Determination of the vertical ultimate load of a pile based on static load tests on pilot piles / test piles (prove di carico statiche di progetto su pali pilota). The ultimate load is evaluated using the hyperbolic method and the exponential curve method.

Pile toe failure / base failure  almost always occurs by punching shear, and it is not possible to derive a well-defined maximum from the curve; therefore, a conventional criterion is adopted to determine the ultimate load.

  1. The ultimate load is assumed as the load at which the head settlement is 2w0, where w0 is the settlement  when the load is equal to 0.9 Qlim.

  2. Hyperbolic interpolation technique, in which the load-settlement curve is interpolated with a hyperbola.

Assuming in fact:

Q=w/(m+n·w)

which can be rewritten as:

w/Q=m+n·w

To determine the hyperbola parameters m and n, the test points are plotted on a diagram where the settlement w is reported on the abscissa and the pile axial flexibility w/Q on the ordinate. If the hyperbola equation approximates the load-settlement curve recorded during the test, the points in the (w,w/Q)plane align on a straight line, whose y-intercept represents m, and the sloperepresents the value of n.

Once m and n, he ultimate load will be:

Qlim=Lim(n->∞)w/(m+n·w)=1/n

This approach can be used for a test that does not reach failure in order to estimate the ultimate load and thus the factor of safety.

Using the least squares method (metodo dei minimi quadrati), the coefficients m and n are obtained.

Qlim=1/n

Applying criterion 1), we obtain that:

Qlim=0.889/n

Another parameter that can be derived from the test is the load corresponding to the mobilization of the shaft resistance / skin friction of the pile:

Q=1/n·(1-((m·E·A)/L)0.5)

Where:

  • E represents the elastic modulus / Young’s modulus of the material
  • A is the cross-sectional area
  • L is the length

 

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