(Continued) Pumping subgrade, but passing compaction tests?

[GGGGeo]

thread158-324442

This is a very interesting subject. Not too many books discuss about identifying and/or dealing with "pumping" of subgrade soils, as well as the technical reason(s) of the phenomenon (may be excessive pore pressure, particular soil grain, or pumping soils situated over dense soils, etc.).

If these pumping soils can not be identified in field investigation and described in the soil report, adding "bridging" materials during grading could sometimes significantly increase costs which definitely not come from contractors. One job 2 feet think of 1-in rock were used to bridge the pumping soils (since 2-in gravel is considered as ballast which become special order) but still not worked, a site can only be stabilized by mixing with cement.

Could any knowledgeable person can share his/her knowledge on how to identify potential "pumping" soils during the geotechnical field investigation or share a direction of any technical papers discussed of this pumping soils?

Thanks a lot in advance!

 

[oldestguy]

If you keep the vehicles off it, there is no "pumping". Typically heavy in silt content and fully saturated is a key that disturbance is likely to result in this TEMPORARY condition. Trucks take different "tracks". Back and forth of tracked excavators also to be avoided. Let it rest (maybe over night) and place base course or other material with as little disturbance as possible, by thick layers. Changing methods such as lighter equipment maybe needed. When I'd get a call from a contractor about the unstable mess he had, I usually said "get yo ur front end loader out of there". The reply generally was "How did you know that".

 

[Ron]

Pumping is a common phenomenon in many soil types, particularly relatively fine cohesionless soils. The primary reason is excessive pore pressure. Loading happens faster than the pore pressure can dissipate, thus a hydraulic pressure response takes the load. Since that hydraulic pressure has a poorly defined constraint barrier (it depends on the permeability of the material), the whole mass can move in a "wave", thus pumping.

Loaded, rubber-tired vehicles will readily show pumping conditions. We routinely recommend "proof rolling" with either a rubber-tired vehicle (loaded dump truck or front loader, for instance), or a vibratory compactor.

You can alleviate the poor pressure by several means. You can wait for the moisture in the soil to reduce (slow process) or you can reduce the applied compactive effort to allow dissipation to occur during the loading. This will result in longer compaction times, but is usually effective. If you can achieve compaction in lower depths, you can do as OG recommended and increase the lift height of material prior to compaction. Another effective method is to till or disc the soil to aerate it and reduce moisture.

 

[fattdad]

a clean, poorly-graded sand (or crushed rock), will often yield free water at or near optimum water content (i.e., from the proctor in the lab). In the field, the compaction of lift 3 over lift 2 will result in water draining out of lift 3 and into lift 2. If the contractor is using a vibratory compactor, the density of lift 2 (i.e., when lift 3 is being compacted) will not remain stable, 'cause water is draining out of lift 3 and lift 2 is feeling the overlying compactive effort. So, lift 3 is great, but lift 2 and maybe even lift 1 just got all screwed up.

Delivering specified materials to the owner is the contractor's job. The contractor owns means and methods.

f-d

ípapß gordo ainÆt no madre flaca!

 

[BUGGAR]

The problem I've experienced is the contractor bringing in different material without a Proctor. A good person on-site is worth whatever the cost.

 

[GGGGeo]

Thanks to all your valuable inputs: "oldestguy" suggestion- "get that front end loader out of there". "Ron" always provides detail suggestions: increase lift thickness and drying method did work before. I did experience conditions described by "fattdad", and the lift 3 was the most expensive aggregate base (cracking, heaving, and probing through easily but nuc results still show 95%).

Nowadays, more cases that the field technician is hired by contractor. When the subgrade soils cracked, heave, probed for 24 inches deep, but tested as 90-95%, the owner's soil engineer will need to ready himself/herself to answer questions from the owner, contractor, and field technician (and his/her engineer). When a soil engineer proposes one of above recommendations, he/she is on his own and pray for that mitigation works.

Using "bridge" material (most cases not called out in soil report), specify particular equipment (rental cost), left to dry out (extend construction duration) will all have cost involved. If jobs is for paving, leave it as-is and increase road base and asphalt left may work but everyone will look at you with doubt. Soil engineer needs to be on-site to watch how it was done. Nevertheless, this "leave it" option may not work for footing excavation.

I did notice that most cases were red-orange-brown very fine silty sand/sandy silt near optimum moisture content in the field and underlain by either1) high groundwater (suck up water) or (2) underlain by hard impervious soils such as highly weathered bedrock or hard clay (so these upper near wet soils are "rolling" within this "bowls" and moisture was squeezed out by compaction but no where to be dissipated).