How to Measure Excavation Production Rates
Outline
Why the heck does it cost so much to move dirt?
The last posts in the series considered measuring dirt on a volume-basis. This post considers how long it takes to move material. There’s many factors that affect how fast an excavation company can move material. The saying is more true in earthwork than probably anywhere else: time is money.
Simply getting the equipment to site can cost thousands of dollars. An extra day of equipment over plan can burn margins, so any way to improve production could greatly reduce the cost of a project.
Various factors influence how quickly production excavation can take place:
- Cycle times.
- Distance to move material.
- Site conditions and geology.
- Rehandling.
- Site Access.
Let’s break these down:
1. Cycle times
Simply put, a cycle time is how long it takes for an excavator to complete one complete digging motion, and should be measured in seconds. If a bucket is 0.5 CY, and a cycle time to load a heap from a stockpile is 30 seconds, you could expect up to 60 CY moved per hour. Of course this doesn’t include inefficiencies but can be used for production excavation situations. When measuring truck loads, it’s best to measure in minutes per truck, and consider their round trip cycles as well.
2. Distance to move material
How far does the material need to be hauled? Is it an hour? Is it five minutes? Increase Haul distance drastically decreases excavation production by requiring more trucks on the road to match cycle times with truck arrivals, or causing the excavator to wait. Trucking is costly, but this can sometimes be optimized if your disposal location for material is the same place that you need to bring in materials from.
3. Site conditions and geology
Lightly cohesive soils have a way of sticking together that make massive heaping buckets. Clayey soils can stick together and be stubborn or muddy, and cause issues for tracks. Poorly graded sandy soils are easier to excavate and can create opportunities for equipment like scrapers, but wear on equipment tracks faster. Rock creates an additional layer of complexity to projects, and it helps to have understanding of rock strengths. Schist might only have a break strength of 2,000 psi, compared to a typical Granite rock at 20,000 psi. All this time influences excavation production rates for the contractor to consider. Sources like the USGS map here can be used to help inform site surface geology.
4. Rehandling
Early in my career on a mass excavation site, a superintendent told me: “if we touch it once, we might make money, if we have to touch it twice, we’re losing money”. This effort can be planned for, but it’s important to include in an estimate when considering production rates, and durations for tasks.
5. Site access
Site conditions are worth adding to the discussion also. There is added difficulty when trying to avoid the tree that’s in your swing path, compared to when excavating an open field. More space allows equipment to be staged, allows trucks to cycle easily, and can stockpile excess material, reducing costs and improving excavation production.
If we don’t have good site access, we’ll have to pull out the shovels and that wouldn’t be very fun.
Conclusion
Moving dirt is an art and a science. The variables to excavation production are endless, but with proper planning and a good team, a constructable, cost-effective project can be performed. Let us know if you have any questions, it’s our pleasure help. Helpful spreadsheet tools are available that can be helpful, just ask.