addition on pile/existing structure on strip footings

[bpcgeo]

Hello, this is my first post but I've read a lot of posts and really like this forum.

I would just like to get some feedback from this forum on additions on piles. Project: Addition to Existing fire station founded on strip footings on very loose to loose sand down to depth of about 42 ft (where dense till was encountered). Preliminary liquefaction analyses based on SPT indicates sand is susceptible to liquefaction and settlements could be in range of 300 to 400 mm.

1) If we recommend piles for addition, I'm worried about the performance of the existing building connected to the addition on piles during earthquakes. If there is an earthquake, won't the two structure behave differently and wont it make it worst that the addition is on piles?

2) If so, would recommending pilling the whole structure the only solution or is it excessive?

3) i have the same question for additions to building founded on soft clays (where major settlement cracks were observed on the existing building), Do you have experience with pilled additions? I have seen structure with soft clay consolidation settlements damages being partially underpinned (piles to rock) and have performed well but I'm always worried about performance during an earthquake. I would like feedback because I have seen some projects (where we were not involved) where additions were built on piles and existing structure most likely on superficial foundations as settlement cracks were observed. I'm wondering if our way thinking is excessive.

thanks

 

[oldestguy]

I'd be more concerned with the driving of piles causing vibration and resulting in nearby footings then settling.

There may be other alternatives, such as improving the density or settlement potential of that sand under old and then treating new areas, and no piles driven.

 

[bpcgeo]

How do you increase sand density under an existing structure? and how do you control performance of existing structure if settlements are to be expected from the increased density. Dynamic compaction and rapid impact usually stay away from existing structures not to cause settlement below existing structures.

 

[oldestguy]

Chemical injection is common and no change in sand grain configuration since voids only are filled. Limited compaction grout injection works also if you monitor the footing elevations that may be affected. Contact those specialty firms and see what they may say. They advertise in Engineering News Record now and then.

 

[oldestguy]

I just did this Google search and many showed up.

Chemical injection soil improvement

 

[oldestguy]

This link has a section view of just what you might do.

http://www.haywardbaker.com/WhatWeDo/Techniques/Grouting/ChemicalGrouting/default.aspx

 

[bpcgeo]

I did not know about injection soil improvement. Thanks for the help, this seams like a good solution.

 

[EireChch]

You are almost guaranteed to have differential settlement/movement with having two foundation types beneath the one building during an earthquake event. I have worked in Christchurch in New Zealand and this was a major issue. One common design feature is to ensure the structure is not structurally connected and that it includes a movement joint. In the event of an earthquake the piles portion should perform to with in ULS and SLS limits while the existing portion will most likely be damaged (possibly beyond repair).

Given this is a fire station and a building of high importance, there is most likely scope to future proof the structure. Low Mobility Grout columns (LMG) are one option which can be installed with minimal disruption (i.e. removing floor coverings etc) with the existing building in place, however they have divided opinion in ChCh as some people say that there isn't enough confining pressure at the surface to provide compaction. Essentially the columns expand at the surface and push the soil away causing some heave at the surface and little compaction. This was an issue for green field sites however less of an issue with retro fitting existing buildings. They are also used to re-level existing buildings which have settled. I understand that buildings which have settled by 350mm have been re-levelled to within normal building tolerances. Two companies which have good information on the topic are RE-LEVEL and URETEK.

Good Luck.

 

[Fayoub]

Consider the added weight of the chemical injections, making sure that it does not overstress the underlying soils.
I believe deep foundations such as helical piles are the only solutions here but to minimize differential settlement, you may need to drive piles to rock. If driving piles is a vibration concern, then choose mini piles.