Grading Design
Balancing Dirt Onsite?
When a land planner or civil engineer is starting a grading plan design, one of the prime goals is to make sure the site balances. What this means essentially is that the cuts and fills are equal and that no import/export of dirt is required. Moving soil around with scrapers and bulldozers onsite is typically more cost effective, while having to transport dirt in trucks can be much more costly. But the balancing of dirt is not always just a simple calculation of the cuts and fills.
A competent grading designer will always take into account the soils report. If a site has alluvial soils, which are not compacted enough for safe building construction, the recompaction of this type of soils generally creates the need for more dirt – referred to as “shrinkage”. Conversely, if a site has expansive soils, the grading will create additional dirt quantities – referred to as “bulkage”. As well, the management of rock onsite can be a factor. Thus, the soils report is an important tool in managing grading quantities.
Another factor is the “spoils” of dirt that are generated when the house footings are trenched and when underground utilities are installed. This post-grading construction dirt needs to be placed somewhere or be exported offsite in those expensive trucks. A common dirt strategy is to leave a portion of the site lower than its intended finished grade, usually the last phase, so that the spoils can be transported locally onsite. The goal would be that the spoils finish off the last phase grades and avoid the export costs.
But in many cases, the ultimate grading design may not be able to avoid importing or exporting dirt. The value of lots can outweigh the costs of a site not balancing. I seem to hear more often that importing dirt can be cheaper, as other developments tend to have excess dirt and will deliver to a site free of charge. A lot has to do with timing, and some markets will have some “dirt brokers” who track the developments and coordinate the import/export of soil.
As always, feel free to share your experiences, comments, and questions below.
