Road settlement 3

[janithahb]

is there any method to find the settlements occur on roads constructed on reclaimed land areas (i.e Ports) and the compaction depth required for a heavy vehicle road when preparing the subgrade of the road? most of the pavement design methods doesn't explain it deeply. please can anyone help me since i'm in a tight situation..

 

[Ron]

Settlement in road construction is usually caused by the weight of fill material, not from traffic. Traffic loads are transient.

Traffic loads can cause additional compaction, but that influence is usually only in the upper 12 to 18 inches.

Traditional pavement design methods still apply; however, I would suggest elastic layer analysis down to the point of your potentially compressible layer.

You need to consider a higher modulus material as your fill, in order to "bridge" the softer soils. Unfortunately higher modulus usually means greater compaction and higher unit weights. To counteract this, I have specified cellular concrete fill, which has a unit weight of about half that of soil, for such applications. I designed a truck haul road for a paper mill over a reclaimed swamp area about 20 years ago. Worked fine. Also designed a high modulus fill without reduced weight for a similar application but almost all car traffic for a municipal street, about 25 years ago. Both lasted at least their anticipated design lives.

I used a conventional flexible pavement section for both applications, above the high modulus fill.

 

[janithahb]

Thank you very much for the great insight Ron, the problem that i'm tackling is that we have reclaimed the land in two layers, each with a thickness of 2.5m. the layer beneath the top 2.5m thick layer is not compacted and the top layer will be compacted in 0.25m thick layers. there is a concern that the bottom layer will settle due to the heavy loading from the machines (i.e container transporters, stack lifts)and the material used in the reclamation is sand dredged from the sea.
it is necessary to provide proof that the road will get deformed due to the traffic. i tried using the Westergaard’s formula to find the stress variation due to traffic. the results were that up to 0.5m the stresses are high and from then onwards the stresses are not that high.

 

[Ron]

janithahb....the lower 2.5m layer will likely settle due to the placement of the layer above it, but most likely this will occur during construction. Prior to compaction of the second layer, have the contractor "proofroll" the first layer using very heavy, rubber-tire equipment (such as a pan or large front loader with the bucket loaded). This will show up soft spots in the fill which can be corrected prior to moving on with the compaction.


Your Westergaard analysis is as expected. Pavement loads dissipate quickly with depth.

 

[BigH]

As you are using sand fill (dredged sand), settlements will occur rather quickly, like Ron implied, with the placement of the upper layers. You didn't say if the 5 m of fill was being placed above existing grade or dig out and replace scenario.

A couple of things - make sure you have some drainage provisions - the dredged sand will be wet and will want to drain when placed unless it has "lost" its moisture during transport, etc.

Secondly, and assuming that the placed fills are wholly or partly above the existing ground, the outer perimeters of the dredged sand layers need to be of a material that will help confine the dredged sand if the sand has a potential to "run". I have seen "clay' being used but the problem with this, unless you have some drainage holes through it, that the clay will act making the structure act as a bathtub which raises the groundwater level within the fills. I had wanted to use prefabricated wick drains laid horizontally to act as rainwater infiltration outlets through the clay confining zone but client and engineer didn't see the need. Time will tell, I guess.

On compaction, with dredged sand, think of checking the compaction of the previous lift rather than the lift you are placing as an acceptance criterion.

Another "toy" that might be considered is to mix in a couple of percent cement in the compacted sand layer - to increase the strength for the heavy loads. Mixing it with sand and a farm harrow is easily done as we did with a mix of fly ash and sand.

 

[Ron]

BigH...haven't seen anyone mention the "layer below" concept in 20 years or more, but you are exactly right!!

 

[janithahb]

The layer beneath the 5m layer is an hydraulic fill which is believed to be compacted during the construction stage and the reclamation process.

 

[FixedEarth]

You need to do 4 m deep borings with SPT, and obtain moisture and hand penetrometer readings, say every 50 m along the road. Take some on left side of the centerline and some on the right side. Then look to see if you have SPT > 10. If you hit clay, look for qu > 1.0 kg/cm^2. Take few Atterberg tests if you hit clay in the upper 5 m. In my opinion, quick soils investigation is warranted and proofroll test with a loaded semitruck (15 Tons net) is only a secondary requirement.

As for settlement calculations, you can use Boussinesq method or the simple 60 degree approximation method. The Atterberg tests will give you liquid limit value and this will enable you to approximate Cc which wil give you settlement magnitude.

I have seen few failed pavements, they are not pretty nor are they cheap to fix.

 

[fattdad]

Install some piezometers to gauge whether there are excess pore pressures greater than atmospheric conditions (i.e., underconsolidated). If the layers below demonstrate an atmospheric phreatic surface then that's one thing you don't have to worry about.

Sand compression (elastic) will occur quickly, as already mentioned. That said, whatever you would calculate using the "gravity turn on" approach, I'd consider an additional 20 percent of this value to occur after the initial compression. Duncan's settlement manual (Virginia Tech CGPR) gives parameters for long-term compression of granular soils.

I agree that the traffic loading is moot below 2 ft or so. That said, if the soils below 2 ft are weak, you'll have a heck of a time actually getting specified compaction. Geotextile and open-graded aggregate may be needed to get a firm-enough base for specified compaction.

All posts above represent good advice, just don't want to retype what's already been credited to the knowledgable.

f-d

¡papá gordo ain’t no madre flaca!

 

[oldestguy]

So far I didn't find any info on what this fill will be placed on. If that is a typical near-ocean sediment, then the settlements to be concerned with are those that take place beneath the fill being placed. That also brings up a whole different picture, such as problems if wharf edges slipping into the unfilled areas, etc.

Can more info on the site situation be presented to help clarify this part?

 

[BigH]

janithahb - you misunderstood the "layer" concept, I believe. What we are saying, is when checking the compaction of a fill, say done in 200 mm loose lift thicknesses, when you are placing, say, layer 4, the compaction check should be on layer 3. When you are placing layer 5, the compaction check should be on layer 4 - this is because of the "loosening" of river and fine grained sands due to compaction equipment "gripping" and loss of confinement.