Does Rain Hurt Compaction?

[harmsgundam]

Hi everyone. My question concerns the backfill on our pipe trenches. Our Contractors always say that the reason they are not utilizing the native material as backfill to our trenches is the problem with rain. They are saying that when their compacted backfill is exposed to rain (I expect continous pouring), it will never pass the field density test. In our specification, allowed native material must fall between class A-1 to A-3 of AASHTO soil classification. Considering such backfill material, we go back to the question "does rain hurt compaction?"

 

[Okiryu]

Grade of compaction is based on the soil water content. So, yes, rain will affect compaction (negatively). However you have granular soils (A-1 and A-3 soils) which drain well so the negative impact is less in your case.

Also, if I understand it correctly, your native soils are composed of gravels and sands...you are very lucky to have these type of soils as your native soils !

 

[harmsgundam]

@ Okiryu

Thanks for your reply. So what you mean is that even the soil is already compacted, its maximum dry density can change significantly when exposed to rain prior to field density test? Or you mean that rain can have an effect if the backfill material was exposed to it prior to compaction?

May I also confirm, does water has less effect on achieving maximum dry density on Class A-1 to A-3 materials compared to Class A-4 to A-7? When talking about the moisture vs. density curve (from proctor test), does the curve for A-1 to A-3 will appear more flat than that for A-4 to A-7?

 

[Ron]

The field density test is related to the maximum DRY density of your moisture-density relationship (Proctor); therefore the moisture content after compaction occurs (neglecting swelling, since you have a freely draining material), the moisture content is irrelevant. If the high moisture occurs while trying to compact the material you will have difficulty with compaction

 

[Okiryu]

harmsgundam,

So what you mean is that even the soil is already compacted, its maximum dry density can change significantly when exposed to rain prior to field density test? Or you mean that rain can have an effect if the backfill material was exposed to it prior to compaction?

In my experience, the MDD of good granular materials after compaction do not get affected by rain. You can prove this by yourself with some cone tests. Just suggest to move forward fast (after good compaction is verified) to try to no expose the soils to very long periods. And of course, compaction gets negatively affected due to rain prior to compaction.

May I also confirm, does water has less effect on achieving maximum dry density on Class A-1 to A-3 materials compared to Class A-4 to A-7? When talking about the moisture vs. density curve (from proctor test), does the curve for A-1 to A-3 will appear more flat than that for A-4 to A-7?

The compaction curve for A-4 to A-7 soils (fine-grained soils) typically have a noticiable peak (less flatter curve) which means that their compaction degree is more sensitive to moisture changes. A flatter compaction curve is typically for granular soils (this may be due to their good drainage properties).

 

[fattdad]

If it's raining DURING earthwork, yes. If the soil is properly placed and compacted and then it rains, no.

If it's properly placed and compacted and then it rains and then they resume construction, the previously placed and properly compacted fill will likely become disturbed, 'cause the new construction on saturated fill will render it disturbed.

We test strength of soil and its behavior in a soil laboratory under saturated conditions. Our assumptions should be that the earthwork becomes saturated. Our assumptions should also be the contractor returns the specified materials when the job is done. If the contractor cannot perform that service, then it's a contractor problem. Placing and compacting soil only to be responsible for its subsequent ruin should not fall on the owner.

Properly placed and compacted native soil, clay, etc. should perform just fine after it gets wet. Heck, how would embankment dams survive!

Then again, if the earthwork is subject to dynamic loading, that's different. We need to keep moisture-sensitive soils away from dynamic loads - i.e., not too close to traffic loading!

f-d

ípapß gordo ainÆt no madre flaca!

 

[dik]

Our Gran 'A' base, when properly compacted will actually 'pond' water for a while...

Dik

 

[Okiryu]

Good compaction = less voids = less space for water going into the well compacted soil...

 

[geomane]

With it being A-1 and A-3 soils, moisture is not going to be as big of a concern as if you were dealing with A-4 and A-7 soils, due to the good drainage characteristics.

Excess water during compaction will affect your density. Adding water after your soil has been compacted will not affect your soil types. However, adding water after you have achieved compaction and then subjecting the soils to repeated traversing of heavy, rubber-tired construction equipment will disturb your soils.

Soils can be over-compacted. If you're driving heavy machinery over your compacted soils, it can increase the saturation, which would cause your soils to start pumping.

 

[harmsgundam]

So if my backfill on our pipe trenches was properly compacted at some point on the trench depth then it suddenly subjected to a heavy downpour of rain, water from the rain may pond from some time at the compacted layer reflecting the effectivity of compaction done right? What do we need to do to continue adding another layer of backfill? Will we wait for the water ponding to subside before compacting another layer?

 

[EireChch]

Like anything in geotech, it’s not good to place fill or concrete when there is ponding water. Should be written in your spec. If you’re placing in a heavy down pore your soils will be wet of optimum and you will not achieve your compaction.

If you place when fill in damp/wet conditions you may not achieve your compaction when testing on that day but the following day if you test you may hit compaction levels. I’ve seen this before

 

[fattdad]

"What do we need to do to continue adding another layer of backfill? Will we wait for the water ponding to subside before compacting another layer?"​

If yesterday's rain has left the subgrade wet, i.e., over optimum, and you then place another lift, the compaction effort on the new lift will likely disturb the underlying, "Wet" subgrade. You see that subgrade will feel some of the compaction efforts and the moisture content will be too great!

f-d

ípapß gordo ainÆt no madre flaca!

 

[harmsgundam]

@ fattdad

So what can you suggest to mitigate the problem? Since we have an unfinished backfill and we still need to put several lifts before we reach our subbase course (our trench is located on major roadways subjected to Truck loadings) but then it was expose to a heavy rain, what is/ are the best solutions/ methodologies in this case?

 

[fattdad]

scarify and let dry.

f-d

ípapß gordo ainÆt no madre flaca!

 

[killswitchengage]

Also even if a compacted material receives rain then you place upon it a new lift, you should bare in mind that the first layer's water can wet the new layer , meaning the water content of the new layer will be the sum of the water content you added plus the suction water that raises from underneath if you know what i mean.

 

[harmsgundam]

@killswitchengage

Will this problem occur when the compacted fill layer is granular in nature or free draining like sand? Does water ponding really happens in a compacted granular fill?