Construction of Road with Heavy Load Traffic 3

[bumjames]

I have a question and I'm hoping someone can provide some answers =) Thanks in advance!!

I am not an engineer of any sort, but do work in the construction field. Here is the problem I'm trying to solve:

We did a public works contract requiring sewer line replacement for a small street and an parking lot next to it.

Excavated, replaced sewer line, then backfill with compaction no less than 90%. This was done with approved materials and all field compaction tests exceeded the required 90% compaction.

The street portion started to sink in couple different areas with the parking lot portion showing no signs of sinking at all. The street however, is mainly used by trailer trucks with heavy load coming in and out of a construction material warehouse. The street lead to the warehouse and that is about it, there is no "normal" public traffic on this street.

Question: Are there different specification for roads mainly used for heavy load traffic or they mostly the same? The fact that parking lot is perfectly fine and the street sinking; while both compacted using same materials and standards and both passing all compaction tests only the street is sinking. Can this be due to the differing loads on both areas or just a coincidence?

 

[bb29510]

yes, most heavy hauls road are 95 modified subbase with 4 inches of rock base 100% modified and 2 1/2 of asphalt. and they still tear up over time

 

[Ron]

A municipal street is not typically designed for repetitive, high load traffic...they are usually designed for light traffic with a small percentage of trucks, usually 1 to 5 percent. As an example ONE trip by a concrete transit mixer or loaded 18-wheeler is equivalent to between 3000 and 10,000 cars.

A pavement section that receives heavy truck traffic and has not been designed for it will fail prematurely. I've seen this numerous times on local roads and even highways where trucks go into a borrow pit and come out loaded. The pavement distress is very clear.

 

[msucog]

think we'd need to know test frequency and testing protocol. ie. was testing personnel onsite fulltime during the installation and was testing began 1' above pipe and performed every 2' vertical say every 50-100' along the trench? a passing test at the top doesnt help loose crappy backfill down deeper. maybe the road was backfilled faster due to traffic control issues and some loose stuff got left deep. also how long has it been since the work was performed? 2 months or 2 years? i seriously doubt that the different traffic had all that much to do with it although i do prefer to see higher compaction at least up top. was the trench backfill spec 90% standard or modified Proctor? was the upper foot or so required to be higher degree of compaction?

 

[Ron]

I should have read the original post a bit more carefully. My other post certainly applies, but I noticed that you have a 90% compaction spec for the backfill. We generally require a 98% compaction in the upper 12 to 18 inches of subgrade for a pavement. How thick is the base material? How thick is the pavement surface? Was the subgrade stabilized before the base was compacted?

What is the pattern of the depressions? Are they linear and along the pipe alignment? Are they isolated and round?

If you only have 90% compaction, the heavy traffic, both from load and vibration, can induce additional compaction.

Give us a bit more info and we'll try to help. Post a couple of photos if you can.

 

[oldestguy]

My question: What is meant by "sinking"? Rutting? Pavement breakup? Where is it? Over the trench? Soil type?