SAND AS TRENCH BACKFILL MATERIAL AND APPLICABLE COMPACTION METHOD TO BE USED

[harmsgundam]

Hi All,

I am working as a Standards Manager in a water utility Company and currently we are having issues on the applicability of the backfill material we are using on our trenches and the right compaction method to be applied. I have attached a drawing of a typical trench section. As you can see, for the bedding and pipezone area, we are using Sand Class S1 (0-6mm size). For the backfill above the pipezone, it's either we use native materials (if suitable)/ imported backfill/ Sand Class S1 (0-6mm in size). The following are my concerns:

1. For the bedding and pipezone area.

a.) Is the Sand that I specify really suitable?​

b.) The only compaction method we are applying in this area is by "flooding", is this suitable?​

c.) If flooding is ok, how can we know that the resulting compaction is sufficient? Our specifications states that Field Density test is not required in the pipezone area. Is this because of the difficulty compacting this portion of the trench? Any thoughts?​

d.) Can anyone suggest any other efficient compaction method in this area?​

2. For the backfill above the pipezone.

a.) If the native material/ imported backfill (other than Sand (0-6mm in size)) will be used, conventional procedures are
implemented that is, taking the sample for laboratory testing (proctor test) to determine optimum moisture content and
maximum dry density. The material will then be compacted on site (mechanical means) every 150mm lifts then an FDT will be
performed to know if the Contractors achieved 95% compaction. Did I miss something? Can we allow larger lifts like 200mm,
250mm, 300mm, etc.? What might be the implication of allowing larger lifts?​

b.) If the backfill material is Sand Class S1 (0-6mm in size):

b.1. Is this material really suitable?
b.2. Is the conventional procedure I mentioned in 2.a above is still applicable?
b.3. Our specification allows compaction of Sand(0-6mm in size) by "flooding".Is this really suitable or efficient? How can
we know that the resulting compaction is sufficient enough? Can the actual compacted density by flooding be compared
with the maximum dry density taken from the proctor test? Does compaction by flooding be able to achieve 95% compaction?
How long after the application of water by flooding can you perform a Field Density Test?​

The trenches I am referring to are situated on highway pavements subject to truck loadings (HS20 loading). Sorry for my long querry at least I think I provided the needed details for you more experienced professionals to provide your opinions/ answers to my questions. I am looking forward to your responses.

 

[Ron]

I'm not familiar with your S-1 sand designation, but will offer a few comments about pipe trenches in general.

1. It makes no sense to only provide compaction above the pipe. Proper bedding of pipes, particularly below pavements is important. The soil below the pipe should be compacted to at least 95 percent of the maximum dry density as determined by ASTM D1557 or AASHTO T-180. Field density tests should be run to verify. Use a direct field density method such as the drive sleeve or, if using a nuclear density gage, make appropriate trench corrections.

2. Immediately below the pipe provide a gravel bedding. It does not have to be thick...just enough to allow seating the pipe and its joints.

3. Compact around and above the pipe to at least 98 percent of ASTM D1557 or T-180.

4. Based on the size designation you give for S-1, it should be ok as long as it is compacted properly. It appears to be more of a concrete fine aggregate than a sand fill material.

5. Flooding will not provide sufficient compaction to prevent the pipes from settling.

 

[PEinc]

"5. Flooding will not provide sufficient compaction to prevent the pipes from settling."
Or possibly floating!

http://www.PeirceEngineering.com

 

[harmsgundam]

@ Ron

1. I think the reason why our specification indicates the use of flooding because of the limited width of our trenches. We only have 6 inches allowance on both sides of the pipe. I think no Mechanical compactors can perform on such a small space. But can we do manual tamping then?

2. We do actually apply gravel bedding for our pipe sizes 600 mm and above for both steel pipes and DI pipes.

3. Will the 98% compaction be easily achieved on site specially on the haunches (near the bottom of pipes)?

4. Since the gradation that I have mentioned is ok, does the 150mm lifts for compaction is a more safe assumption since we do not specify particular mechanical compaction equipment?

5. The reason why I made this thread because I am worried that I cannot find any reference justifying the use of flooding as an effective method of compaction and yet it is what is written in our specification.

 

[Ron]

1. Why is the width of the trenches so limited. You need adequate room to properly install and support the pipe.
If you must limit the width that way, you might consider flowable fill rather than compaction.

2. Use gravel bedding for ALL pipes.

3. 98% is certainly achievable, above and below the pipe. Use thin lifts to achieve, such as your 150mm lift thickness.

4. Yes.

5. That's because it is not an acceptable method of compaction. In a few cases it might work, but more often than not, it will fail to achieve the desired compaction. Do a few field density tests and you'll see how inconsistent it can be. Your goal of pipe bedding is to achieve consistency and proper support.

 

[dik]

That's because it is not an acceptable method of compaction. In a few cases it might work, but more often than not, it will fail to achieve the desired compaction. Do a few field density tests and you'll see how inconsistent it can be. Your goal of pipe bedding is to achieve consistency and proper support.

Only in extreme circumstances is that suitable bedding... consider CLSM... very costly.

Dik

 

[oldestguy]

There are compactors that will work in narrow places. One is what looks like a long pipe maybe a five feet long attached to the air hose with valve and a "button" like head doing the compaction. In time using only flooding be prepared for some settlement and perhaps a law suit some day with experts like those here on the other side.

 

[dik]

And that type of compactor work on sand? Poor construction if pipe is to carry any hazardous material.

Dik

 

[harmsgundam]

Can Sand cone method be performed in a compacted granular fill material like sand upto maximum gradation size of 6mm? Are there any other more efficient means of determining field density for such material?