Drainage Question 3

[ponderosa]

I am working on drainage project for a condominium complex that parallels the edge of 100' almost vertical bluff. The edge of the porches are only 20-30' away from the bluff. In between the bluff and the porches is a lawn area. To try and help slow down the rate of erosion, they want to control the drainage as much as possible by connecting the roof downspouts to a storm drain system, install an underdrain (perforated pipe with permeable material around it) by the bluff edge to catch subsurface water from watering the lawn, and have a few strategically located drainage inlets in the lawn area.


Does anyone see a problem with using the underdrain as the storm drain system also (i.e. the downspouts would be connected to the underdrain along with the drainage inlets). Due to space constraints it is difficult to fit in both an underdrain and a regular pipe ( I don't want to excavate too close to the bluff and there are utilities to contend with). One concern I have about doing this is the leaves from the downspouts might get into the underdrain and clog the holes. The other option is to have the under drain run parallel to the storm drain system and it would empty into the storm drain at the drainage inlets. Does anyone have any practical experience with this?

 

[KRSServices]

My first reaction to this, based on experience, is to run for your life! Seriously, if erosion is already a problem, then a detailed geotechnical evaluation is warranted in my opinion, because eventually, there's going to be a lawsuit as a result of the foundation slipping, and it could start with seemingly sound principals you described in your thread.

Don't believe me, then check out the mess with the City of Edmonton, when homes and residences built on the edge of a vertical embankment, certified by competent engineers as sound (complete with underdrains and systems such as you describe), were condemned and soon afterward slid into the river valley some 20 years later due to mother nature's relentless change! If you want further information email me for some contacts and historical avenues for reasearch.

KRS Services

http://www.krs-services.com

 

[sc]

Condominium? multi storey?

Danger! Danger! Danger!

Assuming you have had some good geotech advice that says you can stand a multi storey structure on what is essentially the edge of a cliff (very doubful), then you had better worry more about reinforcing the cliff than simple drainage erosion as the surcharge load from the building will be significant.

This is one project worth staying clear of as simple drainage fixes alone will not ensure erosion is reduced, what about wind erosion?

regards
sc

 

[jheidt2543]

The problem I see is, if the downspouts are connected to the underdrains, they will be surcharging the underdrain system. The underdrains will be acting in reverse and you will be adding to the potential for erosion. I would keep the two separate or drain them into a common, standard storm sewer.

 

[GeoPaveTraffic]

I agree with all of the above comments. Run, don't walk, away from this project. Nothing good can come of it for you, the owner, the buys, or anyone else connected with the project.

 

[BigH]

Ponderosa: What is the nature of the Bluff? What is the base of the condominium sitting on?

KRSServices gives one example - one of many - where building on or near existing bluffs subjected to lake or river erosion/action can be a problem - check out the Scarborough Bluffs in Toronto Ontario. However, given favourable site conditions (e.g., Bluffs far removed from potential scouring/undercutting/erosion due to rivers/lakes/oceans; given favourable Bluff stratigraphy (beds not dipping down away from face - Scarborough Bluffs has this and it led to several failures) etc., then with sound and experienced consultants (geotech especially), you should be able to build - but I agree with the others - such construction is frought with potential problems. But, homes/structures are built atop/at the base of slopes (Seattle, California) - there always are problem areas but it is not universal that you will have a problem; just a heavy dose of potential.

Get a good geotech who is fully conversant with stability of bluffs/slopes and drainage. Get him involved; on a personal level this might be better for you than your trying it yourself.

and keep us informed.

 

[ponderosa]

Thanks for all the helpful advice. I refunded the client's deposit and ran from the job as suggested. I've had a bad feeling about it all along and should've acted upon it earlier. Seeing your comments was enough to secure my decision. Thanks for the input.

 

[Ron]

Good move, Ponderosa. For what it's worth, the outfall rate of an underdrain and the inflow rate from a downspout don't match AT ALL!. I've seen downspouts back up and fall off the building from the weight, or blow out the bottom joint from the head pressure.

 

[ANTHAE]

USER ASSUMES RESPONSIBILITY FOR USE OF THESE OBSERVATIONS

We reviewed such a situation as you described, with the
downspouts connected to the perimeter foundation drains.
Apparently, it's actually mandated in some jurisdictions.
Not only did surface drainage leach down the foundation
and cause paving subsidence, as fines were pulled out of
the backfill, but the perf pipe subdrain eventually was
plugged/kinked, then the roof drains surcharged it to a
pressure enough to overcome foundation waterproofing,
filling the ground floor with water, and slowly sliding
the building downslope with constant moans and pops. ; )

USER ASSUMES RESPONSIBILITY FOR USE OF THESE OBSERVATIONS

 

[Focht3]

Don't connect the downspouts to the underdrains. Period.

And the issue of liability is a big deal; about eight years ago as a co-owner of a geotechnical consulting firm, I was asked to submit a geotechnical study on an apartment complex to be built on the Colorado River in New Braunfels, Texas. I refused - it looked like a lawsuit waiting to happen. My (now ex-) partner is a good friend of the project structural engineer; he convinced "partner" to do the study anyway.

As I recall, the buildings were built within 30 feet of the edge of the river; some failures began to manifest themselves before the complex was completed. By this time, I was no longer involved with that firm. They were (rightfully, in my opinion) sued - and settled.

And the water depth was only 30 or 40 feet, with a bank height of 8 to 10 feet. Nowhere near your 100 foot cliff height.

Don't walk away - [red]RUN LIKE HELL![/red]