Anchors for concrete block retaining wall 4

[ivanlocke]

I am designing a ~10' tall retaining wall to a rock hopper (wall will need to support loader dumping into hopper) using 2'x2'x4' concrete blocks with interlocking bumps - somewhat like legos. I am looking at what options I have for anchors.

One option I am looking at is a geogrid laid between the concrete blocks back into the soil as we backfill. Would anyone know where I could find a coefficient of friction value between the geogrid and concrete?

Another option I would like to look at is the metal anchors with a "T" that pulls against the face of the block and a shaft lined with teeth which is backfilled over. Would anyone know the name or where I could find information about such anchors?

Any other ideas would also be appreciated.

Thanks.

 

[jdmm]

We use a 2.5x2.5x5 foot concrete block around here. It is made with return concrete and known as Lock Blocks. Are your blocks the same?

If they are then we have built many walls using Tensar UX1500 and stronger placed between the blocks in the same fashon as say Allen Blocks. There is usually some damage to the Tensar so you would want to use a higher than normal installation damage reduction factor. I have used Paragrid a pvc coated polyester as well. We have built walls to 30 feet without a problem.

 

[ivanlocke]

Yes, the Lock Blocks are what I am using.

 

[jdmm]

I did a little connection test with the inventor of the Lock Block back in about 1988. The test involved placing a 2 foot wide layer of Tensar UX1500SB between two blocks sitting on the ground. We connected to a piece of the geogrid and tried to measure the force required to pull out the geogrid. The geogrid failed progressively because the individual strands were pinched between asperities in the concrete surface. We got a very low connections strength. We then puddled high early grout over the geogrid before placing the block on top and were able to get 100% connection strength.

You probably don't need the full connection strength. The only program that I know of that can properly model the Lock Block plus geogrid is ReSSA. You can do it by hand by looking at all of the two wedge failure surfaces at each level assuming that the lock block is a passive wedge and the active wedge passes through the geogrid. You need to analyze it for all of the potential active wedges as they pass through the layers of geogrid. Assigning a connection strength will allow you to determine if the blocks want to peal off or if the failure is through the grids.

 

[ivanlocke]

Thanks for all your help jdmm.

When you place the geogrid over the lockblock, did you cut and "x" or some kind of hole where the interlocking bumps are? We were throwing around the idea of using a caulking as an adhesive between the blocks with the geogrid, as mortar be harder to get an even thickness over the length of the wall and keep the blocks level. Would we need to completely fill voids with the caulk to gain sufficient strength? If so the cost would be more preventative than the use of the mortar.

 

[jdmm]

ivanlocke

I initially tried cutting the grid to fit but eventually I ended up just letting the grid buckle at the back of the block. As I said earlier, Tensar UX1500 and UX1600 are normally used but recently I have been using a PVC coated polyester called PARAGRID. The spacing of the grid is wider and it is more like straps that are flexible. They seem to fit better over the crosses. The product is marketed by Maccaferri the gabion people. It comes in very high strengths such as Paragrid 80/15 to 150/15. I used it to create a double lock block wall barrier against rock avalanches along side a railway in the Rocky Mountains. The wall was 30 feet high and 15 feet thick.

 

[geomo]

If you use a cementatious grout in contact with the geogrid, be careful about chemical degradation of the polymers. Some geogrids can handle it and some can't. HDPE grids, like Tensar, are OK. Some polyester and polypropylene grids have problems. PVC is probably OK, but installation damage to the coating is hard to prevent, leaving the primary reinforcement polymer potentially vulnerable. Manufacturers should be able to answer any questions.

 

[doresh]

Hello to you,

As a consultant for Keystone reinforced earth retaining walls systems, i prefer to use the Fortrac geogrid which is made of polyesther and introduce high tensile resistant up to 110 Kn/m (ultimate). We use the BS 8006 as the leading code for this kind of walls, Also, the Australian code can be used.
The Fortrac geogrid has been teste dby The BBA and as result one can find all the design properties of the material related to reinforced earth. In any case, you have to determine the soil you are going to use in the reinforced zone and also limit your friction angle to a maximum of 34 degrees.

 

[kmerl]

Manta Ray anchors are a system of "T" anchors that are preloaded to the working capacity and offer a positive wall anchor contact. These terminate at the soil wall interface with any anchor that can be attached ot a"all thread." I would not suggest using the geogrids as a method to hold the wall from toppling. If you use the geogrid design them to stablize the slope as if the wall wasn't there. If the grid system cannot support the soil face you have to rely on the wall to do the work.

In that case you can calculate the modified effect phi angle of soil post- geogrid installation and determine if the block are heavy enough under there own weight to resist sliding. Otherwise a new system is needed.

 

[ivanlocke]

kmerl,

I take your suggestion as I should model the wall and retained soil completely separate as if there is no connection strength between the wall and geogrid. How would I calculate this modified effect phi angle? If I test the connection strength and can be assured of an adequate amount, your method of analysis would be overly conservative?

thanks

 

[kmerl]

Correct, but if I understand the problem correctly the problem is that the concrete blocks cannot support the weight of the equipment based on the strength parameters of the soils porposed to be placed behind them. I apologize for not expaining myself throughly enough at first.

IF you can desing a grid system that can support the slope then you don't have to worry about a connection that can be difficult to verify during construction

If this is the case the geogrids can be designed to effectively adjust the strength parameter of the soil, that is the basic idea fo reinforced earth slope. The most conservative way is to design a geogrid layer that could effectively support that weight without the wall, however that may not be possible.

So rather than relying on a positive ocnnection between the grid and the block to act as a achor, design a grid system that increases the phi angle of the soil so the block (using the interlocks they have ) can support the effective horizontal loads. To calculate the new phi you can use any slope stability modeling software capable of using geogrids in the soil parameters. Many manufactures of geogrids will assist you or offer you programs based on there products and research to calcualte the effective change.

It is true that if you know the actual pullout failure of the grid and block system that it could be used with factors of saftey, I have no problem with that, just offer up alternative solutions. Keithe J. Merl

http://www.princetonhydro.com

 

[layfieldman]

I'm not sure if you are stuck on using the block that you mentioned because you already have it but there are blocks out there with a built in positive connection with certain types of geogrids. Many geogrid manufacturers also have tons of connection data for various blocks as it is how they market their product. If you have the program SRWall, sold by the NCMA, many companies have downloadable data files that not only include block and geogrid specifications but also connection strength data. If you do not have SRWall most companies still have this data in a table or graphical format that you can download off their websites.

 

[ivanlocke]

Are you talking about blocks like Keystone and Allenblock? They are nice, but from my pricings, the lock block wall is about 30% the cost.

 

[layfieldman]

That is true due to the existing patents on thos proprietary blocks but it does make construction easier and with a positive connection it practically eliminates bulging and local sliding failure. I agree with jdmm that the connection strengths are very low as the irregularities create uneven load distribution on the geogrid and you result in local failures. I have seen the testing on some geogrids at the Bathurst/Clarabut testing facility in Ontario and you could see the local failure of geogrids at such locations.

 

[DRC1]

Have you considered building a gravity wall with the blocks?

 

[ivanlocke]

I did take a look at a gravity wall. It was fairly comparable with doing the geogrid wall, although much more excavating and larger concrete pad. I think we were looking at the base needing to be 5 or 6 blocks deep, which would be 12.5'-15'.

 

[Tryan]

What's with all the stars?? One of them is for a question!!

 

[MSEMan]

Ivanlocke,

Some of your problems may be arising out of your choice of facing material and the compatablility with the reinforcements you want to use.

We did a 15m high cursher headwall in Ghana a few years ago. It carried Volvo A40 dumpers, (125 tonne loaded weight.)

The key design concerns with this type of structure is soil/reinforcement strain (which you want to be low) and accomodating the very heavy dumper loads. If your dumpers are very heavy (as ours were) you may want to consider a reinforced concrete raft at the discharge location. You can then analyse the loads from the dumper using a bousinesq distribution, as you would for a bridge abutment.

For the facing, we used a steel reinforcing mesh facing with geotextile behind it to retain the fines. This won't bulge if you keep the reinforcement spacings reasonable. For soil reinforcement we used ladders made from steel mesh, bent the free ends on site into hooks to attach to the facings, which provides a positive connection.

All very cheap and cheerful, and more importatnly very effective. It was built with local labour in the East Ghana jungle.

 

[wallguy]

There is another large block gravity system on the market. Go to

http://www.westblocksystems.com

and look at their modular block system. It is also a lego crib type system.