Econmic Design? 1

[normm]

I find that designs produced by Civil / Structural Engineers are more and more being scrutinised and commented on by project engineers / value engineers etc. I give an example below:

In structures under light axial load and high uni-directional moment like retaining walls where piling is appropriate, I have always provided 2 rows of piles - one for tension and one for compression. See attached Fig 1.

Now I am being asked why not do away with compression pile and provide tension pile only. Compression being resisted by bearing on soil. See attached Fig 2.

I have never seen this single row pile design before and instinctively resist it. But I will be interested if any of you have come across this proposal on grounds of economy?

 

[asixth]

Engineers should be very wary of the economy of their designs because the major factor which the client is concerned with is cost.

Is the retaining wall subject to high surchage loads and what are the typical retaining heights. In my part of the world, we would try to get away with no piles and size the footing to resist overturning.

 

[normm]

Thanks asixth.

The height of the wall about 7-8 m. Surcharge 10 kN/m2.

Whether to use piles or not depends on soil condition below the base. If soil is poor, you will have to pile.

 

[bootlegend]

If you have poor soil how are you even going to be able to handle the compression bearing? I would think that if you can handle the bearing pressure surely you can adjust the heel length and thickness to prevent overturning.

I don't know much about statistics, but I do know that if something has a 50-50 chance of going wrong, 9 times out of 10 it will.

 

[asixth]

The retaining dimensions and surcharge loading are significantly greater than what I normally design. I assume it is a highway structure?

 

[normm]

Asixth you are correct it is a highway structure and there is some site restriction on how far the heel can project.

It seems that you will never reccommend a piled base retaining wall.

 

[ash060]

In the building code down here they specifically say that a foundation has to be pile supported or soil supported. You can not have a hybrid foundation.

 

[Qshake]

Another item to consider is the rigid body transformation of forces that are used to give compression and tension pile. With out one of the two the other is moot.

As an example, if your soil, upon which the toe of the footing rest, settles, then all that will happen is a simple rotation of the footing about the supposed tension pile. The tension pile will never see the actual vertical tension force it is deigned for. Ultimately the wall will fail.

Poor soil conditions and the adverse effects of them are the reason pile foundations are used with highway agencies. Sure they could, in some situations, use different foudnations but the issue of settlement and the problems emanating from that are often costly once traffic is allowed on the facility that no one wants to get back in there to fix anything.

Often times those espousing alternates and value engineering don't really understand the principles of engineering to begin with.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[BAretired]

If the soil is poor at the underside of footing, two rows of piles as shown in Fig. 1 are necessary. The passive pressure against the footing must be capable of resisting horizontal forces. If not, battered piles may be needed.

BA

 

[mijowe]

When i have desinged retaining walls on piles the horizontal pressure was what goverened, ie i did need to batter the piles, and the pile spacing was goverend by the lateral, not the uplift.

Are permanant tiebacks a possibility?