Per the current state of practice, a pile design is typically completed by checking the following design items:

1. Static settlement which includes elastic compression of pile, immediate and consolidation settlements of soil;
2. Structural and geotechnical bearing capacity of pile in static and seismic conditions;
3. Seismic settlement under downdrag forces induced by the surrounding soil during post-liquefaction settlement;
4. Lateral capacity of piles in static and seismic conditions.

The article intends to address some drawbacks of the current practice to assess item #2 – geotechnical bearing capacity and provides an alternative approach.

I have encountered several cases where estimated pile settlements were within the design tolerances, however the pile bearing capacity, calculated using traditional methods, was not sufficient to carry the design loads. Therefore, although the estimated settlement was within tolerable ranges, I had to deepen the piles to achieve a sufficient bearing capacity. Evidently, something did NOT sound right there. Let’s elaborate on this by answering the questions below:

Question #1: why is there a discrepancy between settlement and bearing capacity analysis results? In reality, insufficient bearing capacity would result in excessive settlement.

Note that the settlement and bearing capacity analyses are performed based upon two totally different design philosophies. The main differences are:

1. Settlement is estimated based on service state design approach where service loads (un-factored) are applied. On the other hand, the bearing capacity analysis is performed based on  the ultimate limit state concept where a hypothetical failure mode is considered to estimate ultimate capacity and then a factor of safety of 2.0 and/or 3.0 is used to approximate the performance at service state. The safety factors 2.0 and 3.0 are some empirical numbers that may not be necessarily applicable to the site condition under question.
2. The input parameters for the two analysis are not even the same. We generally use soil secant stiffness modulus for settlement calculations and soil strength parameters (friction angle and cohesion) for bearing capacity calculations. Evidently, excluding soil stiffness in bearing capacity analysis implies that soil deformations have nothing to do with soil bearing capacity which is absolutely unrealistic.

Question #2: What method should be adopted as an alternative?

Strain-based analysis approach has been shown to be an appropriate alternative. In this approach settlement and bearing capacity are modeled simultaneously. The analysis inputs include both stiffness and strength parameters, and most importantly friction and end bearing capacities are mobilized as pile deforms.

The strain-based approach follows the performance-based design (PBD)  method in which the performance of the structural element is of interest. Any yielding, crack, and/or damage is acceptable as long as system performance is in accordance with design requirements. You can either use Beam on Nonlinear Winkler Foundation (BNWF) or continuum modeling approaches to simulate soil pile interaction. The spring approach (BNWF) may not be applicable if foundation soil is highly compressible because consolidation cannot be simulated by a set of springs. The continuum modeling approach would be a better simulation tool for such cases.

Please watch the presentation of James Niehoff in ASCE Deep Foundation Web Conference in August 2017. He presented a case history where piles were designed 80 feet deep with allowable capacity of 100 tons using the traditional approaches. With the aid of strain-based analysis method, the project design team was able to reduce the depth of the piles to 45 feet with allowable capacity of 200 tons. Please see below the YouTube link. James’ presentation starts at time 1:22:00 and ends at 1:50:00.

Last but not the least, note that the traditional methods were developed decades ago where there were no computational tools in any engineering offices. Engineers had to come up with some simple tools to quickly complete their designs. We, geotechnical engineers, must take advantage of recent advances in computer science and recently-developed engineering softwares in order to improve our design approaches. Sticking to old methods just because of the liability issues is not acceptable. Having said that, it is important to keep in mind this wisdom quote from Issac Newton:

“If I have seen further than others, it is by standing upon the shoulders of giants.”     

– Isaac Newton (1642-1727)