tension capacity of piles

[sfat]

does the tension capacity mentioned in soil rports includes self wt of pile also? or its just the shaft resistance given?

i have to make a stability check for my case.Piles are connected via SOG and design hydrostatic pressure is due to the water head of 4.5 meter.Dewatering is proposed untill minimum two floors are built to balance this pressure but due to uncertain conditions,i have to check that what happens if, theres only SOG built (which is connected to pile caps) and further dewatering is not done, the piles are able to withstand that uplift pressure or not?i have taken pile's tension capacity as mentioned in soil report and checked against the uplift pressure.IF SOG and Pile cap weight are accounted,then i am Ok,but FOS comes around 1.1 which is low.i was just wondering if i can add pile weight to tension capacity of pile ,then FOS will be satisfactory.Is this a right approach?please advice.

 

[oneintheeye]

i would think if precast piles yes, if jointed then maybe not. It depends on how much your relying on it I suppose. for new build I would just make cap deeper.

 

[moe333]

You need to ask the geotech. who provided the recommendations if the weight of the pile has been included in the recommended capacity. I always make it very clear in a report if it is included or not so this type of confusion does not happen.

 

[sfat]

moe333,
thanks..i agree that i should do that as this is critical for my case.
mine case is reinforced cast-in-place piles.i was just wondering also that the values of skin friction in soil report are based on the bore hole logs they got from existing ground level.my piles top has to be some 7m below that existing level.so the actual value of bearing capacity has to be confirmed thru load tests.but just for checking purpose at this stage when no pile test has been done,i should get all these factors to be discussed with the geotech person who provided the report.
Another thought is, that it make sense if pile weight is included in calculating tension capacity as pile size,length are known in case of precast piles,(as "herewegothen" said)..but for cast in place piles,tension capacity should not include self wt of pile as length of pile is uncertain factor.

 

[moe333]

You should have a good idea of the length of cast in place piles as well. Why not? You can make a conservative estimate of the diameter, and as long as you have sufficient rebar to the bottom of the pile, you should be able to count on it for tension capacity. Only use adequately reinforced length for your estimate.

 

[oneintheeye]

i would think if its that close the piling contractor would accept anyway. Perhaps reducing factor of safety from a designed say 3 to a 2.9 maybe.

 

[BAretired]

The tensile resistance of a cast-in-place pile is a very rough approximation based on incomplete knowledge of soil strata. If your factor of safety is as low as 1.1, then it is too low.

Forget about the weight of the pile. It is not significant. Increase the length of pile to provide sufficient resistance without relying on pile weight.

Best regards,

BA

 

[sfat]

agreed.weight of pile is not improving FOS much.problem is that i cant go further deeper as already i am asking for 5m embeddment in rock(total pile length 34m approx) and contractor finding it difficult already (doing it in similar adjacent projects)..have to live with this contractor and his equipment,no choice..
Problem started with the uncertainity,that what if dewatering couldnt be done and project is abandoned.
if only piles are cast,and dewatering stopped,i am safe as i can leave piles nailed in ground and project can restart any time later.
if piles caps+SOG is cast, then i am in trouble as hydrostatic pressure is developed enormously...bad time to stop a project.

Well..we now suggesting that
The best time to abandon the project is right after piling,
OR
Make partial pile caps(uplift pressure is not critical with this smaller pile cap area) ,leaving dowels for columns and cast the dowels for protection
OR
Built basements and ground floor at least + filling some sand/earth on ground floor to make FOS desirable.(ground floor has been designed for construction load).

 

[BAretired]

sfat,

I don't agree that you have to stick with a contractor who is incapable of doing the job properly. If you have determined that his equipment is unable to provide a reasonable factor of safety, then you must find another contractor.

Let's have a look at your three options.

(1) Abandoning the project after installing piles would seem like a foolish expense, but would not produce uplift problems.

(2) Building pile caps will not cause uplift problems because the water will not be confined, i.e. water level can rise above the pile caps.

(3) You could build the ground floor, leaving openings for the water to seek its level above the floor. It would happen only if de-watering cannot be achieved and if that is the case, the project will be abandoned anyway.

It seems to me that you must make certain that de-watering can be achieved before proceeding with the project.

You say that ground floor has been designed for construction load. I hope it has also been designed for uplift pressure because otherwise, the slab will fail as a result of upward pressure, no matter how many stories are built above it.

Best regards,

BA

 

[STVU]

I can't recall the paper right now, but I use 35 to 42% of downward capacity as a maximum tension capacity, ignoring the weight of the pile. Look at chapter 5 of NAVFAC Design Manual 7.02 for more details.

 

[sfat]

BAretired,

regarding your 1st comment;
abandonig project after piling is not foolish expense,cuz partial piling has already been done and the project has to be stopped right now for some reason but it HAS to be completed sometime later.leaving piles in ground is not a bad choice.
as for comment 2;
yes,water level will rise above pile caps.thats OK if i protect dowels of coulmns for that situation.
as for point 3;
my building profile is like this

------- podium level 2

------- podium level 1

------- Ground floor,0m
--NGL,-1.0m
--Water table,-3m
------- basement-1

------- SOG,foundation level(basement-2)

SOG is the slab critical for uplift pressure.and it is designed for this pressure.If building is built upto ground floor ,i just have to worry about overall foundation failure.structure is already confined with boundry retaining walls ,and water table is 3m below ground floor.so ground floor slab design itself is not critical from uplift point of view.

 

[BAretired]

The slab on grade (SOG) must be heavily reinforced to resist such a large upward pressure. Two further possibilities come to mind:

(1) Increase the thickness of SOG to get additional weight and at the same time decrease the reinforcement because of larger effective depth.

(2) Use additional piles at midspan to help resist uplift and reduce the span of the SOG.

Best regards,

BA

 

[sfat]

Thanks BA,
points noted.