Pre-Excavation Guidelines and Necessary Actions

Before beginning excavation, several essential steps must be taken to ensure safety and structural stability, especially when using truss-type retaining walls:

1. Geotechnical Study Review

The supervising engineer must thoroughly review the geotechnical report, which includes:

  • Soil mechanical parameters

  • Groundwater level and its height

  • Presence of qanats and wells

  • Suitable excavation slope

  • Borehole logs, etc.

2. Stability Check of Adjacent Structures

The supervising engineer must initially assess:

  • The stability of shared boundary walls

  • The load-bearing capacity of neighboring buildings

Ideally, the retaining structure should stabilize the foundation soil of adjacent buildings. The height of the truss column should extend approximately 1.5 meters above the foundation level.

3. Additional Support for Weak Neighboring Structures

If adjacent buildings lack structural stability due to aging or deterioration, secondary retaining structures must be installed. Their type and design must be discussed with the structural engineer before implementation. The goal of a truss retaining system is to secure the excavation wall adjacent to existing buildings.

4. Arch Formation Assumption and Soil Type

This method assumes that part of the soil is restrained and the rest forms a natural arch to transfer load. In weathered soils, where arches may not form properly, this method is not recommended. For high cohesion soils, timber lagging or shotcrete is preferred.

5. Truss Walls in Urban Construction

Truss retaining walls are common in urban projects, often without detailed geotechnical studies. Therefore, layer changes during excavation must be monitored. Observations should include:

  • Cracks

  • Soil layer variations

  • Movement of adjacent structures

  • Subsurface water flow

These signs must be immediately addressed if detected.

6. Contact Between Truss and Excavation Wall

Because soil failure in this method requires minimal movement, avoid any gap between the truss and the excavation wall. If a gap exists, it must be filled with materials stronger than native soil—e.g., lean concrete, bricks, or gravel stabilized with gypsum and cement.

7. Optimal Truss Location

The truss should be installed near the columns of adjacent buildings and right at the excavation site’s edge. Proximity reduces building movement and prevents localized sliding.

8. Column Spacing Based on Soil Type

In loose, cohesionless soils, column spacing should be minimal. In denser, cemented soils, spacing may be increased.

9. Proper Truss Installation Method

Correct implementation begins with digging pits for vertical truss members at natural ground level, before excavation. After erecting the vertical elements and securing a suitable soil berm, excavation proceeds step by step while completing the rest of the truss system.

🚫 Incorrect Method: Excavating to the final depth before installing trusses is improper and must be avoided.

10. Modifications to the Retaining System

If the supervising engineer deems it necessary to strengthen or weaken the structure, this must be officially communicated by the owner/contractor to the structural engineer and followed up through project completion.

11. Dismantling Time

The removal timing of the truss is critical and must be pre-evaluated during the planning phase.

12. Use of Recycled Steel

If using old/recycled steel is unavoidable due to site limitations, material quality must be inspected and approved by the supervising engineer before use.

13. Owner Refusal to Implement Retaining System

If the owner/contractor refuses to implement the retaining structure, the supervising engineer must refer to Guide No. 1 for Supervising Engineers by the Tehran Province Engineering Organization.