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So you want to build a retaining wall and you&#;re researching geogrid? That&#;s good, because geogrid is a fantastic tool that will make your retaining wall much more robust. This article focuses on small block retaining walls typical of DIY residential retaining wall projects that a lot of homeowners build. These blocks don&#;t require equipment to move and can be hand placed.

There are two main types of retaining wall &#; gravity retaining walls and reinforced retaining walls.

A gravity retaining wall is a retaining wall that relies solely on the mass of the retaining wall blocks to hold back the retained soil. Quite rare for small block retaining walls &#; only the shortest of walls should be built as a gravity wall when using small blocks.

&#;Gravity&#; Retaining Wall Example Design Cross Section

In this article we will be focusing on reinforced retaining walls. A reinforced retaining wall is a retaining wall that relies on both the mass of the retaining wall blocks and the geogrid that is installed behind them. If you&#;re not sure what geogrid is or how it works (in a retaining wall or otherwise), check out this article to brush up.

&#;Reinforced&#; Retaining Wall Example Design Cross Section

Reinforced small block retaining walls can be engineered to be very robust &#; geogrid is extremely important to the strength of reinforced retaining walls!

When Should I Use Geogrid In A Retaining Wall?

The best person to answer this is a geotechnical engineer familiar with the many different site conditions that might be present on your property. While this isn&#;t exactly helpful if you&#;re looking to DIY &#; don&#;t worry. We&#;ll still give you some guidance in this article.

Depending on where you live, your governing body will have rules regarding retaining wall heights and the maximum height you can build without having an engineer involved. In Calgary, any retaining wall over 1.2 m needs to be designed by an engineer.

If you&#;re not sure what the rules are where you live you can find out by calling your local permitting office or by calling a local geotechnical engineer or retaining wall builder. Once you&#;ve got them on the line, briefly explain your project to them and let them know the maximum wall height you&#;re planning on building. At Factor, we are happy to help homeowners and DIYers over the for questions like this (free of charge of course!)

If your wall is taller than the maximum allowed height, you&#;ll either need to hire an engineer, or you&#;ll need to change up your plans.

I Don&#;t Need An Engineer! Now What?

Since your wall is short enough that you don&#;t need an engineer you can use a rough rule of thumb for geogrid length in your wall.

Geogrid Length = 0.8 x Retaining Wall Height

So if your wall is 5 feet tall you&#;ll want 4 foot long geogrid layers. For small block walls, geogrid is typically installed every second block layer, starting from the top of the bottom block.

Just be aware that different types of blocks have different requirements and the recommendations given here may not be applicable to every situation and block type. You should check with the block manufacturer&#;s recommendations for geogrid lengths.

Drainage

For more information, please visit Shuangcheng New Material.

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I know this article is about geogrid, which doesn&#;t really have anything to do with drainage&#; but your geogrid layers are going to interfere with your drainage blanket.. and installing the drainage blanket is a total pain with geogrid.

In case you don&#;t know what a drainage blanket is &#; it is a 300 mm (1&#;) thick layer of washed gravel completely wrapped in geotextile filter fabric installed directly behind your retaining wall blocks. The drainage blanket is very important for the performance of your wall &#; it prevents the build up of excess water pressure caused by rain, groundwater, snowmelt, irrigation, etc. Excess water pressure on your wall can cause a whole bunch of problems, including complete failure.

In this image (same as the one at the start of this article), you can see the individually wrapped sections of drainage blanket being intersected by the geogrid layers.

Drainage Blanket Sections Individually Wrapped at Geogrid Intersections

How Do I Install Geogrid In A Retaining Wall?

Assuming you have completed the very important first steps to building a retaining wall (excavated organic materials under the wall alignment and built a smooth, compacted gravel levelling pad), you are ready to place your first layer of blocks.

Making sure your first layer of blocks is level and has been sufficiently backfilled on the front side of the wall (200 mm or 8 inches is usually good for walls under 1.2 m) you can begin the installation as follows:

1. Build your first section of drainage blanket
2. Place backfill behind the wall to the height of the first block and to at least the length of the geogrid you will install
   Geogrid Length = 0.8 x Retaining Wall Height
3. Once the backfill is sufficiently compacted and is the same height as the first block layer, lay the geogrid on the first block layer
   • The edge of the geogrid on the lower block should be placed as far forward on the lower block as possible without sticking out of the face of the wall (you shouldn&#;t be able to see it once you&#;ve built the wall).
   • If you&#;ve bought biaxial geogrid (a.k.a two way geogrid), you can just roll it out along the length of the wall, assuming the roll width is wide enough to satisfy the Geogrid Length equation.
   • If you&#;ve bought uniaxial geogrid (a.k.a one way geogrid) you will have to cut it to the length you need and lay it so that the thick or stiff strands in the geogrid are perpendicular to the wall (going into the blocks).
4. Once you&#;ve covered the entire first block layer with geogrid of the correct length, you can now place the second block layer to pin the grid in place.
5. With the grid pinned in place by the second row of blocks, carefully begin building your next drainage blanker completely wrapped in geotextile and adding backfill on top of the grid, making sure that the grid stays tight
6. Compact the backfill in place
7. Add the third row of blocks
8. Place backfill and compact in place to the height of block row 3
9. Place geogrid on the third row of blocks the same as you did in Step 1/2/3
10. Continue until you&#;ve reached your wall height, remembering to compact every backfill layer, building your drainage blanket, and placing geogrid on every second layer

Roadworks are fast-moving with unexpected ground conditions that emerge once the topsoil is stripped and soft spots are identified. With the pressure on to keep projects on track, a common question asked is &#;how can geogrids help keep my project on time?&#; 

Roadworks are fast-moving with unexpected ground conditions that emerge once the topsoil is stripped and soft spots are identified. With the pressure on to keep projects on track, a common question asked is &#;how can geogrids help keep my project on time?&#; 

A good rule of thumb is for low CBR subgrades, geogrids will reduce the subbase by 1/3 or increase ESA&#;s by 1/3. 

The next question is &#;which geogrid do I need?&#;

Geogrid selection works in two ways:

1. Refined Calculation

The design engineer calculates a pavement design considering subgrade CBR, road base, ESA&#;s and the geogrid&#;s mechanical properties. Reputable geogrids will have a strong track record, with independent testing carried out in both the lab and full-scale field testing such as that done by the US Army Corp of Engineers.

The design engineer should have all the mechanical properties of the geogrid to input into standard commercial design software. Note that the use of proprietary manufacturers&#; design software should only be used once design output legal liability is verified. This is because liability for the design needs to be clear should a pavement not perform. 

Once the parameters have been entered, the design will then specify a subbase thickness and a tensile capacity. Typical generic grids have biaxial tensile capacities of 20, 30 or 40 kN/m.

2. Trial and Error

In some cases, a project begins and unexpected soft spots are encountered on site. Due to the time and cost involved to stop works and then engage geotechnical engineers and design engineers, it is not practical or cost-effective to design a complete from-scratch solution. 

In these cases, the project manager on site can work with the geogrid technical team to trial a grid in order to overcome the issue quickly and cost-effectively. 

Typically a grid will be laid in at subgrade level and 300mm of subbase laid over the top. At this point, the layer is proof-rolled and if deemed satisfactory, usually with a differential settlement of less than 20mm (or other field criteria), the next compaction lift is carried out. 

If it is still soft with rutting occurring under wheel loads, then another geogrid is laid and a subsequent 300mm of subbase is laid. 

There is an art to choosing the correct starting biaxial tensile capacity of the grid: something that an experienced geosynthetics supplier can assist with. Some criteria to take into account include the road&#;s importance, its lifespan, traffic volume, installation time, costs and subbase availability. 

As an example, a 40kN geogrid is preferred on very soft CBRs to minimise the subbase depth and maximise construction speed. Multiple 20kN grids are preferred on higher subgrade CBRs or where subbase thickness will exceed 300mm. Geogrid will typically stabilise up to around 300mm of subbase. 

In all cases, it&#;s wise to refer to your local State Road Authority guidelines, for example, QLD MRTS 58 for Road Subbase Reinforcement using Geogrids/Geocomposites.

Other geogrid selection criteria:

There is a range of other factors that go into choosing the right geogrid, including: 

• Subbase particle distribution &#; generic biaxial grids will stabilise a well-graded subbase up to 70mm. 
• Use of geotextile-laminated geogrids or a separate geogrid and separation geotextile 
• Special consideration for pH ranges beyond the 4 to 9 limits
• Roll width to reduce the potential for wastage: geogrid widths vary but are typically 4m to 6m. Selecting geogrids with the appropriate width will help minimise wastage.

Advice on choosing geogrids for pavement applications 

When chosen correctly, geogrids over offer substantial time and cost savings. 

For more information on product selection, view the Polyfabrics range of geogrids or get in touch with the technical team at Polyfabrics. 

For more information, please visit Composite Geogrid.