Cable Bolt Pattern Calculator
The pattern is treated as a regular rectangular grid over the plan area.
Enter Values
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the length and width of the back or wall area to be cable bolted, in metres.
- Enter the ring spacing (bolt-to-bolt distance along the drive) and the in-ring / collar spacing (across the drive).
- Optionally enter the cable length per bolt to get the total cable metreage to order; read off the bolt count, grid layout and support density.
How it works
The pattern is treated as a regular rectangular grid over the plan area. The number of rings is floor(length / ring spacing) + 1 and the number of bolts per ring is floor(width / in-ring spacing) + 1; multiplying them gives the installed cable-bolt count. The +1 in each term places a bolt on the first and last row and column so the edges of the support zone are not left bare.
Two support densities are reported. The nominal density is 1 / (ring spacing x in-ring spacing) bolts per square metre, i.e. one bolt per grid cell, which is what design charts usually quote. The installed density is the actual bolt count divided by the plan area and is typically higher because the shared end rows and columns are counted. Total cable length is simply the bolt count multiplied by the cable length per bolt. This is a geometric layout estimate only and does not size the cables for ground load.
Worked example
Cable bolting a 30 m x 5 m back on a 2.0 m x 1.5 m grid. A jumbo-mined drive back 30 m long and 5 m wide is to be secondarily supported with cable bolts on a 2.0 m ring spacing (along the drive) and 1.5 m in-ring spacing (across). Rings = floor(30 / 2.0) + 1 = 16; bolts per ring = floor(5 / 1.5) + 1 = 4; total = 16 x 4 = 64 cable bolts over 150 m2. Nominal density = 1 / (2.0 x 1.5) = 0.333 bolts/m2, while the installed density is 64 / 150 = 0.427 bolts/m2 because the end rows and columns are included. At 6 m of cable per bolt that is 64 x 6 = 384 m of cable to order.
Common mistakes
- Confusing ring spacing with in-ring spacing. Ring spacing is measured along the drive (from one ring of cables to the next); in-ring spacing is measured across the drive between adjacent cables in the same ring.
- Expecting bolt count = area / (spacing x spacing). That gives the nominal grid-cell figure and undercounts a real pattern, which includes bolts on both end rows and columns.
- Reading the cable length output as ground support capacity. This tool lays out a pattern and totals the cable metreage; it does not check that the cable rating, embedment or pattern is adequate for the rock mass or stress.
Frequently asked questions
What is the difference between the nominal and installed density?
Nominal density is one bolt per grid cell, 1 / (ring spacing x in-ring spacing), and matches the value most design charts quote. Installed density is the actual bolt count divided by the plan area. Because the pattern places bolts on both end rows and columns, the installed density is usually a little higher than the nominal figure for a finite area.
Does this tool size the cable bolts for the ground load?
No. It is a geometric pattern and quantity calculator only. Cable capacity, plate and barrel-and-wedge selection, embedment length and pattern adequacy must come from a site-specific geotechnical ground-support design and be signed off by a competent professional.
Why does the bolt count add one to each direction?
A regular pattern needs a bolt at the start and the end of every span, so an area covered by N whole spacings actually carries N + 1 bolt positions in that direction. The floor() function counts the whole spacings that fit, and the +1 adds the final edge bolt.
Can I use this for rock bolts or resin bolts instead of cables?
Yes. The grid geometry is identical for any pattern-installed support element, so it works for rock bolts, resin bolts, split sets or friction bolts. Just enter the spacing and element length for those bolts. Only the terminology differs.
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