Behavior of foundation piles resisting lateral forces in soft soil

The role of the foundation in construction is crucial. Foundation planning aims to ensure that it can support loads up to a specified safety limit, including the maximum load that may be encountered. The foundation serves as the lower structure that interacts with the soil, providing stability and support for the superstructure. In this study, the analysis focused on the bearing capacity of the pile foundation under lateral forces. The Brinch Hansen method was employed to determine the allowable lateral bearing capacity, while the computer application was used to calculate the lateral deflection. Various pile lengths, such as 20m, 25m, 30m, 34m, and 40m, as well as different dimensions, including circular piles with diameters D30, D35, D40, and square piles measuring 30x30, 35x35, and 40x40, were considered. The analysis revealed that square cross sections exhibited greater allowable lateral bearing capacity compared to circular cross sections. The square sections showed smaller lateral deflection values in comparison to the circular sections.


INTRODUCTION
The foundation plays a critical role in civil construction as it is responsible for transmitting the loads from the structure and the building above it to the underlying subsoil. It serves as an underground structure that supports both the lower structure and the superstructure. By interacting with the soil, the foundation ensures stability and security for the entire construction.
Piles are a type of deep foundation that can be constructed using various materials, including wood, concrete, steel, and composite materials. In addition to being designed to withstand vertical or axial loads, pile foundations must also consider lateral loads.
These lateral loads can originate from various sources, such as earth pressure acting on retaining walls, wind forces, seismic forces, and eccentric loads on columns. Therefore, the design of pile foundations involves addressing both the vertical and lateral load requirements to ensure the stability and integrity of the structure.
The estimated value of the lateral bearing capacity of a pile foundation can be determined by analyzing the physical characteristics of the foundation and soil parameters, using principles of mechanics. Conventional methods, such as the Brinch Hansen method is suitable for calculating the ultimate lateral resistance of short piles and offers the advantage of applicability to various soil conditions, including homogeneous soils, soils with c-Ø (cohesion and friction angle), and layered soils. However, it should be noted that this method is applicable only to short piles and may not be suitable for analyzing the behavior of longer piles.
This study investigated the impact of pile depth and dimensions on the design of pile foundations to resist lateral forces at the Customs Building in Banjarmasin. This study enhances our understanding and determines the influence of pile length and foundation dimensions on the design of pile foundations for lateral force resistance. The design process utilized the Conventional Brinch Hansen method and the Allpile V 7.3B Computer Application method to evaluate the pile foundation's ability to withstand lateral forces acting on the foundation.

Conventional Method (Brinch Hansen)
The method described is particularly valuable for assessing the deflection of piles under moderate lateral loads. In this calculation, the pile is considered as a fixed cantilever structure at a depth of zf.
The deflection of a free pile can be determined using the equation.

= ( + ) 3 3
On the other hand, the deflection of a fixed-end pile can be expressed by the equation:

= ( + ) 3 12
Where: H = lateral load (kN), Ep = elastic modulus of pile, Ip = inertia of pile, E = distance of lateral load on ground surface, zf = distance of pinch point from ground Surface.
The Brinch Hansen (1961) method was used to calculate the ultimate lateral resistance of short piles. This method is based on earth pressure theory and offers the advantage of being applicable to various soil conditions, including homogeneous soils, soils with cohesion and friction angle (c-Ø), and layered soils. The equation for calculating the ultimate lateral resistance (Psu) of short piles is given as:.
Where Kc and Kq are functions of Ø and x/D shown in Figure 1 below. Where: e = distance of the force H from the ground surface, zf = distance from the ground surface to the pinch point The zf distance is unknown at this stage. For practical purposes, zf is taken to be 1.5 m for sandy/stiff clay soils and 3 m for soft clay/silt soils.

P-Y Method on Computer Program
The p-y curve method is one of the settlement methods used to analyze the lateral deflection of piles. This method establishes the relationship between lateral load and deflection between the soil and the pile, which is represented by the p-y curve. The p-axis represents the lateral soil resistance per unit length of the pile, and the y-axis represents the lateral deflection of the pile.
This equation was used to solve the problem using the p-y curve method.

RESULTS AND DISCUSSIONS
The soil investigation data utilized for the analysis comprises two types, namely Cone Penetrometer Test data and NSPT data. These two investigation data sets are presented in Figure 6, conducted by the service provider PT. Somif Borneo Perkasa on August 2, 2020. Additionally, supporting data such as the parameter values for each soil layer are provided in Table 1. To acquire secondary data concerning the lateral load of a single pile, a recalibration is performed using the values of the group lateral load, namely V = 50 kN, H = 1500 kN, and Mmax = -67.6761 kN.m. The calculation outcomes are presented in Table 2 as follows. Following the acquisition of secondary data on the lateral load of a single pile, the data were subsequently entered into the software to compute the lateral deflection. By performing the calculation of the lateral bearing capacity for various types of pile dimensions (circular and square), including circular pile sizes D30, D35, D40, and square types 30x30, 35x35, 40x40, alongside different pile lengths such as 20m, 25m, 30m, 34m, and 40m, the resulting calculations are presented in Table 3 as follows.    Table 4 presents the results of the lateral pile deflection calculation conducted using the P-Y method in the computer program application for various pile variations. The calculations include circular type piles with diameter variations of D30, D35, D40, and square type piles with cross-sectional size variations of 30x30, 35x35, 40x40. The pile lengths considered in the calculations are 20m, 25m, 30m, 34m, and 40m. The calculation results for all variations in pile length and dimensions can be found in the appendix. Additionally, the 35x35 square cross-section exhibits slightly larger lateral deflections compared to the circular D40 cross-section. Furthermore, when comparing the 35x35 square cross-section, the D35 circular cross-section, and the 30x30 square cross-section, it is observed that the D30 circular cross-section has significantly larger lateral deflection values across variations in the length of the foundation piles (L=20m to L=35m).

The Relationship between Lateral Deflection and Pile Length
Conversely, for a pile length of 40m (same length), the graph reveals that pile foundations with circular sections D30, D35, and square sections 30x30 are closely grouped together, indicating relatively similar deflection values with minimal variation.

CONCLUSIONS
Based on the results of calculations and graphical analysis using the computer program method and the Brinch Hansen method for the case study of the Customs Building in Banjarmasin, South Kalimantan, the following conclusions can be drawn