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Reinforced concrete slab activities/production rates

22 replies [Last post]
chris eaglen
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Dear Sirs,

I wish to schedule 5 basement slabs each 260mm deep and 72m by 36m rectangular. These will be installed using the top down excavation method through two excavation locations, one at each end of the 72m long side.

The ground floor will be installed first. Then there with be some excavation 8m to the -1 basement floor and then to each floor in turn -2, -3 -4 making the 5th slab.

The box is produced using a 36m diaphragm wall with deep punch piles installed from the ground level prior to forming the ground floor slab.

Would anyone be able to provide a planning schedule for the slab formation times and the activities. The concrete will be brought onto site as premix with a concrete pump. Guidance on the productivity rates and the extent of each excavated area to form the on grade sections making up each basement floor will be appreciated.
Thank you
Best regards
Chris Eaglen London UK

Replies

Daniel Limson
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Hi All,

Reading throught the comments on this thread, Chris must have been more confused than enlightened. Anyway, I think he must have found the solutions already by this time and already finished the work by now since the question was posted last March. Anyway, what are the advantages of a top down method. I can only think of 2

1st - you can save time because you can work in 2 directions up & down

2nd - you can save cost on formworks and temporary supports (ELS) Strutting and Wailing since you can cast your slab using the ground and ELS no longer required.

Steve Austin
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The applicability of acoustic emission (AE) technique for evaluation of fatigue damage in reinforced concrete (RC) slabs under cyclic loadings in both laboratory and as a structure in service is studied. The fundamental test performed in laboratory shows that the cracking process can be practically monitored by the measurement of AE signals. Analysis of the relationship between loading phase and AE activity indicates that the final stages of the fracture process can be evaluated by detecting AE signals generated near the minimum loading phase. Concrete Trucks

Samer Zawaydeh
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Dear Anoon,

It is good to hear about different construction methods all over the world. Thus the Engineering Innovation that I am refering to.

Chris and Mike can tell us more. It seems that they can go up to -100 meters (way too much). But in high rise, you try to have the excavated weight equal to the weight of the building, hence reducing the settlement due to load.

I am sure that it is an expensive techniques. But they seem to be well documented. I think that Chris can dig up more case studies.

With kind regards,

Samer
Mike Testro
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Hi Anoon

Yes it is built in both directions at the same time.

Best regards

Mike Testro
Anoon Iimos
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Dear Samer,

I believe you have explained everything (nothing left for me to elaborate anymore).

I’m just not sure if Mike has better idea than yours?

Honestly, I don’t know anything about this type of construction (Top Down), I’m just here to listen.

Thank you.
Samer Zawaydeh
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Dear Anoon,

The essence of Value Engineering is to ensure that the client is getting what they want.

This is usually done at the pre-design stage where the engineers from all disciplines site with the Owner and the VE specialist and determine the best way to achieve the client’s requirements.

The stages of VE are
1. Information
2. Functional Analysis
3. Creativity
4. Evaluation
5. Development
6. Report

I think that you are associating Value Engineering with "Cost Reduction". The best example for Value Engineering is the "Paper Cup" next to the water cooler. It serves the purpose with the lowest cost.

Let us consider what options they had to do this Construction from a Structural point of view:

1. Do not build. Use it as a parking lot and rent it per hour.
2. Do a single story light structure (steel structure) with brick cladding. Maybe 2-3 floors max. This will require a small amount of excavationl; say 2-3 meters with isolated or strip footing.
3. Since Londong is an expensive place and the price of land is high. They wanted to get the maximum built-up area. I guess the zoning specified how much you can go above ground, but does not specify how deep you can go.
Once this was determined, they wanted a structural solution that will permit them to build deep into the ground.

3. We have collapsable soil. What are the options.
3.a Top Down; what they selected.
3.b Bottom up;You have to install the sheet piling all around then dip and anchor the sheeting horizontally inside the surrounding ground. This depends on the surrounding buildings and structures around the site from teh 4 sides. Chances are you can’t anchor the sheet piling horizontally.

In short, the cost of this approach might be the cheapest safe approach that will enable the developer to build what they want.

If you have better ideas please let us hear them. It would be interesting.

With kind regards,

Samer
Anoon Iimos
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Dear Samer,

How do you relate your "Innovative approach" to value engineering (I mean in terms of costs, including designers and consultants)?

Hi Mike,

Do you mean they have done the upper levels (+) and lower levels (-) at the same time?
Mike Testro
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Hi Anoon

We do not have much of seismic problem in the UK.

Hi Chris

You have not yet said if you do face a clay heave problem.

At Nr 1 Poultry they had to get to level 6 before reaching -5.

Best regards

Mike Testro.
Samer Zawaydeh
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Dear Anoon,

Yes the Raft can be casted in segments.

Yes also, this method is developed for commerical buildings of course. Innovative approach for situations that calls for it. For normal buildings under EXCEPTIONAL site conditions that calls for innovative methods.

With kind regards,

Samer
Anoon Iimos
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Hi Samer,

"The Raft can be casted in segments."

I’m not a structural engineer so I cannot comment about that, but I just guess that the structural designer who can do the design like that, loves to play majong, so that the seismic joints can be perfectly designed between the tile joints.

Haven’t seen yet the websites as mentioned by Chris, I just assumed that they’re doing it in mining construction, and maybe in construction areas where space is limited and excavation would be a special case.

But are you going to do it in normal / typical construction of any commercial establishment?

Regards
Samer Zawaydeh
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Dear Anoon,

You need to look at the websites that Chris mentioned. You will come to understand the process from the pictures.

They install the sheet piling all around and drill the columns in the middle up to the foundation, then they cast the slab at level zero. They leave holes in the slab and keep digging down (Mining). And keep going Top Down. The Raft can be casted in segments.

With kind regards,

Samer
Anoon Iimos
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Hi Mike,

Just a clarification if I may (with regards to post # 4), when you say "Level 0", do you mean the natural ground level?

Chris,

Is this Top down construction means your pile heads are above natural ground level? and your raft foundation(s) will be checquered (i’m not sure of the spelling)? but I mean the raft foundation will be cut into pieces so as to proceed with the excavation(s)? and later put into place as one (final basement level)? Where do you do this things, in fault lines?

Thanks in advance
Samer Zawaydeh
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Dear Chris,

Chequer Board is not required. You need to review the Concrete Code that you are following (BS I assume) for the maximum spacing of expansion joint, then you can cut the concrete surface in 8x8 or 5x5 squares the next morning to prevent crack propogation.

I think that you need a brain storming session with the construction and the excavators at site to figure out a way to increase the productivity. This will not be possible to determine until you have lived at site a bit and experienced the difficulties associated with it.

Casting the 0.00 Slab and then moving down with a (Mining technique) is innovative, but very expensive.

Maybe you can ask your structural engineer to study the possibility of using the deep I-beam and casting a bay in the 36meter direction interchangeably. This way you will not do mining and you will have access from top along the 36m. Ask them to be more innovative and reduce the cost.

With kind regards,

Samer
chris eaglen
User offline. Last seen 10 years 22 weeks ago. Offline
Dear Mike and Samer,

Many thanks for the information. The work of S P Marchand is in the links which if Googled as deep basement construction. The sequence of laying each slab depends on the bending moments in the piling or diaphragm wall. We have seen a good example by Skanska Cementation for the Knightsbridge Palace Hotel which is also on the web.

The problem we have been considering for a different project was the faster excavation was not sustainable because of a constrained site with inadequate space for the plant and lorries. Given that there are constraints on speed of progressing the works it is possible to work in parallel with the slab construction at the same time as slower excavation. We found it was not possible to speed up but wise to slow down also. With the indications of the rates and the chequer board approach this helps. Do I assume the chequer board is 8m by 8m sections for the large column piles areas or use larger multiple sections.

Thank you for the help.

Best regards
Chris Eaglen

Deep basement construction
Top down construction uses the floor slabs to brace the walls. To achieve this, the slabs must be cast before excavating the basement. ...
www.esdep.org/members/samplecourse/basement/index.htm - Cached - SimilarTop Down Excavations - Deep Excavations
Top/down or up/down construction methods are another method for constructing deep excavations. In this case the basement floors are constructed as the ...
www.deepexcavation.com/supportsystems_topdown.html - Cached - SimilarA deep basement in Aldersgate Street, London. Part Contractor’s ...
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Samer Zawaydeh
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Dear Mike,

I imagine that the solution must be holistic. An innovative method to carry the huge loads, especially when you have 5 floors going down.

If they use deep steel perforated I- beam (say 1.2m depth) for the 36 meter direction, then they can have a corrugated sheets with concrete slab. I think this will be quicker and cheaper. The steel would be fabricated off site, and you can dig the piles around the perimeter to the required depth, make a cofferdam like structure, and keep lowering these deep I-beams, until they are in place. I imagine that you do not need a raft also. You need a Slab on grade with water insulation to prevent water from getting in.

Its a nice project. It would be interesting to assist Chris and give him the information that he needs.

With kind regards,

Samer
Mike Testro
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Hi Samer

Perhaps Chris would be the best to answer your query but to my understanding the top down principle is that you cast the ground floor slab on the piling and then work both upwards to the above ground levels and downwards to the basement levels.

Normally you would excavate right down to the lowest level and then build up but it is possible to go down one level at a time and cast each slab on the grade and tie it to the piles and then excavate underneath to the next sub level.

This method is very time consuming but does help if you are trying to overcome pressure release sub clay heave.

I have only ever been involved with the method once on the Nr 1 Poultry site in the middle of the City of London in the late 80’s early 90’s.

Best regards

Mike Testro.
Samer Zawaydeh
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Dear Mike,

I am trying to imagine the Top Down approach for a slab! It must be only for the perimeter. You end up having a hole 5 stories deep, then you build it bottom up. Correct.

Yes I can imagine the construction laws are very tough.

You can’t allow for the big machinary to enter city center during the day. We were allowed to have big machinary from 8pm - 6am. So we had concrete pumps, concrete trucks, excavators, loaders, and dump truck all operating at night only.

With kind regards,

Samer
Mike Testro
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Hi Samer

Don’t forget this is bottom down working to level -5 so unless there is an open void all the way down then it is all hoist and hand labour.

In the UK working rules and traffic regulations often forbid round the clock working in city centres.

It would help if Chris told us where this project is located - within the bounds of confidentiality of course.

Best regards

Mike Testro.
Samer Zawaydeh
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Dear Chris,

In order to answer the title of the thread, the activities for reinforced concrete slab are:

1. Vertical & Horizontal Supporting Formwork
2. Bottom and side Shuttering
3. Steel Reinforcement installation
4. 1st fix MEP installation (if any)

Production Rates are site, environment, equipment (Tower and mobile cranes) and time of day dependent. So it is bit more difficult to predict.

I have been in similar situation last November. The building was divided into segments, and we were casting on average 100m3 per night. We were working 20 hours/day in one of the busiest location. Huge crashing of activities in order to meet site requirements.

The best advice would be: be prepared 24 hours a day for emergencies, always work in teams (each position have 2-3 people), and spend all you time planning ahead as per site requirements.

Hope the information helped.

With kind regards,

Samer
Mike Testro
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Hi All

This is a response I made to a private message from Chris - I thought It would be good to share it.

Hi Chris

You seem to have a better understanding of the situation than I do.

If I was looking at a rebar fixing rate where it has to be lowered and assembled and tied under floodlight where there is no chance of prefabrication then maybe a factor of 3 would be reasonable over and above the standard rebar fixing rate.

You lay the slabs between the construction joints - chequerboard style - since you are pumping down the rates should be close to normal.

As a side issue why are you even considering top down working?

Get your piling in place up to Level 0 - cast the slab and excavate underneath to Base level -5 and work upwards - meanwhile work upwards from level 0.

This was the method used by Laing at the Nr 1 Poultry site in the City - which also incorprated an aercheology examination.

Best regards

Mike Testro.
Mike Testro
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Hi Chris

If you are in London then you need to allow for pressure reduced heave on the clay belt.

Your piling will have to be sheathed to reduce friction lift.

If you are 5 levels down this will become apparent before you reach grade level.

Best regards

Mike Testro.

PS - Since you have been planning for 37.2 years you should know about these things.
Samer Zawaydeh
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Dear Chris,

I have heard or the top down method and we were going to use it on of one of the projects, but the Structural Engineer rejected it.

Basically, you have to drive piles into the ground every 1-2 meters and allow for horizontal beams to slide into them. This will have to be done along the perimeter. Once you have excavated few meters into the gound, you will have to support the excavation with the horizontal beam along the perimeter and install the diaphram. This is as per the design of the Structural and Soil Engineer. You keep doing that until you reach the bottom.

The horizontal beams along the side will keep dropping as the excavation progresses, and you will keep adding more beam from the top.

I would recommend that you read the soil report and understand well what is required. The Contractor should be the one telling you what he is planning to do. The idea is simple as described about. But the application is complicated. So you excavate and slide until you reach the Bottom of Foundation, then it is business as usual. Raft, walls and columns, then slab every month or so.

If you have more specifics, maybe we can assist more.

With kind regards,

Samer