Minimum Unit Weights in Fills

[BigH]

I am hoping that my friends and peers have some comments on the following. In India, for road construction, they follow a specification that states for subgrade soils, the minimum unit weight, for the maximum modified Proctor dry density shall be not less than 17.5kN/m3 (1.78g/cc). Any soil being non-expansive (not defined) but taken (by us) as PI<15 (basically) can be used. We have a requirement of CBR=6 at 95% modified Proctor max. dry density. Rather than use the poor clayey soils, contractor is getting river sand – fine to medium sand, trace silt. But, the MDD at modified Proctor is only 1.65g/cc. This is under the 1.78 required. We have, at my behest, waived the minimum unit weight restriction (which we are permitted to do at our discretion) since the CBR values are > 15%. Recently, we have encountered an MDD of 1.56g/cc – still high CBR values (>10). I believe that this is too far “off” the minimum unit weight requirement and “off” their historical norms. We probably will reject this. There is similar minimum MDD for embankment fills (15.2kN/m3 or 1.55g/cc) but clays with PI<45 and LL<70 are permitted.

Does anyone have any rational idea why the Indian specifications require a minimum MDD requirement for fill? I have not seen this before. (I am thinking that this might be to ensure that there is no substantial organic content in minute amounts – but for our material this is a reach.) Your thoughts are appreciated.

Again – I hope that many of you will take the time to write the “Threadmaster” and request that there be threads in the geotechnical/civil area for flexible and rigid pavements. There is a paucity in the threads in these areas – and there is much work overseas/domestically dealing with highway/road construction. Please help out this dinosaur and favour the inclusion.

Thanks for you considerations.

 

[KRSServices]

Does the specification relate to a particular type of &quot;preferred&quot; material suitable for use? I appears to me that rather than prescribe the end characteristics of the subgrade, such as deflection, CBR, load bearing capacity and compaction requirement, they are instead trying to prescibe a certain material which will conform to a predetermined requirement.

It's like saying that all aggregate for use in asphaltic pavement should have a minimum MDD of &quot;x&quot;, regardless of the mix design. KRS Services

http://www.krs-services.com

 

[dicksewerrat]

One thing to rimember about river sand. All the particles are rounded. they don't compact well, but they do drain well. The spec was probably writtten so the engineer's 'uncle' could sell the aggregates.

 

[BigH]

KRS - if they are trying to obtain a certain material, clearly there is a better way, eh? If by specifying a &quot;high&quot; MDD, they may be trying to inject coarse sand and fine gravels - but they don't come out and say it. I dislike such - if you want 10% gravel - say it. The designer actually never thought that sand would be used - he figured on &quot;soil&quot; which here is taken as clayey material (loam, for instance).

Dicksewerrat: Yeah, the river sand does damage easily. But, in such materials, you are really compacting the preceeding layer rather than the top layer. (See D'Appolonia's work - circa 1973 or so). In sands, of course, we do test at the &quot;surface&quot; - but if it falls a bit below and I know that they have put the effort to it, I will scrape down some 4 inches and retest. Usually, you will find the density is good. When putting down the GSB (granular subbase) we water and reroll the subgrade sand and then do not drive the tippers on it. The biggest problem we have is the cattlefoot compactors that ply the route every day from the market to the railyards. (see paper in 1967 or so on using elephants for compaction in Thailand!!! - it was a joke - if you read the same author's book - heartily recommend the book - called something like Geotechnical Engineering and Tales of Getting Sued).

 

[valfer]

Big H,

I can suggest an explanation for this specification on MDD :

Everyone here certainly observed the influence of fine proportions, plasticity index and maximum size of particles on the maximum density (modified or standard Proctor).

I personaly think that the clay content and its specific surface area are major parameters that are too rarely measured though they bring much information. Thus, I think that the more the clay content and the finer the clay content, the less is the maximum dry density (standard or modified). This way, if a specification sets a minimum value for the MDD, it can be a way to avoid using active clayey soils which could be very sensitive to water content changes.

If anyone has an opinion, I'm interested ...

 

[BigH]

Valfer - good point; we are using sand though (<10% fines), NP - so point on clay not relevant to MY pariticular soil. You are right in that the subgrade needs to be non-expansive and maybe this is the way they covered it. But, would be better to have specified a specific desireable material rather than a minimum unit weight - could get a &quot;poor&quot; clayey soil good by adding a bit of crushed stone to get the MDD up!

Focht3 - haven't checked that site yet. Hope your kids are fine. My girl gives up on me &quot;taking care&quot; of her as mommy does EVERYTHING better!!!! - except of course rubbing her head!! She's 11 and attending a convent school here in our town. Only has been studying 2 years now in English - first 4 years was in Chinese. Good kid; even learning Hindi and Bengali now!!

Hey everyone - take time and

 

[UdayRB]

The specified minimum dry density is often for the purposes of precluding the use of low density silts (which compact with a very high void ratio/optimum moisture content). This is often the case with with micaceous silts. These soils are very moisture sensitive and with a low CBR value. In a country such as India, where the weather is composed of 8 months of dry weather followed by 4 months of monsoon rains this can be a problem. If is is demonstrated by CBR tests that the values are satisfactory and reliable and there are no time related (seasonal) soil deterioration/strength loss problems, the minimum density specification becomes less relvent to the long term performance of the subgrade.

 

[UdayRB]

One other comment relative to the CBR and unit weight discussed in my previous note. Due to extreme seasonal weather changes, the CBR test would have to simulate potential for extreme drying of the subgrade. Althouth, the subgrade is covered with the road base and pavement, there is a possibility of subgrade moisture loss during prolonged dry periods. The CBR value therefore should be considered at a moisture content that is several percent dry of optimum. This will yield a value that is significantly lower than CBR tests on samples compated near optimum moisture.

 

[reddog99o]

Be careful of the material he is importing. It is not unusual with soils of those characteristics to not be able to come up to such rigid standards, but forgetting that, if that area is to be traveled upon, subjected to cyclical loading, expect it to pump somewhat, this could cause a maintenace problem as time goes by. You may want to consider wrapping the new fill in filter fabric on the bottom and both vertical faces.

Good Luck R.A. Hassett, P.E.
rah1616@hotmail.com