THE ELECTRICAL ESTIMATOR

UNDERGROUND

Getting Down and Dirty.

Underground

Underground can be priced in 3 ways depending on the scope of the works.

  1. Day rate
  2. Linear meter rate
  3. Volume m3 rate

Day Rate

The simplest and lowest risk is a day rate. It is simply the cost of day hire of any machinery plus labour and materials required.

This is best suited for small jobs where regardless of how small the required trenching, there will be fixed hire rates for machinery. The fixed costs are then added to the labour rate and materials required to be installed.

Finding rates for trenching with machinery, get in contact with your machine supplier and ask for their average trenching times so you have an idea of how long a given size of trenching will take.  

This will help to confirm the days required and your labour.

Linear Meter Rate

Next method would be a linear meter rate. This is where a cost per meter of trenching is used. It is a simple and fast. It is best utilized when you are doing continuous familiar trenching where you know what your costs are from previous projects.

Volume (m3) Rate

Finally is a volume rate. This is a little more complex method however and will expand on it more below. It will yield the most accurate and flexible pricing method and applicable to any amount of complexity.

Design and Pricing

When approaching trenching with any of the above methods, you want to always keep the actual trenching component separate from any materials to be installed. Don’t bundle, for example, the costs and labour for trenching and conduits together into one rate.

Keeping it all separate give you flexibility to changes to scope and as projects increase in complexity.

There are 3 main steps to pricing any trenching and your pricing should also be grouped this way:

  1. Excavation
  2. Installation
  3. Back Fill

Excavation

This is the digging part (obviously). The factors to consider with excavation are; the length of trenching, the depth, the width, the soil condition and any possible existing services.

Costs will be your digging method (shovel, machine, pliers…) and labour (apprentice, operator, spotter).

When calculating the minimum size of your trench allow 50mm either side of the trench plus the conduit size. If multiple conduits allow 50mm between each conduit. For shared services be aware of the separation requirements in AS/NZS 3000 such as 100mm from a communications conduit (if running comms and power in shared trench).

For example: If you are installing 4 x 100mm electrical conduits in a single layer of 4 wide your trench size will be;

Depth:

  • 50mm (bottom of trench covered with sand)
  • 100mm (conduit)
  • 500mm (cover)  
  • Total 650mm depth.

Width:

  • 50mm (side of trench to first conduit)
  • 100mm (first conduit)
  • 50mm (Space between first conduit and second conduit)
  • 100mm (second conduit)
  • 50mm (Space between second conduit and third conduit)
  • 100mm (third conduit)
  • 50mm (Space between third conduit and forth conduit)
  • 100mm (forth conduit)
  • 50mm (Space between forth conduit and side of trench)
  • Total 650mm width.

For this trench the volume of material to be moved per linear meter would be 0.65 x 0.65*1.0 = 0.42m3 per linear meter of trench.

Keep in mind that you don’t want to excavate more than 1.5m deep (sometimes shallower in loose soil or sand) or stepping and shoring will be required. If possible make the conduit configuration wider and add width to the trench to avoid, where possible, excessively deep trenching.

Installation

Conduits… whatever you are installing in the ground. Materials and labor for conduits, bends, pits, marker tape.

Pretty straight forward. A simple take off of material quantities and labour to install.

Back Fill

Back fill will depend on the soil and site requirements. It could simply be filling the hole back in with the excavated material or you may need to allow sanding of conduits or the site requirements may call for the despoil of spoil and a particular backfill material provided.

When backfilling with the excavated material only a labour component will need to be considered.
If additional material is required, calculate the required amount in m3 by multiplying the width of the trench by the length by the depth of fill required.

For example: If you are required to lay a 50mm bedding of sand and a sand cover of 50mm with a conduit depth of a single layer of 100mm conduits. So sand depth would be 50+100+50 = 200mm.

If we continued with the first example (a trench width of 650mm) the required sand would be 0.2 x 0.65 x 1.0 = 0.13m3 per linear meter of trench.

Putting it Together

Now we have the principals it’s just a matter of measuring off the physical length of trenching and what materials are in them.

Applying the most applicable pricing strategy (Day Rate/Linear/Volume).

Punch in the numbers (excavation + materials & installation + backfill) utilizing machine higher rates, material costs and labor rates and you’re off.

Labor rates can be a combination of personal time records or looking up rates of labour from industry standards such as NECA.

Tips

Projects with multiple different trenches.

Assign a type (number/letter) to each different trench and take of the total length for each type.