Detecting source of water damage - from water main? 2

[civilman72]

I observed a site yesterday where observable ground movement has started to cause displacement and overturning in deck post footings and adjacent retaining walls. This building complex was built almost 30 years ago, and this recent movement has only occurred in the last two-three weeks. It appears this movement is being caused by an increase in hydrostatic pressure, seemingly due to a significant increase in below-surface water flow. It has been very cold (-20F to 10F) here, so it is highly unlikely that this is a natural spring popping up, or a natural rise in the groundwater level.

My first reaction was a water line leak, but the HOA representative said the Water District came out and tested the water for chlorine and found none. So they conclued it was "not their problem." I vaguely recall a conversation years ago with an individual who stated that a lack of detectable chlorine levels in water does NOT preclude this water coming from a treated water pipe. Anyone know more about this?

An issue similar to this one popped up about five years ago at a single-family house and the water district (another one) tested the water for chlorine, found none and said it wasn't their problem. After months of finger-pointing, it turned out a leaky water main was the culprit, so needless to say I'm hesitant to trust this most recent investigation by the water district.

Discussion and thoughts would be appreciated.

 

[civilman72]

GD,

1. You're right - I should have taken photos on my first visit.

2. Rocky Mountains - Colorado - 9,000 feet. According to the property manager (for the last 5 years) they have not seen any issues with the integrity of the retaining walls, until the last month. Melting/freezing water was also never observed, but is now overtopping the wall and inundating the adjacent concrete sidewalk (interestingly enough, the building crawl space is dry). While this could be a case of fatigue over time, the wall movement seems to be directly related to the recent events.

3. The snow will not normally melt in this area until June. Considering the significant recent movements, it does not seem prudent to wait four months to further investigate. We feel that if we can control the hydrostatic pressure behind the walls now, we may be able to salvage the walls by repairs (in the spring/summer). If we wait, there's a good chance they will have to be completely replaced.

 

[cvg]

too busy skiing, let me guess Silverthorne or Keystone, Vail, Dillon or Frisco or Breckenridge (no wait, Breckenridge is at 10,000). Good luck with this, it is going to get very mushy in March and April.

 

[civilman72]

cvg - all good guesses... This is about 5 minutes from Winter Park.

Are you really skiing right now?

 

[civilman72]

As requested, here are some photos.

Don't have any "big picture" photos - mostly just up-close ones of the walls.

 

[civilman72]

Update:

We dug two test holes yesterday about 50' uphill of the retaining walls - 15' in depth. Found a seam of moisture at about 6'-7' below grade, with soaked granular/sandy/boulder soils beneath that. A few hours after the holes were dug there was 1'-2' of standing water, but seepage from the sides of the trench had significantly decreased.

I took some basic measurements and developed a rough cross-section of the site, and created an assumed "water table" elevation through the site, starting from the water level in the holes down to the adjacent stream. It indicates that at the current water level in the holes, hydrostatic pressure will be induced on the back of retaining walls adjacent to the building. It also indicates that if we can control the water level in the test hole area to 10' below grade that there's a good chance the hydrostatic pressure behind the walls will be eliminated.

Over the next few days, the contractor will install temporary sumps pumps in the holes. These will discharge into a tank to determine how much water is pumped, and the downhill retaining walls will be observed to see if this area appears to dry up. If the results are positive, we are currently discussing the option of installing a french drain in the current hole location (at least 10' deep), with a daylight to surface past the buildings in order to permanently control the groundwater level.

One concern I have is the potential for building settlement if we potentially reduce/change the current hydrostatic pressure beneath the building footings. Comments?

Any thoughts or additional comments would be appreciated.

 

[dicksewerrat]

Drill holes through the face of the wall every 3-5 feet about 6 feet in. Stuf a perforated PVC pipe into the holes and push some filter fabric in the pipe to filter the drainage. Wait till spring thaw.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[civilman72]

Dick - Interesting idea, but the wall is next to a concrete sidewalk, attached to the building, so there would be no place for the drains to (safely) discharge. Plus, in this climate, all of those outlet points would freeze and be impossible to maintain.

 

[civilman72]

Final Update (hopefully):

It appears that the pumps are working. The first day the pumps ran they filled a 500 gallon tank overnight. The flows have stabilized and have been pumping a steady flow of 220-240 gallons per day. Prior to the pumping, ice buildup around the lower retaining walls was thawed. Since that time this area has remained dry.

The permanent fix will be to install french drains in the current location of the uphill pumps along with drains a few feet behind the existing retaining walls.