Soil Behavior in Deep Excavation Structures

Soil Behavior in Deep Excavation projects is one of the most critical factors determining the success or failure of a civil engineering project. A misunderstanding of how the soil responds to changes in stress and loading conditions can lead to wall slippage, severe settlement, or even the collapse of adjacent structures. With the increasing depth of excavations in urban projects and limited space for implementing retaining structures, the importance of accurately analyzing Soil Behavior has significantly grown.

In this article, you will become familiar with Excavation Stability analysis and modern numerical methods for predicting Soil Behavior. It also covers effective geotechnical software, relevant codes and standards, and solutions for stabilizing excavations in different soil types. We will address key and practical questions that every civil engineer, geotechnical engineer, or project manager must know.

❓ Questions you will find answers to:

• How is Soil Behavior assessed in Deep Excavation projects?

• What are the most important factors affecting Excavation Stability in clayey soils?

• Which software is more suitable for numerical analysis of Deep Excavation?

• How can Excavation Stability be improved in weak soils?

• What codes and standards are used for designing excavation support systems?

🔍 If you’re looking for precise answers to these questions, don’t miss the rest of the article.

Soil Behavior in Deep Excavation Projects: An Introduction to Geotechnical Analysis

Understanding Soil Behavior in Deep Excavation projects is one of the fundamental pillars of success in geotechnical engineering. When excavating at depths greater than 5 meters in civil engineering projects, the pressures exerted by adjacent soil layers, wall instability, and unexpected deformations can pose serious threats to the safety of structures. In such cases, a precise analysis of Soil Behavior based on mechanical properties, soil type, moisture, and stress history is essential.

Studies have shown that in fine-grained soils, such as clay, creep and swelling phenomena can cause significant settlement in excavation walls. For example, in the Tehran Metro Line 7 project, the instability of one excavation was caused by an incorrect estimate of Soil Behavior and the pore water pressure, leading to soil displacement and cracks in surrounding buildings. This case clearly demonstrates the importance of understanding the soil’s stress-strain behavior.

In Soil Behavior analysis, parameters such as modulus of deformation, internal friction angle, permeability coefficient, and drained and undrained shear strength play a key role. For instance, a study published by Sharif University (2019) revealed that in dry sandy soils with high density, the Excavation Stability was 25% higher than in saturated sandy soils. This study confirms the importance of accurately understanding soil type and its conditions in Soil Behavior analysis.

On the other hand, numerical methods like PLAXIS and GeoStudio allow engineers to predict the dynamic Soil Behavior under real loading conditions through advanced modeling. In urban excavations, which are typically conducted in densely built environments, accurate modeling of Soil Behavior helps prevent serious accidents and extensive financial losses.

Soil Behavior is influenced by factors such as overburden pressure, horizontal stresses, groundwater, and even the history of previous loadings. Therefore, a comprehensive geotechnical analysis must consider all of these factors. Inadequate analysis of Soil Behavior can sometimes result in complete excavation wall failure and collapse, as reported in projects in the Mazandaran province, where walls collapsed due to neglecting soil parameters.

In conclusion, understanding and properly analyzing Soil Behavior is the first step in designing safe and stable Deep Excavation projects. This foundational knowledge should be prioritized before any structural design or implementation of support systems to prevent financial, human, and environmental damage.

The Role of Soil Type in Excavation Stability: Why Understanding Soil Behavior is Crucial

In Deep Excavation projects, the type of soil has a direct and decisive impact on the stability of the excavation and the performance of the walls. Each type of soil has its unique characteristics that affect how the soil behaves under excavation stresses. Therefore, accurately understanding the soil type in the early stages of geotechnical design can significantly reduce potential risks.

For instance, clay soils have adhesive properties and high load-bearing capacity, but they swell and lose strength when saturated. On the other hand, sandy soils are highly sensitive to groundwater and, if not properly drained, increase the risk of excavation wall sliding. Studies conducted at the Petroleum Industry Research Institute have shown that in 70% of Deep Excavation projects with wall collapse, the soil type or incorrect analysis of Soil Behavior played a central role.

In the Shariati Hospital project in Tehran, initial soil sampling revealed a layer of saturated clay at a depth of 12 meters. Engineers, with a correct understanding of the soil’s behavior and the weakness of this layer in shear strength, used a combined nailing and anchoring system to ensure excavation stability. The result of this precise geotechnical analysis was the successful completion of the excavation without any settlement or cracking in nearby buildings.

One of the common methods for understanding soil type and assessing Soil Behavior is conducting in-situ tests like SPT (Standard Penetration Test) and CPT (Cone Penetration Test). These tests provide accurate data on the relative strength, compaction, and settlement potential of the soil. With this data, engineers can decide whether the excavation requires specific stabilization methods.

According to a study published in the ASCE Geotechnical Journal (2022), selecting the appropriate support system in Deep Excavation projects should be directly linked to the type of soil. This research, which reviewed over 40 international projects, found that in fine-grained soils, delayed collapses were more common, while in coarse-grained soils, sudden collapses were more frequent. Therefore, an accurate analysis of Soil Behavior is essential to predict critical scenarios.

In conclusion, soil type not only influences the type of excavation support system but also determines the project’s execution timeline, final costs, and safety risks. Without a correct understanding of Soil Behavior and its effect on excavation stability, any operational action may result in failure or accidents.

The Impact of Groundwater on Soil Behavior in Deep Excavations

Soil behavior is directly influenced by hydrological conditions, particularly the groundwater table. In Deep Excavation projects, the presence or rise in the groundwater table can lead to a significant decrease in excavation stability. The primary cause of this phenomenon is the increase in pore pressure within the soil and the reduction in effective shear strength, ultimately resulting in the sliding or collapse of excavation walls.

According to Clause 7-3-3 of the 7th section of the Iranian National Building Code (2020 edition), effective shear strength of soil is defined as follows:

$\tau = c’ + \sigma’ \cdot \tan(\phi’)$

Where:

• $\tau$ is the effective shear strength,

• $c’$ is the effective cohesion,

• ${\sigma }^{\mathrm{\prime }}$ is the effective stress (total stress minus pore water pressure),

• $\phi’$ is the internal friction angle of the soil.

An increase in the groundwater level causes an increase in pore pressure

$u$

$\sigma’ = \sigma – u$

which directly reduces the shear strength of the soil. This reduction in strength is one of the main reasons for failure in Deep Excavation projects in areas with high groundwater levels, such as northern Tehran and riverbanks.

In a project in Rasht city in 2021, a sudden decrease in excavation stability at a depth of 8 meters, due to the unexpected rise in groundwater levels after heavy rainfall, resulted in the sliding of part of the wall. After geotechnical investigations, it was found that the effective strength of the soil had decreased from 45 kPa to 18 kPa, which was a clear sign of failure in the initial Soil Behavior analysis and neglecting the impact of groundwater.

In Eurocode 7 – Geotechnical Design (EN 1997-1), Clause 2.4.1 emphasizes that pore water pressure should be fully accounted for in design analysis, and the use of “worst-case” conditions, i.e., the highest groundwater level, is recommended. According to this standard, the safety factor for excavation stability should be at least 1.5 in the presence of groundwater.

To mitigate the negative effects of groundwater on soil behavior, measures such as horizontal drainage systems, pumping wells, the use of cut-off walls, or cement grouting are implemented. For example, in the Deep Excavation project of Iran Mall, diaphragm walls with a depth of 40 meters were used to control water infiltration, which significantly enhanced the stability of the excavation.

In conclusion, groundwater is one of the most complex and crucial factors affecting soil behavior in Deep Excavation projects. Accurate hydrogeotechnical analysis and the use of design standards such as Eurocode 7 and the 7th section of the Iranian National Building Code should be incorporated at every stage of design and execution to ensure excavation safety and prevent extensive damage.

Stress and Strain Analysis in Surrounding Soils: The Foundation of Proper Deep Excavation Design

In Deep Excavation projects, accurate analysis of the stresses and strains developed in the soil around the excavation is a fundamental step in evaluating the excavation’s stability. Soil behavior under loads caused by excavation changes, and depending on the soil type, the amount of strain from wall settlement or sliding can disrupt the performance of the support system.

According to Clause 7-5-3 of the 7th section of the Iranian National Building Code (2020 edition), it is recommended to perform stress-strain analysis of the soil in excavation projects using limit state design methods or numerical methods such as FEM (Finite Element Method). Additionally, Eurocode 7 (EN 1997-1, Section 9.3) stresses that nonlinear behavior of soils and the effect of strain on resistance should be considered in the design of retaining structures.

Key Formulas for Stress and Strain Analysis:

Effective Stress:

$$\sigma’ = \sigma – u$$

Where:

• • σ′ is the effective stress,

  • $\sigma$ is the total stress,

  • $u$ is the pore water pressure.

Shear Strain ( $\gamma$) is defined as:

$$\gamma = \frac{\Delta u}{h}$$

​Where:

• • $\Delta u$ is the horizontal displacement of the retaining wall,

  • $h$ is the excavation depth.

For a better understanding, note that clayey soils exhibit hardening behavior with low strain but become softening at higher strains. This greatly impacts the excavation behavior. For example, in dense sandy soils, displacements caused by excavation may create small strains, which, as the stresses increase, may lead to sudden settlements in adjacent structures.

Real Engineering Example:

In the construction of the Valiasr Metro Station in Tehran, excavation to a depth of 22 meters was carried out in fine-grained soil (clay-silt). Initial analysis based on triaxial tests and plate loading tests revealed that shear strains in the excavation walls could reach up to 2%. Numerical analysis (using Plaxis) showed that tensile stresses in the lower layer of the retaining wall exceeded the allowable limits. As a result, the initial design of the bracing system was revised, and combined nailing with jet grouting was used to ensure the excavation’s stability.

According to the 7th section of the building code, the allowable displacement of retaining walls in urban projects should not exceed 0.5% to 1% of the excavation depth. This criterion is crucial in strain design and analysis, as excessive strain not only reduces excavation stability but also causes settlement in adjacent buildings.

Accurate stress and strain analysis around the excavation is a key tool for predicting wall behavior, evaluating settlements, and optimally designing retaining systems. Neglecting soil behavior in strain conditions can lead to sudden and costly failures in Deep Excavation projects. Utilizing valid codes like the 7th section of the Iranian National Building Code and Eurocode 7, along with advanced numerical modeling, provides a precise and scientific solution for achieving sustainable safety.

Numerical Analysis of Soil Behavior with Geotechnical Software in Deep Excavation Projects

In large-scale deep excavation projects, numerical analysis serves as an advanced tool for accurately modeling soil behavior and assessing the stability of the excavation. Numerical methods allow for precise predictions of phenomena such as settlement, wall sliding, stress and strain development, lateral pressures, and groundwater effects.

Importance of Numerical Analysis in Excavation:

Numerical analysis typically employs methods like the Finite Element Method (FEM) or the Finite Difference Method (FDM). These methods allow for modeling different soil layers, structural characteristics of the support system, groundwater effects, variable loads, and complex boundary conditions. Selecting the appropriate soil behavior model (such as Mohr-Coulomb, Hardening Soil, or Soft Soil) is critical for simulating accurate soil behavior.

Notable Software for Deep Excavation Analysis:

Regulatory References:

According to Eurocode 7, Section 2.4.7, and also Chapter 7 of the National Building Regulations, Clause 7-6-4, it is recommended that numerical analysis complement limit equilibrium analyses in high-risk projects or deep excavations. These regulations emphasize that the chosen behavior model must be based on soil mechanics tests (such as triaxial, consolidation, and plate loading tests).

Real Example:

In the construction project of the Line 7 Tehran Metro Tunnel near Beryanak Square, Plaxis 3D was used for simultaneous analysis of both station excavation and access tunnels. The modeling results indicated that with the implementation of a nailing system along with concrete rings, the lateral displacement of the walls remained below 1% of the excavation depth, ensuring excavation stability.

Numerical analysis is a powerful and essential tool for assessing soil behavior in deep excavation projects. Choosing the right software and behavior model, along with a complete understanding of the site’s geotechnical conditions, can scientifically ensure the safety of the excavation. Given the growth of large urban projects in Iran, mastering these software tools and utilizing them in conjunction with recognized international and national regulations is essential for geotechnical engineers.

Improvement Solutions for Excavation Stability Through Understanding Soil Behavior

The stability of deep excavations is directly influenced by the behavior of the surrounding soil. A proper understanding of this behavior allows for the selection and implementation of appropriate measures to improve stability. The following outlines several effective methods in this regard:

Improving Soil Mechanical Properties Using Injection and Mixing Methods

One of the effective methods for enhancing excavation stability is improving soil mechanical properties through techniques like jet grouting and deep cement mixing. A study in 2023 showed that using jet grouting in soft soils resulted in reduced wall deformation and surface settlement. The combination of these methods led to a 15% reduction in horizontal wall deformation.

Optimization of Retaining Systems Design

The use of advanced numerical models for analyzing soil behavior and optimizing retaining system designs plays a crucial role in enhancing excavation stability. A study in 2024, using hardening soil models, demonstrated that precise analysis of stresses and strains in soft clay soils could lead to a more optimal design of retaining systems.

Controlling Groundwater Levels

Groundwater levels have a significant effect on soil behavior and, consequently, excavation stability. Lowering groundwater levels can increase effective stress and reduce stability. Therefore, monitoring and controlling groundwater levels using drainage wells and pumping systems are critical.

Using Combined Retaining Systems

In situations where the soil has poor mechanical properties, using combined retaining systems such as diaphragm walls with steel bracing can improve excavation stability. A 2024 study showed that combining these systems in silty soils reduced wall deformations and increased safety.

Advanced Numerical Evaluation and Analysis

Using numerical analysis software like PLAXIS and FLAC for accurately modeling soil behavior and predicting deformations and stresses around the excavation allows for more optimal designs and reduces risks associated with excavation. A 2024 study showed that choosing the appropriate behavior model for the soil had a significant impact on the accuracy of the results.

A proper understanding of soil behavior and the use of appropriate improvement methods plays a key role in increasing the stability of excavations in deep excavation projects. Combining methods to enhance soil mechanical properties, designing optimal retaining systems, controlling groundwater levels, and utilizing advanced numerical analyses can reduce risks and increase safety in these projects.

Frequently Asked Questions (FAQs) about Soil Behavior and Deep Excavations

How is soil behavior assessed in deep excavation projects?

Soil behavior in deep excavation projects is assessed through geotechnical studies, field tests (such as SPT and CPT), and laboratory tests (such as triaxial tests). This information is used to determine the soil’s mechanical properties (such as permeability, internal friction angle, shear strength, and modulus of elasticity). Soil behavior is then modeled using software like PLAXIS, GeoStudio, and FLAC, and stress-strain conditions, settlement, and excavation stability are analyzed.

What are the main factors affecting excavation stability in clay soils?

In clay soils, factors such as the type of clay (soft or hard), saturation level, presence of weak layers, pore water pressure, and creep behavior significantly affect excavation stability. Additionally, poor drainage, climatic changes, and loads from adjacent structures can lead to failure or sliding of excavation walls. Using retaining systems such as piles and braces and controlling groundwater pressure is recommended to mitigate these issues.

Which software is best for numerical analysis of deep excavation?

Among geotechnical software, PLAXIS is the most popular choice for deep excavation analysis due to its advanced modeling capabilities, consideration of nonlinear soil behavior, and stress-strain analysis. GeoStudio is effective for modeling slope stability and drainage analysis. FLAC3D is recommended for 3D analysis and modeling complex structures. Software choice depends on the project type, required accuracy, and the user’s modeling proficiency.

How can excavation stability be improved in loose soils?

To improve excavation stability in loose soils, methods such as cement grout injection, stone columns, jet grouting, soil nailing, and combining retaining systems can be used. Additionally, controlling groundwater levels with horizontal drainage or pumping wells is crucial for increasing stability. These improvement methods are optimized based on numerical analysis of soil behavior and geotechnical testing.

What regulations are used for designing excavation retaining systems?

The key regulations for designing retaining systems in deep excavation projects include:

• AASHTO LRFD (2017) for designing retaining walls and piles

• Eurocode 7 (EN 1997) for geotechnical soil analysis and slope stability

• ASTM D5778 and ASTM D1586 for soil testing methods

In Iran, publications 282 and 608 from the Planning and Budget Organization and regulations from the Engineering System Organization are used.

These regulations provide precise guidelines for loading, safety factors, allowable soil resistance, and stabilization methods.

Understanding Soil Behavior: A Determining Factor in Project Safety

In this comprehensive article from civilPBD, we explored various aspects of soil behavior in deep excavations, from geotechnical assessments to numerical analysis using specialized software, international regulations, and methods for improving excavation stability. This information will help you make more informed decisions in real-world projects and avoid costly problems.

Your Opinion Matters!

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