Straight Shaft Drilled Piers

[fargofarmer]

Hi,

When we are designing Straight shaft drilled piers especially for light light loaded structures, what parameters we need to consider other than skin friction and axial load? Is there any standard limit for minimum length of the drilled pier? Can some one clarify my doubt.

Thanks

 

[BigH]

** lateral loads
** end-bearing if rock or till is close
** uplift loads - especially if in areas of frost heave and the depth of the drilled shaft is not much (may require sleeving through frost zone)
** some would argue that the depth to diameter must be greater than 5 or else it is a shallow foundation.
** "no support" if upper zone is peat or very soft clay
** support for equipment to install (drilling pad) if upper zone is peat, other paludal deposits or very soft clay.
** nature of the soils - in determining the "skin friction" or adhesion (see Tomlinson's alpha coefficient or Srinivasin/Focht's lamba coefficient).
** serviceability limit - i.e., amount of settlement or lateral deflection that is permitted by the structure without damage or loss of service
** probably others but some of our colleagues can add to the list.

 

[STVU]

Minimum practical depth is about 20 ft. As a rule of thumb, 1 ft diameter, 20 ft deep in medium dense sand or very stiff clay gives 20 to 25 ton capacity(no groundwater).

 

[Drumchaser]

standard length limit? I am not aware of any.

Your design should be based on your boring logs-pen test data along with the reqt.s of what you will be supporting.

The last several years I have seen an asterisk and note on some Bridge layout/ shaft layout ..."Shafts shall penetrate a minimum of 2 diameters into hard blue shale" (or other matl.s as req'd) in other words no design is bullet proof if the folks on the ground are not well versed in what they are doing and paying attention to materials.
If your boring logs reflect a water table elev.- have it shown in the plans...this may change the methods by which the work will be accomplished.

 

[fargofarmer]

Thank you for your response and suggestions. The structure is a 2-story wood frame residential building. The Front side of the lot has firm residual soils (N>20) below 4 to 5 feet below the ground surface at the front side and N value is 50 at apprxoimately 12 to 15 feet below the groundsurface at the rear of the lot. The material above the frim residual soils is uncontrolled fill consisting of debris. I am going to negelect skin friction for this protionof the soil. Somebody told me that we can design the straight shafts even for lenghts between 5 to 10 feet? Based on my understanding, deep foundations should be longer than 10 feet.

Thanks

 

[Ron]

Wouldn't it be better to remove the debris laden soil and bring in select fill and compact it? The expense of the shafts, grade beams and reinforced slab are likely greater than filling and compacting to put a slab on grade. Do you know that the debris is pervasive?

 

[fargofarmer]

You are right. The onsite geotechnical engineer recommended to replace the soil or support the structure on mini caissons. The client wants to use piers/mini caissons and grade beams. I do not know if the fill is pervisive or not?

 

[csd72]

Have you considered screw piles, they would be much cheaper.

 

[lovethecold]

Have you checked on the price of a mini caisson vs. excavate and replace vs. helical piers? It seems to me that excavating and replacing would be the cheaper option in this case.

 

[fargofarmer]

you are right lovethecold. There is groud water table at 12 feet below the ground surface. The client doesn't want to replace the exsisting fill material. The other option is only helical piers. Anyhow thanks for your suggestion.

 

[DRC1]

Would a footing on the firm residual soil and a frost wall make sense? Easy to build lots of contractors to bid, very straigt forward.

 

[fargofarmer]

What is a frost wall?

 

[worldofmud]

given the near surface uncontrolled fill, the need of temporary liners may come into play-this will probably slow the job and 'up' the price, as well as the groundwater table and potential need to tremie the grey stuff.

 

[DRC1]

Sorry, been traveling, went to IFCEE '09 great show & conference.
A frost wall is simply a wall that takes a continous wall footing below frost level, or in your case down to the N=20 soils.