Grading, in its most basic sense, is the redirection of surface run-off, setting of elevations and grades for pedestrian and vehicle traffic, pads for structures and amenities, and your site's interface with the existing surroundings. It is the flesh of the project body. It is the genesis of a 2-D site plan into a 3-D reality. Many good site plans fall apart once the grading is taken into consideration!
While streets may seem like a completely different topic than grading, such as a community park, the concepts are intertwined. Often a smaller project, such as an apartment complex, won't have a true street plan. It will most likely just be designed in the "grading style" without the use of alignments and profiles. The subsequent posts on this topic may deal with these sub-topic separately when appropriate. For now we will mostly focus on grading of a mostly level site.
Although there are many places you can begin when starting a new project, none are as important to the rest of the project as grading, except perhaps
site plan design. This topic will be the most robust. A proper grading concept can make or break a project for a client. For sure, grading design will have the most variation from engineer to engineer. Many techniques can be implemented and certain goals may shape the outcome dramatically.
In order to start any grading concept, you must first know your perimeter constraints. It is true the design within your boundary can be mostly as you want, but at the perimeter you are limited, especially if the perimeter is a public
right-of-way. Start by researching the grades off your site. Find any conditions which will dramatically affect your on-site design such as existing retaining walls, existing cut or fill slopes, existing buildings or structures closer than say 10 feet to your property line.
In the image above you can see there are some buildings right up against our property lines.
You must have a moderate level of confidence regarding the disposition of these items including their condition, their elevation, limits of additional loading from on-site grading operations, etc. Establish any unknowns with additional field survey. As always, it's best to visit the job site at least once, and take many photos for reference later. Even photos of things you may not think are important at the time.
Since changing site grading can redirect storm water, it's important to also know if there are any locations where perhaps your project currently allows off-site
run-on (sometimes also called run-off) to enter the property. By common law, you must either maintain that condition in it's existing state, or provide an engineered alternative means, such as a
Storm Drain system, to accept that run-on. Quantifying the run-on is an important step and is covered more in-depth in the
Hydrology topic.
A corollary to the above paragraph, common law says you cannot introduce more surface
run-off from your site than is currently leaving at any one point along your perimeter. For instance, if your site currently allows 2 cubic feet per second (
cfs) run-off to an adjacent flood control channel during a 25-year frequency storm event, then you cannot exceed that amount with your new design (this may be caused by added impervious surfaces such as pavement, or by increased area tributary to that location with grading).
In the image above, you can see there is a concrete gutter on our project and it connects to a parkway culvert to remove storm water from our site. It is dumped directly onto the public street with no water quality or flow controls. Many jurisdictions do not allow this condition once you decide to redevelop your site.
Once you have identified the above items and conditions you next need to understand the general slope of the existing site condition within your property lines. This will play a part in your conceptual grading design. You cannot, for instance, completely change the direction the site generally slopes without incurring huge costs due to added retaining walls, graded slopes, and import or export of huge amounts of soil. Likewise, any low points in the center of the project will generally need to be raised.
Now that you have identified the perimeter conditions including: run-on, adjacent structures, and also identified the existing grades within the project, you can now begin your proposed design. These previous steps may seem obvious, but you would be surprised at how many engineers simply don't start with this basic information and end up having to revise their design as a result.
Check back for the next post where we will deep dive into this vast and complicated process!
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