10 m High Embankment on Soft Clay+Stone Columns 5

[AlaaElnahas]

Dear All,
I am designing a road embankment (35 m wide at its crest )with its height increasing from 4 to 10 m along 420 m to cross a bridge. After the bridge the embankment height goes down from 10 to 3 m. The underlying soil consist of: about 0.9 m Top soil+Man Made Ground , underlain by 2.5-6.2 m Recent Alluvium which consist of V. Soft to Soft Clay, V. Soft to Firm Silt, and Peat. The clay and silt have some traces of peat and organic substances. The SPT in the clay can be as low as 2, and cu of 8-30 kPa with average of 20kPa. The peat thickness is up to 2 m and its bottom level can be as low as 3.9 m below the ground surface. In the geotechnical report, it is mentioned to rising embankment (420 m long)For the Raising embankement, the geotechnical report suggested improving the recent alluvium using band drains below the first 280 m length, and Vibro Concrete Columns with tension membrane for 110 m + CFA piles with tension membrane for 50 m. We are amending the geotechnical report and can make some changes. The cost of the VCC, CFA piles and tension membranes for the embankment and the other one on the other side of the bridge is about 1.2-1.5 million pounds. The tension membrane alone would cost 0.9 Million pounds and the rest for the VCC and CFA piles. I am thinking of using the stone columns with bands drains for the shallow depths of the embankment (about 3-5 m height), and have the following questions:

1- How to make sure that the clay will not be squeezed inside the stone columns and block the drainage path by filling the stone voids?

2- How to choose thye stone grading (is it a filter grading)?

3- How to make quality control on the stone columns (SPT or Plate Load??). Is the SPT is meaningful in stones?

4-How to make sure that the lateral clay displacements below the embankment slope during the clay primary consolidation will not cut the stone columns and subsequently jeoparadise the embnakment slope stability?

5- Is the stone columns a good solution, given the fact that we have peat (fibrous and in some places pseudofibrous) and organic clay and silt, which means good chances for long secondary consolidation? If the answer No, is there a cheaper solution than the Vibro concrete Columns and CFA piles with tension memberane?

6- Is there a design manual to design the improved ground with stone columns?

Thank you
Ala'a

 

[BigHarvey]

You have Cu values too low for stone columns. The columns need soils which can provide a minimum lateral strength in order to compact the stone.
For places where you have less than 5 m of compressible soil, you can think of dynamic replacement ( dynamic compaction family ) with or without band drains.

 

[eric1037]

I have experience with stone columns in soft clays and organic soils. They don't work well and you could get long-term, excessive settlement due to the issues that BigHarvey brings up.

 

[AlaaElnahas]

Thanks for your replies. The reduction in settlement rate was probably due to the reduction of Cv, as the soil between the columns was disturbed during the columns' installation. However, I think that the settlement magnitude was probably halved (due to the stress concentration on the columns). Prof. Das in his book "Principles of Foundation Engineering" mentioned that the technique work more effectively when it is used to stabilize large areas with cu=10-50 kPa. If you look at the Book of M. Kitazume "the sand compaction piles", you will find that the technique has been used in Japan in areas with SPT blows about 2-4. The average of the clay in my site is about 20kPa, but we have 8kPa measured by hand vane in extracted samples. The Peat gave N=5 in some borings. If I use Stone Columns with high area ratio (as=0.6), would it be possible to build a 10 m high embankment on top of it without failure (even if no improvement in the rate of settlement), and without cutting or clogging the columns below the embankment slope with the sequeezed clay during primary consolidation? If the answer is yes, how would the columns affect the long term settlement (creep and secondary consolidation)?

On the other hand, are their case studies or design manuals for using the dynamic compaction with band drains in clay?

 

[BigH]

I'll tell you how we recently did it in very soft alluvial soils in the subcontinent. Site conditions were 6 to 8m of very soft clay (some peaty pockets) - N values in the range of 0 to 2-3. Su values 20kPa on average. Underlying the very soft clay was a firm clay (N values in the range of 8 to 10 - Su values in the order of 35 to 50. Below about 12 to 13 m was dense silty sands/sandy silts, clayey silts, etc. - quite competent material.
Computations indicated that we could build the embankment to 4.5m or so (4 or 5 m depending on who was analyzing the situation)and have some reserve of safety. We utilized band drains (i.e., pvd wick drains) at spacings of about 1.5 m rectangular. We construted in stage fashion - 4m initially, wait for consolidation to 90% or so, utilized the increase in the Su value to place 3m more; again waited, added 2m more and waited and then topped it off. We were able to build this (and 11m high RE Walls successfully). Settlements ranged up to 800mm for the RE Wall construction - slightly less for embankments. Waiting time was about 45 to 50 days between stages. Stages were built 'not so fast' due to fill availability etc. You could reduce the waiting time if you put the pvd's in at 1.2m spacings.
Due to the time constraints, we didn't have 'time' to build out the settlements of the pavement layers - I would suggest that you overbuild your embankment by 1 m (maybe another stage), let it sit for the waiting period, then remove and replace with the pavement structure.
There will be some long term settlements with time, but you would put on regulating layers as needed for several years - especially at the bridge end slab.
It is buildable without resorting to methods other than the vertical drains - but need to think through the settlement issue on the final construction.
On issues like this you will find several possible choices - and it depends on experience and comfort levels of the individuals proposing the various options - you will need to find your own way - good luck.

 

[singingk]

In germany stone columns were sometimes used for ground improvement in soft soils with an undrained shear strength down to 15 kPa. Approved methods for the design of embankments founded on stone columns are given by Heinz Priebe (settlement and slope stability). The Method for estimating (!) the settlements caused by primary consolidation (!) is described here

http://www.vibroflotation.com/06.pdf

Unfortunately I can’t give a reference in english for the estimating of the improvement of the shear strength, may the experts of keller-grundbau can give some advice. The settlements caused by secondary consolidation can only be crude estimated (maybe according to Buismann).
If there is enough time staged construction and surcharge in combination with drains like proposed by BigH will be the cheapest way to build the embankment. According to our experiences the surcharge at the abutements should be the 3/10 of the height of embankment to minimize different settlements between the embankmant and the abutement caused by secondary consolidation especially of the peat.

 

[rochplayer]

We performed a similar embankment construction program as outlined by BigH. Our site had 8 m of N5-10 ML-CL with fibrous peat layers up to 1.5 m thick, underlain by 8 m of N = 25 to 50 ML-CL, underlain by 30+ m of N = 0 to 2 ML/ML-CL. Very interesting geology. Embankment widths were 100 m wide and up to 10.5 m high at the bridge approaches.

We used Su/p ratios based on index properties to estimate the shear strength increase with increased load and with time - as pore pressures dissipated an incremental increase in strength is gained. We timed the staged constructed based on the minimum strength needed to add the next lift and installed pore pressure monitors to confirm our calculations. We also installed settlement monuments at the base that continued up through the fill, with extensions added with each lift. These were monitored to track settlement changes with time as well.

NAVFAC DM7.2 has graphs for determining pore pressure dissipation during staged construction.

Good luck!

 

[rochplayer]

Another option is to use geogrid as base reinforcement to tie the roadway fill together and prevent slope failure as the fill is brought up. Tensar geogrids have been used very successfuly to build high embankments over thick peat deposits.