Dynamic Equations and Cohesive Soils

[foreng]

On rural (resource) access roads in our local it is common to install pipe pile foundations for small simply supported structures (12m-21m) with a dynamic equation such as Engineering News (ENR). We are dealing mainly with cohesive soils and friction piles; the use of dynamic equations has lead to some very conservative designs. I'm advocating the use of a PDA at a minimum, but they can be hard to find in a remote location. Unfortunately re-striking is not a common practice, as the time suggested for set up (3-4 days) does not fit into the construction schedule (5-8 days for structure installation).

Any comments on the use of dynamic equations to predict the capacity of friction piles?

I've noted that the FHWA is advocating a modified Gates and Washington State has adopted their own dynamic equations, what has been the experience to date with these systems?

 

[aeoliantexan]

I worked for 25 years in a midwestern practice where we drove thousands of friction piles (pipe, concrete, timber, ,and thin shells) through weak loess or alluvium into cohesive glacial till. We ignored end bearing and designed the piles for an allowable skin fiction somewhat less than half the undrained shear strength. We counted the blows to verify at what depth the till was encountered and then drove to a calculated penetration in the till. The final driving resistance was often not much greater than the resistance 2 or 3 feet below the till surface. This practice was very succesful and was confirmed by load tests when higher capacities were used.

The practice you describe is wasting money.

 

[foreng]

Thanks for the comment; it is a real struggle trying to convince my non-technical supervisors of the value of soil sampling. It is common practice to start driving with little to no information on subsurface conditions (i.e. no information on undrained shear strength) and merely a dynamic equation to verify capacity. Again it's about cost, they would rather pay for more piling then spend the money on geotechnical investigation

I've got calls where they have 150-200 ft of pile in the ground and they're phoning to find out when they can consider it a friction pile. The only advice I could give is to get a PDA out to verify capacity or keep driving till you hit something. I was just wondering if anyone else had encountered this situation.

 

[oldestguy]

Here is a rough way to check your capacity, based on those dynamic formulas. I've used this with success many times.

When you have reached the computed depth as calculated by soil skin friction, stop driving and take a lunch break or other delay.

If you can wait longer than an hour, such as over night, fine.

Then count the blows for the first few inches, preferably the first inch. You can do this best if you attach a paper (on a board) to the pile and slowly draw a line horizontally, guided by a horizontal stick, free of the hammer effects. Allow for any changes in hammer energy, if you can.

It is very common to get a "fetch up"in soil support by waiting for excess pore pressures and other factors to stabilize. Most clays are somewhat sensitive to shearing and lose varying percentage of that strength when disturbed, to be regained later.