Foundation System for Poor Ground Condition?

[28620921]

1. Ground Condition

I have a site for building a few nos. of houses of rein-concrete and 3 storeys high of which the lowest storey is a carport.

Previous ground investigation showed the site comprises fill of approx. 2.5m thk. overlying 7m of Alluvium, then a varying depth of Completely to Highly Decomposed Tuff (C/HDT) and Moderately to Slightly Decomposed Tuff (M/SDT). Grade III bedrock was encountered at the levels ranging from 21m to 50m below existing ground.


2. Founding Criteria for Raft Footing

In view of the nature of the top layer of Alluvium, it is considered that the footings founded at shallow depth will induce large settlement and angular distortion. Therefore, it is suggested to expose dense stratum for the founding layer of the raft footing.

Also, the design maximum bearing pressure of the raft footing for the structures is about 150kPa with reference to the local Code of Practice of Foundations. By reviewing the general soil conditions of this site, the engineer advised if raft footing construction is adopted, the estimated founding level is about 4m deep from the existing ground.

Alternatively, the engineer also recommended to use piling such as mini-piles due to the saving of extensive shoring works for bulk excavation and is less susceptible to unveiled ground conditions and adverse weather in particular we are at tropical Asian areas with rainy season.

3. The essence of time is also part of our consideration, what would you advise to consider raft footing vs. piling?

 

[jdonville]

28620921,

Time to start talking to local contractors to suss out cost and time for each type of foundation.

Jeff

 

[asiandude]

Hi there 28620921,

I believe that what your facing is queit a common problem that engineers usually face especially when at the end of the day it all boils down to cost.

But I would like you to consider the practical view at stake here. First of all you have a fill layer which is founded on what I would say is compressible soil or in simple terms soil conditions which are bound to have settlement related problems. Having known this fact I would say that the best answer for your problem is to construct your foundation on piles rather than raft foundation.

As you mentioned previously, if you go for a raft foundation approach you will have to construct your foundation's formation level approximately 4.0m deep which will add on other related matters to dwell on. Its like what I mentioned above, lets just be practical about it and get back to basics:- Go for the piles.It is faster, less hassle and easier to construct.

HOWEVER, please do take into account of negative skin friction along the alluvial deposits. When choosing the type of pile to use make sure that the structural pile capacity can take the load of your structure and at the same time the load coming from the negative skin friction effect. There is no straight forward answer how much load is coming from the negative skin friction but I would say that 75% of the compressible material thickness will cling on to the pile and add up as the negative skin friction load.

Do some further checking on the pile concept that you are looking at. Is it end bearing? Is it skin friction? The choice you make may effect your total cost as your site has an uneven rock level.

Mmmmm other than that, all structural elements must be suspended.

Hope you will think about it and have fun designing!

 

[BigH]

Where is the groundwater level? How compact or dense is your alluvium - and what is it (silty sand, sandy silt,)? Knowing these parameters will help in devising suitable foundation system.

 

[ShawnR]

Micro piles...maybe. Why not cast in sitiu piling? You would be able to install a much smaller # of piling although the grout costs will be higher than micr but cheaper than driven.

 

[Zambo]

I would expect precast concrete piles to be cheaper than cast-insitu. Depnding on the loads the size could be 250mm x 250mm or 300mm x 300mm. Driving to refusal at 21m would be no problem, for the 50m areas you should check with the piling contractor whether he normally drives such lengths or whether you should redesign as friction piles.

 

[DRC1]

A more ecomical option is to drive timber piles into the tuff.

 

[Patgeotech]

This thread just reminded of a similiar job, you say that for a raft footing founding is 4m below grond level.

The idea we have used on other jobs concentrated on improving the upper fill layers by either rapid impact compaction or remove the fill and recompact in engineered layers and then found shallow - the engineered soil layer will provide a 'soil raft' for your foundations and meet the 150 kPa bearing capacity requirement.

Hope this is not too late for you!