Lateral load resistance of pile by various approaches and uplift capacity of pile or anchor.

Submitted by

2K13/STE/22,25,26

M. Tech(Str.)Department of Civil EngineeringDelhi Technological University

What is meant by load carrying capacity of the pile foundation?

Load carrying capacity of the pile in the context of foundation engineering:

The amount of load the pile can carry without undergoing continuous displacements for insignificant load increments by virtue of its boundary condition (soil condition) and not by virtue of its structural strength.

The assumption for this definition is - the failure of surrounding soil occurs prior to the failure of the pile material especially in the case of concrete piles.

PILE CAPACITIES

What are the various capacities of pile commonly used in practice?

Axial capacity

Lateral capacity

Pullout capacity or Tension capacity

The lateral capacity of a pile is usually defined as the

load corresponding to a specified deflection of pile head from its plumb. The amount of this deflection is usually suggested by the local codes based on the structure(s) for which the pile foundation is designed.

A vertical pile resists lateral load by mobilizing passive pressure in the soil surrounding it.

Causes of lateral forces: For tall buildings and transmission towers, wind action is the primary cause.In the case of bridge abutments and piers, horizontal forces are caused due to traffic and wind movement. Dams and lock structures have to withstand water. Pressures which transfer as horizontal forces on the supporting piles. Defense structures often have to withstand blasts that cause lateral forces. In the case of earth retaining structures, the primary role of piles is to resist lateral forces caused due to the lateral. Pressures exerted by the soil mass behind the retaining wall. Sometimes, piles are installed into slopes, where slow ground movements are taking place, in order to arrest the movement. In such cases, the piles are subjected to lateral forces The horizontal shaking of the ground during earthquakes generates lateral forces that the piles Impact Loads from Ships Eccentric Loads on Columns

LOAD CARRYING MECHANISMS OF PILES End bearing cum friction piles carry vertical compressive loads partly by means of resistance offered by the hard stratum at the tip of the pile and partly by the friction developed between the pile shaft and soil.

Pure friction piles carry the major part of loads only by means of friction developed between pile shaft and soil; and pure End bearing piles only by means of bearing resistance at the tip of the pile.

In both the above cases lateral loads are carried by the lateral resistance offered by the surrounding soil.

Lateral Capacity of Piles

Piles are subjected to lateral loads in addition to axial loads However for simplicity a pile subjected to only lateral load is usually studied for analytical convenience. Unlike axial capacity, the determination of lateral capacity of the pile is a complex problem. The lateral capacity of piles tested in the field is dictated by the lateral deflection criteria of local codes

The laterally loaded pile unlike an axially loaded pile is a three dimensional problem. In case of circular pile, the problem can be analyzed as two-dimensional due to symmetry. A laterally loaded pile can deflect in any direction depending on the direction of the lateral load.

Load carrying mechanism of pile subjected to Lateral Loading and Moment

Y

X

Figure 7

Categories of laterally loaded piles

Laterally loaded piles are divided into two categories based on variation of deflection, shear and moment, as shown below

Rigid pile Figure 14Flexible pile Figure 15

Load Transfer MechanismsIn the case of lateral loads, piles behave as transversely loaded beams. They transfer lateral load to the surrounding soil mass by using the lateral resistance of soilWhen a pile is loaded laterally, a part or whole of the pile tries to shift horizontally in the direction of the applied load, causing bending, rotation or translation of the pile. The pile presses against the soil in front of it (the soil mass lying in the direction of the applied load), generating compressive and shear stresses and strains in the soil that offers resistance to the pile movement

Interaction between piles in the case of laterally loaded pile groups In a laterally loaded pile group, each pile pushes the soil in front of it (in the direction of the applied force). Movement of the piles placed in the first (leading) row in the direction of the applied force is resisted by the soil in front of it. In contrast, the piles in the rows behind the first row (the piles in the trailing rows) push on the soil which in turn pushed on the piles in the rows in front of them. The resistive forces acting on the trailing-row piles are in general less than the resistive forces acting on the leading row.

Analytical Methods for Lateral LoadingThe simplified methods proposed by Meyerhof et al. (1988) and Patra & Pise (2001) has been briefly reviewed in this section. These methods are approximate with considerable assumptions. Meyerhofs methodThe ultimate lateral resistance of rigid pile, Qur is expressed by Meyerhof et al.(1981) as

Where is average unit weight of sand; d is the diameter of pile; L is embedded length of pile; Kb is coefficient of net passive earth pressure on pile using an average angle of skin friction = /3. Where is the angle of internal friction. However the ultimate lateral load resistance of flexible pile was presented by Meyerhof et al. (1988) as

Where Le is the effective embedded length of flexible pile. Meyerhof and Yalcin (1984) suggested that if relative stiffness ratio Krs is less than 10-1 to 10-2 then the pile can be consider as flexible pile.

Where Ep is modulus of elasticity of pile; Ip is moment of inertia of pile; Eh is horizontal soil modulus at pile tip; L is embedded length of pile. Meyerhof et al. (1988) reported that Le/L has an approximate functional relationship with relative stiffness Krs and it can be presented asHowever, Rahman et al. (2003) reported that Le/L can be represent by following relation asThe relative stiffness, Krs can be presented as

Patra & Pise methodPatra & Pise (2001) modified the Meyerhofs equation by multiplying a constant shape factor of 3 with the line of Broms (1964)Pile groupPatra & Pise (2001) reported that the ultimate resistance of the pile group can be represented byWhere, QLg is ultimate lateral resistance of the pilegroup, F is frictional resistance on the vertical planealong the side of the pile group of width equal tocentre to centre distance between external piles andembedded length L and Pp passive earth pressure forthe front pile as shown in Figure 1.

Some other analytical methods for Lateral Loading

Rigid Methods (Broms)(Used for light weight short foundationsSame limitations as rigid methods for mat foundations)

Depth to Fixity Methods (Davisson)(Only considers a certain depth as flexibleStructural engineers could analyse the foundation as a structure once the depth of fixity was known Too simplistic)

Finite Element Analysisp-y curvesHorizontal Modules of Sub grade Reaction MethodSoil as Elastic Continuum Method (Poulos & Hull)

Field test Methods for Lateral LoadingAs per Indian Standard code 2911 series Initial Test- This test is required for one or more of the following purposes. a) Determination of ultimate load capacities and arrival at safe load by application of factor of safety,b) To provide guidelines for setting up the limits of acceptance for routine tests,c) To study the effect of piling on adjacent existing structures and take decision for the suitability of type of piles to be used,d) To get an idea of suitability of piling system, ande) To have a check on calculated load by dynamic or static approaches.

Routine Test- This test is required for One or more or the following purposes. a) One or the criteria to determine the safe load of the pile;b) Checking safe load and extent of safety for the specific functionalrequirement of the pile at working loadc) Detection of any unusual performance contrary to the findings of the initial test, if carried out.

Application of Load:Incremental load each increment being of about 20 percent of safe loadan the pile. For testing of raker piles it is essential that loading is alongthe axis.The next increment should be applied after the rate of displacementis nearer to 01 mm per 30 minutes.

The safe lateral load on the pile shall be taken as the least of the following:

a) Fifty percent of the final load at which the total displacementincreases to 12 mm.

b) Final load at which the total displacement corresponds to 5 mm and

c) Load corresponding to any other specified displacement as per performance requirements.

STIFFNESS FACTORSThe lateral soil resistance for granular soils and normally consolidated clays which have varying soil modulus is modeled according to the equation:p/y = h zwhere p = lateral soil reaction per unit length of pile at depth z below ground level; y = lateral pile deflection; and h = modulus of sub grade reaction for which the recommended values are given in Table 3.

The lateral soil resistance for preloaded clays with constant soil modulus is modeled according to the equation:

p/ y = K Where where k1 is Terzaghis modulus of subgrade reaction as determined from load deflection measurements on a 30 cm square plate and B is the width of the pile (diameter in case of circular piles). The recommended values of k1 are given in Table 4.

Stiffness FactorsFor Piles in Sand and Normally Loaded Clays:For Piles in Preloaded Clays:

CRITERIA FOR SHORT RIGID PILES AND LONG ELASTIC PILESHaving calculated the stiffness factor T or R, the criteria for behaviour as a short rigid pile or as a long elastic pile are related to the embedded length L as given in Table 5.