the quantity of the soil needed to be excavated will be two times larger.

On the other hand, a deeper Cut and Cover tunnel needs stronger and heavier supports for the walls at the side, so the application of Cut and Cover for deep tunnels is very illogical and impossible due to the cost increase in the volume of excavation, fill back and their costs, as well as the cost of more vital earth retention systems needed.

 As cut and cover need to excavate the entire section, any underground structure or any network utility laid between the road surface and desired tunnel location is subjected to modification, dislocation, and relocation, or even permanent alternative provision, which all of them carry a significant cost imposing to the tunnel metro project. NATM is doing this section using the shafts, and there is no conflict with the utility lines or other structures lying between the road surface and desired profile location. Due to lesser conflicts, there is no need to relocate or modify any present utilities.

 The Cut and Cover method destroys all the soil strata bound at its top, and filling backs are subjected to different settlements in the future.

Response of any utility line, network, or structures constructed to that settlement differs based on their natural 153 flexibility to settlements, partially or fully section. But this is not the case for NATM, as it preserves the soil-bound. 

As the soil-bound is lost in the Cut and Cover method, no heavy structure or loading can be applied to the soil in the future as the settlements will occur based on compaction imperfections, whether this is not the case for NATM.

The main aspects of the difference in cost for construction methods are the type of excavation and utility lines challenges. NATM has a much smaller volume of excavation of the soil than Cut and Cover; also, NATM does not interfere with the above network of utility lines which Cut and Cover do. In both cases, NATM shows a more affordable construction cost. Considering all aspects of construction, NATM’s total cost is Tk. 7,507.2 million, and Cut and Cover total cost are Tk. 9,392.99 million. Compared to each other, NATM stands cheaper by 25.12% compared to the Cut and Cover method.

Analyzing all the factors and conditions discussed in this article will make the NATM more efficient, sustainable, and cost-effective for applying to MRT-1 subway construction.