UPDATE: All my assumptions in this post turned out to be wrong. See my report here. It turns out that the contractor did not prepare the soil cement properly. This casued the base to fail, which caused the road to fail.

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What a mess.

I inspected the road that failed as mentioned in this post.

The road has failed everywhere there was some sort of trench excavation. It did not matter if it was an underground power line, a water line or a sewer line. If it was excavated, it failed.

Now I am left to write a report explaining what went wrong and how to prevent it from happening again. I tend to enjoy this sort of thing, but I have a bad feeling that they will not care about my findings. They will want to pin this failure on someone and I fear it will be my boss.

Anyway, below are my findings, including pictures. Even if you are bored to tears with this topic, check out the pictures to see what I am babbling about.

The main culprit appears to be poor soils. …how about piss poor construction. Now, I am no Geotech expert, but I am betting the soils are either Grady or Dunbar. Actually, the area with the worse failure is:

PELHAM (PELHAM, FLOODED)–This very deep, poorly drained soil is along drainageways. The subsoil is loamy and extends to a depth greater than 5 feet. A seasonal high water table occurs at a depth of 0 to 1.0 foot. Flooding is common. Permeability is moderate.

I know enough to know they are not the best for roads.

The other soils show to be Wagram, Red Bay and Ocilla. All three are relatively good draining soils. It may be that the Pelham soils cover a larger area than the soil survey shows. It is possible, but that really leads me to think that it was a combination of poor soils, high water table, drought and piss poor construction.

Here is what happened as best as I can tell:

The subdivision was constructed in late 1999. Our city experienced a significant flood in 1998 and then immediately entered into an almost 7 year drought. When the subdivision was constructed, the underlying soils were compacted appropriately, but the soil conditions were damp dry.

In 2005 we had an exceptionally wet Spring and early Summer. Over one week we had almost 10 inches of rain. The holding pond for this subdivision reached the top of its embankment and I was called out to see what steps could be taken to lower the water level.

Looking at the subdivision design, it is apparent that the inverts of the sewer lines were close to the same elevation as the high water level in the pond. the water in the pond reached 221.00, while the invert of the sewer line near the pond is at 217.29. Since the soils in question are Hydric soils, The water table probably extended beyond the holding pond at roughly the same level. Leaving about 3′ of water above the bottom of the trenches.

This made the Hydric soils become fully saturated. As water takes the path of least resistance, the water followed the trench excavations. This also meant that at locations where there was a change in rigidity, such as at manholes, the water wicked up the structures. The fully saturated soils then began to consolidate. This is called water compaction. The soils in the old trench excavations and around the manholes compacted and settled.

We have been in a mini drought since 2005 and the water table has dropped again. The visible signs of the road failure are now present. It also appears that one of the manholes may have floated.***

My guess is that all the areas that were excavated as trenches have settled. The only thing preventing this settlement from being visible is that the soil cement base of the road is acting as a bridge across the settled areas. However, it is only a matter of time before the bridging fails and additional settlement along the trench lines will become visible.

Here are the pictures:

Settlement at a utility crossing

Settlement at a sewer tap

Settlement at an underground power crossing

Settlement around a manhole - the manhole is 3″ to 4″ higher than the failed road

Settlement along the trenchline of the sewer line

Settlement at a sewer tap

Settlement around a manhole

Inside a manhole - the manholes looked structurally sound and intact, ruling out I/I from the manholes as the cause

So, how do we fix the road? What I will propose is to hire a local soils firm to drill some soils samples and determine if our suspicions of poor, highly plastic, clay soils and poor construction techniques is correct. If so, has the worst of the settlement taken place?

If the majority of the settlement has already occurred, which I believe it has, then I suggest we patch the settled areas and overlay the road with 1″ of asphalt. If the settled areas currently bridged by the soil cement base begin to fail within the next year or two, you could place another 1″ overlay on the road. You may have to mill the edges near the gutter, but it should solve the problem.

I guess all of the above is pretty easy to grasp, but the real issue is how can this be prevented in the future.

My short answer is it can’t.

Our City and County suffered a tremendous exodus of people to the neighboring county in the 1990’s. Developers were lulled with low property taxes, little development regulations and lots of cheap land. As a result, we were encouraged to be as developer friendly as possible to draw development back to the City. That meant we were to protect the City but at the same time not place any undo burden on developers.

In essence we relied on the Engineer designing the project to exercise the appropriate standard of care and to follow standard engineering practices. The same can be said for contractors. This worked fine until all the developable land in the City was taken. What we have been left with for the past 10 years are called infield lots. These are areas within the City that are usually surrounded by existing development. The reason these infield lots were never developed almost always revolves around the land in question being low or containing wetlands. A few years ago, they changed the wetlands laws and that opened up many of these lots for development.

The location of the subdivision in question is a natural depression with no natural outfall for runoff. Thus the Grady and Dunbar soils. The reason no one built there before now was because of the Hydric soils and lack of drainage. But when property becomes scarce, even marginal property is forced into development.

The Planning Department handles zoning and land use for the City. They allowed the subdivision property to be re-zoned and classified as being able to support a residential neighborhood. My guess is that flooding and soils issues never entered into the conversation.

The Engineering Department reviewed the plans as discussed in my previous post, but relied on the Engineer of Record to design the road section and to ensure a proper design. The contractor built it per the plans and per standard construction practices. However, the contractor was a minor contractor that is now defunct. A larger, more reputable contractor would have seen the poor soils and demanded a change order to correct the deficiencies in the design. It is fair to say that the contractor did not compact his backfill for his trenches to an appropriate level of compaction.

I can only think of one thing that may, MAY have prevented to road from failing, outside of not allowing the subdivision: A complete soils report with soil samples and a subgrade design.

That would have added an additional $5,000 to $10,000 to the cost of the development. I have heard stories about the developer in question and I can only imagine how loudly he would have screamed to the City Manager, Commission or Mayor if we had told him he had to do a soils report.

Also, the road section used for this project has been used all over this town with very few failures. It would have been irresponsible of the Engineering Department and it would have placed an undo burden on the developer to require a soils report.

But knowing what we know now, should we require a full soils report or a longer warranty or what?

There is always a balance between being too tough and being too developer friendly. I wonder if something more can be done to protect us in the future? I still think that even with a tougher road section and a full soils report, this road would have failed due to the poor soils and high water table.

One last thought. Our city has 4 inspectors and one supervisor. Currently, there are 35 active projects they are looking after. As you can imagine, the larger projects get most of the inspector’s attention, as they should. At the time this subdivision was being constructed, there was a very large separation project being built in the south of the City. I know that we had an inspector on that job full time. That left fewer inspectors to take care of the rest of the projects. As a result, projects probably did not get the attention they needed and we relied on reputable contractors to do the right thing, but the one constructing this project was less than stellar.

It will be interesting to see how this all turns out.

*** Yes, concrete manholes can float. It is simple buoyancy. Like how a steel ship can float. To prevent this, you can pour concrete ballast around the base of the manhole to add weight to prevent the manhole from becoming buoyant. Usually you see these type of calculations for pump stations where you have a very deep and a very large diameter wet well.