The purpose of a pile foundation is to transfer and distribute load through a material or stratum with inadequate bearing, sliding or uplift capacity to a firmer stratum that is capable of supporting the load without detrimental displacement.
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 type, purpose, and function of the structure affect decisions regarding;
Subsurface Investigation Programs
Construction Procedures and Inspection
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.
Structure or foundation failures can be categorized as:
Functional failure can be due to
Unacceptable Differential Movements
Premature Deterioration due to Environmental Factors
For critical structures, failure to meet functional requirements may be as serious as the actual collapse of a lesser structure. The designers should be aware of the degree of safety against collapse but also of effects of settlement and vibration on the functional performance.
Foundation Testing Program should clearly define soil types and profiles, soil strengths etc.
A well-planned and monitored Pile Load Test Program will allow the designer to utilize a lower FOS or by modifying the required number or length of piles 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.
The designer prepares the specifications and instructions for field personnel to assure the proper execution of the design.
For critical structures a representative of the design office should be present in the field on a continuous basis.
Designers should make frequent visits to the construction site not only to ensure that the design objective is being fulfilled but also to familiarize themselves with construction procedures and problems to improve on future designs.
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 or may be from operating personnel and periodic inspections.
Where gravels or cobbles are expected, some large diameter soil borings should be made in order to collect representative samples to determine their properties.
An accurate location of the soil borings should be made in the field and a map provided in the design
An engineering geologist should be present during the drilling operation to provide field interpretation.
Geologic interpretations should be provided in the design documents in the form of geologic maps and/or profiles. The profiles should extend from the ground surface to well below the deepest foundation element. The accompanying text and/or maps should fully explain the stratigraphy of the subgrade as well as its engineering geology characteristics.
Laboratory Determinations of the shear strength and consolidation properties of the clay should be performed.
For the construction case in clay, UU triaxial shear (Q) test should be performed.
In silts, CU triaxial shear (R) test, with pore pressure recorded, should be performed In sands if samples can be collected, CD triaxial shear test or direct shear test (S) should be used to predict the shear strength.
The sensitivity of these soils should be estimated and the appropriate remolded triaxial shear test performed
Shrink-swell tests is performed, if appropriate
Consolidation tests should be performed throughout the profile so that settlement of the structure may be estimated.
The groundwater should be evaluated in each of the soil borings during the field investigation.
Piezometers and/or monitoring wells should be installed and monitored during the various weather cycles.
A determination should be made of all of the groundwater environments beneath the structure, i.e. perched water tables, artesian conditions and deep aquifers, excess pore water pressure
For the design of pile foundations the highest groundwater table elevation should prove to be the worst case for analysis of pile capacity. However, significant lowering of the water table during construction may cause installation and later service problems by inducing densification or consolidation.
Liquefaction is most commonly induced by seismic loading and rarely by vibrations due to pile driving during construction or from vibrations occurring during operations.
If soils in the foundation or surrounding area are subject to liquefaction, the removal or densification of the liquefiable material should be considered, along with alternative foundation designs.
The first few natural frequencies of the structure-foundation system should be evaluated and compared to the operating frequencies to assure that resonance (not associated with liquefaction) is not induced.
Skin Friction and End Bearing are dependent on the subsurface conditions and soil type.
In soil deposits that contain layers of varying stiffness, the ultimate axial pile capacity cannot be equal to the sum of the peak strength of all the materials in contact with the pile because the peak strengths are not reached simultaneously. Failure is likely to be progressive.
The existence of boulders or cobbles within foundation layers can present driving problems and hinder determination of ultimate axial capacity of a single pile.
The pile load test is intended to validate the computed capacity and to provide information for the improvement of design. Therefore, a test to soil/ pile failure should be conducted in lieu of testing to a specified load of termination.
Data from a test should not be used to lengthen or shorten piles to an extent that their new capacities vary more than 10% from the test load.
Shortening the service piles may place the tips above a firm bearing stratum into a soft clay layer. Then Consolidation and loss in BC may occur.
Extending tips deeper into a firmer bearing may cause driving problems.
At the end of test, the test piles should be extracted and inspected
Instrumentation used during Pile Load Tests to obtain Design Data
Pile Driving Analyzer used to control Quality of Pile Installation
Permanent Instrumentation used to gather Information during the Service Life
The designer should select instrumentation that has sufficient accuracy to measure the required data.
Permanent instrumentation is used to gather data relating to the state of stress and behavior of the pile under service load conditions, the behavior of a particular pile foundation and about analysis and design assumptions in general.