Staged Construction of Embankment

[qhtony]

Hi guys, recently having a problem with staged constructions of high fill embankment. The embankment constructions were done in 2 stages to allow alternating traffic (i.e. stage 1 embankment constructed on one half of the road and then stage 2 on the other half of the road, following the completion of the stage 1).

However, cracking problems (or differential settlement) occurred between the interface of stage 1 and stage 2 embankments, which resulted in a weak zone in between. And I think this is mainly due to the different compaction levels applied for stage 1 and 2 (maybe the contractor did not compact the fill materials property in stage 1). Anyway, they are fixing the problem by applying reinforced fill and geogrid.

My question is: is this problem only related to improper compaction during the construction or there could've been some construction techniques need to be applied to make sure the appropriate joining of embankments done in different stages?

I hope ppl can understand what I'm trying to say. Cheers.

 

[Ron]

When you do 2-stage construction divided by lane divisions, it is difficult to achieve compaction at the center, simply because to can't compact out to the edge of your stage limit.

Has there been any actual settlement and is there faulting at the interface between the two?

Stage 1 would likely have insufficient compaction at the interface, while Stage 2 could be fully compacted against stage 1.

 

[qhtony]

Hi Ron, at the interface between the two, there has been an obvious crack, or faulting. Photo's at work. Settlement does occur in the adjoining area between stage 1 and 2 and I think in exactly the same way as you, "it's difficult to compact this area".

But it gotta be some construction method to overcome this??

 

[Ron]

How thick is the embankment and what is your pavement section? What were the compaction requirements in the embankment and in the pavement section?

 

[qhtony]

Hi Ron, the embankment is about 6m high and pavement is just conventional asphalt, so it's flexible pavement rather than rigid concrete (I'm not so familiar with pavement works). Compaction for subgrade directly under pavement is 98% std and I think for rest of embankment is 95-97% std from my memory. The compaction requirements in the contract complies with state DOT standards so I don't think it's the specification problem but rather construction problem.

 

[dgillette]

I didn't see anything here about the foundation. Could you be seeing an effect from consolidation thereof? If the foundation under the first stage was partially or mostly consolidated before the second-stage fill went in, you could get differential settlement from that.

 

[cvg]

perhaps the edge of the first embankment should have been undercut and benched prior to placing the adjacent second embankment

 

[emmgjld]

A little more information, please.

What is the embankment constructed with?
soil types? How consistent is the fill?
What is the subgrade composed of?
soil types? Soil Density?
How consistent are the soils?
Is there ground water at or near the original subgrade elevation?
What is the general climate for this project?
What weather conditions during construction?

 

[qhtony]

Hi Emmgjld,

To your questions:

embankment is constructed with common fill (mostly cohesionless materials + some cohesive) with soaked CBR specified as > 3.

original subgrade is 1m basaltic clay underlain by basalt. The clay has a moderate swell of 2.5%.

GW is well below the original surface, say 5m below and has little impact on the project.

The area is relatively dry and not so sure about the construction time. It did not appear to be much related to drainage.

The thing I really want to know is, what is the 'correct' construction method/procedure for this type of works where you have to construction embankment in two stages?

 

[Ron]

Do you have more heavy (loaded) traffic in one direction as compared to the other?

You've added 6M of fill over clay. While this would likely produce some settlement, it shouldn't be cause for differential lane settlement unless the clay layer is significantly different in character or thickness from one side to the other (again, not likely, but don't completely rule it out yet).

Has there been any lateral movement of the fill?

 

[qhtony]

There is traffic on stage 1 already and the construction is about to finish (not paved yet) on Stage 2. The crack is already developing in between stage 1 and stage 2.

We didn't notice any lateral movement, maybe it's too little to observe.

 

[Drumchaser]

I am guessing there was not enough room to bench phase 2 into phase 1. Even so I am in agreemnet in principal with cvg.

If the interface layer(s) were not benched or notched-in as fill for phase 2 was brought up... it is almost certain to be a problem.

**With phase construction, this area (the joint) is usually an area well suited for temp. MSE wall or temp. spl. shoring if your fill slope has lateral width restriction(s), even with the shoring/temp.MSE "minimal" cracking can happen under certain conditions.**

If corrective action(s) are employed, cutting down and placing back will be expensive (if you have enough room to do it with traffic)

Do you have a typical or cross-section available? That would be nice.

 

[fattdad]

I would expect a 20 ft fill constructed with compacted fill where a CBR of 3 was tolerated (and if placed wet of optimum) could settle 1 or 2 percent of the fill height. That would be 0.2 to 0.4 ft. This settlement could take a few weeks, perhaps. If Stage 1 was constructed in a week or so and then Stage 2, the first stage may be on a different compression curve than the second stage. This could lead to some strain boundary at the interface.

I agree on compaction problems at the interface (already explained).

From the project description, I'd also agree the foundation soil and the ground water is not an issue.

I'd think if the problem is strain as I described above, geogrid several feet below the finish grade would help, but that would also interfere with your staging.

Make sure you don't have a slope stability problem, wouldn't want to overlook something else. . .

f-d

¡papá gordo ain’t no madre flaca!

 

[hokie66]

From a structural perspective rather than a geotechnical one, some consolidation of a 6 metre high fill ALWAYS occurs after it is placed. This seems to be irrespective of how careful the compaction is controlled. So I doubt this differential settlement problem is preventable with "common fill". Perhaps full height stabilization would help. Just because the work was done to DOT specifications doesn't mean it is without risk.

 

[BigH]

I don't know why anyone is surprised that there is cracking at the interface between the two zones. They may, in the end, end up with the same "settlement" due to the embankment self weight (it seems that the foudation was not a problem) especially since they are using fairly plastic silty clay (based on the low CBR values permitted). Just that the consolidation of the two halves will not be occuring in unison - and I doubt that "keying" or "stepping" into the the first stage would have made any signficant difference. Do you have records of settlement points on both sides with time - monitor some points like you would settlement plates.

 

[qhtony]

Hi guys,

I've got a little bit more information on the project. I've been given a photo of the site(see attached).

The stage 1 embankment has a temporary wire mesh/geotextile faced temporary retaining wall due to the limited space. And during the compaction at near the temp. retaining wall, the geotextile and the mesh tend to "bulge out", and I think this is the main reason for the insufficient compaction.

Is there a construction method which will enable the compaction to be achieved?

 

[cvg]

I'm sorry, but that photo doesn't clarify anything for me. Can you draw a sketch?

 

[qhtony]

Hi guys, see if this illustration helps (excuse my terrible drawing).

 

[hokie66]

The temporary support "wall" was doomed to cause proplems. The more compaction, the more lateral deflection. So the first stage never got compacted, now you have problems compacting the second. Perhaps an adequatedly designed reinforced earth wall with concrete facings, buried in the fill, would have been a better system.

 

[Drumchaser]

ghtony

Your sketch and pic were helpful in getting a better idea about your problem(s).

I cannot be sure this temp wall is the aggregate filled type that we use, and have not encountered temp walls that do not include an aggregate section...that is "not" to say that there are not one million and one out there, just none that I am familiar with.

The temp MSE wall standards we use require a system complete with calc.s and a seal, and are designed with-in very tight guidelines specifying minimum wire size spacing, aggregate size type, spacing and length of straps that project into the fill area. As with any system there are specific criteria including materials used and installation procedures that must be adhered to.

"When constructing wire faced walls it is critical that the area behind the face mat be completely filled with material and properly compacted....failure to do so will result in bulging of the face mats and settlement of the top of wall"...

As far as how to remediate this problem...you might get some input from the supplier of the wall system. I would guess they have encountered some of this in the past.

Good Luck