Pre-Cast Foundation Pads for small structures 2

[richg1]

Hi,

I am trying to find technical design information on the use of pre-cast, ie above ground, concrete pads to support small lightly loaded structures. Something similar to temporary road barriers.

Of my concerns with employing such a structural foundation is:
1) soil settlement
2) soil movement due to shrinkage and swelling

I was also interested in whether anyone was aware of soil nails/anchors used in such an application to improve load capacity and reduce the impacts of the factors mentioned above.

Cheers Rich

 

[MiketheEngineer]

Check with your local code authority. Usually anything over 100 sq ft has to have a foundation.....

 

[lovethecold]

Your concerns of soil settlement and movement will have to be dealt with by a local geotechnical engineer. They would do soil borings and be able to give you the bearing capacity and potential settlement. Is the stucture sensitive to movement at all? This would impact the recommendations. Also, how temporary will the structures be, and what do you mean by lightly loaded. Lightly loaded can mean different things in different parts of the country. Generally, bearing capacity will be a function of the size of the foundation. And the size of the foundation will be a function of the weight of the structure and allowable settlement. Either settlement of bearing capacity will control the size of the foundation, soil borings and the structural information will determine which.

Just out of curiosity, what type of road barrier uses concrete pads?

Hope this helps.

 

[richg1]

To explain further...

By lightly loaded I meant something similar to a pole mounted street sign, only with a larger swept area and therefore higher wind loads.

Preliminary design for such an above ground pad estimates a requirement of a square pre-cast pad ~700x700mm and ~150mm thick. Giving ~175kg of weight to support the attached structure.

Current geotech for the area of concern indicates potential seasonal movement of up to ~40mm, which for the application is of concern, as this is not a temporary installation, and this amount of movement may affect other aspects of the design.

In my limited knowledge of soil mechanics I understand that the first ~200mm of soil should be regarded as providing no bearing support for applied forces.

I am concerned that in addition to ground movement the structure will 'sink' over time. And so was interested in how to determine this?

And with respect to resisting environmental loads (ie wind) I was interested in possibly incorporating moulded-in stakes (i am currently calling them soil nails) to the pre-cast pad. These would be driven into the ground on installation beyond the ~200m top soil depth, primarily to reduce the amount of concrete required and size of the pad to improve on shipping costs etc.

At this stage looking for any similar applications and to expand my knowledge in this area...

Thanks again...Rich

 

[civilperson]

Transformers are placed on pads that are on gap graded gravel to minimize frost heaze potential. Any movement is easily tolerated since flexible conductors connect to the bushings.

 

[richg1]

thx civilperson...I found a few more potential solutions for supporting 'very' small structures...

In addition to the small pre-cast pad idea I was also pursuing screw piles / auger anchors? Possibly to be used in conjuction with a precast pad to increase load capacity. But again have no references to designing and utilising these products. Do these have to conform to any standards etc?

Rich

 

[lovethecold]

Check out this website for more info on helical piers. My guess is that this is what you may be refering to. These can be quick and easy to put in. Installed properly (ie below the frost line) they should be relatively resistance to movement from frost.

http://www.abchance.com/

As to the top 200mm, my guess is that is the typical thickness of topsoil in your area, this should be removed from below structural slabs and footings anyway.

Good luck.

 

[wancho]

Here's another product site to look at:

http://www.pinfoundations.com/

It may be sutible to your application.

Steve

 

[richg1]

Thx LTC and Wancho...good links.

Abchance had some really good info and that pre-cast pin foundation is very interesting.

I passed some interlocked road barriers in the street the other day. Another potential solution that I am considering...

http://www.t-lok.com/

Rich