FUNCTION OF A PILE FOUNDATION

The primary function of a pile foundation is to transfer load to a deeper soil or rock strata.

There can be various reason that can force designers to transfer loads to deeper soil or rock strata.

For example

• The soil at shallow depth is weak, i.e., not able to withstand the load of super structure
• The soil strata at shallow depth are very compressible there by more prone to settlement of structure. More settlement means risk of cracks, instability in structures.
• A water body (like river, sea) is present where shallow foundation is not possible.
• Some time a structure is designed to carry uplift loads. For example, a tall building subject to overturning forces due to wind and wave
• Marine structures subjected to lateral load due to impact of ships and waves
• Also, situations exist when both vertical and horizontal load are expected to act on structures. For example, in case of retaining walls, bridge piers and abutments and machinery foundation.

HOW TO SELECT PROPER TYPE OF PILE FOUNDATION?

Considering all the above situation there exists different type of pile foundations to suit different needs. A designer needs to evaluate all the options in terms of execution, cost, and feasibility.

Different type of piles available for different applications with various soil types and structural requirements are

• Steel H pile
• Steel pipe pile
• Precast concrete piles
• Mandrel driven piles
• Cast in place concrete piles
• Timber piles

PRELIMINARY SELECTION OF PILE TYPE

All identified foundation alternatives should first be evaluated for suitability for the intended application and cost.

For piles, evaluation should be based on the 

• capacity, 
• availability, 
• constructability, and
• expected performance of the various types of piles.

Initial evaluation of non-pile alternatives should be based on similar criteria.

This will limit further studies to those foundation alternatives which are reasonably feasible.

During this initial evaluation, it may also be possible to eliminate from consideration obvious high-cost alternatives.

1. LOAD CAPACITY AND PILE SPACING

Of prime importance is the load carrying capacity of the piles.

In determining the capacity of a pile foundation, it is important to consider the pile spacing along with the capacity of individual piles.

The lateral load resistance of the piles may also be important since lateral loads can induce high bending stresses in a pile.

2. CONSTRUCTABILITY

The influence of anticipated subsurface and surface effects on constructability must be considered.

Piles susceptible to damage during hard driving are less likely to penetrate hard strata or gravel and boulder zones.

Soil disturbance or transmission of driving vibrations during construction may damage adjacent piles or structures.

Pile spacing and batters must be selected to prevent interference with other structural components during driving.

The ease of cutting or splicing a pile may also affect constructability.

3. PERFORMANCE

The pile foundation must perform as designed for the life of the structure.

Performance can be described in terms of structural displacements which may be just as harmful to a structure as an actual pile failure.

The load capacity should not degrade over time due to deterioration of the pile material.

4. AVAILABILITY

Piles must be available in the lengths required, or they must be spliced or cut off.

Project scheduling may make lead time an important consideration since some piles may require up to 6 months between order and delivery.

5. COST

Once a pile type satisfies all other criteria, relative cost becomes a major consideration.

For comparisons between types of piles, it may be adequate to compare the pile cost per load capacity.

A comparison between unit capacity costs may lead to an obvious exclusion of certain pile types.

The cost evaluation should include all expenses related to and dependent on the pile foundation.

Such costs may include additional expense for storage or splicing.

They may include pressure-relief systems used to reduce uplift pressures and thus control pile loads.

In addition, any required modifications to the structure to accommodate the piles should be included in a comparative cost estimate.

For example, an increase in base slab thickness may be required to provide additional embedment for the tops of the piles.

FINAL SELECTION OF PILE TYPE

The final evaluation and selection should be based mainly on relative costs of the remaining alternatives.

This evaluation should include the costs of structural or site modifications required to accommodate the foundation type.

Cost and other factors may be important in the selection.

Differences in delivery or installation schedules, levels of reliability of performance, and potential construction complications may be considered.

When comparing a pile foundation to another type of foundation, it will be necessary to develop a preliminary pile layout to determine a reasonable estimate of quantities.