How can you ensure accurate and reliable pile capacity analyses?

Pile foundation is mostly necessitated when upper layers of soil are weak and unable to support superstructure on a shallow foundation. Therefore a detailed geotech investigation incorporating collection of soil samples from varying soil strata encountered in subsurface is essentially required. Initial theoretical pile design requires calculation of pile theoretical capacity based on parameters of all layers of soil/rock uptil required depth of pile. Also as pile design is complex extensive methods are available for pile design , therefore, Geotech designer will be in a better position to design pile foundation if a detailed geotech investigation and elaborate testing in laboratory is performed.

In calcarious sands, coralline limestones, calcarenite and calcarious sandstones, you must be super conservative in your design. We use a safety factor of 3 instead of 1.5 or 2 for skin friction and FS= 6 instead of 3 for tip resistance. ( Poulos et al ).

Based on my practical experience there is variation among almost all the theoretical methods published in literature regarding pile design. No single method covers all the parameters of soil/rock in order to calculate ultimate capacity of pile. Therefore based on type of pile i.e bored, driven etc , a geotech designer should select pile capacity calculation with caution. Best practise is to calculate pile theoretical capacity based on 2-3 recommended methods in published literature and take their average . This being recommended as final pile design will be governed by field Pile load test anyways any any discrepancy in theoretical design will be improved to a greater extent in field load test.

Knowledge of magnitude and direction of application of pile load will have a direct impact on the geometry , length , settlement and ultimate capacity calculation of pile foundation.

Based on my practical experience analysis on a number of pile design softwares, these should be used for pile design with caution as each one has 1-2 theoretical method /equation at backend and may not cater all complexities of soil-pile interaction. Finite element analysis software such as PLAXIS is a good tool for simulating soil-pile analysis but it also requires expertise in modelling , input of correct soil parameters and most importantly defining the failure criteria for calculation of pile ultimate capacity which is variable as per various codes and published research. Software cannot replace the field load testing , however it can be utilised for pile preliminary design and also comparison of results after load testing.

Final pile design is based on interpretation of load - settlement results obtained from Pile Load Test. A test pile is constructed in field at construction site based on theoretical design. Afterwards test pile is subjected to Pile Load Test in order to obtain load settlement curve for final pile design.Expertise in application of suitable and reliable load-settlement curve interpretation methods will govern the reliability of pile design. Knowledge and practise of interpretation methods beforehand is essential in order to perform a reliable pile analysis.

Pile tests. The best pile test method is the O- Cell , by which you do not need to build a heavy super structure plus three piles. Only one is enough. Pile tests are very important if the piles do not reach and go into the rocky bedrock, but are rather completely friction piles.

The Bi-directional load test method. The procedures used for this will in general accordance with ASTM D-8169/D-8169M, “Standard Test Methods for Deep Foundations Under Bi-Directional Static Axial Compressive Load.”