Question to civil engineers here

[Paul2]

There been a comment that it's hard to build highrises in Dhaka because it's impossible to reach bedrock, and that just a big, and heavy block of concrete is the only suitable foundation for a tall building to secure it in case of a seismic event in BD.

There are for sure cities in China with equally troublesome soils, yet highrises all around. Supertalls are of course an exception (PA tower in Shenzehn sits on a lone island of bedrock, and Shanghai tower, and surrounding skyscrapers too.)

Can anybody with background in civil comment here?

My suspicion is there are just nobody with modern enough civil engineering background to weigh in on foundation designs for highrises in challenging soils. I think Japan got both the most troublesome soils, and the most challenging seismic environment, yet they still do make very tall residential highrises.

 

[mb444]

I do not think it a question simply of engineering.... offcourse you can build highrises in BD but the cost would be prohibitive. Pile foundation is expensive and cassion foundation which I suppose is what BD needs is even more so.

 

[UKBengali]

@bluesky

Are you able to help here?

 

[Vapnope]

It all depends on two factors.
1 - Seismic zone
2 - Type of soil

If your area lies on fault line, then it is advised not to go for high-rise buildings.

In second case, you will have to achieve the AASHTO compaction using crush and suitable earth fill so that proper compaction can be achieved. And when you say hard to build, that surely means cost involved to achieve suitable bearing capacity.

 

[shadowdragon]

When undertaking such projects ..they are analyzed from all aspects including geotechnical and economical point of view.. it can be built but does the cost outweigh the benefits..... These buildings depend on the type of foundation given... That alone depend on various factors..

 

[bluesky]

There been a comment that it's hard to build highrises in Dhaka because it's impossible to reach bedrock, and that just a big, and heavy block of concrete is the only suitable foundation for a tall building to secure it in case of a seismic event in BD.

There are for sure cities in China with equally troublesome soils, yet highrises all around. Supertalls are of course an exception (PA tower in Shenzehn sits on a lone island of bedrock, and Shanghai tower, and surrounding skyscrapers too.)

Can anybody with background in civil comment here?

My suspicion is there are just nobody with modern enough civil engineering background to weigh in on foundation designs for highrises in challenging soils. I think Japan got both the most troublesome soils, and the most challenging seismic environment, yet they still do make very tall residential highrises.

For me, I am just a construction laborer, and I have no right to speak here because you asked for only civil engineers to send opinions. I hope someone with a proper background comes forward with his/ her opinion.

In the case of Shanghai Tower, a total of 955 piles were driven over which a concrete raft (8945 m2) with a depth of 6m has been built. A 5-storied basement is built over the raft. Many of the piles were driven to a depth of (GL - 100m) where the soil is very stiff.

When the N-value of soil is more than 30 strokes by the Standard Penetration Tests (SPT), it is certainly very stiff and the soil 100 m below the Ground Level at the site of Shanghai Tower has been found to be granite rock and very stiff.

We better do not compare Shanghai subsoil with that of Dhaka. Granite or bedrock is absent as far as I know. Moreover, BD itself lies in an earthquake-prone zone, the fault line extends from the Himalayas to Thailand.

However, I believe that the construction of 30 to 40-storied buildings anywhere in BD should be encouraged by using the design experience of other countries.

By the way, do you know that the piles under two of the Piers in the Padma Bridge had to be driven more than 100 m below the river bed because the entire depth is loose silts unable to give good frictional resistance to the piles against the downward pressure of the imposed vertical loads?

@UKBengali

 

[Bilal9]

The fact that base rock is located deep in Bangladesh is not debatable, it is established fact.

German manufacturer Menck was contracted to build the world's largest 3000-kilojoule hammer to drive piles for the Padma bridge footings. The piles were 122 meters long and filled with concrete. These are some of the deepest piles driven in any location for this type of bridge.

MENCK LOGISTICS CONTRACTED AIRCRAFT ANTONOV FOR PADMA MULTIPURPOSE BRIDGE PROJECT
News Date : 05.06.2020

MENCK supplied an MHU 2400S hydraulic hammer system for the challenging pile driving project of the foundations for the Padma Bridge, currently the longest bridge in South Asia. After completion of the bridge foundations, a follow-up project for the parallel running energy supply road was necessary.
After almost three million blows, the 60t hydraulic drive unit of the customer's owned hydraulic hammer was replaced. For the almost uninterrupted operation on the construction site, this was delivered by MENCK Logistic using the world's second largest cargo aircraft - the Antonov 124 - from Kaltenkirchen (Germany) to Dhaka (Bangladesh) within one day. During regular project-integrated maintenance work, the drive unit was installed on the MHU 2400S hydraulic hammer in record of time, supported by the MENCK service team, and was thus able to make another significant contribution to achieving the next project milestone.

 

[bluesky]

The fact that base rock is located deep in Bangladesh is not debatable, it is established fact.

Pray, tell me your source of information about the presence of bedrock in BD. I have not read one yet. However, a few years ago the then Faridpur Mayor told me about its presence 600 meters below the ground level in or near his town. The Municipality was searching for good quality drinking water free of arsenic. This is what he told me.

Whatever it may be, but do you think a structure can be built by taking the piles so deep as 600m? It is prohibitively expensive and technically not feasible.

The bedrock must be within 100m below the GL to build a tall structure over it.

 

[bluesky]

German manufacturer Menck was contracted to build the world's largest 3000-kilojoule hammer to drive piles for the Padma bridge footings. The piles were 122 meters long and filled with concrete. These are some of the deepest piles driven in any location for this type of bridge.

I am happy to read about the pile driving system in the Padma Bridge project. The Pile Sleeve above has a diameter of 2.74 m. This is the first time I came to know that the pile dia was about 2.7 m which is very large.

However, since the longest piles were driven 120m down the river bed, you have assumed that there must be bedrocks at that level. I do not think there is any bedrock at that level.

In only two of the Piers (Pillars), the construction team did not get enough upward resistance that would support the downward vertical and moving loads. So, they increased the number of piles under these two Piers and increased the piling depth to 120 meters.

These piles are not resting on bedrocks because there is no such thing at 120m depth in the Padma. I have never heard anything like this. The piles are resting on two systems. They are:

1) Point Bearing Pile: On the lowest tip of a pile at 120 m depth, the soil is strong enough to resist a part of downward loads. How much it is in the Padma I do not know because I have not read the SPT reports to know the N-value at that depth or the nature of soil there, silt or bedrock.
2) Friction Pile: The soil around a pile puts stiff upward resistance by exerting friction around it. This is why you cannot drive a LOGI (লগি) more than 6" in the ground when anchoring a boat at the bank of a river. The soil exerts a certain amount of "Frictional Resistance" to your efforts and you are unable to overcome it. A friction pile works exactly the same way.

 

[Bilal9]

Pray, tell me your source of information about the presence of bedrock in BD. I have not read one yet. However, a few years ago the then Faridpur Mayor told me about its presence 600 meters below the ground level in or near his town. The Municipality was searching for good quality drinking water free of arsenic. This is what he told me.

Whatever it may be, but do you think a structure can be built by taking the piles so deep as 600m? It is prohibitively expensive and technically not feasible.

The bedrock must be within 100m below the GL to build a tall structure over it.

I phrased the comment wrong. What I meant, is that Bedrock is so deep, that building on bedrock is unfeasible and close to impossible.

This is why Padma bridge pier foundations were built on a hexapod, septa pod or octa pod structure like this (which in civil engg. is known as pedestal pier) driven 122m deep. Even collision with 4000 ton coasters won't damage it supposedly...

 

[Bilal9]

When undertaking such projects ..they are analyzed from all aspects including geotechnical and economical point of view.. it can be built but does the cost outweigh the benefits..... These buildings depend on the type of foundation given... That alone depend on various factors..

Well I am no civil engineer and unqualified to comment. But I heard from my CE friends that you need to have a foundation depth below ground of at least one third or one fourth of the height above ground, of which part of it can be basement parking....

For super-talls like the 111 story tower planned in Dhaka's suburbs, the foundation supposedly will need to be at least 15 or 20 stories deep, maybe more.

Please correct me if I'm wrong.

 

[bluesky]

By watching the inclination of piles, they are regarded as "Batter Piles", a term frequently used by the foundation/ civil engineers irrespective of their number. The Padma Bridge has used this method.

An inclined pile is more able to provide resistance against the lateral/ horizontal forces that want to destabilze the foundation comparing to a vertical pile.

The more it inclines the more it gives horizontal resistance, but at the same time its vertical strength weakens. So, the design engineers take into account this factor when analyzing the pile strength.

 

[serenity]

Some very good answers.

First big picture is about economics and requirement to provide that much housing? How dense is the city, what available transport and do the concentration of people require what types of transport to go to their usual places. First consideration comes from city planning urban design. Then the need results in definite requirement for high rise housing or office in one area which satisfies all requirements then it is engineering problem.

Can Bangladesh do it? Of course. These things are not impossible and can easily get foreign firms to develop the custom solution. Australian firms often use Canadian or even Chinese and German and Spanish or Italian companies for some particular things they are not familiar with.

High rise design against expected wind load, soil types, water table changes and saturation of soil, lateral earth pressures and seismic activity? All can easily be done by foreign consultant or I think even in Bangladesh without any assistance. Piling design depends on so many things but it can be too expensive compared to engineering lower levels and having more sprawl using up more land if the cost and benefit is worth it. If it isn't then deep piling foundations required and suitable frame design for extreme wind and seismic activity. In China there are some designs in cities with constant typhoon and seismic risk. I have seen some very cost effective methods for this for lower level and lower risk buildings. For higher risk ones and higher level which I think Bangladesh does not need in terms of calculating 100 year probability using Boolean or Monte Carlo method to determine risk of catastrophic natural activity exceeding design, or even for the need of a building like let's say Taipei 101 with more custom seismic design.

Cheaper solution will be much more suitable. Why does a poor man living in high traffic shit roads city need a Ferrari right?

So the real reason why so far Dhaka may not feature any super high rise is because no need for them from the urban planning point of view and no need to spend too much money just to have some.

 

[bluesky]

So the real reason why so far Dhaka may not feature any super high rise is because no need for them from the urban planning point of view and no need to spend too much money just to have some.

Yes, you are correct. Dhaka does not need highrise structures yet. But, in my opinion, even if built, the highrises should not exceed 30 to 40 stories. It is because the entire BD land is composed of silts and loose silts. There is no design technology to offset the weakness of loose silty subsoil.

Many million years ago before the Himalayas were created by the Indian plate going below the Eurasian plate, there was no land where BD is located now.

This part was created with the continuous flow of silt and clay accompanied by the water flowing through the rivers coming down from the Himalayas. Today's BD land was created with the deposition of these silts, sands, and clays continuously for many million years. The process still continues and this is why there are new islands being created in the Bay of Bengal near the river mouth of the Padma.

As such, I am not sure of the very north near but BD subsoil has no bedrock. I read one Indian geological feature that says of a depth of 13 km of silt below what is now Bengal/ Bangladesh. This is the reason I think that very highrise buildings are inappropriate to build in BD. Loose silts cannot take heavy vertical as well as lateral loads due to wind or earthquakes.

A lateral load creates an additional vertical load on the piles on the other side of a structure which may be very high when a structure is very high.

The Geological Society

 

[RealNapster]

There been a comment that it's hard to build highrises in Dhaka because it's impossible to reach bedrock, and that just a big, and heavy block of concrete is the only suitable foundation for a tall building to secure it in case of a seismic event in BD.

There are for sure cities in China with equally troublesome soils, yet highrises all around. Supertalls are of course an exception (PA tower in Shenzehn sits on a lone island of bedrock, and Shanghai tower, and surrounding skyscrapers too.)

Can anybody with background in civil comment here?

My suspicion is there are just nobody with modern enough civil engineering background to weigh in on foundation designs for highrises in challenging soils. I think Japan got both the most troublesome soils, and the most challenging seismic environment, yet they still do make very tall residential highrises.

Piles are used to shift building load to "solid rock" underneath some 100-400 feet. Such piles are called bearing piles. But that's not always the case. You don't always have a solid strong rock in this range. Sometimes the rock doesn't exist, sometimes it exist too deep that it's uneconomical to build. So in those cases the issue is solved with the help of another pile type called "friction pile".

So let me differentiate the two. Bearing piles shift building load directly to underneath solid rock base. On the other hand friction piles (as the word friction clarifies) use force of friction with its surrounding soil or sand. Hence building a bond through which the friction piles transfer the load to surrounding soil/sand.

That's what they have been doing in Dubai. I recommend you watch Burj Khalifa and Burj Al Arab construction documentaries.