Culvert replacement-7.5 m of existing embankment on soft clay 1

[Silty]

Hi All,
Could you please share your thoughts about this project.
The culvert and embankment was built few decades ago.
The embankment is 7.5 m high and relatively steep at some areas, generally 1V:1H to 1V:2H.
The underlying soil below embankment, problematic! :
Soft Clay,
SPT N=0 (sinking by weight of hammer), shear vane records available
LL= 55 approx. & moisture content=approx. 60% , so W>LL ,
The slope has low factor of safety, however was there since few decades ago and kind of consolidated the underlying layer to some extent, but still soft
--------------------
Questions:
1- Is trenchless method for culvert replacement a good idea or it will trigger failures either in foundation or slope?
2- is using geofoam to replace embankment is a good idea?
3- what about geo-grids? The objective is not increasing footprint of the slope by adding soil to flatten the slope to avoid over stressing underlying soft clay.
4 what kind of analyses are required? I think about slope stability, bearing capacity, plus hand/excel calculations of settlement.

Thank you for your contribution,

 

[fattdad]

to widen the road you'd need to make sure that there are no utilities that would fail from the settlement. To widen the road with fill seems it'll trigger primary consolidation. Heck, you may have to revisit secondary compression too?

We have a project like this right now. An existing road that needs to be widened. About the same problems too. Step 1: Get the geotechnical properties correct! Step 2: Analyze the behavior. Step 3: Look at schedule and the expectations of the public.

Sometimes, you have no choice but to make a pile supported embankment or use geofoam.

f-d

ípapß gordo ainÆt no madre flaca!

 

[Silty]

Thanks Fattdas, I like the step 1 to 3 that you mentioned.

 

[oldestguy]

Are your dimensions meters or feet? Your slope ratios need correcting. Easy done, just hit "edit" and fix" Also is Fatdad right about widening?

 

[GeoEnvGuy]

From your description I believe that the foundation has consolidated along the normal consolidation line. I also believe that the toe area has not been consolidated and would indicate the starting consolidation state. I also believe that the undrained shear strength has increased by the consolidation probably closer to Tao over sigma = 0.22 plus or minus.

I think immediately after construction your stability was probably closer to 1.0 as the clay strength was not developed and excess pore pressures were applicable. Now that consolidation has occurred the factor of safety has increased.

Relating to question 1. Trenchless will generate excess pore pressures. If your height is 7.5 metres it is unlikely your method would influence that wide across the embankment face, so globally your probably stable. Given the steep slopes your probably going to see shallow sloughs or local toe failures which could retrogress if the situation isn't monitored and doesn't stop operations following movement.

2. Geofoam to replace embankment? Am I to infer you want to remove the embankment, haul it away, and put in a new embankment with geofoam. This is probably very costly I don't think the owner would be interested in paying for that.

3. How do you intend on implementing Geo grids without increasing the footprint? If your looking to prevent a foundation failure plane your working outside the footprint. If your looking to prevent shallow slope failures you would need the geogrid to be in the embankment before you start the replacement. If your looking to place is across the face of the embankment they are usually implemented with soil anchors.

4. Culvert "replacement" not a new design requires you to determine the hydraulic performance of the old culvert size and determine what the new size needs to be and determine the material type suitable, then assemble the necessary standard engineering specs for your jurisdiction. If you are asked to provide a stability assessment ask if the client is intending to spend the money to increase the factor of safety for the embankment from the standards of a couple of decades ago to today. The settlement should be negligible if your not increasing the footprint and this is a true replacement. Any settlement would have occurred in the last decades.

 

[Mad Mike]

When considering the profile of old embankments based on excavations or borings beyond the toe, be careful of the assumptions you make regarding the old embankment foundation...

There may have been significant improvement carried out for that embankment that you're not aware of, most likely pre-loading at the least. Also, if the clays are that soft they wouldn't have been trafficable, so a bridging fill layer (or the remnants of a preload fill) would almost certainly have been used- there may be some coarse rock down there, extruded into the soft foundation clays.

I'm currently involved in a highway widening of several fills over soft clay, and in all cases, there has been significant cracking through the old asphalt- the load of the fill widening drags everything down with it as the foundation clays consolidate. In my case, we used coarse rock-fill as a foundation for the fill widening, but all that does is increase the consolidation rate.

It all comes down to budget I'm afraid...I had recommended a piled geogrid solution, but was scorned by the Roads department paying for the project...then once all the cracking started, I was scorned once again for charging a fee to assess it...Happy Friday!

All the best,
Mike

 

[Mad Mike]

Last point coming out of my post above- stick to your guns once you've come up with a design- so often the Client talks you into adopting a cheaper design and then holds you responsible for its poor performance in the long-term.

 

[Silty]

Thank you all, great points,
oldestguy- I fixed the dimensions and slope- it was 7.5 m, thanks for noting the mistakes

GeoEnvGuy- Great notes, the drilling was done last year, still showing soft clay, whatever consolidation happened, is reflected in the results. In fact, a thin top layer is showing some level of consolidation. Do you think keeping the fill, and first stabilising with soil nail and then using trenchless works? I am concerned nails will interfere with pipe being inserted.

Mad Mike - great points, pieces of wood were found in the samples at the bottom of fill, I guess they put wood plank road first to stabilize the surface, the project is in Northern Ontario Canada and according to my research it was a common practice in northern Ontario at that time.

I totally agree with you about how clients forget what they asked for. I am basically fed up with being treated like that.
Here is my strategy implemented since 2 yrs ago, it may works for you!
I give them options in my report mentioning costs and risks and let the client choose! so, I basically tell the client :if you want a five star work with minimal risk and low maintenance cost, you should pay more now, if not, take the ownership of the decision!
Happy Friday!

 

[GeoEnvGuy]

The project is in northern Ontario which is where I practise. You will find a lot of information about the clay crust with soft underlying clays in the Canadian geotechnical journal and in the works by Lerouil and Tavenas.

As for your plan of using nails that is not what I typically see from the MTO you should discuss with your client on which approach they want to consider. Probably not soil stabilization, only temporary protection during trenchless.

 

[HENRYZAU]

I have similar experience with a highway widening over deep soft clay. Both sides of the highway were widened using surcharge/wick drain preloading, a few cracks as large as 100mm+ were observed along the highway traffic direction. The cracks were filled regularly during preloading then the old pavement was removed and replaced with new one after temporary traffic diversion.

 

[ACtrafficengr]

What kind of culvert is it? Have you considered lining it rather than replacing it?

My glass has a v/c ratio of 0.5

Maybe the tyranny of Murphy is the penalty for hubris. -

http://xkcd.com/319/