Stone Pitching Slope Protection

[BigH]

Thought I would throw one out. In India, they are very very big on stone pitching. Highway specs for high embankments say to use 40kg stones! Now in our situation, we have a contract drawing that is neither signed or checked but it made it in the drawings. It shows a simple 2H:1V slope, a 150mm "filter media" and then the stone pitching layer. The site situation is a large alluvial flood plain - no rivers or creeks flowing at the toe of the slope. The water rises in the monsoon season and then drops as the irrigation department manipulates their weirs. The question is of the filter design. In accordance with the project specs, the filter is to filter the embankment soil; in our case a clay capping layer to sand fill. It is 2m thick. Of course, a sand will do the job - but what keeps the sand from washing out through the large stones? The embankment is basically "dry" - it sits 2 to 6 m above the outside ground level.

What are your takes on
(1) the need for stone pitching
(2) do we need a filter?
(3) do we need graded filter?
(4) can the stone just be placed directly against the clay confinement.
(5) if the filter is/should be used, then what gradation would you use. California/Maine seem to suggest 3/4 inch to 3inch stone (without mention of the "base" soil).

Anyone? and

 

[MRM]

I wonder if the stone pitching is just an aesthetic feature in the design...it sometimes does look better than just a wild grass covered embankment, I guess.

My first thought was that with the clay cover; I didn't see the need for the 150mm thick filter material between the stone and clay. But then again, are these India clays prone to dispersion? If so, the filter media would help to slow runoff down the embankment quite a bit, helping that situation.

Another question in my mind is this: with the clay cap in place, is it certain that no water will find its way, and build up, behind the cap? If there's a possibility of this, there ought to be some outlet for that water. Perhaps the embankment, with the proper protection i.e., stone pitching, and filter fabric/media, doesn't need the clay cap at all. This would at least help keep the embankment internally drained. Unless it is normal practice for the clay capping in India, I have always steered clear of clay caps on embankments unless it is a pond or wastewater basin you're considering. As far as sizing the filter fabric retaining sand or clay, I'd have to check with Cedergren on that.

I've got to split-it sounds like halftime of the Eagles game is over downstairs!

 

[VAD]

Interesting question:

We are thought to believe that cla capping would block drainage and this has been the answer that we apply to many failures where water build up has been observed. However, there are areas that this practice has been used without tse problems. As Poulos once said at a prediction symposium, one is an expert only in his surroundings. While the prectice may not seem sound my experience would tell me taht it may be fine. However this would depend on the examination of roads constructed similarly in the same environment. It depends on the type of clay that is used to cap. High plastic clays - not all high plastic clays are swelling- seem to work well as they prevent water from migrating inwards. If we look at water entering the roadway via the pavement surface, it seems that this factor is not a big concern as drying throgh the sun tends to solve this problem as well cracks in the pavement structure are not deep seated unlike like what we encounter in the North American climate and hence such systems perform well despite our concerns of drainability. While Cedergren's work is exceellent for the North American Climate and generally. I have found some areas that it does not apply fully. Nonetheless his concepts are sound.

Again understanding ofthe behaviour of past pavements in India in the locale amy help in the understanding.

Regards

[Cheers]

 

[BigH]

MRM - made an interesting point - also pointed out by VAD - about the "blockage" aspect of the capping. We used sand for the embankment and of course it wouldn't stand at 2H:1V during the flood - it will slough - they didn't want this so the clay cap. Not really forceable to the contractor as it was not part of the contract requiremetns but he did it. I agree that there might be some buildup of water behind the cap - in our case, I believe that there will be sufficient "holes" in the cap that there will be some outlet. In our flyash/sand mix embankment with clay cap, I had wanted to put in horizontally laid wick drains to ensure water level doesn't build-up higher than mid-height; but not part of BOQ - therefore variation - therefore forget it!

I tried to get rid of the stone pitching as I feel it was put in either as (1) we always do it (2) it looks nice (although harder than a bugger to build). I wanted to do just plain turfing; it holds, the irrigation department uses it, the local embankment roads use it and, with a few problems that get built out in the first few years, it will work - also could have saved over 50,000 Cool Ben Franklins!

The specs actually say that more than one layer may be needed - we need something for the sand (acting as filter to the clay capping) from washing out into the stone - it is in the book, not on the drawing and again, problems will develop by all. Specs also say that if the capping is CH, then you don't need it. I agree that a dispersion test would be nice - but I feel that best if we just put the stone on the slope and let natural turfing develop inside.

I have seen specs from Cal and ME and they indicate to use a filter of 3/4 inch to 3 inch between the stone pitching and the embankment - and are really silent about what the embankment is to consist of.

I am approaching one of the profs at IIT-Kharagpur and will suggest we do a thorough review of stone pitching requirements for highway embankments. Would be very useful here. I get to give an invited lecture at NIT-Rourkela in February and will be pushing the "non-need" of it in favour of grassing.

Thanks for your responses - hope to get some more.

 

[VAD]

Nothing wrong with plain turfing.Instant grass and looks good. Needs care. You may wish to use topsoil, if such exists in the general format. With Clay capping and tropical climates grass catch is often immediate with rains. Clay cap used to avoid erosion problems and if area will be flooded I see the reason for wanting to protect same. You probably need to have some dicussion with the Aggie dudes to get some info on grass types and ground prep. Sometimes this is useful. There are of course many things that one can think about eg use of geotextiles but this becomes costly along highways as we are not dealing with parking lots.

Just an idea coconut fibre can be used and burlap if you are thinking about trying the seeding approach. These should be hopeful cheap in those parts and perhaps encourage industry growth. These products are soil in North America shipped from India. Years ago we did some trials on a sand dune backslope using burlap to cover the grass seeds after seeding so that they woud stay in place. It seemed to work but again so much for one shot trials.

When I first advocated the use of instant turf on a bridge headslope the complaint was watering as this is no longer fashionable. But perhaps in India it is not a problem. Yet in the city environs this approach is used by small operators to ensure that the bridge looks complete immediately afon its opening. Of course it is the City and not the Highway where eyesores are tolerated.

Your research should be of interest, but in North America before the influx of geotextiles many highways were grassed by hydroseeding but now of course the environmental Regs are different, hence proliferation of geotextile use.

[Regards]