In excavation projects, stabilization, and retaining structure implementation, what is meant by the excavation hazard level?l

What Is Meant by Excavation Risk Level?

Excavation risk level in excavation and retaining structure projects is determined by the building designer based on excavation depth, soil type, presence of water, vibration sources nearby, and sensitivity of adjacent buildings. The risk is classified as normal, high, or very high.

Risk level evaluation follows the guidelines of Chapter 7 of the National Building Regulations (Foundations and Substructures). For projects with high or very high excavation risk, pre-excavation meetings called “joint meetings” are held onsite with the owner, supervisor, geotechnical supervisor, building designer, excavation designer, contractor, and municipal technical representative to review and coordinate plans.

Excavation Depth Definitions (See diagram below):

• Free Excavation Depth:
  Distance from the ground or adjacent roadway surface level (whichever is higher) to the excavation bottom (in meters).

• Excavation Depth Below Foundation:
  When there is a building adjacent to the excavation requiring retaining structures, this is the distance from the excavation bottom to the foundation underside of the adjacent building (meters). If foundation bottom elevation is unknown, the lowest finished floor level of the adjacent building is used.

• Effective Excavation Depth Below Foundation (Parameter ):
  To consider the influence of the number of floors in the adjacent building, the effective excavation depth below foundation (in meters) is calculated as:
  (formula involving n = number of adjacent building floors)
  If the building is farther than 1.5 meters from the excavation edge, it is not considered in excavation importance.

Soil Classification for Retaining Wall Projects:

1. Hard Soil:
Cohesive soils with uniaxial compressive strength ≥ 150 kPa, e.g., clay, silty clay, sandy clay. Natural cemented soils and firm soils are usually hard. Not considered hard if:

• The soil is part of a layered slope formation with layers dipping > 4:1 (horizontal:vertical) towards excavation, or

• The soil is subject to local moisture increasing weakness.

2. Medium Soil:
Cohesive soils with strength 50–150 kPa, and granular soils like dense sharp sand, sandy silt. Also includes soils in layered slope with ≤ 4:1 dip inward, or other moderate conditions.

3. Weak Soil:
Cohesive soils with ≤ 50 kPa strength (e.g., silty clay without cement), loose granular soils, cracked soil, fill soil, or layered slope formations with dips > 4:1 towards excavation.

Adjacent Building Sensitivity:

Refers to structures around the excavation (buildings, pipelines, roads) potentially affected by excavation. Sensitivity is determined by the building designer:

• Buildings with proper foundations and independent structural systems, no signs of deterioration, are medium sensitivity.

• Buildings with signs of deterioration are sensitive; severe deterioration makes them very sensitive.

• Very sensitive buildings include:

  • Those relying on the target building for demolition,

  • Attached or sharing common walls with the excavation building,

  • Buildings with high occupancy or special uses (schools, hospitals, mosques, cinemas),

  • Heritage or historic buildings,

  • Masonry or adobe buildings with deterioration signs.

Source:
“Enhancing Safety in Demolition and Excavation Projects in Tehran” – Official Resolution Text