Non engineer asking: where in geotech does it say i need perimeter drainage 2

[Blue1997]

Hi,

I've searched and read and thought someone here could point me in right direction. From the geotech report, i do not see where it says i would need perimeter drainage. The guys say i dont need it, Water proof rep says water table is low enough so i dont need it. But, i would like to understand how to read this. I want to know where to look. Reason: grade is sloped and i have a basement condition.

 

[Ron]

If your geotechnical engineer says you don't need it, then you probably don't!

If you want to intercept surface drainage, it is easier to do so with a swale to direct the surface runoff from the sloped ground surface around your structure.

Assuming your water table is significantly below your basement foundation and slab, there is not much of a reason to put in subsurface drainage.

 

[cvg]

geotech reports provide information and recommendations based on the scope of work. the scope of work may or may not have required the geotech to make a recommendation for drainage. the field investigation should have determined the depth to groundwater, however since groundwater depth varies from year to year, again your scope of work should have requested that be estimated also. regarding surface runoff which might affect your basement, your grading and drainage plan should address any recommendations for swales or drains to manage that water. your architectual plans should handle roof drainage as well as any foundation drains.

 

[GeoPaveTraffic]

If I'm doing a basement, I always recommend a perimeter drain. Once construction is compelete, it is very diffulct and expensive to add it. During construction, it is very easy to install and fairly inexpensive.

Mike Lambert

 

[Blue1997]

Thanks for all the input. We asked the question to the geotechnical firm and they came back with perimeter drain recommendation. I guess it never hurts to ask again.

 

[oldestguy]

Did that geotech firm follow the rules of soil filtering, to keep the drain from plugging with mud? Most architects and contractors have no idea how dumb it is to backfill with gravel. The best backfill is concrete sand. Also, many a geotech prof doesn't teach this. Many a building code also wrongly calls for gravel backfill. A sock on the pipe doesn't do the job for all situations.

The state building law in Wisconsin requires it for every residential building, even 100 feet above a water table.

 

[Blue1997]

Good point oldest guy. Ill double check. Dont laugh at this question, well not too much: in a perf pipe drain do the holes in the pipe go up or down? Does it matter? Perhaps pipes are made such there are holes in both locations? I have no idea but thought I'd ask.

 

[concretemasonry]

Blue -

The holes in the PVC go at 4:00PM and 8:00PM, so they are on the bottom half of the pipe.

As cvg said it is easy and much, much cheapest to install it during construction when it is open.

The concept of clean washed rock by itself is ridiculous and there is no uniform definition of "gravel". I prefer to see a mixture of 3/4" clean rock and clean coarse sand to fill "choke) the voids between the rock and still carry the water to the PVC. I you just use 3/4" clean rock the water velocity is higher and it will fill up the the fines (clay, silt organics, etc.) from the surrounding soil. The percentage of sand depends on the coarseness of the sand.

Ideally if you really want the best, lay filter fabric against the wall, several inches under the PVC and then against the natural soils. Fill between the PVC and fabric with the sand/rock mixture.

A perforated drain system is a long term method to reduce the amount of water and pressure on the walls and slab. High flow rates are not necessary since most water runs off away from the home and hopefully the downspouts are not connected the perforated pipe.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[shobroco]

I have to say, I have never heard of anyone building a basement in our area without exterior perimeter drainage in more than 75 years, maybe 100. It is simple, easy & cheap to do when you are building, and the opposite of that next year when you have problems. And if you do it without problems, you better run out & get your Lotto 649 ticket asap.

 

[shobroco]

Sorry, forgot one comment re hole location: it depends if you are collecting or distributing. I don't want to disagree with concretemasonry but I wouldn't collect with pipes intended for distributing, which is what pipes with holes in one half are for. Use fully perforated pipe for foundation drainage, typically flexible in a roll (Big "O"). Use perforated pvc pipe for a distribution field (weeping bed) & place it holes down.

 

[concretemasonry]

A typical "french drain" uses the perforation to collect water and to distribute it. The perforated PVC runs to a sump that provides positive water removal with the use of a pump or drainage to "daylight". With an acceptable site the perforated PVC is also changed to solid wall PVC to drain to "daylight" instead of a sump. Some people also later switch from solid wall PVC to allow remote distribution at a lower elevation. Using both interior and exterior perforated PVC is the premier system.

I have a friend that built over 3000 homes and routinely installed both interior and exterior linked drains in every home (standard detail) because the cost was so low in comparison to future problems. - Standard details, materials and procedures. - No wet basements in the 3000 homes.


Engineer and international traveler interested in construction techniques, problems and proper design.

 

[jasperrupp]

Regarding Ron’s comment, I’d say that even when your geotechnical engineers say you don’t, it would be better to get a second opinion and ask for an explanation so you can better understand your situation. Just a piece of advice