An experimental study of the effect of lateral static load on cyclic response of pile group in sandy soil

Amal S. Gummar; Jasim M. Abbas

doi:10.1515/jmbm-2022-0253

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An experimental study of the effect of lateral static load on cyclic response of pile group in sandy soil

Amal S. Gummar und Jasim M. Abbas

Veröffentlicht/Copyright: 28. November 2023

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Aus der Zeitschrift Journal of the Mechanical Behavior of Materials Band 32 Heft 1

Abstract

The application of two different lateral loads (i.e., the first is static and the other is cyclic load) at the same time to pile group seems a complex problem. So, this study is concerned with the interaction of these loads together and includes laboratory tests to determine the effect of the static lateral load on the lateral cyclic capacity of the 2 × 2 pile group with three distances (S/D = 3, 5, and 7). The experimental program included small-scale laboratory setup by using hollow piles of aluminum and sand rain method for the preparation of sandy soil medium. The results showed that increasing the static lateral load led to reducing the lateral displacement, and the average value of decrease for the three distances reaches 72%. It can also be concluded that the distance S/D = 3 is more affected by the increase in the static lateral load than other distances, where the average decrease in the lateral displacement was about 83, 74, and 60% when S/D = 3, 5, and 7 sequentially.

Keywords: lateral static loads; cyclic response; pile group; pile distances

1 Introduction

In practice, it can be seen that the pile foundations are inevitable when the position of the strong soil is down from that of the weak soil, to transfer the loads from the superstructure to the strong soil. Often, these piles are subjected to combined loads, vertical and lateral [1]. Currents, traffic, tide, seasons, avalanches, waves, earthquakes, wind, blasting activities, operations of plant, or rotating machinery could all be the loadings’ source [2]. Most often, the vertical and horizontal responses of piles are evaluated separately without taking into account any potential linkages [3]. The source of loading, the distance between the piles, the condition of the soil, and other elements all affect the pile groups’ capacity to resist horizontal loads [4]. The effect of the cyclical loadon the performance of the lateral pile foundation has been tested in a number of prior research studies using laboratory models and finite element model (FEM). Some researchers employ a finite element method during the dynamic investigation of pile groups [5]. The GEOFEM-3D program, which is based on 3D finite elements, was used with sandy soil to study the lateral reaction of pile groups under combined loads. Combined loads have the effect of changing the pile group’s lateral capacity; also, increasing the vertical loads increases the efficiency of the piles [4]. Investigational research has also been done on square groups of piles implanted in different types of clay soil with two values of the relative densities: 10 and 90%. These groups were subjected to cyclic loading in one direction only. It is found that when the number of loading cycles increases, the deflections and the maximum bending moment increase [6]. Another study tested a single pile’s performance under varied static inclined loads till failure. So, from this research, it can be inferred that the inclined load has a significant influence on pile group performance [7]. Through experimental studies and in case the piles being subjected to combined loads: lateral load and vertical load, the effect of an increase in the number of cycles regarding piles’ horizontal displacement under combined loads is less than its effect in case the piles are subjected to the cyclic lateral load only [8]. When pile groups are subjected to two loadings, one vertical and the other cyclically, there will be a disparity in lateral displacement of the head of the pile with a change in the cyclic load ratio; next, the behavior becomes more consistent with an increase in the number of cycles. According to that, the cyclic load ratio (CLR) of 60% indicates the cyclic load critical in this study [9]. In the case of deflection, lateral displacement diminishes as pile spacing increases. All piles and rows display the difference caused by pile spacing. This results from the effect of overlapping between the group’s piles and surrounding soil [10]. The pile group’s lateral resistance will diminish during the initial cycles, and as the number of cycles increases, the group’s deflection will become nearly constant due to the soil’s densification [10]. A piled raft model loaded with pure horizontal cyclic loading once and combined loading again was tested to see how clay soil saturation degrees affected the model’s behavior. The findings demonstrated that this model’s lateral resistance decreases as clay soil saturation increases. The model’s lateral resistance was enhanced by the presence of vertical loads [11]. A static lateral load was applied to groups of piles, a cyclic lateral load was applied to the same groups, and the behavior of these piles was studied under each load separately. The study’s findings showed that the intensity and size of the load have a significant influence on the type and extent of deformation. Additionally, the configuration of the groups and the shape of the piles have an effect on how the foundation behaves [12]. When pile foundations are subjected to inclined cyclic loads, this would result in the division of the applied load into two components, horizontal and vertical. The vertical component acts as a vertical load on the pile groups and increases the stiffness of the pile while the horizontal component acts as the horizontal load on the pile groups [13]. As summarized from previous studies, attention can be drawn to the fact that there are only a few studies that search for two lateral loads applied to a group of piles at the same time, although this is very common in nature, especially the static lateral load synchronous with the cyclic lateral load. Therefore, this study was conducted on a group of piles with different distances in sandy soil because this soil occupies large areas in Iraq, and the lateral cyclic load was bidirectional, with a certain frequency during 100 cycles for the one test. This study aims to determine the effect of the combined load on the lateral deflection of piles, the CLR, the number of cycles on load–deflection, and the effect of the distances between the piles.

2 Materials and test details

2.1 The soil used

Sandy soil was provided from the city of Karbala, southwestern Iraq. All necessary tests were done in the soil laboratories in the College of Engineering University of Diyala, and Table 1 shows the results of those tests.

Table 1

Sandy soil’s physical properties

Property	Value	Standard of the test
Specific gravity, G _s	2.61	ASTM D 854 (2006)
Angle of internal friction, φ°	35.6	ASTM D3040-04 (2006)
Cohesion, c (kN/m²)	0	ASTM D3040-04 (2006)
Maximum dry unit weight, γ _dmax (kN/m³)	17.5	ASTM D 4253 (2006)
Minimum dry unit weight, γ _dmin (kN/m³)	15.1	ASTM D 4254 (2006)
Dry unit weight, γ _d (kN/m³)	16.3	—
Relative density, D _r (%)	70	—

2.2 Model of piles and piles cap

Hollow pipes made from aluminum, circular cross-sections with a diameter of 16 mm have been utilized. These pipes’ length (690 mm), the ratio of the circular pile’s embedded length to the pile’s diameter (L/D) = 40. On pieces of these pipes, tensile tests were conducted according to (ASTM-A370, 2005) to estimate the modulus of elasticity, and the result was E = 68.75. The caps for groups of piles were constructed from sturdy plates with smooth surfaces, and a thickness of (6 mm), plates of Square steel with dimensions of (96 × 96, 128 × 128, and 160 × 160) mm were chosen according to the distances between the piles of 3D, 5D, and 7D, respectively, as shown in Figure 1.

Figure 1

Pile groups distances.

2.3 Test setup

Two lateral loads have been applied: the major one is a two-way cyclic load and the secondary is a static lateral load, the loads are applied directly to the cap of pile groups. As for the dimensions of the container used, they are of 1 m × 1 m × 1 m in order to reduce the boundary effects. The static lateral load tool is positioned that a wire passes through it pulling the pile cap sideways and passing through a pulley to the disk on which the loads are placed. This tool ensures that the wire does not friction with the container, and for cyclic load, it is applied to the cap of pile group laterally in two directions via an actuator that is driven by a gear system. The deflection of the pile head is measured by a type of inductively Linear Variable Displacement Transducers (LVDT), as shown in Figure 2.

Figure 2

Laboratory model system of static and cyclic lateral loading device.

According to Brooms’ theory, the lateral static load capacity is equal to 20% of the pile diameter [14]. The cyclic lateral load is applied to the pile groups at a certain CLR without constant load, and then, the same procedure is repeated but with a constant lateral load added at a ratio equal to the maximum bearing of the pile group. The test period takes five seconds to achieve 100 cycles during a single test at a frequency of 0.2 Hz.

3 Results and discussion

3.1 Effect number of cycles and CLRs on piles displacement

Figure 3 shows the maximum lateral displacement of the pile tip under the effect of four values of CLR = 0.2, 0.4, 0.6, and 0.8 at a frequency of 0.2 Hz. It is observed that with an increase in the number of cycles, the deflection of pile groups rises gradually. In both statuses (pure cyclic lateral load and combined load), the lateral deflection increased faster during the first 50 cycles, but over the second 50 cycles, the deflection increased slowly; the increase in the deflection is due to the occurrence of the phenomenon of conical depression around the piles near the surface of the soil [15]. While the decrease in the speed of the deflection that occurs after that can attribute the reason to an increase in the condensation of the soil as a consequence of the entry of its grains together.

Figure 3

Effect of the CLR on the lateral displacement of a 2 × 2 pile group under cyclic load affected by a static lateral load: (a) S/D = 3, (b) S/D = 5 and (c) S/D = 7.

3.2 Lateral resistance of the piles

Figure 4 represents the load–deflection curve of pile group models 2 × 2 with three distances S/D (3, 5, and 7), where the number of cycles used are1, 5, 25, 50, and 100, representing tat cyclic loading is accompanied by static loading. These curves appear such that in the first cycles, the lateral deflection increases with a greater value than in the last, and as the number of cycles increases, the lateral displacement increases exponentially until the 50th cycle. After which the lateral displacement continues to increase, but with a smaller value. The effect of spacing on lateral resistance is direct; the lateral resistance in the case of pure cyclic lateral loads in the small spacing is less than the wide spacing due to overlapping stresses in the small areas. In the case of combined lateral loads, the lateral resistance at small spacing is greater than at wide spacing, because that static lateral load has a greater effect on smaller spaces than on wide spacing; the rate of decrease in lateral displacement was 83% in S/D = 3, while 74 and 60% for S/D = (5 and 7) sequentially.

Figure 4

Influence of the number of cycles on load–deflection curve of pile groups under cyclic loads affected by a static lateral loads 2 × 2 group: (a) S/D = 3, (b) S/D = 5, and (c) S/D = 7.

4 Conclusion

According to the results of the tests of the combined loads consisting of a cyclic lateral load and a static lateral load, the following can be concluded:

The piles placed in the sandy soil are affected by the cyclic lateral load synchronous with the static lateral load, where the static lateral load reduces the effect of the cyclic lateral load on the lateral displacement of the pile tip.
When the ratio of the pure cyclic loading increases without the static loading, the lateral displacement will also increase, but when it increases with the static loading, the lateral displacement is less than it was without the static load.
The increase in the lateral displacement of the pile tip during the first half of the test occurs significantly, but in the second half of the test, the increase begins to decrease.
The presence of the static load causes the lateral displacement in S = 3D to decrease by 83%, while in S = 5D and S = 7D, the lateral displacement decreases successively by 74 and 60%, indicating that the small area group of piles is more affected by the static load than the large group.

Funding information: This manuscript is not funded by any party.
Author contributions: Both the authors have accepted responsibility for the entire content of this manuscript and approved.
Conflict of interest: The authors state no conflict of interest.

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Received: 2022-05-10

Revised: 2022-06-29

Accepted: 2022-09-05

Published Online: 2023-11-28

This work is licensed under the Creative Commons Attribution 4.0 International License.

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https://doi.org/10.1515/jmbm-2022-0253

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lateral static loads; cyclic response; pile group; pile distances

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