According to 14387 National Standard which itself is arranged according to ASTM 1759 Standard, polyethylene manhole should have the tolerance for horizontal (radial) loads caused by the pressure of the surrounding soil and also vertical (axial) loads caused by the weight of soil and loads forced on it. According to this, the backfill of the manhole should be prepared and implemented in a way in which it would bear these loads alongside the manhole’s polyethylene structure, without retaining its sustainability during years of utilization. If the bed’s soil was infirm and flooring wasn’t possible, pre-made concrete with a thickness of 15 centimeters can be used instead of soil.
This standard, defines the backfill of manhole as being from class I soil with 90% density and manhole’s base with 95% density. This backfill has elastic and unshrinkable qualities and can sustain about 0.8 MPa. Since condensation of the manhole’s surrounding soil is not easily possible, considering that we don’t want to remove too much of the intact surrounding soil, we should use substitute mortar.
According to standard’s definition
In case of having appropriate soil at the site and separating its large stones and rubble, this mortar can be easily used which has a great similarity to the standard’s definitions and doesn’t chemically react with the polyethylene manhole or corrode it at all.
American Concrete Institute, in introducing backfill mortar cement, has introduced a kind of cement mortar which has different applications in different sites with different cement contents, such as the back of foundations or around manholes … which is called CLSM. This backfill suitable for the surrounding (CLSM) is merely a cement mortar and is not considered as concrete at all, because its content is about 70 – 80 Kg/m3 and has the following attributes:
- Has elasticity
- Is self-compacting
- Is unshrinkable
- Is not breakable at all
- Can sustain about 0.4 - 0.6 MPa
- Pressure resistance of 0.1 MPa after one day
- Pressure resistance of 0.5 MPa after 28 days
Manhole Installation General Schematic Alongside Required Substances and Materials
Mixture of the cement mortar ingredients (CLSM)
Gravel (up to 19 mm)
How to fill the area around the manhole:
After the installation and fitting the manhole and also levelling it, the filler material should be layer by layer and evenly poured around the manhole to maintain the vertical alignment and to ensure that the filler is uniformed and without any holes. Avoid pouring all of the filler materials at the same time.
Using the compactor in order to compact the soil around the manhole
In order to position the manhole, a tight base should be provided. So for doing it, you should clean the excavation floor at the first place and then foam and smooth it using compactor. If that is not possible, we should use at least 10cm concrete instead and slick the bottom of the manhole.