General Design Methodology for Deep Foundations.

A single drilled shaft or a group of driven piles is typically designed to support a column load.  The number of driven mpiles in a group is determined by dividing the column load by  the design load of a single pile.  The piles should be arranged in the group to provide a spacing of about three to four times the pile diameter B up to 6B.  The diameter of the piles may be increased to reduce the size of the pile cap if appropriate. Table 1-1 describes a general design methodology.   Other design methodology aspects are the following:

a.  Load factor design.  This publication applies load factors for design (LFD) of the structural capacity of deep foundations.  The sum of the factored loads shall not exceed the structural resistance and the soil resistance.  The LFD, the  structural resistance, and the soil resistance are all related to the load factors as follows:

(1)  Definition.  The LFD may be defined as a concept which recognizes that the different types i of loads Qi that are i applied to a structure have varied probabilities of occurence. Examples of types of loads applied to a structure include the live load QFF , dead load QDL , wind load QWL , and earthquake load QEL. The probability of occurrence of each load is accounted for by multiplying each Qi by a load factor Fi > 1.0.  The value of Fi depends on the uncertainty of the load.

(2)  Structural resistance.  The sum of the factored loads shall be less than the design strength

(3) Soil resistance. The sum of the factored loads shall be less than the ability of the soil to resist the loads, This evaluation may be determined by factors of safety (FS) or by load factors. Factors of safety are often empirical values based on past experience and may lead to a more conservative design than the LFD concept. The FS and the LFD are presented as:

(a) Global FS. The allowable load may be evaluated with global FS

The approach taken throughout this publication is to select a global FS for analysis of soil resistance rather than partial FS or load factors.

(b) Load factor design. Analysis of soil resistance may also be determined by Ilie LFD concept using performance factors

where Φpfq  = performance facto appropriate to the ultimate pile capacity. Performance factors Φpfq depend on the method of evaluating Qu and the type of the soil resistance, whether end bearing, skin friction, uplift or a group capcity. Values for Φpfq and example of load factor analysis are avaible in National Coorperative Highwaw Research Program Report No. 343 " Manuals for the Design of Bridge Foundations". Load factors and factors of safety taken in combination can lead to an  uneconomical foundation can lead t an uneconomical foundation design.

b. Unusual situations. Consideration should be given to obtaining the services and advice of specialists and consultants in foundation design where conditions are unusual or unfamiliar or structures are economically significant. Sone unusual situations for deep foundations, discussed below, include expansivc clay, underconsolidated soil, and coral sands.

(1) Expansive clay. The swell of expansive clay can cause an uplift force on the perimeter area of deep foundations that can force the foundation to move up and damage the structure connected to the deep foundation.

(2) Underconsolidated soil. The settlement of underconsolidated soil can cause negative skin friction on the perimeter area of the deep foundation that can increase the end-bearing load, which results in an mercase in settlement of the foundation.

(3) Coral sands. Piles in coral sands may indicate low penetration resistance dunng driving and an apparent low bearing capacity, but the penetration resistance often increases over time as a result of the dissipation of excess pore pressure. Driving of piles into cemented, calcareous sands can crush the soil and lower the lateral stress, which results in a low value for skin friction and bearing capacity.

c. Computer program assistance. Design of a deep foundation is nonnally accomplished with the assistance of several computer programs.

 Table 1-1 General Design Methodology for Deep Foundations.

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