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Proper retaining wall construction relies on several aspects of a retaining wall to be constructed according to engineered specifications. The labor involved with constructing a retaining wall that will last a lifetime is not seen in the final product, but is the most important aspect of keeping that wall standing upright. One such component of the retaining wall is the backfill. It is important to note that backfilling specifications may vary from retaining wall to retaining wall depending on several variables. However, a retaining wall that is not retaining a heavy static or dynamic loads such as a road or driveway that is below three feet will have similar backfilling requirements. Most manufacturers have their own specifications for their walls constructed below this threshold or will offer those engineering services for retaining walls that will be constructed higher or experience heavier static and dynamic loads.
Retaining Wall Backfill
Properly constructed retaining walls require a specified backfill material and drainage requirements in order to relieve any hydrostatic pressure that may build up over time. Retaining walls experience a surcharge which is the pressure that the wall experiences from behind it caused by static and dynamic loads. Part of the dynamic loads that a retaining wall must withstand is hydrostatic pressure caused by water entering the system. If not backfilled properly and with the correct material, a retaining wall will be pushed forward by this hydrostatic pressure. This can be caused due to the weight and pressure caused by the water and the freeze-thaw cycles that may be experienced.
Best Backfill Material for Retaining Wall
To reduce the pressure, a 3/4″ angular crushed clear stone (ASTM #57) is used to backfill a retaining wall. Clear stone is an important aspect of this aggregate. Clear stone refers to the stone being washed and clear of fines. This allows for the passage of water with minimal resistance. In a retaining wall backfill, the purpose of this is for the water that enters directly behind the wall to work its way down to the base of the wall without being absorbed by a different material and putting lateral pressure in behind the wall. At the base of the wall, a perforated drainage pipe should be installed to collect that water and exited out the face of the wall or to a low spot on either end of the wall far enough that the water that exits will not cause failure to that retaining wall. The importance of the crushed stone being angular is for it to compact to an appropriate rate in behind the wall to reduce any settlement that may be experienced.
How to Backfill a Retaining Wall
After the base material of the retaining wall has been installed on top of a non-woven geotextile and compacted, the base course of the retaining wall can be leveled. Once that base course is leveled, the installer can then begin to build up the retaining wall ensuring that wall blocks are staggered to add to the strength of the wall. Behind the wall, a perforated drainage pipe should be installed at a slope of 1/8″ per foot to collect the water that makes its way to the base of the wall and begin to exit it out of the system. In addition to this, where the water exits should be considered as it should be far enough from the wall to ensure that it does not negatively affect the wall. Every 50 linear feet of wall requires a “T” pipe to be installed and the pipe to exit through the face of the wall at a long enough extension that the water does not flow back to the toe of the wall causing future problems.
Prior to the backfill and drainage pipe installation, if the base material of the wall was constructed using a 3/4″ crushed stone down to fines or Granular “A”, it would be recommended to place a non-woven geotextile on top of this base material prior to backfilling with the 3/4″ clear stone. This will prevent the contamination between the two. It is debated whether the same non-woven geotextile that is used to separate the base material from the subsoil should extend up the backfilled portion of the wall vertically. One side of the debate is that this will prevent contamination of the same soil and clear stone, whereas the other side contends that eventually the non-woven geotextile will become clogged with the fine silt of the soils and begin to add pressure to the back of the wall.
Once the drainage pipe is installed and the wall has two rows constructed, the backfilling process can begin. The rule of thumb is that the backfill should progress one wall block behind the wall construction. Therefore, if the wall construction is at two rows then the backfill construction should be at the level of the top of the first row with the wall construction coming first. With the backfill material placed, it can be levelled and settled with a mid-weight compactor while also not coming too close to the back of the retaining wall block with the compactor.
The use of uniaxial geogrid helps to stabilize the backfill material that is placed on top of it and is installed between two layers of retaining wall block extending into the backfill material. This should be accounted for prior to continuing to backfilling so that you do not have to remove any material in order to install the geogrid. Additionally, this geogrid may be specified to stretch past the 3/4″ crushed stone and into reinforced soil behind the wall where that geogrid will be installed into. This is specified in engineered drawings.
Retaining Wall Backfill Specification
The base of the wall should be installed at a minimum width of 6″ before the face of the wall, the depth of the wall block, and another 12″ behind the wall block. That 12″ behind the retaining wall is a minimum that must be carried up behind the wall to the top of the wall. This is where the 3/4″ crushed stone will be installed. If the engineered drawings call for geogrid, the length of the geogrid will need to be taken into consideration to excavate past this drainage area to allow for the backfilling of reinforced soil that the geogrid will extend into.
At the top of the wall, non-woven geotextile will be placed on the top of the drainage area where low permeability soil will be placed on top followed by topsoil and sod or seed. Creating a swale in behind the wall to collect and / or redirect water is additionally helpful in controlling the flow of water that will be experienced behind a wall.
No two retaining walls are the same and there are many variables that need to be considered when deciding how to construct one that will be able to withstand the loads that it will be subject to. These variables will affect the specifications of the retaining wall. It never hurts to consult with the manufacturer or the engineered drawings already available from the manufacturer.