Design Considerations for Pile Foundation

The pile foundation analysis is based upon several simplifying assumptions which affect the accuracy of the results. The computed results must always be reviewed with engineering judgement by the design engineer to assure that the values are reasonable. Also, the analysis results should be compared with load test results.

The designer, who is responsible for designing a pile foundation, should and must consider the following 5 factors before arriving at any decision regarding design of pile foundation.

1. Functional Significance of Structure

• The type, purpose, and function of the structure affect decisions regarding subsurface investigation programs, analytical methods, construction procedures and inspection, and performance monitoring.
• Generally, the proposed structure should be evaluated on the basis of the consequences of failure, that is, the potential for loss of lives and property, economic losses both local and national, compromising the national defense, and adverse public opinion. The designer must be aware of these factors so that a rational approach may be taken throughout the analysis, design, and construction of the project.
• In order to reduce the potential for failure, as well as to minimize the cost, the designer must apply appropriate factors of safety to the design. These factors of safety are based on the functional significance of the structure, the level of confidence in the foundation parameters, the adequacy of the analysis tools, and the level of construction controls.

2. Definitions of Failure

• Structure or foundation failures can be categorized as an actual collapse or a functional failure.
• Functional failure can be due to excessive deflection, unacceptable differential movements, excessive vibration, and premature deterioration due to environmental factors.
• For critical structures, failure to meet functional requirements may be as serious as the actual collapse of a structure. Therefore, designers should be cognizant not only of the degree of safety against collapse but also of effects of settlement and vibration on the functional performance.

3. Factors of Safety

• Factors of safety represent reserve capacity which a foundation or structure has against collapse for a given set of loads and design conditions. Uncertain design parameters and loads require a higher factor of safety than required when the design parameters are well known.
• For most hydraulic structures, designers should have a high level of confidence in the soil and pile parameters and the analysis. Therefore, uncertainty in the analysis and design parameters should be minimized rather than requiring a high factor of safety.
• For less significant structures, it is permissible to use larger factors of safety if it is not economical to reduce the uncertainty in the analysis and design by performing additional studies, testing, etc. Also, factors of safety must be selected to assure satisfactory performance for service conditions. Failure of critical components to perform as expected can be as detrimental as an actual collapse.
• Therefore, it is imperative that in choosing a design approach, the designer consider the functional significance of the project, the degree of uncertainty in the design parameters and the analytical approach, and the probability of failure due to both collapse and functional inadequacy.

4. Soil-Structure Considerations for Analysis

• The functional significance and economic considerations of the structure will determine the type and degree of the foundation exploration and testing program, the pile test program, the settlement and seepage analyses, and the analytical models for the pile and structure.
• For critical structures the foundation testing program should clearly define the necessary parameters for the design of the pile foundation, such as soil types and profiles, soil strengths, etc.
• Although pile load tests are usually expensive and time consuming, they are invaluable for confirming or modifying a pile foundation design during the construction phase. A well planned and monitored pile load test program will usually save money by allowing the designer to utilize a lower factor of safety or by modifying the required number or length of piles required.
• A pile load test program should be considered for all large structures for which a pile foundation is required.
• Depending upon the type of foundation material, the nature of the loading, the location of the ground water, and the functional requirements of the structure, a detailed seepage analysis and/or pile settlement analysis may also be required to define adequately the pile-soil load transfer mechanism and the resulting parameters necessary for an adequate pile design.
• Where differential movement between monoliths is a concern, an accurate estimate of pile settlement may be crucial, particularly if the monoliths have significantly different load levels.
• It may be satisfactory to analyze the pile foundation for a small, lightly loaded structure based on conservative assumptions for pile parameters and a crude structural model; however, a larger, more important structure would probably require a detailed single pile analysis to establish the proper pile parameters. Perhaps it would even be necessary to use a structural model capable of considering the actual structural stiffness to insure correct load distribution to the piles.

5. Construction and Service Considerations

• No matter how thorough and well researched a design may be, it is only as good as its execution in the field. The proof of the entire design and construction process is in the performance of the final product under service conditions. Therefore, the designer should consider the analysis and design of a structure and its foundation as parts of an engineering process that culminates with the successful long-term performance of the structure for its intended purposes.
• The designer prepares the specifications and instructions for field personnel to assure the proper execution of the design.
• The designer must discuss critical aspects of the design with construction personnel to make sure that there is a thorough understanding of important design features.
• For critical structures a representative of the design office should be present in the field on a continuous basis. One such example would be a major pile test program where the execution of the pile test and the gathering of data are critical for both a successful testing program and verification of design assumptions.
• Another critical activity that requires close cooperation between the field and the designer is the installation of the foundation piling. The designer should be involved in this phase to the extent necessary to be confident that the design is being properly executed in the field.
• As a general principle, designers should make frequent visits to the construction site not only to ensure that the design intent is being fulfilled but also to familiarize themselves with construction procedures and problems to improve on future designs and complete as-built records.
• Once the project is in operation, the designer should obtain feedback on how well the structure is fulfilling its operational purposes. This may require that instrumentation be a part of the design or may take the form of feedback from operating personnel and periodic inspections.