In seismic zones, where earthquakes pose significant risks to infrastructure, choosing the right retaining system is critical for ensuring structural stability and safety. Retaining systems like diaphragm walls and pile walls are widely used to support deep excavations, stabilize slopes, and protect buildings from lateral earth pressures.

However, their performance under seismic conditions varies due to differences in construction, material properties, and interaction with soil.

This article provides a comprehensive comparison of diaphragm walls vs pile walls, focusing on their seismic performance, with specific reference to case studies in Nepal, a seismically active region.

By exploring their design, construction, advantages, disadvantages, and real-world applications, we aim to determine which system is better suited for seismic zones.

Introduction to Retaining Systems in Seismic Zones

Retaining systems are essential for managing lateral earth pressures in construction projects, particularly in urban areas with deep excavations or in regions prone to seismic activity. Seismic zones, characterized by frequent earthquakes, demand retaining systems that can withstand dynamic forces, including ground shaking, soil liquefaction, and lateral spreading.

Nepal, located in the seismically active Himalayan region, has experienced devastating earthquakes, such as the 2015 Gorkha Earthquake, which highlighted the need for robust retaining structures.

Diaphragm walls and pile walls (including contiguous and secant pile walls) are two prominent solutions, each with unique characteristics that influence their seismic performance.

We’ll also examine case studies from Nepal to provide real-world insights and include tables to summarize key differences. By the end, you’ll have a clear understanding of which system is better suited for seismic-prone areas.

What Are Diaphragm Walls?

Diaphragm walls are continuous, reinforced concrete walls constructed in-situ by excavating a trench and filling it with concrete and steel reinforcement. They are typically used for deep foundations, retaining structures, and cut-off walls in projects like metro stations, high-rise buildings, and tunnels. The construction process involves:

1. Excavation: A trench is dug using a mechanical or hydraulic grab or a Hydrofraise®, supported by bentonite slurry to prevent soil collapse.
2. Reinforcement: Steel cages are placed in the trench to provide tensile strength.
3. Concreting: The trench is filled with concrete, displacing the slurry to form a solid wall.

Diaphragm walls are known for their high strength, water-tightness, and ability to minimize ground settlement, making them ideal for urban environments and seismic zones. Their thickness typically ranges from 0.5 to 1.8 meters, and they can extend to depths of 100 meters or more, depending on site conditions.

What Are Pile Walls?

Pile walls, including contiguous and secant pile walls, are retaining systems formed by installing a series of piles (concrete or steel) into the ground. These piles are either placed close together (contiguous) or interlocked (secant) to form a barrier against soil and water. The construction process includes:

1. Pile Installation: Piles are driven or bored into the ground at regular intervals (contiguous) or overlapping (secant).
2. Reinforcement: For concrete piles, steel reinforcement is inserted before concreting.
3. Backfilling: The gaps between piles (in contiguous walls) may be filled with grout or left open, depending on the design.

Contiguous pile walls are cost-effective for softer soils but may lack water-tightness, while secant pile walls provide better sealing due to their overlapping design. Pile walls are commonly used for temporary or permanent retaining structures in deep excavations.

Key Differences Between Diaphragm Walls Vs Pile Walls

To understand their seismic performance, let’s first compare their fundamental characteristics:

This table highlights that diaphragm walls offer superior water-tightness and lower ground disturbance, while pile walls are more cost-effective for certain applications. However, their seismic performance depends on additional factors, which we’ll explore next.

Seismic Performance of Diaphragm Walls

Diaphragm walls excel in seismic zones due to their robust design and ability to mitigate earthquake-induced risks like soil liquefaction and lateral spreading. Their key advantages include:

1. High Rigidity and Strength: The continuous reinforced concrete structure provides excellent resistance to lateral forces, making diaphragm walls less prone to deformation during earthquakes.
2. Soil Stabilization: By acting as a deep foundation, diaphragm walls anchor buildings to stable soil layers, reducing the risk of settlement or tilting caused by liquefaction.
3. Low Ground Disturbance: The use of bentonite slurry during construction minimizes soil disturbance, preserving the integrity of surrounding structures.
4. Ductility: Diaphragm walls can deform under stress without failing, absorbing seismic energy and reducing damage to the superstructure.

However, diaphragm walls have challenges, including high construction costs, the need for specialized equipment, and longer construction times (2-3 weeks per panel). Their complexity also requires skilled labor and precise quality control to ensure structural integrity.

Seismic Performance of Pile Walls

Pile walls, particularly secant pile walls, are widely used for deep excavations but face limitations in seismic zones. Their performance depends on the type of pile wall and site conditions:

1. Contiguous Pile Walls: These are less effective in seismic zones due to gaps between piles, which can allow soil or water leakage, increasing the risk of liquefaction. They are also more susceptible to lateral displacement during earthquakes.
2. Secant Pile Walls: Overlapping piles provide better water-Tightness and structural continuity, improving seismic performance compared to contiguous walls. However, they are less rigid than diaphragm walls and may experience higher deformations.
3. Vibration and Settlement: Pile installation, especially for sheet piles, generates significant noise and vibration, which can disturb surrounding soil and structures, potentially exacerbating seismic damage.
4. Cost-Effectiveness: Pile walls are generally more economical, making them attractive for projects with budget constraints, but their seismic performance may require additional reinforcement or anchoring.

In seismic zones, pile walls may need supplementary measures, such as ground anchors or tiebacks, to enhance stability, particularly for taller walls or deeper excavations.

Comparative Analysis in Seismic Zones: Diaphragm Walls Vs Pile Walls

To evaluate which system performs better in seismic zones, we must consider several factors, including soil conditions, seismic intensity, construction constraints, and project requirements. Below, we analyze their performance across key parameters:

1. Resistance to Lateral Forces

Earthquakes generate lateral forces that push retaining walls sideways. Diaphragm walls, with their continuous reinforced concrete structure, offer superior resistance to these forces due to their rigidity and deep embedment. They act as a barrier, reducing soil movement and stabilizing the foundation.

Pile walls, particularly contiguous ones, are less effective due to potential gaps and lower structural continuity. Secant pile walls perform better but still lack the monolithic strength of diaphragm walls.

2. Soil Liquefaction Mitigation

Soil liquefaction, where saturated soil loses strength during an earthquake, is a major concern in seismic zones like Nepal. Diaphragm walls mitigate this by stabilizing the surrounding soil and preventing water ingress, which reduces the risk of liquefaction-induced settlement.

Pile walls, especially contiguous ones, are less effective due to potential leakage, which can exacerbate liquefaction. Secant pile walls offer better protection but may require additional grouting to ensure water-tightness.

3. Construction Feasibility

In urban seismic zones, construction constraints like noise, vibration, and space limitations are critical. Diaphragm walls are constructed with minimal noise and vibration, making them suitable for densely populated areas.

Pile walls, particularly sheet piles, generate significant noise and vibration, which can disturb nearby structures and increase the risk of settlement. However, pile walls are faster and cheaper to install, which may be advantageous for smaller projects.

4. Cost and Time Considerations

Diaphragm walls are more expensive and time-consuming to construct due to the need for specialized equipment and skilled labor. A typical panel (2.5-3 meters wide, 20-30 meters deep) takes 2-3 weeks to complete.

Pile walls, especially contiguous ones, are quicker and more cost-effective, making them suitable for projects with budget or time constraints. However, their long-term performance in seismic zones may require additional investments in reinforcement or maintenance.

5. Adaptability to Site Conditions

Diaphragm walls are highly adaptable to various soil types, including soft, waterlogged, or rocky soils, making them versatile for seismic zones with diverse geotechnical conditions. Pile walls are effective in softer soils but may struggle in rocky or highly variable ground, where drilling or driving piles becomes challenging.

Case Studies from Nepal

Nepal, located in a seismically active region, provides valuable insights into the performance of retaining systems during earthquakes. The 2015 Gorkha Earthquake (Mw 7.8) caused widespread damage to infrastructure, highlighting the importance of seismic-resistant designs. Below, we examine two relevant case studies:

Case Study 1: Kathmandu Metro Project

The Kathmandu Metro Project, aimed at developing underground transportation infrastructure, utilized diaphragm walls for deep excavations in the city’s soft, waterlogged soils. The walls, constructed to depths of 30-40 meters, provided excellent stability during the 2015 earthquake and its aftershocks.

Their water-tightness prevented soil liquefaction, and their rigidity minimized ground settlement, protecting adjacent structures. The project’s success underscored the suitability of diaphragm walls for urban seismic zones with challenging soil conditions.

Case Study 2: Bhaktapur Retaining Wall Failure

In Bhaktapur, a historical city near Kathmandu, contiguous pile walls were used for a deep excavation project. During the 2015 Gorkha Earthquake, the walls experienced significant lateral displacement due to gaps between piles, leading to soil leakage and partial collapse.

The lack of water-tightness exacerbated liquefaction in the sandy soil, causing settlement of nearby buildings. This case highlighted the limitations of contiguous pile walls in seismic zones and the need for secant piles or additional reinforcement.

These case studies demonstrate that diaphragm walls generally outperform pile walls in Nepal’s seismic zones, particularly in urban areas with soft soils and high water tables.

Design Considerations for Seismic Zones

When designing retaining systems for seismic zones, engineers must consider:

1. Seismic Load Calculations: Use standards like ASCE 7-16 to calculate seismic base shear and diaphragm forces, ensuring the structure can withstand dynamic loads.
2. Soil-Structure Interaction: Model the interaction between the wall and surrounding soil to predict dynamic earth pressures accurately.
3. Redundancy Factor: Apply a redundancy factor (ρ) to account for low structural redundancy in seismic design categories D, E, and F.
4. Ground Improvement: In areas prone to liquefaction, consider ground improvement techniques like grouting to enhance soil stability.

For a deeper understanding of seismic design, refer to the ASCE 7-16 guidelines for comprehensive seismic load calculations.

Advantages and Disadvantages

Diaphragm Walls

Advantages:

• High seismic resistance due to rigidity and continuity.
• Excellent water-tightness, reducing liquefaction risk.
• Minimal ground disturbance, ideal for urban areas.
• Adaptable to various soil types and deep excavations.

Disadvantages:

• High construction cost and time.
• Requires specialized equipment and skilled labor.
• Complex quality control processes.

Pile Walls

Advantages:

• Cost-effective, especially contiguous pile walls.
• Faster construction for smaller projects.
• Suitable for temporary retaining structures.

Disadvantages:

• Lower seismic resistance, especially for contiguous walls.
• Potential for soil and water leakage.
• High noise and vibration during installation.

Conclusion: Which is Better in Seismic Zones?

Diaphragm walls outperform pile walls in seismic zones due to their superior rigidity, water-tightness, and ability to mitigate liquefaction and lateral forces. They are particularly effective in urban areas with soft or waterlogged soils, as demonstrated in Nepal’s Kathmandu Metro Project. However, their high cost and construction time may make them less feasible for smaller projects with budget constraints.

Pile walls, while cost-effective and faster to construct, are less reliable in seismic zones unless designed as secant walls with additional reinforcement. In Nepal’s context, where seismic risks are high, diaphragm walls are the preferred choice for critical infrastructure.

For projects requiring a balance of cost and performance, a hybrid approach, combining diaphragm walls for critical sections and secant pile walls for less demanding areas, may be optimal. Engineers must carefully assess site conditions, seismic hazards, and project requirements to make an informed decision.

Read More: How Groundbreaking Diaphragm Walls are Transforming Nepal’s Foundation Technology