Deep excavations are integral to the construction of foundations for tall buildings, basements, and underground facilities. However, they come with inherent risks to nearby structures, including settlement, cracking, and in extreme cases, structural failure. To safeguard adjacent buildings, it is crucial to implement effective measures during the excavation process. This article explores various strategies to minimize the impact of deep excavations on nearby structures, categorized into three main approaches: utilizing excavation-induced deformation characteristics, enhancing the stiffness of the retaining-strut system, and employing auxiliary methods.
Building Protection Using Characteristics of Excavation-Induced Deformation
Excavation-induced deformation refers to the changes in soil and structural behavior caused by earth pressure during digging. By addressing the causes of this deformation, we can mitigate its effects on adjacent buildings.
Reducing the Unsupported Length of the Retaining Wall
Retaining walls are essential for stabilizing the sides of an excavation. Their effectiveness depends on their support system, typically struts, which reduce deformation by counteracting earth pressure.
- Impact of Unsupported Length: The longer the unsupported length of the retaining wall, the greater the deformation due to earth pressure. Reducing this length minimizes the potential for wall movement and associated ground settlement.
- Recommendations: Struts should be placed as close to the excavation level as possible. Ideally, the distance between the strut and ground level should not exceed 0.5m at each excavation stage. This approach is supported by empirical observations showing that shorter unsupported lengths lead to significantly reduced deformation.
Decreasing the Influence of Creep
Creep is a time-dependent phenomenon where deformation increases under constant stress, particularly in clay soils. If not addressed, creep can exacerbate the deformation of retaining walls and surrounding ground.
- Mitigation Measures:
- Install struts immediately after completing each excavation stage to limit the time creep has to act.
- Apply a 100mm layer of concrete on the excavation surface as a temporary measure to counteract creep until struts are installed. This layer provides a buffer, delaying soil movement.
1.3 Leveraging the Corner Effect
When diaphragm walls are used in excavation, deformation is generally less pronounced at corners and along shorter excavation directions. This phenomenon, known as the “corner effect,” can be strategically utilized to protect nearby buildings.
- Implementation: Diaphragm walls are recommended for excavation sites where nearby buildings are located near corners or along the short sides. This setup naturally minimizes soil movement and reduces the risk of settlement-induced damage.
Building Protection by Increasing the Stiffness of the Retaining-Strut System
The stiffness of the retaining-strut system plays a critical role in controlling excavation-induced deformation. A more rigid system resists soil movement better, offering enhanced protection to nearby structures.
Techniques for Stiffness Enhancement:
Reducing Vertical and Horizontal Span of Struts:
- Reducing the distance between struts increases their effectiveness in counteracting earth pressure. This is the most impactful approach for reducing deformation.
Increasing Retaining Wall Thickness:
- While increasing wall thickness can contribute to system stiffness, its effect is often less significant compared to reducing strut spans.
Enhancing Strut Stiffness:
- Increasing the stiffness of individual struts can further reduce deformation. However, this approach may have diminishing returns if the struts are already highly rigid.
These measures should be tailored to the specific site conditions, ensuring that the balance between cost, practicality, and structural safety is maintained.
Building Protection by Utilizing Auxiliary Methods
Auxiliary methods serve as supplementary strategies to reduce settlement and wall deformation. Their application depends on the unique conditions of the site, the surrounding buildings, and the severity of excavation impacts.
Common Auxiliary Methods:
Ground Improvement:
- Strengthening the soil around the excavation through grouting or soil stabilization techniques reduces the likelihood of settlement and wall deformation.
Counterfort Walls:
- These are reinforced walls designed to resist lateral earth pressure. They are particularly effective in preventing excessive wall movement.
Cross Walls:
- Cross walls are additional vertical barriers within the excavation area, providing extra resistance against soil pressure.
Micropiles:
- These small-diameter piles are drilled into the ground to support structures and prevent settlement.
Underpinning:
- This technique strengthens the foundation of an existing building by extending its depth or breadth. It is ideal for structures directly adjacent to deep excavations.
Considerations for Auxiliary Techniques:
Before implementing auxiliary methods, a detailed analysis is necessary to evaluate their cost, feasibility, and effectiveness. In some cases, poorly executed auxiliary measures can worsen site conditions or significantly increase project costs without yielding the desired results.
Deep excavations present significant challenges for the safety of nearby structures. However, with careful planning and the implementation of effective strategies, these risks can be mitigated. Key approaches include managing deformation through optimal retaining wall configurations, increasing the stiffness of the retaining-strut system, and employing well-assessed auxiliary methods. By adopting these measures, construction projects can proceed with minimal impact on surrounding buildings, ensuring both safety and structural integrity.