Q:

How are earthfills not eroded before they are overgrown with grass?

Almost every railroad and vehicle road landfill has its slopes overgrown with grass which is quite good at preventing erosion. However it takes some time (a year or more) for the grass to grow and emit enough roots to form a reliable mesh that would hold the earthfill surface and rains can cause enough damage to the earthfills before grass grows and becomes efficient against erosion.
How are earthfills not eroded by rainfall before they are overgrown with grass?

A:

The slope angle of such slopes is constructed to be less than the angle of repose for the material used in the slope. The angle of repose is the great angle to which granular materials can be dumped without slumping.

The higher the cohesiveness of the material, the higher the angle of repose. Stability of such a slope will be improved by compacting the material during construction.
The only way to know the angle of repose for different materials is via testing.
The shallower the slope angle, the slower the speed of water run-off, resulting in reduced erosion capabilities of water on slopes. Another method that assists in slope stabilization is to use geotextiles to protect the surface of the slope. Additionally, slopes can be sprayed with a mixture of grass seeds and bitumen. The grass seeds develop roots and stabilize the surface while the bitumen hold the soil in place. Over time the bitumen breaks down.
In arid regions, an initial stabilization is achieved by ensuring a quick growing grass develops a network of roots to lock in the soil. The grass is then deprived of water and allowed to die, but the network of roots remains. Once this happens, the long term stabilizing grass seed is allowed to grow on the slope, establishing it own network of roots.

A:

There are a few methods that are used to control erosion before permanent vegetation is established. The exact method used can depend on location (also country), season, preference, governing laws, etc.
Mulching
This is almost always done. Mulch is frequently straw but can also be ground up newspaper. This helps to initially keep the soil from eroding while plants are growing. It also somewhat helps retain moisture in the soil so that plant will grow easier.
The mulch is sometimes pressed into the soil with tracked equipment, or a type of netting is placed with the mulch. Both of these help to keep the mulch from blowing away.
Hydro mulching is also a method that is frequently combined with hydroseeding.
Seeding
The choice of seed is also a factor. Many times a portion of the seed used is a variety specifically chosen because it is quick growing. This plant may not be a species that will survive the winter, but it serves as a cover crop while the permanent vegetation grows.
Mechanical Means
In addition to the effects of mulch, other measures can be taken to slow down the speed of water flowing down a slope. Slow moving water is more likely to soak into the soil. Water that doesn't soak in will be moving slow enough that it won't cause erosion.
A few of these methods are:

Grade the top of slope so that rain water isn't funneled to one point. Counterintuitively, this means making sure that there isn't a small berm at the top of slope. A small berm will eventually fail and cause water to be directed to one spot, causing erosion.  Without a berm, any water trying to flow down the slope is more evenly distributed.

Install terraces or fiber rolls staked across the slope. These help to keep the water from flowing in a straight line down the slope and causing erosion.

Mats or wattles of natural material can be placed. These function similarly to mulch, but may withstand higher water velocities in areas likely to flood.

Rock of various sizes can be placed. Plants will eventually grow through the rock.

It is usually not recommended, but in the past, an asphalt emulsion was sprayed on slopes to keep them from eroding. The thought was that plants would take root as the asphalt formed cracks.

A:

They lay turf pads down to kickstart the grass coverage. Even the loose pads will have better erosion protection than bare soil.
Other options include laying down a sheet with holes for plants/shrubery to grow through, the sheet will let enough rain through to let the plants grow but prevent the hillside from eroding until the planted shrubs are robust enough to hold the soil.

