Design Block

Prior to carrying out modeling an appropriate design block needs to be defined. The guideline provides two methods of selecting the design block: a “Simplified Approach” and a “Standard Approach”.

The simplified approach may only be carried out if at least one of the following applies:

  • Less than 100 jointed rock bodies are present in the initiation zone
  • Consequence Class is defined as CC1
  • Event frequency falls under EF1 or EF2 (see table below)
Event Frequency ClassOccurrence Frequency
n (1/a)
Design Block
Size Fractile
EF 4
(very high)
n ? 10
(> 10 occurrences per year)
EF 3
1 ? n <10
(1 to 10 occurrences per year)
EF 2
0.03 ? n <1
(1 occurrence per 1 - 30 years)
EF 1
n <0.03
(<1 occurrence per 30 years)

Otherwise the standard approach must be used to determine the block size.

Simplified Approach

For the simplified approach, an expert can define the block size based on their experience and information obtained during the site investigation.

Standard Approach

The standard approach involves carrying out a statistical analysis of measured block sizes in the initiation and/or deposition zones. The design block is obtained as a fractile value of the block size distribution as shown in the above table, based on the event frequency. Thus, where there are 10 or more occurrences per year, the 98th-fractile is used as the design block and the 97th-fractile is used for sites having frequencies between 1 and 10 occurrences.

The design block is to be determined for each homogeneous area.

Next Step


  • Consequence Class

    What level of risk is there?
    Click Here

  • Design Block

    How is the design block selected?
    Click Here

  • Energy

    What system capacity is sufficient?
    Click Here

  • Height

    What system height is required?
    Click Here

  • Anchorage

    How is an anchor verified?
    Click Here

  • Performance

    What other performance criteria are there?
    Click Here

  • Constructive Rules

    How can the system be safely adapted to a site?
    Click Here

  • Maintenance

    What recommended maintenance protocols exist?
    Click Here