Disused quarry landfill

[NJonesUK]

I've been asked to have a look at a landfill project as pictured in the link to check if the design will be stable for a different use other than the purpose it was originally designed for.

http://img115.imageshack.us/img115/9415/landfill1.jpg

The landfill is placed in a disused quarry and the frame that has been designed is built up in 3m sections so as more waste is required in the landfill, another 3m section is added and so on until it reaches the quarry depth of 35m or so.

The HDPE liner is backed with a 3mm wire mesh and a framing system, this in turn is back with 500mm thickness bentonite enriched sand (BES) as an impermeable layer behind the HDPE membrane and then this is backed by an uncompacted rock fill, (say avg 50mm particles) which extends from the back of the BES to the solid rock face which varies (due to the shape of the quarry face) from between 300mm of rock fill to 2500mm of rock fill.

I've shown that the rock bolts are stable and strong enough to take required loadings and that the mesh and frame system is up to scratch however my Director is concerned that the height of the project may cause something to fail within the BES layer. i.e. He wants me to show that the height of the rock fill won't cause a large lateral earth pressure to appear against the BES and 'squeeze' is out the top of the frame system or cause it to fail in another way.

In this round about question, I was wondering how I could show that this is or is not the case as I can't think of a way to show it myself.

Would massively appreciate the help,

Nick

 

[ishvaaag]

I find a solution that however adds cement to bentonite;

Deep Excavations, a practical manual 2nd ed., Malcom Puller
Thomas Telford ed. 2003

gives at p. 39 a relationship

R=10^4x(cement/water ratio)^2 kPa

valid in the range of water to cement ratios .1 to .7

and gives some example of mixes with about 29 kg of cemént by 1 m3. Since the HDPE is to be protected by the wire mesh if close enough maybe adding coarse angulous aggregate will ensure even bigger reliable compression capacities, and depending on the aggregate, to no loss of impermeabilty in the backup. So I would start asking some bentonite mixes providers.

 

[cvg]

I believe adding cement to the bentonite may be the wrong move if it allows it to form cracks which will then allow leachate to escape or groundwater to enter the landfill. The BSE should be maintained in a plastic state to provide good resistance against infiltration. How about mechanically stablizing the BSE and / or the rock fill with a horizontal geonet or a geotextile layer at every lift?

 

[NJonesUK]

I believe there may well be a wire mesh diaphragm every 3m lift. I'm not sure though...

I'm not too interested into trying to stabilise the BES until I am sure that there will be a problem with it. Do you think there is a realistic probability that the backfill will cause an issue such as the one I described in the original question?

Cheers,

Nick

 

[ishvaaag]

I would think not; a 40 m deep foundation wall has been built in Amsterdam without problem (however a short term situation)

http://www.royalhaskoning.com/NR/rd...C9D-75FC2303D3A2/10532/Diaphragmwall_test.pdf

and mixes of bentonite slurry walls are tested to 1000 kPa

http://www.geo-solutions.com/tech-papers/pdf/TP-GeoInst-TimeBehaviorSB05.pdf

and it seems it can attain significant swelling pressures

http://www3.imperial.ac.uk/pls/portallive/docs/1/7293179.PDF

By the way use of evaluation of attainable swell pressures in expansive soils may apply, or variants of. So a look to this aspect in geotech books may be worth the effort.

plus it seems is used for barrettes to 100 m depths

http://www.linkedin.com/pub/antoine-victor/9/7b9/b85

You likely won't need to stabilize and create infiltration rixk, it is mor a matter of finding a proper bentonite slurry provider, I think.

 

[ishvaaag]

errata,

risk,

sorry, a keyboard 1 month old and some letters already invisible. Plus wandering fingers.

 

[Patgeotech]

NJONESUK, an interesting problem,

I agree with ishvaag on adding the cement to the system in a controlled manner, but from a different cement approach. What if the rockfill material is replaced with a cement stabilised sand (say 4% cement by mass) or cement stabilised - BES layer could be left as is to perform its specific function. This would enable easy placement, compaction would not be a major problem - medium type compaction would do the job and it would almost 'self-compact' with time. Settlement would not be a problem with height increases as the cement stabilised sand would result in the lateral pressure acting more into the in-situ rock face rather than in the rock fill.

Once the rock bolts are installed, any movement in the rock fill will result in shear force development along the rock bolts i.e. the rock bolts will start working. This will result in a development of a stressed cone behind the rock bolt face plate - provided the rock bolts are spaced sufficiently, the backfill material has nowhere to go and will be confined or fixed in place.

I would say that there is no need for geonets/geogrids as that function will be sort of performed by the rock bolts themselves (depending on spacing).

One probably also has to be careful with the rock bolt arrangement as well (assuming fully grouted bolts), the grout would crack when called upon to start working, which could potentially lead to groundwater ingress though cracks. Was this aspect discussed in your project? How is the rock bolt arrangement fixed along the BES layer?

Hopefully the above thoughts/comments will aid your thinking.