Pile type comparisons

I am a new engineer trying to design a pile foundation system for a ped bridge. The soild is mainly silty sand with a water level roughly 10 feet below grade. The area is relatively high seismic zone and liquifaction is a possibility. I was hoping someone would be able to either tell me or guide me in choosing what type of pile, i.e. drilled, battered, cast in place, would be best for this type of site with regards to performance and cost. Thanks.

 

[gibsong]

Driven H-piles or precast (and likely prestressed) concrete piles are probably your best bet. Cast in place piles would be difficult to construct due to the caving of the silty sand, and shallow water table. You need to have a good idea of the depth to bedrock for end-bearing piles, and will need to account for downdrag for the soil above the liquifiable zone for designing friction piles. Some pile design programs and manuals are available for download on the internet. Try

http://www.vulcanhammer.net

for more information.

 

Thanks for the info. your reply brings up another question though. It appears from preliminary reports that the liquifaction zone may be a couple of feet. I read somewhere that there is a way to mitigate this zone through densification and other processes, but no specifics were involved. Does anyone have any ideas as to how this may be done? This is important because there is a rail line in close proximity and we are trying to make sure this liquifaction happens as little as possible. Thanks.

 

[JAE]

To mitigate liquefaction - a couple of ways - one would be using dynamic impact on the soil (somewhat costly) to densify. Another is removal/replacement if it is shallow enough - this would even, perhaps, allow use of mat or spread foundations.

In addition to driven piling, an auger-cast pile system could also be considered. This uses a hollow stem auger that drills into the soil and injects grout through the tip of the auger as it is removed - thus the sandy soil doesn't present a problem with installation.

You might want to contact local foundation contractors to see what is the regional, common, types of foundations in the area of the project. This will allow locals to bid on it and help keep costs in line.

 

Thanks for the advice JAE. One question though... there is a railroad line fairly close to the site. My boss is concerned that the vibrations induced by passing locomotives would affect the curing process of the piles. Do you know if this may present a problem in using Auger-Cast piles? AC piles were the preliminary recommendation
(as it is common in the area) but we simply feel it won't work due to the train. If you could provide some additional feedback about this, I would greatly appreciate it. thanks.

 

[JAE]

First my disclaimer - I'm not a vibration expert by any means but if the rail is simply near the site - but not directly adjacent to the piling, I wouldn't think that the higher, vibration frequency delivered by the rail would seriously affect the piling. I know its been shown in the past that even pile driving near a home cannot initiate enough energy to crack foundations, gypsum wallboard, etc.

If it is directly adjacent to the rail, the only alternative would be to use a high-early strength grout in the piling - at least it would minimize the effect of any lateral movement or disturbance - but again, if the rail has been there a while, the vibration you are talking about is high frequency that doesn't carry a lot of energy and therefore would not "move" your pile or the earth around it - rather, simply vibrate through it.

 

[DRC1]

The rail line should not have a significant effect on the curring of the concete. We have done a lot of work in rail yards and although we haven't used auger cast piles, we have poured concrete fo foundations & other types of structures. Auger cast piles in loose wet sands are not ussualy a good combination. I would recommend drilled in mini piles with a casing left in place on the upper 25 feet or so These can be installed with out impacting the railroad, both physically, as there is no vibration to settle the loose soil,and operationally, as minipile installation can be completed in a small area, where as driven piles need a lot of room. Minipiles provide a reliable pile in sands and the steel casing provides moment resistance in liquifable sands during a seismic event.

 

[ayfarm]

A cast in place pile is where u drive a can then fill it up with rebar and concrete. A driven pile is a driven steel can, HP, wood or concrete pile already casted or made. Dont even think of auger piles because of enviro probs. Depending on the demand loads, the least number of pile to install is the cheapest. Maybe a 4 ft cip pile at the middle could carry the load. The installation cost is where the budget goes.