Thursday, November 22, 2012

Factors Influencing Pile Group Behavior.

Piles are normally constructed in groups of vertical, batter, or a combination of vertical and batter piles.  The distribution of loads applied  to a pile group are transferred nonlinearly and indeterminately  to the soil.  Interaction effects between adjacent piles  in a group lead to complex solutions.  Factors considered below affect the resistance of the pile group to movement and load transfer through the pile group to the soil.

a.  Soil modulus.   The elastic soil modulus Es and the lateral s modulus of subgrade reaction Els  relate lateral, axial, and rotational resistance of the pile-soil medium to displacements.  Water table depth and seepage pressures affect the modulus of cohesionless soil.

The modulus  of submerged sands should be reduced by the ratio of the submerged unit weight divided by the soil unit weight.

b.  Batter.   Battered piles are used in groups of at least two or more piles to increase capacity and loading resistance.  The angle of inclination should rarely exceed 20 degrees from the vertical for  normal  construction and should never exceed 26½ degrees.

Battered piles should be avoided where significant negative skin friction and downdrag forces may occur.  Batter piles should be avoided  where the structure’s foundation must respond with ductility to unusually large loads or where large seismic loads can be transferred to the structure through the foundation.

c.  Fixity.   The fixity of the pile head into the pile cap influences the loading capacity of the pile group.  Fixing the pile rather than pinning into the pile cap usually increases the lateral stiffness of the group, and the moment.  A group of f ixed piles can therefore support about  twice the lateral load at identical deflections as the pinned  group.  A fixed connection between the pile and cap is also able to transfer significant bending moment through the connection.  The minimum vertical embedment distance of the top of the pile into the cap required for achieving a fixed connection is 2B where B is the pile diameter or width.

d.  Stiffness of pile cap.   The stiffness of the pile cap will influence the distribution of structural loads to the individual piles.

The thickness of the pile cap must be at least four times the width of  an individual pile to cause a significant influence on the stiffness of the foundation (Fleming et al. 1985).  A ridgid cap can be assumed if the stiffness of the cap is 10 or more times greater than the stiffness of  the individual piles, as generally true for massive concrete caps.
A  rigid cap can usually be assumed for gravity type hydraulic structures.

e.   Nature of loading.   Static, cyclic, dynamic, and transient loads affect the ability of the pile group to resist the applied forces. Cyclic,   vibratory, or repeated static loads cause greater displacements than a sustained static load of the same magitude.

Displacements can double in some cases.

f.  Driving.   The apparent stiffness of a pile in a group may be because the density of the soil within and around a pile group can be increased by driving.  The pile group as a whole may not reflect this increased stiffness because the soil around and outside the group may not be favorably affected by driving and displacements larger than anticipated may occur.

g.   Sheet pile cutoffs.   Sheet pile cutoffs enclosing a pile group group  load capacity.  The length of the cutoff should be determined from a flow net or other seepage analysis.  The net  pressure acting on the cutoff is the sum of the unbalanced earth and water pressures caused by  the greater  than that of an isolated pile driven in cohesionless soi l may change the stress distribution in the soil and influence the cutoff.   Steel pile cutoffs should be considered in the analysis as  not totally impervious.  Flexible steel sheet piles should cause
negligible load to be transferred to the soil.  Rigid cutoffs, such  as a concrete cutoff, will transfer the unbalanced earth and water  pressures  to the structure and shall be accounted for in the  analysis of the pile group.

h.  Interaction effects.  Deep foundations where spacings between individual piles are less than six times the pile width B cause  interaction effects between adjacent piles from overlapping of stress zones in the soil, Figure 5-2 In situ soil stresses from pile loads are applied over a much lager area and extend to a greater depth leading to greater settlement.

i.  Pile spacing. Piles in a group should be spaced so that the bearing capacity of the group is optimum. The
optimum spacing for driven piles is 3 to 3.5B(vesic 1977) or 0.02L + 2.5B, where L is the embedded length of the piles(Canadian Geotechnical Society 1985). Pile spacings should be at least 2.5 B.

 Figure 5-2 Strees zones in soil supporting piles.

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