Settlement problems on fill playing field

[Snatch]

I have been asked to advise on settlement problems at a recreation field that was constructed about 3 years ago with a cut/fill operation, the maximum depth of fill being approx 30’. I am uncertain to the compaction achieved whilst carrying out the initial construction of the field, however there are about 3 areas on the fill section where there are 4”-6” deep depressions. I have been told that these depressions appeared several days after a few days of continuous intense rainfall. There are sub-grade carrier drains that run along edges of the field.

In carrying out remedial works to the playing areas, my initial thoughts would be to excavate the depressed area to sound material and the refill compacting in 6”-8” layers to meet 100% MDD. I am also thinking about adding some sub-grade (perforated pipes) and connecting them into the main carrier pipes. I know this would not guaranty solving the long-term settlement problems however are there comments from anyone on my approach in solving this problem

 

[GeoPaveTraffic]

What was the fill material and what records exist as to the methods used to place and compact the material? I would not start planning a repair until you know a lot more about the material and how it was placed. If no records exist, then get a drill rig and start punching holes.

 

[Snatch]

GeoPaveTraffic,

Thanks for your response. I have done some research and there is no information/ data records on what was carried out during earthworks construction of the playing feild. Several people have told me that compaction was carried out by a 8t roller but no densities were done.

However the slippage appears to be have occured at a depth of 5'. If you like i can forward some photos to you.

Thanks
Snatch.

 

[Chewi00]

I would be willing to guess, the fill was not compacted at optimum moisture content, therefore the rain allowed the material to consolidate more. These depression areas are areas where compaction was not met. The problem is, you never did any density testing to get a compaction % to the maximum dry density. Sounds like primary consolidation due to the added weight of water and lack of pore water dissipation. That would be my best guess without more information.

 

[GeoPaveTraffic]

Snatch,

I hate to say it but, get a geotechnical engineer and drill rig and start drilling. You are going to need several borings and a lot of Shelby tubes. This will allow the engineer to determine the type of soil used in the fill, and make some educated assumptions about how the fill was placed. This is the only way to start the process of preparing a design to correct the problem(s) whatever they may be.

By the way, the reported fact that the site was compacted with an 8 ton roller doesn't provide much real information. If the lifts were 2 inches thick (very unlikly) then an 8 ton roller would like do a lot of good. However, if the lifts were 2 feet thick then the 8 ton roller, or any other roller, wouldn't do much.

Good luck.