Numerical Simulation of Waves Propagated from Driven Piles to Nearby Structures in Basra Soil

When the hammer strikes the pile, energy transfers into the pile and causes pile penetration. Some energy is lost through shear at the pile shaft and compression at the pile tip (Figure 1). Energy transfers away from the pile as traveling stress waves into the ground. This energy depends on the vibrating source and the medium that vibration transfers through it. It directly affects the surrounding vibration receivers such as human beings, equipment, structures, or anything sensitive to vibration [1].

Neglecting energy losses, the velocity of the dropping hammer is calculated as in reference [2]:

$V_{H 0}=(2 g h)^{0.5}$        (1)

where, V_H0: velocity of the hammer (m/s); g: acceleration due to gravity (m/s²); h: height of drop (m).

The velocity of the pile head increases while the velocity of the hammer decreases due to pile impedance, as explained in Eq. (2) [3]:

$Z_H * V_H=Z_P * V_P$        (2)

where, Z_H,P: hammer and pile impedance (kN.s/m), respectively; V_H,P: hammer and pile particle velocity (m/s), respectively.

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Figure 1. Vibration transmission mechanisms between pile and soil

The time (t) required to propagate the wave from the pile head towards the pile toe and reflect to the pile head again could be calculated as:

$t=2 L_p / C_p$     (3)

where, L_p: pile length (m); C_p: velocity of wave in the pile (m/s).

Many research articles have been published regarding the utilization of the finite element method in simulating the problem of vibrations induced due to pile driving. A streamlined axisymmetric model was established by Zhang et al. [4] to replicate the pile driving procedure utilizing Plaxis-2D software. The 15-noded element was selected to represent both the pile and the soil, while their connection was simulated using interface elements. Additionally, a simplified structure, comprised of beam elements, was positioned at a distance where ground vibrations were monitored. The finite element analyses yielded outcomes related to ground vibrations near the building and the building's reactions to the dynamic pile load. Afterward, comparisons were made regarding the structural responses, examining normal and shear stresses against their respective strengths. Andersson Olivecrona and Sulander [5] explored the potential for creating an effective finite element simulation of pile-driving-induced vibrations. The study delves into analyzing and contrasting the vibrations resulting from a single hammer impact at various depths of the pile, juxtaposed with real-world field test data. The successful application of the finite element method enabled accurate prediction of wave propagation and vibrations resulting from impact pile driving. The results indicated that the behavior and intensity of vibrations are significantly influenced by Young's modulus of the soil. Grizi et al. [6] investigated the ground vibration produced by driving a (1.7 m) H-Pile into a small-scale model of sandy soil. The findings indicated a strong alignment between the numerical outcomes and the observed data. Furthermore, there was a swift decrease in the peak particle velocity (PPV) as the distance from the pile increased. When the pile's tip went beyond a depth of (0.5 m), the numerical results underestimated the measured response. Sun et al. [7] utilized ABAQUS software to study the vibration effects on a nearby pre-existing pile, caused by a (1.25 m) diameter and (52.1 m) long new pile driving. The distance between the driven and existing piles varied between (10) times to (150) times the driven pile diameter. The lateral and vertical peak accelerations at the preexisting pile were found to be close to those in the free field. Additionally, within (20) diameters and greater distances, the acceleration at the existing pile diminished consistently as the distance increased.

Vibrations have a direct effect on the surrounding structures, three kinds of problems are induced by pile driving [2]:

(1) Vibration receivers may be human beings or anything sensitive to vibration.

(2) Medium that vibration can transfer through it.

(3) Vibration source.

To guarantee the safety, environmental sustainability, and regulatory compliance of building projects, it is crucial to study the impacts of pile-driving vibrations on nearby buildings, and a knowledge of how these vibrations affect structural integrity. To facilitate sustainable building methods, it is crucial to analyze the environmental effects to undertake thorough environmental impact assessments. Engineers can adapt techniques and tools to unique situations thanks to this information, which is included in building planning and design. The development of effective mitigation solutions, such as changes to building practices and the use of dampening devices, may be facilitated by recognizing possible consequences. Construction projects are more financially viable when they address neighborhood concerns about noise and vibrations. Researching the consequences of pile-driving vibrations helps with responsible construction methods, better decision-making, and more acceptability and success for building projects as a whole.

The research objective is to achieve successful numerical modeling of the problem of pile driving induced vibrations into Basrah soil utilizing the finite element method, and using this model in assessing the safe distance of a hypothetical structure based on AASHTO standards. This study helps us understand and simulate the consequences of pile driving, gives us ideas for particular sites, and gives us practical guidance to make construction sites safer. Three sites are selected to conduct the study, namely Al-Fao, Al-Shuaiba, and Umm Qasr, as shown in Figure 2.

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Figure 2. Locations of the case studies

The free vibration analyses are performed by removing pile hammer loads, to determine the natural frequencies. The results of forced vibration analyses are presented in terms of surface and body wave velocities (Figures 6-8). In addition to that, they are expressed as variations of (PPV) with distance from the vibration sources [16] for various pile penetrations (Figures 9-11).

The reduction in wave intensity (attenuation) occurs due to two factors: mainly on the wave propagation's geometry, known as geometric damping [1], and minorly the materials that the waves traverse, referred to as material damping [17].

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Figure 6. Wave velocities in Al-Fao site: (A) Surface wave; (B) Body wave

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Figure 7. Wave velocities in Umm Qase site: (A) Surface wave; (B) Body wave

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Figure 8. Wave velocities in Shuaiba site: (A) Surface wave; (B) Body wave

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Figure 9. Predicted and measured PPV vs. distance for various pile penetrations (Al-Fao site)

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Figure 10. Predicted and measured PPV vs. distance for various pile penetrations (Shuaiba site)

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Figure 11. Predicted and measured PPV vs. distance for various pile penetrations (Umm Qaser site)

The rise in vibrations near the pile is mostly due to the direct effect of surface waves originating from the pile's surface. Conversely, the increase in vibrations at shallow depths, as opposed to deeper depths near the pile, is caused by the shorter travel of reflected waves from the pile's toe. This leads to a decrease in the reduction of energy as it passes through the soil (geometric damping) and the reduction of energy due to the soil's intrinsic properties (material damping) [14]. Due to the narrow wavelength in close proximity to the source, there is an elevation in the frequency of vibration. As the distance from the source rises, the amplitude of the vibration reduces, the length of the wave grows, and the frequency diminishes. The vibration strength and amplitude converge to a similar level when seen from a significant distance from the source. Based on the data provided in Tables 10-12, it can be shown that the Al-Fao model has the largest damping ratio (A₂/A₁) in comparison to the other models. The increased damping seen in the Al-Fao model may be ascribed to several variables. One of these aspects is the larger depth of the Al-Fao model compared to the other models. Additionally, the horizontal distances from the source also change between the Al-Fao model and the other models, making a direct comparison between them inappropriate owing to variances in circumstances. Furthermore, the Umm Qaser model exhibits substantial damping due to its comparable depths and horizontal distances to the Al-Shuaiba Model. Due to its lower hammer energy compared to other models, the Umm Qaser model exhibits a reduced dissipation of low-energy waves during propagation. Based on the data presented in Figures 6, 8, and 10, it is observed that the Al-Shuaiba model exhibits the highest values of PPV compared to the other models. This is because the Al-Fao model utilizes a tubular steel pile, which differs from the other models. Despite having the maximum hammer energy, the Al-Fao model has lower values of PPV due to the reflection and refraction of waves as they propagate through deeper depths. Additionally, the presence of surface waves has a negligible effect on the PPV compared to the presence of body waves. The Umm Qaser model, although having mostly sandy soil, has the same depths and distances as the Al-Shuaiba model. However, the Umm Qaser model has a greater capacity to transmit waves compared to Al-Shuaiba's soil. On the other hand, the hammer energy in the Umm Qaser model is lower than that of the Al-Shuaiba model.

Table 10. Damping and absorption coefficients at the Al-Fao site

Table 11. Damping and absorption coefficients at the Shuaiba site

Table 12. Damping and absorption coefficients at the Umm Qaser site

Conclusively, it is difficult to undertake a thorough comparison of models, especially when taking into account the peak particle velocity (PPV). The challenge stems from the inherent differences in site circumstances, including changes in pile and hammer types as well as soil properties. Notwithstanding these difficulties, a comparison will be made between the models to clarify the influence of different pile and hammer types, as well as soil variables, on the measured PPV values:

(1) Soil conditions: According to Tables 1-3, the most common types of soil in Al-Fao were silty clay and clay in Al-Shuaiba whereas silty sand in Umm Qaser, the Al-Fao site has less attenuation due to the high depths and energy of the hammer which increase the body wave at a long distance while in Al-Shuaiba site has more attenuation due to the clay soil that tends to damp the vibration.

(2) Hammers: despite the hammer used in Al-Fao having the highest energy, the maximum PPV was detected in the Al-Shuaiba site which has a hammer’s energy less than Al-Fao’s Hammer, this is attributed to surface wave close to the pile in Al-Shiaiba was significant due the shallow depth as compared with Al-Fao Site.

(3) Piles: the pile used in Al-Fao was steel tubular that had the highest wave speed at the surface but due to the high pile impedance the induced vibration was less than the vibration in the Al-Shauiba site.

To evaluate the efficiency of the numerical simulation, regression analyses are performed between the predicted and measured (PPV) variations [18-20] with distance for various pile penetration depths. The results are listed in Table 13. A very good agreement is achieved.

Table 13. Results of regression analyses between the predicted and measured PPV

Table 14. Maximum allowable PPV for transient vibrations to prevent structural damage [AASHTO 1990] [16]

Many standards are available to provide the maximum permitted vibrations for various types of structures. In this study, the AASHTO standard will be used to determine the safe distance from pile driving (Table 14).

Referring to Figures 9 and 11 and Table 14, it can be realized that for a residential building with plastered walls, the safe distance is around 18 times the pile section dimension.

I express my gratitude to GCPI Company for granting me access to the Daewoo Company site, where I obtained the necessary soil data. Additionally, I am thankful to Technital Company for their assistance with the same matter.