Stream Bedding Material Gradation 1

[DDBPE]

We have an order to supply bedding material for a stream restoration project. The specifications indicates percentages of various size materials ranging from 3" up to 27". We are blending several commonly produced quarry materials to achieve the desired gradation.

The Environmental Engineer for the project has been on site and is rejecting some of the material for gradation after it has been placed. The Engineers method for sampling is to have several other staff members on site, they walk around, then look up, close there eyes and reach down and whichever rock they touch is part of the sample. Then they measure the rock. They perform this blind game until they have touched 100 rocks.

The Engineers initially claimed there was no 27" material, although they could see it, they did not touch it during the sampling game. Now they claim there is not enough smaller material, same game method.

Has anyone heard of this sampling method and if so, where is it written?

I have sent them a copy of ASTM D5519-07, Standard Test Methods for Particle Size Analysis of Natural and Man-Made Riprap Materials.

Thank you for any help.

 

[beej67]

I've never heard of that. Sounds fishy. Maybe it's something Rosgen dreamed up?

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[cvg]

gradation is generally checked at the stockpile with an agreed upon sample size, either in tons or yards. Accuracy is lowest when sampling in-situ.

Why would they allow material to be procured, delivered and placed before checking for compliance with the gradation specs? Seems to be bass ackwards.

 

[jgailla]

That's the stupidest thing I've heard today.

 

[civilman72]

They can see the 27" rock but because they did not blindly touch it they can reject the material? My first reaction: stories like this are an embarrassment to engineers.

Did your contract document specify how material testing and pay approvals were to be completed? If it is not specified, you had the right to anticipate that typical industry standard material testing methods would be used for approvals. You should make them show you what standards they used.

 

[Ron]

jgailla...agree!...but then, there's still a few more hours in the day!

 

[DDBPE]

All

Thanks for replying to my thread.

After beej67 mentioned Rosgen in the reply I found a document titled "Rosgen Goemorphic Channel Design" which is Chapter 11 of "Part 654 Stream Restoration Design National Engineering Handbook" published by the US Dept of Agriculture. This guide discusses a pebble count and further mentions a "Wolman Pebble Count".

I found the Wolman Pebble Count procedure with a quick search on the web. The procedure for this method is basically to randomly walk around a stream bed, stop, touch your finger on the rock closest to your big toe, and then measure it. Perform this 100 times and plot the measurements. If the rock is too large or embedded, the procedure says to measure the smallest exposed axis and record (there could be a 15" rock embedded and only have 3" exposed and one would record it as a 3" rock, how accurate is that?).

They did initially check the pile at the source and seemed satisfied, however, after placing, and performing this pebble count, the Engineer is not satisfied with the material. In my mind this goes back to the procedure, if the rock is too large or embedded, measure the smallest axis, which would make large rocks fall into the smaller rock percentages.

We are reviewing the contract documents, but do not recall the Wolman Pebble Count being part of the document.

I will keep you posted.

 

[cvg]

"pebble count" - the name alone implies that the method is not appropriate. you are not supplying pebbles and pebbles are not even an accepted engineering term. sand, gravel, cobbles, boulder counts would be more appropriate... Unless the engineer has some reason for suspecting that segregation occurred when the material was placed, then there is really little reason for any pebble counts. Ask them to do a pebble count at the stockpile to calibrate their method.

 

[Debaser]

Having Googled a little on this, it appears they are using a method designed to track changes in a river bed over a period of time, rather than actual gradation of material to be placed.

Pebble counts have been used for a variety of monitoring
projects and are an important component of stream evaluation
efforts throughout the United States. The utility of pebble counts as
a monitoring tool is, however, based on the monitoring objectives
and the assumption that data are collected with sufficient precision
to meet those objectives. Depending upon the objective, sources of
variability that can limit the precision of pebble count data include
substrate heterogeneity at a site, differences in substrate among
sample locations within a stream reach, substrate variability
among streams, differences in when the substrate sample is collected,
differences in how and where technicians pick up substrate particles,
and how consistently technicians measure the intermediate
axis of a selected particle. This study found that each of these
sources of variability is of sufficient magnitude to affect results of
monitoring projects.

Note the continual use of the word monitoring. When acceptance, or not, of a material is at stake I would have thought those sources of sampling error would be a good rebuttal.

 

[beej67]

HA, I was right.

I sorta hoped I wasn't going to be right, for your sake.

I fear you have stumbled into a situation where you will have to educate those who should have known better. You will have to repeatedly say, "No, this is how real engineers do it" to them, in different, subtle, nonconfrontational ways, over and over until they finally get it. Do be nice, but be firm that no, how you're doing it is how real engineers do it, and that their methodology is not for real engineers.

There are quite a few engineers out there who don't like Rosgen methods at all, largely because he is not, nor does he pretend to be, an engineer. Rosgen methods are good for what they are, but they are not engineering. At their fundamental level, they are purely a Stream Matching Game, where you pick one stream that doesn't suck and you try to fix a stream that does suck by mimicking the one that doesn't.

That's not to disparage the Rosgen process. I know some very good engineers who use it on projects regularly - guys that I trust to get something right. But the good engineers also wouldn't hold your feet to the fire over something like that if you got the gradation right at the batch plant, because good engineers understand that the whole thing is only as good as your reference reach data, which has a tendency to be wildly variable and not often applicable to the project stream in the first place.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[DDBPE]

Thanks again everyone.

I have a meeting with the Engineer on Monday at the quarry.

I will keep you posted.

 

[DDBPE]

Update

The Engineer was on site yesterday at the quarry, sorry, the Engineers rep was at the quarry. We found out today the person who was out here was a technician, not an engineer or EIT.

The first thing we did was review the results from in the field, he noted we needed more larger material represented on the 3", 11.8" and 27.6" sizes. We then mixed the previous blend of material and added some gabion and class 1. These two additions should increase the amount of larger material.

Then we started the test, at around noon (approaching the hottest part of the day). First we spread the material to about an 18" lift. Then the count started, just as previously posted, he stepped, averted his gaze, and touched the stone by his left foot and measured it. This was then recorded in a book 100 times. The book it was recorded in is used by environmentalists and was pre-printed for recording pebble counts. After 1-1/2 hours, we had our results. We had increased the larger sizes, but not enough for his liking.

We then added more gabion and class 1, this time I left and had my assistant go out and watch this, I wanted to share the pain and I was hungry. This count resulted in less larger material than the first count. His thought was that in the stream setting, less than 3" material would settle in the void spaces and figured our test was being skewed due to having more of the fine material on top of the larger material which is not natural.

Now he decided to run a third test, but this time we are officially in the hottest part of the day with high humidity. He wanted us to bring a water truck over and wash the 18" lift to try and simulate a stream to move the fines into the voids. Another count was performed and again, different results, still not quite what he was looking for, but he thought it would work.

In summary, pebble counting from what I have found through web research and what I witnessed is not an exact science. There have been many articles written about the method and the fact that it is not repeatable, the same person cannot repeat the test. This is a method, really only for environmentalists to use to come close to evaluating streams and the behaviors of the stream as it interacts with the bedding materials in place.

We did get the Engineers rep to give us the blend to use, in writing, so hopefully this will clear things up and the river will flow smoothly or something to that effect.

Thanks again everyone for your thoughts.

 

[civilman72]

The only method this seems to follow is a method-to-madness. I'm surprised you kept your cool.

Thanks for the update.

 

[Phidelt218]

in Alaska we have to take samples out of the stream itself and match the gradation of the existing stream bed. Simple and straight forward.

that sucks to have to go through that testing. That is ridiculous.