Selecting the Right Retaining System for Excavation

To propose and choose an appropriate retaining system for excavation stability, the process begins with a thorough review of geotechnical reports and site-specific investigations. Afterward, technical options for stabilization are assessed and compared based on technical, economic, and execution feasibility to select the most efficient solution.

Key Considerations in Urban Excavation Projects

When selecting a retaining system in urban areas, it’s essential to consider both technical and economic factors. If excavation is near existing buildings, controlling deformations in adjacent structures becomes critical, in addition to ensuring overall stability.

Design Approach

The design process begins by using general code recommendations for the preliminary layout of reinforcing elements such as nails, anchors, and piles. This includes their spacing and initial lengths across the depth and surface of the wall. Optimization is then performed using analytical methods based on code-defined criteria.

Advantages of Prestressed Anchors

  1. Execution Assurance
    In anchoring systems, all anchors are tensioned up to 120% of the design load after installation, allowing for complete quality control. Any defects in the strands or grout injection are detected during tensioning.

  2. Deformation Control
    Prestressed anchors increase the stress state in the unbonded length of the soil, improving its shear strength and deformation modulus, which leads to reduced soil displacement.

    Additionally, since prestressed anchors are activated prior to each excavation stage, they eliminate the need for soil deformation to activate the system (unlike soil nails), resulting in minimal deformation.

For projects near adjacent buildings, the Berlin wall with prestressed anchors is often selected for maximum safety and minimal ground movement.

Software Used for Retaining Wall Design

In soil mechanics and foundation engineering, limit equilibrium methods are among the most practical approaches for stability calculations. These methods are also applicable for analyzing slopes and soil walls.

✅ Slope/W (GeoStudio 2007)

One of the most widely used software tools is Slope/W, part of the GeoStudio suite. It applies several limit equilibrium methods to assess slope stability and the impact of soil reinforcements such as:

  • Soil nails

  • Anchors

  • Piles

The software calculates the contribution of reinforcements to the overall factor of safety by accounting for:

  • Pull-out strength

  • Tensile strength

  • Anchor prestress forces

Pull-out strength depends on the length of reinforcement beyond the failure surface and the grout-soil bond strength. Shear resistance from piles is also included as a stabilizing force.

✅ Plaxis (Finite Element Analysis)

In addition to stability analysis, deformation analysis is crucial. This is typically done using finite element or finite difference methods.

Plaxis is widely used for displacement predictions. It uses a Stage Construction method to simulate every phase of excavation and the resulting ground deformations, enabling realistic modeling of actual conditions.