Reinforced soil 1

[a8006]

while constructing a reinforced soil wall
is there any other reason for providing Machine direction(MD) of grid (reinforcement) wrt face of the wall for connection.

what if MD reinforcement provided parallel to the face of wall

 

[Ron]

Geotextiles often have different strengths based on the orientation. The MD and XMD strengths sometimes vary considerably, particularly for non-woven geotextiles. Match the strong orientation to the greater stress need.

 

[a8006]

Thanks Ron
I'm referring to typical HDPE uni axial grids
principal strength direction to be kept perpendicular to face of the wall.
what is the purpose/mechanism behind it
can any one clarify this.

thanks

 

[Erdbau]

Look on any geogrid manufacturer's website, and you'll find pictures of uniaxial and biaxial geogrid. The difference between the two should be obvious. The biaxial grid is (more or less) symmetrical and the uniaxial is not. Anything that doesn't have symmetry will have different strengths in different directions. A retaining wall only needs strength in one direction, so its not necessary to provide significant strength in the other direction. It's sort of like how you wouldn't use an I-beam with the flanges pointing in the same direction as the load.

 

[RFreund]

Agree with the above. The wall is designed so that the grid is holding the wall units in place (usually). Therefore there is tension on the geogrid. The thicker strands (most often) are run perpendicular to the wall. We have come across where this is done incorrectly more than once. Usually because its a long straight wall and the plans call for 5' of grid, the roll is usually 6' wide. So the contractor figures lets just roll it out. Don't do it. The reduction in strength was something around 50% for the couple of products we looked into. But there is no testing to support this claim.

EIT

http://www.HowToEngineer.com

 

[JML4TCM]

Uniaxial geogrids have strength in 1 direction, typically the Machine Direction (MD) or the long direction of the roll. The reinforcement must be place perpendicular to the face because that is the direction of the strain in a reinforced wall, it is a 2-dimensional system. Unfortunately, becasue it is quicker and easier to roll the material out parallel to the face, this is sometimes done by an inexperienced contractor or when there is not sufficient QC/QA oversight of the construction.
The Cross-Machine Direction (CMD or XD) is across the width of the roll and most uniaxial geogrids have a significantly lower strength in the CMD than the MD (often less than half). So placing geogrid in wrong direction will result in an insufficient reinforcement strength and affect the factor of safety of the wall.

 

[PEinc]

The attached photo shows an MSE wall that failed just for the reasons mentioned above. The contractor installed a single pass of uniaxial geogrid with its strong driection parallel with the block facing instead of perpendicular. The grids weren't long enough and were not installed in the strong direction.

www.PeirceEngineering.com

 

[egsa]

Hi a8006

contractors want always find the shortcut to finish the job very quickly and if there is no proper QC on site failures most propably will happen as showed you by Penc.

however sometimes consider the dimensions of the wall you might want to use a bidirectional geogrid that can be unrolled along the wall. you need to consider the following:

- a bidirectional geogrid is about 30-40% more expensive than a monod.
- the wall cannot be very high because the lenght of the reinforcement has to be less than the size of the roll. usually ggr comes in max 4m wide and considering a rule of thumb (L=0.7H - BS 8006) you can use this method for wall not higher than 5.7m

usually for very small walls I saw geotextile nonwoven being used. it is fine but remember that geotextile is not consider as reinforcement because does not show any long term properties, it creeps (stretches under constant loading) and does not have any tensile strength after couple of years.

if your wall is a temporary wall with a high not more than 5m without any restriction deformations, use a geotextile woven at bidirectional.

otherwise ggr monodirectional.

hope it helps

 

[JML4TCM]

egsa,
Good post except for the comment that geotextiles are not considered reinforcement. This is completely false. The highest strength geosyntehtic reinforcement on the market are geotextiles (less open area means more strength) and are used for reinforcement applications.
Nonwoven geotextiles are not reinforcement geotextiles but don't lump all geotextiles into the same pile, they are vastly different. Nonwoven vs. woven, PET vs. PP, yarn vs. fiber, etc. all factor into what each geotextile should be used for. Some are used for drainage, some for separation and some most definitely are used for REINFORCEMENT!