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Site grading minimum slope adjacent to building foundation

sbw (Civil/Environmental) (OP)
20 Dec 10 19:17
California Building Code section 1803.3 requires 5% min slope away from the building foundation or 2% min for impervious surfaces. I have some guesses, but does anyone know for certain the rationalization behind this? Can this required minimum not be met (say 1% for concrete paving) and still meet the intent of the code?

In a nut shell, I have a client who would prefer to use 1% for the concrete flatwork around a residence. The residence is elevated on a sloping site, so beyond the flatwork the grades either meet a retaining wall (ie they drop off in grade) or are sloped at 5% or more.

Thanks in advance.
Dozerman56 (Civil/Environmental)
24 Dec 10 10:09
I'll wager it's done to ensure the area around the building drains away from the building rather than to it. Many times here in NC if something doesn't meet local standards they'll accept it if a PE will sign off on the variation. But I'd doubt that's the case in CA.
oldestguy (Geotechnical)
25 Dec 10 17:12
Another important factor is the height between the top of that earth and the wood or siding.  Some codes call for 8 inches to keep termites from taking over the house.  I have seen that violated to as low as zero or even minus some inches.  
Hoagie (Civil/Environmental)
27 Dec 10 13:01
If the project has a geotechnical report, look for their recommendations. Usually, they recommend a slope for certain lenght (i.e. 3% slope for min. 5-ft).  If you don't have a geo report, then I would recommend that you use the published 2% min for the first 5 ft, then break the slope to 1%....I think your client can accept a 0.05' difference.  
VAD (Geotechnical)
8 Jan 11 19:55
Dozerman's answer is correct. However, the overall expectation is that water from rainfall and snowmelt precipitation does not infiltrate the subsurface via the "wound" that the construction of the building foundation has placed on the earth.

Generally, the earth and concrete stem wall fail to be well knitted and eventually a separation is evident which allows water to enter close to the foundation. However we often take no consideration of the need to have the prescribed slopes during construction and as a result much water enters the foundation soil during the construction period.

With shallow foundations this water entry and often the non use of drainage based on concepts of height of interior slab in relation to height of grade on the outside of the building has been used to determine whether or not drainage at the footing level is required.

This concept is flawed since once there is apassage for water at the contact between the soil and foundation wall water enters the foundation and starts to slowly deteriorate its bearing condition. The worst case is the ready reduction of strength of near surface soils that depend on their strength from matric suction.

That water is constantly entering the vulnerable area is readily seen whan it rains against the sides of a building. If one notes rain precipitation follows the building face down to the exterior building landscape. Significant water from this process gets to the foundation. This can be readily appreciated when we look at when problems tend to occur with cracking etc.i.e afew years after construction and sometimes shortly after

There is a need to place an impermeable liner that can be sealed against the foundation wall so that this could be below the earth or pavved surface which can provide some defense against above ground water. As of yet this concept seems to be disregarded and much to the pleasure of the helical screw pile installers who have to be called in to support foundations.They seem to work hand in hand with new construction

There is much more on this drainage subject such as insufficient grade of drainage pipes at the footing level, improper installation, improper perforations etc etc that appear not to be given much consideration since these will delay production.     

Regarding Hoagie's suggestion about the geotechnical report. It would be of interest to know that most reports have this as astandard section etc without giving much thought to when it is to be used which seems to only be after construction and beutification of the exterior of the building. No thought is given to during construction phase.

Please excuse the lenth but the question asked has more far reaching effects and suggests that continuous maintenance around the building is required to prevent issues from occurring.
TGLG (Civil/Environmental)
27 Jan 11 17:51
While 1% slope on an impervious surface is more than sufficient to provide positive flow away from the foundation, it is important to remember that design standards have safety factors built into them.
  
In the military they say: "No plan survives contact with the enemy," but in engineering and construction a more appropriate statement is: "No plan survives contact with reality."  Minor mistakes happen in the field on a regular basis.  Things rarely go in *exactly* as designed.  But a good design will be able to tolerate these minor mistakes.

If you design for 2%, and if the contractor messes up you might get 1%, or maybe 0.5% and the water will still go where you want it to.
Design for 1%, and if the contractor messes up you might wind up with water flowing *towards* the foundation instead of away from it.

The difference between 1% and 2% is not really discernable without measuring instruments, and the difference in elevation from 1% to 2% over a short run, as Hoagie said above, is minimal.
It's probably best to talk your client into complying with the code.  
   
Boltricity (Civil/Environmental)
28 Jan 11 16:46
sbw,

You need to tell your client that the term "flatwork" should not be taken literally. OK, bad joke.

Seriously, though, I agree wholeheartedly with TGLG in that codes are written to (among other things) provide some factor of safety in construction. We don't live in a perfect world and aside from the fact that stuff almost never gets built exactly right, also consider that the materials (both manmade and natural, such as soil) involved in construction may not always behave exactly as predicted, either.

More to the point here, there is always the possibility of some future settlement of the concrete flatwork around the building. It could be due to water infiltration (maybe through cracks or seams in the concrete?) or for other reasons. If the flatwork is already as flat as 1% away from the building, even some minor settling could result in parts of it ending up "dead flat" with no drainage away from the building, or (even worse) draining towards the building(!).

So even if you had the best contractor in the world and built the concrete exactly at a 1% slope today, consider that future settlement could change that. A 2% slope on the concrete will allow for some settlement and still provide for positive drainage away from the structure.

Bottom line...TGLG is right. Talk to your client.
dik (Structural)
29 Jan 11 11:11
More often than not, soil adjacent to a foundation consolidates and slopes towards the foundation wall.  This is a consequence of dessication of the soil adjacent to the foundation wall and lack of compaction. I typically stipulate 2%.

Dik
brandonbw (Civil/Environmental)
26 Feb 11 10:51
I got the 5% slope on pervious from building on a plan check.  Depending on who is plan checking this will be caught.

B+W Engineering and Design
Los Angeles Civil and Structural Engineering
http://bwengr.com

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