PILE GROUP BEHAVIOUR

Introduction

Many structures need deep foundations to utilize the bearing capacity of deeper and stronger soil layers. Group piles are one particular type of deep foundations most widely used for high structures. Piled foundations are most often designed in a group configuration, and piled foundations that support long-span bridges are no exception. Such a foundation configuration typically contains a large number of closely spaced piles cast into a substantial pile cap, referred to here as a “large pile group.” In the case of a large pile group, the individual responses of piles within the group are certainly influenced by the presence and actions of neighboring piles, and thus pile group effects become an important design consideration.

Piles are driven in groups at a spacing ranging from 3 to 4B where B is the diameter or side of a pile. The behavior of piles in a group may be quite different than that of a single pile if the piles are friction piles. This difference may not be so marked in bearing piles.

At the earlier time, the capacity of pile groups was taken as equal to the sum of the capacities of the individual piles. However, in practice, when piles are placed close to each other, the stresses transmitted to the soil through neighboring piles will overlap, resulting in a considerable change in the group capacity. A method by which the load capacity of the individual piles in a group embedded in sand could be assigned. According to this method, the capacity of a pile is reduced by 1/16 by each adjacent diagonal or row pile. Based on this method, different loads will be assigned to different piles in the group (Singh, 2014).

Group Pile loading and bearing capacity

The nature of the loading and the kind of soil around the pile, are major factors in determining the response of an isolated single pile and the pile groups. According to active loading at the pile head, four types can be identified: static loading, cyclic loading, sustained loading, and dynamic loading. Besides, passive loadings can occur along the pile length due to moving soil, when a pile is used as an anchor.

A typical bearing pile usually penetrates a short distance into a soil stratum of good bearing capacity, and the pile transfers its load to the soil in a small pressure bulb below the pile tip. If the stratum in which the piles are embedded in all strata below it has ample bearing capacity, each pile of the pile group is capable of carrying essentially the same load as that carried by single piles. If compressible soils exist below the pile tips, the settlement of the pile group may be much greater than the settlement observed in the single pile tests, although the bearing pressure may be smaller than the allowable value. This is due to the overlap of the zones of increased stress below the tip of the bearing piles and the pile group is likely to act as a unit. The total stress shown by the heavy line (Fig.1) may be several times greater than that under a single pile. The effective width of the group is several times that of a single pile. However, if the bearing stratum is essentially incompressible and there are no softer strata below the pile tips, the settlement of a group of bearing piles may be essentially equal to the settlements observed in loading tests on isolated piles. In this case, the piles may, if desired, be spaced about as closely as it is practicable to drive them (Taylor, 1948).

Figure 1: Stress condition below tips of piles (a) single pile (b) group of piles

Pile group behaviour under vertical load

Figure 2: stress zones in soil supporting piles

Pile group behavior under lateral load

In addition to the vertical loads that must be carried by the piles, lateral loads may be present and must be considered in the design. These lateral loads can be caused by a variety of sources such as earthquakes, high winds, wave action, ship impact, liquefaction, and slope failure. Concerning their use in practice, piles under lateral loads are termed active piles or passive piles. An active pile is loaded principally at its top in supporting a superstructure such as a bridge. However, a passive pile is loaded principally along its length due to earth pressure, such as piles used as a retaining wall in a moving slope.

The response of a laterally loaded pile group differs from that of a single pile because of interference with the zone of influence of the pile by adjacent piles and their zones of influence. A difference may also exist between the degree of fixity of a single pile and a corresponding pile group; however, this is primarily a theoretical problem and not one of the important behavioral differences in the soil.

Interference of Adjoining Piles Figure 3 shows a plan and profile of a pile group loaded horizontally at the ground line by a load, Q, (Prakash 1962). The dotted lines in Figure 3a indicate schematically how one pile in a group may affect its neighbors. Pile 1 (Figure 3a) stresses the soil outside of the pile group, whereas piles 2 and 3 generally stress the soil immediately in front of their locations. This, coupled with the deflection of piles 1 and 2, causes a lower soil resistance for piles 2 and 3. Therefore, piles 2 and 3 would exhibit less stiffness than pile 1. Application of the above concepts leads to a qualitative comparison of the stiffness of piles in a pile group. For example, the front corner pile should be the stiffest and interior the most flexible.

Fig 3. Pile group (Overlapping zones of stresses (a) Plan (b) Section

The response of a laterally loaded pile within a group of closely spaced piles is often substantially different than a single isolated pile. This difference is attributed to the following three items:

1. The rotational restraint at the pile cap connection. The greater the rotational restraint, the smaller the deflection caused by a given lateral load.

2. The additional lateral resistance provided by the pile cap.

3. The interference that occurs between adjacent piles through the supporting soil. Interference between zones of influence causes a pile within a group to deflect more than a single isolated pile, as a result of pile-soil-pile interaction.

Pile group and spacing

Piles are most widely used in groups as shown in Figure 4. The models that are used for the group piles should reply to two problems:

1- The group efficiency of closely spaced piles that are loaded laterally

2- Load distribution of individual piles in a group.

In the first case, the forces are transmitted through the soil, however, in the second case, the forces are transmitted by the pile cap. In widely spaced pile groups the pile-soil-pile interaction is inconsiderable and a solution is made to reveal lateral load to each of the piles in the group.

Figure 4. Basic formation of pile groups

For lateral load design of long-span bridges, such as is required for seismic loading, it is a question of how to adequately characterize this group interaction and the effect it has on the lateral stiffness of the foundation group as a whole.

In a large group of closely spaced friction piles, the actions of the piles overlap and the distribution of load to the various piles is not uniform.

Settlement Of Pile Groups

The settlement of a group of friction piles is considered to result from three causes (Taylor, 1948):

1. Settlement due to compression of the pile and from the movement of the piles relative to the immediately adjacent soil (Figure 5). When full skin friction is developed, this settlement corresponds to that observed in a loading test on a single pile.

Figure 5. Load-carrying capacity of a pile group in clays: (a) Section (b) Plan

2. Settlement due to compression occurring in the soil between the piles.

3. Settlement Due to compression that occurs in compressible strata below the tips of the piles.

The settlement due to compression of the soil between piles and that due to compression of the strata below the tips of the piles are generally of much larger magnitude than that due to compression of the pile and movement of pile relative to the soil. However, these settlements may occur very slowly in saturated soil because of consolidation and slow dissipation of pore pressure. Since there is partial disturbance to the structure of the soil around the piles, accurate estimates of the amount of settlement occurring under the item are not possible. The disturbance of soil structure during pile driving may result in increased settlements after the final loading of a pile foundation. It is well known that a remolded clay, when subjected to a given load, consolidates to a considerably smaller void ratio than that reached under the same load by the same clay in an undisturbed state (Taylor, 1948). Therefore, structural disturbance results in increased settlements. The magnitude of this settlement increase depends largely on such factors as;

1. the distance the disturbance extends from the pile.
2. the type of soil.
3. the degree to which the soil is disturbed.
4. the details of the action in the complicated consolidation process after driving.

A definite increase in settlements may not be quantitatively defined, but it is possible that in some soils they are much larger than many engineers may suspect (Taylor, 1948). Estimates of items may be made by methods based on Terzaghi’s theory of consolidation.

In loading tests, the settlement of a single friction pile is not representative of the settlements of the pile group. Therefore, such a load test will give information on the failure load rather than the settlements under actual loading conditions of a friction pile. The installation of piles usually alters the deformation and compressibility characteristics of the soil mass in a different way and to a different extent as compared to that around and below the tip of the single pole although this influence extends only to a few pile diameters. Accordingly, the total settlement of a group of driven or bored piles under the safe design load not exceeding one-third to one-half of the ultimate group capacity can generally be estimated roughly as for an equivalent pier foundation Terzaghi and Peck(1967)

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 is 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.

1. Soil modulus: The elastic soil modulus E and the lateral modulus of subgrade reaction E1 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 sand should be reduced by the ratio of the submerged unit weight divided by the soil unit weight.
2. 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 conduction and should never exceed 26 degrees. Battered piles should be avoided where significant negative skin friction and down drag 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.
3. 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 it into the pile cap usually increases the lateral stiffness of the group and the moment. A group of fixed 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 moments 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.
4. Stiffness of pile cap: The stiffness of the pile cap will influence the distribution of structural loads to the individual piles. The stiffness 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 rigid cap can usually be assumed for gravity-type hydraulic structures.
5. 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 magnitude. Displacements can double in some cases.
6. Driving: The apparent stiffness of a pile in a group may be greater than that of an isolated pile driven in cohesionless soil because the density of the soil within and around a pile group can be increased byåiving. 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.
7. 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 the 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 ova-lapping of zones in the soil. In situ soil stresses from pile loads are applied over a much larger area and extend to a greater depth leading to greater settlement.
8. 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.

Analysis of pile group

Many studies have been performed in the analysis of Pile due to lateral, vertical, and combined loading using various methods. Since there is a continuous movement of the tectonic plates, the structures are imposed on lateral loading in addition to vertical loads. These lateral loads increase their shear stress, leading to member failure. In-situ testing of piles has many drawbacks and doesn’t provide us with immediate solutions for the problem. Several FEM and FDM software are available for the simulation of complex Geotechnical problems in a simpler way (Abishek, 2020).

Although isolated single piles may be encountered in some applications, it is more common that a structure foundation will consist of several closely spaced piles (many building codes require a minimum of three piles in a group). The structure/pile/soil system is highly indeterminate and nonlinear. Historically, design methods have been based on numerous simplifying assumptions that render the analytical effort tractable for hand computations. The advent of the computer has allowed solutions to be obtained in which many of the simplifications of the classical design methods are no longer necessary (Reed et al, 2020).

References

• Singh P. K., Arora V. K, Vol. 3 Issue 3, March — 2014, Behavior of Pile Groups Subjected to Vertical Loading (A Comparative Study), International Journal of Engineering Research & Technology (IJERT).
• Abishek, R. R, Janaki Raman, 06 November 2020, A comparative analysis of pile and pile groups imposed on lateral, vertical loads and its combination.
• Reed L. Mosher and William P. Dawkins, 2020, November 2000, Computer-Aided Structural Engineering Project, Theoretical Manual for Pile Foundations.
• Andrew M. Dodds and Geoffrey R. Martin, April 16, 2007, Modeling Pile Behavior in Large Pile Groups under Lateral Loading, Technical Report MCEER-07–0004
• Shamsher Prakash, Hari D. Sharma, 1990, Pile foundations in engineering practice, retrieved from www.knovel.com 27/11/2021.
• Lymon C. Reese, William Van Impe, 2011, Single Piles and Pile Groups Under Lateral Loading, RC Press is an imprint of Taylor & Francis Group.
• Junbo Jia, 2018, Soil Dynamics and Foundation Modeling, Offshore and Earthquake Engineering, Springer International Publishing AG 2018.