Water Table Levels for Basement Construction 1

[FinnB]

My projects often involve undeground basements, usually for carparks of up to 3 underground levels.

My query is that as some of you may know BS 8102 recommends that water table levels for basement design should be as follows:

For basements up to 4.0m deep the design water level should be 1.0m above the existing water table or 2/3's the depth of the basement i.e. 2/3's x 4 = 2.66m head of water.

For basements over 4.0m deep the design water level should be taken as 1.0m below ground level in all cases.

I believe these valuse to be conservative in certain situations considering soil types e.g. in sands/gravels water will not build up behind a retaining wall as could happen with impermeable clay.

There is also the issue of rising water levels which can be a problem in certain areas but if evidence is availale that water levels are not rising this may not be a problem.

Any opinions on this would be greatly appreciated as taking the water table at 1.0m below ground level for every basement means that large hydrostatic pressures will have to be designed for resulting in very deep basement floor slabs.

 

[msquared48]

What happens if your foundation drain clogs with time, as will happen?

Mike McCann
McCann Engineering

 

[FinnB]

I take your point on the foundation drain.

Take an example with a 2 level basement where the water table is 6 meter (20 feet) below ground level and the basemnt floot slab is approximately 1.0m (3 feet) below ground level. In a sand or gravel I would like to disign for a head of water of about 2.0m (6 feet). By the code I must design for a 6.0m head of water which I still think is a lot.

Would you design for a 6.0m head of water.

 

[FinnB]

Apologies yping error in the last post here it is again.

I take your point on the foundation drain.

Take an example with a 2 level basement where the water table is 6 meter (20 feet) below ground level and the basement floot slab is approximately 1.0m (3 feet) below water level. In a sand or gravel I would like to design for a head of water of about 2.0m (6 feet). By the code I must design for a 6.0m head of water which I still think is a lot.

Would you design for a 6.0m head of water?

 

[msquared48]

Regardless of the code, if your Geotech said in a written geotech report that the maximum seasonal Water Table was at a certain elevation, I would design only to that elevation unless my engineering judgment told me otherwise..

Mike McCann
McCann Engineering

 

[FinnB]

Thanks for that, this makes sense to me but the standard design in the UK and where I am in Ireland is to design for large water head even if there is a low water table. For example a two storey basement where the water table is at foundation level is requiring 3 feet deep slabs.

 

[patswfc]

Thats a new one for me finn. i've been involved in the design of several part buried water tanks and didnt know about this clause. We have designed to the max level indicated in the geo report and not at 1m below ground level. From time to time we have increased the depth when our judgement has told us so (as Mike has stated).
Ignorance WAS bliss.

It does seem over the top especially if you have a very low water table but according to the code its what should be done. I'm on an IStructE chartered membership exam prep course at the minute and we have looked through past papers. Many questions have buildings with basements some with a high water table and some not. For the ones with a water table below the basement slab, buoyancy and uplift forces on the slab has not been mentioned. I'm on the course tonight I'll ask the talkers their opinion on this and report back tomorrow.

 

[FinnB]

That's a coincidence because I'm also preparing for the IStructE part 3 and the question came up at the prep course I am attending. It was the lecturer who corrected me and pointed out I should be designing to BS 8102 clause 3.4. He asked the rest of the class do their respective offices use this clause on real projects and the answer was 90% did. I was in shock as none of the building I have worked on over the last 10 years were designed for this clause. We tended to use geotechnical and hydrogeological reports to decide on the water table level we should use.

My opinion is the code is a catch all and does not take account of known data and engineering judgement.

Any opinions you can get from collegues or at your course would be appreciated.

Good luck with the exam. I started preparing for it last week. The amouunt of work that has to be done in 7 hours is scary.

 

[patswfc]

Morning Finn,

I asked the lecturers and they say that the basement should be designed for a high water table as per 8102. I suppose that over the design life of the structure the ground water could rise significantly thus it should be designed for this worst case.

Its a good job that i asked, because as i mentioned in my first post, we have looked at some past paper questions were the water table was below the basement. The lecturers had been telling us that the water would not be an issue. They clarified last night that it wont be an issue for construction, however it is for design. Everyone on the course had interpreted their original statement as, it wouldn't be an issue for design as well as construction.

As for the exam i'm glad to here that you've just started your prep, me too. On the prep course we have been given a timetable that someone used to prepare for the exam. He started studying in December and completed 12 past paper questions (under exam conditions) in the build up to the exam. Needless to say he passed but i was getting worried that i hadnt given myself enough time.

Yeah theres a lot to do in the 7 hours, on our course we have had to prepare a solution to part 1a and present it on the course. Its amazing how fast the time went when doing this exercise.

Have you taken the exam before (its my first time), if so any tips?

Where are you from in Ireland, I'm from Antrim but am working in the North of England at the minute.

Best of luck for the exam.

 

[FinnB]

Hi Pat

I'm in Dublin.Seems that for the exam at least we should follow BS8102. I was told that the design/calcs for the exam should be for the permanent design with some references to the basement construction method.

This is my first time for the exam. I'd say a lot of people who sit the exam have enough experience to handle the questions but its really having the design information at your finger tips and being fast enough to complete the paper. I can see that doing the past papers and getting a feel for the type of issues that come up is really important. I have a lot of work to do before April 3 and hopefully it comes together.

My friend sat the paper last year and only took the Concrete Buildings Scheme Design Manual, Arups Scheme Guide and 1 level arch folder into the exam.

 

[patswfc]

(Good?) Morning Finn

How did you get on in the exam yesterday?

I did Q4 and am reasonably happy that i have passed section 1 but struggled with time in the second part. My member sizes where very big and the drawings rushed but i answered part 2e reasonably well. So am a bit nervous about getting a pass mark in section 2.

Anyway hope it went well for you and you get good news in August.

 

[FinnB]

Hi Pat

I'm not sure how I did in the exam. I did question 5 the concrete tank. I thought the question was straight foward, maybe too easy and maybe I missed something important.

I went over the time limit on the two schemes and was just finished the letter at one o'clock. The calcs I had weren't too complicated but again I went way over the time limit and didn't finish them until 3.30. I was under serious pressure doing the drawing but got 2 plans and a full section done. Drawings were very basic but fairly neat. Started the program at 4.45 and it wasn't great.

Having done the exam timing seems to be the key to passing. The 7.5 hours go by so quickly.
I have no idea whether I have passed or failed. I'm happy I answered the brief but who knows how the exams are marked.

Everybody went to the pub after the exam and most people were fairly happy with there paper but the reality is that more than half will fail. I don't know what standard has to be reached to pass but I know I could have done a better paper if I prepared better for the timing.

Best of luck with the result.

 

[FinnB]

Hi Pat

I got the result letter last Monday and thankfully I passed. I'm very releved I don't have to sit the exam again.