Retaining Wall Maintenance Plan 5

[phamENG]

One of the municipalities near me has instituted a new retaining wall ordinance. When a retaining wall is being built, the engineer of record now has to provide life cycle estimates and a maintenance plan. (The owner then has to file a restrictive covenant on the deed agreeing to abide by the maintenance plan.)

This is a new one for me. Anyone have any suggestions? The wall I'm working on is cast in place concrete. Here's what I'm thinking for the maintenance plan:

Upon Completion of Construction:
Survey to fix location of top of wall and bottom of wall (horizontally and laterally) at points not to exceed 20' along the length of the wall.

Every year:
-Clean the wall of any algae or moss growth
-Visual inspection of the wall to ensure no cracking, weeping of water through joints, or noticeable movement of the wall is occuring.
-Inspect drainage to make sure it isn't clogged. (I'm considering requiring a clean out at the high end of the drain to allow a snake to be run through it- thoughts?)


Every five years:
-Survey the wall to check movement of wall against post construction benchmarks. If 3 consecutive surveys show less than 1/2" of movement over the 10 year period, discontinue surveys. If surveys show movement of greater than H/100, then the wall should be checked more often.


As for life cycle...I know it could last 100 years if built well and maintained...but I feel hesitant to put more than 50 years on it. Anyone have any good data sources for infrastructure lifecycle estimates?

Thanks.

 

[OldDawgNewTricks]

Just a couple thoughts:
1. I would advise you to consult your insurance/legal advisor prior to agreeing to make any statements that could possibly be interpreted as warranties of service life. In my experience, it is not part of the usual standard of care for typical structural projects.
2. If #1 is resolved first, I would still only feel comfortable giving broad and conservative ranges of estimated service life unless actual service life modeling of the concrete is done. My firm requires service life modeling sometimes, but this is typically for very large and important projects in marine environments. It is quite expensive and takes a long time to do the service life modeling.

 

[BAretired]

That ordinance should increase the cost of retaining walls substantially in that municipality.

BA

 

[phamENG]

OldDawgNewTricks - fantastic idea. I've contacted my agent to get that ball rolling.

BA - you have no idea. It also says that a RDP has to oversee construction of the wall. Not sure how that's going to work out just yet...

 

[Craig_H]

Which councillor's kitten got flattened by a falling retaining wall?

I have a feeling that something this ridiculous won't have a long life. Personally, I'd try and wait it out. Chances are, it'll get changed or removed once a few people see how unwieldy it is in practice.

 

[bridgebuster]

I did something similar about 4 years ago for a grade separation project in NY, which required construction of 16 walls, ~1300LF. The town asked for a life cycle cost analysis.

Technical Assumptions
The Town of North Hempstead will be responsible for three tasks: inspecting, maintaining, and repairing the retaining walls. The following general and task specific assumptions were made in the analysis:

General
1. Reinforced concrete walls.
2. Service Life of 75 years.
3. Annual Inflation of 3%.
4. Discount Rate of 4% (per NYSDOT Bridge Deck Evaluation Manual, 1992, Section III E).
5. Walls will be replaced 75 years after the End of Construction (EOC).
6. Replacement costs are not included.

Biennial Maintenance
1. Biennial maintenance includes items such as graffiti removal, vegetation removal, and power washing.
2. Maintenance tasks are performed by two laborers at the prevailing wage, 40 hours each per year.
3. When capital repairs are performed maintenance costs are assumed to be included in the repair costs

Inspection (5 year cycle)
1. Hands on structural condition inspection will be performed every five years.
2. There will be 14 inspection cycles from EOC+5 to EOC+70.
3. The inspection will be performed by a two-person team led by a registered NYS Professional Engineer.

Repairs (15 year cycle)
1. Structural repairs are assumed to be required at EOC+15, EOC+30, EOC+45, and EOC+60.
2. The wall area that will require repairs is assumed to be 5% of the wall surfaces at EOC+15; 10% at EOC+30; 20% EOC+45 and 40% at EOC+60.
3. Typical repairs would consist of patching spalled concrete and sealing cracks.
4. Spalls will account for 50% of the repair area and cracks for the remaining 50% (assuming 1 LF of cracking per square foot of repair area).
5. Engineering costs (design & construction administration) are included in the repair cost as 10% of the construction cost. An additional 5% is added for mobilization and miscellaneous soft costs.

 

[phamENG]

Craig - unfortunately I think I'll be the one to prove the unwieldiness of this one. I'm just peeved they told me about it after I was done. The ordinance essentially negates my entire design. Here in Virginia the building code is state wide and, with a few exceptions like defining minimum wind speed requirements or snow loads for a particular jurisdiction, localities can't change it. This is a zoning ordinance, and normally the site/civil guys handle those as they lay out the site and then I'm free to do my thing. Not in this case since the ordinance is only a couple months old and nobody on the team knew about it. That doesn't change the developer's sense of urgency, though...

bridgebuster - very nice. Thank you. Can I ask where the 75 years came from? Apart from brief discussions in school, real lifecycle modeling isn't something I've had to do. If you or OldDawg have any suggestions on literature I'd appreciate it.

Heard back from my insurance - they said as long as I cover it in disclaimers that it's an estimate and not a warranty, I'm fine on the SOC issue since it's mandated by an ordinance (which effectively elevates the SOC to that level since nobody else could provide any less).

 

[GC_Hopi]

ACI 365.1R Service Life Prediction, State of the Art Report

 

[OldDawgNewTricks]

As GC Hopi mentioned, ACI 365.1 is a good reference. Here's an excerpt:

 

[phamENG]

Nice. Thank you both.

 

[RWW0002]

I have not used it in practice, but there is a tool called Life365 that is geared towards service life decision making for concrete structures. If I am remembering right, it mainly applies to highway structures and is geared towards life-cycle cost and material selection (mix design, epoxy coated rebar, corrosion propagation.. etc.). I have played with it in the past and was impressed, but have never used it in any real sense.

It may not be directly applicable, but it might/. Either way it may give you a starting point for both service life determination and for determining when maintenance cost might exceed cost of replacement.

The software was free at one point, and may still be.

http://www.life-365.org/

 

[phamENG]

Looks promising - thank you.

 

[bridgebuster]

phamENG - my company was hired to prepare 30% design documents for a railroad grade separation project, which was let as a design-build project. The 75 year design life was an owner requirement. as an aside, it seems 75 years (minimum) has become the norm since the adoption of the AASHTO LRFD Bridge Specifications.

I developed the particulars in the life cycle costs for maintenance, inspection, and repair. From there, I estimated the costs for each task on a pay-as-you go basis by estimating the initial cost of each and increasing the costs by 3% annually ( I didn't see Bidenflation coming ). This gave me the total cost over 75-years. Then I calculated the cost a sinking fund to cover the LC's. In the end, I settled on the present worth of each task. Although, the sinking fund is an appropriate way to go (one agency we work with does use the sinking fund), I didn't think the owner would budget money every year for the SF. I converted all of the future costs to present worth; this way the owner funds the work once and that's it. By assuming 4% growth and 3% cost increase, it all worked out. Whether or not money was set aside is anyone's guess; but the owner accepted the analysis.

NCHRP Report 483 (2002) Bridge Life-Cycle Cost analysis is a good reference. The attached file from the NCSPA is a another good reference.

 

[LuK13]

OldDawgNewTricks' concern for an implied warranty got me thinking. Maybe state expected service life in terms of both years and deflection?

"Service life is X years or a deflected of H/Y at top of wall, whichever comes first."

That way the top end of your service life is bound by a serviceability limit state and not just an arbitrary duration that the model codes are targeting? I think it works well with the required surveying at regular intervals.